A complete high performance heterodyne interferometer displacement transducer for microactuator control

Abstract

There is a need for a high performance displacement transducer to be used in the closed loop control of a microrobot. This article describes a laser heterodyne interferometer-based transducer which resolves displacements to 0.12 nm (standard deviation) locally, in a scanning range of 10 mm or more, in a 1 Hz to 10 kHz bandwidth, under normal laboratory conditions. The device combines the long measurement range of conventional optical fringe counting techniques with the high resolution of subfringe metrology interferometers. The sensor is a complete device including optics, custom-built phase demodulator electronics, and computer algorithm for displacement computations using a novel fast lookup table technique. The lookup table algorithm uses the estimated instantaneous uncertainties on both quadrature outputs from the phase demodulator to improve the accuracy of the displacement computation. The optics and electronics are completely characterized in terms of signal-to-noise characteristics and frequency response. Experimental results are shown of an application where the transducer was used in the closed loop control of a nonlinear actuator.