Patent GB-617064-A (GB)
Title: Improvements in torquemeters
IPC: G01L3/10
Filed: 1945-05-14
Abstract: 617,064. Electric torsion meters. SQUIRES, M., MITCHELL, J. H., BRECHNER, D., SHAW, H. W., and GREENHOUGH, V. W. May 14, 1945, Nos. 12115/45 and 15762/46. [Class 40 (i)] [Also in Groups XIX and XXXVI] In a torsion meter comprising two wavegenerators arranged on a shaft at points between which torsional strain arises, the alternating voltages produced by the wave-generators are applied to a multivibrator in such a manner that the proportion of time during which the multivibrator is in one or the other of its two conditions is a measure of the phasedifference between the voltages and therefore of the torsion, and means are provided for indicating this proportion.. The wave-generators may be mounted in an aeroengine, their toothed wheels 16, 17, Fig. 1, being secured respectively to one end of a short tubular shaft 12 and to a coupling shaft 18 which is held in a conical bore 20 at the other end of the shaft 12. Pick-ups 24, 25 are mounted in the casing so that each forms a magnetic circuit with its toothed wheel and, as the latter rotates, the varying air gap between the teeth of one pole 29, Fig. 2, and of the wheel produces cyclic changes in the inductance of a coil 30 which may be converted into an alternating voltage. Alternatively, a steady magnetizing force may be applied so that the modulated flux produces the alternating voltage in the coil 30, or a varying capacity may be used. The wheels 16, 17 may be dowelled together during assembly so that they are out of alignment by a known amount. The outputs of the pick-ups are applied to amplifiers A1, A2, Fig. 3, the outputs of which are waveforms 32, 33, and then converted into pulses 34, 35 by pulse-generating circuits PG1, PG2. The phase-difference between the pulses 34, 35 is equal to that between the outputs of the wave-generators and, after suppressing the positive pulses, the negative ones are applied to the grids of the doubletriode of a multivibrator 2MV. The anode currents 36, 37 of the two triodes are each of rectangular waveform and the duration of current flow, during each cycle, in, say, the triode to which the pulses 34 are applied is equal to the time T1 between the arrivals of a negative pulse 35 at the grid of the other triode and of a negative pulse 34 at its own grid. The mean current 38 in this triode is thus proportional to the time T1 or phase-difference between the outputs of the wave-generators and is indicated by a D.C. milliammeter 40. Alternatively, a two-coil differential milliammeter may be connected in the two anode circuits. The output of one of the valves may be further amplified and/or filtered in order that the indicating meter may be of more robust type. A filter may also be inserted to avoid damage to the meter due to the pulse repetition frequency being close to the natural frequency of the needle. The multivibrator may have its grids and anodes cross-connected by resistances only but preferably, to prevent it remaining indefinitely in one of its two conditions, condensers are inserted in the intercouplings so that, in the absence of signals, the multivibrator alternates between its two conditions at a pre-determined frequency which must be lower than that of the outputs of the wave-generators but above that to which the moving parts of the meter can respond. Since the errors are greater when the phase-difference is small, the wave-generators are initially so aligned that the phase-difference is large throughout the working range. To reduce other errors, the voltage excursions applied to the pulse-generators are made large compared with those required to operate them. The preferred form of pulse-generator comprises a double-triode, the anode of the first section of which is coupled by condensers and resistances to the grid of the second section, and a damped reactance in the anode circuit of the second section which converts the rectangular waveform into pulses. When the output of the wave-generators is of large amplitude and amplitude modulation is small, a negatively over-biassed valve amplifier or an over-loaded amplifier may be used as pulse-generators. A switch allows the connections between the wave-generators and the two parallel circuits to be interchanged.. The first and second Provisional Specifications refer respectively to the use of a cathode-ray oscilloscope to enable torsional vibrations to be photographed and to a modified form of multivibrator controlled by positive and negative pulses applied to one grid only.