how do those optical pattern tracers follow a line?

I saw an old one back about 20 years ago. I don't think that it had
any "high tech" stuff like CCD cameras or digital controls.
Servos maybe. The one I saw had an 'eye' that would turn as it went
around the pattern.

Any one know how they worked. And how the tracer controlled the speed
of the X and Y drive motors to maintain speed at the torch as it went
around.


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Dan H.