DEVICE AND METHOD FOR USE IN INDUCTIVE SENSING

Abstract

An inductive sensing device (22) is for sensing at two different frequencies. The device includes an antenna (24) which comprises at least two loop parts (30, 32), each loop part coupled to a different respective tuning capacitor (36a, 36b) to thereby provide different respective resonant frequencies for the first and second loop parts. The two loop parts are jointly connected to a shared input connection (38) for receiving driving signals for driving both antenna parts. A control means (44) controls a drive circuit (34) to supply the antenna, via the shared input connection, drive signals of each of the frequencies, implementing switching between the two.

Claims

1. A device for inductively coupling electromagnetic signals into and out of a medium, the device comprising: an antenna, the antenna comprising at least a first loop part and a second loop part, each loop part being coupled with a respective capacitor to define a first resonant frequency for the first loop part and a second resonant frequency for the second loop part, the resonant frequencies being different, the two loop parts being jointly connected to a shared input connection for receiving a common driving signal for driving the loop parts; a driving circuit connected to the shared input connection and operable to provide the common driving signal selectively at each of the resonant frequencies to thereby selectively drive the antenna at one or other of the resonant frequencies; and a controller adapted to implement switching of the driving circuit between the two frequencies.

2. The device as claimed in claim 1, wherein each loop part is formed by a single turn loop.

3. The device as claimed in claim 1, wherein the antenna comprises at least one shared loop section which forms a portion of each loop part.

4. The device as claimed in claim 1, wherein a terminal of the shared loop section forms said shared input connection of the antenna.

5. The device as claimed in claim 1, wherein the controller is adapted in at least one control mode to switch the driving circuit alternately between the two resonant frequencies.

6. The device as claimed in claim 1, wherein the first and second loop parts define respective interior areas which are non-overlapping.

7. The device as claimed in claim 6, wherein the antenna is configured such that the first and second loop parts occupy a common plane.

8. The device as claimed in claim 1, wherein the device comprises a planar ground plate element electrically coupled with the antenna for grounding both loop parts of the antenna.

9. The device as claimed in claim 8, wherein the first and second loop parts each define a respective interior area, and wherein the planar ground plate element delimits at least one opening through the plate element, the opening having a planar area which extends transversely across a major portion of one or both of said interior areas.

10. The device as claimed in claim 9, wherein the planar ground plate element comprises a single opening extending across major portions of both interior areas.

11. The device as claimed in claim 9, wherein the ground plate element comprises two spaced openings.

12. The device as claimed in claim 8, wherein the ground plate element performs an electromagnetic shielding function.

13. The device as claimed in claim 1, wherein the sensing device comprises at least one signal processor for deriving one or more signal characteristics associated with signals received at the antenna.

14. The device as claimed in claim 1, wherein the device is a physiological inductive sensing device for sensing one or more parameters of a body of a subject.

15. An inductive signal generation method comprising coupling electromagnetic signals into a medium at two different frequencies, the method making use of an antenna, wherein the antenna comprises at least a first loop part and a second loop part, each loop part being coupled with a respective capacitor to define a first resonant frequency for the first loop part and a second resonant frequency for the second loop part, the resonant frequencies being different, and the two loop parts being jointly connected to a shared input connection for receiving driving signals for driving the both the loop parts; and the method comprising driving the antenna at each of the resonant frequencies, the driving comprising switching between the two resonant frequencies.

16. The device as claimed in claim 11, wherein the two spaced openings each have an area extending across a major portion of a different respective one of said interior areas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0068] For a better understanding of the invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which:

[0069] FIG. 1 schematically illustrates stimulation of a thorax of a subject using a loop antenna;

[0070] FIG. 2 shows an example inductive sensing device according to an embodiment;

[0071] FIGS. 3 and 4 show an example antenna and shielding arrangement for an example inductive sensing device according to an embodiment;

[0072] FIGS. 5 and 6 show a further example antenna and shielding arrangement for a further example inductive sensing device according to an embodiment;

[0073] FIGS. 7 and 8 show a further example antenna and shielding arrangement for a further example inductive sensing device according to an embodiment, the antenna having overlapping loop parts; and

[0074] FIG. 9 shows a block diagram of an example signal processing circuit.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0075] The invention will be described with reference to the Figures.

