METHODS OF ANALYSING APPARATUS

• US 20140180606A1
• Filed: 07/05/2013
• Published: 06/26/2014
• Est. Priority Date: 12/21/2005
• Status: Active Grant

• Current Assignee: Rolls-Royce PLC (Rolls-Royce Holdings Plc)
• Original Assignee: Rolls-Royce PLC (Rolls-Royce Holdings Plc)
• Inventors: ANUZIS, Paul, GELMAN, Leonid Moiseevich
• Granted Patent: US 10,352,823 B2
• Time in Patent Office: Days
• US Class Current: 702/39
• CPC Class Codes: G01H 1/006 of the rotor of turbo machines
• CPC Class Codes: G01M 15/14 Testing gas-turbine engines or jet-propulsion engines
• CPC Class Codes: G01M 5/0066 by exciting or detecting vibration or acceleration vibration testing of structures G01M7/00

Abstract

Engine health monitoring is used to assess the health of an engine, such as a gas turbine engine. Blades mounted on a shaft produce a modal response when excited. The shaft has an order related component that varies with the rotational velocity of the shaft. Modal responses are increased when the natural frequency range of the selected blade mode intersects with one of the order related components. By applying a short time chirp-Fourier transform with a frequency speed that is a function of a rate of change in the rotational velocity of the shaft a selected signal can be isolated. Cracks in the blades can be detected from the isolated signal.

Claims

1. A method for determining an artifact in a component comprising the steps:

a) exciting the component to induce a resonance response;

b) calculating the real and imaginary components of bi-coherence b of a resonance response at pk0 and qk0 wherein p and q are integers and k0 is the component resonant frequency; and

c) determining a location of a point related to calculated components in real and imaginary two-dimensional space, the location of the point determining the presence of an artifact.

2. A method according to claim 1, wherein the location of the point is compared with a discriminate function.

3. A method according to claim 2, wherein the discriminate function is linear.

4. A method according to claim 1, wherein the component is a blade.

5. A method according to claim 4, wherein the blade is mounted on a shaft.

6. A method according to claim 5, wherein the blade is excited by rotating the shaft at a selected rotational velocity such that an order related component of the shaft intersects with the natural frequency range of the blade.

7. A method according to claim 6, wherein

the shaft is operated between a first rotational velocity and a second rotational velocity different from the first rotational velocity with a rate of change of shaft velocity to provide a signal that comprises a plurality of periodic responses,

the signal is transformed using a short-time chirp Fourier transform, thereby isolating a selected periodic response, and

the frequency speed of the short-time chirp Fourier transform is a function of the rate of change of the shaft velocity.

8. A method according to claim 7, wherein the varying rotational velocity occurs during acceleration of the engine.

9. A method according to claim 5, wherein the component is excited by

a) determining at least one order related component of the shaft;

b) determining at least one vibration mode of articles functionally mounted to the shaft;

c) selecting one of the vibration modes;

d) selecting one of the order related components that has an instantaneous frequency which intersects the natural frequency range of the selected mode; and

e) rotating the shaft and sampling response data where the instantaneous frequency of the selected order and natural frequency range of the selected vibration mode intersect.

10. A method according to claim 9, wherein the sampling of the data begins immediately before the instantaneous frequency of the selected order and natural frequency range of the selected vibration mode intersect.

11. A method according to claim 9, wherein the sampling of the data ends immediately after the instantaneous frequency of the selected order and natural frequency range of the selected vibration mode intersect.

12. A method according to claim 9, wherein the selected order is the lowest order having an instantaneous frequency that intersects with the natural frequency range of the selected vibration mode.

13. A method according to claim 9, further comprising determining a true collections of peaks in the sampled response data by:

a) providing sample windows comprising a first window and a second window, each of the sample windows enclosing a portion of the sampled response data, the second window lagging the first window;

b) calculating the mean value of the envelope of the sampled response within each sample window;

c) calculating a ratio of the mean value in the first window to the mean value in the second window;

d) comparing the ratio against a threshold ratio;

e) indicating a single peak where the compared ratio differs by a predetermined amount from the threshold ratio;

f) moving the sample windows along the envelope of the sampled response data and repeating steps b) to e) at least once;

g) calculating the spacing between adjacent single peaks;

h) grouping adjacent single peaks into a collection of peaks, each single peak in the collection being separated from an adjacent single peak by a spacing that is less than a threshold spacing, adjacent collections being separated by a spacing that is greater than the threshold spacing;

i) removing collections of peaks that have a length less than a threshold length;

j) calculating the density of the remaining collections of peaks; and

k) comparing the calculated density of each collection of peaks with a threshold density, wherein the true peaks have a calculated density greater than the threshold density.

14. A method according to claim 13, further comprising the step of providing a third sample window, the third sample window leading the first window with respect to the envelope of sampled response data, wherein

in step c), the ratio of the mean value in the first window to the mean value in the third window is also calculated, and

in step e), a single peak is indicated when both calculated ratios are above the threshold ratio.

15. A method of determining an artifact in a component comprising the steps:

a) exciting the component to induce a resonance response;

b) detecting higher harmonics in the averaged power spectral density of resonance response;

c) calculating the normalised amplitude of two or more higher harmonics of component resonance oscillations; and

d) comparing the sum of the normalized amplitude of the two or more higher harmonics with a threshold value thereby determining the presence of an artifact.

16. A method according to claim 15, wherein the component is a blade.

17. A method according to claim 16, wherein the blade is mounted on a shaft.

18. A method according to claim 16, wherein the blade is excited by rotating the shaft at a selected rotational velocity such that a frequency of order related component of the shaft intersects with the natural frequency range of fundamental harmonic of the selected blade mode.

19. A method for determining an artifact in a component comprising the steps:

a) exciting the component to induce a resonance response;

b) detecting higher harmonics in the averaged power spectral density of the resonance response;

c) calculating the normalized amplitude of two or more harmonics, at least one of the harmonics being the fundamental harmonic;

d) calculating the ratio of the sum of the normalized amplitude of the, or each, higher harmonic and the normalised amplitude of the fundamental harmonic; and

e) comparing the value of the ratio with a threshold, an artifact being determined by comparing the value with a threshold.

20. A method according to claim 19, wherein the component is a blade.

21. A method according to claim 20, wherein the blade is mounted on a shaft.

22. A method according to claim 21, wherein the blade is excited by rotating the shaft at a selected rotational velocity such that a instantaneous frequency of order related component of the shaft intersects with the frequency of fundamental harmonic of the selected blade mode.