Patent ID: 12196633

DETAILED DESCRIPTION

Referring toFIG.1, there is shown a schematic view of an assembly100, according to an example. The assembly100comprises a bearing114having an axis of rotation A1. The bearing114comprises an outer race116A, an inner race116B and a plurality of bearing rollers110between the outer race116A and the inner race116B.

In this example, the assembly100comprises five fibre Bragg grating (FBG) sensors102A,102B,102C,102D,106for sensing strain or temperature of the bearing114. Four of the FBG sensors102A,102B,102C,102D extend in directions parallel to the axis of rotation A1. The FBG sensor102A, the FBG sensor102B and the axis of rotation A1lie in a common plane. In this particular example, in which the axis of rotation A1extends horizontally, the common plane extends vertically. The FBG sensor102C, the FBG sensor102D and the axis of rotation A1lie in a further common plane, which in this example extends horizontally. The further common plane is therefore orthogonal to the common plane.

The fifth FBG sensor106extends about the axis of rotation A1. The FBG sensor106may be arranged to form one or more loops around the bearing114. In this particular example, the FBG sensor106is mounted around the inner race116B of the bearing114. In another example, the FBG sensor106may be mounted around the outer race116A of the bearing114.

Each FBG sensor102A,102B,102C,102D,106is a fibre optic sensor which comprises a plurality of FBG elements104. The FBG elements104are etched directly into the core of an optical fibre. The spacing, between each immediately-adjacent pair of the FBG elements104, may be equal. Alternatively, the spacing between a first immediately-adjacent pair of the FBG elements104may differ from the spacing between a second immediately-adjacent pair of the FBG elements104. In other embodiments, there may be unequal spacing between each of the FBG elements104along the fibre.

Each FBG element104is a distributed Bragg reflector comprised within an optical fibre and comprising periodic variations in the refractive index of the core of the fibre along a section of the length of the optical fibre. The wavelength of a band of light reflected from an FBG element104is dependent on the axial strain of the fibre within which the FBG is located and/or the temperature to which the FBG element104is subjected. FBG sensors are lightweight and the spacing between FBG elements may be below 1 mm. The FBG elements104may form a continuous grating. As a result, a large number of sensing points can be located in a small area. An optical fibre may comprise a large number of FBG elements, for example, over 1000 FBG elements. Alternatively, an optical fibre may comprise a small number of FBG elements, for example, 10 FBG elements.

In this example, each FBG element104is configured to sense strain or temperature of the bearing114. In particular, each FBG element104is configured so that an optical characteristic of the each FBG element110changes in response to changes in strain and/or temperature of the bearing114. When a mechanical force acts upon the bearing114, the mechanical force may cause the FBG elements104of an FBG sensor to experience a strain. Moreover, different FBG elements104may experience different strain. Additionally, the FBG elements104of different FBG sensors102A,102B,102C,102D,106may experience different strain according to the location and direction of the mechanical force. When the temperature of the bearing114changes, the temperature of an FBG sensor may also change. As a result, the refractive indices of the FBG elements104may change. In some cases, changes in the temperature of the bearing and FBG sensor may additionally or alternatively change the thermal expansion of the FBG elements104. The FBG sensors102A,102B,102C,102D,106are used to measure resultant temperatures or strains at the respective sensor locations, and the optical signals output from the FBG sensors102A,102B,102C,102D, FBG sensor106are received at an interrogator (not shown inFIG.1). The interrogator uses the received optical signals to determine the strain and/or temperature of the bearing114.

The FBG elements104of one or more of the FBG sensors102A,102B,102C,102D,106may be configured such that a first FBG element reflects a first range of wavelengths of light that is different from a second range of wavelengths of light that is reflected by a second FBG element. Accordingly, when the sensor is used with a suitable interrogator as discussed below, the interrogator is better able to distinguish light received from one of the FBG elements104from light received from another of the FBG elements104, which simplifies determination of the location of the point of application of, for example, a mechanical force on one or more of the FBG elements104. Additionally or alternatively, the time-of-flight of light received from the FBG elements104may be used to distinguish light received from one of the FBG elements104from light received from another of the FBG elements104.

