The input end of a Komori printing press includes a system for ensuring that either side of the paper is butted up against a known edge. The way it does this is that there is a paper detector setup in line with the edge that detects if the paper is in position as well as a driven side roller that can pull the sheet of paper across and into position. This system is called the side lay and there is one for each side of the machine.
The photo above shows the mounting end of the side lay sensors. The side lay sensor is made up of a light source which is directed into a strand of fibre optic cable, a lens (shown above), a return strand of fibre optic cable and a light detector (housed in the same device as the light source). If enough light bounces off a sheet of paper and back up, through the fibre optic cable and into the light sensor a digital input signal is sent to the main PLC to tell it that there is a piece of paper in place. The components are made by Keyence.
The light sensor has a sensitivity pot that has to be tuned in order to get a reliable reading for when the sheet of paper is in place. For this particular job the sensitivity of the light sensor wasn’t able to be adjusted in order to reliably detect the sheets of paper. The fault was a bit difficult to identify because when you placed a sheet of paper under the sensor by hand it appeared to work. However when the machine was running the detection was unreliable.
I ended up finding that the fibre optic cable had been damaged. Just near the mounting thread one of the cables had been sliced half through. This allowed enough light to leak such that the signal became unreliable. With the new cables fitted I was able to tune the sensor up and get the side lay system working again.