Patent GB-650913-A (GB)
Title: Improvements in or relating to apparatus for the representation of data in a binary digital form
Inventor: BARKER RONALD HUGH
IPC: H03M1/00
Filed: 1948-08-23
Abstract: 650,913. Electric selective signalling. MINISTER OF SUPPLY. Aug. 23, 1948, No. 22215. [Class 40 (i)] [Also in Groups XL (b) and XL (c)] A scale is divided transversely into a number of tracks marked so that any position along the scale can. be represented as a binary number, and the scale tracks, after the first, are scanned in a predetermined order at one of two alternative positions, the operative one of the two scanning positions for any track being determined by the indication received at the scanning of the previous track. The scale readings may represent any magnitude, e.g. displacement, barometric pressure, electric current, &c. and the invention is particularly adapted for telemetering, the various binary digits of a reading being transmitted as the presence or absence of an electric impulse or by short and long impulses such as Morse dots and dashes. The application to pulse code modulation is also mentioned. As shown, Fig. 1, a shaft, the angular position of which is to be translated into digital form, carries a known form of disc 7 divided into six digital rings 1 ... 6, subdivided into alternate opaque and transparent sections, or conducting and non-conducting sections. The rings 6, 5, 4 ... are subdivided into 2, 4, 8 ... 64 sections, respectively, so that each one of the 64 sections of ring 1 is characterized by a unique combination of transparent and opaque sections in a radial direction inwards therefrom. If the disc is then scanned along a fixed radius, e.g. by a light source on one side thereof, a photo-cell on the other side will react to produce a series of electric impulses representative, in the binary system of the angular position of the scanned radius from the zero position 8. Further rings may be used for measuring to a finer fraction of a revolution. To eliminate scanning errors due to rotation of the disc and to slight misalignment of the disc sections, scanning is effected by a mask, Fig. 2, having two series of radial slits 20, 21, 23, ... 29 and 20, 22, 24, ... 30 arranged symmetrically in pairs 21, 22, 23, 24 ... &c. co-operating with the respective track rings 1, 2, 3 ... &c., the total angular spacing between each pair of slits being equal to that of one section on the immediately surrounding ring. The slit 20 is first illuminated and if this lies over a transparent section of ring 1 so that an output is obtained, the slit 21 is next illuminated, whereas if slit 20 lies over an opaque section the slit 22 is next illuminated. Similarly, whichever slit 21 or 22 is illuminated, slit 23 is next illuminated if slit 21 or 22 lies over a transparent section and slit 24 is illuminated if an opaque section is illuminated by slit 21 or 22, and so for succeeding slit pairs. The two series of slits may be replaced by two symmetrically disposed slits 33, 34, Fig. 3 (not shown), which in Fig. 4 are replaced by similarly-disposed linear traces 33, 34 produced by a cathode-ray tube 39, the central portions only of the track rings on the disc 7 being exposed to the traces 33, 34 through arcuate slits 41-46 in a mask 40. The traces are controlled electronically, the appropriate portions only of the respective traces being illuminated in a manner corresponding to the illumination of the various slits 20-30 in Fig. 2, i.e. in dependence on the disc position. Scanning control circuit, Fig. 6. A sawtooth voltage 63 is applied from the time base generator 58 to the Y plates of the C.R.T. 39. The photo-cell 38 produces an output only when scanning a transparent section of disc 7. This output is converted at 50 into a positive pulse 60, amplified and differentiated at 51 to produce the waveform 61 which is shaped by a Miller integrator 52 to produce a saw-tooth wave 62 the peaks of which occur as the scanning spot leaves a transparent section. The output 62 is fed to a Kipp relay 53 in which the triode 53b is conducting and triode 53a non- conducting in the absence of any input 62. In this case the point 55 being at the potential of the H.T. supply causes diode 59a to conduct increasingly as the potential at point 68 drops due to the time base generator 58, so that the trace 34 is produced by the C.R.T. 39. An output impulse 62 reverses the conditions of the two triodes 53a, 53b, whereupon diode 59b conducts and 59a is blocked to switch the C.R. beam to the trace 33. The time base 35 is equal to the time of one radial scan of the disc 7 and the pulses 62 effectively last for the scanning time of one track only. The portions of the respective traces illuminated are therefore determined by the condition (opaque or transparent) of the preceding track scanned as above described. Modifications. The same scanning effect can be produced with a single radial scan by constructing the disc 