MEV 403 Introduction to Mechatronics · MEV 403 Introduction to Mechatronics Module 2: Sensors and Actuators. Santhakumar Mohan, Assistant Professor, MED, NITC ... between the input

Santhakumar Mohan, Assistant Professor, MED, NITC1

MEV 403 Introduction to Mechatronics

Module 2: Sensors and Actuators


Introduction to Sensors

• Sensor is a device that when exposed to a physical phenomenon (temperature, displacement, force, etc.) produces a proportional output signal (electrical, mechanical, magnetic, etc.).

• The term transducer is often used synonymously with sensors.

However, ideally, a sensor is a device that responds to a change in the physical phenomenon.

On the other hand, a transducer is a device that converts one form of energy into another form of energy.


Temperature Sensors

• What is a temperature sensor?An analog temperature sensor is pretty easy to explain, it's a chip / device that tells you what the ambient temperature is!

• Temperature measurement is based on one of the following principles

– Material expansion based on change in length, volume or pressure.

– Change in electrical resistance.

– Contact voltage between two dissimilar metals

– Changes in radiated energy

Zeroth law of Thermodynamics???!!!


Types of temperature sensors

• Thermocouples

• Resistive temperature devices (RTDs, thermistors)

• Infrared radiators

• Bimetallic devices

• Liquid expansion devices

• Molecular change-of-state

• Silicon diodes

• Others

– Temperature sensing using fiber optics

– Temperature sensing using interferometrics

• Contact type

• Non contact type


Thermocouples• Thermocouples are voltage devices

that indicate temperature by measuring a change in voltage. As temperature goes up, the output voltage of the thermocouple rises -not necessarily linearly.

• Often the thermocouple is located inside a metal or ceramic shield that protects it from exposure to a variety of environments. Metal-sheathed thermocouples also are available with many types of outer coatings, such as Teflon, for trouble-free use in acids and strong caustic solutions.


• Thermocouples generate an electromotive force (mV) when temperature of “hot weld” of thermocouple (which is in contact with heat source) is different from temperature of “cold weld” (the “cold weld” being the benchmark) .

• This value corresponds with a temperature according to international standards. An electronic device conGreens this value into a temperature in order to display it (temperature indicator) or regulate it (temperature regulator).

Peltier effect and Seebeck effect


Types of thermocouples

-200°C / +850°C(-360°F to 1562°F)

PT 100 Platinum sensor

0° C / +1260° C(32°F / 2300°F)

Aluminium plated nickel

(or Alumel)

Nickel chromium(or Chromel)

K Thermocouple

0° C / +760° C(32°F / 1400°F)

ConstantanIronJ Thermocouple

Temperature range

NegativePositiveType

There are J type thermocouples (iron/constantan), K type thermocouples (nickel chromium / aluminium plated nickel), S type thermocouples (platinum /rhodium plated platinum), etc.


Laws of thermocouples

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. In other words, temperature changes in the wiring between the input and output do not affect the output voltage, provided all wires are made of the same materials as the thermocouple.

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. So If a third metal is inserted in either wire and if the two new junctions are at the same temperature, there will be no net voltage generated by the new metal.

Law of successive or intermediate temperatures• If two dissimilar homogeneous materials produce thermal emf1 when the

junctions are at T1 and T2 and produce thermal emf2 when the junctions are at T2 and T3 , the emf generated when the junctions are at T1 and T3 will be emf1 + emf2 .


Advantages and Disadvantages with thermocouples

Advantages• Capable of being used to directly measure temperatures up to 2600 ºC. • The thermocouple junction may be grounded and brought into direct contact with the

material being measured.Disadvantages • Temperature measurement with a thermocouple requires two temperatures be measured,

the junction at the work end (the hot junction) and the junction where wires meet the instrumentation copper wires (cold junction). To avoid error the cold junction temperature is in general compensated in the electronic instruments by measuring the temperature at the terminal block using with a semiconductor, thermistor, or RTD.

• Thermocouples operation are relatively complex with potential sources of error. The materials of which thermocouple wires are made are not inert and the thermoelectric voltage developed along the length of the thermocouple wire may be influenced by corrosion etc.

• The relationship between the process temperature and the thermocouple signal (millivolt) is not linear.

• The calibration of the thermocouple should be carried out while it is in use by comparing it to a nearby comparison thermocouple. If the thermocouple is removed and placed in a calibration bath, the output integrated over the length is not reproduced exactly.


