WO2006111877A1

WO2006111877A1 - Method and apparatus for inductively measuring the bio-impedance of a user’s body

Info

Publication number: WO2006111877A1
Application number: PCT/IB2006/051026
Authority: WO
Inventors: Eberhard Waffenschmidt; Claudia Hannelore Igney; Andreas Brauers; Elke Naujokat
Original assignee: Philips Intellectual Property and Standards GmbH; Koninklijke Philips Electronics NV
Current assignee: Philips Intellectual Property and Standards GmbH; Koninklijke Philips NV
Priority date: 2005-04-19
Filing date: 2006-04-04
Publication date: 2006-10-26
Anticipated expiration: 2007-10-19
Also published as: JP2008536606A; CN101160093A; EP1874185A1

Abstract

The present invention relates to a method and apparatus for inductively measuring the bio-impedance of a user' s body. In order to provide a reliable method and apparatus for inductively measuring the bio-impedance of a user's body (14) it is suggested to use the capacitor (5) of the measuring circuit as a resonant capacitive shield for electrically shielding the sensor inductor (2) of the circuit. Therewith the number of structural components is reduced. Thus also the number of possible failures is reduced, leading to a very reliable technique for measuring of bio-impedance.

Description

Method and apparatus for inductively measuring the bio-impedance of a user' s body

The present invention relates to a method and apparatus for inductively measuring the bio-impedance of a user's body.

The inductive measurement of bio-impedance is a known method to determine various vital parameters of a human body in a non-contact way. The operating principle is the following: Using an inductor loop, an alternating magnetic field is induced in a part of the human body. This alternating magnetic field causes eddy currents in the tissue of the body. Depending on the type and conductivity of tissue, the eddy currents are stronger or weaker. The eddy currents cause losses in the tissue, which can be measured as a decrease of the quality factor of the inductor loop. They also cause a secondary magnetic field, which can be measured as an induced voltage in a second inductor loop or using compensated sensors e.g. gradiometers. A fairly simple setup of an apparatus for measuring the bio-impedance comprises a sensor coil as an inductor that induces the magnetic field in the human body. Measuring of the losses of the sensor coil gives information about the bio-impedance. The inductive measurement of the bio -impedance has been shown to allow the non-contact determination of several parameters, e.g. breath action and depth, heart rate and change of the heart volume and blood glucose level, as well as fat or water content of the tissue.

As a problem, the alternating voltage, that is applied to the sensor coil leads to capacitive common mode currents through the surrounding. These currents may deviate the measurement results. Therefore, a capacitive shield may be applied above and below the sensor coil. In existing solutions, the capacity between the sensor coil and the shield is minimized in order to minimize unwanted parasitic currents from the sensor coil to the shield. This is achieved by keeping the shield as far away as possible from the sensor coil.

It is an object of the present invention to provide a reliable method and apparatus for inductively measuring the bio-impedance of a user' s body.

This object is achieved according to the invention by an apparatus for inductively measuring the bio-impedance of a user' s body, the apparatus comprising an inductor adapted to induce an alternating magnetic field in the user' s body, and further comprising a resonant circuit adapted to measure electrical losses in the user's body, the resonant circuit comprising the inductor and a resonant capacitor, wherein the resonant capacitor is adapted so serve as a capacitive shield for electrically shielding the inductor.

The object of the present invention is also achieved by a method of inductively measuring the bio-impedance of a user's body, the method comprising the steps of inducing an alternating magnetic field in the user's body by means of an inductor, and measuring electrical losses in the user's body by means of a resonant circuit, said resonant circuit comprising the inductor and a resonant capacitor, wherein the resonant capacitor serves as a capacitive shield for electrically shielding the inductor.

A basic idea of the present invention is to use the resonant capacitor as a capacitive shield for electrically shielding the inductor. Therewith the number of structural components is reduced. Thus also the number of possible failures is reduced, leading to a very reliable technique for measuring of bio-impedance. With the present invention the size, the reliability and the sensitivity of a contact-less bio-impedance measuring system can be improved, thus allowing an easy and comfortable diagnosis of vital parameters like heart rate, tissue water content or blood glucose level to supervise a user without the need of applying any kind of devices to the user' s body. These and other aspects of the invention will be further elaborated on the basis of the following embodiments, which are defined in the dependent claims.

According to a preferred embodiment of the invention a single inductor is used for inducing a magnetic field and for measuring. This raises the problem to separate the small measurement signal from the large current needed to induce the magnetic field. One way to solve this problem is to distinguish the real and imaginary part of the inductor current. The imaginary part is used to induce the magnetic field, while the real part is attributed to losses. Parts of the losses are the losses in the tissue to be detected. The imaginary part of the current in the inductor is by far the largest contribution to the total current amplitude.

