An improved displacement detector for an encoder

Claim 1

An improved displacement detector for an encoder provided with a scale including a track generative of sine wave signals at the borders of constant domains comprising the first and second sensors arranged closely facing to and movable relative to said scale, generative of level signals cor­responding to the intensity of said sine wave signals, and being spaced apart by a phase lag of 1/4 λ when λ is the wavelength of said sine wave, the first and second A/D converters for digitalizing said level signals generated by said first and second sensors, a function generating and multiplying unit generative of the product of multiplication of the output signal of said first A/D converter with a cosine value corresponding to prescribed data, and the product of multiplication of the output signal of said second A/D converter with a sine value corresponding to said prescribed data, respectively, a reducer for calculating the difference between said products of multiplications, a counting unit which performs counting corresponding to said difference, shifts mode of counting depending on the polarity of said difference, and passes the count value to said function generating and multiplying unit, and means for detecting the number of said domains passed by one of said first and second sensors depending on square pulses which are obtained by wave shape discrimination of said level signals from said first and second sensors, said function generating and multiplying unit, said reducer and said counting unit forming a phase-locked loop, and output signals from said counting unit and said detecting means being used as absolute displacement data.