WO2011054548A1

WO2011054548A1 - Respiration measurement sensor

Info

Publication number: WO2011054548A1
Application number: PCT/EP2010/058703
Authority: WO
Inventors: Varun Akur Venkatesan; Praveen Kumar Vangali
Original assignee: Siemens Aktiengesellschaft
Priority date: 2009-11-05
Filing date: 2010-06-21
Publication date: 2011-05-12

Abstract

A sensor for measuring a respiration of a user, wherein the sensor comprises a capacitor comprising of a first capacitor plate and a second capacitor plate, the capacitor plates being coupled to a skin of the user such that a movement of the skin during the respiration changes a relative arrangement of the first capacitor plate with respect to the second capacitor plate to vary the capacitance of the capacitor and a means to generate an output signal dependent on the capacitance is disclosed. The change in the relative arrangement of first capacitor plate and the second capacitor plate is a change in a distance between the first capacitor plate and the second capacitor plate.

Description

Respiration measurement sensor The present invention relates to a sensor for measuring respiration of a user.

It is known in the prior art to measure respiration, and other body parameters characterized by mechanical movement, with devices mechanically coupled to the body. In most cases the respiration monitoring is based on an impedance

measurement of the patient's thorax. However, other measuring methods are also known, e.g., measuring the temperature variations of the respiration air or pressure variations between body and support or mechanical ventilation or the like .

Such a system is known from U.S. Pat. No. 6,840,907, which discloses a respiratory analysis system for monitoring a respiratory variable of a patient. The system includes an array of sensors attached to the body of the patient for measuring respiratory movement at different locations of the patient's body to generate a set of independent respiratory movement signals. Further, these signals are processed to derive a respective breathing pattern. Disadvantageously, the deployment of the system causes discomfort to the patient as the sensor harness herein induces constraints in physical movement . Another example is shown in U.S. Pat. No. 6,416,471, which discloses a system and method for monitoring vital signs and capturing data from a patient remotely. This includes a cordless sensor band with sensors and transmission circuitry for the detection and transmission of vital signs data.

However, the sensor band is designed to work for only limited period of time, for example 24-30 hours, after which it need to be discarded and replaced by a new sensor band, which in turn incurs cost. The object of the invention is to provide a means which can be utilized in a flexible and cost-effective way for

measuring the respiratory function of an individual. The above object is achieved by a sensor according to claim 1 and a system according to claim 16.

The invention is set forth and characterized in the main claim, while the dependent claims describe preferred

embodiments of the invention.

The coupling of the capacitor to the skin of the user enables accurate sensing of the forces and the movement caused by the user's respiration. The sensor uses only simple circuitry to measure the respiration of the user corresponding to the change in the capacitance of the capacitor. The sensor herein is compact in design and is inexpensive as for system implementation standard components like known variable capacitors is used. The sensor provides flexibility to the user as it does not require any additional harness for its functioning, which may cause discomfort to the person.

According to an embodiment herein, the first capacitor plate and the second capacitor plate are coupled to the skin via a base member fixed onto the skin. This facilitates

transmission of the fluctuations caused during respiration from the skin to the capacitor plates.

According to a preferred embodiment, the base member is an adhesive plaster. This provides for easy attachment or removal of the sensor from the body of the user.

According to a preferred embodiment, the change in the relative arrangement of first capacitor plate and the second capacitor plate is a change in the distance between the first capacitor plate and the second capacitor plate. This varies the capacitance of the capacitor proportional to the

respiration motion. According to another preferred embodiment, the first

capacitor plate and the second capacitor plate are connected to a spacing means which holds the capacitor plates at a tension, wherein the capacitor plates are coupled to the skin such that the movement of the skin exerts a force against the tension to change the distance between the first capacitor plate and the second capacitor plate. This helps to provide the sufficient flexibility to vary the distance between the capacitor plates. According to another preferred embodiment, further comprises a connecting means attached to the first capacitor plate and the second capacitor plate, wherein the connecting means is coupled to the skin in opposite sides of the first capacitor plate and the second capacitor plate such that the movement of the skin exerts forces working from opposite directions on the capacitor plates. This helps to equally distribute the force among both the capacitor plates such that the capacitor plates get closer. According to another preferred embodiment, the sensor further comprises a transmission means attached to the first

capacitor plate, wherein the transmission means is coupled to the skin through the base member such that the transmission means transmits a force provided by the movement of the skin to the first capacitor plate to reduce the distance between the capacitor plates.

