C4: Control Terminologies

8/9/2019 C4: Control Terminologies

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Lecture 3 : CONTROL SYSTEMTERMINOLOGY

By

Oladokun Sulaiman


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Learning Objective:Learning Objective: To be able to describe theTo be able to describe the

common TERMS OF CONTROL SYSTEMScommon TERMS OF CONTROL SYSTEMS

Specific Objectives:Specific Objectives:

SystemSystem

Control componentsControl components

Control elementsControl elements

Performance of control systemPerformance of control system

Classification of control systemClassification of control system


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3.1 SYSTEM

A system can be defined as an arrangement of parts within some

boundary which work together to provide some form of output from

a specified input or inputs.

INPUT PNEUMATICCONTROLLER

FEED BACK

OU

TP

UT

JACKET WATER COOLER


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CONTROL SYSTEM

The operation of a system may be controlled externally (by an

operator) orautomatically (by the system itself). When the control

action of a system is independent of the output, the system is saidto be an OPEN-LOOP control system. If It, it is called a CLOSED-

LOOP or FEEDBACK control system.

In a simplified definition - an input of the required value of some

variable and an output of the variable at the desired value.

CONTROL

SYSTEM

INPUT

The required

value of a

variable

OUTPUT

The variable at

the desired

value


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BLOCK DIAGRAM A useful way of representing a system is as a block diagram.

Within the boundary described by the box outline is the system and inputs to

the system are shown by arrows entering the box and outputs by arrows

leaving the box.

Control system may be represented by a series of interconnected system

elements with each system element represented by a block having a

particular function.

Pneumatic controller Valve positioner Cooler

outlet

let


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CONTROL SYSTEM COMPONENTS A control system consists of a controller and a plant. We use the general

term plant to describe the machine, vehicle or process which is being

controlled. The controller can be a person, in which case we have a manual controlsystem.

Alternatively, in an automatic control system the controller is a device,electronic circuit, computer, or mechanical linkage, etc. Figure below showsthe general arrangement.

The interface between the plant and the controller requires actuators(control elements) to provide the control action. In addition, instrumentation, detectors and sensors(measurement elements)

are needed to provide information about the plant status to the controller. The information passing between the controller and the plant is in the form

ofsignals. These signals can be very diverse, for example electrical,

pneumatic or mechanical, etc. The term transmitter is commonly used to describe the measurementelement in a process control system because the transmitter sends anelectrical or pneumatic signal representing the measured value to thecontroller.


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Control System Components Controllers are usually implemented electronically, either

using analogue circuits or a digital computer(microprocessor).

Pneumatic and hydraulic controllers are also to be found.Actuators are commonly pneumatic, electric or hydraulicdepending on the application and power level required.

The behaviour and performance of a control systemdepends on the interaction or all elements.

The individual components cannot generally be

considered in isolation. The plant itself is probably the most important element in

any control system; the best controller in the worldcannot make an inadequate plant operate well.


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3.2 CONTROL SYSTEM COMPONENTS

Plant a piece of equipment, ora set of machine partsfunctioning together, the purpose is to perform a particular

operation

System - a and perform a certain objective combination ofcomponents that act together

State variable - a quantity that change with time

Controlled variables - these are the variables which quantify theperformance or quality of the final product, which are also calledoutput variables.

Manipulated variables - these input variables are adjusteddynamically to keep the controlled variables at their set-points.

Disturbance variables - these are also called "load" variables andrepresent input variables that can cause the controlled variables

to deviate from their respective set points.


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3.3 CONTROL ELEMENTSControl element: This determines the action to be taken as a

result of the input to the system.

Correction element: This has an input from the controller andgives an output of some action designed to change the variablebeing monitored.

Process: This is the process of which a variable is beingcontrolled.

Comparison element: This element compares the required valueof the variable being controlled with the measured value ofwhat is being achieved and produces an error signal.

Error: Reference value signal measured actual value signal.

