Patent GB-870838-A (GB)
Title: Electrical measuring arrangements
Inventor: FIELDEN JOHN ERNEST
IPC: G05D3/14
Filed: 1958-11-17
Abstract: 870,838. Measuring temperature electrically. FIELDEN ELECTRONICS Ltd. Nov. 4, 1959 [Nov. 17, 1958], No. 36867/58. Class 40 (1). [Also in Group XXXVI] A variable quantity, e.g. temperature, is indicated by a movable member driven by a servomotor, the position of the member and the value of the variable quantity being arranged to determine the state of balance of a bridge circuit. The output and input circuits of the bridge form the input and output circuits of a tuned amplifier part of the output of which, after rectification, is used to drive the servomotor in a direction dependent upon the amplitude of this output. The motor always tends to drive the bridge towards a condition of unbalance which occurs at a predetermined output amplitude of the amplifier. As shown, the output terminals 7, 8 of a bridge circuit are connected to the input terminals 2 of an amplifier 1 whose output circuit includes a parallel tuned circuit 9, 10. Inductor 9 forms the primary of a transformer having secondary windings 14, 15 which each form an arm of the bridge and are so connected that negative feedback to the amplifier is supplied via winding 14 and positive feedback via winding 15. The two remaining arms of the bridge comprise a temperaturesensitive resistor 23 and a range resistor 24 connected to either side of a resistive potentiometer 12 whose slider 11 forms the output terminal 8. The circuit oscillates when the bridge is on one side of balance only. Under these conditions, an output is derived at an auxiliary winding 16 and is fed via a rectifying circuit 18 to an amplifier 25. The output terminals 37, 38 of the amplifier are connected in series with a D.C. power source 26 and a resistance 33 which may be current-sensitive, e.g. a metal filament lamp. Servomotor 39 is connected between terminal 37 and an intermediate point 36, preferably the midpoint, of the power source 26. In the simplest embodiment of the invention, bridge terminal 7 is connected directly to windings 14, 15 and the circuit comprising elements 27, 28, 29, 30, 31, 41, 42, 43, 44 and 45 is omitted. At a predetermined amplitude of oscillation, the output of amplifier 25 is such that no current passes through the motor 39 and the slider 11 remains stationary with the bridge in a state of unbalance. An increase or decrease in oscillation amplitude due to a change in temperature of the resistor 23 will cause a corresponding change in the output voltage of the amplifier 25 which varies between zero and the value of the source 26. The motor 39 is driven to restore the slider 11 to a position corresponding to the original condition of unbalance of the bridge. A pointer 50 indicates the position of the slider 11 and hence the temperature of the resistor 23. The circuit components are chosen so that the servomotor current is a maximum in one direction when the amplitude of oscillation is zero and a maximum in the opposite direction when the amplitude of oscillation is a maximum. The use of a currentsensitive resistor 33 increases the maximum torque available from the motor. To improve stability, two resistors 27, 28 of low and equal value may be connected between the windings 14, 15 and the terminal 7. In parallel with these are arranged two capacitors 29, 41 and two identical diodes 30, 42, poled as shown, whose common point is connected to tapping 36. The common connections between resistors 27, 28 and diodes 30, 42 are at the same potential so far as oscillatory currents are concerned. The diodes, therefore, are effectively in parallel with resistors 27, 28 and windings 14, 15 and in series with resistors 23, 24 between terminals 7 and 8. The other ends of the diodes are connected through resistors 31, 43 to a pre-set potentiometer 45 in shunt with the motor 39. Thus, the sign and magnitude of the bias applied to the diodes depends upon the potential appearing at the amplifier output terminal 37. An increase in the resistance of the resistor 23 tends to increase the motor current but is accompanied by a decrease in the resistance of diode 30 which has the effect of reducing the speed at which the motor readjusts the circuit. Similarly, a fall in resistance of the resistor 23 is accompanied by a fall in the resistance of diode 42, the effect again being to reduce the speed of the motor. The magnitude of the stabilizing effect depends on the setting of the potentiometer 45. In the modification of Fig. 3, the bias of a single diode 30 is provided by a potentiometer 47 across the output of the amplifier 25, the potentiometer being adjusted so that the resistance of the diode is between its extreme values. Changes in the resistance of resistor 23 are accompanied by opposite changes in the resistance of the diode as determined by the output voltage of the amplifier 25. The maximum torque developed by the motor may be increased by connecting it between the outputs of two amplifiers connected for phase inversion. Thus, the motor 39, Fig. 4, may be connected between the collector electrodes of transistors 70, 71. The base of transistor 70 is connected to the output terminal 37 of amplifier 25 and the base of transistor 71 to the collector of the first transistor through a resistor 72. As the output of the amplifier falls the potential on the base electrode of the transistor 70 approaches that of negative terminal 35, the transistor conducts and a substantial current flows through resistor 68. The potential of point A therefore approaches that of the positive terminal 34 causing the current in the transistor 71 to fall to a low value. The potential of point B therefore approaches that of negative terminal 35. Hence a maximum current flows from A to B through the motor 39. When the output from the amplifier reaches its maximum value, a maximum current flows from B to A. Resistors 68, 69 in the collector circuits of the transistors may be replaced by metal filament lamps in order to increase the maximum motor torque. In the circuit of Fig. 4, the steady bias applied to the diode is derived from a source 66, of constant potential fed from the terminals 34, 35. Superimposed upon the steady bias is a voltage of sign and magnitude dependent upon the rate of change of output of the amplifier 25. This is derived through a capacitor 74 connected to the point B or to the output terminal 37. The circuits of Figs. 1 and 3 may be modified to obtain a similar stabilizing effect by inserting a capacitor in the common connection to resistors 31, 43, Fig. 1, or in the connection between the potentiometer 46 and the diode 30, Fig. 3. The movable member 50, Fig. 2, for indicating the temperature of the resistor 23 is mounted upon the same shaft 60 as the contact arm 61 of the balancing potentiometer. The wiper 11 moves over an arcuate resistor 12 and is connected to the external circuit through a hairspring 64. The servomotor 39 is mounted upon a counter-balanced pivoted arm 56 which is biased by a spring 58 to maintain pressure between motor spindle 52 and a friction disc 54 mounted upon the shaft 60.