According to another preferred embodiment, the sensor

comprises a flexible member, wherein the ends of the flexible member is attached to the first capacitor plate and the second capacitor plate, wherein the first supporting means and the second supporting means is coupled to the flexible member such that a movement of the skin exerts a tension on the coupling to change the distance between the first

capacitor plate and the second capacitor plate.

According to yet another preferred embodiment, the first capacitor plate and second capacitor plate is arranged relatively parallel to the skin, wherein the first capacitor plate is coupled to the skin such that the respiratory movement of the skin pushes the first capacitor plate towards the second capacitor plate. This results in a reduction of distance between the capacitor plates, thereby changing the capacitance.

According to yet another preferred embodiment, the change in the relative arrangement of first capacitor plate and the second capacitor plate is a movement of at least one

capacitor plate in a direction parallel to a surface of the capacitor. This causes a change in the capacitance of the capacitor .

According to another preferred embodiment, the first

capacitor plate and the second capacitor plate is flat. This is the most commonly used as it acquires very less surface area .

According to another preferred embodiment, the first

capacitor plate and second capacitor plate is spiral.

According to yet another preferred embodiment, the sensor includes a separation means placed between the first

capacitor plate and the second capacitor plate. This prevents the first capacitor plate and the second capacitor plate from contacting each other.

According to yet another preferred embodiment, the sensor further comprises an antenna to receive an electromagnetic sender signal, wherein the antenna is electronically linked to the capacitor such that an electromagnetic resonance frequency of the antenna depends on the capacitance of the capacitor. The resonant frequency is indicative of the pressure applied on the base member due to the respiration. This in turn helps to detect the mode of respiration.

According to yet another preferred embodiment, the antenna is adapted to receive an electromagnetic sender signal, wherein the antenna is electronically linked to the capacitor such that an electromagnetic resonance frequency of the antenna depends on the capacitance of the capacitor. This helps to receive the signal at any sender frequency with no signal losses .

According to yet another preferred embodiment, the sensor comprises an energy storage adapted to store energy dependent on the electromagnetic sender signal received by the antenna, wherein the signal strength depends on the amount of energy saved in the energy storage. This eliminates the need for any external power supply means thereby reducing the cost

involved in timely replacement or maintenance.

Another aspect of the invention includes a respiratory monitoring system comprising a sensor and a transceiver adapted to transmit the electromagnetic sender signal to the sensor and to receive the electromagnetic response signal from the sensor. This simplifies the detection process as the hardware and/or software in the monitoring system is simple with no complex components.

According to yet another preferred embodiment, the

transceiver is adapted to vary the frequency of the

electromagnetic sender signal . This helps to determine the exact frequency at which the strength of the signal attains a higher value.

According to yet another preferred embodiment, the

transceiver comprises a signal measuring means adapted to measure strength of the electromagnetic response signal and a computing means adapted to determine relative maxima in the strength of the electromagnetic response signal. This helps to detect not only a normal breathing but also an irregular breathing having a frequency different from that of the normal breathing.

According to yet another preferred embodiment, the computing means is adapted to determine the respiratory cycle based on the intervals between the extrema. This helps to continuously measure the respiration of the user with high accuracy.

According to yet another preferred embodiment , the computing means is adapted to determine the resonance frequencies of the capacitor based on the frequency of the electromagnetic sender signal for which the strength of the electromagnetic response signal reaches the relative maxima.

According to yet another preferred embodiment, the computing means is adapted to determine the respiratory cycle based on the development of the resonance frequency over time. This helps to find the breath pattern of the patient. The present invention is further described hereinafter with reference to illustrated embodiments shown in the

accompanying drawings, in which: FIG 1 illustrates a schematic view of a sensor for

measuring the respiration of a user according embodiment of the invention;

FIG 2 illustrates a schematic circuit diagram of a

capacitor for use with the device of FIG. 1 ;

FIG 3 illustrates an alternative arrangement of the

capacitor of the sensor of FIG. 1 ; FIG 4 illustrates an alternative arrangement of the

capacitor of the sensor of FIG. 1 ;

FIG 5 shows an exemplary illustration of an arrangement of the capacitor of the sensor of FIG. 1 ;