[0076] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the apparatus, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the apparatus, systems and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures to indicate the same or similar parts.

[0077] The invention provides an inductive sensing device for sensing at two different frequencies. The device includes an antenna which comprises at least two loop parts, each loop part coupled to a different respective tuning capacitor to thereby provide different first and second resonant frequencies for the first and second loop part respectively. The two loop parts are jointly connected to a shared input connection for receiving driving signals for driving both antenna parts. A control means controls a drive circuit to supply the antenna, via the shared input connection, drive signals of each of the frequencies, implementing switching between the two.

[0078] Embodiments of the present invention operate on the principle of inductive coupling, whereby a coil or wire has induced across it a potential difference due to exposure to a time-varying magnetic field. Embodiments of the present invention use this principle to measure strength of electromagnetic signals generated within regions of a medium or body by sensing changes in characteristics of a current generated in a loop antenna placed in proximity of the body. Changes in the inductance of the coil may be sensed, where these changes are detected for instance based on changing resonance characteristics of the coil circuit.

[0079] Embodiments of the invention may be used to sense electromagnetic signals emitted by a body in response to an electromagnetic excitation signal that is emitted into the body. In advantageous examples, the same antenna may be used to generate the excitation signals as to sense the returned electromagnetic signals.

[0080] This principle is illustrated schematically in FIG. 1, which shows by way of example a loop antenna 10 (not in accordance with the invention) being driven with an alternating current in proximity to a thorax 12 of a subject, so as to propagate electromagnetic signals 16 into the thorax.

[0081] As a consequence, eddy currents 14 are induced within in the thorax. The eddy currents naturally arise due to Faraday's law of induction, whereby an electromotive force (EMF) is induced in a conducting medium in response to presence of a time-varying magnetic field.

[0082] The eddy currents in turn result in generation of a time-varying magnetic flux 18 of equivalent frequency to that generated by the primary antenna 10. These fluxes result in propagation of electromagnetic waves (signals) 18 which can be sensed at the antenna 10.

[0083] In accordance with advantageous examples, the device may be used for sensing physiological parameters and properties, for instance air, fluid and/or tissue movements in the body of a subject. The system may be advantageously applied in particular for sensing breathing movements for instance.

[0084] In these examples, the device can be used to sense air, fluid and/or tissue movements (e.g. caused by breathing or the beating of the heart) by sensing modulations in the reflected inductance of the signal caused by these movements.

[0085] It will be appreciated that movements of tissue in the body can comprise changes in volume of the tissue. These modulations cause amplitude and/or phase modulations of the electromagnetic signal.

[0086] A modulated electromagnetic signal is emitted by the body in response to the electromagnetic excitation signal that is emitted into the body of the subject. As the tissue, air or fluid move or expand and contract, the eddy currents generated in the medium change shape and form resulting in modulation of the returned EM signals. This allows sensing of changes and movement beneath the surface of a body.

[0087] It is of benefit to provide an inductive sensing device capable of performing sensing using inductive electromagnetic signals of more than one frequency, as different structures of the body (e.g. lungs, heart, skin) are more or less sensitive to different frequencies. It is a challenge to provide a simple inductive sensing arrangement capable of generating signals of more than one frequency, where the signals in both frequencies are of high strength.

[0088] The solution of the present invention provides a loop antenna design permitting generation of strong magnetic fields in both a low and high frequency range, to measure in-body eddy currents at two different frequencies, while retaining a single antenna input connection for driving signals.

[0089] FIG. 2 shows an example inductive sensing device according to an embodiment of the invention.

[0090] The device 22 comprises an antenna 24, the antenna having a first loop part 30 and a second loop part 32. Each loop part defines a distinct closed loop and forms a distinct branch of the antenna circuit. The first loop part is coupled with a first tuning capacitor 36a, of capacitance C1, defining a first resonant frequency, f.sub.1, for the first loop part. The second loop part is coupled with a second capacitor 36b, of capacitance C2, defining a second resonant frequency, f.sub.2, for the second loop part. The first and second resonant frequencies are different. This can be achieved by selecting C1 and C2 to be different from one another for example.

