TECHNICAL FIELD

[OOO1] The present disclosure relates generally to a control surface skew and/or loss detection system. More specifically, the present disclosure relates to a wireless skew and/or loss detection system for use in chains of actuable aircraft parts, such as the slats and flaps of aircraft wing structures.

BACKGROUND

[0002] Aircraft wings typically comprise a series of actuable control surface elements. These control surface elements are moveable relative to the fixed wing structure in order to alter the aerodynamic characteristics of the wing. Such control surface elements include leading edge devices such as slats, and trailing edge devices such as flaps.

[0003] Typically, control surface elements such as flaps and slats are actuated at either of their span-wise ends by two separate actuators. It is conceivable that either of these actuators could fail, thus resulting in inconsistent actuation and skew or loss of the relevant control surface. It is important that this skew or loss be detected, and the relevant systems shut down and the pilot of the aircraft notified.

[0004] Patent publication WO2020/144221 Al describes an aircraft moveable element monitoring system, using inductive coupling to detect skew and loss of moveable elements such as slats, flaps, and Kruger Flaps. More particularly, the aircraft moveable element monitoring system includes a signal generator, a signal transmitter coil electrically connected to the signal generator, a signal detector, a signal receiver coil electrically connected to the signal detector, and one or more moveable element signal transmission units. Each moveable element signal transmission unit comprises a first signal transmission unit coil and a second signal transmission unit coil, the first signal transmission unit coil being electrically connected to the second signal transmission unit coil. Each moveable element signal transmission unit is configured to be installed on a respective moveable element of an aircraft. The signal transmitter coil, the one or more moveable element signal transmission units and the signal receiver coil form an inductively coupled transmission line. For example, each coil in the system is configured to be proximate to another coil under normal (e.g. no skew/loss) conditions, such that a series of pairs of inductively coupled coils are provided, across which an electrical signal can be transmitted. The signal generator is configured to provide an electrical signal to the signal transmitter coil and the signal detector is configured to detect the electrical signal (e.g. after having been transmitted across one or more moveable element signal transmission unit) via the signal receiver coil. The signal detector is further configured to determine a condition (e.g. whether there is no misalignment/misalignment within acceptable operating parameters, or if there is unacceptable misalignment, such as skew or loss of a moveable element) of the one or more moveable elements by comparing the detected electrical signal to a predetermined signal characteristic/value (for example, comparing the detected signal strength to a predetermined threshold or value and/or monitoring the detected signal for occurrence of a characteristic property). [0005] Patent application EP 3505451 Al relates to a flight control surface assembly comprising electrical components at gaps between surfaces, wherein electrical energy is transferred across the gaps. A detection circuit detects whether received electrical energy is below a threshold and, if it is, controls a drive unit to stop movement of the flight control surfaces.

SUMMARY

[0006] The present disclosure provides for a signal (a monitoring signal) transmitter apparatus configured to be used in an aircraft moveable element monitoring system, the signal transmitter apparatus comprising a signal transmitter circuit. The signal transmitter circuit comprises: a first electrical path between a voltage input and a first node, the first electrical path comprising an inductor; a second electrical path between the first node and a second node, the second electrical path comprising a signal transmitter coil and a first capacitor, wherein the signal transmitter coil is electrically connected in series with the first capacitor; a third electrical path between the first node and the second node, the third electrical path comprising a second capacitor, such that the second capacitor is electrically connected in parallel to the signal transmitter coil and the first capacitor; and a fourth electrical path between the second node and the voltage input.

[0007] Advantageously, this arrangement provides a resonant circuit, wherein a current waveform having a substantially constant magnitude can be maintained across the signal transmitter coil, even when the moveable elements undergo small differences in separation during normal operation. Additionally, energy transfer across the aircraft moveable element monitoring system is improved, as is the signal to noise ratio of transmitted signals.

[0008] In some embodiments, the signal transmitter apparatus further includes a signal generator comprising: a connection to an aircraft DC voltage supply; an H-bridge configured to controllably connect the connection to the aircraft DC voltage supply to the voltage input; and an H-bridge driver circuit configured to control the H-bridge so as to provide a time varying voltage signal to the voltage input. Beneficially, this enables the leveraging of existing aircraft DC supplies to provide a suitable drive signal, moreover in a manner leading to low electromagnetic emissions. Optionally the H- bridge driver circuit is configured to control the H-bridge to selectively connect the connection to the aircraft DC voltage supply to the voltage input such that the varying voltage signal is a substantially square wave voltage signal. Preferably the H-bridge is configured to drive the signal transmitter coil at resonance, wherein the angular frequency, to, of the substantially square wave voltage signal satisfies the conditions a) = where 1 is the inductance of the inductor, L r _P is the inductance of the signal transmitter coil, is the capacitance of the second capacitor, and C ₂ is the capacitance of the first capacitor.

