Centralized P/PI control system design based on equivalent transfer functions for unstable TITO process

Established equivalent transfer function (ETF) matrix method for stable MIMO systems is extended to unstable systems to design multivariable proportional and integral (PI) controllers. The simplified decoupler decomposes the unstable multi-loop systems into independent loops with ETFs as the resultant decoupled process model having unstable poles. Single loop PI controllers are designed by the synthesis method based on the unstable ETFs. Since the overshoot is higher for the unstable systems, a double-loop structure is used to reduce the overshoot. To stabilize the unstable system, inner loop P controllers are designed for the diagonal elements of ETFs by the synthesis method. Further, for the stabilized inner-loop system, diagonal PI controllers are designed based on the identified simple First order plus time-delay (FOPTD) model. Simulation results show that the designed control system gives a better performance than that reported in literature.