Resistive Temperature Devices

• Resistive temperature devices also are electrical. Rather than using a voltage as the thermocouple does, they take advantage of another characteristic of matter which changes with temperature - its resistance. The two types of resistive devices we deal with at OMEGA Engineering, Inc., in Stamford, Conn., are metallic, resistive temperature devices (RTDs) and thermistors.

• In general, RTDs are more linear than are thermocouples. They increase in a positive direction, with resistance going up as temperature rises. On the other hand, the thermistor has an entirely different type of construction. It is an extremely nonlinear semiconductive device that will decrease in resistance as temperature rises.


Infrared Sensors

• Infrared sensors are noncontacting sensors. As an example, if you hold up a typical infrared sensor to the front of your desk without contact, the sensor will tell you the temperature of thedesk by virtue of its radiation - probably 68°F at normal room temperature.

• In a noncontacting measurement of ice water, it will measure slightly under 0°C because of evaporation, which slightly lowers the expected temperature reading.


Bimetallic Devices

• Bimetallic devices take advantage of the expansion of metals when they are heated. In these devices, two metals are bonded together and mechanically linked to a pointer. When heated, one side of the bimetallic strip will expand more than the other. And when geared properly to a pointer, the temperature is indicated.

• Advantages of bimetallic devices are portability and independence from a power supply. However, they are not usually quite as accurate as are electrical devices, and you cannot easily record the temperature value as with electrical devices like thermocouples or RTDs; but portability is a definite advantage for the right application.


Thermometers

• Thermometers are well-known liquid expansion devices. Generally speaking, they come in two main classifications: the mercury type and the organic, usually red, liquid type. The distinction between the two is notable, because mercury devices have certain limitations when it comes to how they can be safely transported or shipped.

• For example, mercury is considered an environmental contaminant, so breakage can be hazardous. Be sure to check the current restrictions for air transportation of mercury products before shipping.


Silicon Diode

• The silicon diode sensor is a device that has been developed specifically for the cryogenic temperature range. Essentially, they are linear devices where the conductivity of the diode increases linearly in the low cryogenic regions.

• Whatever sensor you select, it will not likely be operating by itself. Since most sensor choices overlap in temperature range and accuracy, selection of the sensor will depend on how it will be integrated into a system.


Change-of-state Sensors • Change-of-state temperature sensors measure just that - a change in the

state of a material brought about by a change in temperature, as in a change from ice to water and then to steam. Commercially available devices of this type are in the form of labels, pellets, crayons, or lacquers.

• Limitations include a relatively slow response time. Therefore, if you have a temperature spike going up and then down very quickly, there may be no visible response. Accuracy also is not as high as with most of the other devices more commonly used in industry.

• However, within their realm of application where you need a non-reversing indication that does not require electrical power, they are very practical.

• Although it is not perfectly precise, it does have the advantages of being a small, rugged, non-electrical indicator that continuously updates temperature.


Small to mediumMedium to smallSmall to largeSize/Packaging

LowMediumHighLead Effect

GoodFairExcellentPoint (end) Sensitive

HighVery low to lowNoSelf Heating

PoorGoodFairLinearity

Medium to fastMediumMedium to fastResponse

Very highMediumLowSensitivity (output)

Fair to goodExcellentPoor to fairRepeatability

MediumHighMediumAccuracy

PoorGoodPoor to fairLong-term Stability

Poor to fairExcellentGoodInterchange ability

Short to medium-100oF+500oF

Wide-400oF+1200oF

Very wide-350oF+3200oF

Temperature Range

LowHighLowCost

ThermistorRTDThermocoupleAttribute


Range sensors

• Special importance in manufacturing automation applications

• Range sensors have been successfully employed in other areas as well, including

– Automatic guidance systems

– Navigation systems

– Collision avoidance

• Digitizers (distance measurement sensors – work-piece inspection)

• Proximity sensors (presence of the object or closeness of one object to another object)


Range sensor techniques• Optical methods

– Basic triangulation principle• Spot sensing• Light stripe sensing• Camera motion• Time of flight• Binocular vision• Optical ranging using position sensitive detectors

– Other ranging techniques (laser interferometric)• Acoustic • Inductive• Electrical field techniques

– Eddy current– Hall effect– Magnetic field


Eddy Current Transducer

• The Eddy Current Transduceruses the effect of eddy (circular) currents to sense the proximity of non-magnetic but conductive materials.

• A typical eddy current transducer contains two coils: an active coil(main coil) and a balance (reference) coil.

• The active coil senses the presence of a nearby conductive object, and balance coil is used to balance the output bridge circuit and for temperature compensation.


