In the conventional methods of measuring radial, axial, and angular motions of a spindle concurrently, complicated reference artifacts with relative large volume, e.g., two balls linked to a cylinder, are required. A simple and small artifact is favorable from the viewpoint of accurate measurement. This paper describes a concurrent measurement of radial, axial, and angular spindle motions using concentric circle grating and phase modulation interferometers. In the measurement, concentric circle grating with fine pitch is installed on top of the spindle of interest. The grating is a reference artifact in the method. Three optical sensors are fixed over the concentric circle grating, to observe its proper positions. The optical sensor consists of a frequency modulated laser diode as a light source as well as two interferometers. One interferometer observes an interference fringe between reflected light from a fixedmirror and 0-th order diffraction light from the grating to measure the axial motion. Another interferometer observes an interference fringe between ± 2nd order diffraction lights from the grating tomeasure the radialmotion. Using three optical sensors, three axial displacements, and three radial displacements of the proper observed position of the grating can be measured. From these measured displacements, radial, axial, and angular motions of the spindle can be calculated concurrently. In this paper, a measurement instrument, a novel fringe interpolation technique using sinusoidal phase modulation, and experimental results are discussed.

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