NIP Roll Control

There are two ways to control the tension of the web for a winding machine. One is dancer, another is nip. In this application profile, we will be discussing nip roll control.

Specific we only control tension T1 as an example. When the speed V2 of the Nip roll is faster than the web speed V1, the tension T1 will be increased and the web will be stretched. When the speed V2 is slower than V1, the tension T1 will be decreased and the web will be loose. It looks like we can control the speed V2 of Nip roll to maintain the tension. But the problem is that the speed error will make a dozen times error on tension. So adjusting speed to control the tension is not a good direction.

If we could control the tension by controlling the torque of Nip roll, then a number error in torque would produce the same number error in tension because torque is 1:1 proportional to tension. Tq=T*R, Tq=Torque, T=Tension, R=Radius of the roll. This is the right direction to control tension.

How to control the torque of Nip roll? We can use VFD to drive the motor of Nip roll and run VFD in torque control mode.

What kind of VFD should be used for this control? Most VFDs have torque control mode. Actually, torque control is the inner loop of VFD control for these kinds of VFDs. Speed control proceeds as one of the torque references. There are two ways to control torque, one is Flux vector control using PWM, and another is Direct torque control DTC.

Flux vector control simulates DC drive. To perform the field orientation process, the flux-vector drive needs to know the spatial angular positon of the rotor flux inside the AC induction motor. It means, feedback (encoder) is needed for better performance and response of torque control. The controlling variables are voltage, current and frequency. Torque is controlled INDIRECTLY.

With the revolutionary DTC technology developed by ABB, field orientation is achieved without feedback using advanced motor theory to calculate the motor torque directly and without using

modulation. The controlling variables are motor magnetizing flux and motor torque.

ABB has ACS800, ACS880 and ACSM1 drives with DTC control. We can use any kind of these type drives for the NIP roll control application.

Once NIP control is put in, part of the torque will match with the line speed of the web, the left torque will make a little change on the speed to maintain the proper tension T1 on the web, which is perceived by the operator through his hand.

Bellow are the setting for ACSM1 for the application in Olympic plastic bag.