Robust decentralised control strategies for large-scale web handling systems

Abstract

In web-transport systems, the main concern is to independently control speed and tension in spite of perturbations such as radius variations and changes of set point. Nevertheless, in these control techniques, tracking properties and perturbation rejection are interdependent and cannot be specified separately. In this paper, the authors first present multivariable decentralised H ∞ controllers with one or two degrees of freedom (DOF), with and without explicit integrator, applied to winding systems. Different controller structures are then considered: centralised and semi-decentralised controllers with or without overlapping. The 2DOF approach significantly improves the disturbance rejection while reducing the coupling between tension and velocity. The use of decentralised control leads to performance deterioration as compared to centralised control. The off-diagonal elements of the plant are represented as uncertainties. This enables to analyse the stability of a decentralised control structure. Finally, multivariable H ∞ controllers are presented, with and without integrator, applied to winding systems and determined by a bilinear matrix inequality (BMI) approach. This method consists in designing state-feedback control with the use of BMI optimisation. Insertion of full or partial integral action is discussed in this paper. Simulation results are given based on a nonlinear model identified on a 3-motor winding test bench. Experimental results have also been obtained with this set-up.