Patent GB-909916-A (GB)
Title: Improvements relating to measuring apparatus
Inventor: WILLIAMSON DAVID THEODORE NELS
IPC: G05B19/33
Filed: 1958-06-11
Abstract: 909,916. Electric copying machines. FERRANTI Ltd. June 10, 1959 [June 11, 1958], No. 18597/58. Class 38 (6). [Also in Group XL (b)] Measuring apparatus responsive to the movement of an object relative to a reference structure comprises a first optical grating secured to the object, a second optical grating in optical registration with the first grating, the gratings being so ruled as to set up in combination with one another a control cyclic optical pattern which is displaceable in phase relative to the reference structure in dependence on object movement, a light source irradiating the pattern, a control photo-electric device irradiated by light from the pattern, a reference source providing a reference signal at a predetermined frequency, scanning means controlled by the reference signal to cause the output signal from the photo-electric device to alternate at the predetermined frequency with a phase displacement relative to the reference signal equal to the phase displacement of the pattern relative to the reference structure, a discriminator deriving a signal in continuous dependence on the phase of the signal from the photo-electric device relative to the reference signal, and utilization means responsive to the discriminator output. In the embodiment illustrated in Figs. 1 and 2, a disc 16 bearing a radial grating 15 is rotated by motor 19 across a grating 11 secured to work-table 13 moved linearly by motor 33, and across a fixed grating 12; the ensuing outputs of photo-cells 21, 22 are at a frequency determined by the speed of motor 19 and of a relative phase proportional to work-table displacement. Coarse gratings, e.g. 100 lines to the inch, are used. A magnetic tape 25 carries "command" and "reference" tracks 26, 27 respectively, the outputs from which are fed to phase discriminators 31, 35 to produce control of motor 33 dependent on the difference of the error signals from the discriminators, thus affording correct magnitude and sign of the motor control voltage. Errors in the measurement of the work-table displacement otherwise arising from imperfect disc drive or like fluctuations from lamp 17 are inherently obviated by the inclusion of photo-cell 22. An error signal from discriminator 35 is utilized to control the speed of motor 19; or the motor may be energized to run above synchronous speed and discriminator 35 output applied as an eddycurrent brake. The grating system may have unequal widths of transparent and opaque bands; one grating of each pair may be formed on a reflective base, Fig. 3 (not shown); all gratings are radial when the work-table is to be rotated; the grating on disc 16 may be spiral to permit direct motor drive, Fig. 4 (not shown), or helical on a rotated cylinder, Fig. 5 (not shown). The disc or its cylindrical counterpart may be fixed and the light source moved, e.g. by using a cathode-ray tube (C.R.T.) spot scanned along a time base synchronized with the recorded reference signal. The reference pattern need not be used, Fig. 11 (not shown). In a further embodiment, Figs. 6 and 7, the " control " light pattern is set up by gratings 51, 52 secured to the work-table and fixed frame 14 respectively, and the " reference " light pattern by similar, fixed, gratings 54, 53. The skew angles of the gratings are adjusted for equality of the wavelengths of the patterns with that of endless film 57 which carries an optical track whose density varies sinusoidally in the direction of film motion. The speed of film-driving motor 59 is controlled as in the first embodiment. Pattern-forming variants are described, Figs. 8, 9 and 10 (none shown). Infra-red radiation may be utilized; and the coarse gratings may be replaced by diffraction gratings.