Vibration analysis

• US 20060150738A1
• Filed: 12/16/2005
• Published: 07/13/2006
• Est. Priority Date: 12/16/2004
• Status: Active Grant

• Current Assignee: Mant Manthey, Diane
• Original Assignee: Commtest Instruments Limited (General Electric Company)
• Inventors: Leigh, Nigel
• Granted Patent: US 7,805,281 B2
• Time in Patent Office: Days
• US Class Current: 73/649
• CPC Class Codes: G01H 1/14 Frequency
• CPC Class Codes: G01M 13/028 Acoustic or vibration analysis
• CPC Class Codes: G01N 29/07 by measuring propagation velocity or propagation time of acoustic waves
• CPC Class Codes: G01N 29/12 by measuring frequency or resonance of acoustic waves measuring frequency or resonant frequency of mechanical vibrations or acoustic waves in general G01H1/06, G01H3/04, G01H13/00; acoustic resonators G10K11/04; vibration or shock testing of structures G01M7/00
• CPC Class Codes: G01N 29/42 by frequency filtering or by tuning to resonant frequency
• CPC Class Codes: G01N 29/46 by spectral analysis, e.g. Fourier analysis or wavelet analysis spectral signal processing per se G06F17/14

Abstract

The present invention relates to vibration analysis and in particular, but not limited to, the derivation of multiple types of vibration signals from one vibration signal for vibration analysis. In the preferred method of the invention the vibrations of an object are measured using at least one vibration sensor, wherein the vibration sensor converts vibrations into an electrical vibration signal. The electrical vibration signal is digitised based on a first frequency, wherein the first frequency is selected from a plurality of possible frequency values. A first type of vibration signal is derived from the digitised vibration signal. A second type of vibration signal is then derived from the digitised vibration signal based on a second frequency. The second frequency is rationally determined from, and lower than, the value selected for the first frequency. The invention also provides apparatus for deriving multiple types of vibration signals from one measured vibration signal for vibration analysis.

Claims

1. A method of deriving multiple types of vibration signals from one vibration signal for vibration analysis, the method comprising the steps of:

measuring the vibrations of an object using at least one vibration sensor, wherein the vibration sensor converts vibrations into an electrical vibration signal;

digitising the electrical vibration signal based on a first frequency, wherein the first frequency is selected from a plurality of possible frequency values;

deriving a first type of vibration signal from the digitised vibration signal; and

deriving a second type of vibration signal from the digitised vibration signal based on a second frequency, wherein the second frequency is rationally determined from and lower than the value selected for the first frequency.

2. The method as claimed in claim 1 further comprising the step of deriving a third type of vibration signal from the digitised vibration signal based on the second frequency and a third frequency, wherein the third frequency is rationally determined from and lower than the value selected for the first frequency.

3. The method as claimed in claim 2 wherein the first frequency is an upper cut-off frequency and the first type of vibration signal is a high frequency waveform of the digitised vibration signal.

4. The method as claimed in claim 3 wherein the second frequency is a lower cut-off frequency and the second type of vibration signal is a low frequency waveform of the digitised vibration signal.

5. The method as claimed in claim 4 wherein the third frequency is a demodulation cut-off frequency and the third type of vibration signal is a demodulation waveform of the digitised vibration signal.

6. The method as claimed in claim 3 wherein the step of deriving a first type of vibration signal from the digitised vibration signal further comprises the steps of:

buffering the digital vibration signal; and

emptying the contents of the buffer as the high frequency waveform of the digitised vibration signal.

7. The method as claimed in claim 6 wherein further processing is performed on the contents of the buffer to produce a frequency-domain signal, the further processing comprising the steps of:

windowing the contents of the buffer;

transforming the windowed buffer contents into a frequency spectrum; and

determining the averaged frequency spectrum.

8. The method as claimed in claim 4 wherein the step of deriving a second type of vibration signal from the digitised vibration signal based on a second frequency further comprises the steps of:

reducing the frequency of the digital vibration signal to at least approximate the lower cut-off frequency using at least one decimation filter;

buffering the filtered vibration signal; and

emptying the contents of the buffer as the low frequency waveform of the digitised vibration signal.

9. The method as claimed in claim 8 wherein further processing is performed on the contents of the buffer to produce a frequency-domain signal, the further processing comprising the steps of:

windowing the contents of the buffer;

transforming the windowed buffer contents into a frequency spectrum; and

determining the averaged frequency spectrum.

10. The method as claimed in claim 5 wherein the step of deriving a third type of vibration signal from the digitised vibration signal based on the second frequency and the third frequency further comprises the steps of:

high-pass filtering the digital vibration signal at the demodulation cut-off frequency to produce a filtered signal;

detecting the envelope of the filtered signal to produce an envelope signal;

reducing the frequency of the envelope signal to at least approximate a frequency rationally determinable from and lower than the first frequency using at least one decimation filter;

buffering the reduced-frequency envelope signal; and

emptying the contents of the buffer as the demodulation waveform of the digitised vibration signal.

11. The method as claimed in claim 10 wherein further processing is performed on the contents of the buffer to produce a frequency-domain signal, the further processing comprising the steps of:

windowing the contents of the buffer;

transforming the windowed buffer contents into a frequency spectrum; and

determining the averaged frequency spectrum.

12. The method as claimed in claim 10 wherein the step of reducing the frequency of the envelope signal to a frequency rationally determinable from and lower than the first frequency comprises reducing the frequency of the envelope signal to at least approximate the lower cut-off frequency.