Real and imaginary part can be separated in a simple and reliably way by using a parallel resonant circuit. Operating at the resonance frequency, the capacitor current inherently compensates the inductive currents in the inductor, such that only the real part of the current flows externally of the resonant circuit. This way, the large imaginary part of the current, which does not contribute to the measurements, is eliminated such that it does not appear outside of the resonant circuit.

It was found, that the resonant capacitor is preferably located close to the sensor inductor in order to avoid additional losses in the interconnections and influences of additional parasitics. Using the capacitive shield as a parallel resonant capacitor allows to integrate the sensor inductor and the resonant capacitor by using the parasitic capacity between the sensor inductor and the shield. This also allows a close placement to the sensor inductor. Thus, according to a preferred embodiment of the invention, the resonant capacitor is positioned in the immediate vicinity of the sensor inductor. In other words the shield is very close to the sensor inductor such that it forms a resonant circuit to allow resonant measuring. This way the unwanted parasitic capacity between the sensor inductor and the shield is turned into a well-defined value used for measuring bio- impedance.

To shield the sensor inductor capacitively, the resonant capacitor is preferably positioned between the user's body and the inductor. This arrangement allows the best possible shielding results.

If a capacitive shield is applied to the sensor inductor, eddy currents may be induced in the shield. These eddy currents might deviate the measurement results. Thus, in another embodiment of the invention, the resonant capacitor comprises a number of non-conductive areas, said areas being arranged orthogonally to the conductor paths of the inductor, thereby avoiding induced eddy currents. Preferably the resonant capacitor is structured and the non-conductive areas are slits in the body of the resonant capacitor. To keep the structured parts of the shield on the same electrical potential, preferably all parts of the shield are interconnected.

The positioning of the resonant capacitor close to the inductor not only tolerates the parasitic currents between the sensor coil and the shield. Additionally a quasi-planar structural shape of the apparatus can be achieved. For this purpose the resonant capacitor preferably exhibits the shape of a layer, enabling a very small installation height. The same result is achieved, if the inductor is planar. In a preferred embodiment of the invention the inductor is integrated into a substrate, preferably made of an insulating material. In this case the apparatus can be manufactured using printed circuit board technology. According to another preferred embodiment the resonant capacitor, preferably in form of a layer, is integrated into the substrate of the sensor inductor. Thus the installation height of the apparatus can be reduced. Preferably the inductor and the capacitive shield are made as laminated copper layers in a printed circuit board, similar to multilayer printed circuit boards used for electronic circuits. Thereby the substrate is preferably made of printed circuit board material due to cost reasons.

According to still another embodiment of the invention, the apparatus comprises at least one additional electronic circuit integrated into or on the substrate. Placing the additional electronic circuit beside the sensor inductor, the same copper layers may be used for the interconnections of the electronic circuits and for the sensor conductor with its shielding. If the electronic circuit is located on top of the sensor inductor, a very small-area apparatus can be obtained. Preferably the copper interconnections are then made from additional copper layers laminated on top of the sensor inductor.

In some applications a very high resonant capacity may be needed. Then for the substrate between the inductor and the resonant capacitor a substrate material is preferably used which exhibits an enhanced dielectric constant. As an example, the material C-Lam, offered by the material manufacturer Isola in Dϋren, Germany, or an equivalent material is use. It is also possible to use ceramic substrates with significantly enhanced dielectric constant.

In another preferred embodiment of the invention the substrate exhibits a flexible structure. In this way, the sensor inductor can be integrated in cloths or fabric or in a bed sheet. As materials e.g. flex foil or a polyamide material can be used. If the inductor and/or the resonant capacitor exhibits a flexible structure, the complete apparatus can be integrated in wearables. Preferably a woven or stitched inductor coil made from thin, insulated wire in the fabric and a further stitched or woven layer on top of it used as capacitive shield.

The capacitive shielding may not only be provided on one side of the inductor (preferably the side that directs to the user's body). In another embodiment of the invention a second resonant capacitor is provided, wherein the two resonant capacitors are positioned on both sides of the inductor. Particularly if additional electronic circuits are positioned near the sensor inductor, the second resonant capacitor is used to shield the sensor inductor capacitively from those electronic circuits. In other words, if a planar design is used, the inductor is preferably surrounded by a double layer shielding. In that case it may even improve the shielding effect, if the top and the bottom shield are connected to each other at the outside of the sensor inductor.

In another embodiment of the invention the second resonant capacitor, which is not positioned between the user's body and the inductor, is made of a softmagnetic material, e.g. Mumetal. In case an additional electronic circuit is located near the sensor inductor, such an softmagnetic shielding leads to a very good shielding effect. Softmagnetic material effectively shields magnetic fields, so that magnetic and electric fields are shielded from the electronic circuit. In addition, the softmagnetic capacitor, which again may be provided in form of a layer, enhances the inductivity of the sensor inductor and thus increases the magnetic field in the sensing area. The softmagnetic shielding layer has to be shaped in a similar way as the non-magnetic layer in order to avoid induced eddy currents in the material.

