H∞-based PI-observers for web tension estimations in industrial unwinding-winding systems

The system under study is a web handling machine composed of an unwinder, several traction motors, several idlers with or without load cells and a winder. All flexible materials such as textiles, papers, polymers or metals are handled on rollers during their processing. Maintaining strip tension in the entire processing line while increasing web speed is a key factor for achieving good final product quality. Due to sources of disturbance and the high coupling introduced by elastic webs, robust multivariable control strategies such H∞-controllers require the knowledge of web tension in each span of the process. Estimators or observers represent a cost-effective method in order to limit the number of load cells or dancers. After a summary of the main laws used for system modeling, two different approaches to design tension observers in a section of a process line are presented and discussed. The first approach is based on a PI-observer calculated in a H∞ sense thanks to optimization techniques. The second approach uses Kalman filtering theory. Both approaches underline the importance of friction in estimation accuracy and propose alternatives to face this issue. This friction on rollers are generally neglected or assumed to be well-modeled in the literature. The proposed observers are also analyzed and discussed with variations in friction torques and nominal set points of the web velocity and tension. Both observers have the major advantage of being easily understandable for industrial applications and can be rapidly programmable in industrial plant controllers.