UNIT 1- MECHATRONICS, SENSORS AND TRANSDUCERS

1.1. INTRODUCTION TO MECHATRONICS SYSTEMS:

Mechatronics is a word originated in Japan in 1980s to denote the combination of technologies which go together to produce industrial robots. The word, mechatronics, is composed of "mecha" from mechanism and the "tronics" from electronics. In other words, technologies and developed products will be incorporating electronics more and more into mechanisms, intimately and organically, and making it impossible to tell where one ends and the other begins.According to the Mechatronics Forum, UK a formal defamation of Mechatronics is "the synergistic integration of Mechanics and Mechanical Engineering, Electronics, Computer technology, and IT to produce or enhance products and systems." W.Bolton defmes Mechatronics as "A mechatronic system is not just a marriage ofelectrical and mechanical systems and is more than just a control system; it is a complete integration of all of them." A graphical representation of Mechatronics, as shown in Figure 1.1, illustrates integrated and inter-disciplinary approach of nature.

Even though many people believe that the presence of mechanical, electrical, electronic components, and computers make a system mechatronics, others do not feel the same as there is nothing wrong with the individual identity. Hence, the term mechatronics should be used to represent a different meaning, namely, "a design philosophy," where mechanical, electrical, electronic components, and IT should be considered together in the design stage itself to obtain a compact, efficient, and economic product rather than designing the components separately. The concept of mechatronics is very important today to meet the customers' ever increasing demands and still remain competitive in the global market. A mechatronic engineer must be able to design and select mechanical devices, sensors and actuators, analog and _.digital circuits," microprocessor-based components, and control devices such as logic gates to design modern systems.

1.1.1. Elements of Mechatronics SystemsVarious elements in typical mechatronic systems are shown in Figure 1.2 and are described here under.

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(i) Actuators and Sensors(ii) Signals and Conditioning(iii) Digital Logic Systems(iv) Software and Data Acquisition systems(v) Computers and Display devices.

(i) Sensors and Actuators:1. Sensors and actuators mostly come under mechanical systems. The

actuators produce motion or cause some action.2. The sensors detect the state of the system parameters, inputs and outputs. 3. The various actuators used in mechatronic system are pneumatic and hydraulic actuators, electro-mechanical

actuators, electrical motors such as D.C motors, A.C motors, stepper motors, servomotors, and piezoelectric actuators.

4. The various types of sensors used in mechatronic system are linear and rotational sensors, acceleration sensors, force, torque, and pressure sensors, flow sensors, temperature sensors, proximity sensors, light sensors.

(ii) Signals and Conditioning:1. The mechatronic systems deal with two types of signals and conditioning: input and output. 2. The input devices receive input signals from the mechatronic systems via interfacing devices and sensors, and

then send to the control circuits for conditioning or processing. 3. The various input signal conditioning devices used in mechatronic system are discrete circuits, amplifiers, analog-

to-digital (AID) convertors, Digital-to- digital (DID) convertors. 4. The output signals from the system are send to output/display devices through interfacing devices.5. The various output signal conditioning devices used in mechatronic system are digital -to- analog (D/A)

convertors, display decoders (DD) convertors, amplifiers, power transistors, power op-amps.(iii) Digital Logic Systems:

1. Digital logic devices control overall system operation. 2. The various digital logic systems used in mechatronic system are logic circuits, microcontrollers, programmable

logic controllers, sequencing and timing controls, control algorithms.(iv) Software and Data Acquisition systems:

1. Data acquisition system acquires the output signals from sensors in the form of voltage, frequency, resistance etc. and inputting into the microprocessor or computer. Software is used to control the acquisition of data through DAC board.

2. The data acquisition system consists of multiplexer, amplifier, register and control circuitry, DAC board. 3. The various data acquisition systems used in mechatronic system are data loggers, computer with plug-in boards

etc.(v) Computers and Display devices:.

1. Computers are used to store large number of data and process further through software. 2. Display devices are used to give visual feedback to the user. 3. The various display devices used in mechatronic system are LEDs , CRT, LCD, digital displays etc.

1.1.2. Types of Mechatronics Systems:

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Japan Society for the Promotion of Machine Industry (JSPMI) classified mechatronics products into following four categories:(i) Class I:Primarily mechanical products with electronics incorporated to enhance functionality.Examples: NC machine tools and variable speed drives in manufacturing machines.(ii) Class II:Traditional mechanical systems with significantly updated internal devices incorporating electronics. The external user interfaces are unaltered.Examples: Modem sewing machine and Automated manufacturing systems.(iii)Class III:Systems that retain the functionality of the traditional mechanical system, but the internal mechanisms are replaced by electronics.Example: digital watch, automatic camera.(iv) Class IV:Products designed with mechanical and electronic technologies through synergistic integration.Examples: Photocopiers, intelligent washers and dryers, rice cookers, and automatic ovens.1.1.3. Examples of Mechatronics Systems:Examples of mechatronics systems are as follows:

1. NC & CNC machine tools, variable speed drives, flexible manufacturing systems (FMS) & automated manufacturing systems, automated guided vehicles, rapid prototyping & robots

2. Computers disk drives3. Photocopiers, Laser printers & fax machines4. VCRJDVD drives5. Automatic washing machines, dish washer, rice cooker, automatic ovens & modern sewing machines6. Automatic teller machine (ATM)7. Coin counter8. Automatic/digital camera, digital watch9. Aircraft flight control systems such as cockpit control, landing gear control etc.10. Automobile applications include electronic engine management system, collision detection, global positioning

system, antilock brake system, keyless entry system, cruise control, parking assistance system and others.11. Medical diagnostic instruments such as CT scan system, automatic blood testing equipment, etc.12. Automatic sliding door. vending machines, and garage door openers13. Aerospace applications include launching, satellite solar plate extending mechanisms, and many more

1.1.4. Advantages and Disadvantages of Mechatronics Systems:Advantages:1. Cost effective and good quality products.2. High degree of flexibility to modify or redesign.3. Very good performance characteristics.4. Wide area of application.5. Greater productivity in case of manufacturing organization.6. Possibility of remote controlling as well as centralized monitoring and control.7. Greater extend of machine utilization.

Disadvantages:1. High initial cost.2. Multi-disciplinary engineering background required to design and imp lamentation.3. Need of highly trained workers.4. Complexity in 'identification and correction of problems in the systems

1.1.5. Measurement Systems:

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1. The word system in mechatronics refers to a group of physical component connected or related in such a manner as to form as entire unit for performing a specific task. For example, this universe is a system consists of large number of

2. Subsystems. 3. Similarly human body is a system consists of large number of

subsystems such as brain, nerve systems, digestive systems etc. All mechatronic devices consist of various systems in which some input data are given to get specified output. A system can be treated as a black box having an input and output as shown in Figure 1.3 (a).

4. For example, an electronic heater may be thought of as a system which has, as its input electric power and as output heat as shown in Figure 1.3 (b).

5. A measurement system involves the precise measurement and display/recording of physical, chemical, mechanical, electrical or optical parameters.

6. It provides a means of describing natural phenomena in quantitative terms. Measurement system provides the input to the control systems of mechatronics.

7. A generalized measurement system comprises of a sensor/transducer, signal processor, and a display/recording device as shown in Figure 1.4.

(i) Sensor or transducer:1. Sensor or transducer is

a device which converts a physical quantity, property or condition into output, usually electrical parameters such as voltage, resistance or capacitance.

2. For example, a thermocouple is a sensor which converts changes in temperature into a voltage.(ii) Signal processor:

1. Signal processor or conditioner receives output signal from sensor or transducer and manipulates or processes into a suitable input signal to control system.

2. Signal processor performs filtering and amplification functions.3. For example, the output from the thermocouple is very small voltage, therefore, amplifier increases the magnitude

of the voltage and the ND (analog to digital) converter changes the analog voltage signal to a coded digital signal.

(iii) Display or recording device:1. Recorder records the output from signal conditioner and display device gives the measured variable in visual or

quantitative form. 2. For example, LEDs, CRT, LCD are the example of display devices which gives measured variable interms of

numbers.Example of measurement system:Consider a digital liquid level measuring system in a tank shown in Figure 1.5. This system incorporates float with resistive potentiometer as a sensor which gives electrical voltage as output depending upon the liquid level in the tank. Signal processor involves an amplifier increases the small voltage into higher voltage, NO converter converts analogue voltage to a digital signal, and digital decoder (DD) decodes the digital data into readable format to display. LEDs display the value of liquid level in terms of specific quantity.

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1.1.6. Control Systems: A control system in mechatronics refers to a group of physical component connected or related in such a manner

as to command direct or regulate itself or another system. The physical components may be of electrical, mechanical, hydraulic, pneumatic, thermal or chemical in nature.

Several key terms & elements of the control system are:1. Reference variable or Input: Stimulus or excitation applied to a control system from an external source, usually in order to produce a specified response from the system.2. Output: The actual response obtained from the system.3. Feedback: That portion of the output of a system that is returned to modify the input and thus serve as a performance monitor for the system.4. Error: The difference between the input stimulus and the output response. Specifically, it is the difference between the input and the feedback.5. Disturbance: Any signal other than the reference which affects the system performance.6. Actuating signal: The difference between the feedback signal and reference signal.7. Control or Feed Forward Elements: Those components directly connected between the controlled output and the referenced input.8. Controlled Output: The variable (temperature, position, velocity, shaft angle, etc.) that the system seeks to guide or regulate.9. Feedback Elements: Those components required to establish the desired feedback signal by sensing the controlled output.

Examples of control system:1. Consider an industrial cooler in a food processing unit which is required to maintain the temperature of unit at

particular predefined level. 2. In this control system, the input is the temperature of the unit at present which is received from temperature

sensor and the output is the particular predefined temperature of the unit, i.e., the required temperature is set in the thermostat or controller and the compressor of the cooler unit adjusts itself by comparison of input data and output data to pump refrigerant through evaporator and so produce the required temperature in the unit as shown in Figure 1.6.

3. This is an example of feedback control in which the sensor signals are feedback from the output in order to modify the reaction of the pump to switch on or off.

Consider another example of steering control in automobiles. 1. The vehicle direction is controlled by wheel orientation which is achieved by controlling steering wheel manually.

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2. This automobile steering system consisting of steering wheel, steering gear, linkages, and wheel, constitutes a control system. In this system, the input is the steering wheel position/rotation and the wheel orientation is the controlled variable or output.

3. The route of the vehicle is determined by the driver and by properly adjusting/controlling the steering wheel the vehicle is maintained to run on the road in the desire direction.

4. The driver monitors and compares the road condition and accordingly takes the decision to control the vehicle direction through steering wheel.

5. This is also an example of feedback control system. The input (steering wheel position/rotation) is modified according to the output (wheel orientation) by visually monitoring (feedback data) the road condition as shown in Figure 1.7.

Non-engineering systems such as human body can also be considered as a control system. Normal functioning of human body is controlled by blood pressure and temperature of the body. Both are kept at constant value by means ofphysiological feedback provided by many other sub-systems. Figure 1.8 (a) & (b) shows the illustration of feedback principle of human body blood pressure and temperature control. Therefore, the human body is a highly advanced feedback control system. This feedback system makes the human body relatively insensitive to external disturbances, thus enabling it to function properly in a changing environment.

Other examples of applications of control systems include, but not limited to:

1. Idle speed control system of an automobile2. Print wheel control system of a printer3. Temperature control of an electric furnace or oven ,4. Sun tracking control of solar collector5. Aircraft rudder control system6. Gun or missile director7. Missile guidance system8. Laser-guided projectiles9. Automatic pilot

1.1.7. Open-loop Control Systems:Control systems are classified into two groups:1. Open loop control systems2. Closed loop or feedback control systems

1. Open loop systems are systems in which the output of a system is not used as a variable to control the system.2. In other words, open loop systems are systems in which input to the system is not controlled by the present

output.

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3. In an open-loop system, the output of this system is not fed back into the input to the system for control or operation.

4. An open system is essentially a feed forward system. The system is an "open" system because it does not have a feedback loop in its control as shown in Figure 1.9.

5. There are many reasons to use open loop control such as simplifying the control system, quicker response of the system, to reduce the possibility of oscillation and sometimes to lower cost.

Examples of open-loop control system:1.The basic elements of this system are an amplifier and a controller as shown in Figure 1.10. The amplifier receives a low- level input signal and amplifies it enough to drive the controller to perform the desired job.

2.As an example consider automatic bread toaster. In this system, when the system is switched ON, the heating element in the toaster heat the bread for particular preset time and then automatically it get switched OFF and ejects the bread. Here there is no feedback of data of whether the bread is toasted properly or not.

3.Another example of an open-loop control system is a chemical addition pump with a variable speed control (Figure 1.12). The feed rate of chemicals that maintain proper chemistry of a system is determined by an operator, who is not part of thecontrol system. If the chemistry of the system changes, the pump cannot respond by adjusting its feed rate (speed) without operator action.

Consider an example of the use of open-loop control system is in the control of the wing surfaces on a modern fighter plane. The closed loop implementation would make the control much slower. However, if there is a disturbance on the output side of the process, control action does not take it into consideration. In order to remove this limitation, feedback

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has to be provided. Open-loop control systems are not as commonly used as closed-loop control systems because they are less accurate. All control systems operated by preset timing mechanisms are open-loop.

Advantages and disadvantages of open-loop control system:Advantages:

1. Simple and cost effective construction.2. Easy maintenance because of no complex electronic circuits.3. Good stability.4. Good reliability.5. Quicker response.6. No calibration problem.7. Convenient when output is difficult to measure or economically not feasible.

Disadvantages:1. Less accurate.2. Presence of non-Iinearities causes malfunctioning.3. Slow because of manual control.4. Optimisation in control not possible.5. System is affected by internal and external disturbances.