[0091] The two loop parts 30, 32 are jointly connected to a shared input connection 38 for receiving driving signals for driving both the loop parts. A common input drive signal drives both loop parts together. A driving circuit, in the form of an oscillator circuit 42, is electrically coupled to the shared input connection and operable to drive the antenna 24 at least at each of the resonant frequencies. The driving circuit is conductively coupled to the shared input connection in this example, for example by a connecting conductor line (e.g. wire).

[0092] There is further provided a control means in the form of a controller 44 operably coupled with the drive circuit 42, the controller adapted to implement switching of the driving circuit between the two resonant frequencies f.sub.1, f.sub.2. In this way the sensing device 22 is adapted to drive the antenna 24 sequentially at the two resonant frequencies. In further examples, the controller may be integrated with the driving circuit.

[0093] Since each loop part 30, 32 is configured with a different resonant frequency, each loop part effectively acts in this arrangement as a separate tuned antenna (or separate resonator circuit), but wherein the two are jointly connected to a common signal input feed 38. Simply by driving the antenna 24 at the first resonant frequency, f.sub.1, or the second resonant frequency, f.sub.2, the first or second loop part respectively is induced to resonate, thereby generating high strength magnetic inductive signals of that frequency for coupling into a medium of interest. Hence signals of two different frequencies can be generated at high magnetic field strength, while retaining a single-connection antenna design.

[0094] The first and second loop parts each are formed by co-axial lines according to the present example. The co-axial line comprises an inner conductor core 52a part co-axially surrounded by a tubular conductor shield 52b part. The core and shield parts are separated by a tubular insulating (dielectric) layer. Although co-axial lines are used for this example, this is not essential to the invention and alternative conducting lines may be used in other examples. Various alternatives will be apparent to the skilled person.

[0095] Both the first 30 and second 32 loops parts are connected to a common ground, in particular the outer shield part of the coaxial cable forming each loop part is connected to the common ground.

[0096] The number of windings of the antenna 24 is preferably kept small, to minimize capacitive effects between the wires. The first second loop parts 30, 32 are each advantageously formed in this example by a single turn loop. However a single winding is not essential and one or both loop parts may comprise more windings in other examples.

[0097] In this example, the antenna 24 comprises a shared loop section 48 which forms one portion of each of the loop parts. The loop parts are connected via the shared loop section.

[0098] The shared loop section 48 facilitates the common driving of two loop parts 30, 32, since drive signals can be supplied via the shared loop section, simultaneously feeding both loops. In particular, in this example the shared input connection 38 is provided by a terminal of the shared loop section 48. However, this is not essential for the invention. Other advantages of the shared loop section include reducing the overall internal resistance of the antenna, by avoiding duplication of the shared section in two separate loops.

[0099] As noted, in use the controller 44 controls switching of the oscillator circuit 42 between generating and inputting to the antenna 24 electrical signals of AC frequency f.sub.1 (the first resonant frequency) and AC frequency f.sub.2 (the second resonant frequency). The controller may control the oscillator to alternate between the two resonant frequencies. The alternating between the frequencies may be at a high frequency.

[0100] The frequency of the switching can be adjusted as desired. In one set of examples, the frequency may be configured in dependence upon the duration of the physiological phenomenon being monitored. In particular, it may be set sufficiently fast that the physiological process is captured by both loop parts. For example the period of the switching cycle (between both frequencies and back again) may be set to be less than the duration of the monitored phenomenon, and preferably less than half of said duration, and more preferably less than one quarter of said duration.

[0101] For typical applications, a switching (sampling) frequency of between 4-200 Hz is suitable for ensuring the phenomenon is captured by both loop parts. Lower frequencies such as 4-20 Hz are useful for relative low-power (low-detail) applications, while 20-200 Hz is useful when monitoring at a higher time resolution, to resolve physiological waveform features at greater detail. To save power, optionally the control means may incorporate an idle time within the switching cycle, between the frequencies, during which none of the loop parts is operational,

[0102] Alternatively, the controller 44 may control switching sequentially from one resonant frequency to the next without any regular alternation back again. Switching may be implemented by the controller in accordance with input commands from a user in some examples. A user input means may be provided to facilitate this. For example, the frequency may be set statically at one (e.g. optimal) frequency for one patient, and set at a different frequency for a different patient.

