What's that Lassie? You say that john fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Mon, 08 Dec 2008 18:04:38 -0500:

It has been quite a while since I worked on an optical tracer head. It
had some miniture lamps the illumininated the line, I think there were
four of them around the bottom of the tracing head. There was a mirror
that was rotated by a motor that directed the focused image of the line
onto two sensors. There was a magnet on the rotating shaft that held the
mirror that gave the angular position of the mirror. Each axis was a
separate circuit and as the mirror would scan across the line, first
from left to right and then right to left the sum of the two scans was
fed into the amp for the servo drive motor. If the head was drifting to
the right the combinded reference voltage would go higher and move the
head to the left and vice versa. The same action operated the other
axis. The speed of the cut was regulated by a control the set the gains
of both amps.

Thanks John. This is the first description involving a mirror I've
heard of. Makes sense. No slip rings to go bad or introduce noise.

--

Dan H.