Quick Answer:
Short Answer
Standard web guiding measures web position relative to the machine frame, not the tool. When the tool moves, a fixed sensor can't see if the correction worked—the feedback loop is "broken."
Standard web guiding steers the web to a fixed position, but in tool tracking applications like slitting to a printed line, the sensor must see the relative position between the tool and the web.
Here's why fixed sensors fail:
- Fixed sensor upstream: The sensor sees web wander and commands the slitter to move—but it never sees the slitter move. The error signal remains, and the control loop is open.
- Fixed sensor downstream: By the time you detect misalignment after the slitter, the cut has already happened. You cannot correct a cut that's already made.
The only solution is to mount the sensor on the moving tool carriage. When the carriage moves, the sensor moves with it, verifying the correction. This closes the feedback loop and enables true chasing.