GB2465575A

GB2465575A - Displacement measurement / rotor blade pitch measurement arrangement

Info

Publication number: GB2465575A
Application number: GB0821250A
Authority: GB
Inventors: Jonathan E Holt; Stephen A Fisher; Stephen Granville Garner; Mark J E Bellis
Original assignee: Rolls Royce PLC; Rolls Royce Corp
Current assignee: Rolls Royce PLC; Rolls Royce Corp
Priority date: 2008-11-21
Filing date: 2008-11-21
Publication date: 2010-05-26
Also published as: GB0821250D0

Abstract

A displacement measurement arrangement comprising a cylinder 34 that rotates about its axis in synchronicity with a component, such as a rotor blade of a gas turbine, and translates longitudinally in accordance with a change in a characteristic of the component, such as a change in the blade pitch, the cylinder having signal generating features comprising at least one longitudinally extending feature 50, 46 and at least one feature at least partially angled 44, 48 relative to the longitudinal feature; and at least one sensor (shown as arrows in the figure) arranged to receive a signal from the signal generating features. The change in the characteristic of the component, eg the blade pitch, is detected by sensing the change in the time delay in the generated signals due to the longitudinal displacement of the cylinder. The cylinder may be coupled to an actuator having fingers that are received in recesses 74, 76 in the cylinder. The signal generating features may include an indexing mark, for instance two closely spaced parallel longitudinal features 50, fig 4. In addition to blade pitch the invention may also be used to measure other characteristics of components that may be expressed as the longitudinal movement of the cylinder.

Description

DISPLACEMENT MEASUREMENT ARRANGEMENT AND METHOD

The present invention relates to a displacement measurement arrangement for a rotating component and a method of measuring parameters of a rotating component using displacement. The present invention finds particular utility in relation to an engine comprising variable pitch propeller rotor blade stages.

A known type of gas turbine engine, particularly for use in aircraft propulsion, is a propeller gas turbine engine or turboprop. This works in conventional form, whereby a core engine comprising compressors, combustion equipment and turbines drives one or more propeller rotor stages via a shaft from a free power, or low-pressure, turbine. The one or more propeller rotor stages may be situated at the front or rear of the engine, where front and rear are defined in terms of the direction of airflow through the engine. The propeller rotor blades extend radially outwardly to describe a larger diameter than the core engine. Each blade is pivotable about its own longitudinal axis to change its pitch and thus its angle of attack relative to the airflow. This variable pitch enables more efficient operation at a variety of operating conditions since the incident angle between the airflow and the blade surface can be optimised for the given airspeed and operating mode of the engine and aircraft.

Pitch angle is defined as shown in Figure 1 wherein a blade 8 is shown in plan view. The blade 8 is one of a set of rotor blades rotating clockwise as viewed from the left.

Thus blade 8 is travelling down the page. Pitch angle p is measured clockwise from top dead centre. The smaller the pitch angle ip, the finer the pitch; a larger pitch angle p means a coarser pitch.

There are benefits to providing two stages of propeller rotor blades that rotate in opposite directions and are connected by a differential gearbox. This contra-rotation ensures that airflow leaving the stages is substantially parallel to that entering the stages.

In order to control the pitch angle it is beneficial to receive a feedback signal corresponding to the current pitch angle cp of each blade, or of a blade that is representative of the whole blade set. One conventional arrangement and method of measuring the blade pitch angle is to provide a tongue-shaped transmitter or target extending from the root of at least one of the blades through an aperture in the rotor disc upon which the blades are mounted. The tongue-shaped blade target is arranged to sweep an arc as the blade is rotated to vary its pitch.

Thus the target rotates in synchronicity with the rotor disc but is advanced or retarded relative to a given position on the rotor disc depending on the current blade pitch angle p. A sensor is mounted to a stationary component and arranged to generate a signal on each revolution of the rotor when the target passes the sensor.

One or more additional transmitters or targets extend from the rotor disc so that they rotate in pure synchronicity with the rotor disc. Thus the blade pitch angle cp can be derived from the timing or spacing sensed between the signals from successive targets, since the relative movement of the tongue-shaped blade target changes the spacing relative to adjacent disc targets. The speed of rotation of the rotor can also be derived from the timing or spacing between successive disc targets or successive blade targets, although the latter requires that all the blades are pitched at the same angle. The angular position of the rotor may be determined by providing an indexing feature on the disc such that a different signal is produced once per revolution.

