EP4602325A1

EP4602325A1 - Inductive coil array

Info

Publication number: EP4602325A1
Application number: EP23790008.9A
Authority: EP
Inventors: Robert Hughes; Alexis Hernandez ARROYO
Original assignee: University of Bristol
Current assignee: University of Bristol
Priority date: 2022-10-14
Filing date: 2023-10-13
Publication date: 2025-08-20
Also published as: GB202215187D0; WO2024079346A1; GB2623361B; GB2623361A

Abstract

An inductive coil array for determining a shape of an object, the inductive coil array comprises: a plurality of inductive coils, comprising an active coil for providing an output signal and one or more passive coils magnetically coupled to the active coil.

Description

Inductive Coil Array Technical field The present invention concerns inductive coil arrays, such as eddy current arrays for non-destructive testing (NDT). Background Eddy currents are electrical currents that are created when changing magnetic fields pass through an electrical conductor. Coils excited with AC current will generate changing magnetic fields which drive eddy-currents (back emf) when incident upon one or more neighbouring coils. The coils are driven at frequencies far below resonance, to get a more stable response. When the coils are close to a sample under inspection, the alternating magnetic field induces eddy currents in the sample. Cracks and other defects or variations in the sample affect the eddy current and can therefore be detected by an eddy current probe. For quicker inspection, multiple coils in an array can be used to scan across the surface of a sample. Figure 1 shows an eddy current array probe 1 comprising an array of inductive coils 2 enclosed in a housing 3. The arrow 4 indicates the scanning direction. Connection wires 5 connect to each individual coil in the array to provide input and output signals. For a conventional array (e.g. consisting of 64 or 128 elements) a large number of connections is required and the signals are electronically multiplexed. Multiplexing is the process by which multiple signals are combined into one signal on a shared medium. When eddy current array signals are time multiplexed, the individual eddy current coils are excited at different times. An undesirable effect known, as mutual inductance (magnetic coupling between coils in close proximity), can be minimized with the use of an internal multiplexing system to carefully program the exact time that each coil is excited to transmit its eddy current signal, by allowing the system to excite all of the coils without exciting any two adjacent coils at the same time. Multiplexing also allows any individual coil (data) channel to be analysed after inspection. Multiplexing allows an increased channel resolution, increased coil sensitivity (through the reduction of mutual inductance), and a reduced noise level. The technology is used in the aerospace and automotive industries for the detection of surface or near-surface defects in materials such as aluminium, stainless steel, copper, titanium, and brass, as well as their alloys. Summary According to a first aspect there is provided an inductive coil array for determining a shape of an object. The inductive coil array comprises a plurality of inductive coils, comprising an active coil for providing an output signal and one or more passive coils magnetically coupled to the active coil. The array may be configured for frequency multiplexing, wherein specific array elements can be excited by driving the active coil at a selected frequency corresponding to a resonant frequency of the array element. The one or more passive coils may be floating. In some embodiments, a subset of the passive coils may comprise an open electrical connection. The inductive coil array may comprise a flexible material arranged to mechanically couple the plurality of inductive coils to each other. For example, the inductive coils may be attached to a structure (e.g. a polymer strip, or fabric) configured to keep the distance between coils fixed while allowing the coils to tilt relative to each other, and to follow the contours of the surface of an object on which it is placed. In another embodiment, the coils may be attached to a structure configured to allow the distance between coils to shift, while keeping the relative angle between coils fixed (no tilting). Typically the plurality of coils consists of fewer than ten coils. Compared to conventional eddy current arrays, a relatively small number of coils may be used when operated at resonance frequencies. In a particular embodiment the plurality of coils consists of two coils, which may be particularly suitable for detecting a state change of an object (e.g. from open to closed and vice versa). In one embodiment, the one or more passive coils are arranged in a hexagonal pattern around the active coil. This layout can allow measurement of a more complex geometry. Typically, the distance between coils is minimised to further increase the magnetic coupling between coils. For example, the closest distance between adjacent sides of coils that are nearest neighbours may be less than 10 mm and preferably less than 5 mm. In one embodiment, all the coils in the array have substantially the same nominal resonant frequency. In another embodiment, the nominal resonant frequency of each passive coil is different. Each passive coil may have a different resonant frequency that is within ± 10% of the resonant frequency