Lecture 1-2_ Introduction to Control System 2010

8/3/2019 Lecture 1-2_ Introduction to Control System 2010

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Process Control Dynamics 1: Lecture 1-2

1.0 Introduction to Controls System

Automatic control has played a vital role in the advance of engineering and science. In

addition to its extreme importance in space-vehicle Systems, missile-guidance systems,robotic systems, and the like, automatic control has become an important and integral

part of modern manufacturing and industrial processes. For example, automatic control is

essential in the numerical control of machine tools in the manufacturing industries, in thedesign of autopilot systems in the aerospace industries, and in the design of cars and

trucks in the automobile industries. It is also essential in such industrial operations as

controlling pressure, temperature, humidity, viscosity, and flow in the process industries.

The evolution of controls system from a physical perspective, is a trend that reflects the

advance in technology and the evolution of the mathematics surrounding the disciplined.

Since advances in the theory and practice of automatic control provide the means for

attaining optimal performance of dynamic systems, improving productivity, relieving thedrudgery of many routine repetitive manual operations, and more, most engineers and

scientists must now have a good understanding of this field.

Definitions:

Control system: is a group of components that maintains a desired result by

manipulating the value of another variable in the system.

Controlled Variable and Manipulated Variable. The controlled variable is the quantity

or condition that is measured and controlled. The manipulated variable is the quantity or condition that is varied by the controller so as to affect the value of the controlledvariable. Normally, the controlled variable is the output of the system. Control means

measuring the value of the controlled variable of the system and applying the

manipulated variable to the system to correct or limit deviation of the measured valuefrom a desired value, in studying control engineering, we need to define additional terms

that are necessary to describe control systems.

Plants: A plant may be a piece of equipment, perhaps just a set of machine parts (such asa mechanical device, a heating furnace, a chemical reactor, refinery) functioning together,

the purpose of which is to perform a particular operation.

Processes. A process may be defined as a natural, progressively continuing operation or

development marked by a series of controlled actions or movements systematically

directed toward a particular result or end.

Disturbances: A disturbance is a signal or change in one or more operating condition or

process variable that tends to adversely affect the value of the controlled variable in the

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system. A sudden change in the load on the system would represent a disturbance input to

the system.

Measuring Transmitter: The measuring Transmitter or sensor senses the value of the

controlled variable and converts it into a suitable signal. This may be a passive device

that converts the controlled variable into a suitable signal for input to the error detector (example a potentiometric displacement sensor used to convert the angular displacement

of a motor shaft into a proportional voltage signal. In a process-control environment, the

measuring transmitter usually represents two components, a primary sensing element anda signal transducer (or signal converter).

The Controller: The controller includes the error detector and a unit that implements the

control modes. The error detector computes the difference between the measured value of the controlled variable and the desired value (or setpoint). The difference is referred to as

the error signal. The control modes convert the error into a control action that adjusts the

controller output signal in a direction that will tend to reduce the error. The three most

common control modes are the proportional mode (P), the integral mode (I), and thederivative mode (D).

Manipulating Element: The manipulating element uses the controller output to regulate

the manipulated variable and usually consists of two parts. The first part is called an

actuator, and the second part is called the final controlling element. The actuator

translates the controller output into an action on the final controlling element, and thefinal controlling element directly changes the value of the manipulated variable. For

example, an Electro-pneumatic convert I/P would convert the controller output current

signal into a proportional pneumatic signal that could be used to drive the final controlelement representing a pneumatic control valve, the valve stem position may be used to

adjust the input flow rate of a fluid (manipulated variable) to maintain a particular liquid

level (controlled variable).

Figure 1

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2.0 Closed-loop versus open-loop controls

A control system must balance the material or energy gained by the process against thematerial or energy lost by the process, to maintain the desired value of the controlled

variable. The material or energy lost represents the load on the process.

To maintain the desired inside temperature, a home heating system must balance the heat

supplied by the furnace against the heat lost by the house. The heat loss is the load on the

control system, and the energy supplied to the furnace is regulated by the manipulatedvariable.

To maintain the desired level in a tank, a liquid-level control system must balance the

input flow rate against the output flow rate. The output flow rate is therefore the load onthe system and the manipulated variable is the input flow rate.

Figure 2 Liquid level control system

Closed-loop control systems. In practice, the terms feedback control and closed-loop

control are used interchangeably. A close-loop system maintains a prescribed relationship between the output(control variable) and the reference input (setpoint) by comparing

them and using the difference (error signal) as a means of control. The controller output

adjustment is computed in a way to drive the output of the system to a desired value,hence reducing the error.