1.1.8. Closed-loop Control Systems:1. Closed-loop system uses on a feedback loop to control the operation of the system. 2. In closed loop or feedback control the controller notices what actually takes place at the output end and drives the

plant in such a way as to obtain the desired output.3. Closed-loop control systems are the type most commonly used because they respond and move the loads they are

controlling quicker and with greater accuracy than open-loop systems. 4. The reason for quicker response and greater accuracy is that an automatic feedback system informs the input that

the desired movement has taken place.

The basic layout of a feedback or closed-loop control system is shown in Figure 1.13. The essential elements of this system are:1. The plant is the system or process through which a particular quantity or condition is controlled. This is also called the controlled system.2. Measuring unit: sensors, estimators and signal conditioners are the part of measuring unit.3. The control elements are components needed to generate the appropriate, control signal applied to the plant. These elements are also called the "controller.”4. Comparison element or Error junction: where the desired system outputs and the measured or estimated outputs are compared to generate the error signal. Error signal is the difference between the reference value and the measured value.5. Correction element or actuator: produces a change in the plant or process to correct the controlled plant.6. The feedback elements are components needed to identify the functional relationship between the feedback signal and the controlled output.

Below are several terms associated with the closed-loop control systems.1. The reference point is an external signal applied to the summing point of the control system to cause the plant to produce a specified action. This signal represents the desired value of a controlled variable and is also called the "setpoint."2. A controlled variable is the process variable that is maintained at a specified value or within a specified range. The controlled output is the quantity or condition of the plant which is controlled.3. The feedback signal is a function of the output signal. It is sent to the summing point and algebraically added to the reference input signal to obtain the actuating signal.4. The actuating signal represents the control action of the control loop and is equal to the algebraic sum of the reference input signal and feedback signal. This is also called the "error signal."5. The manipulated variable is the variable of the process acted upon to maintain the plant output (controlled variable) at the desired value. 6. The disturbance is an undesirable input signal that upsets the value of the controlled output of the plant.

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In this system, the actual output is fed back and compared with the desired response. The resulting error is the basis for the application of a control signal to the plant. The controller generates the control signal on the basis of the error. If aMechanical signal has to be applied to the plant; it is generated by an actuator from the output of the controller. In this arrangement, the control signal takes the actual controlled variable into account including disturbances if any. The plant is driven (by the control signal) until the error is reduced. This is the principle of feedback control in which feedback is negative.From the above description it is clear that a closed-loop control system must be capable of the following:1. Accepting an order that defines the desired result2. Determining the present conditions by some method of feedback3. Comparing the desired result with the present conditions and obtaining a difference or an error signal4. Issuing a correcting order. (The error signal) that will properly change the existing conditions to the desired result5. Obeying the correcting order

Examples of closed-loop control system:Few examples are already discussed in the previous topics such as industrial cooling control system and automobile steering control system. Yet, another example for closed-loop control system is room heating system in western countries [Figure 1.14]. The thermostat (input) calls for heat. The heating coil (output) produces heat and distributes it. Some of the heat is "fed back" to the thermostat. When this "feedback" raises the temperature of the room to that of the thermostat setting, the thermostat responds by shutting the system down until heat is again required. In such a system, the feedback path, input to output and back to input, forms what is called a "closed loop."

In this system, the various elements are:

1. Plant or process - the heating of room by electrical coil2. Controlled variable - the room temperature3. Reference input - the desired room temperature4. Comparison element - the electronic logic Circuit5. Error signal - the difference between the current and required temperatures6. Controller - the switch7. Correction element- the thermostat

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8. 'Measuring element- the temperature sensor attached with thermostat

Advantages and disadvantages of closed-loop control system:

1. Closed loop control, with the appropriate sensor, provides much greater stability.2. Closed loop control will also give much better repeatability.3. Closed loop control overcomes temperature and hysteresis effects.4. Closed loop control can perform a task faster than open-loop.5. Good reliability.6. Optimisation in control is possible.

Disadvantages:1. Generally closed-loop control systems are complicated in construction.2. Cost of the system is higher.3. Sometimes closed loop control systems may become unstable.

1.1.9. Comparison between Open-loop and Closed-loop SystemsA comparison would show the following differences between open loop and closed loop control systems.

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1.1.10. Automatic Control Systems:

An automatic control system is a preset closed-loop control system that requires no operator action. Most of the closed-

loop control systems are automatic in nature. This assumes the process remains in the normal range for the controlSystem. Various applications of automatic control systems are explained under.

Examples of automatic control system:example 1: Automatic tank-level control system:

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1. Figure 1.15 shows an example of automatic tank-level control system. 2. The control system maintains water level in a storage tank. 3. The system performs this task by continuously sensing the level

in the tank and adjusting a supply valve to add more or less water to the tank. The desired level is preset by an operator, who is not part of the system.

4. The level transducer measures the level within the tank by using float and potentiometer arrangement as shown in Figure 1.15 (a). The level transducer sends a signal representing the tank level i.e. feedback to the level control device (motor drive).

5. This feedback is compared with a desired level to produce the required control action that will position the level control as needed to maintain the desired level.

6. The level control device computes how far to open the supply valve to correct any difference between actual and desired tank levels. Figure 1.15 (b) shows the block diagram of this system representing the signal flow to various elements including feedback.

In this system, the various elements are:Plant or process- the water storage tank, Controlled variable- the storage tank level ,Manipulated variable - the flow rate of the water supplied to the tank, Reference input- the desired tank level ,Comparison element - the level controllerError signal- the difference between the current and required water level , Controller- the level controllerCorrection element- the level control valve, Measuring element- the level transducer

Example 2: Automatic temperature control system for lubricating oil:1. Figure 1.16 shows another example of temperature control system for lubricating oil. 2. Figure 1.16(a) shows a schematic diagram of the lube oil cooler and its associated temperature control system.

Lubricating oil reduces friction between moving mechanical parts and also removes heat from the components. As a result, the oil becomes hot.

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3. This heat is removed from the lube oil by a cooler to prevent both breakdown of the oil and damage to the mechanical components it serves.

4. The lube oil must be maintained within a specific operating band to ensure optimum equipment performance. 5. This is accomplished by controlling the flow rate of the cooling water with a temperature control loop.6. The temperature control loop consists of a temperature transducer, a temperature controller, and a temperature

control valve.7. The lube oil temperature is the controlled variable because it is maintained at a desired value (the setpoint).

Cooling water flow rate is the manipulated variable because it is adjusted by the temperature control valve to maintain the lube oil temperature.

8. The temperature transducer senses the temperature of the lube oil as it leaves the cooler and sends an error signal that is proportional to the temperature controller.

9. Next, the temperature controller compares the actual temperature of the lube oil to the setpoint (the desired value). 10. If a difference exists between the actual and desired temperatures, the controller will vary the control air signal to

the temperature control valve. 11. This causes it to move in the direction and by the amount needed to correct the difference.12. For example, if the actual temperature is greater than the setpoint value, the controller will vary the control air

signal and cause the valve to move in the open direction. This results in more cooling water flowing through the cooler and lowers the temperature of the lube oil leaving the cooler.

Figure 1.16 (b) represents the block diagram of temperature control system for lubricating oil . The lube oil cooler is the plant in this example, and its controlled output is the lube oil temperature. The temperature transducer is the feedbackelement. It senses the controlled output and lube oil temperature and produces the feedback signal. The feedback signal is sent to the summing point to be algebraically added to the reference input (the setpoint). The actuating ' signal passes through the two control elements: the temperature controller and the temperature control valve.The temperature control valve responds by adjusting the manipulated variable (the cooling water flow rate). The lube oil temperature changes in response to the different water flow rate, and the control loop is complete.

Example 3: Automatic positioning system for a missile launcher:Another example of automatic control system is automatic positioning system for a missile launcher. Figure 1.17 illustrates this system with block diagram. This is a feedback system designed to position the launcher quite accurately on commands from potentiometer. Potentiometer sends a signal back to the amplifier which functions as an error detector. If there is error exist, it is amplified and applied to a motor drive which adjust the output shaft position until it agrees with the input shaft position and makes the error to zero value. Here the input is the desired angular position, the output is the actual angular position, and the control system consists of the potentiometer power, amplifier and motor gearing between the motor and missile launcher, and the missile launcher.

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Example 4: Automatic speed control system of a DC motor:An automatic speed control system of a DC motor is illustrated in Figure 1.18. The function of this system is to maintain the output speed of the motor relatively constant irrespective of the torque variation. Here a tachometer is used as atransducer which transforms speed to voltage and is also used as a feedback element. When the output speed differs from the desired speed, the comparison element develops an error signal which adjusts the field current of the motor in order torestore the desired output speed.

Example 5: Automatic shaft speed control system:Figure 1.19 (a) shows the schematic of an automatic shaft speed control system.The potentiometer is used to set the voltage to be supplied to the differential amplifier. The differential amplifier is used as a comparison element which amplifies the feedback signal and compares the feedback value and reference value.

The amplified error signal is fed to the DC motor to adjust the speed of the rotat ing shaft. The digital tachometer is used as a transducer to measure the speed of the rotating shaft and it is fed back to the amplifier. Figure 1.19 (b) illustrated the

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working signal flow of this system in block diagram.

1.1.11. Sequential Controllers:1. In many situations, various operations of a plant or process takes place in particular order . A sequential control

involves sequential execution of well defined operations that are performed in a prescribed order . Each operation or activity is called step.

2. Each step may be an open or closed loop continuous process or even a sequential sub-process. 3. For example, while using automatic camera the various basic steps in sequence are switch on, battery check, auto-

focus the image, auto flash on/off, taking the image, saving the image and then switching off the camera.4. Each step of the prescribed sequence usually requires a switching of the equipment configuration and may be

triggered by time or an event (push of a button, completion of an earlier task etc.). The sequential controller may be classified into two types:

1. Event- based and 2. Time-based. a) In event based controllers, the next event or step cannot be performed until the previous event or step is

completed. b) In time-based controllers, the series of operations are sequenced with respect to time. Event-based controllers are

more reliable than time-based controllers. Traditionally such a control could be obtained by an electrical circuit with sets of earn-operated switches or relays which are wired up in such a way as to give the desired sequence.

c) Now-a-days, microprocessor or computer controlled systems are used instead of hard-wired circuits with the sequencing being controlled by software programs. Industrial sequential controllers may employ relay or semiconductor logic;

d) More complicated operations are handled by Programmable Logic Controllers (PLCs). As an example of sequential controllers consider automatic domestic washing machine system as shown in Figure 1.20 in which various processes such as pre-wash cycle, main wash cycle, rinse cycle and spin cycle are performed in a particular sequence as follows.

(i) A pre-wash cycle in which the clothes in the drum are given a wash in cold(ii) A main wash cycle wherein the clothes are washed in hot water,(iii) A rinse cycle where the clothes are rinsed with cold water a number of preset time, and(iv) A spin cycle in which the spinning of drum takes place to drain the water from clothes and the drum.

The various processes of the washing machine as stated above are given in the Figure 1.21.

1. These processes were carried out using cam operated switches in earlier days. In cam operated switch mechanism, the contour of the cam is in such a manner that the different switches are activated at different times.

2. The sequence of instructions used was a function of set of cams used. In modem automatic washing machine, the cam operated switches are replaced with the microprocessor based controllers where the .software programs are fed to perform various sequential operations.

3. In addition to the microprocessor controller, various sensors, and drivers are used to effectively and automatically carry out these operations.

4. The timer installed in the system determines the time for which the cycles to be activated.

5. The various sensors such as level sensor, position sensor, temperature sensor, speed sensor provide input signal to the microprocessor.

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The working of modern automatic washing machine is explained under with the help of block diagram shown in Figure 1.22.Step 1.' Pre- wash cycle:In this cycle an electrically operated valve opens to allow cold water into the drum for a period of time determined by the output from the microprocessor. A level sensor is used to check whether the drum is filled to preset level When the waterreaches a preset level the sensor gives output to the microprocessor which in turn stops the water supply to the drum by switching off the current to the valve. Now the clothes in the drum are given a starting wash with cold water. After completing cold wash for preset time microprocessor operates the drain pump to drain the water from the drum.

Step 2: Main wash cycleWhen the pre-wash IS completed, the microprocessor activates an electrically operated valve to opens and allows cold water into the drum for a period of time. The level is sensed by level sensor and the water shut off when the required level is reached in the drum. Now the microprocessor activates the switch to supply current to electric heater to heat the water for main wash. The temperature sensor gives input to the microprocessor, after reaching particular preset temperature, to switch off the current to the heater. Then the drum motor is activated by the microprocessor to the predetermined time with slow speed and switched off after completion. Finally the microprocessor operates the drain pump to drain the water from the drum.

Step 3: Rinse cycle:When the main wash is completed, the microprocessor gives an output for the rinse cycle; it opens the valve to allow cold water to the drum and closes when it reaches a preset level. Drum motor is operated to rotate the drum and the drain pumpis operated to drain the water after preset time. This sequence is repeated for a number of times.

Step 4: Spin cycle:The microprocessor switches on the drum motor and is signaled to rotate at a higher speed than the rinsing cycle. Due to the centrifugal action the water drains out from the clothes.

1.1.12. Microprocessor based Controllers:1. Microprocessors are essential to many of the products we use every day such as TVs, cars, radios, home

appliances and of course, computers. Microprocessor-based controllers are also called as microcontrollers. Microcontroller is a digital integrated circuit which serves as a heart of many modern control applications.

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2. Microprocessors and microcontrollers are similar but the architecture of both differs in the applications domains. Microprocessors are employed for high speed applications such as desktop and laptop computers where as the micro controllers are employed in automation and control applications such as microwave ovens, automatic washing machines, dish washers, engine management systems, DVD players etc.