of the active coil. For example, the one or more passive coils can be connected to a capacitive load, in order to tune their resonant frequencies. The capacitive load may comprise a capacitor, or another capacitive element such as a piezoelectric element or a cable. When using otherwise identical coils, the capacitive load may be used to separate the nominal resonant frequencies of the coils. For example, a first passive coil of the one or more passive coils can comprise a first capacitive load and a second passive coil of the one or more passive coils comprises a second (different) capacitive load. The capacitive load of the, or each passive coil may be adjustable, allowing the resonant frequencies to be tuned for a particular application. In another embodiment, the inductances of the coils are different to provide different resonant frequencies. By capacitively tuning the coils to exhibit unique resonant frequencies, the frequency spectrum of the output from the active coil may be directly interpretable by a human operator, without requiring further post processing. For example, a shift in amplitude of a particular resonance peak in the spectrum may indicate a defect located under the coil corresponding to that resonance peak. Without tuning, the resonances may be too close to be easily distinguished by the operator. The active coil may be connected to a capacitive load. The capacitive load of the active coil may be at least five times greater than a capacitance of a cable connected to the active coil for providing the output of the active coil. Preferably, the capacitive load of the active coil is at least ten times greater than the capacitance of the cable. The array may comprise one or more printed circuit boards, PCBs, each PCB comprising an inductive coil of the plurality of inductive coils. For a flexible array, each coil may be printed on a separate PCB and attached to a structure (e.g. a flexible material) allowing each PCB to move with one or more degrees of freedom relative to the other PCBs. For example, the PCBs may be attached to a structure configured to allow any two neighbouring coils to tilt relative to each other along a first axis but not along a second axis. For a fixed array, e.g. configured for frequency multiplexing, the plurality of coils may be printed on one (single) PCB. In an embodiment, the inductive coils may be printed directly on a flexible substrate. In other embodiments, the inductive coils may be wound or 3D printed. Each inductive coil may comprise a ferromagnetic core, in order to increase the magnetic coupling between coils. The dimensions of the core and in particular the height of the core may be configured to optimise/maximise the magnetic coupling between coils. According to a second aspect there is provided a method of determining a shape of an object with a flexible inductive coil array. The method comprises fitting the flexible inductive coil array to the object, driving an active coil of the inductive coil array at one or more frequencies and measuring a response, and determining the shape of the object from the response. The step of determining may comprise determining two or more resonant frequencies in the response. The step of determining may further comprise determining an angle between the active coil and a passive coil of the flexible inductive coil array based on the response. The step of determining may comprise determining a plurality of angles between the active coil and a corresponding plurality of passive coils of the flexible inductive coil array. In general, the more complex the shape to be determined the more passive coils may be used. For determining a simple geometry, such as the diameter of a pipe, an array comprising only two coils (one active and one passive) may be used. The one or more frequencies typically cover a frequency range comprising a nominal resonant frequency of the active coil. The nominal resonant frequency of a coil is the resonant frequency of that coil in isolation. The step of driving may comprise exciting the active coil with a wave comprising a superposition of a plurality frequencies. Alternatively, the step of driving may comprise a frequency sweep across the plurality of frequencies (e.g. co called chirping), or a pulse generating a broad band across the plurality of frequencies. According to a third aspect there is provided a method of operating an inductive coil array, wherein the inductive coil array comprises a plurality of inductive coils comprising an active coil for providing an output signal and one or more passive coils inductively coupled to the active coil. The method comprises driving the active coil at, or near, a resonant frequency of the inductive coil array. By operating at resonance and making use of the frequency splitting phenomenon, individual coils can be selected by operating at their respective resonant frequencies. The method may comprise selecting a subset of inductive coils (e.g. one coil) of the plurality of inductive coils, wherein the subset has the resonant frequency. The method may further comprise selecting a second subset of inductive coils of the plurality of inductive coils, wherein the second subset has a second resonant frequency and driving the active coil at the second resonant frequency to selectively drive the second subset of inductive coils. By driving the array at different resonant frequencies corresponding to different subsets