Feedback control systems are not limited to engineering but can be found in various non-engineering fields as well. The human body, for instance, is a highly advanced feedback

control system. Both body temperature and blood pressure are kept constant by means of

physiological feedback. In fact, feedback performs a vital function: It makes the human body relatively insensitive to external disturbances, thus enabling it to function properly

in a changing environment.

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Open-loop control systems. Those systems in which the output has no effect on the

control action are called open-loop control systems. In other words, in an open-loop

control system the output is neither measured nor fed back for comparison with the input.One practical example is a washing machine. Soaking, washing, and rinsing in the washer

operate on a time basis. The machine does not measure the output signal, that is,

cleanliness of the clothes.

In any open-loop control system the output is not compared with the reference input.

Thus, to each reference input there corresponds a fixed operating condition; as a resuIt,the accuracy of the system depends on calibration. In the presence of disturbances, an

open-loop control system will not perform the desired task. Open-loop control can be

used, in practice, only if the relationship between the input and output is known and if

there are neither internal nor external disturbances. Clearly, such systems are notfeedback control systems. Note that any control system that operates on a time basis is

open loop. For instance, traffic control by means of signals operated on a time basis is

another example of open-loop control.

Closed-loop versus open-loop control systems. An advantage of the closed loop

control system is the fact that the use of feedback makes the system response relativelyinsensitive to external disturbances and internal variations in system parameters. It is thus

possible to use relatively inaccurate and inexpensive components to obtain the accurate

control of a given plant, whereas doing, is impossible in the open-loop case.

From the point of view of stability, the open-loop control system is easier to build

because system stability is not a major problem. On the other hand, stability is a major

problem in the closed-loop control system, which may tend to overcorrect errors that cancause oscillations of constant or changing amplitude. It’s therefore advisable that for

systems in which the inputs are known ahead of time and in which there are no

disturbances the use open-loop control is most appropriate. Closed-loop control systemshave advantages only when unpredictable disturbances and/or unpredictable variations in

system components are present. Typically, the close-loop control system will cost more

than an open-loop system.

3.0 Instrumentation Telemetry Overview

Within instrumentation there is often a need for telemetry in order to transmit data or

information between two geographical locations. The transmission may be required toenable centralized supervisory data logging, signal processing or control to be exercised

in large-scale systems which employ distributed data logging or control subsystems. In a

chemical plant or power station these subsystems may be spread over a wide area.Tetemetry may also be required -for systems which are remote or inaccessible such as a

spacecraft, a satellite or an buoy in the middle of the ocean. It can be used to transmit

information from the rotating sections of an electrical machine without the need for slip

rings. By using telemetry-sensitive signal processing and recording, apparatus can be

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physically remote from hazardous and aggressive environments and can be operated in

more closely monitored and controlled conditions.

3.1 Pneumatic Telemetry System

Telemetry has traditionally been provided by either pneumatic or electrical transmission.

Pneumatic transmission, as shown in Figure 3, has been extensively used in process

instrumentation and control. The measured quantity (pressure, level, temperature, etc.) isconverted to a pneumatic pressure, the standard signal ranges being 20 - 100 kPa gauge

pressure (3-15 lb/in2/g, psi) and 20 – 180 kPa (3-27 lb/in2/g, psi). The lower limit of

pressure provides a live zero for the instrument which enables line breaks to be detected,eases instrument calibration and checking, and provides for improved dynamic response

since, when venting to atmospheric pressure, there is still sufficient driving pressure at 20

kPa. The pneumatic signals can be transmitted over distances up to 300 m in 6.35 mm or 9.5 mm OD plastic or metal tubing to a pneumatic indicator, recorder or controller.

Figure 3

A major disadvantage of pneumatic systems is the cost of the pneumatic lines between

the process and the control room. Each control loop required two signal lines: one from

the measuring transmitter to the controller, the other from the controller to the controlvalve. In large plants, these lines measured hundreds, even thousands of feet long. The

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distance is limited by the speed of response, which quadruples with doubling the

distance. Pneumatic signals travel at about 1000 ft per second. A process loop located

500 ft from the control room would suffer a 1 -second delay in the control loop - a 1/2-second delay from the measuring transmitter to the controller and another 1/2-second

delay from the controller to the control valve. The transmission delay in the long

pneumatic signal lines can impair the quality of the controls achievable. Finally, pneumatic transmission Systems require a dry, regulated air supply. Condensed moisture

in the pipe-works at subzero temperatures or small solid contaminants can block the

small passages within pneumatic instruments and cause loss of accuracy and failure.

