Seamless PID–MPC Hybrid Control

It is common that PID control loops function satisfactorily most of the time, but that they have issues with violating input, state, or output constraints. While MPC solves this problem in principle, it is in practice not straightforward to replace a functioning PID controller with an MPC implementation. This is particularly true for loops that are critical to plant operation, where stops associated with controller replacement and tuning can turn out costly. We propose a novel configuration where an MPC controller is designed based on the PID closed-loop, and provides a feed-forward term to its control signal. This configuration enables a smooth transition between unconstrained PID operation and MPC-dominated control, governed by a single scalar parameter α. We demonstrate the approach using a relevant process industrial application: flotation level control in mineral enrichment. Using a practical example, we show how our approach results in a structure where the PID control loop can be kept, while leveraging the constraint-honoring advantage of MPC control.