3. Microcontrollers are embedded inside some other device (often a consumer product) so that they can control the features or actions of the product. Therefore, it is also called as embedded controller.

4. Because of its relatively low cost, it is a natural choice for design. It performs many of the functions traditionally done by simple logic circuitry, sequential control circuits, timers or a small microcomputer. Microcontrollers are generally compact in construction, small in size, flexibility and consume less power.

5. A microcontroller generally has the main CPU core, ROMI EPROM / RAM and some accessory functions (like timers, pulse width modulator, AID convertor and I/O controllers) all integrated into one chip. Microcontroller is a computer on a chip that is programmed to perform almost any control, sequencing, monitoring and display function.

6. Another more adaptable form of microcontroller is the programmable logic controller (PLC). Figure 1.23 shows the basic structure of a PLC. The PLC is a microprocessor based controller consists of the CPU, memory and I/O devices.

7. These components are integral to the PLC controller. Additionally the PLC has a connection for the programming unit, and printer. The CPU used in PLC system is a standard CPU present in many other microprocessor controlled systems. The choice of the CPU depends on the process to be controlled.

8. Memory in a PLC system is divided into the program memory which is usually stored in EPROM/ROM, and the operating memory. The RAM memory is necessary for the operation of the program and the temporary storage of input and output data. Input/output units are the interfaces between the internal PLC systems and the external processes to be monitored and controlled.

9. Programming unit in the PLC systems is a essential component and are used only in the development/testing stage of a PLC program, they are not permanently attached to the PLC. Programming unit can be a dedicated device or a personal computer.

Example 1: Automatic camera:The modem automatic camera using film has the features of automatic focusing and exposure. The basic elements of the microprocessor based control system used in an automatic camera for focusing and exposure are shown in Figure 1.24.The working of auto focusing and aperture control for auto-exposure is explained as follows:Auto focusing:The auto focusing is achieved by using range sensor. When the system is switch on to activation mode, the camera is pointed at the object to take the snap. The microprocessor takes the input signal from the range sensor. This signal is processed to send output signal to the lens position drive to move the lens for achieve auto focusing. The microprocessor gets the feedback. signal about the lens position from the range sensor which is then used to modify the lens position to get the desired position of focus.Aperture control for auto-exposure:The light sensor is used to achieve aperture control for auto-exposure. When the shutter switch is pressed to the initial position the microprocessor calculates the shutter speed and aperture settings' based on the input from light sensor. It then gives output signal to the view finder. When the shutter switch is pressed to the final position the microprocessor gives signal to the aperture control drive to open the shutter to the required position. The shutter is kept open for the preset amount of time and then closed. After photograph has been taken, the microprocessor sends an output signal to the motor drive to advance the film for the next snap.

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Example 2: Engine management system:Engine management system is used in many of the modern cars such as Benz, Mitsubishi, Ford, and Toyota etc. This system uses many electronic control systems involving microcontrollers. The objective of the system is to ensure that the engine is. Operated at its optimum settings. Most of the modern medium range cars use 4-stroke 4-cylinder SI engine as the name implies it consists of 4 cylinders, each of which has a piston and a connecting rod which are connected to a common crank shaft. Figure 1.25 illustrates the sequence of operations of the 4-stroke spark ignition engine.

1. At the beginning of the suction stroke, the piston is at the top most position and is ready to move downwards. As the piston moves downwards, a vacuum is created inside the cylinder. Due to this vacuum, air fuel mixture from the carburetor is sucked into the cylinder through inlet valves till the piston reaches the bottom most position.

2. During the suction stroke, exhaust valve remains in closed condition and the inlet valve remains open. 3. During the compression stroke, both the inlet and exhaust valves are in closed condition and the piston moves

upwards from bottom to top to compress the air fuel mixture. It leads to an increase in pressure and temperature of the mixture instantaneously. At the end of the stroke, the spark plug ignites the mixture which increases the pressure of the mixture suddenly.

4. The sudden rise in pressure of the mixture exerts an impulse on the piston and pushes it downwards. Thus, the piston moves from top to bottom and produces power. This stroke is known as a power stroke.

5. During the exhaust stroke, the piston moves from bottom to top, the exhaust valve is opened and the inlet valve is closed. The burnt gases are pushed out through the exhaust valve when the piston moves upwards. Then the cycle is repeated.

Basic elements of an electronic engine management system are shown in Figure 1 .26. The system consists of many sensors for observing vehicle speed, engine temperature, oil and fuel pressure, airflow etc. These sensors supply input signals to the microprocessor after suitable signal conditioning and provide output signals via drivers to actuate corresponding actuators.

1. The power and speed of the engine are controlled by varying the air-fuel mixture and spark ignition timing. 2. The engine speed sensor is an inductive type sensor attached with the fly wheel. It consists of a coil and sensor

wheel. 3. The inductance of the coil changes as the teeth of the sensor wheel pass it and so results in an oscillating voltage. 4. The ignition coil supplies the electrical signal to the spark plug to produce a spark which ignites the mixture. 5. The feedback signal from a spark plug is sent to a microprocessor to adjust the timing if it is wrong.

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6. The solenoid driver attached to the carburetor is used to control the air-fuel mixture supplied to the cylinder based on input received from an engine temperature sensor and throttle position sensor. Hot wire anemometer is used as a sensor for measuring mass airflow rate.

7. The basic principle is that the heated wire will be cooled as air passes over it. 8. The amount of cooling is dependent on the mass rate of flow. 9. The engine temperature sensor is generally a thermocouple which is made of bimetallic strip or a thermister. 10. The oil and fuel pressure sensors are diaphragm type sensors. According to the pressure variation, the diaphragm

may contract or expand and activate strain gauges which produce voltage variation in the circuit. 11. The various drivers such as fuel injector drivers, ignition coil drivers, solenoid drivers are used to actuate

actuation according to the signals by various sensors.

1.2. SENSORS AND TRANSDUCERS:Sensors are devices which produce a proportional output signal (mechanical, electrical, magnetic, etc.) when exposed to a physical phenomenon (pressure. temperature, displacement, force, etc.). Many devices require sensors for accuratemeasurement of pressure, position, speed, acceleration or volume. Transducers are devices which converts an input of one form of energy into an output of another form of energy. The term transducer is often used synonymously with sensors. However, ideally, the word 'transducer' is used for the sensing element itself whereas the term 'sensor' is used for the sensing element plus any associated signal conditioning circuitry. Typically, a transducer may include a diaphragm which moves or vibrates in response to some form of energy, such as sound. Some common examples of transducers with diaphragms are microphones, loudspeakers, thermometers, position and pressure sensors. Sensors are transducers when they sense one form of energy input and output in a different form of energy.For example, a thermocouple responds to a temperature change (thermal energy) and outputs a proportional change in electromotive force (electrical energy). Therefore, a thermocouple can be called a sensor and or transducer.Figure 1.27 illustrates a sensor with sensing process in terms of energy conversion. The form of the output signal will often be a voltage analogous to the input signal, though sometimes it may be a wave form whose frequency isProportional to the input or a pulse train containing the information in some other form.

1.2.1. Classification of Sensors:Sensors are generally classified into two types based on its power requirement:

1. passive and active: In active sensors, the power required to produce the output is provided by the sensed physical phenomenon itself (Examples: thermocouples, photovoltaic cells, piezoelectric transducers, thermometer etc.) whereas the passive sensors require external power source (Examples: resistance I thermometers, potentiometric devices, differential transformers, strain gage etc.). The active sensors are also called as self-generating transducers. Passive sensors work based on one of the following principles: resistance, inductance and

capacitance.2. Sensors can also be

classified as analog or digital based on the type of output signal.

3. Analog sensors produce continuous

signals that are proportional to the sensed parameter. These sensors generally require analog-to-digital conversion before sending output signal to the digital controller (Examples: potentiometers, LVDTs (linear variable

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differential transformers), load cells, and thermistors, bourdon tube pressure sensor, spring type force sensors, bellows pressure gauge etc.).

4. Digital sensors on the other hand produce digital outputs that can be directly interfaced with the digital controller (Examples: incremental encoder, photovoltaic cells, piezoelectric transducers, phototransistors, photodiodes etc.). Often, the digital outputs are produced by adding an analog-to-digital converter to the sensing unit . If many sensors are required, it is more economical to choose simple analog sensors and interface them to the digital controller equipped with a multi-channel analog-todigital converter.

Another way of classifying sensor refers to as primary or secondary sensors.Primary sensors produce the output which is the direct measure of the input phenomenon. Secondary sensors on the other hand produce output which is not the direct representation of the physical phenomenon. Mostly active sensors are referredas primary sensors where as the passive sensors are referred as secondary sensors.

Furthermore, sensors are classified by their measurement objectives. Table 1.1 lists the various types of sensors for various measurement objectives. Although this list is by no means exhaustive, it covers all the basic types.

Quantity to be measured Type of sensorsLinearlRotational displace met LinearlRotational variable differential

transformer (LVDT IRVDT)Optical encoderElectrical tachometerHall effect sensorCapacitive transducerStrain gauge elementsInterfero meterMagnetic pickupGyroscope

Proximity Inductance sensorEdd y current sensorHall effect sensorPhotoelectric sensorCapacitance sensor

Force, torque, and pressure Strain gaugeDynamometerslload cellsPiezoelectric load cellsTactile sensorUltrasonic stress sensor

Velocity, and acceleration Electromagnetic sensorUltrasonic sensorTacho generatorsResistive sensorCapacitance sensorPiezoelectric sensorPhotoelectric sensorElectron tube

Flow Pitot tubeOrifice plateFlow nozzleVenturi tubesRotameterUltrasonic flow meterTurbine flow meterElectromagnetic flow meter

Level Float Level SensorPressure Level Sensor Resistive sensorVariable Capacitance sensorPiezoelectric sensorPhotoelectric sensor

Temperature Thermocouples

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ThermistorsThermodiodesthermo transistorsResistance temperature detector (RTD)Infrared thermography

Light PhotoresistorsPhotodiodesPhoto transistorsPhoto conductorsCharge-coupled diode

1.2.2. Performance Terminology:1. Static characteristics:Static characteristics of an instrument are the parameters which are more or less constant or varying very slowly with time. The following characteristics are static characteristics.

a) RangeEvery sensor is designed to work over a specified range i.e. certain maximum and minimum values. The design ranges are usually fixed, and if exceeded, result in permanent damage to or destruction of a sensor. For example, a thermocouple may have a range of -100 to 1260°C.

b) SpanIt represents the highest possible input value which can be applied to the sensor without causing unacceptably large inaccuracy. Therefore, it is the difference between maximum and minimum values of the quantity to be measured.

c) Span = Maximum value of the input - Minimum value of the inputd) Error

Error is the difference between a measured value and the true input value. Error = Measured value - True input value

e) AccuracyA very important characteristic of a sensor is accuracy which really means inaccuracy. Inaccuracy is measured as a ratio of the highest deviation of a value represented by the sensor to the ideal value. The accuracy of a sensor is inversely proportional to error, i.e., a highly accurate sensor produces low errors.

f) SensitivitySensor sensitivity is defined as the change in output per change in input. The factor may be constant over the range of the sensor (linear), or it may vary (nonlinear).

g) HysteresisHysteresis is defined as the maximum differences in output for a given input when this value is approached from the opposite direction. It is a phenomenon which shows different outputs when loading and unloading. Simply, hysteresis means that both the loading and unloading curves do not coincide. Figure 1.28 shows that the deviation of unloading from loading condition due to hysteresis effect.

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h) LinearityLinearity of a sensor refers to the output that is directly proportional to input over its entire range, so that the slope of a graph of output versus input describes a straight line. If the response of the system to input A is output A, and the response to input B is output B, then the response to input C (= input A + input B) will be output C ( = output A + output B).

i) Non-linearityNon-linearity of a sensor refers to the output that is not proportional to input over its entire range, so that the slope of a graph of output versus input describes a curve. Non-linearity error is the deviation of output curve from a specified straight line as shown in Figure 1.29.

j) Repeatability and reproducibilityRepeatability may be defined as the ability of the sensor to give same output reading when the same input value is applied repeatedly under the same operating conditions.

k) Reproducibility may be defined as the degree of closeness among the repeated measurements of the output for the same value of input under the same operating conditions at different times.

l) StabilityStability means the ability of the sensor to indicate the same output over a period of time for a constant input.

m) Dead band/timeDead band of a sensor is the range of input values for which the instrument does not respond. The dead band is typically a region of input close to zero at which the output remains zero.

n) Dead time is the time taken by the sensor from the application of input to begin its response and change.o) Resolution

Resolution is defined as the smallest change that can be detected by a sensor . It can also be defined as the minimum value of the input required to cause an appreciable change or an increment in the output.

p) Zero DriftDrift is the variation of change in output for a given input over a period of time. When making a measurement it is necessary to start at a known datum, and it is often convenient to adjust the output of the instrument to zero at the datum. The signal level may vary from its set zero value when the sensor works. This introduces an error into the measurement equal to the amount of variation or drift. Zero drift may result from changes of temperature, electronics stabilizing, or aging of the transducer instrument are the parameters which are varying with time.

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2.The following characteristics are dynamic characteristics.q) Response time

The time taken by a sensor to approach its true output when subjected to a step input is sometimes referred to as its response time. It is more usual, however, to quote a sensor as having a flat response between specified limits of frequency. This is known as the frequency response, and it indicates that if the sensor is subjected to sinusoidally oscillating input of constant amplitude, the output will faithfully reproduce a signal proportional to the input.

r) Time constantIt is the time taken by the system to reach 63.2% of its final output signal amplitude i.e. 62.3% of response time. A system having smaller time constant reaches its final output faster than the one with larger time constant. Therefore possesses higher speed of response.

s) Rise time:It is the time taken by the system to reach 63.2% of its final output signal.

t) Setting time It is the time taken by a sensor to be within a close range of its steady state value.

