Methods of Analysing Apparatus

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.

1 . A method of isolating a selected periodic response from a plurality of periodic responses, the method comprising:
 operating a shaft at a varying rotational velocity to provide a signal that comprises a plurality of periodic responses; and   transforming the signal using a short-time chirp Fourier transform, thereby isolating the selected periodic responses and wherein the frequency speed of the short-time chirp Fourier transform is a function of the rate of change of the shaft velocity.   
   
   
       2 . A method according to  claim 1 , wherein the shaft is one of a plurality of shafts in a gas turbine engine. 
   
   
       3 . A method according to  claim 1 , wherein the varying rotational velocity occurs during acceleration of the engine. 
   
   
       4 . A method according to  claim 1 , wherein the periodic responses are resonant oscillations of vibration modes of articles functionally mounted to the shaft. 
   
   
       5 . A method according to  claim 4 , further comprising:
 determining an order related component of the shaft; and   selecting a vibration mode of articles functionally mounted to the shaft transforming the signal where the order and natural frequency range of the selected vibration mode intersect.   
   
   
       6 . A method according to  claim 4 , wherein the articles are blades. 
   
   
       7 . A method according to  claim 1 , wherein the signal is a vibro-acoustic signal. 
   
   
       8 . A method according to  claim 7 , wherein the vibro-acoustic signal is captured by a transducer 
   
   
       9 . A method of sampling data from a rotatable shaft comprising the steps:
 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 at least one vibration mode;   d) selecting one of the at least one order related component that has an instantaneous frequency which intersects the natural frequency range of the selected mode;   e) rotating the shaft and sampling the 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 shaft is rotated with an increasing or decreasing speed of rotation. 
   
   
       11 . A method according to  claim 10 , 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. 
   
   
       12 . A method according to  claim 10 , 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. 
   
   
       13 . 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. 
   
   
       14 . A method of determining true collections of peaks in an envelope of vibro-acoustic response data, comprising:
 a) providing two sample windows each enclosing a portion of the vibro-acoustic response data, the second window lagging the first window,   b) calculating the mean value of the envelope of the vibro-acoustic response within each window,   c) calculating the ratio of mean value in the first window to the mean value in the second window,   d) comparing the ratio against a thresholds   e) indicating a single peak where the compared ratio differs by a predetermined amount from the thresholds   f) moving the sample windows along the envelope of the vibro-acoustic 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 value, adjacent collections being separated by a spacing that is greater than the threshold value,   i) removing collections of peaks that have a length less than a threshold,   j) calculating the density of the remaining collections of peaks, and   k) comparing the calculated density of each collection of peaks with a threshold value, wherein the true peaks have a calculated density greater than the threshold value.   
   
   
       15 . A method according to  claim 14 , further comprising providing a further sample window, the further sample window leading the first window with respect to the envelope of vibro-acoustic response data; step c) being modified in that the ratio of the mean value in the first window to the mean value in the second window and the ratio of the mean value in the first window to the mean value in the further window are calculated; and step e) being modified in that a single peak in indicated when both calculated ratios are above the threshold. 
   
   
       16 . A method of determining an artifact in a component comprising:
 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 normalised amplitude of the two or more higher harmonics with a threshold value thereby determining the presence of an artifact.   
   
   
       17 . A method according to  claim 16 , wherein the component is a blade. 
   
   
       18 . A method according to  claim 17 , wherein the blade is mounted on a shaft. 
   
   
       19 . A method according to  claim 17 , wherein the blade is excited by rotating the shaft at a selected rotational velocity such that a frequency of an order related component of the shaft intersects with the natural frequency range of the fundamental harmonic of the selected blade mode. 
   
   
       20 . A method for determining an artifact in a component comprising:
 a) exciting the component to induce a resonance responses   b) detecting higher harmonics in the averaged power spectral density of resonance response,   c) calculating the normalised 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 normalised amplitude of the higher harmonic 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 the threshold.   
   
   
       21 . A method according to  claim 20 , wherein the component is a blade. 
   
   
       22 . A method according to  claim 21 , wherein the blade is mounted on a shaft. 
   
   
       23 . A method according to  claim 22 , wherein the blade is excited by rotating the shaft at a selected rotational velocity such that an instantaneous frequency of an order related component of the shaft intersects with the frequency of the fundamental harmonic of the selected blade mode. 
   
   
       24 . A method for determining an artifact in a component comprising:
 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 pk 0  and qk 0  wherein p and q are integers and k 0  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.   
   
   
       25 . A method according to  claim 24 , wherein the location of the point is compared with a discriminate function. 
   
   
       26 . A method according to  claim 25 , wherein the discriminate function is linear. 
   
   
       27 . A method according to  claim 24 , wherein the component is a blade. 
   
   
       28 . A method according to  claim 27 , wherein the blade is mounted on a shaft. 
   
   
       29 . A method according to  claim 28 , 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.