FIG.2shows an aircraft wheel assembly200. The assembly200comprises a wheel208supported on an axle212by a first bearing214A and a second bearing214B. The assembly200further comprises a tyre222supported on the wheel208. Each of the bearings214A,214B is the same as that described above and comprises an outer race216A,216B, an inner race218A,218B and a plurality of bearing rollers210between the outer race216A,216B and the inner race218A,218B.

The wheel208and bearings214A,214B rotate about an axis of rotation A2. The wheel208comprises two halves, an inner half wheel and an outer half wheel. The first bearing214A is mounted to the inner half wheel and the second bearing214B is mounted to the outer half wheel.

The assembly200comprises a brake pack226which is secured to the axle212, the brake pack226being located between an outer fastener224and a spacer228, the spacer228being located between the brake pack226and the inner half wheel of the wheel208.

In this example, the assembly200comprises six FBG sensors for sensing strain or temperature of the bearings214A,214B. More particularly, the assembly200comprises four axial FBG sensors202A,202B,202C,202D, and two radial FBG sensors206A,206B. Although the assembly comprise four axial FBG sensors, two of the sensors202C,202D lie in the same horizonal plane. As a result, one of the sensors202D lies behind the other of the sensors202C inFIG.2.

Each of the axial FBG sensors202A,202B,202C,202D extends in a direction parallel to the axis of rotation A2. The FBG sensor202A, the FBG sensor202B and the axis of rotation A2lie in a common plane. In the example ofFIG.2, the axis of rotation A2extends horizontally and the common plane extends vertically. The FBG sensor202C, the FBG sensor202D and the axis of rotation A2lie in a further common plane. In the example shown inFIG.2, again with the axis of rotation A2extending horizontally, the further common plane extends horizontally. The further common plane is therefore orthogonal to the common plane. Each FBG sensor202A,202B,202C,202D extends along an inside of the axle212. In other examples, the FBG sensors202A,202B,202C,202D may be embedded within the axle

Each of the radial FBG sensors206A,206B extends in a direction about the axis of rotation A2. The first circumferential FBG sensor206A is positioned at the first bearing214A and is sensitive to strain and/or temperature of the first bearing. The second circumferential FBG sensor206B is positioned at the second bearing214B and is sensitive to strain and/or temperature of the second bearing214B. In the particular example shown inFIG.2, each of the radial FBG sensors206A,206B is mounted around the axle212, between the axle212and the inner race218A,218B of the respective bearing214A,214B. In another example, each of the radial FBG sensors206A,206B may be mounted around the outer race216A,216B of the respective bearing214A,214B, or around the inside or outside of the axle212. The outer race216A,216B, the inner race218A,218B and/or the axle212may comprise a groove into which each of the FBG sensors206A,206B is seated. In the example shown inFIG.2, each of the radial FBG sensors206A,206B comprises a single loop. Alternatively, each of the FBG sensors206A,206B may comprise multiple loops.

The axial and radial FBG sensors each comprise an optical fibre comprising a plurality of FBG elements204. The FBG elements204are etched directly into the core of the optical fibre. The spacing, between each immediately-adjacent pair of the FBG elements204, may be equal. Alternatively, the spacing between a first immediately-adjacent pair of the FBG elements204may differ from the spacing between a second immediately-adjacent pair of the FBG elements204. In other embodiments, there may be unequal spacing between each of the FBG elements204along the fibre.

The FBG sensors202A,202B,202C,202D,206A,206B sense strain and/or temperature of the first and second bearing214A,214B. Each of the axial FBG sensors is sensitive to changes in strain and/or temperature of both the first bearing214A and the second bearing214B. By contrast, the first radial FBG sensor206A is sensitive to change in strain and/or temperature of the first bearing214A only, and the second radial FBG sensor206B is sensitive to changes in strain and/or temperature of the second bearing214B only. The strain and/or temperature of the bearings214A,214B are determined by analysing optical characteristics of the FBG elements204of the axial FBG sensors202A,202B,202C,202D, and the radial FBG sensors206A,206B.

