Hello,

Cascading PID controllers

The manipulated variable LMN of the primary controller is connected to input SP_EXT of the secondary controller. The primary controller must also change to tracking mode if the cascade is opened. A QCAS_CUT signal is generated in the secondary controller and interconnected with LMN_SEL input of the primary controller. This disconnection can be caused in manual or tracking mode by setpoint input, or by setpoint tracking of the secondary controller:

QCAS_CUT = NOT(QMAN_AUT) OR LMN_SEL OR SP_TRK_ON OR NOT(QSPEXT_ON)

The tracking input LMN_TRK of the primary controller is interconnected with the SP output of the secondary controller to prevent a step change when the cascade is closed again.

It is also advisable to set a directional lock of the integrator in the primary controller which is activated as soon as the secondary controller reaches a manipulated variable limit. With a positive control action, interconnect the INT_HPOS and INT_HNEG inputs of the primary controller with the QLMN_HLM and QLMN_LLM outputs of the secondary controller.


I made a simple example from my old power plant projects, wish it can be helpful for you.