Photoelectric displacement measuring device for obtaining harmonic free signals

Claim 1

In a photoelectric measuring device for obtaining harmonic free signals up to a predetermined bandwidth said system comprising a measuring scale and a scanning unit wherein the period P of said measuring scale and the period P' of said scanning unit sufficiently differ in a scanning direction such that upon scanning a Moire oscillation pattern having a period M is produced, said Moire period M being greater than the graduation periods P and P' of said measuring scale and said scanning unit respectively, said Moire oscillation being detected and converted into a periodic electric output signal (S1, S2) by a detector system comprising a plurality of photoelements, each photoelement generating an electric output signal, said photoelements being arranged to receive and convert said Moire oscillation into said periodic electric output signal, the improvement which comprises, said device comprises a plurality of detector systems (D1 . . . Dn), wherein each detector system comprises a plurality of photoelements (a1, b1, c1, d1, . . . an, bn, cn, dn) having a size, shape and position according to a Fourier analysis such that harmonics are optically eliminated from said periodic output signal of said detector system upon summing the output signal from said photoelements comprising said detector system.