Quick Answer:
Short Answer
In a rewind application, the sensor is attached to the moving frame because the system is chasing the web, not guiding it.
Unlike other forms of guiding, the goal is to align the rewind roll to the position of the incoming web, ensuring a straight wound roll.
Specific reasons for this configuration include:
- Chasing Control System: Rewind guiding is fundamentally a "chasing control system" rather than a lateral control system. The rewind stand must physically move to position the winding roll directly downstream of wherever the web happens to be.
- Maintaining Relative Position: If the sensor were fixed to the ground (as it is in unwind guiding), the system would not know the relative position between the winding roll and the incoming web. By attaching the sensor to the moving stand, the sensor moves with the carriage. When the sensor detects the web edge, the controller moves the entire stand (and the attached sensor) until the sensor is re-aligned with the web, effectively "chasing" it to maintain the correct relationship.
- Roll Alignment: This setup ensures that the edge of the rewind roll is constantly aligned with the edge of the approaching web, regardless of where the web is drifting laterally.
Ideally, this sensor should be located just ahead of the last fixed idler in the machine and as close to the winder as possible to minimize instability caused by the mechanical arm's stiffness.