One advantage of Pneumatic instruments are that they are intrinsically safe and can

therefore be used in hazardous areas. They provide protection against electrical power

failure, since systems employing air storage or turbine driven compressors can continueto provide measurement and control during power failure. A second advantage of such

system was that pneumatic signals could directly interface with control valves which are

pneumatically operated and thus do not require the electrical-pneumatic converters

required by electrical telemetry systems, although they do suffer from the difficulty of being difficult to interface to data loggers.

3.2 Electrical Telemetry Systems

Increasingly telemetry in instrumentation is being undertaken using electrical, radio

frequency, microwave or optical fibre techniques. The communication channels used

include transmission lines employing two or more conductors which may be a twisted

pair, a coaxial cable or a telephone line physically connecting the two sites; radiofrequency (r.f.) or microwave links which allow the communication of data by

modulation of an r.f or microwave carrier; and optical links in which the data aretransmitted as a modulation of light down a fibre-optic cable. All of these techniquesemploy some portion of the electromagnetic spectrum, as shown in Figure 4a

3.2.1 Direct Analogue Signal Transmission

Analogue signals are rarely transmitted over transmission lines as a voltage since the

method suffers from errors due to series and common mode inductively and capacitively

coupled interference signals and those due to line resistance. The most common form of analogue signal transmission is as current.

Current transmission typically uses 0-20 or 4-20 mA The analogue signal is converted toa current at the transmitter and is detected at the receiver either by measuring the

potential difference developed across a fixed resistor or using the current to drive an

indicating instrument or chart recorder The length of line over which signals can betransmitted at low frequencies is primarily limited by the voltage available at the

transmitter to overcome the voltage drop along the line and across the receiver. With a

typical voltage of 24 V, the system is capable of transmitting the current over severalkilometers.

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(a)

(b)

Figure 4

The advantage of using 4 -20 mA instead of 0-20 mA is that the use of a live zero enablesinstrument or fine faults to be detected. In the 4-20 mA system zero value is represented

by 4 mA and failure is indicated by 0 mA. It is possible to use a 4-20 mA system as a twowire transmission system in which both the power and the signal are transmitted alongthe same wire as shown n Figure 5. The 4 mA standing current is used to power the

remote instrumention and the transmitter. With 24 V drive the maximum power available

to the remote station is 96mw.

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Figure 5

3.2.2 Digital Data Transmission and Smart Transmitters

Figure 5 show the optional use of an handheld interface that can be use to interrogate the

measurement transmitter. This is only possible because of thee evolution in the design of transmitters that has been influenced by developments which have taken place in

microelectronics, materials science and communication technologies. The most

significant advances have resulted from the emergence of low power micro-processorsand analogue-to-digital converters which, in conjunction with the basic sensor circuits,

can function on the limited power (typically less than 40mW) available at the transmitter

in a conventional 4-20 mA measurement circuit. This has provided two distinct routes for improving the performance of transmitters: (i) by enabling non-linear sensor

characteristics to be corrected and (ii) by enabling a secondary sensor to be included; so

that secondary effects on the primary sensor can be compensated. The term 'smart' has

come into use to differentiate the conventional transmitters from those in whichcorrections are applied to the primary sensor signal, using a microprocessor to process

information which is embedded in memory, see Figure 6. Smart transmitters may also

incorporate secondary sensor to derive corrections for the primary sensor signal.

The fact that a microprocessor can be incorporated in a transmitter has also provided an

opportunity to move from a regime in which only the measurement signal is transferredfrom the transmitter to a receiver, such as an indicator or controller, to one in which the

microprocessor not only implements the smart functions mentioned previously, but also

manages a communication facility. This enables data specific to the transmitter itself,

such as its type, serial number, etc-, to be stored at the transmitter and accessed via themeasurement loop in which it is installed, as shown in Figure 9. Other functions, such as

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setting or resetting the zero and span, details of the location and application, and running

diagnostic routines to give warning of malfunctioning, can also be implemented. The

term 'intelligent' has come to be used to identify such transmitters.

