H∞ unwinding web tension control of a strip processing plant using a pendulum dancer

All flexible materials such as textiles, papers, polymers or metals are handled on rollers during their processing. Maintaining web tension in the entire processing line under changing web speed is a key factor for achieving good final product quality. Two approaches are mostly used for tension control: load cells which provide direct web tension measurement or dancers which indirectly regulate web tension via regulation of dancer position. Dancers are mobile mechanisms located in specific area of the processing line to regulate web tension. The focus of this paper is to propose an improved alternative to web processing industry practice (PI controllers) based on H_infin synthesis for pendulum dancers. Even though pendulum dancers are widely used on industrial plants, they have been studied very rarely. The nonlinear and linearized phenomenological models of the subsystem containing the dancer are first proposed. The position controller of the dancer is then synthesized using the standard H_infin framework and the mixed sensitivity approach. The performance of the proposed approach is illustrated by experimental results carried out on a large experimental web handling platform containing four driven rollers and the pendulum dancer in the unwind section. These are the first published results of successful application of an H_infin controller to a real plant containing a pendulum dancer.