3 3 CONTROL ELEMENTS


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3.3 CONTROL ELEMENTS Stability:A system is said to be stable if, when given a

input or a change in input, it has transients which die awaywith time and leave the system in a steady state condition.

The system would be unstable if the transients did not dieaway with time but grew with time and so steady conditionwere never reached.

Detecting Element: Responds directly to the value of thevariable. For example Bourdon tube is pressure detecting

element. Measuring Element: Responds to the signal from the

detecting element and gives a signal representing thevariable value. For example the pointer of the pressuregauge.

Measuring Unit: Comprises of detecting and measuringelement.

Sensor: is a term used for the detecting element. By itsnature, it is essentially a transducer.


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3.3.1 SIGNAL TRANSMITTING BY TRANSDUCER

THERMOCOUPLE AMPLIFIER

VOLTAGE SIGNAL

THAT

REPRESENTS

THERMOMETER

TRANSDUCER

Transducer: A device to convert a signal (representing a physical

quantity) of one form into a corresponding signal of another form. For

example a microphone is a sound transducer.

A transducer may be an integral part of the measuring unit, for example

pressure to displacement in a Bourdon pressure gauge.

It may also be a separate unit converter especially suitable to change

the signal to a better form for remote transmissions, e.g. displacement to

electrical in a differential transformer.


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3.3.2 TELEMETERING

It may be defined as signal transmission over a

considerable distance.

In measurement this involves information transfer from

detecting element to a centralrecording display station.

In control this involves control operating devices and

related signal transfers.

In telemetering systems the measuring unit is often

called the transmitter.

Incorporating a transducer, and the recording unit some

distance away is then referred to as the receiverwhich

may have an associated transducer if required.


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3.3.4 FEEDBACK In everyday life, feedback occurs when we are made aware of the

consequences of our actions. Feedback is so natural that we take itfor granted. Imagine trying to accomplish the simplest of taskswithout feedback, for example, trying to walk without visualfeedback. Feedback not only gives verification of our actions: itallows us to cope with a changing environment by adjusting ouractions in the presence of unforeseen events and changingconditions.

Feedback has similar advantages when applied to automatic

control. Feedback occurs in automatic control systems when thecontrol action depends upon the measured state of the machine orprocess being controlled. Feedback gives and automatic controlsystem the ability to deal with unexpected disturbances andchanges in the plant behaviour.

Human

operator

Tank

Meter reading

Desired

fluid

outputError

-

+Valve

Actualcourse

of travel


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FEEDBACK It is the measured value or

a multiplication ofmeasured value which isbrought back to thecomparator unit andcompared with therequired of reference

value.

To stabilize the system anegative feed back is auseful part of the control.

Negative feed backreduces the overall gain ofthe system and makes the

system more stable.

GX+

H

Y

GHG

XY

+

=

1

Error

3 3 5 TRANSFER FUNCTION


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3.3.5 TRANSFER FUNCTION

A controllers input may be a function of time and of many forms, such asramp, sinusoidal, etc.

As a result output will also be a function of time. Therefore gain will also be a function of time. A controller action may not be only be proportional. Controller action (output) may be a resultant of various actions

proportional of input, integration of input with respect to time and derivativeof input with respect to time.

Hence it is easier to express all the actions in terms ofLaplace transform,

which will give an equation in terms ofs domain. Linearity property allows two Laplace transforms of separate functions to be

added. Gain of controlleris obtained mathematically and expressed in s domain.

Inputs and output described as function of s we define the transfer function

)(

)()(

sX

sYsG =

3 3 6 GAIN


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3.3.6 GAIN

It is the ratio of change of output to change of input.

A controllers input may be a function of time and of many forms, such as ramp,

sinusoidal, etc. As a result output will also be a function of time. Therefore gain

will also be a function of time.

Controller input may be composed of different type of action- proportional,integration and derivative, so as output.

Hence it is easier to express all the actions in terms of Laplace transform,

which will give an equation in terms of s domain.

