Algodoo

Something doesn't quite seem right with this. When I move the target, the Meas value doesn't change. Then when I delete the target, and then move the sensor, the POS value changes but the Meas value still doesn't change.

This sensor stops when the reflected beam straddles the phototransistors. In order for it to track, the reflected beam must contact the phototransistors which are fairly small. If the target moves faster than the speed of the (slow) stage, then the beam will come off of the phototransistors, the stage will stop, and the sensor will stop measuring. The measurement is updated only when the beam is reflected onto the phototransistors. The type B sensor will track the best, since it has a larger target and the speed of the stage is proportional to the laser beam deviation from center of the target.

Oh, that explains it! Yes, I noticed that the B sensor does seem to track very nicely compared to the others.

BTW - Many years ago (I hate to admit HOW many) I worked for a packaging machinery manufacturer, and we in Engineering needed to find some VERY small proximity sensors for a special application, and one of them that I found was an optical sensor that was about the size of a pencil eraser. It had a measuring range of 2 CM with a 0.01mm resolution. It was expensive, but it did what we needed it to do. That was about 25 years ago (there, I said it), so I'm sure there are even smaller and more precise sensors that are available today.