FIG 6A-6B is an exemplary illustration of an alternative

arrangement of the capacitor of the sensor of FIG.1; FIG 7 is an exemplary illustration of an alternative

arrangement of the capacitor of FIG.l;

FIG 8 illustrates functioning of the capacitor of the sensor of FIG.l;

FIG 9 illustrates a block diagram of a respiratory

monitoring system according to an embodiment of the invention ; FIG 10A-10E illustrates an explanatory diagram showing variations in capacitance and frequency induced during respiration; FIG. 11 is a graph illustrating the computed power of the electromagnetic response signal versus frequency of the electromagnetic sender signal in accordance with an embodiment of the invention; and FIG. 12 is a graph explaining the periodicity of the

extrema of the electromagnetic response signal.

Various embodiments are described with reference to the drawings, wherein like reference numerals are used to refer to single elements throughout. In the following description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be evident that such embodiments may be practiced without these specific details.

The present invention thus provides a system that senses the body movements of a person to determine parameters of respiratory-related functions. The aforementioned parameters can be used to diagnose a range of respiratory disorders.

Referring to FIG.l of the drawings, illustrates a schematic view of a sensor 10 for measuring the respiration of a user in accordance with an embodiment of the invention. The sensor 10 includes a capacitor 12 comprising of a first capacitor plate 14 and a second capacitor plate 16. The first capacitor plate 14 and the second capacitor plate 16 are mechanically coupled to the skin 17 of the user via a base member 18 fixed onto the skin 17. The base member 18 herein can be a flexible bandage such as a plaster which can be attached to the body of the user. The base member 18 includes an adhesive on one side which helps to easily attach to the body or to remove the base member 18 from the body of the user at a location where the respiration can be observed.

Here, the expansion and contraction of the skin 17 during the respiration causes the base member 18 to expand and contract simultaneously, thereby mechanically transferring the

respiration motion to the base member 18. The capacitor plates 14, 16 is coupled to the base member 18 such that a movement of the skin 17 during the respiration changes a relative arrangement of the first capacitor plate 14 with respect to the second capacitor plate 16 to vary the

capacitance of the capacitor 12. Here, the change in the relative arrangement of first capacitor plate 14 and the second capacitor plate 16 is a change in a distance between the first capacitor plate 14 and the second capacitor plate 16.

The first capacitor plate 14 and second capacitor plate 16 is connected to the spacing means 20 which hold the plates 14, 16 at a tension. The spacing means 20 is coupled to the base member 18 such that the coupling works against the tension during the respiration to change the distance between the first capacitor plate 14 and second capacitor plates 16.

The first capacitor plate 14 and second capacitor plate 16 are in turn connected to a connecting means 22. the

connecting means 22 is coupled to the skin 17 in opposite sides of the first capacitor plate 14 and the second

capacitor plate 16 such that the movement of the skin 17 exerts forces working from opposite directions on the capacitor plates 14, 16. This cause the capacitor plates 14, 16 to come closer thereby changing the capacitance of the capacitor 12. The sensor 10 further includes an antenna 30 to transmit and receive electromagnetic signals. The antenna 30 is

electronically linked to the capacitor 12 such that an electromagnetic resonance frequency of the antenna 30 depends on the capacitance of the capacitor 12.

The sensor 10 further comprises an energy storage 32 adapted to store energy from the electromagnetic sender signal 34 for the operation of the sensor 10. The antenna 30 is adapted to transmit the output signal dependent on the capacitance of the capacitor 12. The antenna 30 transmits the output signal in the form of an electromagnetic response signal 36 with signal strength dependent on the capacitance of the capacitor 12. The signal 36 strength depends on the amount of energy saved in the energy storage 32.

FIG 2 illustrates a schematic circuit diagram of a capacitor 12 for use with the sensor 10 of FIG. 1. The capacitor 12 includes a first capacitor plate 14 and a second capacitor plate 16. The two ends of the first capacitor plate 14 and the second capacitor plate 16 is connected to a spacing means 20. The spacing means 20 is arranged so as to hold the first capacitor plate 14 and the second capacitor plate 16 apart at a tension. The capacitor 12 arrangement further comprises a connecting means 22. The connecting means 22 holds the ends of the first capacitor plate 14 and the second capacitor plate 16

together. The connecting means 22 can be for instance wires, co-axial cables, springs or the like. The connecting means 22 is further coupled to the base member 18 through the spacing means 20.

