Direct visual servo control achieving nanometer resolution in X-Y-Z

A novel measurement technique, laterally-sampled white light interferometry (L-SWLI) was developed by the authors of this paper and was introduced in (Ibid., "An Ultra Precision Six-Axis Visual Servo Control System", IEEE Transactions on Robotics, unpublished) demonstrating sub-10 nanometer resolution in all three x-y-z motion axes of a visually guided motion control system using a interferometer-equipped optical microscope. This paper presents two major improvements of the L-SWLI as a real-time motion sensor. First, the out-of-plane measurement range is increased from several micrometers to 100 um or above by employing an objective-z-positioner (OZP). Augmented motion control schemes are developed for OZP integration. Second, an interferogram pattern matching method is developed for processing interferograms, leading to an improvement of measurement resolution of out-of-plane motion by one order of magnitude, to under 0.4 nm (RMS). Experimental results demonstrating the improvements are presented. Additionally, a specially designed experiment confirms that with low-pass-filtered measurement, L-SWLI can measure slowly varying thermal drift with an accuracy of 1-nanometer in x-y-z.