Real-time motion control with subnanometer heterodyne interferometry

The precision requirement of ASTROD laser astrodynamical missions and research in the new definition of the kilogram drove us to engage in the development, research and application of sub-nanometer and picometer laser metrology and real-time motion control. We realized mid-point cyclic modulation in real time to suppress the periodic nonlinear errors of heterodyne interferometery, and employed a closed-loop feedback control to compensate PZT nonlinearity and other deviations. Using flexure stages and PZTs, we built up a three-axis (X, Y, theta(z)) nanopositioning system with real-time subnanometer heterodyne interferometry. The experimental results of 12-mum linear motion show that the X-axis positioning error can reach 0.6 nm rms, the Y-axis positioning error can reach 0.06 nrn rms, and the theta(z)-axis positioning error is about 4x10(-9) rad rms. We also used two-stage and three-stage schemes to extend the travel range. A preliminary run using the two-stage scheme can attain a 5 mm travel range with a nonlinearity of 10 nm rms.