Two-dimensional measurement of rotational speed of a diffuse object based on laser Doppler anemometry

A novel measurement method based on laser Doppler anemometry that determines the pure rotational speed of a diffuse object on the condition that its axis of rotation is in a plane perpendicular to the illuminating laser lights is described. The rotating object is illuminated by three parallel laser lights and its rotational speed is calculated from beat frequencies resulting from the mixing of their backscattered lights. To verify the principle, measurements of rotational speed of a brass cylinder were performed for rotational speeds in the range of 30-500 rpm. The measured values have a fairly good agreement with the preselected values over this range. The present method is useful for detection of rotational conditions of machinery components assembled in rotating or vibrating structures.