Robust decoupling control synthesis

The control of multivariable industrial processes is frequently performed using a multiloop configuration, in which several PID controllers are committed to control different channels of the plant. A difficulty with such a strategy arises due to the interaction among the control loops, which may cause the control action in a loop to give rise to significant disturbances in other loops. In some cases, it is mandatory to consider a decoupling control that includes a decoupling precompensator, or decoupler, to guarantee acceptable decoupling among the control loops. This paper presents a new robust decoupling control synthesis procedure for multiloop control systems which aims to decouple the different channels of the multivariable system and to guarantee the tracking response performance. The control problem is stated as a non-convex optimization problem which is formulated directly in the space of the PI/PID controllers and precompensator parameters. Polytopic models represent the system uncertainty. An application example is developed for the control of a quadruple-tank process with emphasis in dealing with the control decoupling when the system is working on a non-minimum phase operating point.