Patent GB-1132999-A (GB)
Title: Apparatus for producing electrical quantities corresponding to mechanical angular displacements
Inventor: CROWTHER DEREK NIGEL
Filed: 1966-04-05
Abstract: 1,132,999. Measuring angular displacement electrically. COMMUNICATIONS PATENTS Ltd. Dec.20, 1966 [April 5, 1966], No.15090/66. Heading G1N. [Also in Division G4] In the measurement of the angular displacement of a shaft, e. g. coupled to the shaft of a servo mechanism forming part of an aircraft control, a D. C. signal varying linearly from zero to a maximum for one complete revolution of the shaft is produced by a circuit to which three D. C. signals, each of which increases from zero to a maximum and then decreases to zero again for one revolution, the maxima of the signals being displaced by 120 degrees from each other, are supplied. These three signals (23), (24), (25), Fig. 2 (not shown), are obtained from terminals 15, 16, 17, Fig. 1, connected to tappings 12, 13, 14 of a toroidal resistive winding 11 on the shaft, a D. C. voltage being applied to the windings by way of brushes 18, 19 connected through slip rings to terminals 21, 22. These signals are then applied via terminals 35, 36, 37, Fig. 5, connected to a three arm resistor network 38, to the bases of transistors 46, 64, 66 respectively, forming pairs of transistors with transistors 47, 63, 65, the bases of these latter transistors being supplied with a signal from junction 45 of network 38, representing the instantaneous average value of the applied signals. The emitters of the pairs of transistors are each supplied with constant current from terminals 53, 56, so that an increase (or decrease) in the collector current of one transistor results in a corresponding decrease (or increase) in that of the other of each pair. The collector currents with respect to angular displacement have the form shown in Fig. 3 (not shown), the transistors being so arranged that the collector currents change linearly with base voltage over a given range of base voltages, i.e. between 8-16 volts for a total range of 0-24 volts. The collector currents are summed to provide two signals on lines 59 and 57 one of which increases linearly from zero to a maximum while the other decreases linearly from this maximum to zero for angular displacements between 0-180 degrees, and vice versa between 180 degrees - 360 degrees. These signals are obtained by connecting the collectors of transistors 47, 64 and 65 to line 59 and those of transistors 46, 63 and 66 to line 57. One of those signals is then applied to a load resistor 61 via diode 60 or 58, dependent on the state of transistors 81, 82 which are under the control of switching transistors 77, 78. These transistors operate so that the signal on line 59 is applied to resistor 61 for angular displacements between 0-180 degrees, the transistor 81 being conductive, to produce an output signal increasing linearly from zero to the maximum over this range of angular displacements. Between 180 degrees - 360 degrees, the transistor 82 becomes conductive, so that the signal on line 57, to which is added a further signal from the collector of transistor 76 arranged to be equal to the maximum of these signals, is applied to resistor 61. Thus the output signal across terminals 62, 54 increases linearly from zero to a value of twice the maximum of the signals on lines 57, 59 for one complete revolution of the shaft. Additionally, the angular position of the shaft may be indicated using an intrument comprising an armature coupled to a pointer and three stator windings which are connected to terminals 15, 16, 17. With this load, the input voltages on terminals 35, 36, 37 will be distorted, Fig. 4 (not shown), this distortion being compensated for in the circuit of Fig. 5 by applying to the base of the second of each pair of transistors a voltage corresponding to the average of the input signals rather than a constant voltage.