[0009] In another aspect of the present invention, there is provided an aircraft moveable element monitoring system comprising: the signal transmitter apparatus above, a signal detector, a signal receiver coil electrically connected to the signal detector, and one or more moveable element signal transmission units. Each moveable element signal transmission unit comprises a first signal transmission unit coil and a second signal transmission unit coil, the first signal transmission unit coil being electrically connected to the second signal transmission unit coil. Each moveable element signal transmission unit is configured to be installed on a respective moveable element of an aircraft, such that the signal transmitter coil, the one or more moveable element signal transmission units and the signal receiver coil form an inductively coupled transmission line.

[OO1O] Preferably, each moveable element signal transmission unit comprises a tuning capacitor, thereby enabling each moveable element signal transmission unit to be tuned to the resonant frequency of the signal transmitter circuit, advantageously improving signal transfer across the aircraft moveable element monitoring system.

[0011] Optionally, the signal detector comprises a third capacitor and an envelope detector, wherein the third capacitor and the envelope detector are each electrically connected in parallel to the signal receiver coil. Beneficially, this arrangement provides a tuned detection circuit for monitoring the amplitude of the received signal. Optionally the envelope detector further comprises a unity-gain buffer.

[0012] Optionally the signal detector comprises an analogue phase comparator configured to measure a phase difference between the time varying voltage signal and a received signal received by the signal detector via the signal receiver coil; or a microprocessor configured to measure a phase difference between the time varying voltage signal and a received signal received by the signal detector via the signal receiver coil.

[0013] In another aspect of the present invention, there is provided an aircraft moveable element monitoring system comprising: a signal generator; a signal transmitter coil electrically connected to the signal generator; a signal detector; a signal receiver coil electrically connected to the signal detector; and one or more moveable element signal transmission units. The signal detector comprises a third capacitor and an envelope detector, wherein the third capacitor and the envelope detector are each electrically connected in parallel to the signal receiver coil. Each moveable element signal transmission unit comprises a first signal transmission unit coil and a second signal transmission unit coil, the first signal transmission unit coil being electrically connected to the second signal transmission unit coil. Each moveable element signal transmission unit is configured to be installed on a respective moveable element of an aircraft, such that the signal transmitter coil, the one or more moveable element signal transmission units and the signal receiver coil form an inductively coupled transmission line. Optionally the envelope detector further comprises a unity-gain buffer. Optionally the signal detector further comprises: an analogue phase comparator configured to measure a phase difference between a time varying voltage signal and a received signal received by the signal detector via the signal receiver coil; or a microprocessor configured to measure a phase difference between the time varying voltage signal and a received signal received by the signal detector via the signal receiver coil.

[0014] In a further aspect of the present invention, there is provided an aircraft comprising a moveable element monitoring system as discussed above.

BRIEF DESCRIPTION OF THE DRAWINGS [0001] The accompanying drawings are incorporated herein as part of the specification. The drawings described herein illustrate embodiments of the present disclosure and are illustrative of selected principles and teachings thereof. However, the drawings do not illustrate all possible implementations of the presently disclosed subject matter and are not intended to limit the scope of the present disclosure in any way.

[0002] Figure 1 shows a schematic cross section of an aircraft wing including a prior art control surface element monitoring system.

[0003] Figure 2 shows a schematic of the prior art control surface element monitoring system of figure 1.

[0004] Figures 3A-3C show schematic cross sections of an aircraft wing including the prior art control surface element monitoring system of figure 1 under different conditions.

[0005] Figure 4A shows a schematic circuit diagram of a signal transmitter circuit for use in an aircraft moveable element monitoring system in accordance with an embodiment of the present invention.

[0006] Figure 4B shows an equivalent circuit diagram for an implementation of the signal transmitter circuit of figure 4A.

[0007] Figure 5A shows a schematic signal transmitter apparatus for use in an aircraft moveable element monitoring system in accordance with an embodiment of the present invention.

[0008] Figure 5B shows an H-bridge for use in the signal transmitter apparatus of figure 5A, in accordance with an embodiment of the present invention.

[0009] Figure 6 shows a schematic circuit diagram of a moveable element signal transmission unit forming part of an aircraft moveable element monitoring system in accordance with an embodiment of the present invention.

[0010] Figure 7 shows a schematic circuit diagram of a signal receiver coil and signal detector forming part of an aircraft moveable element monitoring system in accordance with an embodiment of the present invention.

[0011] Figure 8 shows a schematic representation of an aircraft moveable element monitoring system implemented on an aircraft wing in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

[0012] It is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific assemblies and systems illustrated in the attached drawings and described in the following specification are simply exemplary embodiments. Hence, specific dimensions, directions, or other physical characteristics relating to the embodiments disclosed are not to be considered as limiting, unless expressly stated otherwise. Also, although they may not be, like elements in various embodiments described herein may be commonly referred to with like reference numerals within this section of the application.

