Variable vane control system

1. A variable vane control system for controlling the angle of rotation of a circumferential row of variable vanes of a gas turbine engine, the control system including:

a mechanical linkage operable to rotate the variable vanes;

one or more actuators for operating the linkage;

one or more position sensors for detecting the respective actuation positions of the one or more actuators;

a linkage position signalling switch for signalling that the mechanical linkage is at a calibration position corresponding to a predetermined rotation angle of the vanes; and

a controller for controlling the one or more actuators and thereby controlling the angle of rotation of the vanes, based on the detected actuation positions which the controller correlates with vane rotation angle;

wherein the controller corrects the correlation between the detected actuation positions and vane rotation angle on receipt of a signal from the linkage position signalling switch indicating that the mechanical linkage is at the calibration position.

2. A system according to claim 1 , wherein the controller corrects the correlation between the detected actuation positions and vane rotation angle by:

receiving the signal from the linkage position signalling switch indicating that the mechanical linkage is at the calibration position;

recalling the predetermined rotation angle of the vanes corresponding to the mechanical linkage being at the calibration position;

detecting the actuation positions of the one or more actuators using the one or more position sensors,

calculating the vane rotation angle which correlates with the detected actuation positions; and

calculating a corrective offset for the correlation based on the difference between the calculated vane rotation angle and the predetermined vane rotation angle.

3. A system according to claim 2 , wherein the controller activates an alarm if the corrective offset exceeds a predetermined threshold.

4. A system according to claim 1 , wherein the linkage position signalling switch includes first and second parts, the first part being attached to a component of the engine which is stationary relative to the mechanical linkage, and the second part being attached to and movable with the mechanical linkage such that the parts make signalling contact when the mechanical linkage is at the calibration position.

5. A system according to claim 4 , wherein the first and second parts are electrically contacting parts which make electrical signalling contact when the mechanical linkage is at the calibration position.

6. A system according to claim 4 , wherein one of the first and second parts includes a signalling switch part, and the other of the first and second parts includes a tripping part such that the tripping part changes the state of the switch part when the mechanical linkage moves through the calibration position.

7. A system according to claim 1 , including a plurality of the linkage position signalling switches for signalling that the mechanical linkage is at the calibration position.

8. A system according to claim 1 , including a plurality of linkage position signalling switches for signalling that the mechanical linkage is at respective calibration positions corresponding to respective and different predetermined rotation angles of the vanes;

wherein the controller corrects the correlation between the detected actuation positions and vane rotation angle on receipt of signals from the linkage position signalling switches indicating that the mechanical linkage is at the respective calibration positions.

9. A system according to claim 1 , wherein the mechanical linkage includes a unison ring attached to the vanes such that rotation of the unison ring results in rotation of the vanes.

10. A system according to claim 1 , wherein the vanes are variable stator vanes or variable inlet guide vanes.

11. A system according to claim 1 , wherein the, or each, position sensor is a linear or rotary variable differential transducer position sensor.

12. A system according to claim 1 , wherein the controller is an electronic engine control unit.

13. A gas turbine engine having a variable vane control system according to claim 1 .