A scanner is equipped with three to four track sensors that detect the position of the paper as it moves through the track. The camera may also act as one of these sensors.
Tracking the progress of scanned documents through the paper path
The track sensors in a mechanically fed scanner serve a very important but often overlooked purpose: Monitoring the progress of each piece of paper along the track, and communicating that position to the scanner. This is how the scanner knows when a check has entered the feed path, when it’s reaching the exit pocket, and when it’s OK to feed the next item.
The track sensors can also be a source of unexplained paper jams if they become dirty or obscured. This is because they’ll register a piece of paper in the track when there actually isn’t one, which makes the scanner stop the feeding process.
A simple cleaning with a “waffle” cleaning card is usually enough to fix this issue, although you may also try blowing out the track with compressed air in case the sensors are being obscured by an object that the cleaning card couldn’t reach.