[0013] In the embodiments described below, the invention is described in relation to the detection of skew and/or loss of control surface elements. The present disclosure is applicable to a sequence/chain of high lift leading edge and trailing edge control surface elements, including slats, Krueger Flaps, and flaps. However, it will be appreciated that the principles described herein are also applicable to detecting misalignment, skew and/or loss of any sequence/chain of moveable/actuable aircraft parts.

[0014] Figure 1 shows a schematic representation of part of an aircraft including a prior art control surface element monitoring system 100 as described in detail in patent publication WO2020/144221 Al. Figure 1 shows a portion of an aircraft fuselage 101, and an aircraft wing structure 102 comprising a fixed part 104 and a chain/sequence of moveable control surface elements 106a, 106b, 106c. The moveable control surface elements 106a, 106b, 106c may be, but are not limited to, a sequence/chain of slats, flaps, and/or Krueger Flap devices. As shown in the example of Figure 1, the wing structure 102 has three control surface elements 106a, 106b, 106c, however any suitable number of control surface elements (just one or more than one) can be provided in the control surface element monitoring system 100, depending on the function of the control surface elements, the size of the aircraft, etc. The control surface elements 106a, 106b, 106c are optionally disposed on either the leading or trailing edge of the wing structure 102. Each control surface element 106a, 106b, 106c is configured to move relative to the fixed part 104. In one example, the control surface elements 106a, 106b, 106c are slats, Krueger Flaps, or flaps. In an embodiment, in the case of the control surface elements 106a, 106b, 106c being slats, they may be actuated via a rack and pinion arrangement connected to a torque shaft, the torque shaft being rotatably connected to the fixed part 104 and driven by a power drive unit (PDU) located in the fuselage 101.

[0015] The control surface element monitoring system 100 includes a signal generator 108 electrically connected to a signal transmitter coil 110. In an embodiment, the signal generator 108 and signal transmitter coil 110 are disposed on the fixed part 104 as shown in Figure 1— advantageously, this avoids the need for any wired connections that might be required by the signal generator 108 (for example power, data and/or control cables) to cross a gap between the main body of the wing structure 102 and a control surface element 106a, 106b, 106c. In another embodiment, one or both of the signal generator 108 and signal transmitter coil 110 are instead disposed within a further control surface element. The signal generator 108 is configured to produce a time varying AC or DC output monitoring signal. The signal transmitter coil 110 may be an antenna comprising an inductor.

[0016] The control surface element monitoring system 100 also includes a signal detector 112 electrically connected to a signal receiver coil 114. In an embodiment, at least the signal detector 112 is either part of a central computing unit, or communicatively connected to a separate central computing unit within the aircraft. The signal detector 112 and signal receiver coil 114 may be disposed on the fixed part 104 as shown in figure 1. Alternatively, one or both of the signal detector 112 and signal receiver coil 114 are instead disposed within a further control surface element. The signal receiver coil 114 may be an antenna comprising an inductor.

[0017] The control surface element monitoring system 100 also comprises control surface element signal transmission units 115a, 115b, 115c comprising a first signal transmission unit coil 116a, 116b, 116c and a second signal transmission unit coil 118a, 118b, 118c. Each first signal transmission unit coil 116a, 116b, 116c is electrically connected to the corresponding second signal transmission unit coil 118a, 118b, 118c within the corresponding control surface element 106a, 106b, 106c (for example, by providing wired connections between corresponding ends of the first signal transmission unit coil 116a, 116b, 116c and the second signal transmission unit coil 118a, 118b, 118c within each control surface element signal transmission units 115a, 115b, 115c). The first signal transmission unit coil 116a, 116b, 116c and/or the second signal transmission unit coil 118a, 118b, 118c may be an antenna comprising an inductor.

[0018] Each of the control surface elements 106a, 106b, 106c includes a corresponding control surface element signal transmission unit 115a, 115b, 115c. The signal transmission units 115a, 115b, 115c comprise electronic modules configured to be positioned within one or more respective moveable elements to transmit an electrical signal across the one or more moveable elements. As shown in Figure 1, the first and second signal transmission unit coils 116a, 116b, 116c, 118a, 118b, 118c are positioned such that, in the presence of an electrical current, when the control surface elements 106a, 106b, 106c are not in a skew/loss condition, each first and second signal transmission unit coil 118a, 118b, 118c, 118a, 118b, 118c is inductively coupled to another coil. The control surface element monitoring system 100 comprises no wires running across the gaps between adjacent control surface elements 106a, 106b, 106c, and between control surface elements 106a, 106b, 106c and the fixed part 104.

