Implementation of predictive controllers as outer-loop controllers

The implementation of predictive controllers as supervisory control systems is discussed. In this configuration, common in practice, the manipulated variables calculated by the predictive controller do not directly drive the actuators; instead, it is the command signals from the local PID controllers that drive the actuators that act on the physical system. This structure is known to perform less efficiently when the local controllers are re-tuned, because the model used in the predictive controller differs from the real one. Thus, performance decreases and might even lead to oscillations and instabilities of the overall control system. This paper presents a solution to this problem based on the use of an adequate two-degrees-of-freedom structure as an interface between the predictive controller and the local controllers, which makes the predictive controller robust against variations in the local controller. The application of this technique on a simulated industrial boiler achieves the following advantage: the performance of the predictive control system is maintained despite re-tuning of the inner-loop controller.