Patent GB-568554-A (GB)
Title: Improvements in electrical bridge circuits
IPC: G01R27/02
Filed: 1943-10-11
Abstract: 568,554. Impedance measurements; determining dimensions of materials. REYNER, J. H., FURZEHILL LABORATORIES, Ltd.., and SMITH & SONS (ENGLAND). Ltd., S., (formerly SMITH & SONS (MOTOR ACCESSORIES), Ltd., S.). Oct. 11, 1943, No. 16650. [Class 37] In a self-balancing A.C. bridge circuit for the measurement of impedances, which consist of both resistance and reactance and of the kind including two motors arranged to adjust the magnitudes of two elements of the standard impedance of the bridge, one motor being responsive to the in-phase and the other to the quadrature component of the bridge output, so as both to restore the balance of the bridge and to operate indicators, discrimination between the in-phase and quadrature components of the out-of-balance voltage is obtained by two balanced modulators, one supplied with a reference signal in phase with the input to the bridge and the other with a reference signal in quadrature thereto. Fig. 1 shows a Wheatstone bridge for measuring a capacitive impedance C2 having some loss represented by a shunt resistance P2. The ratio arms are formed by resistors R1, R2 and an adjustable condenser C1 shunted by an adjustable resistor P1 constitute the standard impedance elements. The bridge input is connected to a source S of oscillations while its output is fed to an amplifier A. Two balanced modulators BM1, BM2 are fed with the output of the amplifier A. Each modulator is also fed with a reference oscillation from the source S, the modulator BM1 directly, and the modulator BM2 through a 90 degrees phaseshifting network N. The output of the modulator BM1 controls a reversible motor M1 which actuates the standard condenser C1 and an indicator I1, while the output of the modulator BM2 controls a reversible motor M2 which actuates the standard resistor P1 and an indicator 12. Each modulator comprises four rectifiers D1, D2, D3, D4, Fig. 2, and centretapped resistors R3, R4, and R5, R6. The output from the amplifier is applied across input terminals 10 and the reference oscillation across the centre taps 11. Since the reference oscillation is of the same frequency as the amplifier output, the difference term is of zero frequency, i.e. direct current, and this changes sign abruptly as the phase of the input changes relative to that of the reference oscillation. Hence the output is a D.C. signal, the strength of which depends on the magnitude of the amplifier output and the direction of which depends on which side of the balance point the bridge is operating on. The modulator output is fed to a polarized relay 12 controlling the corresponding motor M1 or M2. The balanced modulators discriminate between the in-phase and quadrature components of the bridge output and so operate the two motors until the bridge is balanced. The indicators I1, I2 then show the capacitance and resistance of the impedance C2, P2. In a modification, Fig. 3, the unknown impedance is the capacitance C2 between a fixed electrode 14 and a test-piece 15 which it is desired to grind to a predetermined dimension, the indicator I1, Fig. 1, being calibrated to show the dimension of the test-piece. This dimension may be obtained without removing the test-piece from the magnetic chuck 16 of the grinding machine by making the machine and test-piece the second electrode. A switch 18, Fig. 1, operated by the motor when a predetermined dimension is reached, may give an alarm or switch off the machine. The arrangement is not restricted to two variables, and any of the motor-operated output members may be combined to drive a common indicator or control.