MULTIVARIABLE CASCADE CONTROL FOR PROCESSES WITH OUTPUT CONSTRAINTS

We present a cascade control strategy to treat output constraints and the switching of controls between automatic and manual in multivariabl e systems. The inner loop of the cascade is a multivariable IMC contro ller implemented in a model state feedback form so as to accommodate p ossible control effort saturation. The inner loop controls the output constrained variables to set points provided by the outer loop. In add ition, the inner loop controls as many of the unconstrained process va riables as the number of degrees of freedom allows. The outer loop adj usts the set points of the constrained variables, within their constra ints, so as to: 1)achieve the set points of the unconstrained process variables, 2) drive excess controls to desired nominal values and 3) f orce controls which have ceased functioning due to equipment malfuncti on or due to being placed in manual operation., to their fixed values. Also, the outer loop may adjust unconstrained process variable set po ints to achieve similar objectives. A single input, two output process , and two, two input, two output processes are used to demonstrate the behavior of the new cascade constraint control algorithm. There is no performance difference between the new algorithm and the traditional override control method for the single input, two output process, with , and without modeling errors. For the two input, two output processes , with and without deadtimes, the new algorithm successfully accommoda tes both control effort and process variable constraints. One control effort can be readily transferred between automatic and manual modes w hile maintaining control of either the output constraint, if it is act ive, or the unconstrained process variable, if the output constraint i s not binding.