Patent GB-1166265-A (GB)
Title: A Graphical Data Digitizer.
IPC: G06K15/22
Filed: 1966-12-20
Abstract: 1,166,265. Graphical data digitizer. CALMA CO. 20 Dec., 1966 [21 Dec., 1965], No. 56909/66. Heading G4H. A graphical data digitizer has means for tracing a graph, digital encoder means for converting increments of movement of the tracing means into digital data representing the direction of the incremental movement and recording means for recording the digital data on a recording medium. The apparatus comprises a tilting table 14, Fig. 1, having a translucent screen 15 beneath which the graph or other curve is placed, illuminated from below. A tracing stylus is mounted on a lockable carriage 22 movable vertically along a rail 21 which, in turn can move across the screen on top and bottom rails. Cables attached to the top and bottom ends of rail 21 are caused to rotate a shaft carrying a coding disc, the rotation of which represents the horizontal, or X displacement. Another cable attached to the carriage 22 is similarly wound round a shaft carrying a second coding disc, the rotation of which represents z, the sum of the horizontal and vertical movements. Each coding disc has a toothed periphery, Fig. 6, and it is sensed by three pick-ups T4-T6, displaced relatively to each other by one third of a tooth pitch P. Regular pulses are applied to the inner coil 501 of each pick-up. The outer coils 500, 502 have voltages induced in them and they are connected in opposition so that the output may be positive, negative or zero according to the position of the pick-up relative to the teeth of the disc. The pick-up heads are pulsed by a driver 250, Fig. 12, four times in each recording cycle of the tape recorder 35, Fig. 1. The signals from each group of three heads are amplified and gated to one-shot circuits. The outputs of these are combined in diode gating circuits. Only states 1, 3 and 5, Fig. 23, are used out of the possible 6 states. The intermediate combinations of outputs N1-N3 from the one-shot circuits are ignored. This prevents small oscillations of the stylus producing a series of change signals. The present and past conditions are compared and a change of state causes a corresponding store 252, 253, 261, 262 to be set to indicate positive or negative increments of X or Z. At the end of each record cycle the stores 252, 253, 261 and 262 are sampled and the increments of X and Z are passed to circuits 270 and 280. In circuit 280 the Z values are modified by subtracting the X values to obtain the Y values. The X and Y increments are stored in elements 271, 272 and 281, 282. The circuits 270, 280 also have connections from a manual keyboard 31 so that values 1, 2, 4, 8 can be entered in the stores 271, 272, 281, and 282. These values are recorded in the same way as are increments of X and Y movement. The keyboard also has an IRG (inter-record gap) button which when pressed applies a signal to the tape recorder to advance the tape a predetermined distance without recording any data. The keyboard also has A and B buttons whereby extra data can be entered on the tape. Gates AA and BB prevent these signals passing while the IRG signal is present. If the recording speed of the system is exceeded through the operator moving the stylus too quickly an alarm device 310 is operated. The overspeed detector responds when any of the stores 252, 253, 261, 262 changes more than once in each recording cycle. Since only one bit is recorded in each channel during a recording cycle if more than one bit is produced, all but the first would be lost. The alarm prevents this error. Similarly if any of the stores changes during an inter-record gap the alarm is operated. Motor driven means is provided for shifting the paper web carrying the graph when necessary. The stylus comprises a circular window in carriage 22 having cross-lines. The carriage is moved by an upstanding lug.