The connecting means 22 is coupled to the skin 17 in opposite sides of the first capacitor plate 14 and the second

capacitor plate 16 such that the movement of the skin 17 exerts forces working from opposite directions on the first capacitor plate 14 and second capacitor plate 16 to change the distance between the capacitor plates 14,16. As the capacitor plates 14, 16 are held by the spacing means 20 which is flexible, it makes it easy to transfer the force generated based on the pull from the connecting means 22 during respiration to the capacitor plates 14, 16. This in turn varies the capacitance and hence the tuning frequency of the capacitor 12.

The capacitor 12 is further provided with a separation means 28 between the first capacitor plate 14 and the second capacitor plate 16. The separation means 28 prevents the capacitor plates from touching each other when they are pulled closer.

The capacitor 12 herein is shaped, sized and contoured to substantially match the planar surface of the base member 18. In the preferred embodiment, the capacitor plates 14, 16 includes a strip of copper, silver or gold or other

conductive materials that may be used to form the capacitor plates 14,16. Alternatively, the capacitor plates 14, 16 may be etched from a copper-clad substrate or screened and fired using thick-film techniques, using procedures well known for the fabrication of printed circuits.

FIG 3 illustrates an alternative arrangement of the capacitor capacitor plate 14 and the second capacitor plate 16 are connected to a connecting means 22. The capacitor plates 14, 16 are spaced apart at a certain tension by the spacing means 20.

The connecting means 22 herein is further coupled to the base member 18 of the sensor 10 attached to the skin 17 of the user. The respiration motion of the user causes expansion of the skin 17. This move of the skin 17 causes a sideward pulling of the connecting means 24 in a direction parallel to the capacitor 12. This sideward pulling exerts a force which works against the tension at which the capacitor plates 14, 16 are held. The sideward pulling of the connecting means 22 forces the first capacitor plate 14 and the second capacitor plate 16 to come closer, thereby reducing the distance between the plates 14, 16. The reduction of distance in turn changes the capacitance of the capacitor 12.

FIG 4 illustrates another arrangement of the capacitor 12 of the sensor 10 of FIG.l. The first capacitor plate 14 and the second capacitor plate 16 are arranged horizontally parallel to each other. The connecting means 22 attached to each of the capacitor plate 14, 16 holds the capacitor plates 14, 16 at a tension certain distance apart. As shown in FIG. 4, a transmission means 26 is attached to each of the first capacitor plate 14 and the second capacitor plate 16.

The transmission means 26 is coupled to a base member 18. The first capacitor plate 14 and the second capacitor plate 16 are coupled to the base member 18 through the transmission means 26 and the connecting means 22. Here the connecting means 22 and the transmission means 26 can be a flexible body such as a string or a spring attached to the base member 18. The expansion of the skin 17 during respiration exerts a stretching force on the transmission means 26 coupled to the flexible member 24. The transmission means 26 attached to the first capacitor plate 14 pushes the first capacitor plate 14 towards the second capacitor plate 16. The force exerted in opposing directions causes the first capacitor plate 14 and the second capacitor plate 16 to come closer, thereby reducing the distance between the capacitor plates. This in turn changes the capacitance and the tuning frequency of the capacitor 12.

Alternatively, the transmission means 26 attached to the second capacitor plate 16 pulls forward the second capacitor plate 16 towards the first capacitor plate 14. This also reduces the distance between the capacitor plates 14, 16.

FIG 5 shows an exemplary illustration of the capacitor 12 of the sensor 10 of FIG.l. The first capacitor plate 14 and the second capacitor plate 16 is held apart at a tension using the spacing means 20. The first capacitor plate 14 and the second capacitor plate 16 are further coupled to a flexible member 24 which functions as a connecting means.

The flexible member 24 can be a flexible elastic band. The expansion of the skin 17 during respiration exerts a

stretching force on the elastic band coupled to the skin. This in turn causes pushes the first capacitor plate 14 towards the second capacitor plate 16. Here the capacitance of the capacitor changed by the movement of at least one of the capacitor plate towards the other thereby reducing the distance between the plates 14, 16.