of coils in the array, the coils can be frequency multiplexed without requiring expensive switching electronics. The method may comprise applying the inductive coil array to a sample, obtaining an output from the active coil, providing a frequency spectrum from the output, associating one or more resonance peaks in the spectrum with corresponding coils of the one or more passive coils, and based on the frequency spectrum determining the location of a defect in the sample. The method may comprise comparing amplitudes of the one or more resonance peaks to nominal amplitudes, wherein a deviation from the nominal amplitude of a resonance peak indicates the presence of a defect under the corresponding coil associated with that resonance peak. The method may comprise comparing the frequency spectrum to a nominal frequency spectrum. The inductive coil array can be scanned across a surface of the sample in order to detect defects in the surface. When scanning, the output from the active coil can be used to provide an image of the surface. According to a fourth aspect there is provided a system comprising an inductive coil array, such as the inductive coil array according to the first aspect. The system may be configured to operate the inductive coil array so as to perform the method of the second or third aspect. The system comprises a control unit for controlling the inductive coil array, and an analysis unit for analysing the response from the array. The system does not comprise multiplexing electronics. The control unit may comprise a function generator, to generate the drive frequency. The system may comprise a single input/output connection for connecting to the active coil of the array to the control unit. For a large array with multiple active coils, there is a corresponding multiple of input/output connections. In general, the number of connections between the array and the control unit can be significantly smaller than the total number of coils in the array. In an embodiment the array comprises wound coils and the control unit is configured to provide a current of less than 100 mA. In another embodiment, the array comprises PCB coils and the control unit is configured to provide a current of less than 1 mA. For higher current, the frequency splitting phenomenon may become difficult to observe and measure. The analysis unit may be configured to determine one or more resonant frequency peaks from the response of the array (from the active coil), and to determine a shift in resonant frequency peaks as compared to the peaks from a response from a nominal array configuration (e.g. when the array is applied to a flat sample). For example, the analysis unit can be configured to determine a shift in the frequency between peaks, and/or a shift in relative amplitude between peaks, and to use the shift to determine a property of the array depending on the magnetic coupling between coils. For example, the analysis unit can be configured to determine one of the relative angle between the passive coil(s) and the active coil, the lateral distance between the passive coil(s) and the active coil. The analysis unit can be configured to determine a geometry or change in geometry of the object based on the determined property. The analysis unit may comprise a neural network trained on data obtained from the array or from a similar array. The data comprises responses from the array when applied to a plurality of different known geometries. This may be particularly useful for large arrays (comprising more than three coils) and complex geometries. The system may comprise a memory unit, which may store calibration data that may be used by the analysis unit. The calibration data may be usable by the analysis unit to determine the geometry of a sample from a response of the array. The analysis unit may then be configured to compare the shift in frequency and/or amplitude of the resonant peaks to calibration data relating the shift to the magnetic coupling and/or the relative angle between coils in the array in order to determine the geometry of the sample. In one embodiment, the control unit is configured to excite specific (passive) coils in the array by driving the active coil at a corresponding resonant frequency, and to switch between subsets of coils in the array by switching to corresponding resonant frequencies, or by exciting a plurality of the resonant frequencies via the superposition of excitation signals. The analysis unit can be configured to determine one or more physical properties of the sample based on the response from the active coil. For example, the analysis unit can be configured to determine a defect in the sample located beneath the subset of coils being excited. The analysis unit can be further configured to process the response from the active coil for a plurality of frequencies corresponding to a respective plurality of subsets of coils in the array and generate an image of the sample from the responses. Brief description of drawings Figure 1 shows an eddy current array probe; Figure 2a shows an inductive coil array according to an embodiment; Figure 2b shows the inductive coil array applied to a non-flat object; Figure 3 shows an equivalent circuit of two coupled coils; Figure 4a shows frequency spectra for two coils with different coupling coefficients; Figure 4b shows the frequency dispersion curves for the two coils; Figure 5 shows a frequency spectrum of a