Patent GB-1208505-A (GB)
Title: A device for measuring distances or angles by photoelectric scanning
IPC: G01D5/347
Filed: 1968-06-27
Abstract: 1,208,505. Measuring position. ERNST LEITZ G:m.b.H. 27 June, 1968 [4 July, 1967], No. 30729/68. Heading G4H. [Also in Division G1] The angular or linear (in one or in two coordinate directions) position of an object is determined by photo-electric investigation of the position of one or more convex mirrors attached to the object. In the arrangement of Fig. 1, a single convex mirror 12 is attached to the top of a scale carrier 10, mounted on an object whose position is to be monitored, carrying markings 11 on its underside, the centre of curvature of the mirror coinciding with the plane of these markings. A reader 111 determines the position of the scale carrier 10 by counting the number of marks 11 from a datum position defined by the mirror 12 being aligned with the optical axis of the photo-electric apparatus mounted above the scale. Light from a lamp 13 is passed through condenser 14 and prism 15 to illuminate a slit 16 in diaphragm 17. A virtual image of the slit is formed via lens 18, a Wollaston prism 19 and objective lens 20, in the plane of the marks 11. The light reflected is directed via lens 20, prism 19 and lens 18 through a further diaphragm 21, prism 22 and lens 23 to a polarizing beam splitter which directs oppositely polarized components of the beam, introduced by Wollaston prism 19 to separate photo-cells 27, 28. The detector outputs feed a differential amplifier 29 and trigger 30. When the inputs a and b are equal the trigger emits the output pulse that is used to initiate counting by the reader 111. The arrangement of the mirror 12 with respect to the scale marks 11 is illustrated in Fig. 1a. An alternative arrangement is to use two mirror segments located either side of scale markings formed on the top surface of the carrier, Fig. 1b (not shown). The apparatus of Fig. 2 is similar to Fig. 1 with the exceptions that instead of slit diaphragms 16 and 21, gratings (35, 35a) are used and that a plurality of abutting convex mirrors (36) are carried by the scale carrier (38). A further difference is that a telecentric diaphragm (37) is positioned between the Wollaston prism (19) and mirror array. The centres of curvature of the mirrors lie in the plane of the central axis of the scale carrier (38). The pulses produced by the detector circuit as the mirrors (36) pass the datum provided by the optical system, are counted to give a measure of displacement of the scale (38). Modifications of the arrangement of Fig. 2 include: provision of adjustment means for altering the datum position provided by the instrument, e.g. by elastic mounting of the grating or lens (20) or use of a sliding lens (20); use of a single photo-cell in association with means to reverse periodically the polarization produced by the Wollaston prism, thereby obviating the need for the polarizing beam splitter (24) associated with two cells. The arrangement of Fig. 3 is for measurement of the position of a number of objects, fixed with respect to one another and carried on the top surface of a carrier member (40) which is provided on its underside with a two-dimensional rectangular or hexagonal array of convex mirrors, so that displacements of the carrier in two dimensions may be measured. The photoelectric system mounted beneath the carrier (40) comprises a lamp (13) and condenser lens (14) illuminating a cruciform or L-shaped aperture, an image of which is formed by a lens (44) on a phase grating (45) of chequer board design. An image of the grating is formed via a field lens (18), Wollaston prism (19) and objective lens (20) in the plane containing the centres of curvature of the mirrors via a telecentric diaphragm (37). The reflected rays pass back through these elements and the grating to a polarizing beam splitter (24). The two differently polarized beams emerging from the latter are focused by lenses (25, 26) on to concave mirrors (46, 47) fitted with central slits for separating the components of the beam according to co-ordinate direction and order of diffraction. Associated with each concave mirror (46, 47) are two sets of photo-cells, one set receiving the beam passed through the slit in the mirror and split up by a prismatic beam splitter attached to the back of the mirror and the other receiving the light reflected from the mirror and split into two beams by a further prismatic splitter. The arrangement of Fig. 4 (not shown) provides a datum position in two co-ordinate directions for scale carrier (72) having a phase cross-grating on its underside by monitoring the position of a single convex mirror (73) on its top surface. The photo-electric instrument mounted above the carrier comprises a lamp (13) and condenser (14) illuminating a cruciform aperture (70) having its two components differently coloured. The emerging beam passes through a beam-splitting cube (71), Wollaston prism (19); telecentric diaphragm (37) and lens (20) on to the convex mirror (73). The reflected light passes back through these elements to the beam-splitting cube (71) where it is directed through a cruciform slit (74), lens (75) and split by a polarizing beam splitter (76) into two differently polarized beams. These are subsequently split by two dichroic plates (77, 78) for detection by two corresponding sets of photo-cells (83, 84 and 85, 86) which are coupled via differential amplifiers to trigger circuits operating on in the Fig. 1 arrangement. In the arrangement of Fig. 5, light from lamp (13) passes through a condenser (14), a beamsplitting cube (88), a lens 90 and diaphragm 89; Wollaston prism 19 and lens 20 to a convex mirror mounted on the top of a scale carrier. Reflected light is directed by beam-splitter (88) to a polarizing beam-splitter (24) and pair of photo-cells. This device operates similarly to that of Fig. 1. Angular displacements of an object supported rotatably about an axis are also measurable by attaching one or more spherical mirrors to the object.