Override Control With Pid_Compact V2 - Siemens SIMATIC S7-1200 Function Manual

4.2.3

Override control with PID_Compact V2

Override control
In case of override control, two or more controllers share one actuator. Only one controller
has access to the actuator at any time and influences the process.
A logic operation decides which controller has access to the actuator. This decision is often
made based on a comparison of the output values of all controllers, for example, in case of a
maximum selection, the controller with the largest output value gets access to the actuator.
The selection based on the output value requires that all controllers operate in automatic
mode. The controllers that do not have an effect on the actuator are updated. This is
necessary to prevent windup effects and their negative impacts on the control response and
the switchover between the controllers.
PID_Compact supports override controls as of version 2.3 by offering a simple process for
updating the controllers that are not active: By using the tags OverwriteInitialOutputValue
and PIDCtrl.PIDInit, you can pre-assign the integral action of the controller in automatic
mode as though the PID algorithm had calculated Output = OverwriteInititalOutputValue for
the output value in the last cycle. To do this, OverwriteInitialOutputValue is interconnected
with the output value of the controller that currently has access to the actuator. By setting the
bit PIDCtrl.PIDInit, you trigger the pre-assignment of the integral action as well as the restart
of the controller cycle and the PWM period. The subsequent calculation of the output value
in the current cycle takes place based on the pre-assigned (and synchronized for all
controllers) integral action as well as the proportional action and integral action from the
current control deviation. The derivative action is not active during the call with
PIDCtrl.PIDInit = TRUE and therefore does not contribute to the output value.
This procedure ensures that the calculation of the current output value and thus the decision
on which controller is to have access to the actuator is only based on the current process
state and the PI parameters. Windup effects for controllers that are not active and thus
incorrect decisions of the switchover logic are prevented.
PID control
Function Manual, 03/2017, A5E35300227-AC
Using PID_Compact
4.2 PID_Compact V2
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