two coupled coils exhibiting resonant frequency splitting; Figure 6 shows two frequency spectra for two coupled coils with different relative angle between the coils; Figure 7 shows plots of the difference in frequency and amplitude of the split resonant peaks as a function of angle between the two coupled coils; Figure 8a shows an inductive coil array with three coils; Figure 8b shows the inductive coil array applied to a non-flat object; Figure 9 shows a frequency spectra from an array with three coupled coils for two different angles between the coils; Figure 10 shows a schematic diagram of an inductive coil array with three coils; Figure 11 shows a schematic diagram of an inductive coil array with passive coils in a hexagonal pattern; Figure 12 shows a flow diagram of a method of determining the shape of an object using an inductive coil array; Figure 13 shows a flow diagram of a method of using an inductive coil array; and Figure 14 shows a schematic diagram of a system comprising an inductive coil array. Detailed Some embodiments described herein provide a flexible inductive coil array for operating at or close to resonance, to make use of the mutual inductance between the coils in the array. The array is configured to make use of resonant frequency splitting in near-field sensor RF frequency applications. Conventional RF coil measurement systems employ sub-resonant frequency excitation to avoid the nearest neighbour interactions and instabilities of resonance. Figure 2a shows a schematic diagram of the simplest (2 coil) inductive coil array 6 comprising an active coil 7 with an input/output connection 8 and a passive coil 9 that is left floating. The diagram is a vertical cross section, so two parts of the coil are illustrated on either side of the central axis, shown as a dotted line. The coils 7 and 9 are substantially identical with height h, inner diameter r_in, outer diameter r_out, and number of turns N. The coils are separated by a distance S between adjacent sides of the coils. In a specific example h = 25 mm, rin = 8 mm, rout = 9.2 mm, N = 42 and S = 5 mm. To optimise the magnetic coupling, the distance S between neighbouring coils is minimised. The array 6 is located on a flat sample 10, and may be used for non- destructive testing, for example to detect surface and near-surface defects in the sample 10. The coils may be wound coils or printed coils (on a PCB) and may have cross section that is circular, rectangular, hexagonal, or oval for example. Figure 2b shows the same array 6 but located on a curved sample 10. The curvature of the sample causes a change in the angle between the coil 7 and 9, which in turn changes the magnetic coupling between the coils. An angle θ between the central axes of the coils is illustrated. The array 6 may be used to detect a change of state of a system. For example, by attaching the array to a flexible surface or to parts with a flexible (e.g. hinged) connection. As the flexible surface changes shape or the two parts move relative to each other the relative angle between the coil changes, which changes the coupling between the two coils 7 and 9. As defined herein, a passive coil is a coil that is not being electrically excited or monitored via wired connectors. Passive coils may have no external connections for providing electrical input or output, or may have input or output electrical connections that are being unused (open). An active coil is a coil that has an external connection for providing an input and output to the coil. Figure 3 shows an equivalent schematic circuit diagram of the inductive coil array illustrated in Figure 2. The mutual inductance K between the coils can be modelled using the equivalent circuit. The active coil is represented by the primary circuit 11 and the passive coil 9 is represented by the secondary circuit 12. The primary circuit 11 comprises a current source 13, a first inductor 14 with inductance L₁, a first resistor 14 having resistance R_L1, and a first capacitor 15 having capacitance C₁. The secondary circuit 12 comprises a second inductor with inductance L2, a second resistor RL2 and a second capacitor C2. Depending on the application, the inductances L can be in the order of nH to H, the resistances can be in order of µΩ to Ω, and the capacitances can be in the order of nF to mF. The active coil inductively couples to the secondary coil. The effect of the secondary coil can be modelled as an inductor, L2, in series with a resistor, R2, and a capacitor, C2. This coupling, parameterised by the coupling coefficient, K, will alter the effective inductance and resistance (L’1 and R’1 respectively) of the active coil and will distort the impedance in accordance with Kirchoff’s laws so as to be where ^_^^ = ^_^^ = ^ = ^_^^_^^_^ is the mutual inductance between coils the first (active) coil 7 and the second (passive) coil 9, ^_^^ is the voltage across the inductive branch of the primary circuit, ^_^ and ^_^ are the currents through the primary circuit 11 and the secondary circuit 12 respectively, = 2^^ is the angular frequency of excitation. The free-space (uncoupled) impedance of each circuit can be defined by two terms as ^ ^_^ = is the dimensionless frequency ratio of the secondary circuit 12, and is the free-space natural resonant frequency of the secondary circuit 12, where the subscript denotes the circuit represented by the quantity. Equation 2 can therefore be rearranged to