1.2.3. Displacement Sensors: Displacement sensors are those sensors which measures the variation of position of a body. Displacement sensors are designed to give a quantitative measurement of the displacement being measured. Measurement of displacement is the basis of measuring position, proximity, velocity, acceleration, stress, force, pressure, thickness etc. The various displacement sensors commonly used in mechatronics systems are:1. Potentiometer displacement sensors

2. Strain gauge .displacement sensors3. Capacitive displacement sensors 4. Inductive displacement sensors (LVDT)

1.2.3.1. Potentiometer displacement sensors:Potentiometer is a primary sensor which converts the linear motion or the angular motion of a shaft into changes in resistance, It is a type of resistive displacement sensor. Linear potentiometers are sensors that produce a resistance output proportional to the linear displacement or position.' Linear potentiometers are essentially variable resistors whose resistance is varied by the movement of a slider over a resistance element. Rotary' potentiometers are sensors that produce a resistance output proportional to the angular displacement or position. They can be either wire-wound or conductive plastic, and either rectangular or cylindrical.

Figure 1.30 illustrates the basic principle of a linear potentiometer. The linear potentiometer employs an electrically conductive linear slide member (also called wiper) connected to a variable wire wound resistor (winding) that changes resistance to be equated to the linear position of the device that is monitored. As the sliding contact moves along the winding, the resistance changes in linear relationship with the distance from one end of the potentiometer. To measure displacement, a potentiometer is typically wired as a "voltage divider" so that the output voltage is proportional to the distance traveled by the wiper. A known voltage is applied to the resistor ends. The contact is attached to the moving object of interest. The output voltage at the contact is proportional to the displacement. The resolution is defined by the number of turns per unit distance, and loading effects of the voltage divider circuit should be considered.

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A rotary potentiometer employs a rotary slide member connected to a variable wire wound resistor that changes resistance to be equated to the angular position of the device that is monitored (Figure 1.31). Other principles of operations are same as that of linear potentiometer.

The potentiometer can be used as a voltage divider to obtain a manually adjustable output voltage at the slider (wiper) from a fixed input voltage applied across the two ends of the resistance wire winding.Figure 1.32 (a) and tb) shows a potentiometer circuit with a resistive load and circuit with equivalent fixed resistors respectively. The voltage across RL can be calculated by:

One of the most common uses for modern low-power potentiometers is as audio control devices. Both sliding pots (also known as faders) and rotary potentiometers commonly called knobs) are regularly used to adjust loudness, frequency attenuation and other characteristics of audio signals.

The following factors to be considered while selecting the potentiometers:1. Operating temperature2. Shock and vibration3. Humidity4. Contamination and seals5. Life cycle6. Dither

Advantages and disadvantages of potentiometersAdvantages:

1. Easy to use2. Low cost3. High-amplitude output signal4. Proven technology5. Rugged construction6. Very high electrical efficiency7. Availability in different forms, ranges and sizes

Disadvantages:1. Limited band width2. Frictional loading3. Inertial loading4. Limited life due to wear

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1.2.3.2. Strain gauge displacement sensors:1. The strain gauge displacement sensor consists of a structure attached with the strain gauge that elastically deforms

when subjected to a displacement as shown in Figure 1.33. 2. Strain gauge is attached to the object by a suitable adhesive. As the member is stressed, the resulting strain

deforms the strain gauge attached with the structure; this causes an increase in resistivity of the gauge which produces electrical signal proportional to the deformation.

3. The change of resistance is very small and is usually measured using a Wheatstone bridge circuit where the strain gauge is connected into the circuit with a combination of four active gauges for full bridge, two gauges for half bridge, or a

4. Single gauge for Quarter Bridge. In the half and quarter circuits, the bridge is completed with precision resistors. 5. Figure 1.33 (b) shows the basic configuration, where the strain gauge is one leg of the bridge i.e. quarter bridge.

As stress is applied to the bonded strain gauge, a resistive change takes place and unbalances the Wheatstone bridge. The change in the resistance of a bonded strain gauge is usually less than 0.5%.

6. This changes of the resistance per unit resistance (MIR) is proportional to the strain E. It is given by the relation,

7. A wide variety of gauge sizes and grid shapes are available. The metallic strain gauge consists of a very fine wire or metallic foil arranged in a grid pattern.

8. The grid pattern maximizes the amount of metallic wire or foil subject to strain in the parallel direction. The cross sectional area of the grid is minimized to reduce the effect of shear strain and Poisson strain.

9. The grid is bonded to a thin backing, called the carrier. which is attached directly to the test specimen.

10. The majority of strain gauges are bonded foil types, available in a wide choice of shapes and sizes to suit a variety of applications and typical examples are shown in Figure 1.34.

11. They consist of a pattern of resistive foil which is mounted on a backing material. They operate on the principle that as the foil is subjected to stress, the resistance of the foil changes in a defined way.

12. Bonded foil strain gauges can be as small as 16 mm2 and have a strain sensitivity or "gauge factor" of 2. Wire wound gauges are made of round wire of copper nickel, chrome nickel or nickel iron alloys, about 0.0025 in

diameter. 13. The length of wire is 25 mm or less. Figure 1.35 shows the example of wire wound strain gauges.

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The environmental considerations focus mainly on the temperature of the gauge. Since the resistance is a function of temperature the strain gauges are susceptible to variations in temperature. Thus, if it is known that the temperature of the gauge will vary due to any influence, temperature compensation is required in order to ensure that the force measurement is accurate.

1.2.3.3. Capacitive displacement sensors: A transducer that uses capacitance variation can be used to measure displacement. Elastic deflection of a membrane due to the applied force is detected by a capacitance variation. A highly sensitive displacement and proximity transducers can be constructed because the capacitive transducer senses very small deflections accurately. Capacitive sensors can directly sense a variety of things such as motion, chemical composition, electric field and indirectly sense many other variables which can be converted into motion or dielectric constant, such as pressure, acceleration, fluid level, and fluid composition.

A capacitance sensor consists of two metal plates separated by an air gap. The capacitance C between terminals is given by the expression:

Different forms of capacitive sensor are shown in Figure 1.36, where one plate of the capacitor inside a probe which is sealed in an insulator and the external target object forms the other plate of the capacitor. The operating principle is based on either the geometry (i.e., the distance d), or capacitance variations in the presence of conductive or dielectric materials. Distance variation of parallel plates [Figure 1.36 (a)] is often used for proximity or motion detection if the distance change is less than the plate size. Transverse displacement is easily detected by overlap or underlap area of the parallel plates [Figure 1.36 (b)]. In the distance variation motion detectors, when displacement increases to the dimension of the plates, measurement accuracy suffers

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from vanishing signal level. Area variation is then preferred. As these plates slide transversely, capacitance changes linearly with motion. Quite long excursions are possible with good linearity, but the gap needs to be small and well-controlled.Used as proximity sensors, capacitive sensors can detect metallic or nonmetallic objects, liquids, or any object with a dielectric constant· greater than air. The dielectric object is kept between the plates as shown in Figure 1.36 (c). As thedielectric object moves between the plates, the capacitance changes linearly with motion.

While using two plates capacitive sensor, there is a non-linear relationship between displacement and the change in capacitance exist. This can be overcome by using three plates capacitive sensor, called push-pull displacement sensor. In this type, the upper pair of plates forms one capacitor and the lower pair forms another capacitor as shown in Figure 1.37. When the central plate moves upward, the separation of upper pair decreases and the separation of lower pair increases. Therefore, the capacitance of a parallel plate capacitor is given by

One form of capacitive proximity sensor is shown in Figure 1.38, where one plate of a capacitor is connected to the central conductor of a coaxial cable, while the other plate is formed by a target object. The operating principle is based on either the geometry (i.e., the distance d), or capacitance variations in the presence of conductive or dielectric materials.Applications:This sensor can be employed for measuring position, displacement, gauging, or any other similar parameter in a machine tool.

Advantages:1. Excellent linearity over entire dynamic range when area is changed (since stray electric fields are small)2. High sensitivity.3. Capacitive displacement detectors can detect 10-14 m displacements with good stability, high speed,

and wide extremes of environment.4. The system responds to average displacement of a large area of a moving electrode .

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5. Freedom of electrode (plate) materials and geometry for demanding environments and applications.6. Fractional change in capacitance can be made large.7. Capacitive sensors can be made to respond to displacements in one direction only.8. The forces exerted by the measuring apparatus are electrostatic and usually small enough so that they

can be disregarded.9. Capacitors are noiseless.10. High accuracy and resolution. A resolution of 2.5xlO-3 can be obtained.

Disadvantages:1. The performance of these sensors is likely affected due to the environmental conditions such as dust,

moisture, vibration etc.2. The metallic parts of the capacitor must be insulated from

each other.

1.2.3.4. Inductive displacement sensor:1. The most widely used variable-inductance displacement

transducer in industry is LVDT (Linear Variable Differential Transformer).

2. It is a passive type sensor. It is an electro-mechanical device designed to produce an AC voltage output proportional to the relative displacement of the transformer and the ferromagnetic core.

3. The physical construction of a typical LVDT consists of a movable core of magnetic material and three coils comprising the static transformer as shown in Figure 1.39.

4. One of the three coils is the primary coil or excitation coil and the other two are secondary coils or pick-up coils.

5. An AC current (typically 1 kHz) is passed through the primary coil, and an AC voltage is induced in the secondary coils.

6. The magnetic core inside the coil winding assembly provides the magnetic flux path linking the Primary and secondary Coils.

7. When the magnetic core is at the centre position or null position the output voltages are equal and opposite in polarity and therefore, the output voltage is zero.

8. The Null Position of an LVDT is extremely stable and repeatable. When the magnetic core is displaced from the Null Position, a certain number of coil windings are affected by the proximity of the sliding core and thus an electromagnetic imbalance occurs.

9. This imbalance generates a differential AC output voltage across the secondary coil which is linearly proportional to the direction and magnitude of the displacement.

10. The output voltage to displacement plot is a straight line within a specified range. Beyond the nominal range, the output deviates from a straight line in a gentle curve as shown in Figure 1.40.

The Rotational Variable Differential Transformer (RVDT) is used to measure rotational angles and operates under the same principles as the LVDT sensor.Whereas the LVDT uses a cylindrical iron core, the RVDT uses a rotary ferromagnetic core. A schematic of RVDT is shown in Figure 1.41.Calculation of output voltage:

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. Motion of a magnetic core changes the mutual inductance of two secondary coils relative to a primary coil

The value of kl and k2 depend on the amount of coupling between the primary and the secondary coils, which is proportional to the position of the coil.When the coil is in the central position, kl=k2

Vout = V1-V2 = 0

Positive or negative displacements are determined from the phase of Vout.

Applications:LVDT can be used to measure the displacement, deflection, position and profile of a workpiece.Advantages:

1. Relative low cost due to its popularity.2. Solid and robust, capable of working in a wide variety of environments.3. No friction resistance, since the iron core does not contact the transformer coils, resulting in an infinite (very

long) service life.4. High signal to noise ratio and low output impedance.5. Negligible hysteresis.6. Short response time, only limited by the inertia of the iron core and the rise time of the amplifiers.7. No permanent damage to the LVDT if measurements exceed the designed range.8. It can operate over a temperature range of - 265°C to 600°C.9. High sensitivity up to 40 V/mm.10. Less power consumption (less than 1W)

Disadvantages:1. The performance of these sensors is likely affected by vibration etc.2. Relatively large displacements are required for appreciable output.3. Not suitable for fast dynamic measurements because of mass of the core.4. Inherently low in power output.5. Sensitive to stray magnetic fields but shielding is not possible.

1.2.4. Position Sensors: Position sensors are those sensors which determine the position of the object of interest with reference to some

reference point. Position sensors can be either linear or angular. Different types of sensors commonly used for position

measurement are:1. Potentiometer2. Capacitive sensors (for linear position)3. Inductive position sensors (LVDT)4. Hall effect sensors5. Photoelectric sensor6. Optical encoderThe first three types are already discussed under displacement sensors. The other types are discussed under.

1.2.4.1. Hall effect sensors: 1. Hall Effect sensor is a type of magnetic sensor. 2. A Hall Effect sensor is a transducer that varies its output voltage in response to changes in magnetic field.

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3. In 1879 Edwin Hall discovered that: "when a conductor or semiconductor with current flowing in one direction was introduced perpendicular to a magnetic field a voltage could be measured at right angles to the current path". The voltage is directly proportional to the number of flux lines passing through the conductor, the angle at which they pass through it, and the amount of current used.

4. When a current-carrying conductor is placed into a magnetic field, a voltage will be generated perpendicular to both the current and the field. This principle is known as the Hall Effect.

5. Figure 1.42 illustrates the basic principle of the Hall Effect. It shows a thin sheet of semiconducting material (Hall element) through which a current is passed.

6. The output connections are perpendicular to the direction of current. When no magnetic field is present as shown in Figure 1.42 (a), current distribution is uniform and no potential difference is seen across the output.

7. When a perpendicular magnetic field is present, as shown in Figure 1.42 (b), a force is exerted on the current.

8. This force disturbs the current distribution, resulting in a potential difference (voltage) across the output . This voltage is the Hall voltage (VH).

The Hall voltage is proportional to the vector cross product of the current (I) and the magnetic flux density (B).

where KH is the Hall coefficient t is the thickness of the Hall element,

The Hall element is the basic magnetic field sensor. It requires signal conditioning to make the output usable for most applications. The signal conditioning electronics needed are amplifier stage and temperature compensation. Voltage regulation is needed when operating from an unregulated supply. Figure 1.43 illustrates a basic Hall Effect sensor.