One disadvantage of this arrangement is that there is a limited range of angles at which the blade target can be sensed. This means that large blade pitch angles, whether coarse or fine, cannot be sensed. This may have particularly undesirable consequences in terms of preventing or controlling rotor overspeed and excessive drag caused by the blades failing too fine and in preventing or controlling the blades resisting rotation by feathering at too coarse an angle.

A further disadvantage of this arrangement is that the blade target is a long component that is vulnerable to vibration caused either by its own relative mass or by vibrations transmitted from other components. These vibrations may be sufficiently severe to cause the target to break away from the blade.

The described conventional arrangement is also prone to mechanical float between the rotating components and the stationary sensor since they are subject to different thermal and strain expansion rates. This reduces the accuracy of measurements made using this arrangement.

The present invention seeks to provide a displacement measurement arrangement and a method of measuring parameters of a rotating component that seeks to address the aforementioned problems.

Accordingly a first aspect of the present invention provides a displacement measurement arrangement comprising a cylinder that rotates about its axis in synchronicity with a component and translates longitudinally in accordance with a change in a characteristic of the component, the cylinder having signal generating features comprising at least one longitudinally extending feature and at least one feature extending at least partially angled relative to the longitudinal feature; and at least one sensor arranged to receive a signal from the signal generating features. This arrangement is advantageous in that it enables a linear movement of a cylinder to represent a change in a characteristic of a component, the linear movement being conducive to accurate measurement by virtue of the signal generating features.

Preferably the cylinder is hollow and the signal generating features are provided on the interior surface of the cylinder. The signal generating features may comprise ridges, grooves or inscribed lines as best suited to the chosen sensor. The at least one sensor may comprise a reluctance type speed probe, particularly felicitous in conjunction with ridges, or an optical probe or optical fibre, which are particularly felicitous in conjunction with grooves or inscribed lines.

The at least one angled feature may extend at a constant or a non-constant angle to the longitudinal feature.

The signal generating features may include an indexing feature from which to reference angular position. The index feature may comprise a pair of parallel longitudinally extending features in close proximity to one another. They may extend purely longitudinally or may be angled to extend with at least a component in the circumferential direction.

Preferably the cylinder is hollow and the at least one sensor is mounted within the cylinder but separate therefrom. More preferably the cylinder and the at least one sensor are coupled to negate differential thermal and strain expansion rates. The couple may be axially flexible, and may comprise an axially flexible bellows.

The couple may further comprise at least one bearing between the cylinder and the at least one sensor.

The cylinder may be coupled to an actuator that actuates the longitudinal translation. The actuator may be a piston, particularly a hydraulic or pneumatic annular piston.

The piston may comprise at least one finger that is received in a complementary recess in the cylinder. The at least one finger may comprise at least one signal generating feature. The at least one longitudinal feature may be provided by the edges of the at least one finger and complementary recess. At least one angled feature may be provided on at least one of the fingers and on each intermediary portion of the cylinder. The relative longitudinal translation obviates errors in the signal from the signal generating features causes by mechanical float between the components.

There may be a processor provided to process the signal received by the at least one sensor. This may be separate to the displacement measurement arrangement and may be located at a distance thereto. It may form part of another processor for processing other signals relating to the component.

In preferred embodiments the arrangement is a rotor pitch measurement arrangement. The characteristic of the component may comprise the pitch of the rotor blades.

Preferably the component is in a rotating frame of reference and the at least one sensor is in a stationary frame of reference.

A second aspect of the present invention provides a gas turbine engine comprising a displacement measurement arrangement as hereinbefore described.

A third aspect of the present invention provides a method of measuring parameters of a rotating component, the method comprising the steps of: a) providing a cylinder that rotates about its axis in synchronicity with the component, the cylinder having signal generating features comprising at least one longitudinally extending feature and at least one feature at least partially angled relative to the longitudinal feature, b) translating the cylinder longitudinally in accordance with a change in a characteristic of the component, c) sensing the time delay between peaks or zero-crossings in the signals produced by adjacent longitudinal and angled features, and d) deriving the change in the characteristic of the component from the sensed time delay.

This method is advantageous because it measures a linear displacement and correlates it with a more complex change in a characteristic of a component.

