CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is the U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2018/050044, filed on 2 Jan. 2018, which claims the benefit of European Patent Application No. 17150641.3, filed on 9 Jan. 2017 and European Patent Application No. 17169584.4, filed 4 May 2017. These applications are hereby incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to an inductive sensing system for sensing electromagnetic signals from a body, and in particular a sensor for inductively coupling with a body for sensing electromagnetic signals.

BACKGROUND OF THE INVENTION

Inductive sensing can be used as a means of non-invasive investigation of properties of a body.

In one advantageous area of application, inductive sensing can be used as a means of non-invasively investigating physiological characteristics, in particular heart and lung dynamics. Inductive sensing is based on magnetic induction and has several advantages over conductive and capacitive sensing.

An advantage compared to conductive sensing, such as bio-impedance measurements, is that adhesive electrodes are not required; sensing may be performed without contact and/or through non-conductive material, such as textile and plastic.

An advantage compared with capacitive sensing is that inductive sensing is based on magnetic fields rather than electric fields and as a result is more sensitive to changes at greater penetration depth inside the body, as opposed to those just occurring at skin level. This is because magnetic fields penetrate deeper into a body than electrical fields, and thus magnetic fields can be used to measure changes in properties deeper inside the body, whereas electrical fields are predominantly useful only for measuring effects at the surface of the skin such as changes in skin properties (e.g. permittivity) or movement of the skin (skin proximity).

Coil-based inductive sensors function by inductively coupling with electromagnetic signals (i.e. electromagnetic waves or oscillations), wherein propagation of the signals through the coil leads to a change in the current through the coil, which can be measured and used to sense properties of the propagated signal (including e.g. frequency spectrum, amplitude and phase pattern).

An electromagnetic excitation signal can be propagated into a body to be investigated. The excitation electromagnetic signal causes magnetic induction in the body, i.e. the generation of eddy currents in the tissue of the body due to the application of an external magnetic field. These eddy currents then in turn generate electromagnetic signals propagated out of the body which interact with the applied fields in a way that allows them to be sensed by the coil.

Movements of tissue in the body can manifest in changes in volumes of local regions of the tissue and in changes of the conductive or dielectric properties of a tissue. These changes then cause amplitude and/or phase modulations of the electromagnetic signal which is emitted out of the body in response to the electromagnetic stimulation. By monitoring these changes, movement and size change of elements within the body can be detected and tracked, and changes in the conductivity and dielectric properties can be tracked. For example, heart contractions manifest themselves mainly as movement of blood, and breathing mainly manifests itself as changes in the conductivity of the lung.

For 50 years, beginning with the first two papers in this field in 1967 (Vas et al) and 1968 (Tarjan et al.), this field has been an area of active research, multiple publications appearing each decade. One active aim in particular has been to obtain strong cardiopulmonary signals using an inductive sensor. Strong signals in general has been an aim.

The ongoing research however has never led to adoption of the technology in commercial applications, since the signals achieved from inductive sensors remain notoriously weak, and often saturated by noise.

Improving the strength of magnetic signals received from the body during inductive sensing is hence a very important area for development. In state of the art systems, signal strength remains prohibitively low, preventing the technology from being usefully applied in commercial applications.

It is an object of the present invention to improve the signal strength of detected electromagnetic signals received from a body during inductive sensing.

SUMMARY OF THE INVENTION

The invention is defined by the claims.

According to an aspect of the invention, there is provided a physiological parameter inductive sensing system for sensing electromagnetic signals emitted from a body in response to electromagnetic excitation signals propagated into said body, the system comprising:

a loop resonator for inductively coupling with said electromagnetic signals emitted from the body, the resonator comprising a loop antenna and an electrically coupled capacitor, the loop of the antenna having only a single turn of circumferential length l, and

a signal generation means adapted to excite the resonator to generate the electromagnetic excitation signals having a radial frequency ω for propagating into said body,

wherein a normalized radial frequency

ω^≡ωωref
of the electromagnetic excitation signals is from 0.025 to 0.50, where ωref=2πc/l and c=the speed of light.

For the avoidance of doubt c=3×108 m/s.

The invention is based on results of a significant program of research undertaken by the inventors by which it has been found that the above parameter of normalized radial frequency {circumflex over (ω)} of the electromagnetic excitation signals is one of the most important parameters in determining the strength of the signal sensed by the system (as well as the absolute frequency f).

