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DIGITAL INDUSTRIAL CONTROLLER
english
The literature 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
intended 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. Tr = 999, Td = 0), initially selecting a low
Kp value
• Set the required setpoint
• Increase Kp until the controlled variable oscillates continuously without
attenuation (Figure 44).
The proportional action coefficient (gain) set at the stability limit is referred as Kcrit.
The resulting oscillation period is referred to as Tcrit.
9.2 Rules for adjusting PID controllers
Figure 44: Progression of the control variable at the stability limit
PV Actual value
Tcrit
t
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