The method may comprise the additional steps of: sensing the time difference between peaks or zero-crossings in the signals produced by successive parallel features, and deriving the speed of rotation of the component from the sensed time difference.

The additional steps may occur before, concurrently with or after steps c) and d) above.

The method may further comprise additional steps of: providing an indexing feature on the cylinder, sensing the indexing feature, and deriving the angular position of the component from the sensed indexing feature.

These further additional steps may occur before, concurrently with or after steps c) and d) above.

The longitudinal translation of the cylinder results in relative longitudinal translation of successive angled features and the method may comprise the additional steps of: sensing the time delay between peaks or zero-crossings in the signals produced by successive angled features, and deriving a change in the characteristic of the component from the sensed time delay.

This aspect obviates mechanical float between stationary and rotating components.

A fourth aspect of the present invention provides a rotor pitch arrangement comprising a cylinder that rotates about its axis in synchronicity with the rotor and translates longitudinally in accordance with a change of pitch of the rotor blades.

A fifth aspect of the present invention provides a gas turbine engine comprising a rotor pitch arrangement according to the fourth aspect.

The present invention will be more fully described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic plan view of a blade showing pitch angle.

Figure 2 is a sectional side view of a gas turbine engine having contra-rotating propeller stages.

Figure 3 is a schematic sectional view of a displacement arrangement according to a first aspect of the present invention.

Figure 4 is a perspective view of a first embodiment of a displacement measurement arrangement according to a second aspect of the present invention.

Figure 5 is a plot showing signal responses derived from the arrangement in Figure 4.

Figure 6 is a schematic sectional view of a second embodiment of a displacement measurement arrangement according to the second aspect of the present invention.

Figure 7 is a perspective view of a third embodiment of a displacement measurement arrangement according to the second aspect of the present invention.

Referring to Figure 2, a twin-spooled, contra-rotating propeller gas turbine engine is generally indicated at 10 and has a principal rotational axis 9. The engine 10 comprises a core engine 11 having, in axial flow series, an air intake 12, an intermediate pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, an intermediate pressure turbine 18, a free power (or low-pressure) turbine 19 and a core exhaust nozzle 20. A nacelle 21 generally surrounds the core engine 11 and defines the intake 12 and nozzle 20 and a core exhaust duct 22. The engine 10 also comprises two contra-rotating propeller stages 23, 24 attached to and driven by the free power turbine 19 via shaft 26 and a differential gear box (not shown) The gas turbine engine 10 works in a conventional manner so that air entering the intake 12 is accelerated and compressed by the intermediate pressure compressor 14 and directed into the high-pressure compressor 15 where further compression takes place. The compressed air exhausted from the high-pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high-pressure, intermediate pressure and free power turbines 17, 18, 19 before being exhausted through the nozzle 20 to provide some propulsive thrust. The high-pressure, intermediate pressure and free power turbines 17, 18, 19 respectively drive the high and intermediate pressure compressors 15, 14 and the propeller rotor stages 23, 24 by suitable interconnecting shafts. The propellers 23, 24 normally provide the majority of the propulsive thrust. In the embodiments herein described the propellers 23, 24 rotate in opposite senses so that one rotates clockwise and the other anti-clockwise around the engine's rotational axis 9.

An exemplary embodiment of the displacement arrangement according to a first aspect of the present invention is shown in and described with respect to Figure 3. Although the present invention is described with respect to blade pitch angle changes of a rotor blade or set of rotor blades of a contra-rotating propeller gas turbine engine 10 and an arrangement and method for measuring such, it is to be understood that the present invention applies equally to other rotating components in such an engine or in another rotating machine.

The displacement arrangement 28 comprises an annular cylinder 30 housing an annular piston 32 that comprises an annulus and a plate. The piston 32 may be hydraulic or pneumatic and actuates longitudinally, parallel to the rotational axis 9 of the contra-rotating propeller gas turbine engine 10. Connected to the end of the piston annulus distal to the piston plate, or integral with the piston annulus, is a cylinder 34. The cylinder 34 is connected, through appropriate gearing and / or drive rods, to the pitch change mechanism of a rotor blade 8 or set of blades forming a propeller stage 23, 24, such that linear movement of the cylinder 34 in the direction of arrowheads 36 parallel to the rotational axis 9 causes proportional angular rotation of each blade about its longitudinal axis to alter the blade pitch angle p. The cylinder 34, piston 32 and annular cylinder 30 rotate in synchronicity with the propeller stage 23, 24 with which they are associated and are therefore in a rotating frame of reference.

