Patent GB-606113-A (GB)
Title: Improvements in electrical bridge circuits
IPC: G01R27/02
Filed: 1945-02-22
Abstract: 606,113. Impedance measurements. MILSOM, F. R., FURZEHILL LABORATORIES, Ltd., and SMITH & SONS (ENGLAND), Ltd., S. Feb. 22, 1945, No. 4518. [Class 37] A bridge circuit includes a primary A.C. bridge network consisting of components of relatively small physical size, a secondary bridge network adapted to measure accurately the instantaneous value of the variable impedance element of the primary bridge network, and a filter network serving to connect the variable impedance element to the secondary bridge network, but to prevent the passage to the secondary bridge network of current at the frequency of the supply to the primary bridge network. Fig. 1 shows a gauging device having a primary high-frequency bridge network HFB and a secondary low-frequency bridge network LFB. The primary network HFB is fed by an oscillator OSC1 and includes capacitive arms C1 and C2 in shunt with C3, which may be the capacity between a condenser element and a work-piece a dimension of which is required to be gauged. The third arm is formed by a resistance R1 and the fourth arm bv a resistor R2 in shunt with a fixed resistor R3 in series with an adjustable resistor R4. The components of the primary network HFB may form part of a measuring head. The balance of the primary bridge is restored by means of a two-phase motor M1 coupled to the control member of the resistor R4 by reducing gearing 10. One phase winding F1 of the motor is excited at 50 c/sec. from a source not shown, and the other phase winding F<1>1 is excited at 50 c/sec. by means responsive to the phase change of 180 degrees in the output of the primary network HFB as it passes through the balance point. The output after amplification at A1, is first demodulated in a ring modu'ator RM1, the output of which has a D.C. component whose sign and magnitude represents the phase and amplitude of the bridge output. The output of the modulator RM1 passes through a phase changer PA1 to a second ring modulator RM<1>1 to modulate a voltage oscillation of 50 c/sec. derived from the source that feeds the winding F1, whereby the phase and amplitude of the output from RM<1>1 are changed in accordance with changes in the sign and magnitude of the D.C. component of the output of RM1. The modulated output of RM<1>1 is shifted in phase by 90 degrees at PS1 and passed to the motor winding F<1>1 so that the fields F1, F<1>1 are in quadrature. The adjustable resistor R4 forms an arm of the secondary bridge network LFB, which is remote from the measuring head. The connecting leads 11 include a filter circuit L, C4 so that the components of the secondary network do not represent an appreciable shunt on R4. The secondary bridge network LFB consists of four resistive arms R4, R5, R6, R7, the last of which is adjusted by a motor M2 through a transmission system 12. The control member of the resistor R7 is also coupled to an indicator 13 which may be calibrated to read the dimension gauged by the measuring head. This network is operated at a frequency of 70 c/sec. from an oscillator OSC2 and is provided with self-balancing means similar to those in the network HFB. In a modification, Fig. 2, the adjustable resistor R4 is replaced by means generating resistance electronically. The effect of a variable resistance is produced by arranging a pentode V to draw current proportional to the alternating voltage impressed between its anode and cathode from terminals 14, 15. The magnitude of this current is determined by a potentiometer R8, R9 connected in series with a condenser C6 between the anode and cathode. The resistance is varied by adjusting the conductance of the valve by varying its grid bias, which is derived from RM1, Fig. 1, and inserted at 20 through a smoothing network R13, C9 and a resistance R10, the grid being isolated from the network R8, R9 by a condenser C5.