Method and system for determining the angular position of a turbojet engine rotor

1. A method of determining the angular position of a first rotor of a turbojet, the method comprising:

generating at least one vibration pulse during a rotation of the first rotor, each vibration pulse being generated when the first rotor passes through a given reference angular position;

detecting the vibration generated thereby;

obtaining the angular position at a given instant of a second rotor of the turbojet relative to the angular position that it occupied at a reference instant representative of detecting a vibration pulse, said second rotor being coupled in rotation with the first rotor and having a speed of rotation different from the speed of rotation thereof; and

determining from the angular position of the second rotor, the angular position of the first rotor at said given instant.

2. A method according to claim 1 , wherein the angular position of the second rotor is obtained from a signal delivered by a sensor and representative of the teeth of a toothed wheel that is positioned on the second rotor going past the sensor.

3. A method according to claim 2 , wherein the instant representative of detecting a vibration pulse is selected as an instant at which the passage of the first rotor through the reference position coincides with a tooth of the toothed wheel going past the sensor.

4. A method of determining the angular position of an unbalance that is detected at a given instant on a first rotor of a turbojet, the turbojet also including a second rotor coupled in rotation with the first rotor and having a speed of rotation that is different from the speed of rotation thereof, said method comprising:

determining the angular position of the first rotor at the instant at which the unbalance was detected, as a function of the angular position of the second rotor by using a method of determining angular position according to any one of claims 1 to 3 ; and

determining the angular position of the unbalance at said instant from the angular position of the first rotor.

5. A system for determining the angular position of a first rotor of a turbojet, the system comprising:

means for generating vibration during a rotation of the first rotor, each vibration pulse being generated on the first rotor passing through a given reference angular position;

means for detecting the generated vibration pulses;

means for obtaining an annular position at a given instant of a second rotor of the turbojet relative to the angular position it occupied at a reference instant representative of a vibration pulse being detected, said second rotor being coupled to rotate with said first rotor and having a speed of rotation that is different from the speed of rotation thereof; and

means for determining the angular position of the first rotor at said given instant on the basis of the angular position of the second rotor.

6. A system according to claim 5 , wherein the means for generating vibration are adapted to cease generating vibration on the first rotor passing through the reference angular position as from a particular speed of operation of the turbojet.

7. A system according to claim 5 or claim 6 , wherein the means for generating vibration comprise a projection and a pawl or a flexible blade, a vibration pulse being generated by the pawl or the flexible blade striking against the projection as the first rotor passes through the reference angular position.

8. A system according to claim 7 , wherein the first rotor includes an annular element placed around an annular stator portion, the pawl or the flexible blade being located on the inside face of the annular element of the first rotor and the projection being located on the outside face of the annular portion of the stator.

9. A system according to claim 5 , wherein the first rotor is the high pressure shaft of the turbojet and the second rotor is a shaft connected to an accessory gearbox.

10. A turbojet comprising at least one system according to claim 5 .