Patent GB-1085523-A (GB)
Title: Construction and control of ruling engines
Inventor: DYSON JAMES
IPC: G01D13/02
Filed: 1963-11-18
Abstract: 1,085,523. Dividing engines; photo-electric interferometers. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Nov. 17, 1964 [Nov. 18, 1963], No. 45507/63. Headings G1A and G1X. [Also in Divisions G2 and G3] A ruling engine for the production of diffraction gratings is provided with a blank carriage 25 which moves continuously on rays 51 driven by a screw 52, the operation of the engine being controlled by an interferometer which measures the longitudinal traverse of the blank 24, variable length evacuated tubes 42, 43 and 44, 45 being included in the paths of the interferometer beams the length of these tubes varying with the movement of the carriage in such a way that the total length of the passage through air is the same for both beams. A lamp 58 e. g. a gas optical maser provides monochromatic light for the interferometer and this passes through a polarizer 62 to a Koster's  prism 57 which splits the light into two beams passing by way of a quarter wave plate 63 through the evacuated tubes to be reflected one by a mirror 36 secured to a stationary part of the engine and the other by a mirror 41 on the blank carriage 25. The screw 52 is driven at a suitable constant speed by a worm drive 53. The evacuated tubes are telescopic and are extended and closed by screws 54a, 54b driven from the screw 52. With the blank carriage 25 moving to the left in Fig. 1,  the screws 54a, 54b are arranged to extend the tubes 44, 45 at half the rate of movement of the carriage 25 and to close the tubes 42, 43 at the same rate so as to maintain the path length in air for the two interferometer beams equal. The beams returning to the Koster's prism pass to photo-cells 66, 67 by way of a phase modulator 64 and an analyzer 65. An A. C. voltage is applied to the modulator 64 so that the photo-cell outputs have an A. C. component modulated by a sinusoidal envelope corresponding to the carriage motion. The light to the photo-cell 67 additionally passes through a quarter wave plate 68 so that the sinusoidal modulations of the photo-cell outputs are in quadrature. These outputs are used to drive a magslip 69 which rotates through an angle dependent on the displacement of the blank carriage. A diamond (not shown) for ruling the blank is  carried by a shank 12 on an arm 13 the arm being mounted on the diamond carriage 16 which is reciprocated on fixed rays 17 by means of a connecting rod 55 and a mechanism 56 serving to move the carriage 16 with constant velocity over one stroke, the diamond being lifted clear of the blank by mechanism (not shown) at other times. The magslip 69 drives a disc 71 which has recorded on its periphery a sinusoidally varying magnetic pattern having say 50 complete sinusoids. Three pick-up heads 72, 73, 74 pick-up voltages from the disc and constitute a three-phase supply to a synchronous motor 75 which drives the mechanism 56, the motor 75 being driven at 50 times the speed of the magslip 69. If a sudden jump in the motion of the blank carriage occurs the magslip 69 will follow this if it does not exceed half a fringe. The motor 75 may not respond fast enough and lose synchronism by a whole number of cycles of its driving supply. A back-up system comprising a rotatable half wave plate 81, phase modulator 83, photo-cell 85 and motor 87 are provided to prevent this and their operation is described in the Specification. To ensure that the diamond remains on its correct path during ruling a small Wollaston prism 18 is mounted on the shank 12, as close to the diamond as possible. Light from a lamp 27 passes through a polarizer 28 to the prism 18 wherein it is divided into two beams, which after successive reflections in fixed apertured mirrors 19, 21 pass to a photo-cell 31 by way of a phase modulator 32 and analyzer 29. If the diamond moves along its correct path, the path difference between the two beams is zero. Due to the modulator 32 the light will be modulated in intensity and the photo-cell output will have an A. C. component having no other frequency component. If the Wollaston prism moves normal to its direction of travel, a frequency component will be introduced into the photo-cell output, and this is detected by a phase conscious rectifier 34 giving a D. C. output which is used to return the Wollaston prism and hence the diamond to its correct path using a Piezo-electric element 35.