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Symptom:
Your controller tuning is not achieving the desired closed loop
performance. The process variable reacts too fast or too slow on its return
to setpoint.
Probable cause:
If the controller gain is not set at the correct value, the loop will not
behave in the manner that you would desire. The speed of the closed loop
reaction is a function of the controller gain setting. This controller
reset setting (see Loop Tip November 2000) must also be set to the correct
value to achieve the desired closed loop behavior.
Corrective action: Based on the open loop response test (see Loop Tip
October 2000), the process time constant, process gain, and deadtime are
known values. The reset speed, in units of time/repeat, is set equal to the
process time constant. The controller gain is calculated by the following
equation:
Controller gain =
process time constant
process gain [deadtime + 3(process time constant)]
This
calculation will result in a conservative tuning. Another advantage
to this method is that the time required for the PV to return to SP is
calculated as follows:
PV return to SP @ 12 (process time constant)
If the
conservative tuning is too slow, the gain can be increased to a more
aggressive value to achieve a faster return to setpoint.
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