A second aspect of the present invention provides a displacement measurement arrangement. Internally of the cylinder 34 and mounted to a stationary component is a plurality of sensors 38. Thus the sensors 38 are in a stationary frame of reference. It will be understood by skilled readers that the whole engine 10 may be translating in use, particularly where the engine 10 is used for aircraft propulsion, and therefore the frame of reference is stationary relative to the engine 10. Any number of sensors 38 may be provided without affecting the scope of the present invention, for example four sensors 38 are provided equi-angularly spaced as shown in Figure 4. In a preferred embodiment the sensors 38 are of the reluctance type as conventionally known for use as speed probes.

These produce a varying electrical signal as a varying metal profile passes through the sensing range of each sensor 38. Alternatively other sensors 38 could be substituted, such as optical probes and optical fibres.

Referring now to Figure 4, the cylinder 34 is shown in greater detail, with the piston 32 and its cylinder 30 omitted for clarity. The cylinder 34 rotates in synchronicity with the propeller rotor stage 23, 24 in the direction of arrowhead 40, for example only. The interior surface 42 of the cylinder 34 comprises a target in the form of signal generating features 44. At equal spacing around the interior surface 42 there are longitudinally extending features 46. Preferably the longitudinal features 46 extend totally or substantially from end to end of the cylinder 34 but in some embodiments they may be shorter than this and extend over only part of the distance between the ends of the cylinder 34. At equal spacing around the interior surface 42, and interspersed between adjacent pairs of longitudinal features 46, are angled features 48. As for the longitudinal features 46, the angled features 48 preferably extend from end to end of the cylinder 34 but may be shorter than this in some embodiments. The angled features 48 are arranged to extend with both longitudinal and circumferential components so that each angled feature 48 makes an acute angle B with the adjacent longitudinal feature 46 at the front end of the cylinder 34 (left in figures) . The angle e may be constant over the full longitudinal length of the cylinder 34 or it may vary so as to emphasise parts of the signal response, as discussed below.

Optionally there is an indexing feature 50 provided in place of one of the longitudinal features 46 to provide a once per revolution position index signal. Preferably the indexing feature 50 is in the form of a pair of closely spaced longitudinal features so that a double peak is generated in the signal.

The signal generating features 44 can take various forms.

In a preferred embodiment the features 44 are ridges protruding from the interior surface 42 of the cylinder 34.

This is particularly desirable when using reluctance type speed probe sensors 38 as a clear signal is generated.

Alternatively the features 44 may be grooves etched or machined into the interior surface 42 or lines drawn thereon. The latter is particularly suitable when used in conjunction with optical probes or optical fibres as sensors 38.

Figure 5 shows the responses received by one of the sensors 38 of the signals generated by the features 44 for a variety of different blade pitch angles p, where pitch angle cp increases (becomes coarser) down the figure as indicated by arrowhead 52. Time extends from left to right of the figure and the signal response for a little over one revolution is illustrated. A double, index peak 54 occurs once per revolution, two of which are shown. Three equally spaced peaks 56 corresponding to the equi-angularly spaced longitudinal features 46 lie between each adjacent pair of index peaks 54. Interspersed between successive longitudinal peaks 56, or between a longitudinal peak 56 and an index peak 54, are peaks 58 corresponding to the angled features 48. In alternative embodiments there may be zero-crossings of the signals that correspond to the signal generating features 44, rather than signal peaks 54, 56, 58. It is to be understood that zero-crossings are included in the alternate whenever signal peaks are referenced herein.

The index peak 54 indicates the time at which the index feature 50 passes the sensor 38, which is stationary relative to the rotating cylinder 34. This allows synchronisation to occur as required and may have other purposes synchrophasing, condition monitoring and rotor trim balancing. The speed of rotation of the cylinder 34, and hence the propeller rotor stage 23, 24, is derived from the timing between pairs of successive longitudinal peaks 56, or between a longitudinal peak 56 and an index peak 54.

Alternatively the speed can be derived from the timing between pairs of successive angled peaks 58. Thus speed is derived from the timing between alternate signal peaks 54, 56, 58 generated by features 44 that are parallel to one another. One such time difference is indicated by arrowheads 60.