13. The method as claimed in claim 5 wherein the step of deriving a third type of vibration signal from the digitised vibration signal based on the second frequency and the third frequency further comprises the steps of:

high-pass filtering the digital vibration signal at the demodulation cut-off frequency to produce a filtered signal;

detecting the envelope of the filtered signal to produce an envelope signal with a frequency rationally determinable from and lower than the first frequency;

buffering the envelope signal; and

emptying the contents of the buffer as the demodulation waveform of the digitised vibration signal.

14. The method as claimed in claim 1, wherein the vibration sensor is an accelerometer and the digitised vibration signal is an acceleration vibration signal, and wherein the method further comprises the steps of:

integrating the acceleration vibration signal to obtain a velocity vibration signal;

integrating the velocity vibration signal to obtain a displacement vibration signal;

selecting one of either the acceleration, velocity or displacement vibration signal; and

deriving multiple types of vibration signals from the selected vibration signal according to the methods as claimed in any one of the preceding claims.

15. An apparatus for deriving multiple types of vibration signals from one measured vibration signal for vibration analysis, the apparatus comprising:

at least one vibration sensor for measuring the vibrations of an object by converting the vibrations into an electrical vibration signal; and

an analogue-to-digital converter to convert the electrical vibration signal to a digital vibration signal, wherein the analogue-to-digital converter has a sampling rate based on a value selected from a plurality of possible values for a first frequency; and

at least one signal processor for deriving a first type of vibration signal from the digital vibration signal and for deriving a second type of vibration signal from the digital vibration signal based on a value rationally determined from and lower than the selected value of the first frequency.

16. The apparatus as claimed in claim 15 where in the signal processor further derives a third type of vibration signal from the digital vibration signal based on the second frequency and a third frequency, wherein the third frequency is selected from values rationally determined from and lower than the value selected for the first frequency.

17. The apparatus as claimed in claim 16 wherein the first frequency is an upper cut-off frequency and the first type of vibration signal is a high frequency waveform of the digital vibration signal.

18. The apparatus as claimed in claim 17 wherein the second frequency is a lower cut-off frequency and the second type of vibration signal is a low frequency waveform of the digital vibration signal.

19. The apparatus as claimed in claim 18 wherein the third frequency is a demodulation cut-off frequency and the third type of vibration signal is a demodulation waveform of the digital vibration signal.

20. The apparatus as claimed in claim 17 wherein the signal processor includes:

a waveform buffer to buffer the digital vibration signal;

wherein the signal processor outputs the contents of the buffer as the high frequency waveform of the digital vibration signal.

21. The apparatus as claimed in claim 18 wherein the signal processor converts the contents of the buffer into a frequency domain signal, the signal processor further comprising:

a windowing means to window the contents of the buffer;

a transforming means to obtain the frequency spectrum of the windowed buffer contents; and

an averaging means to determine the averaged frequency spectrum.

22. The apparatus as claimed in claim 18 wherein the signal processor includes:

at least one decimation or sub-sampling filter to reduce the frequency of the digital vibration signal to at least approximate the lower cut-off frequency; and

a waveform buffer to buffer the filtered vibration signal;

wherein the signal processor outputs the contents of the buffer as the low frequency waveform of the digital vibration signal.

23. The apparatus as claimed in claim 22 wherein the signal processor converts the contents of the buffer into a frequency domain signal, the signal processor further comprising:

a windowing means to window the contents of the buffer;

a transforming means to obtain the frequency spectrum of the windowed buffer contents; and

an averaging means to determine the averaged frequency spectrum.

24. The apparatus as claimed in claim 19 wherein the signal processor includes:

a high-pass filter having a cut-off frequency at the demodulation cut-off frequency to produce a filtered signal;

an envelope detector to generate an envelope signal from the filtered signal;

at least one decimation or sub-sampling filter to reduce the frequency of the envelope signal to at least approximate a frequency rationally determinable from and lower than the first frequency; and

a waveform buffer to buffer the filtered envelope signal;

wherein the signal processor outputs the contents of the buffer as the demodulated waveform of the digital vibration signal.

25. The apparatus as claimed in claim 24 wherein the signal processor converts the contents of the buffer into a frequency domain signal, the signal processor further comprising:

a windowing means to window the contents of the buffer;

a transforming means to obtain the frequency spectrum of the windowed buffer contents; and

an averaging means to determine the averaged frequency spectrum.

26. The apparatus as claimed in claim 24 wherein the decimation or sub-sampling filter reduces the frequency of the envelope signal to at least approximate the lower cut-off frequency.

27. The apparatus as claimed in claim 19 wherein the signal processor includes:

a high-pass filter having a cut-off frequency at the demodulation cut-off frequency to produce a filtered signal;

an envelope detector to generate an envelope signal from the filtered signal, where the frequency of the envelope signal at least approximates a frequency rationally determinable from and lower than the first frequency; and

a waveform buffer to buffer the filtered envelope signal;

wherein the signal processor outputs the contents of the buffer as the demodulated waveform of the digital vibration signal.

28. The apparatus as claimed in claim 15 wherein the vibration sensor is an accelerometer and the digital vibration signal is an acceleration vibration signal, the apparatus further comprising:

at least one integrator to integrate the acceleration vibration signal to obtain a velocity vibration signal;

at least one integrator to integrate the velocity vibration signal to obtain a displacement vibration signal; and

a selecting means to allow selection of either the acceleration, velocity or displacement vibration signal, from which the multiple types of vibration signals will be derived.