Wide-range digital reluctance tachometer

Abstract

A digital tachometer for generating electrical pulses at a rate indicative of the speed of rotation of a shaft-like member. A first toothed disk having N teeth is affixed to the shaft and a housing having a second set of M teeth is fixed with respect to the shaft and positioned around the toothed disk. As the shaft rotates, individual ones of the teeth on the outer periphery of the disk which is affixed to the rotating shaft alternatively pass into and out of close proximity to individual ones of the teeth of the outer housing resulting in periodic increases and decreases in the reluctance of the system. A sensing coil is positioned with respect to the housing so as to generate an electrical current which varies as the reluctance changes from one polarity to another as the shaft rotates. At each point of maximum reluctance and at each point of minimum reluctance, the polarity of the reluctance and correspondingly the slope of the current waveform changes and these changes are detected by a slope-change detector circuit which generates a digital signal in response thereto. The rate of generation of these digital signals is therefore indicative of the speed of rotation of the shaft and the speed of rotation can be sensed over a very wide range from very slow rotational speeds to very high speeds.