Patent GB-1440130-A (GB)
Title: Position indicating system
IPC: G06F3/048
Filed: 1973-10-03
Abstract: 1440130 Position indicating systems INTERNATIONAL BUSINESS MACHINES CORP 3 Oct 1973 [20 Nov 1972] 46095/73 Heading G1N The position of a pick-up probe 8 adjacent to a surface of a dielectric material I below which surface is a plurality of evenly spaced parallel conductors e.g. 21, 22, 23 is determined by sequentially energizing the conductors with pulses, and comparing the ratios of the amplitudes of the picked up induced signals. The tablet of dielectric material I may be overlaid with drawing paper 12 and the probe 8 in the form of a ball-point pen with a metallic tip 11. Since there is a different capacitance between the conductors 21, 22, 23 &c. and tip 11 different induced charges are detected by tip 11 when the conductors are sequentially pulsed. The amplitudes V 1 , V 2 , V 3  of the voltages induced by pulsing three adjacent conductors are measured and since these are related to the distances d 1 , d 2 , d 3  the distance X from the tip 11 from the nearest conductor is calculated by the following equation: - If a further set of orthogonal conductors are provided then by repeating the process the two co-ordinates of the probe can be determined and the signals representative thereof fed to a computer. To increase the speed of the determination the conductors of each co-ordinate are divided into blocks and the conductors are first scanned to determine which block the probe lies within and then the conductors of the respective block is scanned to determine the position of the probe in the block. Block counting The conductors of each respective block e.g. 1 l ....1 m , 2 l ....2 m  &c. are connected together and via leads 54 1 , 54 2  to a pulse distribution circuit 53 under the control of block scanning pulses from controller 6. A 20 KHz pulse generator 52 supplies via scanning controller 51, pulses to pulse distribution circuit 53. Thus pulses are applied sequentially one at a time to lines 54 1 , 54 2  &c. so that the conductors of the respective block are energized. The output of stylus 8 is fed via switching circuit 26 to stores 28a, 28b, 28c connected in series, switching circuit 26 being controlled by timing pulses at 27 to store sequentially the stylus signals in the memory circuits 28a, b, c at a speed synchronized with the pulses applied to the conductors. Block detecting circuit 33 including three level detectors connected to a respective memory circuit and an OR gate, Fig. 7 (not shown), examines whether or not a signal level higher than a predetermined value is stored in the memory circuits. The absence of such a signal causes a negative output to be applied to controller 6 to cause a block counter (not shown) to step one position and cause the next block of conductors to be scanned. If such a signal is present the block counter is stopped, its contents transferred to block number register 36 and a line scanning pulse instead of a block scanning pulse generated on line 37. Line scanning Pulse distribution circuit 36 controlled by scanning controller 51 feeds the conductors, arranged so that all the first conductors 1 1 , 2 1 ,... of the blocks are connected, all the second conductors 1 2 ,2 2  .... are connected together &c. sequentially with 20 KHz pulses. The stylus output is again fed sequentially to the memory circuits and line detecting circuit 38, comprising two level detectors and an AND gate connected to calculating circuit 30, Fig. 7 (not shown), compares the memory circuits outputs. When the level of the signal in store 28b is not greater than both signals in stores 28a, 28c circuit 38 generates a negative output and controller 6 causes a line counter (not shown) to step one position and the next conductor of the blocks to be scanned. If the signal level of store 28b is greater than both the adjacent stores controller 6 causes the line counter to be stopped and its contents to be transferred to line number register 40. Calculating circuit then provides an output in accordance with the equation using voltages V 1 , V2, V3 stored in the memory circuits which is converted to a digital form 31 and applied to co-ordinate register 32. The final co-ordinate form is obtained from the three registers. If the stylus is nearest the first or last conductor of a block then the block has to be scanned a second time to obtain an output. After completion of the cycle it is repeated for the orthogonal array of conductors.