PID21

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APCO/TSD/PPMSB

RMPCT for Process Operators Control Algorithms 1-1

Control Algorithms

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APCO/TSD/PPMSB


Control Algorithms

• Feedback control

• Proportional control

• Integral control• Derivative control

• Controller tuning

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APCO/TSD/PPMSB


Concept of Feedback

Water

Temperature

Error =

Actual-

Desired

Move valve

to left

No action

Move valve

to right

Control

Algorithm

Temperature

Feedback

Desired

Temperature

PV

E

H C

SP ?

I’m too

hot!

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APCO/TSD/PPMSB


Feedback Control

• Advantages : Measures the final product or

variable and so does not need an exact knowledge

of the process to work effectively

• Disadvantages : Deviation from the target musthave occurred before feedback control system will

function

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APCO/TSD/PPMSB


Definitions

• Process Variable

– The measured value of the variable to be controlled

e.g. obtained from orifice plate or thermo-couple

– Given in engineering units – Abbreviated as “PV”

• Setpoint

– The required value of the process variable may be set

by the operator or by the output of another controller(cascaded)

– Abbreviated as “SP”

– Same units as PV

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Definitions (continued)

• Error

– Measure of how far the process variable is from the

setpoint

– Abbreviated as “E”

– Defined as PV-SP

• Manipulated Variable (or Output)

– The variable that is adjusted to correct any error

– Abbreviated as “MV”, Output is abbreviated as “OP”

– Output to valve or secondary controller (cascade)

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APCO/TSD/PPMSB


Proportional control• Change in controller output is proportional to change in

error – MV = Kc.E + C

– Where MV= manipulated variable

– E = error – Kc = controller gain

– C = valve position at zero error (0.5 or 50%)

• Proportional band = 100/Kc – % change in error required to move valve full scale

• Proportional only control results in offset – Never reaches SP

• Action is instantaneous (proportional kick)

– As soon as error is produced, control action is taken

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APCO/TSD/PPMSB


Integral Control

• Change in controller output is proportional to sum

of past errors,

• K C = gain as used in proportional only control

• TI = reset/integral time (mins)

• 1/TI sometimes used (repeats/min)

• As long as error exists, integral action will work to

eliminate the error• Used to Remove Offset

MV K T Edt C I

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APCO/TSD/PPMSB


Derivative control

• Change in controller output is proportional to rate

of change of error

– MV = K c.Td.(dE/dt)

– Where Td = derivative time

• Derivative action amplifies measurement noise

• Very effective if deadtime present and often

under-rated

– But problems if noisy signal

– Sensitive to tuning errors

– Provides an additional “pulse” to push the control in

the right direction

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APCO/TSD/PPMSB


Controller Tuning

• Open loop method

– introduce step change to process in open loop

– derive controller tuning from process response

• Closed loop method

– adjust controller tuning to obtain specifc process

behavior

– derive controller tuning based on process response

• Trial and error method

– always needed for fine tuning

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APCO/TSD/PPMSB


Tuning Objective

• Flow control

– Constant flow required

– Very fast response to setpoint or load disturbance

– Tune as fast as possible without causing excessiveoscillation or valve wear

• Level control

– Keep feed to next unit as constant as possible

– Tune as slowly as possible

– Do not tune so slow as to empty or fill vessel!

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APCO/TSD/PPMSB


Tuning objective (Temperature to flow

cascade)• Flow controller

– Fast response required

– Small lag – Fast tuning possible

• Temperature controller

– Steady temperature required

– Larger process lag and often deadtime

– Tune fast but not so as to cause oscillationFC

TCSP

SP

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Manipulated Variable Overshoot

• Most of the tuning methods do not take account of

effect on manipulated variable

– e.g. fuel valve position

• Excessive control valve movement – causes wear and accelerates failure

• Controller tuning becomes a compromise between

fast approach to setpoint and minimum MV

overshoot

– Max 15% recommended

Documents

PID21