[0019] The first signal transmission unit coil 116a, 116b, 116c of each control surface element signal transmission unit 115a, 115b, 115c is positioned proximate to a first lateral edge 117a, 117b, 117c of the corresponding control surface element 106a, 106b, 106c. Similarly, the second signal transmission unit coil 118a, 118b, 118c is positioned proximate to a second lateral edge 119a, 119b, 119c of the corresponding control surface element 106a, 106b, 106c, the second lateral edge 119a, 119b, 119c opposing the first lateral edge 117a, 117b, 117c. This allows the first signal transmission unit coil 116a, 116b, 116c of each control surface element signal transmission unit 115a, 115b, 115c to be inductively coupled with the second signal transmission unit coil 118a, 118b, 118c of a different control surface element signal transmission unit 115a, 115b, 115c installed on an adjacent control surface element 106a, 106b, 106c. For example, as shown in Figure 1, first signal transmission unit coil 116c of control surface element signal transmission unit 115c is inductively coupled with second signal transmission unit coil 118b of control surface element signal transmission unit 115b.

[0020] The control surface element signal transmission units 115a, 115c in the endmost control surface elements 106a, 106c in the chain/sequence, inductively couple to either the signal transmitter coil 110 or the signal receiver coil 114. As shown in Figure 1, the signal transmitter coil 110 is positioned on the fixed part 104 at a location proximate to the first lateral edge 117a of a first control surface element 106a in the chain. Similarly, the signal receiver coil 114 is positioned on the fixed part 104 at a location proximate to the second lateral edge 119c of a last control surface element 106c in the chain. In this way the signal transmitter coil 110 is inductively coupled to the first signal transmission unit coil 116a in the first control surface element 106a, and the signal receiver coil 114 is inductively coupled to the second signal transmission unit coil 118c in the last control surface element 106c.

[0021] In this manner, the signal transmitter coil 110, the one or more control surface element signal transmission units 115a, 115b, 115c and the signal receiver coil 114 form an inductively coupled transmission line.

[0022] In use, the signal generator 108 generates a time-varying electrical signal, for example a sinusoidal voltage signal. The signal is provided to the transmitter coil 110. In the event that the control surface elements 106a, 106b, 106c are in a normal configuration (i.e. no control surface element is missing 106a, 106b, 106c, and there is no unacceptably high level of skew between control surface elements 106a, 106b, 106c), then the signal is transmitted to the signal detector 112 by means of the inductive coupling between the signal transmitter coil 110 and the first signal transmission unit coil 116a of the first control surface 106a, between successive corresponding second signal transmission unit coils 118a, 118b and first signal transmission unit coils 116b, 116c, and between the second signal transmission unit coil 118c of a last control surface element 106c and the signal receiver coil 114. In this normal condition, the signal as received by the detector 112 has a relatively high strength (e.g. a relatively high peak voltage).

[0023] In the event of an unacceptable skew of one of the control surface elements 106a, 106b, 106c one or both of the first signal transmission unit coil 116a, 116b, 116c and the second signal transmission unit coil 118a, 118b, 118c in that control surface element becomes misaligned with respect to a next coil in the chain/sequence (for example, the first signal transmission unit coil/second signal transmission unit coil in an adjacent control surface element, or the signal transmitter coil 110 or receiver coil 114). At such a misalignment, the inductive coupling between coils is reduced. As a result, the signal as received by the detector 112 has a relatively low strength (e.g. a relatively low peak voltage), or potentially no signal is received by the detector 112 at all. Other properties of the signal as received by the detector 112 may also change as a result of the reduced inductive coupling between coils, such as, but not limited to, the phase of the signal.

[0024] A relatively low received signal strength or no received signal would also occur in the event that one of the control surface elements 106a, 106b, 106c was missing, or had failed to actuate or actuated unexpectedly (in the case that the sequence of control surface elements 106a, 106b, 106c were independently actuated). Similarly, any form of misalignment between control surface elements 106a, 106b, 106c (e.g. skew, damage, deformation, etc.), would also result in a relatively low received signal strength or no received signal at the detector 112.

[0025] Accordingly, the signal as detected at the detector 112 is used to determine whether the chain of control surface elements 106a, 106b, 106c, is in a normal condition or if at least one of the control surface elements 106a, 106b, 106c is misaligned (e.g. in a skew condition or otherwise misaligned), has been lost, or has otherwise not actuated deployed as expected. In some embodiments, the monitoring signal strength is compared to a predetermined threshold value indicative of a maximum acceptable skew within the control surface element monitoring system 100 (for example, the peak voltage or peak root mean squared "RMS" voltage can be compared to a threshold value)— if the threshold is not exceeded, then it is determined that unacceptable skew and/or loss of one or more control surface elements 106a, 106b, 106c has occurred. Alternatively, other characteristic properties of the signal, such as a signal profile over time or a phase of the monitoring signal at the signal detector 112, can be compared to predetermined signal characteristics (e.g. one or more threshold values, a predetermined voltage profile over time, an original phase of the signal as produced by the signal generator 108, etc.), wherein the comparison indicates whether the signal as received at the detector 112 is as expected for a normal configuration, or whether skew/loss of a control surface element 106a, 106b, 106c has occurred.