The Hall effect sensor can also be used to measure fuel level in a fuel tank (Figure 1.44). The float has buoyancy in the fuel. It floats up as the fuel becomes· more. The gap between the magnet and hall sensor will changed. It results in the changing of the output. The springs allow the float to move only vertically.

Applications:1. Hall sensors are used for proximity switching, positioning, speed

detection, and current sensing applications.2. Hall sensors are commonly used to time the speed of wheels and

shafts, such as for internal combustion engine ignition timing or tachometers.

3. They are used in brushless DC electric motors to detect the position of the permanent magnet.4. Typical applications are the detection of a moving part, replacing a mechanical limit switch. Another common use

is in indexing of rotational or translational motion.

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Advantages:1. Relative low cost compared to electromagnetic

switches.2. High frequency operation is possible.3. Multiple purpose usage as displacement, position and

proximity sensors.4. Solid and robust, capable of working in a severe

environmental conditions as they are immune to humidity contamination.

5. No contact bounce problem.Disadvantages: Sensor becomes weak during offset effects caused by misalignment of contact in Hall element and piezo-resistive effects.

1.2.4.2. Photoelectric sensor:1. A photoelectric sensor is a device used to detect the distance, absence, or presence of an object by using a light

transmitter, often infrared or LED, and a photoelectric receiver. 2. Photoelectric sensors respond to the presence of all types of objects, be it larger, small, transparent or opaque,

shiny or dull, static or motion.3. They can sense objects from distance of a few mm up to 100 m.4. Photoelectric sensors use an emitter unit to produce a beam of light that is detected by a receiver. When the beam

is broken by any external object, a presence is detected. 5. The emitter light source is light - emitting diodes (LED) that emit light when current is applied. 6. The photo detector or receiver contains a phototransistor that produces a current when light falls upon it.

There are two modes of detection for photoelectric sensors.1. Through-beam2. Retroreflective

A through beam arrangement consists of separate emitter and receiver elements located opposite each other as shown in Figure 1.45. Therefore the light emitted by the emitter falls directly on the receiver. In this mode, an object is detected when the light beam is blocked from getting to the receiver from the transmitter.

A retroreflective arrangement places the transmitter and receiver at the same location and uses a reflector to bounce the light beam back from the transmitter to the receiver as shown in Figure 1.46. An object is sensed when the beam is interrupted and fails to reach the receiver.

The detecting range of a photoelectric sensor is its "field of view", or the maximum distance the sensor can retrieve information from, minus the minimum distance. A minimum detectable object is the smallest object the sensor can detect.More accurate sensors can often have minimum detectable objects of minuscule size.

1.2.4.3. Optical encoder:1. An optical encoder is a device that converts motion into

electrical pulses. 2. These electrical pulses are encoded into required form for

the measurement of displacement. Encoders have both linear and rotary configurations, but the most common type is rotary which is discussed here.

3. Optical encoders are composed of a glass or plastic code disc with a photographically deposited radial pattern organized in tracks, a photoemitter and a photo detector (Figure 1.47).

4. Typically there is also a stationary mask, with the same pattern as the rotating codewheel, in the light path from the emitters to the detectors. As radial lines in each track interrupt the beam between a photoemitter-detector pair when the disc rotates, digital pulses are produced.

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There are two basic configurations for rotary optical encoders, the incremental encoder and the absolute encoder.Incremental encoder:

1. Incremental rotary encoders are preferred when low cost is important, or when only relative position is needed. 2. The incremental encoder, sometimes called a relative encoder,3. consists of two tracks and two sensors whose out puts are called channels A and B, As the shaft rotates, pulse

trains occur on these channels at a frequency proportional to the shaft speed, and the phase relationship between the signals yields the direction of rotation.

4. Incremental encoders often have a third channel, called index channel, with a single segment slot or reference yields one pulse per revolution which is useful in counting full revolutions.

5. It is also useful as a reference to define a home base or zero position.

6. The code disc pattern and output signals A, B and Index are illustrated in Figure 1.48.

7. By counting the number of pulses and knowing the number of radial lines in the disc, the rotation of the shaft can be measured.

8. The direction of rotation is determined by the phase relationship of the A and B pulse trains, i.e., 9. Which signal leads the other. For example, a rising

edge of A while B = 1 may indicate counterclockwise rotation, while a rising edge of A while B = 0 indicates clockwise rotation. The signals from the two channels are a 1/4 cycle out of phase with each other and are known as quadrature signals.

10. A drawback of the incremental encoder is that there IS

no way to know the absolute position of the shaft at power-up without rotating it until the index pulse is received. Also, if pulses are momentarily garbled due to electrical noise, the estimate of the shaft rotation is lost until the index pulse is received. A solution to these problems is the absolute encoder.

Absolute encoder:1. An absolute encoder uses k photo interrupters and k code

tracks to produce a k - bit binary word uniquely representing 2k different orientations of the disc, giving an angular resolution of 360°/2k (Figure 1.49).

2. For example, if there are 8 tracks, the encoder is capable of producing 256 distinct positions or an angular resolution of 1.406 (360/256) degrees. Absolute encoders contain multiple detectors and up to 20 tracks of segment patterns.

3. Unlike an incremental encoder, for each rotary encoder position, there is a different binary output. Therefore, shaft position is absolutely determined.

4. The most common types of numerical encoding used in the absolute encoder are gray and binary codes.

5. The radial patterns on the tracks are arranged so that as the encoder rotates in one direction, the binary word increments or decrements according to a binary code.

6. Although natural binary code is a possibility, the Gray code is a more common solution. With natural binary code, incrementing by one may change many or all of the bits.

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7. With the Gray code, only one bit changes as the

number increments or decrements. The gray code and natural binary code disc track patterns for a simple 4-track (4-bit) encoder are illustrated in Figures 1.50 and 1.51.

8. The linear patterns and associated timing diagrams are what the photo detectors sense as the code disc circular tracks rotate with the shaft.

The gray code is designed so that only one track (one bit) will change state for each count transition, unlike the binary code where multiple tracks (bits) change at certain count transitions. This effect can be seen clearly in Table 1.2. For the gray code, the uncertainty during a transition is only one count, unlike with the binary code, where the uncertainty could be multiple counts.

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1.2.5. Proximity Sensors:1. Proximity sensors are used to determine the presence (as opposed to actual range) of nearby objects.2. They are essentially non contact two state devices which give n-off outputs.3. A proximity sensor often emits an electromagnetic field or beam and look for changes in the field. The object

being sensed is often referred to as the proximity sensor's target. 4. Different proximity sensor targets demand different sensors.5. For example, a capacitive or photoelectric sensor might be suitable for a plastic target; an inductive proximity

sensor requires a metal target. Such proximity sensors are classified into several types in accordance with the specific properties used to initiate a switching action:I

1. Optical encoders2. Hall effect sensors3. Capacitive sensors4. Eddy current proximity sensors5. Inductive proximity sensor6. Pneumatic proximity sensor7. Proximity switchesThe first three types are already discussed under displacement and position sensors. The other types are discussed under.

1.2.5.1. Eddy current proximity sensors:1. Eddy current proximity sensors detect the proximity or presence of a target by sensing the magnetic fields

generated by a reference coil. 2. Eddy current sensors detect ferrous and nonferrous metals. They can be used as proximity sensors to detect3. presence of a target, or can be configured to measure the position or displacement of a target.4. An eddy current is a local electric current induced in a conductive material by the magnetic field produced by the

sensor or active coil. This is sensed by a reference coil to create an output signal.33

5. When the distance between the target and the probe changes, the impedance of the coil changes correspondingly. This change in impedance can be detected by a carefully arranged bridge circuit as shown in Figure 1.52.

The eddy currents are confined to shallow depths near the conductive target surface. Their effective depth is given by.

The target material must be at least three times thicker than the effective depth of the eddy currents to make the transducer successful. This is because the transducer assumes that the eddy currents are localized near the surface of a semi-infinite solid, and the actual eddy current amplitude decreases quadratically with distance.Advantages:

1. Compact in size .2. Low cost3. High reliability4. High sensitivity for small displacement.

1.2.5.2. Inductive proximity sensor:1. Inductive proximity sensors are today the most commonly employed industrial sensors for detection of ferrous

metal objects over short distances.2. Inductive proximity sensors operate under the electrical principle of inductance.3. Inductance is the phenomenon where a fluctuating current, which by definition has a magnetic component,

induces an electromotive force (emf) in a target object.4. An inductive proximity sensor has four components; the induction coil, oscillator, detection circuit and output

circuit as shown in Figure 1.53. 5. The oscillator generates a fluctuating magnetic field the shape of a doughnut around the winding of the coil that

locates in the device's sensing face.6. When a metal object moves into the magnetic field of detection, eddy circuits build up in the metallic object. 7. These eddy currents produce a secondary magnetic field that interacts with field of the probe, thereby loading the

probe oscillator. The effective impedance of the probe coil changes, resulting in an oscillator frequency shift (or amplitude change).

8. The sensor's detection circuit monitors the oscillator's strength and triggers an output signal from the output circuitry proportional to the sensed gap between probe and target.

1.2.5.3. Pneumatic proximity sensor:

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1. Pneumatic proximity sensor, as shown in Figure 1.54, uses the principle of a gas nozzle to detect the presence of an object without any mechanical contact.

2. Low pressure air is supplied through annular converging nozzle surrounding a sensing hole, called output port. 3. Nozzle may also be of the converging-diverging type, if desired. Sensing hole communicates through hose with

switch chamber,4. which contains an elastic-diaphragm switch, or other type of pressure- sensitive switch.5. Nozzle converts some of the energy of the supply air into kinetic energy. As the air stream from nozzle impinges

upon an object to be sensed, a turbulence and back pressure is created.

6. This action then increases the static pressure at output port, and activates switch.

1.2.5.4. Proximity switches:Proximity switches are used to detect the presence of an object. These can be achieved by the presence of an object in order to give output which is either on or off. These can be classified in to two types:(i) Non-contact type(ii) Contact type.The non-contact type switches detect the presence of an object without physical contact. Examples: Magnetic reed switch, photoelectric sensor, inductive proximity sensor. Photoelectric and inductive proximity sensors are already discussed in the previous articles.

Magnetic reed switch:The simplest form of magnetic proximity sensor is the magnetic reed switch, schematically illustrated in Figure 1.55. A pair of ferromagnetic reeds is cantilevered from opposite ends of a glass tube, arranged such that their tips overlap slightlywithout touching. When a magnetic is brought close to the switch, the subsequent attractive magnetic force pulls the flexible reed elements together to make electrical contact.

Reed switches are faster, more reliable, and produce less arcing than conventional electromechanical switches. Some problems can be encountered with this type of sensor due to contact bounce, structural 'vibration, and pitting of themating surfaces in the case of inductive or capacitive loads. These inexpensive and robust devices are commonly employed as door- and window-closure sensors in security applications.Contact type switches are mechanical devices that detect the presence of an object and give output which is either on or off. Examples: Limit switch, micro switch.

Limit switch:1. The function of a limit switch is to produce electrical signals

corresponding to the position of the mechanical member to be detected.

2. Limit switches have operating heads which incorporate some type of lever arm or plunger mechanism: the selection of which is application dependent.

3. When a limit switch has positive opening of the normally closed contact it is typically suitable for use in machine safety applications.

4. A micro switch is a generic term used to refer to a small electrical switch that is able to be actuated by very little physical force.

5. Internally a stiff metal strip must be bent to activate the switch. This produces a very distinctive clicking sound and a very crisp feel. When pressure is removed the metal strip springs back to its original state.

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6. The defining feature of micro switches is that a' relatively small movement at the actuator button produces a relative large movement at the electrical contacts, which occurs at high speed (regardless of the speed of actuation).

7. They are very common due to their low cost and extreme durability, typically greater than 1 million cycles and up to 10 million cycles for heavy duty models. Common applications of micro switches include computer mouse buttons and arcade game joysticks and buttons.

8. Figure 1.56 shows examples of different ways that a micro switch can be activated.

1.2.6. Velocity and Motion Sensors:Velocity and motion sensors are used to monitor linear and angular velocities and detect motion. Motion sensors are commonly used in high precision systems to monitor the linear or rotary movement of an object with respect to another object.Motion sensors are used in security systems to detect movement in a monitored space. Motion sensors are used to detect the presence of people in a room for the purpose of automatically turning on lights or other devices. The various velocity and motion sensors commonly used are:1. Incremental encoder2. Tachogenerator3. Pyroelectric sensors

1.2.6.1. Incremental encoder:Incremental encoders, described in under position sensors (refer page 1.64), can be used in for the measurement of angular velocity.

1.2.6.2. Tachogenerator:1. Tachogenerators or tachometers are used to measure the angular velocity of a rotating shaft or object.2. An electromechanical generator is a device capable of producing electrical power from mechanical energy,

usually the turning of a shaft.3. A generator specially designed and constructed for shaft speed measurement is called a tachometer or

tachogenerator.

4. Tachometers operate on the principle that a driven motor (a motor operated as a generator) produces a voltage that is proportional to the angular velocity of the motor shaft.

5. The proportionality constant, K, that is used to translate mechanical motion into voltage has typical values of 1 to 30 volts per 1000 RPM.

6. One form of tachogenerator is variable reluctance tachogenerator. It consists of a ferromagnetic toothed rotor mounted on the shaft whose speed is to be measured as shown in Figure 1.57.

7. A magnetic pick up consists of a small permanent magnet with a coil wound around it. Magnetic pick up arrangement is placed near the toothed rotor. As the rotor rotates, the reluctance of the air gap between pickup and the toothed rotor changes and the rise in e.m.f. is induced in the pickup coil.

