Patent GB-1525649-A (GB)
Title: Automatic coordinate determining device
IPC: G06F3/033
Filed: 1976-05-21
Abstract: 1525649 Position encoders DAINI SEIKOSHA KK 21 May 1976 [23 May 1974 28 May 1975 17 Nov 1975 19 Nov 1975 10 Feb 1976] 21182/76 Heading H3H The cartesian coordinates of the position of a probe carrying an inductive winding on a tablet carrying sets of parallel conductors defining X and Y coordinates are obtained by applying an oscillating signal to either the probe or the conductors and sequentially activating the conductors, the addresses of the X and Y conductors giving the maximum induced oscillating signal (in the conductors or probe as the case may be) are obtained by comparing the amplitude values of the induced signal at successive conductor activations. Preferably each conductor is an elongate loop, Fig. 2, adjacent legs of adjacent conductors being close together, and the probe winding 5a has a diameter equal to or greater than twice the conductor pitch r. Activating the X conductors is done by applying forward bias to diodes D1 ... Dn in turn or by enabling respective switching devices (not shown) in turn. In Fig. 8 a plurality of oscillating signals of different frequency are applied simultaneously or in sequence to the probe winding or to respective windings on the probe. Band pass filter 10 selects the frequency used for coordinate sensing and band pass filters 62 select the other frequencies which may represent command signals for a computer, Fig. 14 (not shown). The output of filter 10 is amplified, 11, rectified, 12, filtered, 13, and digitized, 14, the digital values for successive conductor activations being stepped through shift registers 16, 17, 18, Fig. 10. A comparator 19 detects when the value in register 17 first exceeds that in register 16, register 17 then being assumed to hold the maximum value, signal Mx causing latch circuit 57, Fig. 8, to store the address of the conductor giving the maximum value. For interpolating the position of the probe between conductors, a comparator 20 selects 21, the value in register 16 if this is greater than or equal to that in register 18 or minus the value in register 18 if this is greater than the value in the register 16. A divider 22 divides the maximum value in register 17 by the so-selected next highest value thereby eliminating some errors due to irregularities in the conductors or in the probe-conductor spacing, the result addressing a read-only memory 27 to provide an interpolation value to be added to or subtracted from the conductor address. Correction for irregular conductor spacing is provided by read-only memories 55, 56 which convert the conductor addresses (i.e. theoretical positions) to the actual positions. The maximum value detected is also fed back via a digital to analogue converter to control the gain of amplifier 11. In a modification, Fig. 15 (not shown), the non- linear conversion performed by read-only memory 27 in Fig. 8 is done on the analogue signal before it is fed to the analogue-to-digital converter (14). In a second modification, Figs. 16-18 (not shown) only those conductors within a given distance of the last detected position of the probe are scanned, the address counter (52) being decremented by a set amount at the start of a new scan instead of being reset to zero. Several probes associated with different frequencies may be used on the same tablet.