Red-lion PCU Manuel d'utilisateur télécharger pdf (Page 68)

DERIVATIVE TIME

Derivative time is defined as the time, in seconds, in which the output due

to proportional action alone equals the output due to derivative action with a

rampingprocesserror.As long asarampingerror exists, thederivativeaction

is “repeated” by proportional action every derivative time. The units of

derivative time are seconds per repeat.

Derivative action is used to shorten the process response time and helps to

stabilizethe process by providing an output based on the rate of change of the

process. In effect, derivative action anticipates where the process is headed

and changes the output before it actually “arrives”. Increasing the derivative

time helps to stabilize the response, but too much derivative time coupled

with noisy signal processes, may cause the output to fluctuate too greatly,

yielding poor control. None or too little derivative action usually results in

decreased stability with higher overshoots. No derivative action usually

requires a wider proportional and slower integral times to maintain the same

degree of stability as with derivative action. Derivative action is disabled by

setting the time to zero.

OUTPUT POWER OFFSET (MANUAL RESET)

Iftheintegraltimeissettozero(automaticresetisoff),itmaybenecessary

to modify the output power to eliminate errors in the steady state. The output

power offset (OPOF) is used to shift the proportional band to compensate for

errors in the steady state. The output power offset (OPOF) parameter appears

in the unprotected mode, if the integral time equals zero. If integral action

(automatic reset) is later invoked, the controller continues from the previous

output power offset and updates accordingly.

PID ADJUSTMENTS

ToaidintheadjustmentofthePIDparametersforimprovedprocesscontrol,

achart recorder is necessary toprovideavisualmeansofanalyzingtheprocess.

Compare the actual process response to the PID response figures with a step

change to the process. Make changes to the PID parameters in no more than

20%incrementsfrom thestarting value andallow the processsufficient time to

stabilize before evaluating the effects of the new parameter settings.

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Figure 30, Derivative Time

Figure 31, Typical Response Curve

1 2 ... 63 64 65 66 67 68 69 70 71 72 73 ... 95 96

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Red-lion PCU Manuel d'utilisateur Page 68