give an expression for the secondary circuit current, ^^^ (5) ^_^ = ^_^ . ^_^ Combining equations 5 and 1 gives where and The measured impedance ^_^^ of the primary circuit can be found by using the reciprocal impedance rule, such that Equations 8 to 11 can be used to calculate the impedance spectra ^(^) for a given coupled system. Embodiments described herein are configured to operate at or close to resonance, which is in the high frequency regime where ^_^ ≪ ^^_^, whereby the coupling term ^ can be simplified to such that the expressions for the effective inductance and resistance in equations 8 and 9 become ^^{^}^ ^ ^^ ^ ≈ ^_^ ^1 + _^ ^. (14 (^^{^} − ^_^ ^{^} ) )^{^} ^_^ The resonant frequency of the coupled system can therefore be approximated to From equation 15 an expression for the coupling coefficient between neighbouring coils can be derived as In the case where the circuit components are identical (representing identical inductive coils) so that = ^_^ = ^ and ^_^ = ^_^ = ^, so that ^ = ^^, and the resonant frequencies in free space of each coil = ^_^ = ^_^. Then the expressions governing the measured impedance spectra become ^^{^} ≈ ^[1 − ^], (17) such that the resonant frequencies and coupled coefficient respectively become Figure 4a shows the simulated frequency spectra for different coupling coefficients k ranging from 0 (blue line) to 0.45 (yellow line). As the coupling increases, the resonant peak splits into two peaks that move apart. Figure 4b shows the frequency versus coupling coefficient dispersion curves, representing the resonant vibrational modes of the system. The dispersion of resonant frequencies predicted by Equation 19 is shown as white dashed lines. The red dashed line represents the dispersion separation threshold. This threshold is dependent on the q-factor of the systems and as such lower resistance systems exhibit sharper, more easily resolvable dispersion at lower coupling coefficients. There is also a practical upper threshold to the coupling coefficient of a realistic inductively coupled system which is dependent on the geometry of the system and the permeability of the cores used within the coils. As can be seen from Figures 4a and 4b, the resonant frequency splitting depends on the coupling between the inductive circuits. The splitting can also depend on other factors such as the power, which can be controlled for. Identical resonating inductive (electromagnetic) coils placed in close magnetic proximity to one another such that their magnetic fields interact strongly exhibit characteristic resonance patterns, such that the spectrum of one coil shows multiple resonant peaks. The number of resonance peaks may equal the number of coils. These peaks occur due to the increase in the number of degrees of freedom, or stores of energy (magnetic and electric) leading to the generation of resonant modes within the sensors. When the sensors are moved relative to one another i.e. the relative distance or angle between the sensors change, the resonant frequencies shift in a characteristic and distinct way allowing this distance or angle to be determined from the resonance spectrum of a single coil (referred to herein as the active coil). This can allow the array of inductive coils to be used together to determine the geometry of an object that they are attached to or scanned over, by measuring the electrical properties of a single coil in the array. Figure 5 shows the frequency spectrum around the nominal frequency f₀ of an inductive coil array comprising two coils. Because of the inductive coupling between the two coils, the frequency spectrum exhibits a split resonant frequency with a second resonant frequency peak at f1. The difference between the two frequencies is Δf0 and the absolute impedance amplitude difference between the two peaks is ∆|^|. The two coils in the array are substantially identical and the relative angle between them is 0. Figure 6 shows how the frequency spectrum changes as the angle between the two coils in an array increases from 0° to 15°. As the angle increases, the coupling decreases, causing the difference in frequency between the split resonant frequencies to decrease (i.e. the peaks move closer together). Also, the difference in the amplitude of the peaks changes with the angle. Hence, the relative angle between the two coils can be determined from the difference in frequency between the peaks Δf₀ and/or from the difference in amplitude of the peaks. Figure 7 shows the experimentally measured change in frequency and change in amplitude as a function of the relative angle between the central axes of the two coils. Hence, by identifying the resonant frequency peaks and calculating the difference in frequency and/or the difference in impedance amplitude the angle between the two coupled coils can be determined. For a flexible array then, at least a part of the geometry of the object to which the array is applied can be determined. A structure providing a mechanical coupling between the coils can be used to restrict the possible directions of relative tilt between the coils, and thereby assist with interpreting the measurements. Alternatively, if a known shape such as that of a cylindrical body is measured, the data can be interpreted to measure the curvature of the cylinder based on the knowledge that curvature only occurs around the central longitudinal axis of the cylindrical body, combined with a known distance