Linearity property allows two Laplace transforms of separate functions can be

added.

Gain of controller is obtained mathematically and expressed in s domain.

Gain is also termed as Transfer function, G(s).

GAIN


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GAINIt is the ratio of change of output to

change of input.

In a simple proportional only

controller it is a constant controlleraction factor.

Assuming a proportional controller

consists of nozzle and flapper, as the

distance between flapper and nozzle

changes, outlet pressure changes

as shown in the sketch.

Here the gain can be expressed

as 0.02 bar/m It is also expressed as the ratio of

percentage of output range to thepercentage of input range.

In many controllers there is provisionof adjusting gain.

1bar

0.2 bar

20m 60m

DV MV

3 4 PERFORMANCE OF CONTROL SYSTEM


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3.4 PERFORMANCE OF CONTROL SYSTEM

The Standard Terms and Definitions used in Instrumentation and ControlEngineering are listed below.

Accuracy: The accuracy of an instrument reading is defined as thecloseness with which the reading approaches the true value.

Precision: The precision of readings is the agreement of readings takenrepeatedly when the same value is measured several times over.

Sensitivity: Usually denoted the smallest change in the measured value towhich the instrument responds.

Calibration: To calibrate a mechanism is the function of adjustment to givea required accuracy of reading over a given range.

Error: Instrument error is the difference between the indicated and truereading as shown by a standard.

Correction: The correction of an instrument is the amount to be added to orsubtracted from the indicated reading to obtain the correct value.

Tolerance: Is the amount by which an instrument indication may departfrom the true reading and still perform its required function, i.e. permissibleerror.

Hysteresis: Hysteresis of an instrument is the maximum differencebetween readings taken at given points moving up scale, to those takenwhen moving down scale, i.e. hysteresis curves are potted from up scaleand down scale readings.

Performance of Control System


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Performance of Control System Control condition: The physical quantity or condition of the controlled

body, process or machine which it is the purpose of the system to control. Measurement Element: The element which responds to the signal from the

detecting element and gives a signal representing the controlled condition. Desired Value: The value of the controlled condition which the operator

desires to obtain. Deviation: Thedifference between the measured value of the controlled

condition and the command signal. Proportional Action: That range of values of deviation corresponding to

the full operating range of output signal of the controlling unit resulting fromproportional action only.

The proportional band can be expressed as a percentage of the range ofvalues of the controlled condition which is the measuring unit of thecontroller is designed to measure.

Offset: Sustained deviation between measured and desired values. Integral Action/Reset: The action of a control element whose output signal

changes at a rate which is proportional to its input signal. Derivative Action: The action of a control element whose output signal is

proportional to the rate at which its input signal is changing.


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3.5 Classification of Control Systems

Based on working medium

Based on type of signal in the system

Based on system structure

Based on control action

Response of control systems

Proportional band

Integral action time Derivative action time


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3.5.1 Based on working medium

Mechanical Control system

Pneumatic Control Systems

Hydraulic Control systems Electrical and Electronic Control Systems

Hybrid Control systems


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3.5.2 Based on Type of Signals

Analog Control Systems

Digital Control Systems

ON-OFF Control

Fuzzy Logic Control

Neural Network

Adaptive Control

Optimal Control

Stochastic Control


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3.5.3 Based on Structure

Open Loop Control system

Close Loop Control system

ON-OFF Control systems

Cascade Control System

Split Range Control system

Ratio Control system


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Based on Control Action

Proportional

Integral

Derivative PD

PI

PID


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Type of Controller Actions

Two step action: Regulating unit open or shut/ on or off.

Proportional action: Action of regulating unit is proportional to the

deviation, that is, output or

Where c is controller output

Integral action: Rate of change of controller action or

Integrating both sides

C = (Ki/s ) x e

Derivative action: Controller action is proportional to rate of

change of deviation or

edt

dCI

eKCoreC ppp =,

= dteKC II

dt

deKCor

dt

deC DDD =,


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PROPORTIONAL CONTROLLER

DV MV

Why proportional control suffers offset and droop :

Controller action is proportional to the input

signal and can maintain a desired value at certain

loading condition of the plant, accordingly

controller changes- the position of the regulator,valve, etc.

