dan wrote:
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.


One method is to use two photocells, squinted a bit, one to one side,
the other towards the opposite side. They have overlapping fields of
view. Ratio of the two sides can find a line position. In the center the
two signals are equal, and if line is one one direction one cell
decreases signal while other increases.