FIG 6A-6B is an exemplary illustration of an alternative arrangement of the capacitor 12 of the sensor 10 of FIG.l. The figure 6A shows the outer surface of the skin 17 on which the sensor 10 is placed. Here the first capacitor plate 14 and second capacitor plate 16 is arranged relatively parallel to each other on the skin 17 and are mechanically coupled to the base member 18 using a connecting means 22.

The expansion of the skin 17 during a respiration motion exerts a stretch on the base member 18 which induce a tension on the coupling. The stretching pushes the second capacitor plate 16 towards the first capacitor plate 14 as shown in

FIG. 6B . This causes to reduce the distance between the first capacitor plate 14 and the second capacitor plate 16.

FIG 7 is an exemplary illustration of an alternative

arrangement of the capacitor 12 of FIG.l. Here the first capacitor plate 12 and the second capacitor plate 16 are arranged in a spiral form as shown in FIG. 7. The first capacitor plate 14, the second capacitor plate 16 and the gap between the plates and the base member 18 forms a capacitor having a characteristic capacitance.

One end of each of the first capacitor plate 14 and the second capacitor plate 16 is attached to a connecting means 22 such as a string. The end of the first capacitor plate 14 and the second capacitor plate 16 is thus coupled to the base member 18 through the connecting means 22.

The gap between the two spiral capacitor plates 14, 16 are provided with a separation means 22. The separation means 22 prevents the physical contact of the first capacitor plate 14 and the second capacitor plate 16 during the sensor 10 operation. The separation means 22 can be for example sponge or any similar non-conductive material. The respiration motion causes a stretching of the base member 18 which in turn pulls the first capacitor plate 14 and the second capacitor plate 16 closer. This reduces the gap between the plates thereby changing the capacitance of the capacitor 12.

FIG 8 illustrates functioning of the capacitor 12 of the sensor 10 of FIG.l. The first capacitor plate 14 and the second capacitor plate 16 are arranged flat on the base member 18. The first capacitor plate 14 and the second capacitor plate 16 are mechanically coupled to the skin 17 of the user such that a force generated due to the movement of the skin 17 during respiration is transferred to the

capacitor 12.

The movement of the skin 17 during the respiration thus changes a relative arrangement of the first capacitor plate 14 with respect to the second capacitor plate 16 to vary the capacitance of the capacitor 12. The change in the relative arrangement of first capacitor plate 14 and the second capacitor plate 16 is a movement of at least one capacitor plate in a direction parallel to a surface of the capacitor as shown in the FIG.8. Here the sideward shift of at least one capacitor plate in a direction parallel to the other capacitor plate results in a change in the capacitance of the capacitor.

FIG 9 illustrates a block diagram of a respiratory monitoring system 38 according to an embodiment of the invention. The respiratory monitoring system 38 of FIG. 9 comprises a sensor 10 and a transceiver 40 adapted to transmit an

electromagnetic sender signal 34 to the sensor 10 and to receive the electromagnetic response signal 36 from the sensor 10.

The transceiver 40 is adapted to transmit the electromagnetic sender signal 34 at a particular frequency in a direction of the user's body. The antenna 30 of the sensor 10 receives the electromagnetic sender signal 34 at the transmitted

frequency. The sensor 10 analyzes the power at which the electromagnetic sender signal 34 is received. When the frequency of the electromagnetic sender signal 34 received at the sensor matches with the tuning frequency of the capacitor 12, resonance occurs and the power of the electromagnetic sender signal 34 is the highest. The energy storage 32 of the sensor 10 then stores power for the operation of the sensor 10.

The power stored in the energy storage 32 is used to transmit the electromagnetic response signal 36 to the transceiver 40. The strength of the electromagnetic response signal 36 is thus proportional to the amount of energy stored in the energy storage 32.

The transceiver 40 includes a signal measuring means 42 adapted to measure strength of the electromagnetic response signal 36. The transceiver 40 further includes a computing means 44 adapted to determine adapted to determine relative maxima in the strength of the electromagnetic response signal. The computing means 44 is adapted to determine the resonance frequencies of the capacitor 12 based on the frequency of the electromagnetic sender signal 34 for which the strength of the electromagnetic response signal 36 reaches the relative maxima. The computing means 44 further determine the respiratory cycle based on the intervals between the maxima. The

respiratory cycle is determined based on the development of the resonance frequency over time. The computing means 44 analyzes the cycles of the electromagnetic response signal 36 to determine where the signal 36 is increasing and decreasing in frequency and to measure a time interval between extrema of two adjacent cycles of the electromagnetic response signal 36 to determine duration of one respiration motion.

