Patent GB-939870-A (GB)
Title: Improvements in or relating to vibration analysing apparatus
IPC: G01V1/30
Filed: 1961-07-31
Abstract: 939,870. Phase difference measuring. WAYNE KERR LABORATORIES Ltd., and R.W.LEVINGE. July 30, 1962 [July 31, 1961], No. 27767/61. Class 37. Apparatus for measuring the phase difference and the amplitude difference between components of two electrical oscillations having complex waveforms produced by transducers 10, 11 (Fig. 1) in vibration recording means (not shown), comprises a local oscillator 14 producing two pairs of reference oscillations of substantially equal amplitude and frequency, adjustable phase-shifting means 60, circuits 12, 13 in which the pairs of reference oscillations are mixed with the output signals of the transducers, low-pass filters 61, 62, an adjustable-gain amplifier 64, and an oscilloscope 63; alternatively, an adjustable attenuator (not shown) may be used in place of the amplifier. Each pair of reference oscillations are in anti-phase and one pair is in phase-quadrature with respect to the other, both pairs being adjustable in frequency. Each of the circuits 12, 13 include a transformer 40 (Fig. 3) whose primary winding 41 receives the signal from the corresponding transducer 10 (or 11), and transistors 47, 48 to which one pair of reference oscillations is applied through leads 49, 50. The transistors are rendered conducting during alternate half cycles of the corresponding reference oscillations whereby the resultant signal delivered to the corresponding filter 61 (or 62) through lead 53 is a series of substantially uni-directional pulses having an amplitude proportional to the phase difference between the output signal of the transducer and the pair of reference oscillations. The filters 61, 62 have a low cut-off frequency e.g. 1 c/s., whereby sinusoidal oscillations arising from variations in the phase difference between the reference oscillations and the components having a frequency different from that of the reference oscillations, are substantially eliminated. The gain of the amplifier 64 is adjustable to equalize the amplitudes of the two sets of very low frequency output signals from the filters, these signals being applied to the x and y deflection means of the oscilloscope, respectively. The gain of the amplifier and the phase-shift produced by the means 60 are adjusted to equalize the amplitudes and phases of the filter output signals and display a straight line inclined at 45 degrees on the screen of the oscilloscope. The adjusting means for the amplifier and the phase-shifting means are calibrated, and their settings to produce the desired Lissajous figure indicate respectively the amplitude difference and phase difference between the components of the transducer output signals at the frequency of the reference oscillations. The oscillator 14 comprises two amplifiers (Fig. 2) connected in series and made to oscillate by means of a resistance feedback connection between them, each amplifier being provided with resistance input and capacitive feedback circuits. Each amplifier produces one pair of the reference oscillations, the frequency of which is variable by adjustment of the resistance in the input circuit or of the capacitance in the feedback circuit. The oscillator also includes a stabilizing circuit comprising a pair of diodes. The phaseshifting means 60 may comprise two ganged variable potential dividers (Fig. 4, not shown), connected to the outputs of the oscillator, the sliding contacts being coupled to a fixed potential divider whose output terminal is connected to one or other of the transistors in the circuit 12. An alternative phase-shifting means is described and illustrated in Fig. 5 (not shown), which comprises a ring-resistor provided with two diametrically opposite sliding contacts which are connected to the transistors 47, 48 in circuit 12, the outputs of the oscillator being connected to four equally spaced tapping points in the resistor.