8. The output is in the form of pulses and wave shapes. By counting the number of pulses in a particular time interval the angular velocity can be calculated by using the following equation:

9. Tachogenerators can also indicate the direction of rotation by the polarity of the output voltage. 10. When a permanent-magnet style DC generator's rotational direction is reversed, the polarity of its output voltage

will switch. In measurement and control systems where directional indication is needed, tachogenerators provide

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an easy way to determine that. Tachogenerators are frequently used to measure the speeds of electric motors, engines, and the equipment they power: conveyor belts, machine tools, mixers, fans, etc.

1.2.6.3. Pyroelectric sensors:1. The pyroelectric sensor is made of a crystalline material that generates a surface electric charge when exposed to

heat source.2. Example of crystalline material is lithium tantalite. When this type of material is heated below a temperature

known as Curie point, a large spontaneous electrical polarization is exhibited from the material in response to a temperature change.

3. The change in polarization is observed as an electrical voltage signal if electrodes are placed on opposite faces of a thin slice of the material. Figure 1.58 (a) shows that the charges in the pyroelectric material are balanced if there is no infrared radiation from the heat source falls on the materials surface.

4. When the material is exposed to the infrared radiation from the heat source the charges in the pyroelectric material are not balance and hence there is some excess charge in the material as shown in Figure 1.58 (b).

5. The design can be thought of as a typical form of a capacitor circuit. Figure 1.59 shows the equivalent circuit of a pyroelectric sensor. It essentially consists of a capacitor charged by the excess charge with a resistance R to represent the internal leakage combined with the input resistance of an external circuit.

6. For detection of a human motion or intrusion in the country borders, the pyroelectric are sensors used. In such applications, the sensing element has to differentiate between general background heat radiation and a moving heat source.

7. Therefore, a single pyroelectric sensor is not capable to use and dual pyroelectric sensors are used as shown in Figure 1.60.

8. In this dual pyroelectric sensor, the sensing element has the one front electrode and two back electrodes. When two sensors are connected, both sensors receive the same heat signal and their outputs are cancelled.

9. When a heat source moves from its position the heat radiation moves from one of the sensing elements to the other. Then the current alternates in one direction first and then reversed to the other direction. When the amount of infrared radiation from heat source striking the crystal, the electric charge also changes and can then be measured with a sensitive FET device built into the sensor.

1.2.7. Force Sensors :1. Force sensors are used in many mechanical equipments and

aggregates for an accurate determination of forces applied in the system. The force sensor outputs an electrical signal corresponding to the force applied.

2. Force sensors are commonly used in many applications such as automotive brakes, suspension, transmission, speed control, lifts, aircrafts, digital weighing systems etc.

3. Most of the force sensor uses displacement as the measure of the force. The simplest form of force sensor is the spring balance in which a force is applied to the one end of the spring causes displacement of the spring.

4. This displacement is the measure of the force applied. A common force sensor is a strain gauge load cell which is explained under.

1.2.7.1. Strain gauge load cell:

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1. A load cell is an electromechanical transducer that converts load acting on it into an analog electrical signal.2. Load cells provide accurate measurement of compressive and tensile loads. Load cells commonly function by

utilizing an internal strain gauge that measure deflection. Because the modulus of elasticity of a load cell is constant the amount of strain can be calibrated to determine the force upon the load cell.

3. Typically the force creates the train in the load cell which is measured by strain gauge transducer.4. Strain gauge is attached to the object or the strained element where the force is being applied. As the object is

stressed due to the applied force, the resulting strain deforms the strain gauge attached with it. This causes an increase in resistivity of the gauge which produces electrical signal proportional to, the deformation.

5. The measurement of resistivity is the measure of strain which in turn gives the measurement of force or load applied on the object.

6. The change of resistance is generally very small and is usually measured using a Wheatstone bridge circuit where the strain gauges are connected into the circuit.

7. The strain gauges are serving as resistors in the circuit. The Wheatstone bridge circuit produces analog electrical output signal.

8. In a typical strain gauge load cell for measuring force, four strain gauges are attached to the surface of the counterforce and are electrically connected in a full Wheatstone bridge circuit as shown in Figure 1.61.

9. Load cells have different shapes (cylindrical tubes, rectangular or square beams, and shaft) for different applications and load requirements to ensure that the desired component of force is measures, thus strain gauges having different shapes are positioned in various orientations upon the load cell body.

10. The different configurations of strain gauges are already discussed under strain gauges displacement sensors.

1.2.8. Fluid Pressure Sensors:Pressure is an expression of the force required to stop a fluid from expanding, and is usually expressed in terms of force per unit area. A pressure sensor measures pressure of gases or liquids. These sensors generate a signal as a function of thePressure applied by the fluid. Pressure sensors are used in many applications such as automotive vehicles, hydraulic systems, engine testing etc. Pressure sensors may required to measure different types of pressures: 1. Absolute pressure where the pressure is measured relative to the perfect vacuum or zero-pressure, 2. Gauge pressure where the pressure is measured relative to the atmospheric pressure, and3. Differential pressure where a pressure difference is measured.

The devices which are used to measure fluid pressure in industrial processes are:1. Diaphragm pressure sensor2. Capsule pressure sensor3. Bellows pressure sensor4. Bourdon tube pressure sensor5. Piezoelectric sensor6. Tactile SensorThe construction and working principle of these sensors are explained here.

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1.2.8.1. Diaphragm pressure sensor:1. The diaphragm pressure sensor uses the elastic deformation of a diaphragm (i.e. membrane) to measure the

difference between an unknown pressure and a reference pressure. 2. Diaphragm is a thin circular elastic membrane made of generally silicon as show in Figure 1.62. As pressure

changed, the diaphragm moves, and this motion is the measure of differential pressure.3. Diaphragms are popular because they require less space and the motion they produce is sufficient for operating

electronic transducers.4. They also are available in a wide range of materials for corrosive service applications.5. A typical diaphragm pressure gauge contains a chamber divided by a diaphragm, as shown in the Figure 1.63.6. One side of the diaphragm is open to the external targeted pressure. PCX1' and the other side is connected to a known

pressure, PRef The pressure difference, PEx1 – Pref mechanically deflects the diaphragm.7. The diaphragm deflection can be measured in any number of ways. 8. For example, it can be detected via a mechanically-coupled indicating needle, an attached strain gauge [refer

Figure 1.64 (a)], a linear variable differential transformer (LVDT) [refer Figure 1.64 (b)], or with many other displacement/velocity sensors. Once known, the

9. deflection can be converted to a pressure loading using plate theory.

10. Strain gauge arrangement consists of four strain gauges with, two measuring the strain in a circumferential direction while the remaining two measure strains in a radial direction.

11. The four strain gauges are connected to form the arms of a Wheatstone bridge. The sensitivity of pressure gauges using LVDTs is good and, ~ therefore, stiff primary sensors with very little movement can be used to reduce environmental effects. Frequency response is also good.

Advantages:1. Much faster frequency response than U tubes .2. Accuracy up to ±0.5% of full scale .3. Good linearity when the deflection IS no larger than the order of the

diaphragm thickness.

Disadvantages:1. More expensive than other pressure sensors.

1.2.8.2. Capsule pressure sensor:2. In order to improve the sensitivity, two corrugated diaphragms

are combined by arranging these in back-to-back and sealed together at the periphery to obtain shell-like shape as shown in Figure 1.65. These are called as capsules. One of the diaphragms is provided with a central reinforced port to allow the pressure to be measured, and the other is linked to a mechanical element.

3. The difference in pressure between inner and outer surface of the capsule produces displacement.

4. These capsules can also be attached with the LVDT as described in the diaphragm pressure gauge.

1.2.8.3. Bellows pressure sensor:

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1. The bellows is a one-piece, collapsible, seamless metallic unit that has deep folds formed from very thin-walled tubing. It looks like a stake of capsules. It is more sensitive than the diaphragm and capsule pressure sensors.

2. The diameter of the bellows ranges from 1.2 to 30 cm and may have as many as 24 folds. System pressure is applied to the internal volume of the bellows. As the inlet pressure varies, the bellows will expand or contract.

3. The moving end of the bellows is connected to a mechanical linkage assembly. The deflection can be measured in any number of ways. For example, it can be detected via a mechanically-coupled indicating needle [refer Figure 1.66 (a)), a linear variable differential transformer (LVDT) as described in the diaphragm pressure gauge [refer Figure 1.64 (b)), a potentiometer [refer Figure 1.66 (b)], or with many other displacement sensors.

4. As the bellows and linkage assembly moves, either an electrical signal is generated or a direct pressure indication is provided. Figure 1.66 shows a bellows pressure sensing element along with the potentiometer.

5. The potentiometric bellows pressure sensor provides a simple method for obtaining an electronic output from a mechanical pressure gauge.

6. The device consists of a precision potentiometer, whose wiper arm is mechanically linked to a bellows or Bourdon element. The movement of the wiper arm across the potentiometer converts the mechanically detected sensor deflection into a resistance measurement, using a Wheatstone bridge circuit. The flexibility of a metallic bellows is similar in character to that of a helical, coiled compression spring. Up to the elastic limit of the bellows, the relation between ~increments of load and deflection is linear. In practice, the bellows must always be opposed by a spring, and the deflection characteristics will be the resulting force of the spring and bellows.

1.2.8.4. Bourdon tube pressure sensor:1. The bourdon tube pressure instrument is one of the oldest

pressure sensing instruments in use today. 2. It is widely used in applications where inexpensive static

pressure measurements are needed. The bourdon tube consists of a thin-walled C shaped tube that is flattened diametrically on opposite sides to produce a cross-sectional area elliptical in shape, having two long flat sides and two short round sides.

3. The tube is bent lengthwise into an arc of a circle of 270 to 300 degrees.

4. Bourdon tube is open to external pressure input on one end and is coupled mechanically to an indicating needle on the other end. as shown schematically in Figure 1.67. Pressure applied to the inside of the tube causes distention of the . flat sections and tends to restore its original round cross-section.

5. This change in cross-section causes the tube to straighten slightly. Since the tube is permanently fastened at one end, the tip of the tube traces a curve that is the result of the change in angular position with respect to the center. Within limits, the movement of the tip of the tube can then be used to position a pointer or to develop an equivalent electrical signal to indicate the value of the applied internal pressure.

6. The deflection of the Bourdon tube can be measured in any number of ways.

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For example. it can be detected via a mechanically-coupled indicating needle [refer Figure 1.67], a linear variable differential transformer (L VDT) as described in the diaphragm pressure gauge [refer Figure 1.64 (b) ] a potentiometer [refer Figure 1.66 (b)], or with many other displacement sensors.To increase their sensitivity, Bourdon tube elements can be extended into spirals or helical coils [Figures 1.68 (a) and (b)]. This increases their effective angular length and therefore increases the movement at their tip, which in turn increases the resolution of the transducer.

Advantages:1. Portable2. Convenient to use3. No leveling required

Disadvantages:1. Limited to static or quasi-static measurements .2. Accuracy may be insufficient for many applications. A mercury

barometer can be used to calibrate and check Bourdon Tubes.

1.2.8.5. Piezoelectric sensors:A piezoelectric sensor is a device that uses the piezoelectric effect to measure pressure, acceleration, strain or force. When pressure, force or acceleration is applied to piezoelectric materials such as quartz crystal, PZT ceramic, tourmaline, gallium phosphate, and lithium sulfate, an electrical charge is developed across the crystal that is proportional to the force applied (Figure 1.69 (a)). When pressure is applied to a crystal, it is elastically deformed. This deformation results in a flow of electric charge (which lasts for a period of a few seconds). The resulting electric signal can be measured as an indication of the pressure which was applied to the crystal.The net electrical charge (q) produced in the crystal is proportional to the deformation of the crystal (x) due to the applied pressure and the stiffness of the material (k). Since the deformation is proportional to the applied pressure or force(P), the net electric charge is given by the equation;q=k×x=S×Pwhere S is the charge sensitivity.The piezoelectric sensors are attached with the diaphragm pressure sensing element to measure the pressure as shown in Figure 1.69 (b).The output electrical signal of the piezoelectric sensor IS

related to the mechanical force or pressure as if it had passed through the equivalent circuit as shown in Figure 1.70. The model of the equivalent circuit includes the followingcomponents:C represents the capacitance of the sensor surface itself;R is the insulation leakage resistance of the transducer; and q is the charge generator.If the sensor is connected to a load resistance, this also acts in parallel with the insulation resistance.The fundamental difference between these piezoelectric sensors and static-force devices such as strain gauges is that the electric signal generated by the piezoelectric sensors decays rapidly. This characteristic makes these sensors unsuitable for the measurement of static forces or pressures but useful for dynamic measurements.

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Piezoelectric pressure sensors do not require an external excitation source and are very rugged. These sensors, however, do require charge amplification circuitry and very susceptible to shock and vibration.The desirable features of piezoelectric sensors include their rugged construction, small size, high speed, and self-generated signal. On the other hand, they are sensitive to temperature variations and require special cabling and amplification.

1.2.8.6. Tactile sensors:1. Tactile pressure sensors are used to detect the pressure distribution

between a sensor and a target. They are often used on the robot grippers or flat tactile arrays to identify whether the finger is in touch with the target object or not.

2. These sensors are also used in touch screen display of laptops, ATM machines, mobiles etc.

3. Most tactile pressure sensors use resistive-based technologies where the sensor acts as a variable resistor in an electrical circuit. A small deflection of the diaphragm causes implanted resistors to exhibit a change in resistance value.

4. The sensor converts this change in resistance into a voltage that is interpreted as a continuous and linear pressure reading.

5. When tactile pressure sensors are unloaded, their resistance is very high. When force is applied, their resistance decreases. Pressure sensitive film is used to create a direct, visual image of the pressure distribution. Active pressure sensor arrays consist of multiple sensing elements packaged in a single sensor.

