Ideally in this case the contact between the roller and the web is only a contact line with zero wrap angle. But this components needs a warp angle to work correctly. If you add a small displacement in the y direction of your nip roller you get a contact arc and the model runs. This would have not much influence on your model.

1. First, in this example the speed of the web line is defined by the master speed of the nip roller in the middle: ω=v/r with r=radius of the nip roller. 
   It is the same for the unwinder. But the radius of the unwinder must be known. (In practice it can be measured directly or indirectly.) This is the speed definition in this example.

2. Second, the control of the angular of the dancer is a possibility for a tension control. The angular corresponds to the tension.
   If the tension is too high for this side the dancer moves up. If the tension is too small the dancer moves down in this example. Based on this the controller calculates a change for the speed of the unwinder. If you need more tension the unwinder must rotate slower. If you need less tension the unwinder must rotate faster.