Achieving Decoupling with Predictive Controllers

Predictive controllers such as Dynamic Matrix Control (DMC) and Internal Model Control (IMC) are formulated as true multivariable control systems rather than as traditional multiloop systems. For many processes such predictive controllers perform well with a minimum of interaction between controlled variables. However, for some processes strong coupling between outputs can occur when predictive controllers are used to implement step changes in set points. A new technique is presented which yields decoupled set-point changes while retaining the original load disturbance rejection properties of the controller. The technique uses a particular time-varying set-point trajectory to enforce decoupled set-point changes in the closed-loop system.