Pete Kepf, Certified Vision Professional -
Advanced
Problem
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
Solution
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.