[0103] As noted, the first resonant frequency and second resonant frequency are different. The resonant frequency of each loop part 30, 32 depends on the circumference of the respective loop and on the capacitance of the respective capacitor 36a, 36b. Both parameters together define the resonant frequency. Hence, tuning each part to a desired resonant frequency may involve appropriately adjusting both parameters to achieve the given frequency. Where the circumference of both loop parts is the same, tuning can be achieved by selecting C1 and C2 to be different from one another for example.

[0104] For the example of FIG. 1, each loop part 30, 32 has a rectangular configuration. The shared loop section 48 provides one side of each of the rectangles. A rectangular shape is not essential and any other closed loop or annular shape may alternatively be used in further examples, for example rounded (regular or irregular), e.g. circular or oval, or triangular or any other loop shape. A regular shape is preferred, for generating symmetric electromagnetic field patterns.

[0105] The two loop parts may have the same shape or different shapes. The two loop parts may be the same size or different sizes, e.g. the same circumference or difference circumferences or define (bound) the same interior area or different interior areas.

[0106] According to a preferred set of embodiments, the antenna is configured in a flat or planar configuration, wherein the first and second loop parts occupy a common plane. This may be achieved for example by forming the antenna structure from printed circuit board (PCB) material using commonly used production techniques. This provides it a rigid structure.

[0107] In certain embodiments, the first and second loop parts each bound a respective interior area, wherein these areas are non-overlapping. This arrangement is shown in the example of FIG. 2 for example. However, in other examples, the first and second loop parts may each bound a respective interior area, where these areas do overlap. One loop may be inset inside the other for example.

[0108] As noted, the signal sensing device 22 comprises a driving circuit 42. The driving circuit may be or may comprise a signal generating means. In the example of FIG. 2, the driving circuit is in the form of an oscillator circuit. The oscillator is arranged to supply oscillatory drive signals to the common input connection 38 to excite the antenna 24 with an oscillatory current in order thereby, in use, to generate a sinusoidal electromagnetic signal (a sinusoidal electromagnetic wave) for propagation into the body to be stimulated.

[0109] Use of an oscillator circuit is not essential. In other examples a different form of drive circuit may be used, for example a drive circuit adapted to generate pulsed electrical signals having a pulse frequency corresponding to one of the loop part 30, 32 resonant frequencies f.sub.1, f.sub.2.

[0110] The sensing device 22 may further comprise signal processing means for analyzing signal characteristics of signals coupled into the antenna 24 from a medium being investigated. This is not essential the invention however, which is concerned primarily with improvement to the means for coupling signals into and out of a medium (in particular to facilitate coupling at two different frequencies), this being for use in performing inductive sensing. Signal processing may be performed for example by an external unit or device.

[0111] Where signal processing means are provided, the signal processing means may be provided in the form of a signal processing unit. Alternatively, in some examples, the signal generation (driving) and signal processing may be facilitated by a single unit. In accordance with particular examples, the functionality of both these elements may be facilitated by a Vector Network Analyzer, containing an oscillator for exciting the antenna 24 and impedance measurement means for sensing received signals.

[0112] In further examples, the signal processing means may be integrated with the control means, for example with the controller unit 44 in the present example.

[0113] The signal processing means analyses the characteristics of response signals received at the antenna 24 from a medium being excited by the antenna. In particular, the signal processing means may process received signals to derive a measure of changes in the damping factor and/or the resonant (or natural) frequency of the resonator circuit which is formed by each respective loop part 30, 32 of the antenna 24. When electromagnetic signals are received back at an antenna they detune the characteristics of the antenna circuit, and this allows the received signals to be measured.

