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eachtype,sothesametypeofsignalcannotbeusedfortransducerfeedbackandsetpointreference.For

example,a4-20mAsignalfromatransducercouldnotbeusedasafeedbacksignalifthesetpointisbeing

controlled by a 4-20 mA signal from a PLC.

19.5 TUNING THE PID CONTROL

Once the PID control is set up correctly, it needs to be tuned in order to maintain the process set point.

First,settheIntegralandDifferentialGainsto zero,andincrease theProportionalGain(Parameter77)

untilthesystembecomesunstable,thenlowerthegainuntilthesystemstabilizesagain.SettheProportional

Gainabout15%lessthanthatvaluethatstabilizesthesystem.IfonlyProportionalGainisused,andthe

systemisoperatinginasteady-statecondition(setpointisfixedandprocessvariablehassettledtoafixed

value),therewillalwaysbeacertainamountoferrorinthesystem.Thisiscalledthesteady-stateerror.

 IntegralGain(Parameter78)isusedtoforcethesteady-stateerrortozerobyincreasingtheoutputspeed

commandwithrespecttotime.Overtime,theerrorwillbeforcedtozerobecausetheIntegraltermwill

continue to change the speed command, even after the Proportional term reaches steady state and no longer

affects the speed command. The Integral Gain affects the rate of rise of the output speed command from

the Integral term. Small amounts of Integral Gain can cause large changes in PID performance, so care must

betakenwhenadjustingIntegralGain.ToomuchIntegralGainwillresultinovershoots,especiallyiflarge

step changes in error occur.

Typically, Proportional and Integral Gain are all that is needed to fine-tune the system. However, it may

benecessarytouseDifferentialGain(Parameter79)tofurtherstabilizethesystem,especiallywhenquick

responsesarerequired.TheDifferentialtermrespondstotherateofchangeoftheerror,nottheactualerror

itself.DifferentialGainactslikea“shock-absorber”todampenovershootsthatcanoccurwhenthePID

triestoreactquicklytochangesinerrororsetpoint.ThisallowsfastPIDresponse,withreducedriskof

becoming unstable due to overshoots. The Differential term is very sensitive to electrical noise on the

feedbacksignalandtodigitizingerrors,soitmustbeusedwithcaution.

The other parameter setting that affects the response of the PID control is Parameter 80 - PID ACC. This

setstheacceleration(anddeceleration)rateofthesetpointreferenceintothePIDunit.Whenthesetpoint

changes,thisfunctionwill“filter”theinputtothePIDunitbyrampingthesetpointreferencefromthe

previous value to the new value. This will help prevent overshoots that can occur when the PID control

attempts to respond to step changes in set point, resulting in smoother operation. If PID ACC is set to 0.0

seconds, it is effectively disabled.

19.6 MICRO SERIES DISPLAY IN PID MODE

1 2 ... 76 77 78 79 80 81 82 83 84 85 86 ... 91 92

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