Patent GB-1161235-A (GB)
Title: Acoustical Analyser for Indicating Faults in Operative Parts of High Speed Engines.
IPC: H03B21/02
Filed: 1966-08-31
Abstract: 1,161,235. Analysing sounds; automatic phase control. CURTISS-WRIGHT CORP. 31 Aug., 1966 [6 Dec., 1965 (2)], No. 38831/66. Headings H3A and H4D. In a system for detecting mechanical faults in rotating parts of a high-speed engine running substantially at a predetermined reference speed, by comparing the level of sound energy of a discrete characteristic frequency radiated by a selected rotating component with the corresponding level for a standard counterpart, a signal whose frequency is independent of engine speed and whose amplitude corresponds to the level of the said characteristic frequency is derived and applied to an indicator which shows the operative condition of the selected component as compared with the standard counterpart. For analysing discrete frequencies f 1  . . . f n  ... produced by, e.g., bearings and compressor stages in a jet engine E, the output of either microphone A or B is applied through amplification and stepped attenuation stages 28 to a data channel in which it is mixed at 33 with a high-frequency (# 200 kc/s.) carrier signal from unit 34. The mixer output is applied through intermediate frequency (I.F. = 200 kc/s., bandwidth = 15 c/s.) amplifier and filter stages 36-39 to a detection stage and indicator 40. When analysing the frequency f n  (½ 200 kc/s.) the carrier signal from unit 34 is made equal to 200 kc/s. + f n  so that the filtered I.F. output comprises a signal of frequency 200 kc/s. + f n -f n , i.e. 200 kc/s., with amplitude corresponding to that of the f n  signal, other side-band signals being rejected by the I.F. filtering. To obtain the required carrier-signal frequency of 200 kc/s. + f n , a signal of frequency f L  corresponding to the engine revolution rate is derived from the second microphone (B or A) or a tachometer and passed through bandpass filter 45 (with centre frequency corresponding to the predetermined reference engine speed and with band-width encompassing expected deviations from the reference speed) to phase detector 48 for comparison with the output of a local oscillator 49 whose frequency is initially set manually to correspond to the reference speed. An amplified D.C. error voltage derived from the phase detector output is then fed back to control the frequency of oscillator 49 to reduce the error, and an indication of the error is given at 52. The output of oscillator 49 is thus phaselocked to the engine revolution rate signal and provides a noise-free output signal of frequency f L  which is applied to unit 34. The ratio, K, of f n  to f L  is known from, e.g., gear ratios in the engine or the number of blades in the compressor rotors, and is set into unit 34 which frequency-multiplies the f L  signal by the factor K to give a signal of frequency Kf L  = f n  which is then superimposed on a 200 kc/s. carrier. The analyser is calibrated by connecting a relatively broad band-width filter NF (of 200 kc/s. centre frequency and 400 c/s. bandwidth) in shunt with I.F. filter stage 38 and adjusting the amplification of stages 28 such that a predetermined indication on indicator 40 is given for a known microphone placement and standard engine. The phase lock detector 46 (Fig. 3) comprises local oscillator 49 (stages V 4 , V 5 , V 6 ) whose output is applied to one input 49a of balanced phase detector 48. The signal from band-pass filter 45 (Fig. 1) is amplified in stages V 1 , V 2 , V 3  and applied to the second input 47a of phase detector 48. The sum and difference of the two signals, derived from transformers T 1  and T 2 , are fed to envelope detectors CR 1 , CR 2  whose outputs are smoothed in filter 50 and subtracted in a differential amplifier (stages V 7  to V 10 ) to give a D.C. phase error signal. This error signal is used at VC to modify the inductances of coils 1 and 2 in frequencydetermining circuits of local oscillator 49.