When a mechanical force is applied to one of the FBG elements204of the FBG sensors202A,202B,202C,202D,206A,206B, the optical characteristic of the FBG element204changes. In particular, when the mechanical force is applied, the spacing of the grating of the FBG element204changes. Therefore, the emission wavelength of the FBG element204changes. For example, if a compression is applied to the FBG element204, the spacing of the grating is reduced, engendering a blue shift in wavelength. In contrast, if a tension is applied to the FBG element204, the spacing of the grating is increased, engendering a red shift in wavelength.

The assembly200is part of a system comprising one or more light sources and an interrogator220. The one or more light sources are configured to direct light towards the FBG sensors202A,202B,202C,202D,206A,206B. The interrogator is configured to receive the optical signals output from the FBG sensors202A,202B,202C,202D,206A,206B, and determine a strain and/or or temperature of the bearings214A,214B based on the received optical signals. The optical characteristics of the FBG elements204of each of the FBG sensors may be represented as optical spectra. The optical characteristics may represent a pattern or spacing of the grating of the FBG elements204. In use, the FBG sensors receive light from the one or more light sources emitting a wavelength full optical spectrum. Considering, for example, the FBG sensor202A, the light passes through the optical fibre of the FBG sensor202A and reaches a first FBG element204of the plurality of FBG elements204of the FBG sensor202A. A characteristic of the light is modified by the first FBG element204of the FBG sensor202A and a first optical spectrum is obtained. The characteristic of the light may be, for example, a spectrum of wavelengths. Similarly, the light with the full wavelength spectrum of emitted by the one or more light sources reaches a second FBG element204of the plurality of FBG element204of the FBG sensor202A. The characteristic of the light is modified by the second FBG element204and a second optical spectrum is obtained, and so on. An optical spectrum is obtained for each FBG element204of the plurality of FBG elements204of the FBG sensor202A. The optical signal output by the FBG sensor202A may therefore be said to comprise a plurality of optical spectra, each spectrum generated by a respective FBG element204of the sensor202A. The interrogator220then analyses the optical signal from the FBG sensor202A to determine the strain and/or the temperature of the FBG sensor202A. The same analysis can be performed for each FBG sensor202A,202B,202C,202D,206A,206B.

As explained above, the assembly200comprises the interrogator220configured to receive optical signals from each FBG sensor202A,202B,202C,202D,206A,206B. The interrogator220is configured to analyse the optical signals output by the FBG sensors to determine a strain and/or temperature of the first bearing214A and the second bearing214B.

Broadly speaking, the interrogator220comprises at least one light measurer, and the interrogator220is configured to determine a strain and/or temperature of the bearings214A,214B based on the optical signals received at the light measurer from the FBG sensors202A,202B,202C,202D,206A,206B. The or each light measurer may be, for example, a wavelength meter, a photodetector, or a photodiode, such as an avalanche photodiode or a PIN diode.

For example, the interrogator220may comprise a primary optical splitter, a plurality of secondary optical splitters, one or more light sources, a processor, a plurality of light measurers, and an analogue to digital converter. The processor is communicatively connected to the light measurers.

The one or more light sources are configured to direct light towards the FBG sensors202A,202B,202C,202D,206A,206B. The one or more light sources may be tuneable lasers. In another example, the one or more light sources may be broadband light sources or broadband light sources with tuneable a narrowband filter on the output.