In yet another embodiment of the invention the apparatus is adapted such, that a change of the resonance frequency corresponds to the distance to the user' s body. In this case the apparatus can be used for distance measuring. These and other aspects of the invention will be described in detail hereinafter, by way of example, with reference to the following embodiments and the accompanying drawings; in which:

Fig. 1 is a cross sectional view of a planar resonant bio-impedance sensor with integrated resonant capacitive shielding layers,

Fig. 2 is a schematic picture of a planar spiral winding of a bio- impedance sensor,

Fig. 3 is a schematic picture of a first capacitive shielding layer, Fig. 4 is a schematic picture of a second capacitive shielding layer, Fig. 5 is a schematic arrangement of a planar resonant bio-impedance sensor with integrated resonant capacitive shielding,

Fig. 6 is a circuit equivalent to the schematic arrangement of fig. 5 Fig. 7 is a schematic arrangement of a planar resonant bio-impedance sensor with integrated resonant capacitive shielding, adapted for distance detection,

Fig. 8 is a circuit equivalent to the schematic arrangement of fig. 7

Fig. 1 shows a planar bio-impedance sensor 1. Such a sensor 1 is preferably used in a contact less medical diagnostic system that measures inductively the bio-impedance of a user's body. The bio-impedance sensor 1 comprises a single inductor coil 2, which is adapted to induce an alternating magnetic field in a user's body 14, see Fig. 5. The inductor 2 shows the form of a planar spiral copper winding, see Fig. 2. For operating the system the centre pin 3 of the inductor 2 is connected to ground 12 and the outer pin 4 is connected to an AC voltage source 13, see Fig. 5.

The bio-impedance sensor 1 further comprises a parallel resonant circuit adapted to measure electrical losses in the user's body 14, the resonant circuit comprising the inductor 2 (for serving as sensor inductor) and a first resonant capacitor 5, wherein the first resonant capacitor 5 is adapted so serve as a capacitive shield for electrically shielding the inductor 2. For this purpose the first resonant capacitor 5 is positioned between the user's body and the inductor 2. On the opposite side of the inductor 2 a second resonant capacitor 6 is located. The second capacitor is also adapted so serve as a capacitive shield for electrically shielding the inductor 2. Both the inductor 2 and the capacitors 5, 6 are provided as laminated copper layers. Both are integrated into a planar substrate 7, made of printed circuit board material, such that a monolithic member is formed. The capacitors 5, 6 show a slightly larger diameter as the inductor 2.

The first and the second capacitor 5, 6 are positioned in the immediate vicinity of the inductor 2 such that it forms a resonant circuit to allow resonant measuring. Both capacitors 5, 6 are connected to each other at the outside of the inductor 2 by means of copper tracks 8. Each capacitor 5, 6 comprises a number of slits 9 arranged orthogonally to the conductor paths 10 of the inductor 2. In other words the structure of the capacitors 5, 6 comprises radial copper stripes separated by radial slits 9 to avoid eddy currents. However, all parts of the capacitors are interconnected. A first embodiment of such capacitors 5 comprising an outside connection design is shown in Fig. 3. Thereby a number of capacitive stripes are interconnected at the outer edge of the capacitor 5 except on one position. A second embodiment of such capacitors 5' comprising an centre connection design is shown in Fig. 4. To achieve a parallel resonant circuit, the capacitor 5, 6 is connected to one of the connections of the inductor, as shown in Figs. 5 and 6. To achieve a good shielding effect to the surrounding one pin 11 of the inductor 2 is connected to ground connection 12 of the driving AC voltage source 13. The capacitor 5 is connected to the same pin 11, i.e. the capacitor 5 is connected to the ground pin of the inductor 2. The capacitor 5 is located between the user's body 14, which is to be examined, and the inductor 2. There is no potential difference between the capacitor 5 and the ground layer 12 of the voltage source 13 and thus no common mode currents are induced.

According to another embodiment (not shown), the capacitive area is increased by enlarging the track width of one or more dedicated turns to enhance the capacity of the system. The highest effect of capacity increase is reached by enlarging that turn, which is connected to the AC voltage source 13. In order to avoid eddy currents, slits are provided in that turn.

If the inductor 2 and the capacitor 5 are not interconnected, a geometric capacity C_GEO between both can be measured according to the classical plate capacitor equation:

/ d, where So is the electric field constant (βo = 8.8542 10^"12 As/(Vm) ), ε_r is the dielectric constant of the substrate material, A is the area where the conductor winding and the capacitive shield face each other and d is the vertical distance between the winding and the shield.