[0026] Figure 2 shows a schematic representation of electronic components in the prior art control surface element monitoring system 100 of Figure 1, and as described in detail in patent publication WO2020/144221 Al. The control surface element monitoring system 100 includes the signal generator 108, signal transmission coil 110, first signal transmission unit coils 116a, 116b, 116c, second signal transmission unit coils 118a, 118b, 118c, receiver coil 114 and signal detector 112. Figure 2A shows an additional control surface element signal transmission unit 115d corresponding to an additional control surface element 106 in the sequence, including a first signal transmission unit coil 116d and a second signal transmission unit coil 118d. As noted above, any suitable number of control surface elements 106 (one or more) can be provided in the system 100, depending on the function of the control surface elements 106, the size of the aircraft, etc. Optionally a capacitor and a resistor are included in each of the control surface element signal transmission units 115a, 115b, 115c, 115d.

[0027] Figures 3A-C show schematic cross sections of an aircraft wing as described in detail in patent publication WO2020/144221 Al, including the control surface element monitoring system 100, 200 under different conditions. Figure 3 shows four control surface elements 106a, 106b, 106c, 106d, though a different number may alternatively be provided. The situation shown in Figures 3A-C may correspond to any particular degree of actuation of the control surface elements 106a, 106b, 106c, 106d. For example, the control surface elements 106a, 106b, 106c, 106d may be in a fully retracted configuration, a fully extended configuration, or an intermediate configuration. Other elements of the system 100, 200 (such as the signal transmitter coil 110 and the signal receiver coil 114, etc.) have been omitted from Figures 3A-C in the interest of clarity of explanation.

[0028] In Figure 3A, all control surface elements 106a, 106b, 106c, 106d are in perfect alignment. No control surface element 106 is missing, nor is there any skew or other misalignment between adjacent control surface elements 106. This represents an ideal case. In this case, there is good coupling between the signal transmitter coil 110, the signal transmission units 115a, 115b, 115c, 115d and the signal receiver coil 114, resulting in a relatively strong monitoring signal 320 being detected at the detector 112.

[0029] In Figure 3B, the control surface elements 106a, 106b, 106c, 106d are not in perfect alignment, however the amount of misalignment is within acceptable operation parameters. In this case, the coupling between the signal transmitter coil 110, the signal transmission units 115a, 115b, 115c, 115d and the signal receiver coil 114 is not as good as in the ideal case, but is still sufficient to effectively transmit the electrical monitoring signal to the detector 112. In this case, the detected signal strength 322 is less than the ideal case, but may still satisfy an associated signal strength threshold indicating that any misalignment is within acceptable operation parameters.

[0030] Both the situations in Figures 3A and 3B can be considered "normal" conditions, in that they represent situations in which the alignment between control surface elements 106a, 106b, 106c, 106d is within acceptable operation parameters.

[0031] Figure 3C shows the situation in which a skew condition has occurred. Two control surface elements 106b, 106c are misaligned relative to one another introducing a skew 326 between them. In this case, the coupling between the signal transmitter coil 110, the signal transmission units 115a, 115b, 115c, 115d and the signal receiver coil 114 is poor (i.e. worse than the coupling for any case in which misalignment is within acceptable operation parameters). In particular, the inductive coupling between the second signal transmission unit coil 118b of the second control surface element 106b in the sequence, and the first signal transmission unit coil 116c of the second control surface element 106c is reduced as compared to a normal condition.

[0032] In this case, the detected signal strength 324 is relatively low, and does not satisfy a signal strength threshold, thus indicating that there is misalignment that is not within acceptable operation parameters.

[0033] In the event that it is determined that unacceptable misalignment/loss of one or more control surface elements 106a, 106b, 106c has occurred (for example, if a skew is detected as shown in Figure 3C), the detector 112 may be configured to provide an indication to a central computing unit within the aircraft, which in turn notifies the pilot and/or other relevant personnel of the skew/loss condition.

[0034] The present invention provides a signal transmitter apparatus configured to be used in a control surface element monitoring system of the type shown in figures 1 to 3C and as described above. The present invention also provides a control surface element monitoring system including a signal transmitter apparatus. Embodiments further include control surface element signal transmission units and signal detectors configured for use with the signal transmitter apparatus.

[0035] Figure 4A shows a schematic circuit diagram of a signal transmitter circuit 400 for use in an aircraft moveable element monitoring system in accordance with an embodiment of the present invention. The signal transmitter circuit 400 is configured to receive an input varying voltage signal from a signal generator (not shown) at voltage input 403, and to transmit a modified monitoring signal inductively from a signal transmitter coil 412 to a neighbouring first signal transmission unit coil (not shown).