Eddy Current Transducer

Pros:-Non-contacting measurement. -High resolution. -High frequency response.

Cons:-Effective distance is limited to close range. -The relationship between the distance and the impedance of the coil is nonlinear and temperature dependent. Fortunately, a balance coil can compensate for the temperature effect. As for the nonlinearity, careful calibrations can ease its drawback. -Only works on conductive materials with sufficient thickness. It can not be used for detecting the displacement of non-conductive materials or thin metalizedfilms. -Calibration is generally required, since the shape and conductivity of the target material can affect the sensor response.


Hall effect Sensor• A Hall effect sensor is a transducer that varies its output voltage in

response to changes in magnetic field. Hall sensors are used for proximity switching, positioning, speed detection, and current sensing applications.

• In its simplest form, the sensor operates as an analogue transducer, directly returning a voltage. With a known magnetic field, its distance from the Hall plate can be determined. Using groups of sensors, the relative position of the magnet can be deduced.

• Electricity carried through a conductor will produce a magnetic field that varies with current, and a Hall sensor can be used to measure the current without interrupting the circuit. Typically, the sensor is integrated with a wound core or permanent magnet that surrounds the conductor to be measured.


• Frequently, a Hall sensor is combined with circuitry that allows the device to act in a digital (on/off) mode, and may be called a switchin this configuration. Commonly seen in industrial applications such as the pictured pneumatic cylinder, they are also used in consumer equipment; for example some computer printers use them to detect missing paper and open covers. When high reliability is required, they are used in keyboards.

• Hall sensors are commonly used to time the speed of wheels and shafts, such as for internal combustion engine ignition timing, tachometers and anti-lock braking systems. They are used in brushless DC electric motors to detect the position of the permanent magnet. In the pictured wheel carrying two equally spaced magnets, the voltage from the sensor will peak twice for each revolution. This arrangement is commonly used to regulate the speed of disc drives.


Advantages over other methods

• This has several advantages; no additional resistance (a shunt, required for the most common current sensing method) need be inserted in the primary circuit. Also, the voltage present on the line to be sensed is not transmitted to the sensor, which enhances the safety of measuring equipment.

Disadvantages compared with other methods• Magnetic flux from the surroundings (such as other wires) may diminish or

enhance the field the Hall probe intends to detect, rendering the results inaccurate. Also, as Hall voltage is often on the order of millivolts, the output from this type of sensor cannot be used to directly drive actuators but instead must be amplified by a transistor-basedcircuit.


MEV 403 Introduction to Mechatronics

Module 2: Systems and Control


Introduction

• System

– It is a collection, set, or arrangement of elements (subsystems)which will give some meaningful output

• Subsystem

– It is a part of system which will give meaningful output independently

• Element (component)

– It is the smallest part of a system that can be treated as a whole (entity)

• Block

– It is a set of elements that can be grouped together, with overall characteristics described by an input/output (I/O) relationship


Control Fundamentals• Control

– Control means to regulate, direct, or govern

• Automatic

– It implies self-action without any human intervention

• Control system

– It is a group of physical components arranged to control themselves or another system

• Automatic control system

– It is a control system that is self-regulating without any human intervention

• Manual control system

– It is a control system regulated through human intervention


Types of Control systems

• Open-Loop control system

– Is a control system in which the control (regulating) action is independent of the output

• Closed-Loop control system

– Is a control system in which the control (regulating) action is influenced by the output

• Regulator system

– Is a control system where the reference (input) is normally fixed

• Servomechanism (follower system)

– Is a control system where the reference (input) caries continuously and the system operates so that the output follows the output


• Linear System– Is a system where input/output relationships may be

represented by a linear differential equations– It follows superposition principle (additive and scaling)

• Non-linear System• Time-Invariant System

– Is a system described by a differential equation with constant coefficients

• Time-Variant System– Is a system described by a differential equation with

variable coefficients


• Single-Input, Single-Output (SISO) System

– Is a system where only one parameter is entered as input and only one parameter represents the output

• Multiple-Input, Multiple-Output (MIMO) System

– Is a system where several parameters may be entered as input and output represented by multiple variables

SISO

MIMO

input output

outputsinputs


• Signals– Are variables, which evolve, or change, with respect to an

independent variable, usually time.• Parameters

– Or coefficients, are variables whose values are constant• The behaviour of the signal is often considered in two regions

– Transient region• The signal derivatives dominate its shape, the region

between two steady state values– Steady state region

• All signals derivatives die out, leaving only the offset value


System representation

• Transfer function form– A transfer function (also known as the system function or network function) is a

mathematical representation, in terms of spatial or temporal frequency, of the relation between the input and output of a (linear time-invariant) system.

