Patent GB-862994-A (GB)
Abstract: 862,994. Measuring rotary unbalance electrically. GENERAL DYNAMICS CORPORATION. Feb. 18, 1959 [Feb. 18, 1958], No. 5664/59. Class 40 (1). [Also in Groups XXXIX, XL (b) and XL (c)] The rotary unbalance of a machine in each of two compensating planes is determined by mounting the machine 10 (Fig. 1) on four pressure - sensitive transducers 22-25, whose output signals are modified at 50- 53 in accordance with the relative positions of the transducers with respect to the machine, and combining the modified signals to obtain an indication in a valve voltmeter 70 of the unbalance force in each compensating plane, the angular positions of the unbalance forces being determined by a stroboscopic lamp 16 which is illuminated when the magnitude of the force signal is zero, i.e. when the unbalance force is horizontal. A mathematical analysis of the relationship between the individual transducer signals, the spacing and the unbalance force magnitude and position is given. The signals from the four pressuretransducers 22-25 are adjusted in networks 50-53 (Fig. 5A, not shown), for differences in sensitivity, and may be added directly to give an indication of the total static unbalance of the machine. For dynamic balancing, each signal is multiplied by a predetermined factor, depending on the spacing of the transducers relative to the machine, and added algebraically by a double triode valve (Fig. 5B, not shown), and fed through a variable gain negative feed-back amplifier 60, which may be by-passed by operating ganged switches 59a, 61a, to a filter network 62 (Fig. 5C, not shown) comprising three triode valves with a negative feed-back link consisting of two parallel-T networks adjustable by a ganged switch to reject a particular frequency, whereby all other frequencies are fed back and eliminated. To ensure proper adjustment of the filter network, the input signal is fed to a phase discriminator together with a quadrature signal derived subsequently, where the sum and difference of the quadrature signals are obtained and rectified, and the difference shown on a valve voltmeter. If the signals are correctly 90 degrees out-of-phase the voltmeter shows zero, and any indicated error signal is amplified and fed to a servomotor to adjust elements of the filter network to obtain the correct relationship. The single frequency output of the filter network is taken through two similar integrators 65, 68 each consisting of a pentode and two cathode coupled triodes with adjustable feed-back, the first of which supplies the quadrature signal for the phase discriminator described above, and is finally fed to a valve voltmeter 70 to indicate the magnitude of the unbalance in the selected compensating plane. To determine the position of the unbalance, the single frequency output of the filter network is fed to a cross-over detector 66 wherein a pulse is produced each time the sine wave input signal goes through zero from maximum to minimum value, which is used to fire a gas strobe tube 16, whose firing time is adjustable. The strobe tube illuminates the motor at the instant when the unbalance force is horizontal. In practice the angular position of the unbalance is determined with units 68, 70 out of circuit, and the magnitude of the unbalance determined with amplifier 60 by-passed. Each transducer 22 (Fig. 2) consists of a pair of circular plates 32, 33 with studs 26, 27 between which is located two piezoelectric elements 37, 38 of barium titanate crystals in countersunk openings 39, 40 separated by a silver disc 41. The plates are bolted together to a predetermined loading on the crystals to give a linear pressure response.