Burkert 8630 Operating Instructions Manual page 133

8.3 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).
The proportional action coefficient set at the stability limit is referred as Kcrit. The resulting oscillation period is referred to as Tcrit.
Actual value
X
Figure 8.7 : Progression of the control variable at the stability limit
On the basis of Kcrit and Tcrit, the controller parameters can then be calculated in accordance with the following table.
Parameter settings according to Ziegler und Nichols :
Controller Type Parameter settings
P controller Kp = 0,5 Kkrit
PI controller Kp = 0,45 Kkrit
PID controller Kp = 0,6 Kkrit
The Ziegler and Nichols adjustment rules were determined for P systems with a time delay of the first order and a dead time. However,
they apply only to controllers with a disturbance response, but not to controllers with a command response.
Adjustment rules according to Chien, Hrones and Reswick (manipulated variable methode)
When using this method, the controller parameters are adjusted on the basis of the control system's transition response. A 100%
change in the manipulated variable is output. The time Tu and Tg are derived from the progression of the actual value of the control
variable (following figure).
Top Control Positioner
Istwert
-
Tn = 0,85 Tkrit
Tn = 0,5 Tkrit
T krit
T
krit
-
-
Tv = 0,12 Tkrit
Zeit
Time
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