Considering for example the two FBG sensors202A and202B, the one or more light sources may be arranged to output light into the primary optical splitter. The primary optical splitter splits the light received from the one or more light sources into two portions, and these two portions are fed into the first and second secondary optical splitters, respectively. From the first secondary optical splitter, the portion of light received from the primary optical splitter is sent through the first FBG sensor202A. From the second secondary optical splitter, the portion of light received from the primary optical splitter is sent through the second FBG sensor202B. The reflected light from the FBG sensor202A passes back through the first secondary optical splitter to the first light measurer, and the reflected light from the FBG sensor202B passes back through the second secondary optical splitter to the second light measurer.

In embodiments in which the one or more light sources are broadband light sources, the, or each of the, light measurer(s) may be a wavelength meter configured to measure the wavelength of the light reflected from the FBG elements204of the respective sensors. The processor processes the wavelength(s) measured by light measurers, and converts the measured wavelengths (for example via a calibration) into a mechanical force or thermal energy applied to the sensors.

In embodiments in which the one or more light sources are tuneable narrow band optical sources, such as a tuneable laser as in the present embodiment, the, or each of the, light measurer(s) may be a photodetector or a photodiode such as an avalanche photodiode or a PIN diode configured to measure the intensity of light reflected from the FBG elements204of the respective sensors. The analogue to digital converter is arranged to convert the analogue photodiode signals into digital signals, which are then provided to the processor. In this example, the processor is arranged to control the tuneable laser to emit light successively at different wavelengths, and at the same time monitor light intensity signals received from the photodiodes. The processor can thus determine for which emitted wavelength the highest intensity of reflected light is detected, and thereby determine the wavelength of light most reflected by the FBG elements204of the sensors at that point in time. The processor may then convert the determined wavelengths (for example via a calibration) into a mechanical force or thermal energy applied to the sensors.

It will be appreciated that although two secondary optical splitters and two associated light measurers are described in the examples above, in other examples there may be any number of optical splitters and associated light measurers. For example, there may be one pair of secondary optical splitters and an associated light measurer for each FBG sensor. In some examples, as noted above, there may be only one light measurer and associated secondary optical splitter for all FBG sensors.

In an example, the assembly200may comprise two or more light sources, each emitting at a different wavelength. As noted above, the range of wavelengths reflected by one of the FBG elements204of an FBG sensor may be different from the range of wavelengths reflected by another FBG element204of the FBG sensor. Therefore, a first FBG element204may be transparent to a range of wavelengths needed to interrogate a second FBG element204, and the first and second FBG elements204may be transparent to a range of wavelengths needed to interrogate a third FBG element204, and so on. Therefore, by using two or more light sources, each emitting at a different wavelength reduces the weight and complexity of connections needed to interrogate each FBG sensor.

As indicated above, the interrogator220receives optical signals from the FBG sensors202A,202B,202C,202D,206A,206B to determine strain and/or temperature of the first bearing214A and the second bearing214B. As will now be described, the determined strains and/or temperatures of the bearings214A,214B may be used to determine, for example, shear and/or lateral forces acting on the wheel, mass imbalance of the wheel, or the condition of the bearings.

In this embodiment, the FBG sensor202A and the FBG sensor202B are used to determine vertical forces, such as a ground force applied to the wheel208or a vertical force depending on the aircraft weight. Each of the FBG sensor202A and the FBG sensor202B may alone be used to determine such forces. However, when a vertical force is applied to the FBG sensor202A and the FBG sensor202B, the force has a different impact on each FBG sensor202A,202B. In particular, in response to a vertical force, one of the FBG sensors202A,202B will be in compression and the other will be in tension. As a result, the optical signals output by the FBG sensors202A,202B will be different. For example, when one of the FBG sensors202A is in tension, the spectra of the optical signal output by the FBG sensor202A may be shifted to higher wavelengths. By contrast, when the other of the FBG sensors202B is in compression, the spectrums of the optical signal output by the FBG sensor202B may be shifted to lower wavelengths. Accordingly, by analysing the optical signals of the two FBG sensors202A,202B, the interrogator220is capable of sensing the direction and magnitude of vertical forces. For example, when an upward vertical force is applied to the wheel208and axle212(e.g. ground force), the upper FBG sensor202A will be in tension, and the lower FBG sensor will be in compression. Conversely, when a downward vertical force is applied to the wheel208and axle212(e.g. weight of the suspended wheel in flight), the upper FBG sensor202A will be in compression, and the lower FBG sensor will be in compression. The FBG sensors202A,202B may therefore be used to sense the weight of the aircraft. The sensed weight of the aircraft may be used in a high integrity system and automatically entered into a flight computer, or the sensed weight may be compared to an aircraft weighted entered by a pilot. The FBG sensors202A,202B may also be used to detect landing forces and identify if a hard landing may have occurred. For example, a peak detection algorithm may be used to determine an impact by analysing significant shift in the spectrums of the optical signals output by the FBG sensors202A,202B.