More particularly, research has found that at values of {circumflex over (ω)} below 0.025, the signal to noise ratio of the sensed electromagnetic signal is significantly reduced, leading to higher motion sensitivity. More particularly, below this level, the signal strength is much decreased, and in most practical applications saturated by noise emanating from electronic noise, electromagnetic interference and noise from capacitive coupling with the surface of the body (i.e. direct coupling via electric fields and electrically induced charges).

However, despite the highly significant signal strength advantages which the inventors have managed to achieve with this invention, frequencies as high as those claimed have never been explored in the state of the art. This is most likely due to a strong prevailing belief in the field that (absolute) frequencies above around 10-29 MHz lead to a significant reduction in achievable penetration depth due to skin effects. The assumed penetration depth reductions would render the system ineffectual for probing physiological parameters. The inventors however have found that this prevailing prejudice is a misconception, as even though the skin effect is real, it becomes only significantly detrimental at much higher frequencies than those used in the present invention.

It has further been found that a value of 0.50 for {circumflex over (ω)} is the highest that is physically possible while still achieving effective inductive sensing. This is the highest possible normalized frequency of the first resonance of a (single) loop antenna, which is the resonance at which precisely half a wavelength fits around the circumference of the loop.

Above the first resonance, a highly non-uniform current is induced in the loop of the resonator, and the loop will carry a stationary oscillating pattern of accumulated charges. This fringe pattern of accumulated charges will typically capacitively couple to the surface of the body, thereby inducing surface charges on the surface of the subject's skin.

In this case, the sensor is rendered highly sensitive to motion of the body surface, where even very small movements induce parasitic signals which fully drown out the inductive signal being sensed (namely the magnetic inductive signal originating from beneath the surface of the body). This renders the inductive sensor ineffective for most practical applications. Hence, by keeping {circumflex over (ω)} below a value of 0.5, signal strength can be maintained at a level suitable for practical applications.

The range of values for {circumflex over (ω)} provided by the present invention therefore maximise the signal strength achievable in magnetic inductive sensing.

It has further been found in the course of investigation that, contrary to expectation, increasing the number of coil windings (coil turns) N above one (a single loop) provides no detectable improvement in signal strength. Moreover, increasing the number of windings is in fact detrimental to the object of maximizing signal strength because increasing the number of windings limits the maximum realizable value of {circumflex over (ω)} due to capacitive coupling between the windings. This in turn restricts the available increase in signal strength.

The two features of the above defined {circumflex over (ω)} range and the single coil antenna are therefore fundamentally interrelated. Where the antenna comprises more than a single turn (N>1), the maximum frequency {circumflex over (ω)} that can be used with the loop is significantly reduced below {circumflex over (ω)}=0.5 due to parasitic capacitances between the loops and the longer total length of the coil wire Nl. This makes achievement of the beneficial normalized frequency range claimed in the present invention, and the accompanying increases in signal strength, impossible. Hence, only by reducing the number of coil windings to one are the beneficial higher {circumflex over (ω)} values possible.

However, restricting the coil windings to one is contrary to prevailing assumptions in the art, which generally assume that multiple coil windings increases signal strength by enhancing the flux linkage. This has further contributed to the reluctance in the field to explore frequencies in the range presently claimed.

Multiple windings also add cost and complexity to the sensor. Reducing to a single loop simplifies construction and operation of the device and reduces form factor.

More detailed physical explanation and grounding of the particular range identified will be presented below. In brief however, the claimed invention is based on the finding that reflected inductance (inductance of the antenna resulting from eddy currents induced in the body in response to electromagnetic stimulation) is the key determinant in the resulting received signal strength at the antenna. By first normalizing this quantity, so as to render it effectively dimension-independent, and then simulating variation of various parameters upon which it is dependent, normalized radial frequency of the sensed signals was found to be the most significant parameter determining reflected inductance. The optimal range of values for {circumflex over (ω)} claimed in the present application was then identified based on modelling and experimentation.

The system of the present application is a physiological parameter inductive sensing system. The system is hence for sensing electromagnetic signals received from a body of a subject in response to electromagnetic excitation signals applied to the body of the subject.

The system may in particular be a vital sign sensing system. Vital signs may include for instance heart rate, pulse rate, breathing capacity, breathing rate, stroke volume, stroke volume variations, cardiac output, or aortic or arterial pulse height/pressure/diameter modulations.

It is noted that although the present application is directed to physiological parameter sensing, the inventive concept embodied by the invention is not in principle limited only to the measurement of physiological parameters (such as vital signs). The inventive concept is applicable more broadly to any inductive sensing system. Examples will be described in the next section.

