Tilt Trays Employ Photoelectric Sensors From Leuze

Beumer Maschinenfabrik's tilt tray systems, employed in parcel handling, book and CD/DVD logistics centres and airports, now use PRK 25B retro-reflective photoelectric sensors from Leuze. Beumer's high-speed sorters use a conveyor running at more than 2m/sec to carry a letter lying in a tray, for example. A control system tracks the tray and the letter until it reaches the correct output portal, where the tilt tray deposits the letter sideways onto the target chute, in this example into a mail bag, sorted for a particular part of the country.

The PRK 25B sensor was chosen to enable the system to detect whether a tray is occupied or not. The company uses four sensors mounted side by side above the conveyor to check if each tray is occupied, even by small goods down to a minimum volume of 50 x 50mm. Detecting small items is a challenge because the speed of the conveyor means that small objects only interrupt the light beam of the sensors for a very short time - around three tenths of a second.

The sensors target reflective strips that are integrated into the tilt trays, but these present another challenge because they are covered with Perspex for their protection. In selection trials, Beumer tested sensors and reflectors from various suppliers with Perspex sheet, which was scratched with sandpaper to simulate wear. Leuze's PRK 25B performed well in trials because of its high-performance reserve.

The sensors face another challenge because there is a pattern of ridges on the tilt tray that is needed to prevent the static charging of goods wrapped in plastic film, which might, in turn, cause the item to adhere to the tray and not tip into the correct output chute. The size of the light spot projected by the sensor and its high power prevent unintended switching, which overcomes the challenge of these ridges. Additional features such as Brightvision make Leuze's sensors simpler and faster to align, which is important to Beumer because, depending on the sortation system's size, it can use several hundred sensors.

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