When the vertical force acting on the wheel208and axle212is uniform and balanced, the FBG elements204of the upper FBG sensor202A experience the same strain, and the FBG elements204of the lower FBG sensor202B experiences the same strain. However, the FBG sensors202A,202B may also be used to determine unbalanced forces, such as shear forces or localised forces acting on the wheel208and axle212. For example, in response to a vertical shear force or vertical moment acting on the axle, the FBG elements204of the FBG sensors202A,202B may experience different strains. As a result, the optical signals output of the FBG sensors202A,202B may be used to sense the location of the vertical forces as well as the magnitude and direction. The FBG sensors202A,202B may therefore be used to sense wheel mass imbalance, runway irregularity or vibrations indicative of wear to the bearings.

In this embodiment, the FBG sensor202C and the FBG sensor202D are used to determine horizontal forces such as a drag force caused by wheel drag or a deceleration force caused by the brakes. Each of the FBG sensor202C and the FBG sensor202D may alone be used to determine such forces. However, when a horizontal force is applied to the FBG sensor202C and the FBG sensor202D, the force has a different impact on each FBG sensor202C,202D. Similar to that described above for the vertical FBG sensors202A,202B, in response to a horizontal force, one of the FBG sensors202C,202D will be in compression and the other will be in tension. As a result, the optical signals output by the FBG sensors202C,202D will be different. For example, when one of the FBG sensors202C is in tension, the spectra of the optical signal output by the FBG sensor202C may be shifted to higher wavelengths. By contrast, when the other of the FBG sensors202D is in compression, the spectra of the optical signal output by the FBG sensor202D may be shifted to lower wavelengths. Accordingly, by analysing the optical signals of the two FBG sensors202C,202D, the interrogator220is capable of sensing the direction and magnitude of horizontal forces. For example, when a horizontal force acts on the wheel208and axle212in a rearward direction (e.g. drag force during acceleration), the front FBG sensor202C will be in compression, and the rear FBG sensor202D will be in tension. Conversely, when a horizontal force acts on the wheel208and axle212in a forward direction (e.g. brake force during deceleration), the front FBG sensor202C will be in tension, and the rear FBG sensor202D will be in compression. The FBG sensors202C,202D may be used to determine, for example, a braking force on the wheel208. Further, the impact of the brake force may be analysed to determine or predict a brake condition. Similarly, the drag force may be analysed to determine or predict a wheel condition.

Additionally, as noted above in connection with the vertical FBG sensors202A,202B, the horizontal FBG sensors202C,202D may be used to sense shear forces or localised forces acting on the wheel208and axle212. Such forces can be used to measure dynamic behaviours such as bearing condition, wheel imbalance, shimmy or runway irregularity.

