Patent GB-690326-A (GB)
Title: Improvements in or relating to the electrical measurement of mechanical strains
IPC: G01R1/30
Filed: 1947-08-15
Abstract: 690,326. Electric measuring systems. ELLIOTT BROS. (LONDON), Ltd., and YATES, J. G. Aug. 5, 1948 [Aug. 15, 1947], No. 22756/47. Class 40 (i) [Also in Groups XXXVI and XL(c)] To measure a physical effect, e.g. strain, a number of resistance strain gauges mounted in different positions are connected in pulse-energized bridge circuits having their outputs applied to a cathode ray tube. The pulses may be D.C. or A.C. and in the latter case may be produced by feeding a frequency modulated wave through band pass filters or piezo-electric filters to the gauge circuits. Alternatively the resistance strain gauges may be replaced by temperature, humidity, light-sensitive frequency or magnetic-field sensitive elements or capacitor or inductor transmitters may be used to measure displacement. A number, e.g. ten, bridge-circuits 1 having their outputs connected in parallel are energized sequentially from a pulse-generator 3, comprising eleven flip-flop connected double-triodes arranged to trigger one another in a closed chain Fig. 5 (not shown). The eleventh stage allows for fly back in the C.R.T. time base. The shape of each pulse is regulated by a shaping circuit 5 controlled by a master oscillator 6. A square pulse output from the shaping circuit is used as a brightening pulse for the C.R.T. and is so adjusted that the initial disturbed portion P2 of the bridge-output-pulse due to reactances in the circuits, is left dark. A third output synchronizes a stepped-voltage-generator 7. In one position of the switch 8 the stepped voltage is connected to the X-plate-ampliner 9 and the bridge output is amplified at 2 and connected to the Y-plateamplifier 11. to produce traces as in Fig. 2. In the other position of the switch 8 the stepped voltage is superimposed on amplified bridge output fed to the Y-plates, and a saw tooth wave is applied to the X-plates whereby expanded traces as in Fig. 3 are obtained. For photographic recording the time base is disconnected and the film traversed at a convenient speed. An alternative method of pulsing the bridgecircuits 1 comprises a frequency-modulated oscillator 30 Fig. 9 which increases continuously or in steps from say 500-1000 Kc/s over a 100 microsecond period and returns to 500 Kc/s in 10 microseconds Fig. 10A, said oscillator feeding the bridge circuits through bandpass filters each tuned to accept a band such as OA, AB, &c. When the strains are inaccessible members such as an aeroplane propeller the gauge and band-pass filter units may be mounted on the propeller and connected to common input and output leads terminating on the prop-shaft in slip rings which have instead of a brush, a capacitive coupling to the oscillator and amplifier, Fig. 13 (not shown).. The output pulses may modulate a radio transmitter co-operating with receiver attached to the indicator unit on the ground. When using a high p.v.f. to display high-frequency strains the resistance unit is preferably made equal to characteristic impedance of the balanced co-axial or twin screened cable to which it is connected Figs. 14, 16 (not shown).