[0036] The signal transmitter circuit 400 comprises an LCC circuit, wherein the signal transmitter coil 412 is connected in series with a first capacitor 414, and a second capacitor 416 is connected in parallel with the signal transmitter coil 412 and the first capacitor 414. An inductor 406 is connected between the voltage input 403 and the signal transmitter coil 412 and first and second capacitors 414, 416. Put differently: a first electrical path 402 is provided between the voltage input 403 and a first node 404, the first electrical path 402 comprising the inductor 406; a second electrical path 408 is provided between the first node 404 and a second node 410, the second electrical path 408 comprising the signal transmitter coil 412 and the first capacitor 414; a third electrical path 415 between the first node 404 and the second node 410, the third electrical path 415 comprising the second capacitor 416; and a fourth electrical path 418 is provided between the second node 410 and the voltage input 403.

[0037] Advantageously, use of an LCC circuit in conjunction with the signal transmitter coil 412 means that the signal transmitter circuit 400 exhibits resonant properties. During normal operation, the separation between neighbouring control surface elements (e.g. control surface elements 106a, 106b, 106c) may change slightly. For example, the gap between control surface elements, and/or the gap between an endmost control surface element and a fixed part of the aircraft wing, may be different when the control surface elements are extended than when they are in a retracted position. Similarly, changes in temperature, and the effect of air moving over the control surface elements may also change the separation slightly. These slight changes do not represent unacceptable misalignment or skew. In the present embodiment, when a time varying voltage is applied to the voltage input 403 at the resonant frequency of the signal transmitter circuit 400, the magnitude of the current (e.g. the RMS value of the current) passing through the signal transmitter coil 412 can be held substantially constant, even in the event of small changes in separation between the signal transmitter coil 412 and an adjacent first transmission unit coil (e.g. first transmission unit coil 115a). Specifically, by driving the signal transmitter circuit 400 at its resonant frequency, effects resulting from changes in the mutual induction, caused by changes in separation between the signal transmitter coil 412 and an adjacent first transmission unit coil, become negligible.

[0038] Thus the present arrangement provides the ability to drive the signal transmitter coil 412 at constant magnitude current, irrespective of small variations in separation between the signal transmitter coil 412 and an adjacent first transmission unit coil (e.g. first transmission unit coil 115a). This in turn provides further advantages. Firstly, it simplifies signal detection analysis by mitigating the effects of variable mutual induction in operation. Secondly, it allows the signal transmitter coil 412 to be driven constantly at its maximum current rating, thereby maximising energy transfer between the signal transmitter coil 412 and an adjacent first transmission unit coil. Thirdly, it improves the signal to noise ratio at the end of the signal chain, i.e. at the signal detector (e.g. the signal detector 112 described above).

[0039] An additional advantage of the arrangement of figure 4 is that the signal transmitter coil 412 current magnitude can be directly increased, (e.g. to increase signal to noise ratio), by increasing the magnitude of the voltage applied to the voltage input 403.

[0040] Figure 4B shows an equivalent circuit that models the arrangement shown in figure 4B in operation. A time varying voltage Vin is applied to the voltage input 403, with a corresponding time varying current ly. The inductor 406 has an inductance of value Li and produces a series resistance component of RLI . The first capacitor 414 has a capacitance of C2, and the second capacitor 416 has a capacitance of Ci, with a potential difference Va over the second capacitor 416. The signal transmitter coil 412 has an inductance of L _P and produces a series resistance component of RL _P . The effects of reflected signals from the first signal transmission unit coil (for example the first signal transmission unit coil 116a described above) neighbouring the signal transmitter coil 412 can be modelled by a further series resistance RR, giving a total effective series resistance for the signal transmitter coil 412 of R = RL _P + RR. The resistances of the first and second capacitors 414, 416 are small compared to RL _P and RLI, and are modelled as being negligible under resonant conditions. Under this model, the resonant angular frequency, c o, of the signal transmitter circuit 400 is governed by equations (1) and (2) below:

(2)

[0041] In one example, values of Li, C2, Ci, and L _P are chosen such that the resonant angular frequency c o corresponds to a resonant frequency of between 10 and 100kHz, and more preferably around 50kHz. Advantageously, such a resonant frequency has been found to provide effective signal transfer over the size of gaps typically found between control surface elements such as slats. Moreover using a resonant frequency of between 10 and 100kHz, preferably 50Hz, reduces the risk of interference with radar and communications systems that may be present on the aircraft.