If load of the controlled plant changes the

same amount of regulating action will not bring

back to desired value.

e = MV-DV


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Response of controller action

Change in load

Proportional only control

Proportional + Integral

Control

Proportional + Integral +

Derivative Control

offset

No

offset

Less recoverytime


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PROPORTIONAL BAND

FLOAT

ADJUSTING

SCREW

20 M

=1M UP

V=0.05 MM

DOWN

P SET

-k=V/ = 0.05MM/M

TRAVEL 10 M

VALVE TRAVEL= 0.5 MM


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Proportional Control system

Proportional Band


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Proportional Band

P control Response


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P-control Response


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Example of offset

In the following figures control valve opens on a decrease in

controller output. Controller output decreases with its liquid level

decreases.

Desired level is 0.6 meter flow through regulating valve is 60

litre/min. Assuming the regulating valve at 0.2meter, level remains

full open when it passes 120 litre/min. At 1m level the valve will be

fully closed.

Desired value

0.6m

Process

Regulating

unit

Controller

e

1m

0.6m

0.2m


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Example of offset

The system is trimmed so that with the valve half open, inflow and

outflow are the same, i.e. 60 litre per minute.

Now if the outlet valve is further opened so that the outflow is now

90 lpm. With 60 lpm flowing into tank the level will obviously drops,

and will not stop dropping until the input flow is 90 lpm. This can

only occur if the level is much below than 0.6 m.

The controller will now be in equilibrium with the control point offset

from the desired value

The level can not rise any more because at that value the controller

will regulate to cause less inflow than 90 lpm, subsequently level

falls.


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Integral Control Action

Integral Control (I)

Controller output proportional to integral of

error signal e

C e dt


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Integral Control Response


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(P + I ) Control

(P+I) Control

Controller action combination of (P)

and (I) actions

C ( e + e dt)

C = (Kp + Ki/s) e

(P + I) Control System


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(P + I) Control System

P+I C t ll t t


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P+I -Controller output


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PROPORTIONAL + INTEGRAL CONTROLLER

DV MV


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contd

(P + I + D) Control

Controller action is combined effect of (P)

(I) and (D) control actions.

c ( P + e dt + de/dt)

C = (Kp + Ki/s + Kd x s) eMost accurate control, but very complex


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(P + I +D ) Control system


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PNEUMATIC THREE TERM CONTROLLER (P + I + D)

DV MV

INTEGRAL BELLOW DERIVATIVE BELOW

DERIVATIVE

RESTRICTOR

INTEGRAL RESTRICTOR

ONE CAN ADJUST THE OVERALL GAIN OF THE CONTROLLER BY ADJUSTING THE INTEGRAL AND

DERIVATIVE RESTRICTOR.

CORRECTING

UNIT

RELAY VALVE


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SINGLE ELEMENT CONTROL

3 way

valve

Fresh water Return

Fresh water supply

Controller

SW IN

SW OUT

COOLER

AIR SUPPLY

SENSING ELEMENT

Single elements means the controlled element (variable) is controlled by a

single sensing element- For example it senses the Jacket Cooling Water inlet

temperature and tries to maintain this temperature at constant set level


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Based on Structure

Open Loop Control system

Close Loop Control system

ON-OFF Control systems


Split Range Control system

Ratio Control system


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Open loop Control systems

TYPES OF CONTROL


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TYPES OF CONTROL

Broadly classified into two types:

1) Open loop: Controlled manually. No automatic

feedback from the measured value of the plant output(being controlled).

2) Closed loop: Feedback of measure values is

compared with desired value and the error is used as

input signal of the controller.

