Shaft shear detection through shaft oscillation

1. A method for detecting a shear of a rotating shaft of a gas engine, the shaft positioned between a source and a load, the method comprising:

storing in memory a shaft oscillation signature determined as a function of known characteristics of the shaft and associated with a shaft shear event, the shaft oscillation signature comprises a signature modulation frequency;

monitoring a rotational speed of the shaft comprising obtaining a signal from a speed sensing device;

detecting from the rotational speed an oscillation wave superimposed on the rotational speed, the oscillation wave having a wave modulation frequency and a wave modulation amplitude;

comparing the shaft oscillation signature to the oscillation wave comprising comparing the signature modulation frequency to the wave modulation frequency;

detecting the shaft shear event when the oscillation wave corresponds to the shaft oscillation signature; and

in response to detecting the shaft shear event, commanding a fuel shutoff of the engine.

2. The method of claim 1 , wherein the oscillation signature further comprises a signature modulation amplitude, and wherein comparing the oscillation wave to the shaft oscillation signature further comprises comparing the signature modulation amplitude to the wave modulation amplitude.

3. The method of claim 2 , wherein the oscillation signature comprises a range of signature modulation frequencies and a range of signature modulation amplitudes, and wherein comparing the oscillation wave to the oscillation signature comprises determining if the wave modulation frequency and the wave modulation amplitude fall within the range of signature modulation frequencies and the range of signature modulation amplitudes, respectively.

4. The method of claim 2 , wherein detecting from the rotational speed an oscillation wave comprises decomposing the rotational speed into a carrier wave and a modulation wave, the carrier wave corresponding to the rotational speed and the modulation wave corresponding to the oscillation wave, and extracting the wave modulation frequency and the wave modulation amplitude from the oscillation wave.

5. The method of claim 1 , wherein detecting from the rotational speed an oscillation wave comprises detecting a first period below a lower threshold and a second period above an upper threshold, and detecting a rate of occurrence of the first period and the second period.

6. The method of claim 1 , wherein the speed sensing device comprises a phonic wheel sensing assembly; and wherein monitoring flail the rotational speed of the shaft comprises determining the rotational speed using flail the phonic wheel sensing assembly.

7. The method of claim 1 , further comprising confirming detection of the shaft shear event when the oscillation wave corresponds to the oscillation signature for a predetermined amount of time.

8. The method of claim 1 , further comprising monitoring a torque of the shaft and performing the method only when the torque meets a minimum threshold.

9. A system for detecting a shear of a rotating shaft of a gas engine, the shaft positioned between a source and a load, the system comprising:

a memory storing a shaft oscillation signature determined as a function of known characteristics of the shaft and associated with a shaft shear event, wherein the shaft oscillation signature comprises a signature modulation frequency; and

at least one of:

(a) at least one processor configured for executing program code; and

(b) a circuit;

the at least one of (a) and (b) configured for:

monitoring a rotational speed of the shaft comprising obtaining a signal from a speed sensing device;

detecting from the rotational speed an oscillation wave superimposed on the rotational speed, the oscillation wave having a wave modulation frequency and a wave modulation amplitude;

comparing the oscillation signature to the oscillation wave comprising comparing the signature modulation frequency to the wave modulation frequency;

detecting the shaft shear event when the oscillation wave corresponds to the oscillation signature; and

in response to detecting the shaft shear event, commanding a fuel shutoff of the engine.

10. The system of claim 9 , wherein the oscillation signature further comprises a signature modulation amplitude, and wherein comparing the oscillation wave to the oscillation signature further comprises comparing the signature modulation amplitude to the wave modulation amplitude.

11. The system of claim 10 , wherein the oscillation signature comprises a range of signature modulation frequencies and a range of signature modulation amplitudes, and wherein comparing the oscillation wave to the oscillation signature comprises determining if the wave modulation frequency and the wave modulation amplitude fall within the range of signature modulation frequencies and the range of signature modulation amplitudes, respectively.

12. The system of claim 10 , wherein detecting from the rotational speed an oscillation wave comprises decomposing the rotational speed into a carrier wave and a modulation wave, the carrier wave corresponding to the rotational speed and the modulation wave corresponding to the oscillation wave, and extracting the wave modulation frequency and the wave modulation amplitude from the oscillation wave.

13. The system of claim 9 , wherein detecting from the rotational speed an oscillation wave comprises detecting a first period below a lower threshold and a second period above an upper threshold, and detecting a rate of occurrence of the first period and the second period.

14. The system of claim 9 , wherein the speed sensing device comprises a phonic wheel sensing assembly for sensing the rotational speed of the shaft.

15. The system of claim 14 , wherein the phonic wheel sensing assembly comprises a phonic wheel having phonic teeth, and the phonic teeth are spaced such that a lowest sampled wave modulation frequency is at least five times a highest possible signature modulation frequency.

16. The system of claim 9 , wherein the circuit is provided on a dedicated circuit board inside an aircraft electronic engine controller.

17. The system of claim 16 , wherein the at least one processor communicates the shaft oscillation signature to the circuit for comparing with the oscillation wave.

18. The system of claim 9 , wherein the gas engine is a gas turbine engine; and wherein the source is a turbine and the load is a fan, and the shaft is a lower pressure shaft of the gas turbine engine.

19. A system for detecting a shear of a rotating shaft of a gas engine, the shaft positioned between a source and a load, the system comprising:

a memory storing a shaft oscillation signature determined as a function of known characteristics of the shaft and associated with a shaft shear event, the shaft oscillation signature comprises a signature modulation frequency;

a speed sensing device for monitoring a rotational speed of the shaft;

at least one processor configured for executing program code for:

detecting from the rotational speed an oscillation wave superimposed on the rotational speed, the oscillation wave having a wave modulation frequency and a wave modulation amplitude;

comparing the oscillation signature to the oscillation wave comprising comparing the signature modulation frequency to the wave modulation frequency;

detecting the shaft shear event when the oscillation wave corresponds to the oscillation signature; and

in response to detecting the shaft shear event, commanding a fuel shutoff of the engine.