Velocity detecting apparatus employing a pair of modulation-type magnetic heads on which carriers are phase modulated by flux from a moving magnetic scale

Claim 1

A velocity detecting apparatus comprising a movable magnetic scale having a magnetic signal thereon of alternately changing polarity at regular intervals in the direction of movement of the scale, a pair of modulation-type magnetic heads disposed fixedly adjacent said scale and being spaced apart along the latter by a distance which is (n+1/4) times the length of said interval between adjacent different polarities of said magnetic signal and in which n is a whole integer, oscillator means generating an AC signal which is applied to each of said modulation-type magnetic heads to provide respective carriers which are modulated by electromotive forces induced in said heads by magnetic flux from said scale to provide a respective phase-modulated carriers as the outputs from said heads, means to shift the phase of one of said outputs with respect to the other of said outputs, mixing circuit means receiving both said one phase-shifted output and the other of said outputs and adding the outputs to provide a composite phase-modulated signal output, frequency discriminating means receiving said composite phase-modulated signal and producing a voltage corresponding to the frequency of said composite phase-modulated signal, and means indicating said voltage as a function of the velocity of movement of said scale.