Electromagnetic excitation signals having a radial frequency ω may be generated by exciting the resonator at a radial frequency of ω. By this may be meant exciting the resonator so as to resonate at a frequency of ω. By this may in particular be meant inducing in the antenna a resonating current having radial frequency ω.

This may be achieved by providing a resonator having a natural resonance frequency equal to ω. The natural resonance frequency of the resonator can be set by appropriately selecting the capacitance of the capacitor.

Additionally or alternatively, exciting the resonator so as to induce in the antenna a resonating current having radial frequency ω may be achieved by exciting the resonator with a current of frequency ω.

The normalized radial frequency {circumflex over (ω)} of the electromagnetic excitation signals is dependent upon the radial frequency ω of the electromagnetic excitation signals and a reference frequency ωref=2πc/l of the antenna (which is the radial frequency at which the loop circumference 1 equals one free-space wavelength (i.e. where λfree space=l)).

Hence embodiments of the invention may require that the antenna, capacitor and/or signal generation means are together configured such that the normalized radial frequency is within the defined range.

By way of example, this may be achieved for instance by providing a resonator having a capacitance selected such as to set the natural resonance frequency of the resonator at ω, and choosing an antenna with the correct circumferential length l. Where the resonator has a given natural resonance frequency, it may be induced to resonate at that frequency by exciting it, for instance using a free-running oscillator (one without a fixed or forced oscillation frequency).

The system in accordance with the present application may be for sensing electromagnetic signals emitted from a body of a subject, i.e. a physiological, diagnostic or medical sensing system. However, the system is applicable more generally to sensing electromagnetic signals received from any body, where body is to be understood broadly as meaning any physical object or entity, or any region of such an object or entity.

The sensing system comprises a signal generation means for exciting the resonator to generate electromagnetic signals of a radial frequency ω for propagating into the body.

By exciting the resonator, a resonating current is induced to flow back and forth through the loop antenna into the capacitor.

The resonator may be excited by driving an (alternating) current through the antenna such as to stimulate generation of oscillatory electromagnetic signals (waves).

By driving the antenna at a current with frequency ω, electromagnetic excitation signals of frequency ω may be generated.

The signal generation means may comprise a driver means for driving the antenna, for instance at a radial frequency ω, i.e driving the antenna with an alternating current of frequency ω. The driving means may be oscillator for instance.

The same antenna is used to generate the excitation signals as is used to sense the electromagnetic signals received from the body in response.

For the avoidance of doubt ‘electromagnetic excitation signals’ simply means electromagnetic signals for propagating into the body for the purpose of exciting or stimulating generation of eddy currents within the body for in turn stimulating emission of electromagnetic signals back out of the body which can be sensed by the sensing system.

By ‘electromagnetic signals’ may generally be meant electromagnetic radiation emissions or electromagnetic oscillations and/or electromagnetic waves.

In accordance with one set of embodiments, the normalized radial frequency {circumflex over (ω)} of the excitation signals may be from 0.025 to 0.25

Within this range, current amplitude when sensing the signals received back from the body is substantially constant (homogeneous) over the antenna loop. Above {circumflex over (ω)}=0.25, the current amplitude varies to a greater extent over the antenna loop. Phase of current is also relatively constant around the antenna loop. These factors are significant limiting factors in achieving high signal strength and high quality signal.

Inhomogeneous currents in particular are highly disadvantageous because these result in accumulated charges in the antenna loop which in turn lead to capacitive coupling with the medium being examined. Capacitive coupling with the medium is a primary source of signal distortion (motion artefacts) in inductive sensors.

In accordance with one set of embodiments, the normalized radial frequency {circumflex over (ω)} of the excitation signals may be from 0.025 to 0.20. Below an upper limit of 0.20 for {circumflex over (ω)}, radiation resistance is kept optimally low (<0.5Ω), current phase is relatively constant over the antenna loop and uniformity of current amplitude is further improved (<20% difference in current amplitude between any two points around the loop).

This range is also particularly advantageous since above {circumflex over (ω)}=0.20, some electrical effects begin to occur, resulting in the coil becoming sensitive to the electric field component of the electromagnetic (EM) signals received back from the body (which interferes with the sensitivity to magnetic field signals). Below {circumflex over (ω)}=0.20, it has been found that the sensing system remains strongly inductively sensitive to the induced electromagnetic signals emerging from the body, without building up significant electrical charges along the single antenna loop wire.

