By Mitchell Belser
I was working for a large manufacturing company during the early to mid nineties. The company produced a line of multi-function copiers that were used primarily used to make working copies of environmentally sensitive documents.
Several units had been shipped to customers in Europe. The initial reaction from the customers was positive and more units were being prepared for shipment. Then complaints began to come in from the field. The copier was equipped with a document sensor that would alert the user that a document was left in the machine (A useful feature considering some of the documents being reproduced were maps that were several hundred years old). The copiers started reporting that documents were in the copier when no document was actually present. The problem was intermittent. Sometimes the document sensor would indicate the presence of a document that wasn’t present. The presence of a document was always sensed correctly. The problem started showing up in the assembly plant as well. Needless to say, the customers were not pleased with the erratic behavior and the lack of a known cause and a solution was not making management happy either.
I was part of an in-house software and electronics support group, so I was assigned to the project to provide manufacturing support. I traveled to the manufacturing plant to meet with the assembly technician about the problem. He showed me several machines that indicated a document was present and later the problem would mysteriously disappear. The problem didn’t seem to be correlated with the time of day or line voltage fluctuation. The environmental conditions were well controlled.
I opened one of the copiers to get a look at the sensor. It was an opto-reflective sensor that contained an LED transmitter and a photo diode receptor in a shared housing. The sensor was located behind a sheet metal platen that separated the sensor electronics from the document. When operating correctly, light from an LED passed through a hole in the platen and was reflected back to the receptor if a document is within sensing range. I tested the sensor, which operated flawlessly time after time. I obtained the data sheet for the part to see if there was something obvious that I was missing. The transmission cone for the LED was about 45 degrees! This was part of the problem. Upon closer inspection I noticed that the hole through which the paper was to be sensed had a curved edge extending toward the sensor. The hole had been created with a punch press which created a curved edge. I suspected that the transmitted light was being reflected from the curved edge and being picked up by the receptor. I asked the technician to obtain a piece of flat black construction paper. A hole slightly smaller than punched hole was cut in the paper. The paper was placed between the sensor and the metal plate. Voila! The phantom document was exorcised and the problem solved.
The sensor transmission parameter coupled with the curved edge of the punched hole resulted in intermittent reflections that made the document appear to be present and then later not present.