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A new method is described for non-contact rotational speed measurement. It utilizes the natural irregularity of the surface pattern on a rotating object. The method employs an optical system and a spatial filter. The spatial filter is a radial array of photo-sensors having equal angular pitches. With this arrangement of photo-sensors, the filter acts as a narrow-bandpass spatial filter which generates a narrowband random signal whose center-frequency is proportional to the rotational speed. First, the spatial frequency characteristics of the spatial filter are analytically examined in the polar co-ordinate system. Next, a practical configuration of the measuring system is presented. In the measuring system, the spatial filter is realized by using a silicon solar cell of which the surface is etched to form the specially designed pattern. The experimental results showed that our method gave excellent characteristics in terms of accuracy, resolution and measuring range. Furthermore, the effects of measuring conditions are examined and evaluated, among which the effect of nonalignment between the center axis of spatial filter and the center axis of the projected image of the rotating object is not neglisible. It turnes out that this offcenter effect can be reduced if the parameters of the spatial filter pattern and the method of the center-frequency measurement are properly selected. 1. ま え が き

Non-Contact Rotational Speed Measurement and Integrated Spatial Filter Detector