[0114] A significant advantage of the antenna arrangement of the present invention is that regardless of which of the loop parts is being driven at resonance, the circuit properties of that antenna (and so the detuning characteristics of that antenna) can be directly measured from the output of the single feed point 38. This is because only one loop part is ever resonant at one time, meaning the output feed from the antenna is always dominated by the circuit properties exhibited in the resonant antenna. This makes detuning of the resonant circuit properties detectable regardless of which loop part is active.

[0115] The signal processing means may comprise a phase-locked loop (PLL) circuit according to one or more examples.

[0116] There may optionally be provided data communication means (not shown in FIG. 2) for facilitating communication between the controller 44 and an external device such as an external computer or data store. This may facilitate communication of signal processing results derived by a provided signal processing means to an external computer. It may also facilitate communication of control commands for instance to the controller from an external control means such as a computer.

[0117] The data communication means may comprise a wireless communication means or wired communication means. The communication means may implement or operate in accordance with any suitable communication protocol or medium such as for example Bluetooth, Wi-Fi, Near Field Communication (NFC), ZigBee, or any suitable wired communication protocol.

[0118] In use, the antenna 24 is held in proximity to a body or medium of interest, and the antenna is excited by the driving circuit 42 to generate electromagnetic excitation signals.

[0119] In accordance with advantageous examples, the inductive sensing device 22 may be used for sensing physiological parameters and properties, for instance air, fluid and/or tissue movements in the body of a subject. The system may be advantageously applied in particular for sensing breathing movements for instance.

[0120] In these examples, the device permits sensing of air, fluid and/or tissue movements (e.g. caused by breathing or the beating of the heart) by sensing modulations in the reflected inductance of the signal caused by these movements. This was explained with reference to FIG. 1 above.

[0121] The configuration of the antenna 24 of the system may take different forms.

[0122] A set of examples comprising a planar antenna configuration and including electromagnetic shielding for the antenna will now be described with reference to FIGS. 3-6.

[0123] FIGS. 3 and 4 schematically depict a first flat-plane antenna 24 arrangement, wherein the sensing device 22 further includes a planar ground plate element 62 electrically coupled with the antenna for grounding both loop parts of the antenna. The electric coupling with the ground plate is purely capacitive: no connection line is required. The antenna may be connected to the ground plate via the outer shield part 30, 32. The antenna may be formed from PCB material to facilitate a stable, flat shape.

[0124] The planar ground plate element 62 delimits an opening 64 through the plate element. The opening has an extended planar area such that the opening effectively forms an open window area through the ground plate, or a missing portion of the ground plate. The opening extends from one planar side of the plate to the other. The (material of) the plate extends the whole way round the opening, effectively defining an annular shape around the opening, i.e. the opening is fully bounded or surrounded by the material of the plate.

[0125] In the example of FIGS. 3 and 4, the planar ground plate 62 delimits a single opening 64. The opening has a shape which is similar or substantially similar (in a mathematical sense) to an outer perimeter of the antenna 24, i.e. the opening has the same shape in this case as an outer or outline shape of the overall antenna, but has an opening area which is smaller than the interior area bounded by the outer perimeter of the antenna.

[0126] The opening 64 is configured such that a planar area of the opening 64 extends transversely across a major portion of an interior area defined or bounded by each of the loop parts 30, 32 of the antenna 24. The opening is positioned such that (its planar area) maps spatially onto a central region of an interior area bounded by an outer perimeter of the antenna. In the particular example illustrated, an outer perimeter of the opening is concentrically aligned with an outer boundary of the antenna, such that the opening is centrally aligned within the interior area defined by the outer perimeter of the antenna.

[0127] The ground plate 62 acts as an electromagnetic shield (Faraday shield), shielding components of the antenna 24 from electromagnetic waves. The hole (opening) 64 permits propagation through the shield of magnetic field components both from and to the antenna.

[0128] The purpose of the (leaking) ground plate shield is to transfer the outgoing and, more importantly, incoming electromagnetic wave impedance, from high to low impedance, from electric field to magnetic field. In this way, in received signals, the electric field part is minimized as much as possible, as it is only the magnetic field part which contributes to the useful sensing signal. The electric field part merely generates noise in this sensing signal. Despite this, there always remains a residue of electric field; to block the electric components completely would also lead to blocking of the useful magnetic field parts.

