The principle method for XY measurement was previously developed and demonstrated at iAi.
The method has then been extended to tip-tilt (RX-RY): this was tested first in our lab and then at NIAOT (China) on a 1-m size hexapod. The setup requires placing a small flat mirror on the hexapod, through which the patterned target fixed to the external frame is seen by the camera.
Concurrent X-Y-RX-RY measurements with two USB cameras: left in our lab, right at NIAOT (China) measuring a 1-m hexapod of novel type.
The accuracy achieved in our lab without particular effort is about 1 μm, 3 μrad. The method appears also quite robust: although the test conditions in China were far from optimal (no isolated table, flimsy support structure), we nonetheless achieved accuracies in the order of a 2-3 μm, 10 μrad (which would already be close to adequate for the ATS hexapods).