Producing rolls of a fixed length is a great selling point and may even be a requirement. Web length measurement is normally accomplished with a length or footage counter. This is a wheel of known circumference (1 ft or 0.25 meter) that rides along the surface of the web on a roller near the winder. The length counter can signal the winder to perform an index and cut at a specific length. Slitters may be signaled to slow at a certain length and then stop at the target length.
How accurate can this be? It cannot be exact. There are too many assumptions.
- The web may shrink or expand after being measured.
- The thickness of the web affects its length – do we measure on the roller or the outside of the web?
- Does the web ever travel in the reverse direction (ex. roll back before starting after a cut).
- We assume the measuring wheel doesn’t slip.
- We assume the measuring wheel is the same temperature each day.
- The length counter wheel may wear.
- The PLC has a scan time which introduces error.
I have made very accurate rolls for some customers. That would be +80 mm/-0 mm on a 30-meter roll. These were calibrated by rolling them out on the floor and measuring with a tape. Helping to achieve this accuracy involved a slow line speed, and stopping to make a square cut (no serrated knife cut a full speed). The roll was also shorter than the building so we could unroll it in the aisle.
The next question to ask is how does your client know your roll is too long or too short?
The length measurement can be improved, but error cannot be eliminated. Other technology such as laser interferometers promise higher accuracy at the time the web passes the sensor. Please see my article Achieving precise roll-length measurement and control in Converting Quarterly 2018 Quarter 2, pp. 46-52.