The radial FBG sensors206A,206B are capable of sensing radial forces acting on the bearings. The radial FBG sensors206A,206B are therefore capable of sensing both vertical forces and horizontal forces, as well as radial forces acting in other directions. Depending on the directions of the applied forces, some FBG elements204of each radial FBG sensor206A,206B will be in compression and other FBG elements204will be in tension. For example, when an upward vertical force is applied to the wheel208(e.g. ground impact on landing), the FBG elements204at the bottom of the FBG sensors206A,206B will be in compression and the FBG elements at the top of the FBG sensors206A,206B will be in tension. Similarly, when a horizontal force acts on the wheel208in a forward direction (e.g. brake force during deceleration), the FBG elements204at the front of the FBG sensor206A,206B will be in tension and the FBG elements204at the rear of the FBG sensor206A,206B will be in compression. Each of the radial FBG sensors206A,206B may be used to determine both horizontal and vertical forces, as well as other radial forces. Moreover, by comparing the optical signals of both the first radial FBG sensor206A and the second radial FBG sensor206B, it is possible to determine, for example, unbalanced forces such as shear, or lateral forces acting on the wheel, as well as wheel asymmetry or wheel mass imbalance. For example, when a vertical shear force is applied to the wheel208, one bearing will experience a strain than differs in magnitude to that of the other bearing. As a result, one radial FBG sensor will experience a strain that differs from that of the other radial FBG sensor. More particularly, the FBG elements at the top of the radial FBG sensor experiencing a downward force will be in compression, and the FBG elements at the bottom of the FBG sensor will be in tension. In contrast, the FBG elements at the top of the other radial FBG sensor, experiencing an upward force, will be in tension, and the FBG elements at the bottom will be in compression. In another example, a lateral force may be applied to the wheel208. As a result, the loading or preload of each of the bearings may differ. In particular, the loading or preload of one bearing may increase, and the loading or preload of the other bearing may decrease. Which of the two bearing experiences increased loading or preload will then depend on the direction of the lateral force. Changes in the loading or preloading of the bearings214A,214B may in turn translate to differences in strain on each of the radial FBG sensors206A,206B. For example, the FBG elements104of the radial FBG sensor around the bearing having a higher loading or preload may experience a higher strain.

As explained above, the axial FBG sensors202A,202B,202C,202D, and/or the radial FBG sensors206A,206B may be used to determine the temperature of the first bearing214A and the second bearing214B. For example, changes in the temperature of the first bearing214A may cause the refractive index of the FBG elements204of the first circumferential FBG sensor206A to change, which in turn causes the spectra of the optical signal to change. Likewise, changes in the temperature of the second bearing214B may cause the refractive index of the FBG elements204of the second radial FBG sensor206B to change, which in turn causes the spectra of the optical signal to change. Differences in the temperatures of the two bearings214A,214B, as sensed by the FBG sensors206A,206B, may suggest a loss of lubricant or wear of one or more of the bearings214A,214B. Therefore, the temperature may be used to determine or predict a bearing condition, or as preventative maintenance in order to extend the operational lifetime of the bearings214A,214B.

By comparing the optical signals of the axial FBG sensors202A,202B,202C,202D, and/or the radial FBG sensors206A,206B and determining differences between the emission spectra of the optical signals, it is possible to determine the strain caused by different mechanical forces acting on the bearings214A,214B, as well possible temperature changes in the bearings214A,214B. By employing a combination of axial and radial sensors, changes in the emission spectra due to changes in strain, temperature or both may be better resolved. For example, changes in the emission spectra of one of the radial sensors may arise due to changes in strain or temperature of the respective bearing. However, by additionally providing one or more axial sensors, and by comparing the emission spectra of the axial and radial sensors, a better determination may be made as to whether the changes observed in the emission spectra of the radial sensor are due to strain, temperature or both.

In an example, the radial FBG sensor206A,206B may be used to determine the rotational speed of the wheel208. When the wheel208is rotating, each FBG element204of a radial FBG sensor206A,206B, experiences increased strain as each bearing roller210passes the FBG element204. The interrogator220is then able to sense this periodic increase and decrease in strain and from this determine the speed of the bearing214A,214B and thus the speed of the wheel208. The assembly200may therefore be used as a tachometer to obtain the ground speed of the aircraft.