[0042] Figure 5A shows a schematic signal transmitter apparatus 500 for use in an aircraft moveable element monitoring system in accordance with an embodiment of the present invention. The signal transmitter apparatus 500 links a signal transmitter circuit 504 (for instance the signal transmitter circuit 400 of figure 4A as described above) to an DC voltage supply, preferably an existing voltage DC voltage supply.

[0043] The signal transmitter apparatus 500 includes an H-bridge 502 (shown in more detail in figure 5B), which is in electrical communication with a signal transmitter circuit 504 (e.g. signal transmitter circuit 400) and a connection 506 to an aircraft DC voltage supply. The H-bridge 502 is controlled by an H-bridge driver 508. The H-bridge controller 508 is preferably configured to cause the H-bridge 502 to selectively connect the aircraft DC voltage supply connection 506 to the voltage input 505b of the signal transmitter circuit 504 (e.g. the voltage input 403 of the signal transmitter circuit 400 described above) via a voltage output 505a of the H-bridge 502. In particular, the H- bridge controller 508 is preferably configured to cause the H-bridge 502 to provide a square wave time-varying voltage to the voltage input 505b of the signal transmitter circuit 504. [0044] The H-bridge 502 thus provides a switched mode inverter, giving efficient production of the necessary alternating current with minimal power loss in switching. This further allows the use of pre-existing low voltage DC aircraft power supplies - no additional power source is required.

[0045] Preferably the signal transmitter circuit 504 is identical to the signal transmitter circuit 400 described in relation to figure 4A above. In this case, the square wave voltage output by the H-bridge 502 advantageously has an angular frequency substantially equal to the resonant angular frequency c o- The LCC arrangement of the signal transmitter circuit 400 advantageously filters out high frequency components of the square wave voltage output by the H-bridge 502, producing a substantially sinusoidal time-varying current through the signal transmitter coil 412. Accordingly the signal transmitter apparatus 500 ultimately produces a substantially sinusoidal signal for propagation along the inductively coupled transmission line i.e. along signal transmission units (e.g. signal transmission units 115a, 115b, 115c, 115d) and the signal receiver coil (e.g. signal receiver coil 114). Beneficially, by filtering out high frequency components the risk of interference with other aircraft wireless systems (e.g. radar, communications systems) due electromagnetic emissions effects is further reduce.

[0046] Thus the use of the signal transmitter apparatus 500 using the signal transmitter circuit 400 not only provides the advantages described above in relation to the use of a time varying voltage at the resonant frequency, but does so in a particularly effective manner, leveraging existing low voltage DC aircraft power supplies in a simple manner that also reduces electromagnetic emissions effects. This arrangement can be used in place of the prior art signal generator 108 and signal transmission coil 110.

[0047] Figure 6 shows a schematic circuit diagram of a moveable element signal transmission unit 615 forming part of an aircraft moveable element monitoring system in accordance with an embodiment of the present invention. The moveable element signal transmission unit 615 is configured to be used in conjunction with the signal transmitter apparatus 500 including the signal transmitter circuit 400, for example in place of a signal transmission unit 115a, 115b, 115c as described above and known from the prior art.

[0048] The moveable element signal transmission unit 615 includes a first signal transmission unit coil 616 and a second signal transmission unit coil 618, as known in the art. The moveable element signal transmission unit 615 also includes a capacitor 650 in series with the first and second signal transmission unit coils 616, 618.

[0049] Advantageously, the presence of the capacitor 650 allows the moveable element signal transmission unit 615 to be tuned to the resonant angular frequency a)o of the signal transmitter circuit 400. In particular, the capacitance of the capacitor 650 is chosen relative to the inductances of the first and second signal transmission unit coils 616, 618 such that the moveable element signal transmission unit 615 is resonant when the angular frequency of the signal received by the first signal transmission unit coil 616 from the signal transmitter coil 412 (or from a second signal transmission unit coil from an adjacent moveable element signal transmission unit, as applicable) is substantially the resonant angular frequency c o as defined above. By tuning the moveable element signal transmission unit 615 in this manner, the magnitude of the time-varying current induced in the moveable element signal transmission unit 615 is increased, ultimately increase the efficiency of signal transmission across the aircraft moveable element monitoring system.

[0050] Figure 7 shows a schematic circuit diagram of a signal receiver coil 701 and signal detector 700. Advantageously, this arrangement may optionally be used in an aircraft moveable element monitoring system employing the signal transmitter circuit 400 and optionally the signal transmitter apparatus 500 and/or the control surface element signal transmission unit 615 described above in relation to figures 4A to 6 respectively.

[0051] The signal detector 700 includes a capacitor 702 connected in parallel with a signal receiver coil 701 (such as the signal receiver coil 114 described above). The signal detector 700 also includes an envelope detector 704 connected in parallel with the capacitor 702 and with the signal receiver coil 701. Any envelope detector as known in the art may be used, for example that shown in figure 7 comprising a diode 706, a further capacitor 708, a resistor 710 and a unity-gain buffer 712.

