Patent GB-929758-A (GB)
Title: Scale reading device for measuring scales and the like
IPC: G01D5/26
Filed: 1960-09-26
Abstract: 929,758. Photo-electric scale reading apparatus. M. HENSOLDT & SOHNE OPTISCHE WERKE A.G. Sept. 26, 1960 [Oct. 10, 1959], No. 32935/60. Class 40 (3). [Also in Group XX] In an arrangement for reading the setting of an index with respect to a relatively movable scale, in particular, a scale indicating the position of a machine tool, an image of the scale 1 (Figs. 1, 2, 3), which is illuminated by a light source 21 (Fig. 2) and an optical system including beam splitting cubes 4, 7 is formed in a viewing window 6 and, also, on a prism 10 the light transmitted by which is collected by two photo-electric cells 12, 13. The light from source 21 is periodically interrupted by an electromagnetically operated shutter 24 co-operating with an apertured diaphragm 23 so that the cells 12, 13 generate A.C. output voltages which, after amplification in 14, 15 and rectification in 16, 17, are supplied to a cathode-ray tube indicator 18 producing two " shadow-bands " the respective heights of which are determined by the voltages from rectifiers 16, 17. When the image of a scale index mark is precisely located on the apex of prism 10 these two voltages are of equal value and the " shadow-bands " are also of equal height. In operation, this setting is effected by adjustment of the position of lens 3 (Figs. 2 and 3) which moves in a slide 3<SP>1</SP> along the length of the scale 1 and carries an index 34 (Fig. 1) enabling its position to be read with respect to a fixed scale 35. The operating shaft 30 (Fig. 2) for producing the traverse of lens 3 carries a calibrated drum 31 (Fig. 3) and the setting of the latter is read in a window 32 (Fig. 1) with respect to a fixed index 33. The complete reading is thus: 150 (window 6); 3 (fixed scale 35) and 0.35 (window 32) giving a reading of 153.35. If a third decimal place is required the drum 31 is positioned so that the nearest scale division is accurately aligned with index 23 and the resulting departure from equality of the heights of the " shadow-bands " 19, 20 read on a scale 41. The complete reading is thus 153.355. Fig. 4 shows a modification in which the interruption of the light is effected in the optical path " after " prism 10, the interruption in this case being such that the two halves of the beam from prisms 10 are alternately interrupted. In this arrangement a single photoelectric cell 54 is employed the output of which after amplification in 55, is rectified in a rectifier 56 operated synchronously with the electron mechanical interrupter drive 24 so that when the image of a scale index mask is accurately positioned on the apex of prism 10 the output from cell 54 is a direct-current voltage which is not passed by amplifier 55. Departure from such position then results in unbalance and the resultant A.C. voltage is effective to vary the lengths of the " shadow-bands " on the indicator tube 18. A further arrangement which allows the setting to be read to three places of decimals utilizing gear coupled calibrated drums and in which the indicator tube is employed merely to ensure correct setting of the index is described with reference to Figs. 7 and 8 (not shown).