Improved constrained cascade control for parallel processes

This paper addresses, first, the problem of constraint handling for a syste m with one input and multiple outputs, where one output must reach a given set point and the other outputs must lie between lower and an upper limits. Three algebraic solutions based on cascade control are outlined. One metho d employs the traditional cascade controllers, applied to serial transfer f unctions. The second uses cascade controllers applied to parallel transfer function processes. The latter method shows sensitivity to disturbance and tuning of inner loops. A third innovative method, called a pseudo-cascade c ontroller, is introduced for parallel transfer functions. The new method al lows independent tuning of the controllers, and requires no special anti-re set windup feature. An extension is also given for decoupled two-input proc esses. A simulated example and a distributed control design for an industri al application are given to illustrate the proposed methods. (C) 1999 Elsev ier Science Ltd. All rights reserved.