between the coils. While the above description focuses on an array with two coils, which may be easier to model, the concept can be expanded to a greater number of coils that are inductively coupled. In particular, an array may comprise one active coil and a plurality of passive coils. Each coil in the array may provide a separate resonant frequency peak. For example, an array with three highly-coupled coils can split the nominal resonant frequency peak in three. The distance (i.e. the frequency difference) between the peaks and the difference in amplitude between the peaks can be used to determine the respective angles between the active coil and the passive coils. To provide high coupling between the passive coils and the active coil, the passive coils are preferably arranged around or adjacent to the active coil. In general, the active coil can be located substantially in the middle of the array. For a large array, configured to cover a large area, multiple spaced apart active coils may be used. For example, for an array with more than ten coils, the array may comprise two or more active coils, each with an associated subset of passive coils. Importantly, measurements are only taken from the active coil(s), which therefore allows even a large array to operate with only a small number of external connections. Figure 8a shows a schematic diagram of an inductive coil array 6 comprising three coils. The array has an active coil 7 with an input/output connection 8 and a two passive coils 9a and 9b. The coils 7, 9a, 9b are substantially identical with height h, inner diameter rin, outer diameter rout, and number of turns N. Adjacent coils are separated by a distance S. In order to provide a high coupling between the active coil and each passive coil, the passive coils are arranged on either side of the active coil (rather than on the same side of the active coil). Figure 8b shows the same array 6 as in Figure 8a but located on a curved sample 10. The curvature of the sample causes a change in the angle between the active coil 7 and each of the passive coils 9a and 9b, which in turn changes the magnetic coupling between the coils. The frequency spectrum of the active coil may be analysed to determine the relative angles between the coils. For example, the measured frequency peaks may be compared to calibration data. Calibration data may be obtained by measuring the frequency spectrum from the array or a similar array on a range of samples with different shapes. To determine the shape of a sample, the measured frequency spectrum and in particular the resonant frequencies can be compared to those from the calibration data. The calibration data may be corrected by fitting a polynomial. Interpolation or extrapolation may be used if the measure sample has a shape that falls between or outside the shapes contributing to the calibration data. For example, a straight-line calibration between neighbouring points of the calibration data (or the corrected calibration data) may be used. Alternatively, or in addition, simulated calibration data may be provided and used in the analysis. For example, an equivalent circuit model as described above with reference to Figure 2b may be used to provide calibration data for a range of distances or angles between coils in an array. Additional analytical models, and or finite element simulations may also be used to provide calibration or geometry inversion data. Machine learning and AI may also be employed to evaluate the geometry of more complex arrays. Figure 9 shows the finite element simulated frequency spectra for an array with three coils, such as the array shown in Figures 8a and 8b. The first spectrum 20 is from a flat sample (i.e. all three coils are parallel) and the second spectrum 21 is when the passive coils each have a relative angle of 16° the active coil (θ1 = θ2 = 16°). As can be seen, each spectrum comprises three resonant peaks (one for each coil) and as the angles between the coils change, the peaks shift relative to each other. Figure 10 shows a schematic diagram of a flexible inductive coil array 22 configured to measure the angle between coils along two perpendicular axes (e.g. along x and y). The array comprises an active coil 23, a first passive coil 24 next to the active coil along a first axis, and a second passive coil 25 located next to the active coil 23 along a second axis that is perpendicular to the first axis. The active coil 23 comprises an external connection 26 for input and output. The coils are attached to a structure 27 configured to keep the distance between the coils constant while allowing the coils to tilt relative to each other along axes predetermined by structure 27. For example, the structure 27 may comprise a flexible material (e.g. a polymer or textile fabric) or may comprise a rigid material with hinges between elements in the array, or may represent faces of a structure of interest. Figure 11 shows another embodiment, which may be particularly useful for determining complex shapes. The flexible inductive coil array 22 comprises an active coil 23 with external connection 26 at the centre of the array 22 with six passive coils 24 arranged around the active coil 23 in a hexagonal pattern. The coils are fixed together by a structure 27 comprising a flexible material. The hexagonal layout of coils allows the tilt between the passive coils 24 and the active coil 23 to be determined along six