[0129] In addition, the shield performs other functions. Due to its electrical coupling with the antenna, it provides a contribution the tuning each part of the loop. Furthermore, in examples, the shield may be arranged in such a way that it also shields the series capacitors 36a, 36b from incoming electric fields. Further, the shield may be arranged so as to reduce or prevent the build-up of electrical charges along the line of the antenna 24, which can thereby reduce the effect of capacitive coupling with the medium. Capacitive coupling with the medium is detrimental to performance, since it affects the magnetic sensing signal, and also leads to motion artefacts due to its sensitivity to the relative distance between the body and the antenna.

[0130] The positioning of the opening 64, having a planar area aligned with an interior area of the antenna, means that fields are still able to propagate across this antenna interior area, thus permitting inductive coupling of signals into the loops 30, 32. The same is true for signals generated by the antenna, which are induced within this interior region of the antenna, as illustrated in FIG. 4.

[0131] FIG. 4 shows a perspective view of the antenna 24 and ground plate (shield) 62 arrangement, and schematically depicts the generation of electromagnetic signals by the antenna. disposed above the ground plate in co-planar fashion, the two separated by a small clearance which may be filled with an insulating (i.e. dielectric) material or medium. The ground plate is thus arranged facing one planar side of the antenna. The magnetic field lines 70 of the generated signals are schematically illustrated.

[0132] An alternative example configuration is illustrated in FIGS. 5 and 6. This arrangement includes the same antenna 24 as in the arrangement of FIGS. 3 and 4, but comprises a planar ground plate element 62 delimiting two openings 64a, 64b. The openings are the only difference between the ground plate of FIGS. 5 and 6 and that of FIGS. 3 and 4. Its electrical and structural properties, and its electrical interaction with the antenna (as a grounding element) are the same.

[0133] The planar ground plate element 62 delimits two openings 64a, 64b through the plate element, each opening again having an extended planar area so as to effectively form an open window area through the ground plate, or a missing portion of the ground plate. The two openings are spatially separated from one another. Each opening extends from one planar side of the plate to the other. The (material of) the ground plate 62 extends the whole way round each opening, effectively defining an annular shape around each opening, i.e. the openings are each fully bounded or surrounded by the material of the plate.

[0134] As noted above, the first and second loop parts 30, 32 of the antenna 24 each bound a respective interior area. The openings 64a, 64b are configured such that a planar area of each opening extends transversely across (or occupies) a major portion of a (different) respective one of said interior areas. The first opening 64a spans across a major portion of the interior area defined by the first loop part 30. The second opening spans 64b across a major portion of the interior area defined by the second loop part 32.

[0135] Each opening 64a, 64b is positioned such that (its planar area) maps spatially onto a central region of an interior area bounded by the outer perimeter of one of the respective loop parts 30, 32. In the particular example illustrated by FIGS. 5 and 6, an outer boundary of each opening is concentrically aligned with an outer boundary of a respective one of the loop parts, such that each opening is centrally aligned within the interior area defined by this outer perimeter.

[0136] Each of the openings 64a, 64b has a shape which is similar or substantially similar (in a mathematical sense) to an outer perimeter of a different respective one of the loop parts 30, 32 of the antenna 24, i.e. the opening has the same shape in this case as this outer perimeter.

[0137] The planar ground plate element 62 optionally further delimits two slots or gaps 65a, 65b which extend longitudinally from an interior of each of said planar openings 64a, 64b in the ground plate to the edge of the plate. Each hence defines an open path from the interior of a respective one of the planar openings to the exterior of the ground plate. Each defines an interruption in the ground plate material. Each gap traverses (extends from one side to the other of) a line of one of the loop parts 30, 32.

[0138] The purpose of the gaps 65a, 65b is to block eddy-currents from forming in the annular-shaped sections of the ground plate which extend around each of the two planar openings 64a, 64b. The gaps hence form interruptions or breaks in each of these annular-shaped sections of the ground plate. It is advantageous to prevent the formation of such eddy currents (due to the propagation of back-reflected magnetic signals) in these annular sections, to avoid the consequent inducement of magnetic fields by the currents which are in opposition to those generated (or sensed) by each antenna loop part 30, 32.

