Fixed structure H_∞ control of couple tank system and anti-integral windup PID control strategy for actuator saturation

This paper considered two main issues. First is about the complete mathematical modelling and Fixed-Structure _∞ robust controller design for couple tank system in frequency domain. This system is nonlinear and classical Proportional Integral Derivative (PID) controller may not give appropriate performance. Although classical PID controllers are more practical due their simplicity and linearity but they have some limitations. Especially in case of nonlinear system or small variations in parameters, they may not give desired transient specification. The proposed controller is a robust controller which contains a fixed structure and fixed order, and its parameters are optimized using non smooth H_∞ optimization in frequency domain. Also Fixed-Structure H_∞ controller also gives satisfied performance in case of parameters variation. Couple tank system is widely used in industry. For example its most important applications are water purification, effluent treatment, dairy filtration, chemical processing and also this system is used in pharmaceutical industry. Actuator saturation is a major nonlinearity that may exist in couple tank systems. And this nonlinearity may lead to performance degradation or instability of controller and whole system. This problem is known as integral windup and to deal this problem, an Anti-Integral Windup PID control strategy is introduced in the second part of this paper.