Each blade pitch angle p corresponds to a longitudinal position of the cylinder 34 and of the actuating piston 32.

Thus, each blade pitch angle cp corresponds to a spacing between any pair of adjacent features 44 and hence to a timing between the corresponding pair of adjacent signal peaks 54, 56, 58. As is apparent from Figure 3, when the piston 32 is most compressed the sensors 38 are aligned with the distal portion of the cylinder 34 and the spacing from each longitudinal feature 46 or index feature 50 to the adjacent angled feature 48 is small. When the piston 32 is actuated, the cylinder 34 translates longitudinally to the left and the sensors 38 are aligned with a portion of the cylinder 34 that is nearer to the piston 32. This means that the equivalent spacing is larger. Thus the blade pitch angle p can be derived from the time delay, indicated by arrowheads 62, between any longitudinal or index peak 56, 54 and the adjacent, subsequent angled peak 58. For small blade pitch angles p it may be more accurate to measure the time delay 64 between any angled peak 58 and the adjacent, subsequent longitudinal or index feature 56, 54, as this is a longer period than the period 62.

Although the angled features 48 have been described as being at a constant angle e so that the signal peaks 58 are linear for the incrementally increasing blade pitch angles p shown in Figure 5, they may have a varying angle e between the ends of the cylinder 34. This enables greater resolution of the signals and therefore timing differentiation within a blade pitch angle cp range of interest in a particular application. For example, in normal flying operation of a contra-rotating propeller gas turbine engine 10 the blade pitch angle cp is typically varied between 20 to 65 degrees. At angles greater than around 85 degrees the blades 8 are driven to feather so that all that is required by an associated control system is a signal indicating that the blades 8 have feathered and not the precise pitch angle p. Thus the angled features 48 can be curved so that a small change in blade pitch angle p, mirrored by a small change in longitudinal position of the cylinder 34 results in a small spacing difference for blade pitch angles cp greater than 85 degrees and a relatively large spacing difference between adjacent signal generating features 44 and thus between adjacent signal peaks 54, 56, 58 for blade pitch angles p within the range of interest. This means that the time delay 62 or 64 is unambiguously different for small changes in blade pitch angle p within a predetermined range of interest. These blade pitch angles p are exemplary only and should not be considered to limit the scope of the invention.

Referring now to Figure 6, a second embodiment of the displacement measurement arrangement is shown, which comprises the displacement arrangement 28 described with respect to Figure 3. The sensors 38 are mounted to a stationary component 66 that is coupled to the rotating piston cylinder 30 by bearings 68. This arrangement ensures that the sensors 38 are held stationary (rotationally free) but are otherwise locked to the rotating components. The stationary component 66 is also connected to another stationary component, for example the central oil tube 70 of the engine 10, by a flexible arrangement comprising flexible bellows 72. The bellows 72 compress and stretch in response to differential thermal and strain expansion rates between the stationary and rotating components to absorb such differentials. The bellows 72 compensate any longitudinal movement, axial misalignment or vibration whilst holding component 66 stationary. In addition, the bellows 72 can house cables and the like to transfer the electrical signals from the sensors 38 to suitable processing means situated in the engine 10 or mounted thereto.

A third embodiment of the displacement measuring arrangement is shown in Figure 7, which shows cylinder 34 and piston cylinder 30. The cylinder 34 is modified in comparison with the first and second embodiments by the addition of longitudinal recesses 74 that are equi-angularly spaced around the inner surface 42. These recesses 74 are complementary to fingers 76 that extend longitudinally from the cylinder 30 and are of sufficient length to seat fully within the length of the recesses 74 of the cylinder 34 when the piston 32 is fully compressed and the sensors 38 are aligned with the distal portion of the cylinder 34. The fingers 76 and complementary recesses 74 are so arranged that they slide longitudinally with respect to one another when the piston 32 actuates the longitudinal movement of the cylinder 34 to change the blade pitch angle cp of the rotor blade 8 or set of rotor blades.

The signal generating features 44 in this embodiment are provided on both the fingers 76 and the intermediate portions of the interior surface 42 of the cylinder 34.

