Minimal controller structure realization in decentralized control of unknown systems

The following type of problem is considered. Given a linear time-invariant system, assume that there exists a solution to the robust servomechanism problem for the system, for the case of a given class of disturbance/reference input signals. Then it is desired to implement a controller for the system, which solves the robust servomechanism problem, and such that the resulting controller has the property that the number of input/output interconnections required to implement the controller is minimized. Such a controller is said to have a "minimal controller structure realization". The motivation for studying this problem arises in large scale systems, e.g. chemical process control, where, in general, one would like to find the "simplest possible controller", i.e. one which has a minimal controller structure realization, which satisfactorily regulates the system.