Siemens SIMATIC S7-1200 Function Manual page 94

Using PID_Compact
4.2 PID_Compact V2
Both controllers must share one valve as shared actuator. The logic that decides which
controller gets access to the actuator is implemented by a maximum selection of the output
value (in Real format, parameter Output) in this case. Because the output value corresponds
to the opening of the valve, the controller that requires the larger valve opening gets the
control.
Note
Activate inversion of the control logic
Because a decrease of the actual value (pressure) is to be achieved with the pressure
regulator PID_Compact_2 when the output value increases (valve opening), the inversion of
the control logic must be activated: PID_Compact_2.Config.InvertControl = TRUE.
In normal operation of the plant, the actual value of the flow rate corresponds to the setpoint.
The flow controller PID_Compact_1 has settled on a stationary output value
PID_Compact_1.Output. The actual value of the pressure in normal operation is significantly
below the high limit that is specified as setpoint for PID_Compact_2. The pressure regulator
therefore wants to close the valve even further to increase the pressure, which means it will
calculate an output value PID_Compact_2.Output that is smaller than the output value of the
flow controller PID_Compact_1.Output. The maximum selection of the switchover logic
therefore gives the flow controller PID_Compact_1 continued access to the actuator. In
addition, it is ensured that PID_Compact_2 is updated by means of the assignments
PID_Compact_2.OverwriteInitialOutputValue = PID_Compact_1.Output and
PID_Compact_2.PIDCtrl.PIDInit = TRUE.
If the pressure now approaches the high limit or exceeds it, for example due to a fault, the
pressure regulator PID_Compact_2 calculates a higher output value to open the valve even
further and thus reduce the pressure. If PID_Compact_2.Output is greater than
PID_Compact_1.Output, the pressure regulator PID_Compact_2 receives access to the
actuator through the maximum selection and opens it. It is ensured that PID_Compact_1 is
updated by means of the assignments PID_Compact_1.OverwriteInitialOutputValue =
PID_Compact_2.Output and PID_Compact_1.PIDCtrl.PIDInit = TRUE.
The pressure is reduced while the flow rate increases and can no longer be kept at the
setpoint.
Once the fault has been remedied, the pressure will continue to drop and the opening of the
valve is reduced by the pressure regulator. If the flow controller calculates a larger opening
as output value, the plant returns to normal operation so that the flow controller
PID_Compact_1 once again has access to the actuator.
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Function Manual, 03/2017, A5E35300227-AC
PID control