If the capacitor 5 is connected to one of the pins of the inductor 2 a parallel capacity to the inductor coil is obtained. This effective value C_p is smaller than the geometric capacity C_GEO- Assuming an equal induced voltage in each turn and an equal turn length of the different turns of the winding, the following equation gives a good approximation of the effective capacity:

3. A more precise calculation method may take into account a varying turn length for a planar winding.

In a further embodiment the capacitor 5 is not connected to the ground pin of the inductor 2 but to the AC- voltage pin 15. In this case a capacitive path from the capacitor 5 through the user's body 14 to the ground plane 12 of the voltage source 13 is provided, as illustrated in Fig. 7. This capacity C_BODY depends on the distance of the capacitor 5 to the user's body 14. As Fig. 8 shows, the capacity is located in parallel to the sensing resonant circuit. A change of this capacity will change the resonance frequency of the resonant circuit. Thus, the resonance frequency will change, if the distance to the user's body changes. The variation of the bio-impedance, however, will only change the quality factor and the amplitude in resonance, but not the resonance frequency. Therefore, a change of the resonance frequency can be attributed to the distance between the sensor and the user's body. The losses in the user's body, which are measured to determine the user' s bio-impedance, depend on the distance between the sensor and the user's body. By using a distance information, motion artefacts during the measurement can be compensated by means of an appropriate compensation system.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. It will furthermore be evident that the word "comprising" does not exclude other elements or steps, that the words "a" or "an" do not exclude a plurality, and that a single element, such as a computer system or another unit may fulfil the functions of several means recited in the claims. Any reference signs in the claims shall not be construed as limiting the claim concerned. REFERENCE LIST

1 bio-impedance sensor

2 inductor

3 center pin

4 outer pin

5 first capacitor

6 second capacitor

7 substrate

8 interconnection

9 slit

10 conductor path-

11 contact pin

12 ground

13 AC voltage source

14 User' s body

15 voltage pin

Claims

CLAIMS:

1. An apparatus (1) for inductively measuring the bio-impedance of a user's body (14), the apparatus (1) comprising

- an inductor (2) adapted to induce an alternating magnetic field in the user's body (14), and - a resonant circuit adapted to measure electrical losses in the user's body (14), the resonant circuit comprising the inductor (2) and a resonant capacitor (5), wherein the resonant capacitor (5) is adapted so serve as a capacitive shield for electrically shielding the inductor (2).

2. The apparatus (1) as claimed in claim 1, wherein the resonant capacitor

(5) is positioned in the immediate vicinity of the inductor (2).

3. The apparatus (1) as claimed in claim 1, wherein the resonant capacitor (5) is positioned between the user's body (14) and the inductor (2).

4. The apparatus (1) as claimed in claim 1, wherein the resonant capacitor (5) comprises a number of non-conductive areas, said areas being arranged orthogonally to the conductor paths of the inductor (2).

5. The apparatus (1) as claimed in claim 4, wherein the non-conductive areas are slits (9) in the body of the resonant capacitor (5).

6. The apparatus (1) as claimed in claim 1, wherein the resonant capacitor (5) exhibits the shape of a layer.

7. The apparatus (1) as claimed in claim 1, wherein the inductor (2) is planar.

8. The apparatus (1) as claimed in claim 1, wherein the inductor (2) is integrated into a substrate (7).

9. The apparatus (1) as claimed in claim 8, wherein the resonant capacitor

(5) is integrated into the same substrate (7).

10. The apparatus (1) as claimed in claim 8, wherein the substrate (7) is made of insulating material.

11. The apparatus (1) as claimed in claim 8, comprising at least one additional electronic circuit integrated into or on the substrate (7).

12. The apparatus (1) as claimed in claim 8, wherein the substrate (7) exhibits a flexible structure.

13. The apparatus (1) as claimed in claim 8, wherein the substrate material between the inductor (2) and the resonant capacitor (5) exhibits an enhanced dielectric constant.

14. The apparatus (1) as claimed in claim 1, wherein the inductor (2) and/or the resonant capacitor (5) exhibits a flexible structure.

15. The apparatus (1) as claimed in claim 1, comprising a second resonant capacitor (6), wherein the two resonant capacitors (5, 6) are positioned on both sides of the inductor (2).

16. The apparatus (1) as claimed in claim 15, wherein the resonant capacitor (6), which is not positioned between the user's body (14) and the inductor (2), is made of a softmagnetic material.

17. Use of an apparatus (1) as claimed in claim 1 for distance measuring.

18. A method of inductively measuring the bio-impedance of a user' s body (14), the method comprising the steps of: - inducing an alternating magnetic field in the user's body (14) by means of an inductor (2),

- measuring electrical losses in the user' s body (14) by means of a resonant circuit, said resonant circuit comprising the inductor (2) and a resonant capacitor (5), wherein the resonant capacitor (5) serves as a capacitive shield for electrically shielding the inductor (2).