Automatic control of unwind tension in film finishing applications

Tension control is an important function of film finishing machines. A dancer roll assembly is one common method for controlling the tension in the unwind section of such machines. The dancer roll imparts a constant tension on the film as long as it does not reach its physical limits of travel. Therefore, the stockroll´s rotational position and velocity must be controlled so that the dancer remains near the center of its travel regardless of any disturbances to the system. This must be done in a way that keeps acceleration forces from greatly affecting the film tension. The changing stockroll diameter and changing inertia due to the unwinding film makes the control of such a system more complex. It is the intent of this paper to analyze a simplified version of this system. The system consists of a motor driven unwind feeding a dancer roll assembly. A controller is developed to meet the dancer position and acceleration requirements based on computer simulations