Photoelectric position detector with offset phase grating scales

Claim 1

In a photoelectric position detector comprising illuminating means, a scale member provided with phase grating scale means, a scanning member, which is operable to perform relative to said scale member a scanning movement along said phase grating scale means and is provided with a scanning phase grating, which is arranged to transmit light from said illuminating means onto said phase grating scale means and subsequently to receive said light from said phase grating scale means, wherein said light is diffracted by said phase grating scale means and by said scanning phase grating before and after said light has been incident on said phase grating scale means and an interference pattern is thus generated, which comprises diffraction groups of different orders, and a plurality of photodetectors are provided, each of which is arranged to receive only diffraction groups of a predetermined order of said interference pattern and in response to the reception of said diffraction groups is operable to generate phase-displaced photodetector signals, which are cyclically changed by said scanning movement, the improvement residing in that said phase grating scale means consist of two juxtaposed phase grating scales, which have a predetermined grating constant and are offset by 1/8 of said grating constant and are both arranged to receive light from said illuminating means through said scanning phase grating, one pair of said photodetectors are associated with each of said scales and arranged to receive only diffraction groups of respective predetermined orders of said interference pattern from said associated scale and operable to generate phase-displaced photodetector signals in response to such reception, and corresponding photodetectors of said two pairs are arranged to generate photodetector signals which are displaced 90° in phase owing to said offset of said scales.