9.2 Closed-Loop Control

A dancer roller is a roller that pushes into the web with a controllable force. The dance of a dancer roller, in either a pivoting or sliding motion, is how it moves to accumulate or dispense web in response to any speed variations between the nearest upstream and downstream driven elements. If the incoming web is faster than the outgoing web, the dancer will accumulate. If the incoming web is slower than the outgoing web, the dancer will dispense.

When used as a tension feedback device, the position of the dancer is monitored and used to adjust the speed of one driven element relative to another, maintaining a relatively constant tension within the range of accumulation and preventing the tension extremes of web breaks or slackness. The dancer is usually wrapped with a 180-degree wrap, so the tension created by the roller load is relatively consistent through the accumulate and dispensing range. A dancer has a null position, usually defined as the center of its travel range.

The feedback of the dancer is the distance it moves from the null position, usually monitored with a linear or rotational transducer. If the input web is at speed V1 and the output web is at V2, the amount of web that needs to be accumulated or dispensed is (V1-V2) x time. The greater the speed differential and longer the differential exists, the more dancer range is needed. In a closed-loop tension control system, the dancer position is fed to a controller that quickly adjusts the input our output speed to drive the dancer back to its null position. In operation, a dancer roller will constantly dither between above and below the null point.

Dancers need to dance (accumulate and dispense), but this motion will be opposed by Newton’s first law (sometimes called the Law of Inertia).

An object at rest will remain at rest unless acted upon by an external and unbalanced force.
An object in motion will remain in motion unless acted upon by an external and unbalanced force.

So for a dancer to dance, the forces applied to it must overcome inertia. Inertia is related to mass, so reducing mass improved a dancer roller’s performance. Besides inertia, dancers also need to avoid high friction, and for dancers using pneumatic cylinders to increase or decrease the load on the web, the pneumatic system should avoid significant restrictions to the dancer’s dance.

In general:

• Minimize friction and inertia (dancers are the most logical application of low inertia idlers)
• Avoid using weights to counter-balance a dancer roller. Instead, use either a horizontal dancer motion or a pneumatic counterbalance.
• To avoid hysteresis, design any pneumatic system with sufficient piping diameter, high volume flow controllers, and air accumulators.
• Reduce geometry-related tension variations with 180-degree wrap and sufficient entry and exit span lengths.
• Magnify the measurement of the dancer roller motion by gearing it with a mechanical advantage over the rotational encoder.
• Pivoting dancer assemblies will almost always have fewer stick-slip and misalignment problems than linear dancers.
• As with any pivoting element, make sure your equipment is design for ease of measuring and aligning the dancer roller’s pivot shaft. A dancer roller’s parallelism starts with a well-aligned pivot shaft.
• Sufficient range to accumulate and dispense the length of web required based on DV x time. Usually, a dancer range of 200-250 mm (8-10 inches), storing 0.4-0.5 m (16-20 inches) of web, is enough.