GB1493576A - Interferometric shaft angle or shaft torque encoder - Google Patents

Abstract

1493576 Measuring angular displacement photo-electrically HAWKER SIDDELEY DYNAMICS Ltd 18 Nov 1974 [27 Nov 1973 4 March 1974 7 March 1974] 54872/73 9615/74 and 10205/74 Heading G1A Angular displacement measuring apparatus comprises optical means for converting an angular displacement into a difference between two path lengths, and interferrometric means for measuring the difference and including a beam splitter providing two differently-directed coplanar beams to pass through the optical means along separate paths, the angular deflection of the optical means being in the common plane of the two beams. A beam from a mono-chromatic source 1 Fig. 2, is split by a half-mirror 2 into divergent beams 3, 4 which pass through a glass slab 5 rotatable about an axis inclined to the plane of the beams 3, 4, and are retroreflected by respective mirrors 6, 7 and recombined at 2 into a beam 9. Rotation of the slab 5 varies the difference between the path lengths 3 and 4 whereby a pattern of interference fringes sweeps across a photo-cell 8 in the path of the beam 9. As illustrated, the slab 9 is mounted at an angle α to the end of a shaft 11 so that its normal axis at the point of attachment nutates about the shaft axis as the shaft rotates. A zero reference angle may be established by light from a polychromatic source 14 blended with that from the source 1. Such light will excite a maximum response in the detector 8 when the paths 3 and 4 are symmetrical, but will not produce well-defined ambiguous variations in intensity at other angular positions. Either or both of sources 1 and 14 may be modulated to enable the detector 8 to distinguish between them. A plurality of detectors may be arranged Fig. 3 (not shown) to provide signals having phase separation representing the sense of the displacement. If the detectors (17, 18) cannot be separated by sufficiently small spacing, further beam-splitting means (16) may be used. In another embodiment, Fig. 5, a beam splitter 2 comprising rhomboidal prisms 21, 22 causes the beams 3, 4 to converge on an aperture 28 of an annular mirror 27 which, with an opposing mirror 29, constitutes a rotatable assembly mountable at an angle to the axis XX of a shaft (not shown). The rhomboidal prisms 21, 22 may also replace the half-mirror beam-splitter 2 of the first embodiment, so as to cause the beams 3, 4 to converge on a common region of the glass slab (5), Fig. 4 (not shown). The apparatus may be used to measure torque, the displacement-converting element and the remainder of the apparatus being mounted on respective zones spaced apart along a shaft undergoing torsion.