Method and system for determining the torque induced in a rotating shaft

1. A method of determining a torque induced in a rotating shaft, the shaft having a torsional oscillation frequency that is dependent on a stiffness of the shaft, wherein the torsional oscillation frequency and the stiffness are dependent upon operating conditions of the shaft, the method comprising:

measuring the torsional oscillation frequency of the rotating shaft;

measuring a twist induced in the rotating shaft by the torque;

using the measured value of the torsional oscillation frequency and the measured value of the induced twist to determine the torque induced in the shaft,

wherein the torsional oscillation frequency of the shaft and the induced twist are measured at a first set of operating conditions; and

determining a torsional oscillation frequency of the shaft at a second set of operating conditions at which the stiffness of the shaft is determined to determine the torque induced in the shaft.

2. A method as claimed in claim 1 wherein the torque induced in the shaft is determined using the measured torsional oscillation frequency and the induced twist at the first set of operating conditions, and the measured torsional oscillation frequency and the stiffness at the second set of operating conditions.

3. A method as claimed in claim 2 wherein the torque is determined as follows:

T

1

=

θ

1

⁢

ɛ

2

⁡

(

ω

1

2

ω

2

2

)

where; T 1 is the torque at the first set of operating conditions,

θ 1 is the induced twist at the first set of operating conditions,

ε 2 is the stiffness of the shaft at the second set of operating conditions,

ω 1 is the measured torsional oscillation frequency at the first set of operating conditions,

ω 2 is the measured torsional oscillation frequency at the second set of operating conditions.

4. A method as claimed in claim 3 wherein the stiffness of the shaft at the second set of operating conditions is determined by applying known torques to the shaft and measuring the resultant twists.

5. A method as claimed in claim 1 wherein twist in the shaft is measured using two sensors spaced axially along the shaft.

6. A method as claimed in claim 5 wherein the sensors are multipole sensors.

7. A method as claimed in claim 1 wherein the sensors used to measure the induced twist are also used to measure the oscillation frequency.

8. A method as claimed in claim 1 wherein a temperature of the shaft varies between the first and second set of operating conditions.

9. A method as claimed in claim 8 wherein a change in a moment of inertia of the shaft between the first set of operating conditions and the second set of operating conditions is negligible.

10. A method as claimed in claim 1 wherein the shaft is in a gas turbine engine.

11. A system for determining a torque induced in a rotating shaft, the shaft having a torsional oscillation frequency that is dependent on a stiffness of the shaft, wherein the torsional oscillation frequency and the stiffness are dependent upon operating conditions of the shaft, comprising:

a pair of sensors for measuring an induced twist in the shaft, spaced axially along the shaft;

measuring means for measuring the torsional oscillation frequency of the shaft; and

a processor for using the measured value of the torsional oscillation frequency and the measured value of the induced twist to determine the torque induced in the shaft,

wherein the pair of sensors are operable to measure the induced twist and the measuring means are operable to measure the torsional oscillation frequency at a first set of operating conditions, and

wherein the measuring means are also operable to measure a torsional oscillation frequency of the shaft at a second set of operating conditions at which the stiffness of the shaft is determined to determine the torque induced in the shaft.

12. A system as claimed in claim 11 wherein the processor uses the measured torsional oscillation frequency and the induced twist at the first set of operating conditions, and the measured torsional oscillation frequency and the stiffness at the second set of operating conditions to determine the torque induced in the shaft at the first set of operating conditions.

13. A system as claimed in claim 12 wherein the processor determines the torque as follows:

T

1

=

θ

1

⁢

ɛ

2

⁡

(

ω

1

2

ω

2

2

)

Where; T 1 is the torque at the first set of operating conditions,

θ 1 is the induced twist at the first set of operating conditions,

ε 2 is the stiffness of the shaft at the second set of operating conditions,

ω 1 is the measured torsional oscillation frequency at the first set of operating conditions,

ω 2 is the measured torsional oscillation frequency at the second set of operating conditions.

14. A system as claimed in claim 11 further comprising means for determining the stiffness of the shaft at the second set of operating conditions by applying known torques to the shaft and measuring the resultant twists.

15. A system as claimed in claim 11 wherein the sensors for measuring the induced twist also measure the torsional oscillation frequency.

16. A system as claimed in claim 11 wherein the sensors are multipole sensors.

17. A system as claimed in claim 11 wherein a temperature of the shaft varies between the first and second set of operating conditions.

18. A system as claimed in claim 17 wherein the shaft is such that any change in moment of inertia of the shaft between the first set of operating conditions and the second set of operating conditions is negligible.

19. A system as claimed in claim 11 wherein the shaft is in gas turbine engine.