Patent GB-1520111-A (GB)
Title: Electrical position responsive arrangements
IPC: G01B7/004
Filed: 1976-06-08
Abstract: 1520111 Electrical position resolver J A TURNER and G J RITCHIE 8 June 1976 [8 March 1975] 9733/75 Heading G1N An electrical position resolver includes a resistive tablet connected to electrodes 22, 23, 24, 25 disposed along its four sides. Electrodes 22, 25 receive a supply voltage waveform V 21  from a generator 30 and electrodes 23, 24 receive a supply voltage waveform V 22  from generator 31. The waveforms V 21 , V22 are exactly similar but the latter is delayed relative to the former by a predetermined time, and each waveform has at least one part during which the voltage-time relationship is linear. Application of the supply voltages to the electrodes is controlled by semi-conductor switches 21, those for a first pair of parallel electrodes 22, 23 being gated by a voltage V 1  1  from a pulse generator 26 and those for the other pair 24, 25 being gated by a voltage V 12  from a pulse generator 27. A movable probe 5 is either capacitively coupled to the resistive layer forming the tablet or is in direct contact with it. The voltage V23 sensed by the probe is dependent on the voltages V 21 , V22 as a function of its X or Y co-ordinates on the tablet, which co-ordinates can be determined sequentially but not simultaneously. During the period P when a gating pulse V 11  is applied to the switches for electrodes 22, 23 the voltage waveform V 21  is applied to electrode 22 and the voltage waveform V22 is applied to the electrode 23. The horizontal or X ordinate of the probe position i.e. its perpendicular distance from electrode 22 is proportional to the time delay t 1  of the zero-crossing of the probe voltage waveform V23, during a period Ti in which both waveforms are linear, relative to the corresponding zero-crossing of the waveform V 21  on the electrode 22. During a next following period P 2  a gating pulse V 12  is applied to the switches for electrodes 24, 25 and voltages V 22 , V 21  respectively are applied to these electrodes. The vertical or Y ordinate of the probe position i.e. its perpendicular distance from electrode 25 is proportional to the time delay t2 of the zerocrossing of the probe voltage waveform V23, during a similar period T to that during period P 1  in which both waveforms are linear, relative to the corresponding zero-crossing of the waveform V 21  on electrode 25. The time delays ti', t 2 ' of zero-crossing of the probe voltage V 23  during periods T 1 ' when the voltages V 21 , V 22  are linearly decreasing are also proportional to the horizontal and vertical ordinates of the probe. If the averages of delays t 1 , t 1 ' and of t2, t2' are taken the results are independent of noise and of variations of quiescent voltages. Noise may also be compensated for by restoring the voltage level of probe voltage V 23  to a reference voltage such as zero when the waveforms V 21 , V 22  are both at constant levels during periods T 4 . The time of zero-crossing of V 23  is then defined as the instant when it passes through this reference voltage. Compensating networks 28, 29 compensate for the effect of probe-resistive layer capacitance which alternates the low-frequency components of the voltages sensed by the probe. These networks may be substituted by direct connections. A-D conversion To convert the sensed probe voltage V23 to a digital representation of the X, Y co-ordinates of probe position it is passed via buffer amplifier 34 to a clamp circuit 36 and thence to a gate 37 which also receives the voltage V 21 . The gate opens when voltage V 21  passes through zero at the start of periods T 1  and T 1 ' and closes when the probe voltage V23 next passes through zero producing output pulses corresponding to the delays t 1 , t 1 ', t 2 , t 2 '. These pass to gate 38  receiving high frequency pulses from an oscillator 40. The resulting bursts of pulses are directed either to counter 42 or counter 44 by gates 46 or 48 according to which electrode is receiving voltage V 21 . In operation the probe 5 is held in a fixed position for a time equal at least to P 1  plus P2 and the totals in counters 42, 44 are read off at the end of this time. The clamping circuit 36 is connected to the reference voltage level through a switching transistor during periods T4 under the control of a timing unit 32. Connection to the resistive layer to form the electrodes is made by groups of high conductivity dot contacts.