Patent GB-886347-A (GB)
Title: Improvements in measuring apparatus
Inventor: PERDUE ROBERT JOHN
IPC: G01D5/38
Filed: 1958-11-10
Abstract: 886,347. Electric selective signalling systems. DE HAVILLAND AIRCRAFT CO. Ltd. Nov. 10, 1959 [Nov. 10, 1958], No. 36043/58. Class 40 (1). In an apparatus for making fine linear measurements and deriving electrical signals therefrom, comprising a pair of members movable relatively in accord with the measurement to be made and carrying overlapping optical gratings having lines which are substantially normal to the direction of movement but of which the lines of one grating are mutually inclined at a small angle to those of the other so as to produce an interference pattern comprising alternate light and dark bands extending parallel with the direction of movement and moving bodily along the lines as a result of such movement, in conjunction with photo-electric means arranged to be scanned by the light bands, the optical gratings are cylindrical, one being tubular and disposed around the other so as to be movable telescopically relatively thereto. A machine-tool slide or sliding table 10 carries an opaque housing 13 provided with a cylindrical transparent liner 15 carrying an optical grating 16 constituted by a multi-turn helix comprising six helical lines each with a pitch P and having a total width Q. The liner 15 snugly engages a transparent tubular liner 17 having a circumferential optical grating 18 comprising a six-turn spiral of opposite hand to the helix of grating 16 and formed by opaque lines 36 and intervening spaces 37. The movement of the member 10 through a distance Q provides a total of twelve illuminated bands as indicated by A, B, C . . . in Fig. 4. The housing 13 is formed with radial holes 22 communicating with axial holes 21 for eleven photo transistors or photo-cells 23 ... 33, equally spaced circumferentially, thus providing a vernier arrangement in connection with the illuminated bands A, B, C . . . L so that each cell produces a pulse during an axial movement of P. Additionally, axial holes 21 are provided at positions between those for the cells 23, 24 ... for further cells 23b, 24b, 256 ... 33b and further axial holes 21a are formed in the (opposite surface of the housing 13 to accommodate eleven photo-transistor cells 23a, 24a, 25a . . . located intermediately between the cells 23 and 23b, 24 and 24b, 25 and 25b . . . The photo-cells 23a ... are axially displaced from the cells 23, 23b . . . by the pitch P or a multiple of that distance. In the arrangement shown in Fig. 5, the cells 23, 24 . . . spaced at 1/11 of the circumference in association with the twelve circumferentially spaced transparent bands A, B . . . provide a vernier arrangement in connection with the operation of the phototransistors 23 . . . 33. The auxiliary cells 23a, 23b, 24a, 24b . . . are utilized to provide an indication of the direction of movement of the table or slide 10. Pulses from the photo-cells 23, 23b operate a bi-stable circuit acting as a gate for pulses from the cell 23a. An initial pulse from cell 23 operates the gate to direct a subsequent pulse from the cell 23a to a pulse adder. The pulse from cell 23a also operates the gate circuit to change its condition. A subsequent pulse from the cell 23b does not alter the condition of the gate, but if the apparatus reverses before such a pulse, the ensuing pulse from the photo-cell 23a is directed to a pulse subtractor. If the initial pulse comes from photo-cell 23b, the gate is set to direct a subsequent pulse to the pulse subtractor, this pulse also changing the condition of the gate D. In a modification, Fig. 6 (not shown), the optical grating 16 on the liner 15 comprises narrow annular bands with corresponding, e.g. front, edges spaced by P in association with an optical grating formed by a six-turn helix of pitch P, similar to grating 18, on the fixed tubular member 17. In this case the number of transparent bands corresponding to A, B, C ... of Fig. 4 is six and for the vernier arrangement five groups of photo-transistors are utilized. The inner cylinder may be reflective rather than opaque and transparent.