6.controller tuning

8/9/2019 6.Controller Tuning

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Controller Tuning


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Proportional Band and Controller

Response

A high proportionalband setting tends toproduce a process

reaction curve thatstabilizes withminimum oscillations.

The curve is said todamp outas thecurve would beproduced by a lowsetting of proportionalgain.


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Response

A slightly lower

proportional band

setting produces a

curve that damps out,but with more

oscillations before

achieving stability. A

similar curve wouldbe produced in

increasing the gain.


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Response

Narrowing the

proportional band

even more produces

a curve that oscillatesin even cycles around

the set point. This

type of curve would

be produced byincreasing the

proportional gain.


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Response

A low proportionalband setting producesa curve in which the

oscillations grow insize as timeprogresses. This typeof curve is veryunstable. A curve ofthis type would alsobe produced by avery high setting ofproportional gain.


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Objectives forProcess Control

The purpose of process control is to keep the controlled

variable of the desired value (set point), despite load

changes and disturbances in the system.

There are three objectives for control in any processcontrol system. Safety The process must ensure the safety of personnel and

prevent damage to equipment.

Production rate The process must be able to meet the

production demands of the facility.

Product quality The process must be able to make a product

that meets the quality specifications of the product.


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Controller Response and Product

Quality


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Controller Response and Product

Quality In controlling the quality of a product in a

process control system, it is important tokeep the parameters of the process within

the quality limits . This means that the controller response to

disturbances and load changes should :

Limit the size of the initial deviation from theset point, and

Return to the set point as quickly as possible.


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One-Quarter Decay Ratio Curve A commonly used

guideline for controllerresponse is a processreaction curve with adecay ratio of .

The decay ratio refers tothe size of the peaks inthe reaction curve.

In a curve that has a

decay ratio of , the sizeof peak B is the size ofpeak A. The size of peakC is the size of peak B,and so on.


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One-Quarter Decay Ratio Curve

The decay ratiocurve is used as aguideline for good

control because it issaid to have aminimum area underthe curve.

Keeping the area to aminimum should reducethe amount ofsubstandard productproduced.


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Optimum Controller Response

The decay ratio curve also represents a

compromise between the size of the initial

deviation form the set point and a quick return to

the set point. The decay ratio curve was developed in 1942

by two engineers named Ziegler and Nichols.

This curve is the basis of two controller adjusting

procedures processes, adjusting the controller to

produce a decay ratio curve produces the

optimum controller response.


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Minimum disturbance

Some processescannot tolerateoscillations in the

process response. However, adjusting

the controller forminimum disturbance

may produce offset inthe process responseand may produce alarge initial deviationfrom the set point.


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Minimum Amplitude

Amplitude is the termused to describe the sizeof the peaks produced inthe process response

curve. However, the process

may be able to tolerate acertain amount ofinstability caused by

smaller oscillations in theprocess reaction curve.

In this case, the processmay be adjusted forminimum amplitude.


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Guidelines forGood Control

The guidelines for good control vary from

one process to another because of

the design of the process system, the nature of the process,

and the type of product produced.

You should always follow your facility

guidelines when adjusting the controller

response.


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Stabilizing a Process on Manual

Control Before tuning any controller, the controller

should be set for manual control and the

process should be stabilized.

If the process cannot be stabilized in manualcontrol, the tuning procedure will probably not be

successful.

A process that cannot be stabilized may have

other problems that are affecting the controller.

These problems should be identified and

corrected before the controller is tuned.


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Tuning Method

Trial and ErrorMethod

UltimateMethod or Closed-Loop

Method Reaction curve Method orOpened-

Loop Method


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Trial and ErrorMethod

The key to the trial and error method isanalysis of process reaction curves. Thecurve must be analyzed to see whether a

particular setting causes the controller torespond to properly control the process.

By examining the curve produced, and

experienced technician can make theproper adjustments to produce a one-quarter decay ratio curve.


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Interpreting Process Reaction

Curves A Proper proportionalsetting produces acurve with a decay

ratio of one-quarter.The period betweenthe peaks of the curveis usually constant.However, in a

properly-tunedproportional-onlycontroller, some offsetmay be evident.


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Curves A proportional band

setting that is too high

produces a curve that

has a large initialpeak. The curve will

return to a stable

value without

sustained oscillations.


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Curves

A proportional band

setting that is too low

is slow in returning to

stability. The curve produced

will oscillate before it

becomes stable, orit

may continue to

oscillate and not

stabilize.


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Tuning by Trial and Error

Each successive adjustment comes closer

to the correct setting.

O

ne control mode should be adjusted at atime.

The following steps are appropriate to the

proportional mode.

Reset and derivative control action should

be eliminated from the controller before

adjusting the proportional band.


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Put the proportional mode at a starting

point setting that gives minimum control

action.

Put the controller on automatic.

Make a small change in the set point.(or

any other disturbance) Observe the

process reaction curve produced.


