Quick Answer:
Short Answer
For optimal performance and stability, the sensor must be located as close as possible to the exit roller of the web guide. The specific placement depends on the type of guiding application.
Intermediate Guides (Displacement & Steering Guides)
For web guides located within the machine process, the sensor must be placed in the exit span (the web span immediately following the guide roller).
- The "1/3rd Rule": The sensor should be located in the upper 1/3rd (or at least the first half) of the exit span.
- Avoid Delays: Placing the sensor too far downstream or in the next span creates time phase lag (delay). This causes the guide to continue moving even after the error is corrected, leading to system instability and oscillation.
- Steering Guides: The sensor should be placed as close as possible to the exit roller within the exit span.
Terminal Guides (Unwind & Rewind)
Because terminal guides move the entire roll stand, the sensor mounting acts differently:
- Unwind Guides: The sensor must be fixed to the machine frame (it does not move with the stand). It should be placed immediately downstream of the last shifting idler on the unwind stand.
- Rewind Guides: The sensor must be attached to the moving rewind stand so it moves with the carriage (chasing the web). It should sense the web position just ahead of the last fixed idler using a mechanical arm connected to the rewind stand. Ideally, this is as close to the winder as possible to minimize instability caused by mechanical arm stiffness.
Plane Change Considerations
If the motion of the web guide causes the web plane to twist or shift significantly (common in line guiding), it can cause focus issues for optical sensors. In these cases, a backup roller or dead bar should be installed, and the sensor should be mounted to look at the web directly over this stabilizer to maintain a constant focal distance.