[0052] Advantageously, the capacitor 702 is used (in combination with the signal receiver coil 701) to tune the signal detector to the resonant angular frequency c o, again increasing the current induced by the received signal and enhancing monitoring signal detection.

[0053] The diode 706 and the resistor 710 form a half wave rectifier and, in combination with the further capacitor 708, form an envelope detector. The capacitance of the further capacitor 708 and the resistance of the resistor 710 determine the time constant of the envelope detector 704. The envelope detector 704 provides an output that corresponds to the amplitude of the received signal, that can be used by further detection electronics (not shown) to determine whether a skew condition has occurred. Use of the unity-gain buffer advantageously provides a high impedance output for further detection electronics, while ensuring the signal detector 700 presents a low impedance to the one or more control surface element signal transmission units.

[0054] Preferably the signal detector 700 further includes further detection electronics (not shown) connected to the unity-gain buffer 712. In some embodiments, the further detection electronics includes an amplitude detector configured to compare the amplitude of the output from the envelope detector 704 to a threshold amplitude for determining whether a skew condition is present.

[0055] In some embodiments, in addition to the envelope detector 704, a phase detector (for example an analogue phase comparator or a microprocessor configured to determine the phase of the received signal) is provided in addition to the envelope detector 704. In this case, the phase detector compares the phase of the monitoring signal as received at the signal receiver coil 701 to the phase of the monitoring signal at the signal transmitter coil 412, to determine whether a phase shift indicative of a skew condition is present.

[0056] Figure 8 shows a schematic representation of an aircraft moveable element monitoring system 800 implemented on an aircraft wing 802 in accordance with an embodiment of the present invention. The aircraft wing 802 comprises a fixed part 804 and one or more moveable elements 806a, 806b, 806c. The one or more moveable elements may be high lift devices such as slats, flaps, Krueger Flaps, or other moveable elements. The moveable elements 806a, 806b, 806c are shown in an extended position in figure 8, and in normal operation are moved in unison between retracted and extended positions.

[0057] The aircraft moveable element monitoring system 800 comprises a signal transmitter apparatus 808 that includes the signal transmitter circuit 400 described above in relation to figure 4A. Preferably, the signal transmitter apparatus 808 is the signal transmitter apparatus 500 described above in relation to figures 5A and 5B.

[0058] The aircraft moveable element monitoring system 800 also comprises one or more moveable element signal transmission units 810a, 810b, 810c, each disposed in/on a corresponding moveable element 806a, 806b, 806c. Preferably, each moveable element signal transmission unit 810a, 810b, 810c is a moveable element signal transmission unit 615 as described above in relation to figure 6.

[0059] The aircraft moveable element monitoring system 800 also comprises a signal receiver coil 812 (preferably the signal receiver coil 701 described above in relation to figure 7), and a signal detector 814 (preferably the signal detector 700 described above in relation to figure 7). The signal detector 814 preferably includes an amplitude threshold detector configured to compare the amplitude of the received monitoring signal to a threshold and thereby determine whether at least one of the moveable elements 806a, 806b, 806c is in a skew condition. Optionally, a phase detector 818 is provided in communication with the signal receiver coil 812 (for example an analogue phase comparator or a microprocessor configured to determine the phase of the received monitoring signal) configured to determine a phase difference between the monitoring signal as received by the signal receiver coil 812 and the monitoring signal as transmitted at the signal transmitter circuit 400, thereby determining whether a phase difference corresponding to at least one of the moveable element 806a, 806b, 806c being in a skew condition is present.

[0060] In an alternative embodiment, the signal detector 814 may be the signal detector 700 described above in relation to figure 7, and the signal transmitter apparatus 808 may be different to the signal transmitter circuit 400 described above in relation to figure 4A, for example including a signal generator 108 electrically connected to a signal transmitter coil 110 as described in relation to figures 1 and 2 above.

[0061] Whilst the presently disclosed subject matter is described in relation to embodiments for monitoring the condition control surface elements, the present disclosure can equally be applied to other movable/actuable aircraft components, wherein a signal transmission unit comprising first and second transmission unit coils are provided for each movable/actuable component, and are used to carry an electrical signal from a signal generator and transmitter coil to a signal receiver coil and detector using the principles set out above. For example, the movable/actuable component may be a door or sequence of doors.

[0062] One or more features of the embodiments described above may be combined to create additional embodiments which are not depicted. While various embodiments of the presently disclosed subject matter have been described above, it should be understood that they have been present by way of example, and not limitation. It will be apparent to persons skilled in the relevant arts that the disclosed subject matter may be embodied in other specific forms, variations, and modifications without departing from the scope of the appended independent claims. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive. The present invention covers all equivalents falling within the scope of the appended independent claims.