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.
John