| 摘要 |
1,227,941. Measuring position photoelectrically. GEORGE RICHARDS & CO. Ltd. March 14, 1968 [March 16, 1967], No.12496/67. Heading G1A. Apparatus for generating a signal sinousoidally related to the position of a grating comprises means defining two spaced local areas of the grating spaced apart by an odd number of nth fractional parts of half the pitch of the grating and means arranged to combine signals derived from examination of the respective local areas to provide a combined sinousoidal signal from which the nth harmonic of the fundamental frequency has been substantially illiminated. In one preferred embodiment the third harmonic is eliminated by making apertures defining the spaced areas rectangular and one third of a grating pitch in width and the fifth harmonic is eliminated by spacing the apertures 9/10ths or 11/10ths of a grating pitch apart. In another embodiment the apertures or grating spaces are in the form of trapezia to reduce higher order harmonics as well. Higher order harmonics may be further reduced by using additional apertures with different mutual spacings. The grating may be of transmission or reflection type. |
