What's that Lassie? You say that Joseph Gwinn fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Sun, 07 Dec 2008 13:03:03 -0500:

The classic approach is two photocells (or one split photocell) feeding
a differential amplifier. If the line is halfway between the cells,
output is zero. It the line strays one way the output is negative, and
the other way positive. To do full X-Y, there will be four cells in a
square, and two differential amplifiers.

This was enough for the gantry servo to steer by.

Do you mean that's what rotated the pickup head? And the position of
the pickup head controlled the X and Y for the gantry?

Or was the head stationary? With a stationary head, what would
determine witch direction to go on the line?
--

Dan H.