In general, dancer control or compensator mechanisms and their position sensors are incorporated into velocity mode drive applications for several reasons.
They can provide accumulation or storage of material. When located between two driven sections of a process that may accelerate or decelerate at different rates, the dancer can absorb or store excess material or give up stored material to provide a more stable operating tension level.
How much material a dancer can store in its acceptable range of movement is “running time storage.” More running time storage allows longer response times in the controlled drives and motors and usually results in more stable operation.
With a conventional gravity-operated “swing arm” type dancer, maximum storage equals the length of web material required to drop the dancer from its highest possible position to its lowest possible position. This range of movement is rarely acceptable in real life and would usually cause a dancer “travel limit” fault to occur. True “running time storage” is the length stored in the range of movement that is acceptable by the person(s) qualifying successful operation. Systems supplying 0.5 to 1.0 second or greater running time storage seldom encounter set-up difficulties, though less storage can still be successful with careful adjustment and by minimizing the range of the dancer control circuit.
Since the force exerted by a dancer sets the Tension in the zone where it’s located, the dancer can be used as a direct Tension controlling device when used with adjustable weights, counterweights or pneumatically controlled actuators. Electrically controlled pneumatics can be used to provide Taper Tension control.
A dancer roll tension control system can use the principle of inertia compensation to absorb upsets in tension while keeping web tension consistent throughout the process. Engineers can create inertia compensation by balancing the inertias of the up-and-down motions of the dancer with the rotating inertias.
How dancer control can best be utilized depends on the answers to several questions:
The most common sensor used to signal the dancer position is a potentiometer. There are many “pot” types with different materials and construction features used in their manufacture. Unfortunately, standard pots meant for manual operation are often used in these applications with several negative results:
How the dancer operates mechanically will significantly affect its ability and stability of control. The dancer control mechanism must move freely and track robustly with the material. Pneumatics used to control dancer operating force must not restrict or dampen movement with a “shock absorber” effect.