Patent GB-579299-A (GB)
Title: Improvements relating to electrical apparatus
IPC: G05B11/01
Filed: 1944-04-06
Abstract: 579,299. Electric control systems. COSSOR Ltd., A. C., and JOFEH, L. April 6, 1944, No. 6489. [Class 40 (i)] [Also in Group XXXVI] The invention relates to a circuit for producing a direct voltage varying in magnitude and sense in accordance with the amplitude of that component of an input alternating voltage which has the same frequency as, and is in phase with, an alternating reference voltage of relatively large value. The circuit is particularly applicable to A.C. bridges where the derived voltage may be used to actuate a servo-mechanism for automatic balancing of the bridge. According to the invention, one side 12 of the input voltage V2 is connected to a point having the mid-potential of the reference voltage VI, if necessary tnrough the intermediary of a transformer 30, two rectifier load resistors 14, 17 are each connected in series with a pair of rectifiers 13, 15-16, 18, (one rectifier being connected at each end of each load resistor) to form parallel but oppositely conducting paths across the reference voltage source and the other side 31 of the input voltage V2 is connected to the common end of four equal resistors 19, 20, 21, 22 the other ends of which are connected respectively to the four ends of the load resistors 14, 17. The direct voltage to be measured is developed between the midpoints of the load resistors 14, 17 and measured, e.g., by a D.C. micro-ammeter 32 in series with a resistance 33. For a linear response of the output voltage relative to the co-phased component of the input voltage V2, the latter voltage amplitude must be small relative to that of the reference voltage, and the rectifiers, in their conductive direction, must have a small resistance relative to the load resistors 14, 17. Fig. 2 shows the output voltage developed across the midpoints of the load resistors 14, 17 applied through a filter 26, 27, 28 to remove any H.C. component and then through a filter network 40-44 to produce a push-pull direct voltage between the grids of amplifying valves 23, 24 between the anode circuits of which the armature 25 of a separately excited servomotor is connected. The motor operates a control (not shown) to vary the amplitude of the co-phased component of the input voltage V2 in the sense to reduce any co-phased component present. To avoid oscellation of the system, "first derivative control" is effected by the network 40-44 whereby the voltage between the grids of valves 23, 24 represents the derived voltage (from 14, 17) plus a component proportional to the rate of change thereof. Further amplifying stages may be used if more power is needed to operate motor 25. The "first derivative" network is dispensed with in the arrangement described in the Provisional Specification. Fig. 3 shows the invention applied to the automatic balancing of an A.C. bridge in which the capacity of a condenser 49 is being measured by adjustment of a condenser 53 and resistance 54 in the comparison arm. The bridge is energised from a source 9, e.g., 50 cycle mains, which also supply the reference voltage to a device 55 similar to the circuit arrangements of Figs. 1, 2. The input voltage to the unit 55 is derived through leads 60, 61 from the bridge output. The motor 25 of the unit 55 is coupled through reduction gearing to the control of the variable condenser 53. A device 55' similar to 55 receives its reference voltage from source 9 through a quadrature phase-shifter 56, its input voltage from the bridge output, and its servo-motor 25<1> adjusts the resistance 54. Both motors 25, 25<1> thus separate until the bridge is balanced.