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Low Cost Automation Tutorial

#091 Motion Mechanism Design - 8 : Detection of Moving Workpiece

Category : Drive mechanism design
October21, 2011

This section explain the method of detecting workpieces moving on a work transfer conveyor (Photo 1) as a typical application of non-contact type photoelectric sensors.

[Photo 1] Typical configuration of moving workpiece detection sensor system

(1) Detection of moving workpieces

The following three different methods of workpiece detection are available: (1) contact detection, (2) non-contact detection, and (3) close contact detection. Non-contact type sensors are selected here for moving workpiece detection for the following features:

  1. No possible damage to the workpiece
  2. Capable of detecting workpieces during machine operation

It should be noted, however, that the detection light may behave unpredictably depending on the shapes of the mechanical parts around the sensors and may cause detection errors, leading to a reduced operating ratio of the machine.

- Precautions for use of non-contact type photoelectric sensors
  • The light emitters/receivers of the sensors must be kept free of foreign matter, such as debris, dust, water droplets, working fluid droplets, and chips. Install sensors facing in the direction with the least likelihood of adhesion of foreign matter. For example, avoid installing sensors facing upward (so that debris will not fall on them).
  • Carefully select the sensor mounting locations to prevent interference between neighboring sensors.
  • Shield sensors from incident light emitted from the surrounding structures (external incident light and reflected sensor light).
  • Avoid mixing opaque and transparent workpieces. Failure to comply may result in inaccurate detection due to changes in surface light reflectance. (This is particularly important with reflection-type sensors.)
  • Select fast response sensors for detection of fast moving workpieces.

Photo 1 shows a work transfer conveyor (second from the right bottom row in "Typical Applications of Standard Components: 1" on the center-page spread of the catalog) with three photoelectric sensors (beam sensors: PEX13B) performing the following functions:

  • Detects workpieces when feeding onto the conveyor belt (see Photo 1 above).
  • Controls the positioning station in the middle of the conveyor to adjust the orientation of the workpieces.
  • Detects the presence or absence of a workpiece at the end of the conveyor and limits the number of workpieces on the conveyor to the specified value.