In accordance with one set of embodiments the normalized radial frequency {circumflex over (ω)} of the excitation signals may be from 0.04 to 0.25. A lower limit of 0.04 is preferred since it results in higher signal strength (than 0.025 for instance) while still maintaining the benefits of very low radiation resistance (˜0.01Ω), constant current phase over the antenna loop and constant current amplitude over the antenna loop.

It has been found that a normalized radial frequency {circumflex over (ω)} above 0.04 provides a signal of strength sufficiently high that the signal is reliably robust against noise across a wide range of practical applications. In certain scenarios (although not all), electromagnetic signals below this normalized frequency can become distorted by noise artefacts originating from capacitive coupling with the tissue surface (i.e., electric coupling with the surface of the body due to electrically induced charges). For conveniently-sized sensors of e.g. radius=1 to 3 cm, it has been found that signal strength is doubled compared to signals using normalized radial frequency of around 0.025. Breathing signals (signals indicating breathing function) have in particular been found to have significantly higher signal strengths. This will be explained in more detail in the next section.

As noted above, in examples, the signal generation means may be adapted to excite the resonator to resonate at a radial frequency ω in order to generate the excitation signals having radial frequency ω.

The resonator (circuit) may be provided having a natural radial frequency of ω in order to facilitate exciting of the resonator at a radial frequency of ω, and optionally wherein the capacitor is selected for tuning the natural radial frequency of the resonator to ω.

The system may comprise a signal processing means adapted to process signals received at the antenna. The signal processing means may be adapted to sense changes in a natural resonance frequency of the resonator (i.e. detuning of the resonator), and use these changes to derive a signal output. The signal processing means may additionally or alternatively sense changes in a damping factor of the resonator circuit and use these changes to derive a signal output.

Sensing of changes in the natural resonance frequency may be preferred because measuring damping factor changes requires measuring the imaginary part of reflected inductance which might require additional complexity in the circuitry of the signal processing means. However, measurement of the imaginary part, and hence the damping factor, is possible in accordance with one or more embodiments.

In accordance with at least one set of embodiments, the system may comprise a signal processing means adapted to process signals received at the antenna to downscale a frequency of the signals by mixing the signals with a reference oscillatory signal of a different frequency, and applying a differential filter to derive a signal having a frequency being the difference between the frequency of the reference oscillatory signal and the received signals.

The object of such embodiments is to reduce energy consumption and required computing power of any signal processing performed by the system. In particular, digital dividers and counters (for processing the received signals for analysis) draw current proportional with the operating frequency. Hence, to save power, in accordance with the present embodiments, the frequency of the received signals is first reduced, allowing the subsequent signal processing to be performed at lower power.

The advantage of the particular embodiment proposed, of first mixing a second signal and then finding a differential frequency signal, is that mixing does not result in a loss of resolution. This is contrast for instance to dividers (which can also be used to reduce frequency) which do reduce resolution. The invention is still compatible with use of dividers in other embodiments however.

The reference oscillatory signal frequency is preferably very close to the frequency of the received signals. In preferred examples, the frequency of the oscillatory signal and the frequency of the received signals are from 10 to 20% apart.

As explained above, the received signals will typically have frequency equal to ω, i.e. the radial frequency of the electromagnetic excitation signals.

In accordance with one or more embodiments, the signal generation means may be adapted to control the antenna to generate electromagnetic excitation signals at a frequency of from 30 MHz to 1000 MHz.

The frequency here refers to absolute frequency f, rather than radial frequency ω.

In accordance with these embodiments, an absolute frequency requirement is provided. Accordingly, since

ω^≡ωωref⁢⁢and⁢⁢ω=2⁢⁢π⁢⁢f,
a corresponding requirement is imposed upon ωref=2πc/l, meaning that the circumferential length l of the antenna loop must be configured to achieve the desired f. In particular, these embodiments require l={circumflex over (ω)}c/f.

Applying excitation signals to a body at such high frequencies is not known in the prior art. This is because it is commonly understood in the art that frequencies above around 30 MHz lead to significant reduction in achievable penetration depth, while increasing the required operating power. However, inventors of the present invention have found that the reduction in penetration depth does not become prohibitive to inductive sensing (in particular for physiological parameter sensing) until much larger frequencies are reached. This is particularly the case in applications pertaining to investigating breathing function. These embodiments hence improve signal strength, and with no prohibitive reduction in penetration depth.

This frequency range is beneficial because signal strength becomes significantly larger above f=30 MHz. This is particularly so for relatively smaller sized loops (radii in the range of 1-3 cm). In this case, where for instance the signal is measured by changes in resonance frequency of the resonator (as described above), the frequency shift signal (real part of reflected inductance) quickly becomes much stronger when going into this range, by both an increase in signal strength, and also a movement in the phase of the reflected inductance towards the real part.

