In some cases, the manipulated parameter of one control loop can impact the controlled parameter of another control loop as illustrated below.
To allow both loops to operate in automatic mode, loop interaction is most often addressed by simply detuning one of the control loops by reducing the proportional gain. The valve (or another final control element) associated with the detuned loop will change position slowly. Thus, the two loops will tend not to interact but at the expense of higher variability in the de-tuned control loop.
When model predictive control, MPC, is used to address in a control application, the interaction of the manipulated inputs and controlled outputs may be addressed automatically while providing the best performance for all control loops. As addressed in Chapters 11 and 12 of Advanced Control Foundation – Tools, Techniques, and Applications, the impact of each manipulated input parameter on each controlled output parameter is identified by the step response model used in MPC block generation. Thus, any interactions are automatically compensated for when using model predictive control.