[0139] FIG. 6 schematically illustrates the magnetic field pattern generated according to this arrangement. For simplicity, the field lines for only one loop part 30 are illustrated, corresponding to the first loop part 30 being driven at resonance. However, a similar, symmetrical pattern is created also for the second loop part 32 during times when it is driven at resonance. It can be seen that for the arrangement comprising two openings in the ground plate shield 62, both parallel 70 and perpendicular 72 magnetic field components are generated. The same circulatory-like field pattern 70 is generated circulating between the two loop parts (and also around the outer wing 76 of the antenna part), but in addition a parallel field component 72 induced, and able to propagate through a central region of the opening 64a aligned beneath it.

[0140] Although in the above described arrangements, electromagnetic shielding is provided on only one side of the antenna 24, in particular the side situated in use between the antenna and the medium to be investigated, in further examples shielding may be provided on both sides. According to one set of examples for instance, a continuous shielding plate (with no openings) may be provided on the alternate side of the antenna (the environment-facing side, away from the medium). In this way, the antenna is maximally shielded from environmentally originating electromagnetic signals, which could interfere with the coupling of signals of interest, originating from the medium being probed.

[0141] However, in further examples, the shielding on the alternate (environment-facing) side may comprise one or more openings. It may for example comprise the same configuration of openings as is provided for the shielding on the medium-facing side of the antenna.

[0142] In each of the above examples (FIGS. 2-6), the antenna 24 is configured such that the two loop parts 30, 32 are non-overlapping with one another. In particular, the first and second loop parts each bound a respective interior area, wherein these areas are non-overlapping.

[0143] However, in other examples, the first and second loop parts may overlap with one another. An example is illustrated in FIGS. 7 and 8. Here the first loop part 30 is inset inside the second loop part 32. Three sides of the first loop part extend concentrically with the corresponding sides of the second loop part 32, though concentricity is not essential. One side of each of the loop parts is formed by a common section 33 of antenna line. This common loop section 33 is connected to the shared input connection 38 for the two loop parts, to facilitate driving each of the loop parts.

[0144] Simulations have found that the performance of the antenna 24 is not affected by overlapping of the loop parts 30, 32 in accordance with the arrangement illustrated in FIG. 7. Hence the overlapping loops arrangement provides equivalent performance in inductive sensing at two different frequencies as the separated loop arrangement of the examples described above. Furthermore the overlapping loops arrangement provides advantages in terms of reduced form factor, and so may be advantageous where space saving is a concern. As discussed, the inductive sensing device may optionally include signal processing means for analyzing received signals from a medium being investigated. By way of illustration of this feature, one example implementation of signal processing will now be described in detail, with reference to FIG. 9. This represents just example signal processing approach, and other approaches and apparatuses will be apparent to the person skilled in this field. In accordance with the presently described example, the signal processing means is implemented by a phased locked loop. An example of a phased locked loop circuit which may be used is shown in FIG. 9.

[0145] In this embodiment, a phase locked loop (PLL) is used to drive an antenna circuit according to the invention 24, and a control signal for the PLL provides an output signal representing the movement of air, fluid and/or tissue in the body of the subject. Hence, the circuit of FIG. 9 implements the functionality of both the drive circuit 42 and optional signal processing means.

[0146] FIG. 9 shows signal generation and processing circuitry 80 for the antenna 24, and comprises a reference oscillator 81, a PLL 82 that is connected to the reference oscillator 81 and that outputs an analog control signal (known as V.sub.tune) to a voltage-controlled oscillator (VCO) 84. The V.sub.tune signal is a result of a comparison of the signal from the reference oscillator 81 to the signal from the VCO 84. In response to the PLL analog control signal the VCO 84 generates an excitation signal at a required frequency and provides this to the antenna 24 so that the antenna emits the electromagnetic excitation signal(s). As noted above the electromagnetic excitation signal will induce eddy currents in the body of the subject, and these eddy currents will induce a magnetic flux which is sensed by the antenna 24. This generated flux results in a reflected inductance component L.sub.r in the inductance of the antenna coil. This can be sensed via detuning of characteristics of the coil, in particular the damping and natural frequency of the coil.

