In
machine vision contrast is king. Creating contrast between what you
want and do not want to see makes it possible to apply edge tools
that are the basis of just about every machine vision application.
(Yes I know about blob analysis, but that too relies on creating
contrast. And pattern-matching is little more than fancy edge
detection.)
But
there is another way. By measuring distance to the target, laser
triangulation 3D eliminates the variation in contrast resulting from
changes in surface reflectivity. And in the real world the things we
need to inspect often vary slightly. Ink dries faster or slower,
there’s natural grain in the finish, or the condition of molding
dies changes through a production run. All these can upset a
conventional front lighting approach.
Note
though that low angle and back lighting techniques, by virtue of
using geometry rather than surface appearance, are largely immune to
changes in appearance.
Take
corks as an example, the kind that keep wine in bottles. Being a
natural product (I hope you’re not using those awful ‘plastics’,)
they have lots of lines on them, so how you do distinguish between
lines and edges? With 3D vision of course.
Now
I didn’t just happen to pick on corks; they are the subject of an
application study in Inspect-online. “No
Leakage”
(June 27th,
2013,) describes a system developed by Spanish integrator baxicat
visiĆ³n
S.L. This uses two camera-laser pairs to inspect the ends of corks as
they pass by in a kind of Ferris wheel arrangement.
The
article gives lots of good details on how the task was accomplished,
which included enlisting the assistance of point cloud specialists
Aqsense.
If you’re struggling with a surface that varies in appearance, I
suggest you read it.
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