New inverter rewinding software is so good it's a wind up

Rewinding in machines that use a web, such as printing and paper manufacturing or those using a warp such as in textile machines, can create problems in machine control that can lead to reduced cycle times or even web breakages. In response to this long standing problem, Yaskawa has developed special software for its range of AC inverter drives to optimise rewinding control with a quick and easy PID set up.

The net results of Yaskawa's development for the user are reduced acceleration and deceleration ramps, increased line speed, improved control of the winding tension, improved product quality and no web breakages even in emergency stops.
In rewinding the line speed is kept constant and a "Dancer" control provides the required constant tension. Yaskawa's inverters have a built in PID function that controls the Dancer response by adjusting the motor speed. However, the PID control loop must be carefully adjusted to minimise oscillation and obtain the highest possible line speed.

To keep a constant line speed as the diameter of the reel increases, the motor speed clearly must decrease. This creates a problem in that as the reel diameter increases the Dancer position error feedback tries to compensate using the same output frequency (speed) adjustment as it used for the smaller diameter reel. This causes instability and it is clear why when one considers that if the rear reel diameter is, say, 10cm, a 5Hz change in frequency causes a 5m/min line speed increase. Similarly, if the rear reel is 100cm diameter, a 5Hz change in frequency creates a 50m/min line speed increase!
Hence, if the PID gain is set to be optimised for the smallest reel diameter, it is potentially unstable when the reel is at full capacity and vice versa. At the same time, if the rewinder speed is ramped up or down, the high PID gain settings cause an overshoot of the Dancer and this can be sufficient to break the web.

Yaskawa's software enables the inverter's output frequency to be set inversely proportional to the reel diameter. To do this, the software provides two P gain settings, the first for the minimum frequency and second for the maximum. The actual gain is therefore linear between these two points and thus provides the optimum gain settings for the entire speed range. If the machine speed is ramped up, the gain remains low and the chance of Dancer overshoot is minimized. At the required speed, the high gain controls the Dancer position with highly stable response.
The Yaskawa drive's control software increases productivity (line speed), improves quality (the Dancer controls the line tension better) and obviates downtime (the Dancer control remains stable).

Released: November 2002