Decentralized control of coupled nonlinear dynamic systems with application to quadruple-tank process

This paper presents a novel method of control system design for nonlinear interconnected large-scale systems. Conventional control design techniques often involve linearization of the system model with respect to an operating state, which may lead to incorrect control performance for systems that are highly nonlinear and interconnected. In this paper, a decentralized control design consisting of a PI controller with constant feedback gains is considered. Then a nonlinear optimal control method based on integral minimum principle of Pontryagin is developed. In this method, the controller performance is defined using a standard quadratic cost function, and the control law is defined using the PI structure with constant feedback gain. The effectiveness of the developed control scheme shows that the controller is capable of maintaining the desired water level in the four tanks.