General

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. 

• Manual Sensor Respositioning: If the web is guided to the centerline position using single-edge methods, the sensor must be moved for every width change, increasing setup time and complexity. 
• Possible Operator Error: Physical repositioning relies on the operator to correctly locate the sensor relative to the machine and securely lock it in position. Mistakes here can lead to misalignment. 
• Fork Sensor Issues: Conventional fork stype sensors could snag the web material if the sensor is not moved before the width change. And even after width change the sensor needs to be repositioned to ensure that the web is inbetween the two sensing arms. 

Legacy systems attempt to solve this with mechanical sensor positioners (moving sensor center guides or even moving a single sensor), but these introduce additional mechanical wear and tear. Roll-2-Roll® Sensors solves this with sensors and adjusting the electronic guide point using Roll-2-Roll® Controller, eliminating the need to physically move the sensor when the web width changes. 

More importantly the Roll-2-Roll® Sensors are single sided completely eliminating the possibility of snag or web threading issues during width change over.