Red Lion PAX2C - Manual (Page 51)

51
AUTO-TUNE PROGRESS
The controller will oscillate the controlling output(s) for four
phases. The bottom display will flash the phase number. Parameter
viewing is permitted during Auto-Tune. The time to complete the
Auto-Tune cycles is process dependent. The controller should
automatically stop Auto-Tune and store the calculated values when
the four phases are complete. If the controller remains in Auto-
Tune unusually long, there may be a process problem. Auto-Tune
may be stopped by entering NO in Auto-Tune Start tUNE.
TIME
SP SP
TIME
INPUT INPUT
OVERSHOOT AND OSCILLATIONS SLOW RESPONSE
TO DAMPEN RESPONSE:
- USE SETPOINT RAMPING.
- USE OUTPUT POWER LIMITS.
- RE-INVOKE AUTO-TUNE WITH A
HIGHER AUTO-TUNE CODE.
- INCREASE PROPORTIONAL BAND.
- INCREASE INTEGRAL TIME.
- INCREASE DERIVATIVE TIME.
TO QUICKEN RESPONSE:
- INCREASE OR DEFEAT SETPOINT RAMPING.
- EXTEND OUTPUT POWER LIMITS.
- RE-INVOKE AUTO-TUNE WITH A
LOWER AUTO-TUNE CODE.
- DECREASE PROPORTIONAL BAND.
- DECREASE INTEGRAL TIME.
- DECREASE DERIVATIVE 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 (ProP) to 10.0% for temperature models
(Temperature) and 100.0% for process models (Voltage/Current).
2. Set both the Integral Time (Intt) and Derivative Time (dErt) to 0 seconds.
3. Set the active PID Power Filter (FLtr) in the PID Menu to 0 seconds.
4. Set the Output Cycle Time (CYCt) in the Digital Output Menu to no higher
than one-tenth of the process time constant (when applicable).
5. Place the controller into Manual Control Mode (MAN) via the trnF parameter
in the PID Menu and adjust the % Power to drive the process value to the
Setpoint value. Allow the process to stabilize after setting the % Power.
6. Place the controller in Automatic (Auto) Control Mode via the trnF parameter
in the PID Menu. If the process will not stabilize and starts to oscillate, set
the Proportional Band two times higher and go back to Step 5.
7. 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.
8. Fix the Proportional Band to three times the setting that caused the oscillation
in Step 7.
9. Set the Integral Time to two times the period of the oscillation.
10. Set the Derivative Time to 1/8 (0.125) of the Integral Time.
11. Set the Output Dampening Time to 1/40 (0.025) the period of the oscillation.
TIME
INPUT
1 4
AUTO-TUNE
START
AUTO-TUNE
CONTROL
POINT
SETPOINT
AUTO-TUNE COMPLETE, PID
SETTINGS ARE CALCULATED
AND LOADED INTO MEMORY
2 3
ON OFF ON OFF Output 1 (OP1) :
PHASE
½ HYS *
AUTO-TUNE OPERATION
(REVERSE ACTING)
½ HYS *
* - On/Off Control Hysteresis
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.