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Analyze the curve to see whether theproportional mode setting is too high or toolow.

If an initial proportional band setting wastoo high, decrease the setting by half.

Make another small change in the set

point .(or any other disturbance) andobserve the response on the chartrecorder.


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Analyze the curve to see whether thesetting was too high or too low.

Readjust the proportional mode setting by

half. Continue to check each successive settingby analyzing the curve produced by asmall set point change.

Readjust the setting each time by half. Stop adjusting the proportional mode

when one-quarter decay ratio curve is

produced.


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Interpreting Proportional-Plus-

Derivative Curves

The trial and error method can be used toadjust the derivative mode of a controller.

It is more difficult to recognize the effectsof tuning adjustments on a two-modecontroller than on a proportional-onlycontroller. This is because the modesinteract.

The derivative mode should reduce theinitial overshoot of the process after adisturbance.


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Derivative Curves

A properly-tuned PD

controller will produce a

decay ratio curve.

The process shouldstabilize fasterthan with a

P-only controller.

The size of the initial

peak on the curve will be

smallerthan the initialpeak on the curve of a

properly-tuned P-only

controller.


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Derivative Curves

When the derivative

setting is too low, the

curve will resemble

one produced by aP-only controller.

The response will not

be improved

significantly, the sizeof the initial peak will

not be noticeably

reduced.


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Derivative Curves

A PD-controller with aderivative setting thatis too high will exhibit

instability. There will be toomany oscillations fora decay ratio curve.

This is becausederivative setting thatis too high makes thecontroller overreact tothe disturbance.


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Reset Curves

The purpose of reset on a

P controller is to eliminate

offset.

A properly-tunedP

Icontroller will produce a

decay ratio curve in

response to a disturbance

change.

The addition of resetreduces or eliminates

offset and the process

indicator returns to the

set point.


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Reset Curves

Too little reset action


returns to the set

point too slowly. The end of the curve

may appear to trial

toward the set point

after it stopsoscillating.


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Reset Curves

Too much reset

action produces a

curve that oscillates

excessively beforebecoming stable.

The curve will not

have a decay ratio of

because of theexcessive oscillations.


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Reset-Plus-Derivative Curves

A properly-tuned PID-

controller will produce

a1/4 decay ratio curve in

response to a disturbance

change.

The curve will exhibit the

speeded-up response

and small initial peak of

derivative control The addition of reset

changes the curve by

reducing or eliminating

offset.


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Too little reset action


is slow in returning to

the set point after adisturbance.

This type of curve

would be produced by

a reset time settingthat was too high.


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Too much reset

action produces a

curve that overreacts

to the disturbance byproducing excessive

oscillations.

This type of curve

would be produced bya reset time setting

that was too low.


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Ultimate Method or Closed-Loop

Method

Set the controller on manual.

Eliminate the reset and derivative modesfrom the controller.

Set the proportional band at its highestvalue. Or lowest value of proportional gain.

Transfer the controller from manual to

automatic. Create a slightly set point or disturbance in

the process, approximately 2-5 % of span.


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Method

Return the set point or disturbance to its

original value as soon as a change in the

variable registers on the controller. This

will keep the disturbance to a minimum.

Watch the response of the process on the

chart recorder.

Reduce the proportional band setting by

one-half, watch the response.


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Method

If the process reaction

curve produces

oscillations that

increase in amplitudegradually increase the

proportional band (or

decrease the

proportional gain)until sustained

oscillations occur.


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Utimate Proportional Band,

Ultimate Proportional Gain

Ziegler-Nichols

formulas for properP-

control action

PB = PBu x 2

Or

Kc = Su x 0.5


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Ultimate Period

PI-Setting

PB = PBU X 2.2

OR

KC = SU X 0.45

TI = PU / 1.2OR

RI = 1.2 / PU


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Derivative Control


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PD Setting

PB = PBU X 1.7

OR KC = SU X 0.6

Td = Pu / 8


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PID Setting

PB = PBU X 1.7OR

KC = SU X 0.6

Ti = Pu x 0.5

OR

Ri = 2 / Pu

Td = Pu / 8


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Reaction Curve Method,

Opened-Loop Method

Obtaining a ProcessReaction Curve


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Process Reaction Curve


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Calculating the Process Reaction

Rate


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Calculating the Process Reaction

Rate

The formula for the

slope of the tangent

and the formula for

the process reactionrate (R) are the same.

R = B / A

Example

R = B / A = 10% / 1

= 10 %


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Calculating the Unit Reaction Rate

The reaction rate of a

process varies. The

percent the valve

opens determines thereaction rate.

Unit Reaction rate is a

measure of how much

the process reacts foreach percent of valve

change.


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Calculating the Unit Reaction Rate

Unit reaction rate (R1)

R1 = R / X

= 10% / 5%

= 2


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Calculate Process Lag

Process lag is the

value that represents

the total lags in the

process.


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Reaction Curve Method


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Cascade Control


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Cascade Control

6.controller tuning

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