Patent GB-1287462-A (GB)
Title: Improvements in or relating to apparatus for measuring angular displacement about an axis
IPC: G01C19/66
Filed: 1969-09-23
Abstract: 1287462 Optical apparatus PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 23 Sept 1969 46805/69 Heading G2J [Also in Division G1] In an apparatus for measuring change in angular position, e.g. of a radial grating 50 provided on a disc 10, light from a source 1 is focused on an aperture 51 in a concave mirror 3 and the resultant diverging beam falls on an optical system comprising a plano-convex lens 11, which collimates the beam, a reflecting prism 15 and a further reflecting prism 13 which deviates the beam through 90 degrees and directs it on to a first zone of the grating - see Fig. 2b. The +1th. and -1th. orders of diffraction are returned through the optical system and are incident on a respective electro-optical crystal 53, 54, the principal direction of crystal 53 being perpendicular to that of crystal 54. The two beams 31 (+1) and 32 (-1) have equal and opposite phases as a result of the diffraction, and are each composed of two components polarized at right angles to one another. Considering beam 31, on passage through the potassium dihydrogen phosphate crystal 53 its two components will be phase-shifted with respect to one another by an amount depending on the A. C. voltage applied to the crystal. These components are separated by an interface 6 of a beam-splitting cube 5 and after reflection at mirrors 3 and 4 are recombined at the interface 6 to travel along path 37. This recombination is possible because double passage through quarter wave plates 8 and 9 rotates the plane of polarization of each component through 90 degrees, so whereas interface 6 was initially transmitting to a particular component, on second passage it is reflecting and vice versa. A similar division is effected on beam 32, which eventually recombines as beam 38. The two beams 37, 38 then pass through an optical system 21, 22, 23 identical to 11, 12, 13 and are reflected from a second zone of the grating to emerge as a single beam 39. This beam contains four phaseshifted components which are finally divided by a polarization separating prism 18, the two components having a plane of polarization in the plane of the drawing say, being received by detector 27, while the other two components polarized perpendicular thereto are received by detector 26. These two signals are phaseshifted with respect to one another, and rotation of the grating causes a change in the phase relationship between the two signals, which is indicated on a counter. The phase difference between the two signals may be set initially to zero, either by altering the orientation of one of the convave mirrors 3, 4, or by introducing an inclined birefringent plate between the crystal and the lens 11. In another embodiment Fig. 5 (not shown), the light source emits a beam parallel to the rotational axis of the grating. This beam is turned through 90 degrees by a first prism (62) whence it travels to a second prism (64) which directs it to the grating surface collects the two diffracted beams Fig. 4a (not shown), and directs them to two electro-optical crystals (66, 67). The two beams are split further and then recombined into two beams by a cube (69), and after a second reflection from the grating a single combined beam with four components is finally sub-divided Fig. 4b (not shown) as in the previous embodiment and is received by two detectors.