[0147] The excitation signal is also provided to the PLL 82 as part of a feedback loop. The analog control signal from the PLL 82 is also provided to an analog-to-digital convertor (ADC) 86 which converts the analog control signal into a digital signal, and this digital signal is provided to a controller 88. The controller 88 determines a digital control signal for the PLL 82 and provides this to the PLL 82. As the skilled person will be aware, in a PLL system, if the phase of the VCO 84 differs from the phase of the reference oscillator 81, the digital control signal corrects the VCO phase.

[0148] Movements of air, fluid and/or tissue in the body effectively detune the characteristics of antenna 24 (because of the reflected inductance), and the digital control signal counters this detuning and corrects the phase of the VCO 84. The digital control signal therefore carries information regarding the movements of the air, fluid and/or tissue, and the controller 88 determines an output signal 90 from the digital control signal that represents or contains the information on the movements of air, fluid or tissue in the body of the subject. Although this output signal 90 does not carry the actual phase and amplitude information, the physiological characteristics (e.g. heart rate, breathing rate) are clearly observable.

[0149] The correction signal V.sub.tune that is required to keep the VCO 84 at the required frequency is used to measure amplitude and/or phase shifts due to movements of air, fluid and/or tissue in the body of the subject. The phase shifts tend to dominate the amplitude changes. The PLL correction signal (the digital control signal output by the controller 88 derived from the analog PLL correction signal) is used to determine the output signal 90. For example the output signal 90 can correspond to the digital control signal with suitable filtering and/or down-sampling to improve the signal-to-noise ratio.

[0150] The output signal hence carries a signal representative of the received electromagnetic signals, derived based on changes in the natural resonance frequency of resonating loop part 30, 32 which is currently active, this being reflected in the correction signal. When the antenna is being driven at the resonant frequency, f.sub.1, of the first loop part, the PLL will provide a correction signal that relates to the detuning of f.sub.1. When driven at the resonant frequency f.sub.2 of the second loop part, the PLL will provide a correction signal that relates to the detuning of f.sub.2.

[0151] Embodiments of the invention outlines above and defined in the claims provide an inductive sensing device which permits sensing at multiple frequencies, each with a high field strength. Sensing at different frequencies permits selective tuning of the sensitivity of the system toward different structures of the body, such as the lungs, heart, and skin. For example, high-frequency measurements are predominantly sensitive to the skin (and therefore motion). Medium- and/or low-frequency measurements have optimized sensitivity to the heart and/or lungs (but also sensitive to motion). These two frequency measurement could therefore be used in combination to compensate the heart and lung measurements for motion artefacts. Furthermore, using multiple frequencies enables better identification of the individual contributions of the heart and lungs to the modulations in the signals.

[0152] In addition, measurements at multiple depths in the lung are possible using multiple frequencies. This for example may facilitate better estimation of the breathing depth of a subject. It may also permit determination of the location of an infection (accompanied by local lung edema).

[0153] As discussed above, embodiments make use of a controller 44. The controller can be implemented in numerous ways, with software and/or hardware, to perform the various functions required. A processor is one example of a controller which employs one or more microprocessors that may be programmed using software (e.g., microcode) to perform the required functions. A controller may however be implemented with or without employing a processor, and also may be implemented as a combination of dedicated hardware to perform some functions and a processor (e.g., one or more programmed microprocessors and associated circuitry) to perform other functions.

[0154] Examples of controller components that may be employed in various embodiments of the present disclosure include, but are not limited to, conventional microprocessors, application specific integrated circuits (ASICs), and field-programmable gate arrays (FPGAs).

[0155] In various implementations, a processor or controller may be associated with one or more storage media such as volatile and non-volatile computer memory such as RAM, PROM, EPROM, and EEPROM. The storage media may be encoded with one or more programs that, when executed on one or more processors and/or controllers, perform the required functions. Various storage media may be fixed within a processor or controller or may be transportable, such that the one or more programs stored thereon can be loaded into a processor or controller.

[0156] Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.