Patent GB-743058-A (GB)
Title: Improvements in or relating to means for controlling the movement of a light beam orlight spot
Inventor: WILLIAMS FREDERIC CALLAND
IPC: G11C13/04
Filed: 1950-11-10
Abstract: 743,058. Automatic control systems; alignment. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Nov. 7, 1951 [Nov. 10, 1950], No. 26037/51. Divided out of 731,342, [Group XIX]. Class 38 (4). [Also in Group XIX] In an arrangement for controlling the repetitive movement of a light beam over a chosen path having a control area at each end, means are provided for deriving separate electric signals from the passage of the light beam over each of the control areas in turn, and from these signals error signals are developed for correcting any deviation of the light beam from the chosen path. In the described embodiment the invention is applied to an electronic calculating machine (see Group XIX). Means are provided to ensure that the light spot produced on the screen of the C.R.T. 17, Fig. 1b, is accurately controlled in its scanning movement whereby the resultant signals obtained from a photo multiplier tube PMT are in proper synchronism with the calculating machine. A stack of cards 26 includes information cards 20, Fig. 4, and interleave cards 30, Fig. 5, and preferably one only of the latter is provided with additional perforations 70, 71, 72, 73, 74, 75 at both ends of each row, those at the four corners being triangular and arranged as shown. The rest of the cards have larger holes 76 in these positions to provide a clear passage for the light from the C.R.T. The movement of the light spot along each row of perforations is controlled by a comparison of the signals resulting from the passage of the light spot over the additional perforations with timing signals derived from the calculator. Control of the progressive deflection of the light spot in a direction at right angles to the row scanning motion is effected by comparing with each other the relative time durations of the signals resulting from the passage of the light spot over the triangular perforations at either end of the first row, and similarly in the case of the last row. The effect of misalignment of the scanning level of the light spot is illustrated in Fig. 7; when correctly aligned with the centre of the light spot moving along a path p, the output pulses from the photo tube (after amplification and shaping) are of equal width (diagram f); when the path of the light spot lies above p the widths of the output pulses are unequal (diagram g); when the path lies below p they are unequal in the opposite sense (diagram h). The X-shift servo control comprises two triodes V24, V25 with anodes connected to each end of a centre-tapped transformer winding and cathodes connected in parallel and through a resistor R30 to the anode of a " tail " valve V26. The secondary winding of the transformer is included in a circuit having two opposed diodes D17, D18 and a condenser C17. Valve V26 is normally held cut off but is rendered conducting by a pulse at terminal T16 derived from the passage of the C.R.T. beam over one of the holes 70, 72 or 74, Fig. 5. This pulse alone would render V25 conducting, but it is normally preceded by a strobe pulse at terminal T15 which raises the bias potential of the control grid of valve V24 so that this valve and not V25 is rendered conducting as a result of the light pulse at terminal T16. The resulting pulse of current through the transformer secondary winding and one of the diodes D17, D 18 alters the charge on the condenser C 17 by a certain amount. The termination of the strobe pulse at terminal T15, arranged to occur, in normal conditions, at the mid-point of the light pulse, cuts off valve V24 whereupon valve V25 conducts and a pulse of current passing in the opposite direction through the transformer windings and diode D17 or D18, alters the charge on the condenser C17 in the opposite sense, normally just cancelling it. It any charge remains on the condenser C17 after this action it is amplified and used to adjust the X-shift control of the time base appropriately. The X-rate servo control comprises a circuit similar to that shown in Fig. 12a, but in this case the signals resulting from the scanning of the holes 71, 73 and 75, are split by a corresponding strobe pulse from the calculator to control the rate of scan of the X-time base. The Y-shift servo arrangements are generally similar to those of the X-shift, but in a modified circuit shown in Fig. 17 a waveform from the calculator, applied to the grid of the " tail " valve V54, renders the latter conductive during the scanning of the first line only. The + 75V bias on both valves V52, V53 allows very little current to flow until the control grid of valve V53 is driven positively by the pulse due to the passage of the C.R.T. beam over the first triangular hole 72, Fig. 5, when a pulse of current will flow through the transformer winding as described for the X-shift servo. When the C.R.T. beam passes over the triangular hole at the end of the first line the control grid of the valve V52 is driven positively and a second pulse of current flows in the opposite direction through the transformer. Any resultant remaining on the charging condenser, representing the vertical misalignment of the scanning level as explained with reference to Fig. 7 (e) ... (g), is applied as a corrective shift voltage to the Y-time base circuit. The Y-rate servo arrangements are similar but are controlled by the scanning of the triangular holes 74, 75 at either end of the last line in the card. Any resultant signal on the charging condenser alters the Y time-base run-down appropriately. Means comprising a phaseadvance network for correcting baseline shift (illustrated by diagrams c and d, Fig. 7) and distortion in the photo-multiplier tube output, are also described; alternatively a shortcircuited delay line may be employed for this purpose. Other types of cards may be used, and the apparatus may be adapted to produce telegraphic or other signals. Instead of a C.R.T. light source a rotating disc or mirrortype scanner may be employed. Specifications 705,479, 731,341, 731,342 and 742,522, [all in Group XIX], are referred to.