different axes separated by 60°. In alternative embodiments, configured to measure the distance between two parts or the stretching of one part, the coils of the array may be attached to a structure comprising a flexible material (e.g. an elastomer) that can be stretched, so as to allow the lateral distance between coils to change. The relative angle between coils can be fixed. As the coils move apart, the coupling between the coils decreases, which changes the frequency spectrum. The change in the frequency spectrum can be used to determine the distance or change in distance between the coils. Figure 12 is a flow diagram illustrating the steps of a method of determining the geometry of an object using a flexible inductive coil array. The method comprises fitting the flexible inductive coil array to the object (S1), so that the flexible array conforms to the shape of the object and each coil in the array has a magnetic axis this substantially perpendicular to the underlying surface of the object. For example, if the coils are flat (e.g. PCB printed coils) then the coils are substantially lying flat against the surface of the object. The method further comprises driving an active coil of the inductive coil array at one or a plurality of frequencies and measuring the relative separation between frequency peaks from the response (S2), and determining the shape of the object from the response (S3). Typically, the response is an impedance spectrum that covers the resonant frequency of the active coil. Due to coupling between the active coil and one or more passive coils in the array, the resonant frequency is split into multiple peaks as explained with reference to Figures 6 and 7 above. The respective relative distances or angles between each passive coil and the active coil can be determined from the peak separation, which indicates the shape of the object. Closely packed coils exhibit the resonant frequency splitting phenomenon, resulting in specific array elements resonating at different frequencies. This can allow selection of individual coils by exciting the specific resonant frequency of that coil. The selected coil(s) can be operated by exciting a single coil (the active coil) within the array at that frequency. This can eliminate the need for expensive electronic multiplexing or other switching electronics. Unlike conventional multiplexing, which is used so to reduce magnetic inductance, embodiments described herein are configured to increase the magnetic inductance between the coils. Figure 13 shows a flow diagram illustrating the steps of a method of operating an inductive coil array using frequency selection. The method comprises selecting a subset of inductive coils of the plurality of inductive coils (S4), wherein the subset has a first resonant frequency, and driving the active coil at the first resonant frequency of the inductive coil array (S5). The method further comprises selecting a second subset of inductive coils of the plurality of inductive coils, wherein the second subset has a second resonant frequency (S6), and driving the active coil at the second resonant frequency to selectively drive the second subset of inductive coils (S7). The method may be used for non-destructive testing such as eddy current array scanning of a component, in both rigid and flexible arrays. The elements of the array can thereby be excited according to any desired pattern via “frequency multiplexing” without requiring electronic multiplexing or other switching technology. Figure 14 illustrates a system 30 comprising an inductive coil array 31, a control unit 32 for controlling the array 31 and an analysis unit 33 for analysing the output from the array 31. The system further comprises a memory unit 34, which may store calibration data that may be used by the analysis unit 33. The system 30 may be configured to perform the method described in relation to Figure 12 or the method described in relation to Figure 13. The inductive coil array 31 may be any array as described herein, comprising an active coil and one or more passive (floating) coils. The active coil is connected to the control unit 32 which generates and supplies a drive signal to the active coil. The control unit may be configured to drive the array 31 at a plurality of frequencies covering a range comprising a resonant frequency of the array 31. For example, the control unit 32 may comprise a function generator and may be configured to supply a chirp signal. The frequency splitting effect may diminish and eventually disappear as the power of the drive signal is increased. The control unit 32 may be configured to provide a current of less than 1 mA. The output from the array 31 is transmitted to the analysis unit 33. The analysis unit 33 may perform a Fourier transform of the output (time) signal from the array 31 to provide a frequency spectrum. The analysis unit may determine one or more properties of an object to which the array 31 is applied. For example, the system may be configured to determine a geometry of an object, and/or may be configured to find surface or near- surface features, material properties or defects in the object. While specific embodiments have been described herein, it will be appreciated that further embodiments may be provided that fall within the scope of the appended claims. The features of one embodiment may be appropriately combined with those of one or more further embodiments.

