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PID TUNING EXPLANATIONS
AUTO-TUNE
Auto-Tune is a user-initiated function where the controller automatically determines the
Proportional Band, Integral Time, Derivative Time, Digital Filter, Control Ouput Dampening
Time, and Relative Gain (Heat/Cool) values based upon the process characteristics. The
Auto-Tune operation cycles the controlling output(s) at a control point three-quarters of the
distance between the present process value and the setpoint. The nature of these oscillations
determines the settings for the controller's parameters.
Prior to initiating Auto-Tune, it is important that the controller and system be first tested.
(This can be accomplished in On/Off Control or Manual Control Mode.) If there is a wiring,
system or controller problem, Auto-Tune may give incorrect tuning or may never finish.
Auto-Tune may be initiated at start-up, from setpoint or at any other process point. However,
insure normal process conditions (example: minimize unusual external load disturbances) as
they will have an effect on the PID calculations. Auto-Tune cannot be initiated while running
a profile.
Start Auto-Tune
1. Enter the On/Off Control Hysteresis value.
(For most applications, 10 is a suggested value.)
2. Enter the Deadband value, if using OP2.
(For most applications, 0 is a suggested value.)
3. Enter the Setpoint value.
(If Auto-Tune overshoot is unacceptable, then lower the value and restart.)
4. Enter the Auto-Tune Code. (See Figure for details)
5. Enter '1' in the Auto-Tune Start register . (Channel A 40011/Channel B 40027).
6. The Auto-Tune LED will come on.
Auto-Tune Progress
The controller will oscillate the controlling output(s) for four cycles. The cycling
phase can be monitored from the Auto-Tune Phase Register (Channel A 40012/
Channel B 40028). 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 cycles are complete. If the controller remains in Auto-Tune unusually
long, there may be a process problem. Auto-Tune may be stopped by entering '0' in
Auto-Tune Start Register (Channel A 40011/Channel B 40027).
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
OVERSHOOT AND OSCILLATIONS
INPUT
SP
TO DAMPEN RESPONSE:
- INCREASE PROPORTIONAL BAND.
- INCREASE INTEGRAL TIME.
- USE SETPOINT RAMPING.
- USE OUTPUT POWER LIMITS.
- RE-INVOKE AUTO-TUNE WITH A
HIGHER AUTO-TUNE CODE.
- INCREASE DERIVATIVE TIME.
- CHECK CYCLE TIME.
SETPOINT
AUTO-TUNE
CONTROL
POINT
AUTO-TUNE
START
Output 1 (OP1) :
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.
PROCESS RESPONSE EXTREMES
INPUT
SP
TIME
19
AUTO-TUNE CODE FIGURE
INPUT
SP
2
1
0
AUTO-TUNE OPERATION
(REVERSE ACTING)
INPUT
½ HYS *
½ HYS *
PHASE
1
2
ON
OFF
* - On/Off Control Hysteresis
SLOW RESPONSE
TO QUICKEN RESPONSE:
- DECREASE PROPORTIONAL BAND.
- DECREASE INTEGRAL TIME.
- INCREASE OR DEFEAT SETPOINT RAMPING.
- EXTEND OUTPUT POWER LIMITS.
- RE-INVOKE AUTO-TUNE WITH A
LOWER AUTO-TUNE CODE.
- DECREASE DERIVATIVE TIME.
TYPICAL RESPONSE CURVES WITH
AUTO-TUNE CODES 0 TO 2.
TIME
AUTO-TUNE COMPLETE, PID
SETTINGS ARE CALCULATED
AND LOADED INTO MEMORY
TIME
3
4
ON
OFF
TIME
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