Patent GB-915078-A (GB)
Title: Improvements in and relating to the production of scales for measurement
Inventor: BURCH JAMES MORRISS
IPC: G02B5/18
Filed: 1958-05-05
Abstract: 915,078. Photo-electric pulse generators. NATIONAL RESEARCH DEVELOPMENT CORPORATION. May 5, 1959 [May 5, 1958], No. 14359/58. Class 40 (3). Linear or circular scales or diffraction gratings are produced under the control of a signal generated photo-electrically by the " fringe " pattern which results from relative motion between two similar " master" scales or gratings which need not have the same graduation spacing as required in the one to be produced. Figs. 1 and 2 show (in section and plan respectively) the arrangement for producing a linear scale &c. in which the latter is formed on a strip of photographically-sensitive material 21 carried by a moving cradle 15 which also carries one of two similar " master "  scales &c. 19, closely adjacent to the second " master " 22 which is supported in a fixed position. To produce the (moire) fringe pattern light from a source 23 is directed through the scales 19, 22 via mirrors 24, 25 and the transmitted light is then returned via mirrors 26, 27 and 24 to a photo-electric cell 28 the output (pulse) signals from which (due to the relative motion of scales 19 and 22) trigger a flash tube (not shown) to produce corresponding graduations on the strip 21 via the optical system 33, 37, 34, 35, 36. The width of the light beam from source 23 (in the direction of length of scales 19, 22) is such that an appreciable length of the scales is illuminated (three sources 23, mirrors 24 and photo-electric cells 28, as shown in Fig. 2, being preferably employed) so that, due to the integrating effect of the photo-electric cells, slight irregularities in the spacing of the graduations on the " master " scales 19, 22 are averaged out and the copy scale produced on strip 21 is therefore of a higher order of accuracy than the " masters." If the copy scale is required to have a larger or smaller number of graduations per unit of length than the " master " the output signals from the photo-electric cell(s) are suitably multiplied or divided respectively before application to the control circuit of the flash tube. The element 37 in the optical path from the flash tube to the strip 21 may be a slit but, preferably, consists of a small stationary scale or grating having the same graduation spacing as the " master " (producing a large number of slit images on the strip 21) so that in addition to increasing the quantity of light incident on the strip 21 slight variations in the triggering of the flash tube are averaged out. During production of the copy the cradle 15 is moved by means of a belt drive 29 along a rigid support comprising cylindrical members 11, 12 and horizontal straight edges 18. Fig. 3 shows a modification in which the photographically-sensitive strip 21a on which the copy is to be made is mounted in end-to-end relationship with the movable " master " 19a and the concave mirrors 24 (Figs. 1 and 2) replaced by collimating lenses 31, 32. Since, in this arrangement, the strip 19a moves partly in front of the stationary "master " 22a further photographic exposure of the former (already exposed to light from the flash tube 33a) to light from the sources 23a is prevented by using a flash tube 33a producing green light and employing red light sources 23a and appropriate filters 42 in front of the photo-electric cells 28a. Further precautionary measures include the provision of screens 39 and a mask 41. Instead of employing a flash tube a constant light source interrupted by a mechanical shutter driven by a synchronous electric motor controlled by the output of the photoelectric cells may be employed. Fig. 5 shows an arrangement for the production of circular scales in which light from a constant source 77 is directed via a rotatable shutter 44 and a slit 79 on to the periphery of a circular photographically-sensitive plate 21b on which the copy is to be produced. This plate is rotated by means of a shaft 59 the upper end of which carries one of the " master " circular scales 19b which thus moves with respect to the (fixed) second " master " 22b the latter being supported by means of a helicoidal condenser 74 which together with an annular totallyreflecting prism 75 produces illumination of the whole periphery of the two "master" scales from a single light source 23b and returns the modulated light to the photo-electric cell 28b. The output signals from the latter are then amplified at 76 and supplied to the control circuit of a synchronous motor 43 driving the shutter through a gear train 45 the ratio of which and the number of apertures in the shutter may be varied when it is desired to produce a copy scale having graduations of different spacing from those of the " masters." Instead of employing a flash tube or a mechanically interrupted light source the printing light may be produced from a constant light source which images a small stationary grating on to the photographically-sensitive material via an optical system comprising a relatively rotatable polarizing and analysing pair and two quarter wave plates arranged to produce circularly polarized light in opposite directions. The relative rotation between the polarizer and analyser is produced by an electric motor controlled by the output of the photo-electric cell receiving the moire fringes and by giving an appropriate speed to this rotation the image of the fixed grating may be caused to move in the same direction and at the same speed as the photographically-sensitive material (Fig. 4, not shown). Specification 912,881 is referred to.