FIG 10A - 10E illustrates an explanatory diagram showing variations in capacitance and frequency. The various patterns of the movement of the skin 17 due to the respiration motion and the corresponding change in the relative arrangement of the capacitor plates 14, 16 is shown in FIG 10A - 10E.

Different respiratory states of the user produce different patterns of the capacitor plate displacements.

The waveforms in 10A-10E shows frequency of the

electromagnetic response signal 36 from the sensor 10

corresponding to the capacitance of the capacitor 12. The waveforms indicate frequency distribution for various

respiration patterns such as normal inhalation, normal exhalation, obstructed inhalation, early inhale/exhale, late inhale/exhale associated with respiration of a user.

The transceiver 40 analyzes the respiratory motion from these waveforms to determine the resonant frequency. FIG. 11 is a graph illustrating the computed strength of the electromagnetic response signal 36 versus frequency of the electromagnetic sender signal 34 in accordance with an embodiment of the invention. The signal measuring means 42 of the transceiver 40 measures the output power of the electromagnetic response signal 36 corresponding to the capacitance of the sensor capacitor 12 representing the respiration of the user. The computing means 44 associated with the transceiver 40 further scans the power of the electromagnetic response signal 36 at various

frequencies of the electromagnetic sender signal 34 at which it is transmitted to the sensor 10. The computing means 44 further estimates the frequency of the electromagnetic sender signal 34 where the maximum power of the electromagnetic response signal 36 is scattered.

This frequency is noted as the peak frequency. The peak frequency is for example, a frequency having maximum power in case where the sensor signal is converted into the power spectrum. Here, the peak frequency is obtained at a resonance when the scanned frequency matches with the transmitted frequency of the electromagnetic sender signal 34. FIG. 11 is a graph explaining the periodicity of the relative maxima of the electromagnetic response signal 36. The

processor 46 computes the respiration on the basis of the electromagnetic response signal 36 outputted from the sensor 10.

The movement of the skin during respiration causes a change in the resonant frequency. This in turn changes the frequency of the maximum power transferred. The plot of peak frequency of the electromagnetic response signal 36 versus time

interval between the extrema of two adjacent cycles gives the respiration cycle of the user.

The processor 46 analyze the cycles of the electromagnetic response signal 36 to determine where the signal is increasing and decreasing in frequency. The breathing cycle includes multiple extrema including maximum values and minimum values. The measurement of the time interval between extrema of two adjacent cycles of the electromagnetic

response signal 36 provides the duration of one respiration motion. This helps to analyze how much time lag is respective succession of inhaling or exhaling.

The system 38 described herein is highly accurate owing to the mode of sensing, does not require extra power storage and hence makes it more user friendly, environment friendly and requires less maintenance. The motion artifacts do not hamper the system performance since the sensor only measures the stretching and contraction. The usage of wireless

transmission obviates the need for line of sight. The sensor is very compact in a bandage form with thin form factor less than 1mm thickness and poses less discomfort to the user. Multiple such sensors can be placed onto the patient to obtain accurate measurements of the respiration rate.

The sensor can be used for continuous respiration monitoring as it is attached to the persons body. This makes it easy to alert the doctor using standing communication protocols such as mobile phone when the respiration rate shows anomalies. The circuitry of the transceiver is small and is adapted to obtain information from a multitude of sensors in the

vicinity. The cost of the system is less, especially

appealing to mass markets. The embodiment described herein finds extensive application in healthcare areas. For instance, it can be used to monitor breathing rate of babies and infants especially in the case of pneumonia. The breath rates of each baby can be wirelessly monitored and can be displayed on a computer or an alarm can be generated. This facilitates monitoring of the patients remotely from elsewhere both when they are asleep or in motion. Further, respiration rate of sports players and athletes can be continuously determined to study and improve their performance.

Claims

Patent claims

1. A sensor (10) for measuring a respiration of a user, wherein the sensor (10) comprises:

- a capacitor (12) comprising of a first capacitor plate (14) and a second capacitor plate (16), the first capacitor plate (14) and the second capacitor plate (16) being coupled to a skin (17) of the user such that a movement of the skin (17) during the respiration changes a relative arrangement of the first capacitor plate (14) with respect to the second

capacitor plate (16) to vary the capacitance of the capacitor ( 12 ) ; and

- a means to generate an output signal (36) dependent on the capacitance .