Figure 6

If the analogue data is converted to a digital signals at the remote locations, all

communication with the central unit will be digital. Digital signal are transmitted over

transmission lines using either parallel or serial communication. For long-distancecommunication serial communication is preferred method. Because of the highcapacitance of the twisted-pair and coaxial cables the length of line over which digital

signal can be transmitted using standard digital output (0 –5.0V) from TTL devices, is

typically limited to a length of 3m at 2Mbits/s. In order to drive digitals signals over longlines special purpose line drivers and receiver circuits are available.

Modems are used to overcome the limitations of the public telephone lines, the basebandsignal is used to modulate a carrier wave which is then transmitted down the line. The

three basic modulation schemes illustrated in Figure 7 are special case of the general

modulation schemes looked at earlier. These are amplitude-shift keying ASK, frequency

shift keying FSK and Phase shift keying PSK. FSK is generally preferred for low-speedapplications ( data rate ≤ 1200bits/s). ASK and PSK are most popular for higher speed

data transmission.

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The model 3051C transmitters shown in Figure 6 communicates via the HART protocol,

which uses the industry standard Bell 202 frequency shift keying (FSK) technique to

perform digital data transmission. The digitally modulated carrier is superimposed on the4-20 mA output signal. The Rosemount implementation of this technique allows

simultaneous communications and output signalling without compromising the integrity

of the loop. The Hart protocol allows the user easy access to the configuration, test andformat capabilities of the Model 3051C.

Figure 7

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ASK transmission

Binary 0 - transmitted using

sinusoidal carrier wave of Amplitude

A1

Binary 1 - transmitted using

carrier of Amplitude A2

Note that A1 < A2.

FSK transmission

Binary 0 - transmitted usingsinusoidal carrier wave of frequency

f 1Binary 1 - transmitted using

carrier of frequency f 2

Note that f 1 < f 2.

PSK Transmission

Binary 0 -transmitted using a

sinusoidal carrier wave with zero phase

Binary 1 -transmitted using aanti-phase sinusoidal carrier (phase

angle = 180 deg.s)



Example

Suppose the temperature range of 20◦ C to 120◦ C is linearly represented by the standardcurrent range of 4 to 20 mA. What current will result from 66◦ C? What temperature does

6.5 mA represent?

3.0 Supervisory, Distributed and Direct Digital Control

Process control began with sensing elements connected directly to recording controllers,

which, in turn, were connected directly to the control valve. The control loop intelligence

was distributed near the process it controlled. This distribution of loop intelligence produced good control of individual process variables; but operators could not adequately

monitor all of the control loops, especially in spread-out processes such as oil refineries,

paper mills, steel mills, and chemical plants. Control engineers could only dream of advanced control concepts, because there was no way to use inputs from several process

variables to improve the control of critical variables in the process.

The digital computer entered the process control scene in 1959, the advent of thistechnology provided the control engineer with a tool to implement advanced control

concepts. The input of all process variables into the computer memory meant that every

process variable was available for use in a control algorithm. Figure 8 shows threedifferent schemes for employing computers in industry.

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Figure 8

One particular method of using computers for control was to replace the analog

controllers with a single large digital computer. The term direct digital control (DDC)

was used to describe this type of system. This meant the conversion of the transmitted

analog signals into digital format using ADCs for presentation to the computer system.

Reliability was a problem, however. If the computer failed, the entire process was out of control. This happened with enough regularity that some form of backup was in order.

One backup scheme used a standby computer that was ready to take over in case the primary computer failed. In extremely critical applications such as the space program,

two backup computers were used. Another method used analog controllers for backup in

case the DDC computer failed, see Figure 8c. Yet another method used analog controllersfor loop control with the digital computer used in a supervisory role, see Figure 9.

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Figure 9

The microprocessor gave us the means to move the control loop back to the plant floor

and the ability to communicate with this distributed intelligence. We can have the ‘shortloop’ advantages of distributed control and the fully informed operator of a centralized

control system. Most modern control system therefore incorporate multiple control

system architecture and hierarchy as shown in the hierarchical computer based control

system block diagram above. Figure 10 gives an example of a fully integrated distributedcontrol system consisting of both process control and machine control. The path that

crosses the top and runs down the left side represents the communication network. Localcontrol units can handle continuous processes, batch processes, and robotic work cells. In

addition, other units, such as programmable logic controllers (PLCs), can be interfaced

with the communications network. Local operators, central control room operators, plant

maintenance, plant engineering, and plant management all have access to all the infor-mation via color display and keyboards.

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Figure 10

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Lecture 1-2_ Introduction to Control System 2010