The longitudinal features 46 may be provided by a small clearance of the edges of the fingers 76 within their respective recesses 74, provided to permit the relative sliding movement. Preferably one of these longitudinal features 46 is adapted, by provision of an additional longitudinally extending feature in close circumferential proximity, to provide an index feature 50. Angled features 48 are provided alternately on the fingers 76 and the intermediate portions of the interior surface 42 of the cylinder 34.

When the cylinder 34 translates longitudinally this causes alternate angled features 48 to translate longitudinally so that any adjacent pair of angled features 46 has one translating and one stationary angled feature 46. This is advantageous because it automatically compensates for mechanical float between the sensors 38 and the rotating components by enabling measurement of the relative timing of alternate signal peak pairs rather than the absolute timing of the signal peaks. Thus, the difference in longitudinal position between the cylinder 34 and fingers 76 is measured rather than absolute longitudinal movement of the cylinder 34. This is particularly beneficial over time since mechanical float is exacerbated by wear on the bearings 68.

In all the embodiments described the signals received by the sensors 38 are processed by processing means.

Typically this will be a separate processor, which may form part of the engine control system electronics or be independent thereof. The processor processes the signals and derives the blade pitch angle p, speed of rotation and rotor position therefrom.

Although specific embodiments of the displacement arrangement, displacement measurement arrangement and method of measuring a change in a parameter by proportional displacement have been described and illustrated it will be apparent to the skilled reader that various modifications and variations can be made within the scope of the invention as claimed.

The number of signal generating features 44 illustrated and described is exemplary only. There may be more or fewer longitudinal 46 or angled 48 features as appropriate for the application. Typically there will be only one indexing feature 50. For example, although a piston 30 has been described, other linear actuators are substitutable with equal felicity. Although a flexible bellows 72 has been described, alternative flexible arrangements could be substituted that act to absorb vibrations, longitudinal movement and axial misalignment.

Figure 5 shows a series of signal responses at discrete blade pitch angles cp but these are exemplary only. A plot could be produced for any given blade pitch angle cp or a look up table used or the angle cp could be calculated directly from the measured timing rather than using a look

up table or plot.

The index feature 50 may comprise a pair of closely proximal, straight, parallel features or a pair of closely proximal, parallel features that are angled or curved over at least some of their length.

Although the present invention has been described in relation to changing the blade pitch angle p of a blade 8 or set of blades forming a propeller rotor stage 23, 24 of a contra-rotating propeller gas turbine engine 10 it may also be applied to different applications. For example, any change in a characteristic of a component in a gas turbine engine 10, whether or not a contra-rotating propeller gas turbine engine, that can be expressed mechanically as the longitudinal movement of the cylinder 34 can use the displacement arrangement and measurement arrangement and method of the present invention.

The present invention can also be applied in different fields such as ship propellers or thrusters, wind turbines, horizontal axis wave or tidal powered turbines, or vertical axis or hydro-electric turbines.