– The transfer function is commonly used in the analysis of single-input single-outputsystems, for instance.

• Block diagram form– A graphical representation relates the various subsystems or parts of a system through

functions, data, or interfaces.

• State space form– is a mathematical model of a physical system as a set of input, output and state variables

related by first-order differential equations. – To abstract from the number of inputs, outputs and states, the variables are expressed as

vectors, and the differential and algebraic equations are written in matrix form (the last one can be done when the dynamical system is linear and time invariant).

– The state space representation (also known as the "time-domain approach") provides a convenient and compact way to model and analyze systems with multiple inputs and outputs.


System performance measures• Stability

– A stable system is one that produces a bounded, or finite, response when subjected to a bounded input.

• Accuracy

– Or steady state error, is the error between an input and an output signal in the steady state

• Transient response

– Is the shape of a signal as it moves between two steady state points. It is quantified with two parameters such as damping ratio and undamped frequency

• Sensitivity

– Is the measure by which controlled signals are influenced by disturbances, which include parameter variations within the plant and external signals such as noise


Specifications• Rise time

– the time required for the response to rise from x% to y% of its final value, with 0%-100% rise time common for overdamped second order systems and 10%-90% for underdamped

• Overshoot– the maximum peak value of the response curve

measured from the desired response of the system.• Settling time

– the time required for the response curve to reach and stay within a range of certain percentage (usually 5% or 2%) of the final value

• Delay time– The delay time is the time required for the response

to reach half the final value the very first time• Peak time

– The peak time is the time required for the response to reach the first peak of the overshoot.

• Steady-state error– the difference between the desired final output and

the actual one" when the system reaches a steady state, when its behavior may be expected to continue if the system is undisturbed.


Types of system inputs

• Impulse input

• Step input

• Ramp input

• Sinusoidal input

• Exponential input


Second order system response

• Un-damped– Damping ratio (ζ) is zero and roots

are only imaginary (s=±iβ)

• Under damped – Damping ratio ζ is between zero to

one and roots are negative real part with imaginary parts.

• Critically damped– Damping ratio ζ is equal to one and

roots are repetitive negative real parts.

• Over damped– Damping ratio ζ is more than one and

roots are negative real parts.


Types of analog control

• On – off control

• Proportional control

• Integral control

• Derivative control

• Proportional integral (PI) control

• Proportional derivative (PD) control

• Proportional integral derivative (PID) control

• Lead compensator

• Lag compensator

• Lead-Lag compensator


On – off Control

• Bang–bang Controller (on–off controller), also known as a hysteresis controller,– It is a feedback controller that switches abruptly between

two states. – These controllers may be realized in terms of any element

that provides hysteresis. – They are often used to control a plant that accepts a binary

input, for example a furnace that is either completely on or completely off.

– Most common residential thermostats are bang–bangcontrollers.

– Cheap and effective


PID Controller

• The PID controller calculation (algorithm) involves three separate parameters, and is accordingly sometimes called three-term control: the proportional, the integral and derivative values, denoted P, I, and D.

• The proportionalvalue determines the reaction to the current error, the integralvalue determines the reaction based on the sum of recent errors, and the derivativevalue determines the reaction based on the rate at which the error has been changing.

• Heuristically, these values can be interpreted in terms of time:P depends on the presenterror, I on the accumulation of pasterrors, and D is a prediction of futureerrors, based on current rate of change


Proportional gain, KpLarger values typically mean faster response since the larger the error, the larger the proportional term compensation. An excessively large proportional gain will lead to process instability and oscillation.

Integral gain, KiLarger values imply steady state errors are eliminated more quickly. The trade-off is larger overshoot: any negative error integrated during transient response must be integrated away by positive error before reaching steady state.

Derivative gain, KdLarger values decrease overshoot, but slow down transient response and may lead to instability due to signal noise amplification in the differentiation of the error.


Effects of increasing a parameter independently

Improve if Kd small

No effect in theoryMinor

decreaseMinor

decreaseMinor

decreaseKd

DegradeDecrease

significantlyIncreaseIncreaseDecreaseKi

DegradeDecreaseSmall change

IncreaseDecreaseKp

StabilitySteady-state errorSettling timeOvershootRise timeParameter

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MEV 403 Introduction to Mechatronics · MEV 403 Introduction to Mechatronics Module 2: Sensors and Actuators. Santhakumar Mohan, Assistant Professor, MED, NITC ... between the input