Patent GB-733824-A (GB)
Title: Improvements in or relating to apparatus for measuring force
Inventor: PAYNE ROBERT ERLE
IPC: G01N15/04
Filed: 1953-02-25
Abstract: 733,824. Measuring force electrically. SHARPLES CORPORATION. Feb. 25, 1953, No. 5262/63. Class 40 (1). A force to be measured rocks a pivotal member to vary differentially the inductive couplings of two transformers. The transformer secondaries are connected in series opposition and their resultant error voltage output is zero in a normal position of the member, and varies in magnitude and phase corresponding to the deviation and sense of the deflection of the member from normal. The error voltage after amplification and rectification is used to apply a restoring force to the member, the magnitude and sense of this restoring force being indicative of the original force. As shown, for determining the particle size distribution of material freely dispersed in a gas, by measuring the rate of fall of the particles and the percentage amounts having various rates of fall, the particles fall in a sedimentation tower 23, on to a scale pan 12 carried by a beam 10 pivotally carried by a torsion wire 35, Fig. 3, on a base-plate 27 slidable within a casing 22. The beam 10 carries conducting plates 20, 21 disposed between the windings of respective fixed transformers 15-17, 16-18. Normally the secondaries 17, 18 connected in phase opposition are balanced. Particles falling on the scale pan upset the balance and produce a corresponding variation of the current flow in a coil 13 also carried by the beam which coacts with permanent magnets 54, 55 to exert a force on the beam to restore it to its normal position. The current in the balancing coil is then a measure of the weight in the scale pan. The indication is reduced to zero at the end of successive tests without removing the particles from the scale pan by raising a pin 42, against the bias of a spring 44, through a manually adjustable sciew 53 until the pin relieves the beam 10 of a portion of the weight of a chain supported ball 40 sufficient to zeroize the indicator. Circuit arrangements, Fig. 6. The transformer primaries 15, 16 are fed from a Colpitt's oscillator 14 through a buffer valve 58 to eliminate unwanted harmonics. The error signal derived from the series opposed secondaries 17, 18 is applied to an amplifier 19 (62, 63, 64) in which a cathode follower stage 65 is included for impedance matching and a lowpass filter 66 for further removal of harmonics. The amplifier output is transformer coupled at 70 to a phase-sensitive detector 20a fed with its reference voltage from the oscillator 14. The detector unidirectional output which varies in magnitude and polarity according to the magnitude and sense of the deviation of the weigh beam is fed over cathode follower power amplifiers 85, 86 to the balance coil 13 in series with a microammeter 87 which indicates the weight. Delayed A.V.C. is provided in the amplifier 14 by a biased diode 67 which becomes operative in the event of excessive deflection of the balance beam to limit the amplifier output voltage. The detector includes a phase-shifting network 75-82 which compensates for any phase-shift of the error signal introduced in the preceding circuits so that the restoring force is applied by the coil 13 with practically no time delay to stabilize the servo-system. The particle size distribution is determined by analyzing the recorded weights at timed intervals. Specification 656,205 is referred to.