FIG.3shows a schematic of the functioning of the assembly200according to an example. In particular, this example illustrates the functioning of the FBG sensor202A and the FBG sensor202B ofFIG.2. In this example, the FBG sensors202A,202B experience an upward vertical force which may be a ground force applied to the wheel (e.g. due to ground impact on landing).

As explained above, one or more light sources direct light towards the FBG sensors202A,202B. Each FBG sensor202A,202B emits an optical signal which is transmitted to the interrogator220. A first optical spectra330A is obtained for the FBG elements204of the FBG sensor202A, and a second optical spectra330B is obtained for the FBG elements204of the FBG sensor202B. Each spectrum shows a peak representing the optical spectrum output by each of the FBG elements204. Before use, the emission of each FBG element204is calibrated in order to determine the emission “at rest”, that is the emission when no force is applied to the FBG element204. As explained above, the emission of the FBG elements204changes when a force is applied. In other words, a shift in wavelength is obtained when the spacing of the grating of the FBG elements204is changed. By comparing the emission of the FBG elements204with the emission “at rest” of the FBG elements204, it is possible to determine if the emission has shifted and therefore if a force is being applied.

The interrogator compares each peak to the corresponding peak “at rest”. Each peak of the FBG elements204of the FBG sensor202A indicates a same shift in wavelength, indicating that the FBG sensor202A experiences a balanced force. Similarly, each peak of the FBG elements204of the FBG sensor202A indicates a same shift in wavelength. However, the shift in wavelength of the FBG elements204of the FBG sensor202A is different from the shift in wavelength of the FBG elements204of the FBG sensor202B. Specifically, the optical signal output by the FBG sensor202A is shifted to lower wavelengths, and the optical signal output by the FBG sensor202B is shifted to higher wavelengths. The interrogator220analyses the spectra of the FBG sensors202A,202B in order to obtain band spectra340A,340B representing each peak (i.e. each FBG element204). The exact wavelength of each peak is computed in order to represent the strain at that FBG element204. The magnitude of each band represents the strength of the strain. The peaks of the band spectra340B of the FBG sensor202B are at a higher wavelength than the peaks of the band spectra340A of the FBG sensor202A, which indicates that the FBG sensor202A is in tension whereas the FBG sensor202B is in compression.

In the example illustrated inFIG.3above, each of the FBG elements204of each FBG sensor202A,202B experiences the same strain. In another example, the FBG elements of each FBG sensor may experience different strains. As a result, the optical signals output by each FBG sensor202A,202B will indicate different shifts in wavelength (one of the sensors being in tension and the other one being in compression). This would be caused by shear forces and would indicate, for example, wheel imbalance or runway irregularity which would cause sear forces.

FIG.4shows a schematic of the functioning of the assembly200according to an example. In particular, this example illustrates the functioning of the first radial FBG sensor206A ofFIG.2.

In this example, the radial FBG sensor206A experiences an upward vertical force which may be a ground force applied to the wheel (e.g. due to ground impact on landing). As explained above, one or more light sources direct light towards the circumferential FBG sensors206A. Each FBG element204of the sensor emits an optical spectrum which is transmitted to the interrogator220and analysed to determine the strain experienced by each FBG element204.FIG.4shows the time-average strain experienced by the FBG sensor206A when the bearing experiences the upward vertical force. The FBG elements204at the bottom of the FBG sensor206A are in compression. In contrast, the FBG elements204at the top of the FBG sensor206A are in tension.

As described above, as a bearing roller passes by an FBG element204, the FBG element204experiences an increase in strain.FIG.4shows a time average of the strain. However, in reality, because the bearing is rotating, the strain experienced by each FBG element would oscillates at a frequency that depends on the speed of the bearing (from a maximum value when FBG element204is in contact with the bearing roller to a minimum value when the FBG element204is not in contact with the bearing roller). By measuring the frequency of oscillation, it is possible to determine the speed of the bearing.

Referring toFIG.5, there is shown a schematic side view of an example of an aircraft, according to an embodiment of the invention. The aircraft1000may comprise any of the assemblies100,200described herein.

