Patent GB-830896-A (GB)
Title: Improvements in or relating to electrical bridge networks
Inventor: HIGHAM EDWARD HALL
IPC: G01R27/02
Filed: 1955-06-20
Abstract: 830,896. Measuring bridges. UNILEVER Ltd. July 2, 1956 [June 20, 1955], No. 17811/55. Class 37. Apparatus for measuring the reactive or the resistive component of an impedance containing both components comprises an A.C. bridge having arms containing the unknown impedance and a variable impedance for balancing the bridge for the component required to be measured, a phase-sensitive detector responsive to the bridge unbalance signal, means for supplying a reference alternating voltage to the detector in phase with the unbalance signal component due to the impedance component not required to be measured, and a thermionic vacuum tube connected in the bridge circuit having a control grid responsive to the unidirectional phase detector output so as to produce a corresponding change of impedance to produce bridge balance for the impedance component not required to be measured. In Fig. 1, bridge 1 comprises an unknown impedance arm 2 comprising resistance and capacitance in parallel, a variable capacitance arm 3, and ratio arms 7, 8 comprising closely coupled primary windings of unbalance transformer 9, centre tapped to earth; the bridge being energized from A.C. generator 11. The unbalance voltage from the secondary of transformer 9 containing voltage components respectively in phase and in quadrature with the energization is amplified at 17, 18 to excite output transformer 20 having secondaries 21, 22 feeding phase-sensitive detectors 23, 24 supplied at their centre taps 33, 30 with reference alternating voltages, respectively in quadrature and in phase with the resistive component unbalance voltage, from amplifiers 32, 29 respectively energized over integrating 90 degree phase lag amplifier 31, and directly, from cathode follower 28 and phase adjusting network 27, which is adjusted to compensate for the phase shifts in amplifiers 17, 18 and transformer 20. Phase detector 23 generates a unidirectional voltage in response to the capacitance unbalance of the bridge to drive a servosystem (not shown) to readjust capacitance 3 for balance of the reactive component and to operate a capacitance responsive indicatorrecorder or process control, while phase detector 24 generates a unidirectional voltage in response to resistance unbalance which is superimposed on a voltage from potentiometer 26 and applied to the grid of triode 5 whose anode circuit is shunted across capacitance 7, to vary its dynamic resistance and so balance the bridge for the resistance component. The cathode of triode 5 or the variable capacitor return may be returned to taps on winding 7, to introduce a valve ratio. The apparatus is applicable to measuring and controlling the moisture content of cereal products as a function of the permittivity of a sample as dielectric of a capacitance cell, through which the material may be continuously forced; the resistance and capacitance components being maintained in separate balance. In a modification of the bridge circuit (Fig. 2) for determining the value of a resistance component, a bridge comprises an unknown impedance 2 having capacitance and resistance in parallel and resistance 35 in series with variable balancing resistance 36; with the centre tapped primary of transformer 9 and excitation generator 11 as before. The anode circuit of pentode 37 is effectively shunted across 35, 36, while the anode is retroactively coupled to the grid over a 90-degree lagging phase shift circuit comprising capacitance 50 in series with resistance 51 and capacitance 52 so that the pentode effectively presents a capacitance across resistances 35, 36, variable by a control grid bias derived from phase-sensitive detector 23 (Fig. 1) to balance the bridge for the capacitative component, while resistance 37 is adjusted by a servo system (not shown) responsive to the output of phase detector 24 to balance the bridge for the resistive component.