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.