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Control Problem/ Solution: Two Camera Time Delay

[#6] Last update 28 Jan 19
, Certified Vision Professional - Advanced
Machine Vision application with a two-camera time delay


Puffed propylene pellets are widely used in the pharmaceutical industry for filter material. The pellets are somewhat fragile and may be subject to spalling at the edges. Another potential defect in manufacture is slight discoloration, which indicates a reduction in the effectiveness of the material. Because these defects are difficult to see with the human eye and the quantities produced are very high (millions per day), the manufacturer implemented a number of automated inspection machines.

Two cameras were used to inspect the parts. A side-view color camera detected the presence of discoloration and a top-view monochrome camera detected spalling. A structured light cast a dense pattern onto the top of each pellet.

The control challenge was to store the inspection results (pass/ fail) of the first camera, the inspection results (pass/ fail) of the second camera and reject that part at a downstream mechanism. If either inspection failed, then the part was to be rejected.

Major technical issues were:

  • A part handling system for inspection needed to be developed
  • Two sides of the parts were to be inspected
  • Discoloration and spalling were to be detected sequentially
  • Required inspection rates were up to 400 parts per minute


The picture at the top shows a simplified layout. The actual installation consisted of two adjacent cameras sufficiently far apart so as to preclude the light sources from interfering with one another.

A sensor detects the part at the uppermost position. A sensor pulse and an encoder pulse are sent to the controller; the controller stores the relative position of the “current” part. When that part reaches the first location, the controller sends a pulse to the side-view camera and light. The inspection is performed, and the results stored. The distance between the two cameras in pulses is known, so at the appropriate encoder count the top view camera acquires an image and its results stored. The distance in pulses between the top-view camera and the reject mechanism is known. If the value of either inspection is a “fail”, then the controller sends a pulse to the reject mechanism. The controller/ timer provided a simple method to sequence control without a PLC.