The Target Laboratory has a Target Scanner (picture) for automatical measurement of thickness and surface profiles.

The description (soft- and hardware) for controlling the scanner you can find here. The project finished in 2000. In the last years sometimes the target motion motor moves, although the length gauge precicion instrument are not in end position. These instruments are very expensive and you must very quickly press emergency stop button.

To became a safety system, two optical sensors should control the end position of length gauge. Only if both in end position, the motor may drive. The realisation was very easy. Here you see the principe wiring diagram.