6. There are many different forms of tactile sensors. One form of tactile pressure sensor includes upper and lower conductive layers separated by an intermediate insulating layer which is formed as a separating mesh (Refer to Figure 1.71).

7. The upper conductive layer is of negligible resistance. The lower conductive layer is formed of a plurality of conductive strips (A-F) separated by insulating strips. Each conductive strip (A-F) has a known resistance.

8. An electrical signal is applied to the conductive strips (A-F) in turn and the electrical path between the upper and lower

9. conductive layers then determined.10. The electrical resistance of the conductive path establishes the location of the pressure point at which bridging

occurs and from this it is possible to establish the location and size of the pressure area.

11. Figure 1.72 shows another form of tactile sensor. 12. It uses piezoelectric material of polyvinylidene fluoride (PVDF)

film. Two layers of PVDF films are used and they are separated by a soft film which transmits vibrations.

13. When the alternating voltage is supplied in the lower PVDF film it results in mechanical oscillations of the film.

14. The intermediate film transmits these vibrations to the upper PVDF film. Due to the piezoelectric effect the vibrations formed cause an alternating voltage to be produced across the upper film. So, pressure is applied to the upper PVDF film and its vibrations affect the output voltage.

1.2.9. Liquid Flow Sensors:1. Flow measurement is an important process measurement to be

considered in operating fluid systems. For efficient and economic operation of these fluid systems, flow measurement is necessary.

2. Most of the traditional flow measuring methods such as orifice meter, venture meter, and Pitot tube, use the Bernoulli's principle which says that the downstream pressure in a pipe line after an obstruction will be lower than

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the upstream pressure before. According to Bernoulli's principle, there is a relationship between the pressure of the fluid and the velocity of the fluid.

3.

When the velocity increases, the pressure decreases and vice versa. To understand orifice and venturi meters it is therefore necessary to explore the Bernoulli Equation.

4. Bernoulli's equation is an equation relating the conservation of energy between

5. two points on the same streamline. Bernoulli's equation at two sections A-A and B-B

6. before and after obstruction (refer to Figure 1.73) in a horizontal pipe is given as:

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Thus, for a given geometry, the flow rate can be determined by measuring the pressure difference (PI - P2). There are number of flow measuring devices available.The following very important and basic devices are described in detail here:(i) Orifice meter,(ii) Venture meter, and(iii) Turbine flow meter

1.2.9.1. Orifice meter:1. The Orifice meter is one of the many devices used to measure the volume or mass flow rate of fluids flowing in a

closed conduit (pipe). The orifice meter is recommended for Clean and dirty liquids and some slurry services. 2. The Orifice meter consists of an orifice place. An orifice plate is basically a thin plate with a hole in the middle. It

is usually placed in a pipe in which fluid flows. 3. As fluid flows through the pipe, it has a certain velocity and a certain pressure. When the fluid reaches the orifice

plate, with the hole in the middle, the fluid is forced to converge to go through the small hole. 4. The point of maximum convergence actually occurs shortly downstream of the physical orifice, at the so-called

vena contracta point (refer to Figure 1.74).

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5. Because of the convergence, the velocity and the pressure of the fluid change. Beyond the vena contracta, the fluid expands and the velocity and pressure change once again. Small access pressure ports, or pressure taps are required on each side of the orifice plate to allow the measurement of the pressure change across the plate when the fluid is flowing. By measuring the difference in fluid pressure between the normal pipe section and at the vena contracta, the volumetric and mass flow rates can be obtained from Bernoulli's equation. The discharge

Advantages of orifice meter:1. The relative cost is low .2. Ease of installation and replacement.3. Requires less space as compared with venturirneter.4. Can be used in wide range of pipe sizes (0.01m to 1.5m).

Disadvantages of orifice meter:1. High loss of head2. Co-efficient of discharge has a low value3. Susceptible to inaccuracies resulting from erosion, corrosion and scaling .4. The viscosity 'effect is high

1.2.9.2. Venturimeter1. In the venturi meter (refer to Figure 1.75), the fluid is accelerated through a converging cone of angle (2α1= 21±

2°). 2. The pressure difference between the section before the convergent cone and the throat is measured which

provides a signal for the rate of flow. 3. The fluid slows down in a divergent cone with smaller angle (2α.2 = 5 to 15°) where most of the kinetic energy is

converted back to pressure energy. 4. Because of the cone and the gradual reduction in the area there is no "Vena Contracta" as in the case of orifice

meter. The flow area is minimum at the throat. 5. Two small access pressure ports are placed, one at upstream side of the venturi meter and the other at the throat,

to allow the measurement of the pressure change across the venture meter. 6. High pressure and energy recovery makes the venturi meter suitable where only small pressure heads are

available. The pressure recovery, is much better for the venturi meter than for the orifice plate. Although venturi meters can be applied to the measurement of gas, they are most commonly used for liquids.

1.2.9.3. Turbine flow meter:

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1. Turbine flow meters measure the rate of flow in a pipe. The turbine flow meter consists of a turbine wheel which has blades around the periphery of a rotor.

2. The turbine wheel is set in the path of a fluid stream in the pipe.

3. As the fluid flows through a pipe, it impinges on the on the turbine blades, imparting a force to the blade surface and setting the rotor in motion.

4. When a steady rotation speed has been reached, flow through the pipe is proportional to the number of revolutions per unit of time.

5. The rotational speed can be measured by any motion sensors such as magnetic pick-up, photoelectric cell, etc.

6. Figure 1.76 (a) shows the measurement of rotational speed from the electrical impulses generated by interrupting light directed at a photocell at each turn of the vane. Another method of measurement is shown in Figure 1.76 (b) which uses magnetic pick-up.

Advantages of turbine flow meter:1. Medium initial set up cost.2. Accurate, reliable. time tested proven technology.

Disadvantages of orifice meter:1. Usage restricted for clean fluid only2. Low to medium pressure drop

1.2.10. Liquid Level Sensors:The measurement of level of the liquid in the tank. or any container is very essential in many times. There are number of devices are used to measure the level of the liquid. Liquid level measuring devices are classified into two groups:(a) Direct method: In this method, the liquid level is directly measured by monitoring the liquid surface. An example of the direct method is the dipstick in your car which measures the height of the oil in the oil pan. Other example includes floats. (b) Inferred or indirect method: In this method, the liquid level is indirectly measured by measuring some variables related to the height of the liquid level. The most important variable related to the liquid level is weight whichcan be measured by using load cells.

The weight of the liquid = Ahρgwhere A is the cross-sectional area of the vessel,h is the head of the liquid,ρ is the density of liquid, andg is the acceleration due to gravity.From this equation it is clear that the height of the liquid vanes proportionally with the weigh. An example of the inferred method is a pressure gauge at the bottom of a tank. which measures the hydrostatic head pressure from the height of the liquid.

1.2.10.1. Gauge glass:

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A very simple means by which liquid level is measured in a vessel is by the gauge glass method (Figure 1.77). In the gauge glass method, a transparent tube is attached to the bottom and top (top connection not needed in a tank. open to atmosphere) of the tank. that is monitored. The height of the liquid in the tube will be equal to the height of water in the tank..

1.2. 10.2. Floats:The float method is a direct liquid level measurement mechanism. The most practical design for the float is a hollow metal ball or sphere. However, there are no restrictions to the size, shape, or material used. The design consists of a ball float attached to a rod, which in turn is connected to a lever arm. A slider attached tc the lever are is in contact with the potentiometer as shown in Figure 1.78 (a). Another arrangement of reading the lever arm movement is using strain gauge load cell as shown in Figure 1.78 (b).

The operation of the ball float is simple. The ball floats on top of the liquid in the tank. If the liquid level changes, the float will follow. This rotates the shaft and changes the position of the slider. The slider moves across the potentiometer which gives the electrical output signal related to the height of liquid. In load cell arrangement, the deflection of lever arm is sensed by strain gauge which in turn givesthe electrical output through Wheatstone bridge circuit.

1.2.10.3. Differential pressure liquid level detector:1. The differential pressure detector method of liquid level

measurement uses a differential pressure cell connected to the bottom of the tank being monitored.

2. The higher pressure, caused by the fluid in the tank, is compared to a lower reference pressure (usually atmospheric).

3. This comparison takes place in the differential pressure cell. Figure 1.79 illustrates a typical differential pressure detector attached to an open tank:. The tank is open to the atmosphere; therefore, it is necessary to use only the high pressure connection on the differential pressure cell.

4. The low pressure side is vented to the atmosphere. Therefore, the pressure differential is the hydrostatic head, or weight, of the liquid in the tank Not all tanks or vessels are open to the atmosphere. Many are totally enclosed to prevent vapors or steam from escaping, or to allow pressurizing the contents of the tank.

5. When measuring the level in a tank that is pressurized or the level that can become pressurized by vapor pressure from the liquid, both the high pressure and low pressure sides of the differential pressure cell must be connected (Figure 1.80).

1.2.11. Temperature Sensors:1. Temperature measurements are most widely monitored parameter in

science and industry.2. Temperature is defined as the average kinetic energy of the individual

molecules that comprise the system. As the temperature increases, the molecular activity also increases, and thus the average kinetic energy increases.

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3. Most often, a simple mercury thermometer is needed to measure the fluid or process temperature.4. This uses the principle of expansion or contraction of liquid to measure the change in temperature. Most of the

temperature measuring or monitoring system uses the principle of expansion or contraction of liquids, gases, or solids. There are also other techniques such as change in electrical resistance of conductors and semiconductors,

5. and thermoelectric e.m.f.s used to measure the temperature. The following are the common methods used to measure the temperature which are described in detail.1. Bimetallic Strips2. Resistance temperature detectors (RTDs)3. Thermistors4. Thermocouples5. Thermodiodes and transistors

1.2.11.1. Bimetallic Strips:1. Bimetallic strip thermometers are mechanical thermometers. They are widely used in industry for temperature

control because of their robustness, temperature range and simplicity.2. It consists of a bimetallic strip which is made of two dissimilar metals bonded together with one end fixed and the

other free. A bimetallic strip is used to convert a temperature change into mechanical displacement. 3. The principle is that as the temperature changes one strip expands more than the other, causing the pair to bend at

the free end. Most bimetallic strips use a high thermal expansion alloy, such as steel or stainless steel, coupled with a low thermal expansion alloy such as Invar. Steel and copper, or in some cases brass instead of copper is also used for bimetallic strips.

4. Figure 1.81 shows the configuration of bimetallic temperature controlled switch or thermostat.5. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is

heated and on the inner side when cooled. 6. When the temperature of the switch is increased the high thermal expansion material is expand faster than the

other side low thermal expansion material. 7. This causes the strip to bend upward, making

contact so that current can flow. By adjusting the size of the gap between the strip and the contact, the temperature can be adjusted or set.

Advantages of bimetallic strips:1. Power source not required .2. Low cost.3. Robust construction4. Easy to use and can be used upto 500°C.

Disadvantages of bimetallic strips:1. Less accurate2. Limited to applications where manual reading is acceptable, e.g. a household thermometer. 'i· .

3.Not suitable for very low temperatures because the expansion of metals tend to be too similar, sothe device becomes a rather insensitive thermometer.

1.2.11.2. Resistance temperature detectors (RTDs) :2. When a metal wire is heated the resistance increases. So, a temperature can be measured using the resistance of a

wire. TheRTD incorporates pure metals or certain alloys that-increase in resistance as temperature increases and, conversely,

3. decreasein resistance as temperature decreases. ·4. RTDs act somewhat like an electrical transducer; converting changes in temperature to voltage signals by the

measurement of resistance.

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5. The metals that are best suited for use as RTD sensors are pure, of uniform quality, stable within a given range of temperature, and able to give reproducible resistance-temperature readings. Only a few metals have the properties necessary for use in RTD elements.

6. RTD elements are normally constructed of platinum, copper, nickel or nickeliron alloys. 7. These metals are best suited for RTD applications because of their linear resistance-temperature characteristics as

shown in Figure 1.82, their high coefficient of resistance, and their ability to withstand repeated temperature cycles.

8. The linear relationship of resistance-temperature is given by the equation:

9. R =Ro (l +αT)where R is the resistance at a temperature T°C

Ro is the resistance at 0°c, and a is the temperature co-efficient of resistance

10. The coefficient of resistance is the change in resistance per degree change in temperature, usually expressed as a percentage per degree of temperature. The material used must be capable of being drawn into fine wire so that the element can be easily constructed.

11. RTD elements are usually long, spring-like wires surrounded by an insulator and enclosed in a sheath of metal for protection. Figure 1.83 shows the internal construction of an RTD.

12. In the figure platinum is used as RTD element that is surrounded by a porcelain insulator.

13. The insulator prevents a short circuit between the wire and the metal sheath. Inconel, a nickel-iron-chromium alloy, is normally used in manufacturing the RTD sheath because of its inherent corrosion resistance.

14. When placed in a liquid or gas medium, the Inconel sheath quickly reaches the temperature of the medium.

15. The change in temperature will cause the platinum wire to heat or cool, resulting in a proportional change in resistance. This change in resistance is then measured by a precision resistance measuring device that is calibrated to give the proper temperature reading. This device is normally a bridge circuit.

Advantages of RTDs:1. Suitable for measuring high temperatures2. High degree of accuracy .3. Good stability and repeatability .4. Do not need a reference temperature junction.

Disadvantages of RTDs:1. Size is more than the thermocouple .2. Power supply required .3. Need auxiliary apparatus to get required form of output.4. Resistance element is more expansive than a thermocouple .5. Possibility of error due to self-heating and thermo-electric effect of the resistive element.