Claims

Claims 1. A displacement measurement arrangement comprising a cylinder that rotates about its axis in synchronicity with a component and translates longitudinally in accordance with a change in a characteristic of the component, the cylinder having signal generating features comprising at least one longitudinally extending feature and at least one feature at least partially angled relative to the longitudinal feature; and * at least one sensor arranged to receive a signal from the signal generating features.
2. A displacement measurement arrangement as claimed in claim 1 wherein the cylinder is hollow and the signal generating features are provided on the interior surface of the cylinder.
3. A displacement measurement arrangement as claimed in claims 1 or 2 wherein the signal generating features comprise ridges.
4. A displacement measurement arrangement as claimed in claims 1 or 2 wherein the signal generating features comprise grooves.
5. A displacement measurement arrangement as claimed in claims 1 or 2 wherein the signal generating features comprise inscribed lines.
6. A displacement measurement arrangement as claimed in any preceding claim wherein the at least one angled feature extends at a constant angle to the longitudinal feature.
7. A displacement measurement arrangement as claimed in any preceding claim wherein the at least one angled feature extends at a non-constant angle to the longitudinal feature.
8. A displacement measurement arrangement as claimed in any preceding claim wherein the signal generating features comprise an indexing feature.
9. A displacement measurement arrangement as claimed in claim 8 wherein the indexing feature comprises a pair of parallel longitudinally extending features in close proximity to one another.
10. A displacement measurement arrangement as claimed in any of claims 1 to 9 wherein the at least one sensor comprises a reluctance type speed probe.
11. A displacement measurement arrangement as claimed in any of claims 1 to 9 wherein the at least one sensor comprises an optical probe or optical fibre.
12. A displacement measurement arrangement as claimed in any preceding claim wherein the cylinder is hollow and the at least one sensor is mounted within the cylinder but separate therefrom.
13. A displacement measurement arrangement as claimed in any preceding claim wherein the cylinder and the at least one sensor are coupled to negate differential thermal and strain expansion rates.
14. A displacement measurement arrangement as claimed in claim 13 wherein the couple is axially flexible.
15. A displacement measurement arrangement as claimed in claim 13 or 14 wherein the couple comprises an axially flexible bellows.
16. A displacement measurement arrangement as claimed in any of claims 13 to 15 wherein the couple further comprises at least one bearing between the cylinder and the at least one sensor.
17. A displacement measurement arrangement as claimed in any preceding claim wherein the cylinder is coupled to an actuator that actuates the longitudinal translation.
18. A displacement measurement arrangement as claimed in claim 17 wherein the actuator comprises at least one finger that is received in a complementary recess in the cylinder.
19. A displacement measurement arrangement as claimed in claim 18 wherein the at least one finger comprises at least one signal generating feature.
20. A displacement measurement arrangement as claimed in claim 18 or 19 wherein the at least one longitudinal feature is provided by the edges of the at least one finger and complementary recess.
21. A displacement measurement arrangement as claimed in claim 19 or 20 wherein at least one angled feature is provided on at least one of the at least one fingers and at least one angled feature is provided on each intermediary portion of the cylinder.
22. A displacement measurement arrangement as claimed in any preceding claim further comprising a processor to process the signal received by the at least one sensor.
23. A displacement measurement arrangement as claimed in any of claims 1 to 22 wherein the arrangement comprises a rotor pitch measurement arrangement.
24. A displacement measurement arrangement as claimed in claim 23 wherein the characteristic of the component comprises the pitch of the rotor blades.
25. A displacement measurement arrangement as claimed in any preceding claim wherein the component is in a rotating frame of reference and the at least one sensor is in a stationary frame of reference.
26. A displacement measurement arrangement substantially as hereinbefore described with reference to the accompanying drawings.
27. A gas turbine engine comprising a displacement measurement arrangement as claimed in any preceding claim.
28. A method of measuring parameters of a rotating component, the method comprising the steps of: a) providing a cylinder that rotates about its axis in syrichroriicity with the component, the cylinder having signal generating features comprising at least one longitudinally extending feature and at least one feature at least partially angled relative to the longitudinal feature, b) translating the cylinder longitudinally in accordance with a change in a characteristic of the component, c) sensing the time delay between peaks or zero-crossings in the signals produced by adjacent longitudinal and angled features, and d) deriving the change in the characteristic of the component from the sensed time delay.
29. A method as claimed in claim 28 wherein the method comprises the additional steps of: a) sensing the time difference between peaks or zero-crossings in the signals produced by successive parallel features, and b) deriving the speed of rotation of the component from the sensed time difference.
30. A method as claimed in claim 28 wherein the additional steps occur before, concurrently with or after steps 28.c) and 28.d).
31. A method as claimed in any of claims 28 to 30 wherein the method comprises the additional steps of: a) providing an indexing feature on the cylinder, b) sensing the indexing feature, and C) deriving the angular position of the component from the sensed indexing feature.
32. A method as claimed in claim 31 wherein the additional steps occur before, concurrently with or after steps 28.c) and 28.d)
33. A method as claimed in any of claims 28 to 32 wherein longitudinal translation of the cylinder results in relative longitudinal translation of successive angled features and the method comprises the additional steps of: a) sensing the time delay between peaks or zero-crossings in the signals produced by the successive angled features, and b) deriving the change in the characteristic of the component from the sensed time delay.
34. A method substantially as hereinbefore described with reference to the accompanying drawings.
35. A rotor pitch arrangement comprising a cylinder that rotates about its axis in synchronicity with the rotor and translates longitudinally in accordance with a change of pitch of the rotor blades.
36. A rotor pitch arrangement substantially as hereinbefore described with reference to the accompanying drawings.
37. A gas turbine engine comprising a rotor pitch arrangement as claimed in claim 35.