process control introduction1

8/9/2019 Process Control Introduction1

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Control systems

Control systems are used to maintainprocess conditions at their desired valuesby manipulating certain process variablesto adjust the variables of interest.

A common example of a control systemfrom everyday life is the cruise control onan automobile. The purpose of a cruisecontrol is to maintain the speed of the

vehicle (the controlled variable) at thedesired value (the set point) despitevariations in terrain, hills, etc.


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(disturbances) by adjusting the throttle, orthe fuel o to the engine (the

manipulated variable). Another common example is the home hot

ater heater. The control system on thehot ater heater attempts to maintain the

temperature in the tan! at the desiredvalue by manipulating the fuel o to theburner (for a gas heater) or the electricalinput to the heater in the face ofdisturbances such as the varying demandon the heater early in the morning, as it iscalled upon to provide ater for the daily

shoers.


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"rom these examples, e can deducethat there are several common

attributes of control systems#a) The ablity to maintain the process

variable at its desired value in spite

of disturbances that might beexperienced (this is termeddisturbance rejection )

b) The ability to move the processvariable from one setting to a nedesired setting (this is termed set

point tracking )


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The controller compares the measurementsignal of the controlled variable to the set point(the desired value of the controlled variable).

The di$erence beteen the to values is calledthe error.

%rror & 'et ( point value) (easurement signalof controlled variable)

*epending upon the magnitude and sign of the

error, the controller ta!es appropriate action bysending a signal to the +nal control element,hich provides an input to the process to returnthe controlled variable to the set point.


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"igure # -eneralied process controlsystems


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The concept of using informationabout the deviation of the system

from its desired state to control thesystem is called feedback control

/nformation about the state of the

system is fed back to a controller,hich utilies this information tochange the system in some ay.


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The type of control system shon in "ig. termed a closed-loop feedbac! control

system.Closed-looprefers to the fact that the

controller automatically acts to return

the controlled variable to its desiredvalue.

/n contrast, an open-loop systemould have the measurement signal

disconnected from the controller, andthe controller output ould have to bemanually adjusted to change the value

of the controlled variable.


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An openloop system is sometimes saidto be in manual mode as opposed to

automatic mode (closedloop). Negative feedbackis the most

common type of signal feedbac!.

Negativerefers to the fact that theerror signal is computed from thedi$erence beteen the set point andthe measured signal.

The negativevalue of the measuredsignal is 0fed bac!1 to the controller andadded to the set point to compute the

error.


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Example 1. Hot water tank controlsystem. As a speci+c example, let us

consider a hot ater heater for a home ("ig. 2 ) and examine its control system,using the same type of diagram ( "ig.3 ).

"ig 2


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The desired hot ater temperature isselected by the homeoner, and

typically it is in the neighborhood of24 to 54 ". 6et us assume that theset point is 34 ". The thermocouple

measures the temperature of theater in the tan! and sends a signal tothe thermostat indicating the

temperature. The thermostat(controller) determines the error as


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/f the error is positive (>4), the measuredtemperature is loer than desired and

the thermostat opens the fuel valve tothe burner hich adds heat to the tan!.

/f the error is ero or negative ( 4), thethermostat closes the fuel valve and noheat is added to the tan!.

*isturbances to the system, hichdecrease the temperature of the ater in

the tan!, include ambient heat lossesand hot ater demand by the householdhich is replaced ith a cold ater feed.


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"igure 3# 7loc! diagram of a hot aterheater control system.


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Types of Controllers

The thermostat on the hot ater

heater is called an 0on8o$ 1 type ofcontroller.

*epending on the value of the

errornsignal, the output from thecontroller iseither 0full on1 or 0full o$1 and the fuel valve is full open or full

closed9 there are no intermediatevalues of the output.


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mportant de!nitions

"lock diagram :*iagram that

indicates the o of informationaround the control system and thefunction of each part of the system.

Closed loop :/n closed loop, themeasured value of the controlledvariable is fed bac! to the controller.

Controlled variable :The processvariable that e ant to maintain ata particular value.


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Controller :A device that outputs a signalto the process based on the magnitude of

the error signal. Aproportional controlleroutputs a signal proportional to the error.

#ist$rbance re%ection :;ne goal of acontrol system, hich is to enable the

system to 0reject1 the e$ect of disturbancechanges changes and maintain thecontrolled variable at the set point.

#ist$rbances :Any process variablesthat can cause the controlled variable tochange. /n general, disturbances arevariables that e have no control over.


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Error :The di$erence beteen thevalues of the set point and the measured

variable. &anip$lated variable :


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'pen loop :/n open loop, the measuredvalue of the controlled variable is not fed

bac! to the controller. )ositive feedback :/n positive feedbac!,

the measured temperature is added to theset point. (This is usually an undesirable

situation and fre=uently leads to instability.) *et point :The desired value of the

controlled variable.

*et point tracking :;ne goal of a controlsystem, hich is to force the system tofollo or 0trac!1 re=uested set pointchanges.


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'pen loop :/n open loop, the measuredvalue of the controlled variable is not fed

bac! to the controller. )ositive feedback :/n positive feedbac!,

the measured temperature is added to theset point. (This is usually an undesirable

situation and fre=uently leads to instability.) *et point :The desired value of the

controlled variable.

*et point tracking :;ne goal of a controlsystem, hich is to force the system tofollo or 0trac!1 re=uested set pointchanges.

process control introduction1

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