Red Lion PXU - Manual (Page 24)

24 24
PID Adjustments
In some applications, it may be necessary to fine tune the Auto-Tune
calculated PID parameters. To do this, a chart recorder or data logging device is
needed to provide a visual means of analyzing the process. 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% increments from the
starting value and allow the process sufficient time to stabilize before evaluating
the effects of the new parameter settings.
In some unusual cases, the Auto-Tune function may not yield acceptable
control results or induced oscillations may cause system problems. In these
applications, Manual Tuning is an alternative.
TIME
SP SP
TIME
INPUT INPUT
OVERSHOOT AND OSCILLATIONS SLOW RESPONSE
TO DAMPEN RESPONSE:
- USE SETPOINT RAMPING.
- USE OUTPUT POWER LIMITS.
- INCREASE PROPORTIONAL BAND.
- INCREASE INTEGRAL TIME.
- INCREASE DERIVATIVE TIME.
TO QUICKEN RESPONSE:
- INCREASE OR DEFEAT SETPOINT RAMPING.
- EXTEND OUTPUT POWER LIMITS.
- DECREASE PROPORTIONAL BAND.
- DECREASE INTEGRAL TIME.
- DECREASE DERIVATIVE TIME.
- CHECK CYCLE TIME.
PROCESS RESPONSE EXTREMES
MANUAL TUNING
A chart recorder or data logging device is necessary to measure the time
between process cycles. This procedure is an alternative to the controller’s Auto-
Tune function. It will not provide acceptable results if system problems exist.
1. Set the Proportional Band () to 10.0% of the input range for temperature
inputs and 100.0% for process inputs.
2. Set both the Integral Time (I) and Derivative Time () to 0 seconds.
3. Set the Output Cycle Time in Output Module  to no higher than one-tenth
of the process time constant (when applicable).
4. Place the controller in Manual () Control Mode () and adjust the %
Power to drive the process value to the Setpoint value. Allow the process to
stabilize after setting the % Power. Note:  must be set to  in
Parameter Lockouts Module 3C.
5. Place the controller in Automatic () Control Mode (). Place the value
of % power into the Output Power Offset (
OPOF
). If the process will not
stabilize and starts to oscillate, set the Proportional Band two times higher
and go back to Step 4. Also put Output Power Offset (
OPOF
) back to zero.
6. If the process is stable, decrease Proportional Band setting by two times and
change the Setpoint value a small amount to excite the process. Continue
with this step until the process oscillates in a continuous nature.
7. Set the Proportional Band to three times the setting that caused the oscillation
in Step 6.
8. Set the Integral Time to two times the period of the oscillation.
9. Set the Derivative Time to 1/8 (0.125) of the Integral Time.