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Title: Photoelectric grating displacement measuring apparatus
Claims: What I claim is: 1. Apparatus for measuring or detecting the motion of a first member relative to a second member, which includes a scale grating of period p.sub.1 attached to one of the two members and extending in a direction parallel to the direction in which relative motion is to be detected with the grating lines at an angle .alpha. with an optical axis, an index grating of period p.sub.2 carried by the other of the two gratings with the grating lines at an angle .beta. to the optical axis, the distance along the optical axis between the two gratings being a, means for illuminating the scale grating directly with radiation such that radiation passing from a scale grating to the index grating is at least partially diffuse, and detector means located at a distance b along the optical axis from the index grating and operable to detect the resulting spatially periodic fringe pattern of period p.sub.3, the components of the apparatus being arranged so as to satisfy the following two relationships ##EQU3## where n.sub.1 and n.sub.2 are small integers which do not have a common integral divisor. 2. Apparatus as claimed in claim 1 in which the scale grating is a reflecting grating. 3. Apparatus as claimed in claim 1 in which the means for illuminating the scale grating is carried on the said other relatively movable member. 4. Apparatus as claimed in claim 1, in which the detector means includes a third grating of different pitch to the spatially periodic distribution of radiation. 5. Apparatus as claimed in claim 1 in which the detector means includes a third grating of the same pitch p.sub.3 as the period of the fringe pattern and being skew with respect to the said periodic distribution of radiation. 6. Apparatus as claimed in claim 1 which includes an optical of focal length f located at a distance b.sub.2 along the optical axis from the index grating and at a distance b.sub.3 along with the optical axis from the detector means, and to focus the spatially periodic fringe pattern onto the detector means, the value of b.sub.3 being given by the expression ##EQU4## and the period p.sub.4 of the fringe pattern now being given by ##EQU5## where b and p.sub.3 are as defined in claim 1. 7. Apparatus as claimed in claim 6 in which the optical element is a lens. 8. Apparatus as claimed in claim 1 in which the gratings are linear gratings. 9. Apparatus as claimed in claim 1 in which the detector means includes four radiation-sensitive devices arranged to respond to different parts of the spatially periodic distribution of radiation derived from the same area of the scale grating. 10. Apparatus as claimed in claim 1 which includes means for enclosing the major part of the space between the scale grating and the index grating to prevent contamination of the index grating. Other info: Inventors: Brake, David George (Welwyn Garden City, EN) Application Number: 716229 Filing Date: 1976-08-20 Publication_date: 1978-03-14 Assignee: Ferranti Limited (Hollinwood, EN) Primary Class(es): 250/237G 356/619 Other Classes: US Patent Ref:
Other Refs: Primary Examiner: Helms, David C. Assistant Examiner: Attorney: Cameron, Kerkam, Sutton, Stowell & Stowell |
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