The aircraft may also comprise other assemblies comprising any number of FBG sensors. Specifically, although the previous examples were described with five or six FBG sensors, it is possible to use any number of FBG sensors. Each FBG sensor described above may be used individually. For example, each individual FBG sensor may be used to determine a vertical force or a horizontal force, such as such as ground force applied to a wheel, a vertical force depending on the aircraft weight, a drag force caused by wheel drag, a deceleration force cause by brakes. Each individual FBG sensor may also be used to determine a temperature. Having multiple FBG sensors enables forces having different directions to be detected, as well as shear or unbalanced forces. Having multiple FBG sensors also increases the accuracy of the strain and/or temperature determination. Additionally, as explained above, having an FBG sensor mounted around each bearing of a wheel enables determining an asymmetry in the wheel. Similarly, by having an FBG sensor mounted around each bearing of both wheels of the aircraft, it is possible to determine more precisely any asymmetry in the aircraft load. Additionally, having an FBG sensor mounted around each bearing of a wheel arranged to form multiple loops may also increase the accuracy.

The aircraft may also comprise other assemblies comprising any number of FBG sensors that extend in a direction parallel to the axis of rotation of the bearing. Specifically, although the previously described examples comprise pairs of axial sensors that lie in vertical and horizontal planes, the direction and magnitude of applied forces may be determined using an alternative number and/or arrangement of axial sensors. For example, the direction and magnitude of applied forces may be determined using three axial sensors spaced evenly around the axle. In another example, a large number of axial sensors may be employed around the axle to increase the resolution with which the direction and magnitude of the forces may be determined.

The aircraft may comprise an assembly comprising one or more axial sensors that extend helically about the axle. A single helical axial sensor may, for example, be used to provide the same functionality (i.e. determination of strain and/or temperature) as that of multiple, linear axial sensors, by virtue of having FBG elements distributed around the inner circumference of the axle.

With the assemblies described above, the FBG sensors may be used to determine one or more of: weight on wheel, geometric weight imbalance, shimmy, loss of bearing lubricant, braking force measurement, wheel pre-stressing and post-stressing, peak load detection for hard landing detection, braking temperature, cantilever forces, bending forces and local strain.

The embodiments of the present invention have been described with FBG elements. In other examples, other fibre-based sensors may be used and the FBG elements may be replaced with other optical elements, such as a distributed sensor (for example, operating according to the principles of Rayleigh, Brillouin or Raman scattering), a long period grating senor or a tilted grating sensor.

The assemblies and systems described herein present inherent immunity to electromagnetic interference (EMI) or ground reference, and may be suitable for use in harsh environments. Other advantages include high sensing accuracy at least due to the lack of EMI noise, simplified routing of signal transmissions, removal of the need for lightning protection circuits since the assemblies can be non-electrically-conductive, limited failure modes due to the passive nature of the sensors, and weight-saving as compared to electrical systems since optical fibres and lighter than e.g. copper wires.

While some embodiments of the present invention have been described in the context of use within an aircraft, it should be appreciated that the invention has utility in other applications, including in vehicles other than aircraft, such as spacecraft, automobiles, railway vehicles, and watercraft, in machines, such as robots, machines for assembly, and machines for manufacture, such as machine tools, drills, as well as in structures with motors or pumps.

Additionally, the embodiments of the present invention may be used for testing or maintenance checks, for example, a wheel supported on an axle by a first bearing and a second bearing, a first fibre optic sensor extending around the first bearing for sensing a strain or temperature of the first bearing, and a second fibre optic sensor extending around the second bearing for sensing a strain or temperature of the second bearing, such as the wheel described with reference toFIG.2, may be part of a system which does not include an interrogator. A separate interrogator may be used when testing the system or during maintenance checks.

The above embodiments are to be understood as illustrative examples of the invention. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

It is to be noted that the term “or” as used herein is to be interpreted to mean “and/or”, unless expressly stated otherwise.