What's that Lassie? You say that Don Stauffer fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Sun, 07 Dec 2008 11:06:06 -0600:

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.

I get the eye part. And how to steer the pickup head. But how to
control the X and Y axes to move in the direction that the pickup head
is pointing? That is where I get lost. If the axes didn't have to be
able to change direction, then I see how they could do it with a
non-rotating head that just controls the speed of one axis to keep up
with the other.

But in a tracer, the X and Y axes need to be able to reverse as needed
to follow the line. And the speed of each needs to be adjusted to
keep the torch constant as the lines angle changes.

--

Dan H.