The "fixed sensor rule" is fundamental to unwind guiding physics and is the opposite of rewind chasing:
The Logic: In unwind guiding, you are positioning the web to enter the machine at a specific target location. The sensor acts as that target. The controller moves the shifting stand until the web edge aligns with the fixed sensor position.
The Common Mistake: If you mount the sensor on the moving stand (as you would in a rewind chasing application), the sensor moves with the error. When the stand shifts left, the sensor shifts left too—so from the sensor's perspective, nothing has changed. The system cannot detect or correct the misalignment relative to the machine frame.
Proper Configuration:
- Sensor: Fixed to the machine frame (floor or fixed structure)
- At least one idler roller: Mounted on the shifting stand (moves with it)
- Sensor position: Immediately downstream of the shifting idler
This configuration ensures the sensor sees true web position relative to the machine, while the shifting idler maintains a consistent web plane as roll diameter changes.