This frequency range is also particularly advantageous when using a signal processing means in the form of a mixer-counter. The absolute changes in the resonator resonance frequency due to, for instance, breathing when using f<30 MHz can be below 1 kHz. When using a slope counter, the quantization noise of the number of counts in the signal is readily visible at this operating frequency.

When increasing the absolute frequency above f>30 MHz the influence of quantization noise on the signal will in most cases cease to be problematic. This is described in greater detail below.

In addition, it is beneficial to maintain frequency below 1000 MHz in order to optimize penetration depth. At frequencies above 1000 MHz, penetration depth of electromagnetic signals start to become prohibitively small for measuring physiological parameters, e.g. lung or heart signals.

In accordance with one or more embodiments, the signal generation means may be adapted to excite the resonator to generate electromagnetic excitation signals having a frequency of from 100 MHz to 1000 MHz.

At frequencies above 100 Hz, penetration depth is still sufficiently deep while signals become much stronger. Cardiopulmonary signals in particular have been found to become much stronger. In addition, the phase of the reflected inductance moves further toward the real part. This is beneficial where the signal generation means is adapted to measure changes in the resonator resonance frequency, which requires sensing the real part of reflected inductance.

In addition, above a frequency of 100 MHz, the quantization noise of e.g. a slope-counter signal processing means (at e.g. integration times of around 0.05 seconds) is almost negligible relative to the signal strength. This is described further below.

In accordance with one or more embodiments, the signal generation means may be adapted to excite the resonator to generate electromagnetic excitation signals having a frequency of from 30 MHz to 500 MHz.

This absolute frequency range optimizes penetration depth, in particular for muscle. The signal strength at 500 MHz remains very high, while penetration depth is maintained relatively large (˜5 cm for muscle—and higher for other media).

This frequency range may also be preferred where power consumption is a concern, e.g. battery powered sensors. Below 500 MHz, the frequency is low enough that it does not significantly contribute to signal processing power consumption.

In accordance with one or more embodiments, the signal generation means may be adapted to excite the resonator to generate electromagnetic excitation signals having a frequency of from 100 MHz to 500 MHz.

This combines the benefits 100 MHz lower boundary and 500 MHz upper boundary discussed above.

Exciting the resonator to generate signals of the above frequencies can be performed in accordance with the example methods outlined above.

In particular advantageous examples, the loop of the antenna may have a radius of between 15 mm and 20 mm.

In further advantageous applications, the loop of the antenna may have a radius of between 90 and 110 mm.

Loop radii of 15 or 20 mm are desirable because larger loops have been found to be more sensitive to movement of objects in the environment since the distribution of the electromagnetic field scales proportionally with the loop size where the relative frequency is kept constant. Loops with radii smaller than 15 mm are possible, but the obtained signal strength decreases.

Antenna loops of radius between 90 and 110 mm are advantageous for instance in cases where sensing over a relatively long distance is required. For instance, an antenna of such a size may be embedded in a mattress or a chair, and allowing sensing through the materials of the furniture. An antenna loop having radius of 100 mm has been tested and found beneficial in such applications for instance.

In accordance with one or more embodiments, the system may comprise a processor for processing signals sensed by the antenna to derive one or more physiological parameters.

Examples in accordance with a further aspect of the invention provide a physiological parameter inductive sensing method comprising sensing electromagnetic signals emitted from a body in response to electromagnetic excitation signals propagated into said body, the method comprising:

exciting a loop resonator to generate the electromagnetic excitation signals and directing the signals into said body, wherein the resonator comprises a loop antenna and electrically coupled capacitor, the loop antenna having only a single-turn loop of circumferential length l; and

using the loop resonator to inductively couple with the electromagnetic signals emitted from the body in response to the excitation signals,

wherein a normalized radial frequency

ω^≡ωωref
of the electromagnetic excitation signals is from 0.025 to 0.50, where ωref=2πc/l and c=the speed of light.

All terms are to be interpreted in accordance with the discussions above in relation to the sensing system aspect of the invention.

In accordance with at least one set of embodiments, the method may comprise processing signals received by the antenna to derive physiological parameter information.

In accordance with one or more embodiments, the normalized radial frequency {circumflex over (ω)} of the electromagnetic excitation signals may be from 0.025 to 0.25.

Any of the options or embodiments outlined above in relation to the sensing system aspect of the invention may also be advantageously applied or incorporated into the present method aspect of the invention.

