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.

What is claimed is: 
     
         1 . A method of determining an artefact in a component comprising the steps:
 a) exciting the component to induce a resonance response;   b) detecting higher harmonics in an averaged power spectral density of the resonance response;   c) calculating a normalised amplitude of two or more of the detected higher harmonics;   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 artefact.   
     
     
         2 . A method according to  claim 1 , wherein the component is a blade. 
     
     
         3 . A method according to  claim 2 , wherein the blade is mounted on a shaft. 
     
     
         4 . A method according to  claim 3 , wherein the blade has one or more fundamental harmonics induced at a respective range of natural frequencies, 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. 
     
     
         5 . A method according to  claim 3 , wherein the higher harmonics are captured as periodic responses in a signal, a method of isolating a selected periodic response from a plurality of periodic responses, comprising
 acceleration of the shaft to provide the signal that comprises the plurality of periodic responses, the acceleration having a rate of change;   transforming the signal using a short-time chirp Fourier transform, thereby isolating the selected periodic response; wherein the frequency speed of the short-time chirp Fourier transform is a function of the rate of change of the shaft velocity.   
     
     
         6 . A method according to  claim 5 , wherein the shaft is one of a plurality of shafts in a gas turbine engine. 
     
     
         7 . A method according to  claim 5 , wherein the periodic responses are resonant oscillations of vibration modes of one of more articles functionally mounted to the shaft. 
     
     
         8 . A method according to  claim 5 , wherein the signal is a vibro-acoustic signal. 
     
     
         9 . A method according to  claim 8 , wherein the vibro-acoustic signal is captured by a transducer 
     
     
         10 . A method for determining an artefact in a component comprising the steps:
 a) exciting the component to induce a resonance response;   b) detecting a plurality of harmonics in the averaged power spectral density of resonance response, one of the harmonics being a fundamental harmonic and one or more being a higher harmonic;   c) calculating a normalised amplitude of two or more of the harmonics, at least one of the harmonics being the fundamental harmonic;   d) calculating the ratio of the sum of the normalised amplitude of the, or each, higher harmonic and the normalised amplitude of the fundamental harmonic;   e) comparing the value of the ratio with a threshold, an artefact being determined by comparing the value with a threshold.   
     
     
         11 . A method according to  claim 10 , wherein the component is a blade. 
     
     
         12 . A method according to  claim 11 , wherein the blade is mounted on a shaft. 
     
     
         13 . A method according to  claim 12 , 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. 
     
     
         14 . A method according to  claim 12 , wherein the or each higher harmonic and the fundamental harmonic are captured as periodic responses in a signal, a method of isolating a selected periodic response from a plurality of periodic responses, comprising acceleration of the shaft to provide the signal that comprises the plurality of periodic responses, the acceleration having a rate of change;
 transforming the signal using a short-time chirp Fourier transform, thereby isolating the selected periodic response; wherein the frequency speed of the short-time chirp Fourier transform is a function of the rate of change of the shaft velocity.   
     
     
         15 . A method according to  claim 14 , wherein the shaft is one of a plurality of shafts in a gas turbine engine. 
     
     
         16 . A method according to  claim 14 , wherein the periodic responses are resonant oscillations of vibration modes of one of more articles functionally mounted to the shaft. 
     
     
         17 . A method according to  claim 14 , wherein the signal is a vibro-acoustic signal. 
     
     
         18 . A method according to  claim 17 , wherein the vibro-acoustic signal is captured by a transducer.