A2: Rules For Adjusting Pid Controllers - Burkert 8205 Notice D'utilisation

APPENDIX
PH CONTROLLER 8205

A2: Rules for adjusting PID controllers

The litterature on control systems specifies a series of adjustment rules with which a favorable
adjustment of controller parameters can be achieved experimentally. To avoid bad adjustments,
the conditions under which the respective adjustment rules have been elaborated must always
be observed. In addition to the characteristics of the controlled system and of the controller itself,
it is important to know whether it is intented to balance out a disturbance change or a command
variable change.
Adjustment rules according to Ziegler and Nichols (oscillation method)
When using this method, controller parameters are adjusted on the basis of the control loop's
response at the stability limit. In doing so, the controller parameters are adjusted so as to ensure
that the control loop begins to oscillate. A conclusion as to a favorable adjustment of the controller
parameters is reached from critical characteristic values occurring in this case. It goes without
saying that, when using this method, it must be possible to bring the control loop to oscillation.
Method:
- Set the controller as a P controller (i.e. Tn = 999, Tv = 0), initially selecting a low Kp value.
- Set the required setpoint.
- Increase Kp until the controlled variable oscillates continuously without attenuation (see following
figure).
X
Actual value
Tcrit
t
Figure : Progression of the control variable at the stability limit
The proportional action coefficient set at the stability limit is referred as Kcrit. The resulting
oscillation period is referred to as Tcrit.
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