Shaft resonance control

1. A method of actively controlling torsional resonance of a rotating shaft of an engine, the shaft having a rotational velocity characterised by (i) a low frequency, rotational velocity term and (ii) a high frequency, oscillatory term superimposed on the low frequency term, the oscillatory term being caused by torsional resonance, the method comprising:

measuring the rotational velocity of the shaft;

extracting the oscillatory term from the measured rotational velocity; and

applying a torque component to the shaft based on the extracted oscillatory term, the torque component being modulated at a same frequency as the torsional resonance to counteract the torsional resonance, the torque component being applied by modulating a flow rate of fuel to the engine, which includes an engine fuel control system that generates a fuel flow demand signal in response to an acceleration demand signal and a steady state fuel flow requirement, the flow rate of fuel to the engine being modulated by frequency modulating the fuel flow demand signal to generate the torque component at the same frequency as the torsional resonance.

2. The method according to claim 1 , wherein:

the measurement of the rotational velocity is performed at a frequency that is higher than the torsional resonance frequency of the shaft.

3. The method according to claim 1 , wherein the extraction of the oscillatory term from the measured rotational velocity includes demodulating the measured rotational velocity.

4. A system for reducing torsional resonance of a rotating shaft of an engine, the shaft having a rotational velocity characterised by (i) a low frequency, rotational velocity term and (ii) a high frequency, oscillatory term superimposed on the low frequency term, the oscillatory term being caused by torsional resonance, the system comprising:

a device configured to measure the rotational velocity of the shaft; and

a control unit configured to perform:

extracting the oscillatory term from the measured rotational velocity; and

generating and issuing a command to apply a torque component to the shaft based on the extracted oscillatory term, the torque component being modulated at a same frequency as the torsional resonance to counteract the torsional resonance, the torque component being applied by modulating a flow rate of fuel to the engine, which includes an engine fuel control system that generates a fuel flow demand signal in response to an acceleration demand signal and a steady state fuel flow requirement, the flow rate of fuel to the engine being modulated by frequency modulating the fuel flow demand signal to generate the torque component at the same frequency as the torsional resonance.

5. The system according to claim 4 , wherein the device for measuring the rotational velocity of the shaft includes:

a phonic wheel that is mounted coaxially to the shaft for rotation therewith, the phonic wheel having a circumferential row of detectable features; and

a sensor configured to detect a passage of the row of detectable features by generating an alternating measurement signal having a frequency that is a multiple of the rotational frequency of the shaft.

6. The system according to claim 4 , wherein:

the measurement of the rotational velocity by the device is performed at a frequency which is higher than the torsional resonance frequency of the shaft; and

the extraction of the oscillatory term from the measured rotational velocity by the control unit includes filtering the measured rotational velocity within a frequency range including the torsional resonance frequency.

7. A gas turbine engine having the system for reducing torsional resonance according to claim 4 .