7, as shown in Fig. 7, which is similar to the Fig. 1 construction except that each track other than the first track 71 is divided radially into equal inner and outer portions 72a, 72b, 73a, 73b, &c. and the respective portions of the second and subsequent tracks are offset circumferentially in opposite directions a distance equal to half the width of a section of the next outer track. Thus, e.g., the track portions 72a, 72b are offset anticlockwise and clockwise respectively a distance equal to half the width of a track section 71, and so on. Scanning is effected by a C.R.T. or mirror drum device controlled in such a way that if the outermost track sector 71 scanned is opaque the indication from the outer half only of track 72 is considered whereas the inner half only of track 72 is considered if the track sector 71 scanned is transparent. If the indication from track 72, whichever half is operative, is opaque, then the indication from the outer half of track 73 is next considered. The converse applies if the indication from ring 72 is transparent and so on for the succeeding inner tracks. The control circuit is shown in Fig. 8. The photocell output 38 is amplified at 80 and applied to pentodes 82, 83, a delay network 81 delaying the application of the input to pentode 82 by half the time taken to scan one track. The screens of valves 82, 83 are pulsed by a voltage train 87, the pulses coinciding with the instants that the scanning spot is at the centre of the inner halves of the various tracks 71, 72 ... 76. The valves 82, 83 can conduct only during the pulse periods 87 and the valve to conduct is determined by a Kipp relay 84 comprising valves 84a, 84b, valve 84a being conducting in the stable condition of the relay and valve 84b when unstable. Valve 82 only may conduct in the stable condition of the relay and valve 83 in the unstable condition. Assuming position 78, Fig. 7, is being scanned, the first half of the outermost track sector 71 being transparent produces an impulse 88 which is passed by valve 82 and delay network 85, similar to 81, and then differentiated, the resulting wave 89 being applied to the grid of the valve 84b to flip over the relay 84 and hence reverse the valves 82, 83 as the scanning spot passes to track 72 so that during the scan of track 72 only the inner half 72b of the track can pass a pulse (through valve 83). The Kipp relay remains in its unstable position for a period slightly greater than the time of scanning one track and so reverts to the stable position if not further pulse is received during the scanning of the track 72. In the example mentioned the inner half of track 72 scanned is transparent so that the relay is maintained in its unstable position for the duration of scan of track 73, the inner half only of which is considered. This also is transparent so that in a similar manner the inner half of track 74 is considered but as this is opaque the relay returns to the stable position as the scanning spot passes to the next track 75 where the first half only is considered and so on. The output 86 from the circuit 84, i.e. the shaft position in digital form, consists of a negative pulse corresponding to each pulse 88 and lasting for the time that the relay 84 is flipped over to " unstable." In alternative control circuit, Fig. 10, the outputs of cascade connected multivibrators 110, 111, 112 are combined at 116 to produce a stepped voltage waveform, the steps producing the dashes in the trace on the C.R.T. 39, in positions covering the centres of the outer track sections 71a, 72a, ... of the disc 7, Fig. 7. The output from the photo-cell 38 arising when a transparent section is scanned, is converted into a pulse amplitude equal to half the rise of each voltage step which is applied to the plate Y1 for the duration of the succeeding step so that the trace dash reads the centre of the inner half of the next track. This is achieved by delaying and suitably shaping the photo-cell output at 100, Fig. 12 (not shown), mixing this delayed output respectively with positive pulses 119 and negative pulses 120 coincidental with the voltage steps and derived from the vibrator 110 and shapers 118, 117. The sum voltages so derived are limited at 101, 102, the combined output therefrom operating a relaxation device 103 giving an output waveform similar to that of the trigger circuit 50 actuated by the cell 38 delayed by the duration of a track scan which, as stated above, is applied after suitable amplitude-limiting to the Y1 plate. The output 127 from 103 representative of the angular position of shaft 7 is also suitable for transmission to a distance. The invention may be applied to straight and fixed scales co-operating with movable scanning devices. The scales may be non-linear, e.g. a disc arranged to give cosine readings at a position displaced by 90 degrees to enhance accuracy of the reading and an additional ring may be used in such a case to indicate sign.