1.2.11.3. Thermistors:Thermistor, a word formed by combining thermal with resistor. Thermistors, like RTDs, are temperature-sensitive resistors. Thermistors are non-linear devices their resistance will decrease with an increase in temperature, but at a much faster rate than that of RTDs. The resistance can change by more than 1000 times. As a result, thermistors can sense minute changes in temperature that are otherwise undetected by RTDs and thermocouples. The basic equation is:

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The equation uses a reference temperature and resistance, with a constant for the device, to predict the resistance at another temperature. The expression can be rearranged to calculate the temperature given the resistance.

Thermistors are small, inexpensive devices that are most commonly made of metal oxides such as those of chromium, nickel, manganese and cobalt. The metals are oxidized through a chemical reaction, ground to a fine powder, then compressed and subject to very high heat. Theses oxides are semiconductors.

There are two types of thermistors based on the lead attachment: 1. beads and2. metallized surface-contact.

Bead types have platinum wires sintered into a ceramic body (bead) as shown in Figure 1.84 (a). Metallized surface-contact thermistors are called chips or flakes. In contrast to bead types, leads are not sintered directly into the ceramic. Instead, the sintered ceramic is coated with a metallic contact as shown in Figure 1.84 (b). Either the chip manufacturer or user attaches leads to this contact.One advantage of chip thermistors over bead types is that the chips are easily trimmed by cutting or grinding. Thus, they are easy to match and, therefore, are interchangeable. While matched bead thermistors are available, they cost more thaninterchangeable chips.

Thermistors can be classified into two types depending on the temperature coefficient of resistance (k). If k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC)thermistor, or posistor. If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor. NTC thermistors are mostly used in temperature sensing devices where as the PTC thermistors are mostly used in electric current control devices.

Thermistors respond quickly to temperature changes, and they have a higher resistance, so junction effects are not an issue. Typical accuracies are 1%, but the devices are not linear, have a limited temperature/resistance range and can be self heating. Compared to other sensors, thermistors have a limited measuring range, typically from -80 to 150aC. Also, because they are often made from semiconductors or sintered mixtures of metal oxides, they can sustain permanentdamage at temperatures above their specified operating range.

Advantages of thermistors:1. High and fast output.2. Suitable for the usage in remote location.3. Can be manufactured in almost any shape and size.4. Very high degree of accuracy.5. Good stability and repeatability.6. Has the ability to withstand mechanical and electrical stresses.

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Disadvantages of thermistors:1. Highly non-linear behavior over its range of operation.2. Have a limited measuring range.3. Self heating may occur.4. Power supply required. 5. Fragile in nature

1.2.11.4. Thermocouple:The thermocouple is a device that converts thermal energy into electrical energy. Thermocouples are very simple and durable temperature sensors. Thermocouples use a junction of dissimilar metals to generate a voltage proportional to temperature.Thermocouples are based on the Seebeck effect. In 1821, a physicist T.J Seebeck discovered that "when two conductors of dissimilar metals, say A and B, are joined together to form a loop, and two unequal temperatures are interposed at the junctions, then an e.m.f will exist between the two points A and B, which is primarily a function of the junction temperature". This is known as the thermoelectric effect or Seebeck effect. Figure 1.85 illustrates Seebeck effect, where two dissimilar metals A and B are used to close the loop connecting junctions at two different temperatures T1 and T2. The e.m.f. produced is found to be almost linear in temperature and very repetitive for constant materials. The e.m.f. produced by the thermocouple loop is approximately given by:A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. The amount of current that will be produced is dependent on the temperature difference between the measurement and reference junction; the characteristics of the two metals used; and the characteristics of the attached circuit.

Thermocouples come with different pairings of materials

allowing for a very wide range of applications. The different compositions are standardized into thermocouple types. The different types are given letter names which are standardized across the industry. The list in Table 1.3 shows different thermocouple junction types, and the normal temperature ranges. Both thermocouples, and signal conditioners are commonly available, and relatively inexpensive. They are usually selected based on the temperature range and sensitivity needed. Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower resolutions. Other selection criteria include the inertness of the thermocouple material, and whether or not it is magnetic.

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Figure 1.86 shows the temperature-voltage relationships of some commonly used types of thermocouples.

For convenience of measurement and standardization, one of the two junctions is usually maintained at a known

temperature of reference. The most common reference is 0°C, which is the temperature of an ice bath. A 0°C reference insures repeatability and accuracy because the ice point of water is a constant. A thermocouple can be used with the reference junction other than 0°C. In such case, a correction has to be used before using the standard table of reference by using thermocouple law, called law of intermediate temperatures.Law of intermediate temperatures If two dissimilar materials produce thermal e.m.f. E12 when the junctions are atT1 and T2 and produce thermal e.m.f. E23 when the junctions are at T2 and T3, the e.m.f. generated when the junctions .are at T1and T3 will be E12+ EZ3• Figure 1.87 illustrates the concept of law of intermediate temperatures.

The following are the other laws of thermocouple:Law of homogeneous material :A thermoelectric current cannot be sustained in a circuit of a single homogeneous material by the application of heat alone, regardless of how it might vary in cross section.

Law of intermediate materials:The algebraic sum of the thermoelectric forces in a circuit composed of any number of dissimilar materials is zero if all of the junctions are at a uniform temperature.An ice bath may not always be convenient, however. In that case, the most common alternate method of determining reference voltage is with an integrated circuit temperature sensor. The Ie sensor is placed near the reference junction andmeasures local temperature. From its temperature reading, reference-junction voltage may be calculated.

Figure 1.88 shows the internal construction of a typical thermocouple. The leads of the thermocouple are encased in a rigid metal sheath. The measuring junction is normally formed at the bottom of the thermocouple housing. Magnesium oxide surrounds the thermocouple wires to prevent vibration that could damage the fine wires and to enhance heat transfer between the measuring junction and the medium surrounding the thermocouple.

Figure 1.89 illustrates a simple thermocouple circuit. Heating the measuring junction of the thermocouple produces a voltage which is greater than the voltage across the reference junction. The difference between the two voltages isProportional to the difference in temperature and can be measured on the voltmeter (in mill volts). For ease of operator use, some voltmeters are set up to read out directly in temperature through use of electronic circuitry.

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A series of thermocouples connected together in series produces a higher voltage and is called a thermopile. In thermopile, all the hot junctions are exposed to the higher temperature and all the cold junctions to a lower temperature. The voltages of the individual thermocouples add up, allowing for a larger voltage and increased power output , thus increasing the sensitivity of the instrumentation. Readings can approach an accuracy of 0.5%.

Applications of thermocouples:

1. Two common applications of thermocouples are measuring room temperature and monitoring the presence of a pilot light in gas-fed heating appliances such as ovens and water heaters. The other applications are listed below:

2. Type S, R and K thermocouples are used extensively in the steel and iron industries to monitor temperatures and chemistry throughout the steel making process.

3. Thermopiles are used for measuring the intensity of incident radiation, typically visible or infrared light, which heats the hot junctions, while the cold junctions are on a heat sink.

4. Thermocouples can generally be used in the testing of prototype electrical and mechanical apparatus. Example: monitoring the switchgears during its heat run test.

5. Chemical production and petroleum refineries use number of thermocouples for logging and limit testing the many temperatures associated with a process.

Advantages of thermocouples:1. Simple in construction .2. Inexpensive .3. Rugged in construction .4. Wide variety to choose for particular applications .5. Wide temperature range .6. Has the ability to withstand mechanical and electrical

stresses.

Disadvantages of thermocouples:1. Highly non-linear behavior over its range of operation .2. Capable of generating low voltage .3. Low stability .4. Reference source is required .5. Least sensitive.

1.2.11.5. Thermodiodes and Transistors:

1. ThermodiodeA junction semiconductor diode is widely used temperature-measuring instrument. The mobility semiconductor diode changes whenever the temperature changes. This affects the rate at which electrons and holes can diffuse across a PNjunction. The difference in voltage and current through the junction is a function of the temperature. The measurement of the voltage across a diode at constant current can be used as a measure of the temperature. Such a sensor is compact in size and has the advantage of giving a response which is a linear function of temperature.

2. ThermotransistorThe base-to-emitter voltage drop of a transistor operating at a constant current is a simple function of absolute temperature. Thus, any transistor can be used as a temperature sensor. In practice, this is difficult to build thermally stable electronics than a convenient means of measuring temperature. Integrated circuits are available that monitor the collector current, amplify, and linearize the base-to-emitter voltage to yield an output that is proportional to absolute temperature. Common integrated circuit temperature sensors are available with outputs of 10 mV/K, or 1µA/K. The temperature range

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over which they may be used is limited to -50°C to 150°C by the construction techniques of integrated circuits. This makes them very useful for referencing one junction of the thermocouple and most ambient temperature measurements.Thermodiodes and transistors are also called as IC temperature sensors.Advantages of IC sensors:

1. Most linear output.2. Inexpensive .3. Highest output.4. Compact in size .5. High accuracy.

Disadvantages of IC sensors:1. Applicable for the measurement of less than 150°C .2. Power supply is required .3. Slow output.4. Problem of self-heating.

1.2.12. Light Sensors:A light sensor or detector converts the radiant power it absorbs into a change of a device parameter such as resistance, surface charge, current, or voltage. Some signal conditioning electronics may also be needed to convert the basic output from the detector into a more useful voltage signal, for example, for digitization by an . analog-to-digital converter (ADC). This may be integrated into the detector or require external components. There are several types of light sensors in common use.The principles of operation and characteristics of the most widely used, including photoresistor, photodiode, and phototransistor are summarized in this section. These light sensors depend on the generation of free charge by the absorption of individual photons. This photon-induced charge causes a change in device resistance, in thecase of photoresistors, or an output current or output voltage, in the case of photodiodes and transistors.

1.2.12.1. Photoresistor:A photoresistor consists of a slab of semiconductor material on the faces of which electrodes are deposited to allow the resistance to be monitored. The increase in conductivity, caused by the absorption of photons increasing electrons and holes, is the basis for the operation of the photoresistive detector. Cadmium sulfide is commonly used as a detector of visible radiation because it is low cost and its response is similar to that of the human eye. Other photoconductive materials include lead sulfide, indium antimonide, and mercury cadmium telluride. A simple light detector circuit employing a photoresistor is shown in Figure 1.90. An increase in light illumination causes the resistance of the photoresistor to decrease and the output voltage to increase. The photon-induced current is proportional to the length of the electrodes and inversely proportional to their separation. Therefore, the typical comb-like electrode geometry of photoresistors is used. The comb-like pattern typically employed in photoresistors gives a relatively large active area of photoconducting material and a small electrode spacing resulting in high sensitivity.Photoconductive devices used for the detection of long wavelength infrared radiation should be cooled because of the noise caused by fluctuations in the thermal generation of charge.

1.2.12.2. Photodiode: In photoresistors, the rate of generation of electrons and holes pairs by the absorption of radiation results in an Increase in free charge and therefore electrical conductivity. In photodiodes and phototransistors, newly generated p-n airs separate before they can recombine so that a photon-induced' electric current can be detected.The separation of electrons and holes takes place in the electric field associated with a P-N junction fabricated in a semiconductor material, which is usually silicon.

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The basic structure of a typical silicon photodiode is illustrated in Figure 1.91. The substrate material is lightly doped n-type silicon. This contributes free electrons to the conduction band of the silicon leaving the impurity atoms ionized and with a positive charge. A region of heavily doped p-type silicon is formed on the top face of the substrate. The P-N junction is the boundary surface between the p-type and n type regions. A space charge, or depletion region, is formed by the diffusion of mobile charge across the surface between the p-type and n-type silicon. It extends furthest into the n-type silicon because this is more lightly doped than the p-type silicon. Any electron hole pairs generated in this region are prevented from recombining by the presence of the electric field. This sweeps them apart and allowing them to contribute to the photon generated current. The p-type region is made thin to allow photons to penetrate into the depletion region.

1.2.12.3. Phototransistor:The phototransistor has a light-sensitive collector-base p-n junction. When there is no incident light radiation on the transistor there is a very small collector-to-emitter current. When incident light radiation fall on the transistor, a base current is produced that is directly proportional to the intensity of light. A simple phototransistor light detector circuit is shown in Figure 1.92. Photon-generated current flowing in the base-collector diode may be amplified several hundred times by transistor action. Although the photon-generated current is much larger than in an equivalent photodiode, response time of the phototransistor is much longer.

1.2.13Selection of Sensors:A number of static, dynamic and other factors must be considered in selecting a suitable sensor to measure the desired physical parameter. The following factors are considered while selecting sensors:1. Accuracy required: It is the difference between the measured value and the true value. Accuracy of the sensor should be as high as possible.2. Precision: It is the ability to reproduce repeatedly with a given accuracy. It should be very high. Error between sensed and actual value should approach zero.3. Sensitivity: It is the ratio of change in output to a unit change of the input. It should be chosen to allow sufficient output.4. Operating range: It is the difference between the maximum and minimum value of the sensed parameter. Sensors should have wide operating range and good accuracy over the range.5. Resolution: It is the smallest change the sensor can differentiate. Sensors should have high resolution.6. Speed response: Time taken by the sensor to respond should be minimum. Response time should be very less.7. Reliability: Reliability of the sensor should be high. Mean time to failure (MTTF) should be high. It results in increased life. Maintenance should also be easy and frequency of maintenance required should be less over the period.8. Calibration: Sensors need frequent calibration for many reasons. Hence it should be easy to calibrate. Drift should be as minimum as possible.

9. Cost: Cost of the sensor should be low.10. The nature of output required from the sensor whether digital or analog has to be considered while selecting a sensor.11. Linearity: Sensor's curve should linear. Percentage of deviation from the best-fit linear calibration curve should be less.12. Environmental conditions: Sensors should operate over wide environmental conditions such as temperature, corrosion, pressure, shocks etc.13. Interfacing: Sensors should be compatible with different instruments for interfacing.14. Size and weight: Sensors should have small size and less weight.

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