Speed measurement system with means for calculating the exact time

Abstract

A digital speed measurement circuit for an engine includes a magnetic pick-up positioned adjacent an engine-driven gear having a substantial number of teeth so that the pick-up produces a number of speed pulses per revolution of the gear. These pulses are shaped into square waves by a signal conditioning circuit and connected through a synchronizing circuit which responds to the trailing edge of each square wave to produce a triggering pulse, with each triggering pulse supplied to a tooth counter. A high speed clock is connected to a second counter which is reset with each triggering pulse. Sampling pulses are produced in an associated control system which is not coordinated with the speed pulse but which are longer in duration than any of the speed pulses and which control the flow of tooth counts to the tooth counter. Memory circuits retain the high speed counts measuring the intervals between a triggering pulse and the beginning of a sampling pulse and between the end of the sampling pulse and the triggering pulse immediately preceding. Another counter counts the high speed counts occurring during the sampling pulse period. These factors are combined as described to provide a high speed count of a new measuring interval exactly related to the speed pulse count and which is combined with the speed pulse count to provide a digital signal which accurately represents a desired function of engine rotational speed.