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One of the challenges in working with tension control systems isworking with closed loop dancers. While dancers are a seeminglysimple mechanism, there are complexities below the surface that canimpact system performance significantly. This article will definethe key parts of a dancer system and how they can interact toprovide a system that works well or poorly.
A well-built, well-installed dancer working with a propercontrol can provide a flexible closed loop means of controllingtension in both unwind and rewind applications.
We will use a simple unwind system to discuss the various partsof a dancer system.(Please note that there are many possible dancerdesigns, but for clarity's sake, we will restrict ourdiscussion to the simple design in Figure 1.) As a web of material(or strand of wire) unwinds, it passes over a roller, around thedancer roll, and then over a third roller.
The dancer roll is allowed to float (rise/fall) as materialfeeds into the process. As the dancer roll rises and falls, thetension system is adjusting for minor changes in tension. In mostwell-designed systems, the dancer roll will rise and fall in smallincrements constantly. In short... it dances!
In a closed loop dancer unwind system, there are several keycomponents: the dancer itself, a feedback potentiometer, a control,and the brake or drive. The goal in a dancer system is for thedancer to remain at the mid-point of travel. Pressure on the dancerestablishes the tension for the web. The brake in the systemcontrols how fast material is allowed to unwind. Brake pressurechanges based on feedback from the dancer potentiometer to thecontrol and from the control to the brake or drive.
If the process is consuming material faster than the unwind isreleasing...