WTI Inc. manufacturers single-stand and dual-turret winders. Typically dancer-controlled, center-driven, these units are powered by dc drives. As the web passes through a dancer, Figure 1, the magnitude of the pressure in the air cylinder establishes the web tension.
The dancer position sensor trims the speed of the dc motor to maintain constant web tension by keeping the dancer in its center position. If the dancer falls, an error signal is created that increases the roll speed. If the dancer rises, the error signal slows the roll speed.
While testing new winders with higher line speeds and larger build-up ratios, common winder problems rapidly surfaced. When the diameter of the roll was small and near empty core, the winder performed well. As the winder started, it wound the material and lifted the dancer to its center position. The winder could accelerate to 2,000 fpm without any problems. While the roll diameter is small, there was very little dancer movement.
However, as the diameter approached 30 in., the winder performance deteriorated creating a telescoping roll (created by excessive tension) or one with air gaps (caused by too little tension). As the roll diameter approached 52 in., the performance was poor and the roll was unacceptable. Also, when the winder was enabled at zero speed with a nearly full roll, the control would try to wind the material and lift the dancer, but it was unable to raise the dancer to the required center position. If such a winder were in a production installation, the line speed would have to accelerate at a slow rate. Of course, this constraint is unacceptable.
If the control was tuned so it delivered proper response with a large roll, the winder reacted so fast that it broke the web when the winder was started with a small roll. At other roll diameters, the winder was unstable causing the dancer to hit its travel limit and produce poor roll quality. One possible cause of the problem could have been an undersized motor. However, a computer simulation at Amicon Inc., Charlotte, N.C., confirmed that the motor was properly sized.
If the motor is sized properly, poor winder performance generally has two causes, both related to the change in mechanics as roll diameter increases.
• The mechanical system gain changes as the roll diameter increases, Figure 2. At empty roll, one revolution of the shaft with an empty 3-in. core winds 9.4 in. of material. By contrast, one evolution of a full roll winds 157 in. of material — a mechanical system gain change of 1 to 16.7.
• As roll diameter increases, so goes roll inertia. As Mark Dudzinski, CEO of Amicon points out: “Although an empty core weighs 50 lb and a full roll weighs 4,000 lb, the inertia increases as the square of the radius. In this case, an empty core has an inertia of 0.4 lb-ft2, and the inertia of a full roll is 8,680 lb-ft2, a factor of 1 to 22,000.”
Therefore, with increasing mechanical system gain, the drive-system gain must also adjust as the roll builds up. Moreover, the inertia compensation in the control circuit must increase as the roll inertia changes. Without these capabilities, roll quality will suffer.
Controller changes gain
To enable their new winders to produce quality rolls at faster speeds and higher build ratios, the engineers at WTI Inc. selected a RollPerfect Winder Control from Amicon as the standard system for controlling the operation of dc drives on their winders.
In operation, the control generally receives signals that are proportional to line speed, dancer position, and winder motor speed. The microprocessor-based control combines this information with parameters entered by the operator and establishes the instantaneous motor speed.
Because WTI manufacturers a wide range of winders and unwinders, the engineers select the applicable RollPerfect controller to meet the needs of the specific unit.