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machine vision system design

Posted on 1997-11-05
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Last Modified: 2011-10-03
I need to design a machine vision system to inspect molded plastic
tail light covers.  These covers are made from red translucent plastic. They are
approximately 70 x120 mm in size. Like most tail light covers, these have
a regular arrangment of small pyramid structure on their back sides. These
small structure allow the covers to reflect light so that head lights    
hitting the covers will cause a reflection even when the tail and/or brake
light is not on. All the covers to be examined are of one design. You are
to find discolorations and cracks. You must also make sure that the four
holes that allow the tail light to attached to the car are all present at
the  appropriate location and are of the appropriate dimension. Finally
you should be able to detect areas that might not have been molded
correctly and are too thin. While the rotational orientation of the covers
cannot be controlled, they will always have the same surface facing up.
What hardware, imaging a nd lighting geometry and algorithms do I need?

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Question by:az110597
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by:tabit
ID: 1016278
thats a 5.000 US$-question, not a 200 points one!
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Expert Comment

by:cymbolic
ID: 1016279
You can take the mars lander approach.  You don't need to inspect for all the details, you only need a backlight reference measurement off a good lens as a comparison. Then, position each lens in front of a standard light source, backlight each lens, read its composit image with a photocell array, and compare it to the reference good lens image.  Calculate the degree of difference between reference good image and current snapshot.  Use multiple test runs to set the sensitivity, then reject any lenses outside of the acceptable limit.  This will work for your  install holes as well since the light shining through the holes will vary with position and hole size.  

Your sensor and readouts are your main problem.  Try the cheap things first, like a personal scanning system!  
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Author Comment

by:az110597
ID: 1016280
I need to be able to evaluate the type of failure: whether it was
due to discoloration, crack, uneven thickness, improper holes,
misaligned pyramid structure, etc.  I also need to know the various types of algorithms.
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Author Comment

by:az110597
ID: 1016281
This is a 200 point question so the answer should be long
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Accepted Solution

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delphinus earned 400 total points
ID: 1016282
Since the covers are plastic, you can illuminate then with
polarized light and watch for the polarization patterns.

Illuminate the cover with polarized light, then place it in an
apparatus which applies a constant pressure on the cover.  The
cracks in the cover will produce higher localized strain which
will show up as a different color under plane polarized light.

As for misalignments of the pyramid structure, you can use a
laser beam to measure the retroreflectivity of the corner reflector.  Merely sweep the cover with a laser beam through
all solid angles, and measure how close the beam comes to
reflecting straight back into the laser.

Finding improper hole placement and size is a relatively easy
task.  Place the cover on a highly reflective surface, then
illuminate it with green or blue light.  The cover will absorb
this wavelength, but the light will pass through the holes,
reflect off of the surface, and return to the camera.  Merely
use any simple image package to measure the hole diameters and
spacings.

For uneven thickness, you may just want to try a mechanical
approach.  Place the cover flat face down on a table, and run
a measurement wand of some sort over the other side.  The
deflection of the wand will determine the thickness at that
point.  Merely plot this on a computer and verify that the
thickness distribution is within acceptable tolerances.

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