Process

Regulatingunit

Measuring

unit

Desired value

Process

Regulating

unit

Measuring unit

Controller e


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Cascade Control

2. CASCADE

Used for contr


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Ratio Control System

One parameter is controlled in a fixed ratio

to another parameter of the system

Boiler air-fuel ratio control is an exampleof ratio control system


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SplitRange Control System

Control signal divided into two range ofmagnitude

Lower magnitude signal controls one

corrective action input Higher magnitude signal controls second

corrective action input

Control of lubricating oil temperature issplit range control system


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contd

DV

Master Controller

M/ESlave Controller

Steam Valve SW VV

Steam Heater Sea Water Cooler

L.O Pump

LO Sump

ATOATC


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Digital Control System

All the signals flowing through control systemare converted into digital form

This is done to simplify processing of plantparameters to achieve better control

All the signals flowing into the final controlelements are reconverted from digital to analogform

System utilises A/D and D/A converters Control functions are distributed rather than

centralised



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5. DIGITAL CONTRO

Consists of a number of microprocessors based control

modules that work together

Control modules distributed geographicallyReduces risk and improves reliability

DCS Configuration


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DCS Configuration

Essential feat res of Distrib ted


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Essential features of Distributed

Control Syetm(DCS)

Distributes its functions into smaller sets ofsemi-autonomous subsystems whichcover specific process or area of the plant

Performs following functions- Process analysis and supervision

Data Logging

Process control

Storage and retrieval of Data Presentation of information and reports


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Basic DCS functions

DCS is connected to primary controlelements such as temperature andpressure transmitters, flow meters, gas

analysers etc From these field devices, it receives

electrical signals, such as 4-20 mA, 1-5 VDC etc

DCS converts these signals intoequivalent digital form


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Contd

Digitized signals can be used by thecomputer to variety of use, such as

Control loops

Execute special programmable logic Monitor inputs

Set alarm operations through use limiters

Trend,Log and report Data Perform many other functions to implem-

ent superior control strategy


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Feedback Control

Process or

System to be

Controlled

Measured

outputsControlled signal

Control system involve with the analysis, design, modeling,

estimation, identification, and control of physical systems or

processes


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Feedback Control

Process orSystem to be

Controlled

Measured

outputs

Control System(analog or

digital)

Controlled signal

+-

Desired

Value

Error


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Motor Control Process

Motor Assembly

Angle

Microcontroller

Motor Voltage

+-

Desired

Angle

Error

Process Control


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From R. Stenz and U. Kuhn, Automation of a Batch Distillation Column Using Fuzzy and ConventionalControl," IEEE Transactions on Control Systems Technology, vol. 3, no. 2, June 1995, page 172.

Power System


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Power System

From:P. Kundar,Power System

Stability and Control, McGraw Hill,

1994, page 9.


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Printed

CircuitBoard test

From: J. Shim, H. Cho and S. Kim, "An

Actively Compliable Probing System,"IEEEControl Systems Magazine, vol. 17, no. 1,

February 1997, page 15.


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From: R. Dorf and R. Bishop, Modern Control Systems, Addison

Wesley, 7th edition, 1995, page 706.

Biomedical


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Baking Processes


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Applications

Power Systems

Manufacturing

Robotics

Process Control Flight Control and Navigation

Network Control

Biomedical Process Scheduling (in Computers)


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Courses

Software Fundamentals for EngineeringSystems

Neural Networks and Fuzzy Logic in Control

Industrial Controls and Manufacturing Computational Computer Vision

Embedded Control Systems


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DCS General Architecture

Input/Out put module to scan and digitise process instrument input/output

Local I/O bus link Input/output links to controller modules

Controller modules to perform control calculations and logic from field data

User interfaces include human interface system,Engineering and workstations

Data Highway, a plant wide communication network

Communication modules, provide a link between Datahighway and other modules


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Summary

The following has been discussed:The following has been discussed:

General control systemGeneral control system

Control system componentsControl system components

Control system elementsControl system elements

Performance of control systemPerformance of control system

Classification of control systemClassification of control system


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