Patent GB-1366990-A (GB)
Title: Coordinate digitizer system
IPC: G06F3/044
Filed: 1972-02-29
Abstract: 1366990 Digitizing position W L COTTER 29 Feb 1972 [5 April 1971] 9275/72 Heading G4H In apparatus for digitizing the position of a stylus over a platen, the platen comprising a coarse set of parallel conductors and the stylus including an electrical conductor to provide capacitive coupling between the conductor and the stylus, the conductors are pulsed sequentially and a counter is simultaneously incremented, a detector, receiving the stylus output, causing readout of the count when the output signal from the stylus is at its maximum. Preferably the platen includes both coarse and fine sets of wires in two perpendicular directions, the fine wires being connected in four groups. The coarse and fine sets may be separate or every fourth fine conductor may serve as the coarse conductor. As described an output signal from detector 57 (Fig. 25) sets bi-stable FF1 and enables fine grid pulse generator 54 so that each of the four groups of fine X conductors are scanned sequentially. Four associated sample and hold circuits (33-36, Fig. 19, not shown) are sequentially enabled to store the output signal from the stylus, the outputs of alternate pairs of groups being combined in summers (37, 38) to derive as the stylus moves across the platen sine and cosine signals which are applied to comparators (41-48) the outputs from which are square wave signals phase displaced from each other by 22¢ degrees, the outputs of two of the comparators (41, 45) being in phase with the sine and cosine signals respectively. The outputs from the comparator are combined to derive binary signals representing the fine X position of the stylus, these binary signals being stored. The output of the first comparator (41) may be used to determine which of two coarse conductors the stylus is nearer since its output changes at the mid point of the two conductors. Similarly by combining the outputs of two of the comparators (41, 45) in an exclusive OR gate (Fig. 21, not shown) the interval between two coarse conductors is subdivided to four equal parts. By using all the comparator outputs the interval is divided into sixteen parts. After the fine position has been found, an X counter 19 is incremented by clock pulses from a generator 18, the counts being decoded and used to scan sequentially the X conductors. The resulting signals from the stylus are fed to sample and hold circuit 30 and differentiator 31, the output from which is added to the output of the first comparator (41) and fed to detector 57 which when triggered enables gate G7 to transfer the count in counter 19 to X store 21. The detector output also resets bi-stable FF1 so that the process is repeated for the fine and coarse Y conductors. The addition of the most significant bit of the fine signal (from comparator 41) to the output of differentiator 31 ensures that the least significant bit of the coarse signal and the most significant bit of the fine signal change states simultaneously. The operation of the detector 57 results in the triggering of a monostable vibrator so that the X and Y counters are counted down for a number of pulses before the next digitization. As described the scan pulses are such that the trailing edge of one scan pulse is coincident with the leading edge of the next so that by differentiating the scan pulses the stylus receive the signal which is the difference between the negative signal from a wire being turned off and a positive signal from the next wire being turned on. Consequently the stylus output constitutes a series of positive spikes as the scan nears the conductor to which the stylus is adjacent followed by a series of negative spikes as it moves away from the conductor. Modifications.-The spacing of the coarse grid wires may be logarithmic. The co-ordinate axes need not to be orthogonal, digitization being effected in for example polar co-ordinates. For the fine digitization the stylus may be pulsed instead of the fine conductors.