Claims

CLAIMS: 1. An inductive coil array for determining a shape of an object, the inductive coil array comprising: a plurality of inductive coils, comprising an active coil for providing an output signal and one or more passive coils magnetically coupled to the active coil.

2. An inductive coil array according to claim 1, wherein the inductive coil array further comprises a flexible material arranged to mechanically couple the plurality of inductive coils to each other.

3. An inductive coil array according to claim 1 or 2, wherein the plurality of coils consists of fewer than ten coils.

4. An inductive coil array according to claim 1, 2 or 3, wherein the plurality of coils consists of two coils.

5. An inductive coil array according to any one of the preceding claims, wherein the one or more passive coils are connected to a capacitive load.

6. An inductive coil array according to claim 5, wherein the capacitive load is adjustable.

7. An inductive coil array according to any one of the preceding claims, wherein a first passive coil of the one or more passive coils comprises a first capacitive load and a second passive coil of the one or more passive coils comprises a second capacitive load, and wherein the first capacitive load is different from the second capacitive load.

8. An inductive coil array according to any one of the preceding claims, further comprising one or more printed circuit boards, PCBs, each PCB comprising an inductive coil of the plurality of inductive coils.

9. An inductive coil array according to any one of the preceding claims, wherein each inductive coil comprises a ferromagnetic core.

10. An inductive coil array according to any one of the preceding claims, wherein the one or more passive coils are arranged in a hexagonal pattern around the active coil.

11. A method of determining a shape of an object with a flexible inductive coil array, the method comprising: fitting the flexible inductive coil array to the object; driving an active coil of the inductive coil array at one or more frequencies and measuring a response; and determining the shape of the object from the response.

12. A method according to claim 11, wherein the step of determining comprises: determining two or more resonant frequencies in the response.

13. A method according to claim 11 or 12, wherein the step of determining comprises determining an angle between the active coil and a passive coil of the flexible inductive coil array based on the response.

14. A method according to claim 11, 12 or 13, wherein the step of determining comprises determining a plurality of angles between the active coil and a corresponding plurality of passive coils of the flexible inductive coil array.

15. A method according to any one of claims 11 to 14, wherein the plurality of frequencies cover a frequency range comprising a nominal resonant frequency of the active coil.

16. A method according to any one of claims 11 to 15, wherein the step of driving comprises exciting the active coil with a wave comprising a superposition of the plurality of frequencies.

17. A method according to any one of claims 11 to 15, wherein the step of driving comprises a frequency sweep across the plurality of frequencies.

18. A method of operating an inductive coil array, wherein the inductive coil array comprises a plurality of inductive coils comprising an active coil for providing an output signal and one or more passive coils inductively coupled to the active coil, the method comprising: driving the active coil at a resonant frequency of the inductive coil array.

19. A method according to claim 18, the method further comprising: selecting a subset of inductive coils of the plurality of inductive coils, wherein the subset has the resonant frequency.

20. A method according to claim 19, the method further comprising: selecting a second subset of inductive coils of the plurality of inductive coils, wherein the second subset has a second resonant frequency; driving the active coil at the second resonant frequency to selectively drive the second subset of inductive coils.