GB576639A - Improvements in or relating to circuits for measuring electrical resistances in strain gauges - Google Patents

Abstract

576,639. Strain-gauges. AUGHTIE, F. Feb. 29, 1944, No. 3747. [Class 40 (i)] [Also in Group XXXVI] In an apparatus for remote indication of the change of value of a resistance, or of change of relation between two resistances, which may be an element or elements in a strain gauge, the variable controls the phase of an alternating output voltage with respect to a reference voltage, the change of phase being used as the indicator. A change in resistance R1, Fig 1a, causes the voltages across the resistance and condenser to change, thus altering the phase of the output 1 relative to output 2. The transformer shown may be replaced by a valve. Where the change of two resistances is to be measured, the resistances constitute two limbs of a circuit such as a bridge circuit, Fig. 2a, or a TT or T circuit or a bridged T or parallel T circuit. In the modification, Fig. 3 (not shown), the output from the bridge network is fed through an amplifier and a phase shifting circuit to the input, so that the apparatus forms an oscillator whose frequency is dependent upon changes of phase in the bridge circuit. The impedances in the bridge are so chosen that the feedback does not vary greatly, is sufficient to ensure oscillation, and is negative to minimise harmonics in the waveform. The frequency change produced can be altered by frequency conversion and multiplication and the outputs from a number of bridge circuits may be summed algebraically to a function, which may be non-linear and represents the several variable quantities. A series of variables may be transmitted through the same channel by multiway switching means. Specifications 560,260, and 565,462, [both in Group XXXVI], are referred to.