Patent GB-1282048-A (GB)
Title: Apparatus for determining the relative movement of an object by optical means
IPC: G01B11/00
Filed: 1969-08-05
Abstract: 1282048 Selective signalling PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 5 Aug 1969 [8 Aug 1968] 39155/69 Heading G4H A displacement measuring system of the type in which light is passed twice or more through, or reflected twice or more from, a diffracting grating to produce interference patterns which are detected photoelectrically, is characterized in that a constant modulation is superimposed electrically upon the light source so that the resulting output signal can be A.C. amplified. The system described is a modification of the system described in Specification 997,405, which is referred to, and in which an oscillating mirror is used to modulate the light, the present system having the advantage that as the modulation is done purely electrically it can be done at a higher frequency. As described, light from a source 2, Fig. 1, is modulated 3, 4, 5, 6, 7, to produce two plane polarized beams 18, 19, the planes of polarization being orthogonal. These beams are passed via a diffraction grating 1 to a mirror 9, from where they are reflected through the grating for a second time, separated by an analyser 10 into two beams corresponding to the two directions of polarization, detected by photoelectric means 11, 12, and processed in order to control a counter 13. Monochromatic light of angular frequency # passes from the source 2 via a plane polarizer 3 and a quarter wave plate 4, to produce circularly polarized light which then passes via two electro-optical or magneto-optical crystals 5, 6 controlled by a generator 14 of angular frequency # to produce a plane polarized beam of light, the plane of polarization rotating at a speed of ¢#. This beam then passes via a further quarter wave plate 7 to produce two orthogonally polarized beams of light 18, 19 whose angular frequencies differ by #. The modulating arrangement 5, 6 is described and claimed in Specification 1,185,970 which is referred to. The composite beam 18, 19 is passed via the grating 1 but only emerging means of order -1 and +1 are utilized subsequently. The beam 18<SP>1</SP>, 19<SP>1</SP> of order -1 is arranged to pass twice through a further quarter wave plate 8 so that its directions of polarization are reversed. The component 18<SP>1</SP>, which is polarized normally to the drawing becoming beam 28<SP>1</SP> which is polarized parallel to the drawing. The composite beam 28<SP>1</SP>, 29<SP>1</SP> is passed through the diffraction grating again and of the emerging beams only that of order -1 is utilized, this being indicated in the drawings by the symbol (-1, -1). In a similar way the beam 18<SP>11</SP>, 19<SP>11</SP> of order + 1 first emerging from the grating is reflected at the mirror to pass through the grating again, and of the emerging beams only that of order +1 is utilized. The resulting composite beam is resolved into two beams by the analyser 10 and each of these two beams itself comprises two sub-beams 1 from each of the groups (+1, +1), (-1, -1) so that finally the photoelectric detectors 11, 12 each produce signals of frequency Q but of a phase determined by the position of the diffraction grating, the phases being such that as the diffraction grating moves through one pitch period the phase of one of the signals advances by four cycles and the phase of the other drops back by four cycles. Thus by examining the way in which the output signals from the photoelectric detectors vary in phase, the displacement of the diffraction grating can be determined. Various modifications in detail are described including (Fig. 5, not shown) the use of a reflective diffraction grating.