2. The sensor (10) according to claim 1, wherein the first capacitor plate (14) and the second capacitor plate (16) are coupled to the skin (17) via a base member (18) fixed onto the skin ( 17 ) .

3. The sensor (10) according to claim 1 or 2, wherein the change in the relative arrangement of the first capacitor plate (14) and the second capacitor plate (16) is a change in a distance between the first capacitor plate (14) and the second capacitor plate (16) .

4. The sensor (10) according to claim 3, wherein the first capacitor plate (14) and the second capacitor plate (16) are connected to a spacing means (20) which holds the capacitor plates (14, 16) at a tension, wherein the capacitor plates

(14, 16) are coupled to the skin (17) such that the movement of the skin (17) exerts a force against the tension to change the distance between the first capacitor plate (14) and the second capacitor plate (16).

5. The sensor (10) according to claim 4, further comprises a connecting means (22) attached to the first capacitor plate (14) and the second capacitor plate (16), wherein the

connecting means (22) is coupled to the skin (17) in opposite sides of the first capacitor plate (14) and the second capacitor plate (16) such that the movement of the skin (17) exerts a force working from opposite directions on the capacitor plates (14, 16).

6. The sensor (10) according to any of the claims 3 to 5, further comprises a transmission means (26) attached to the first capacitor plate (14), wherein the transmission means (26) is coupled to the skin (17) through the base member (18) such that the transmission means (26) transmits a force provided by the movement of the skin (17) to the first capacitor plate (14) to reduce the distance between the capacitor plates (14, 16).

7. The sensor (10) according to any of the claims 3 to 6, further comprises a flexible member (24), wherein the first capacitor plate (14) and the second capacitor plate (16) is coupled to the flexible member (24) such that the respiratory movement of the skin (17) pushes the first capacitor plate (14) towards the second capacitor plate (16).

8. The sensor (10) according to any of the claims 1 to 7, wherein the first capacitor plate (14) and second capacitor plate (16) is arranged relatively parallel to the skin (17), wherein the first capacitor plate (14) is coupled to the skin (17) such that the respiratory movement of the skin (17) pushes the first capacitor plate (14) towards the second capacitor plate (16).

9. The sensor (10) according to any of the claims 1 to 8, wherein the change in the relative arrangement of first capacitor plate (14) and the second capacitor plate (16) is a movement of at least one capacitor plate (14,16) in a

direction parallel to a surface of the capacitor (12) .

10. The sensor (10) according to any of the claims 1 to 9, wherein the first capacitor plate (14) and the second

capacitor plate (16) are flat.

11. The sensor (10) according to any of the claims 1 to 10, wherein the first capacitor plate (14) and second capacitor plate (16) are spiral.

12. The sensor (10) according to any of the claims 1 to 11, further comprises a separation means (28) placed between the first capacitor plate (14) and the second capacitor plate (16) to prevent contact of the first capacitor plate (14) and second capacitor plate (16).

13. The sensor (10) according to any of the claims 1 to 12, further comprises an antenna (30) to receive an

electromagnetic sender signal (34), wherein the antenna (30) is electronically linked to the capacitor (12) such that an electromagnetic resonance frequency of the antenna (30) depends on the capacitance of the capacitor (12) .

14. The sensor (10) according to claim 13, wherein the antenna (30) is adapted to transmit the output signal in the form of an electromagnetic response signal (36) with a signal strength dependent on the capacitance of the capacitor (12) .

15. The sensor (10) according to claim 14, further comprises an energy storage (32) adapted to store energy dependent on the electromagnetic sender signal (34) received by the antenna (30), wherein the signal strength depends on the amount of energy saved in the energy storage (32) .

16. A respiratory monitoring system (38) comprising:

- a sensor (10) according to any of the claims 14 to 16; and

- a transceiver (40) adapted to transmit the electromagnetic sender signal (34) to the sensor (10) and to receive the electromagnetic response signal (36) from the sensor (10) .

17. The system (10) according to claim 16, wherein the transceiver (40) is adapted to vary the frequency of the electromagnetic sender signal (34).

18. The system (10) according to claim 16 or 17, wherein the transceiver (40) comprises:

- a signal measuring means (42) adapted to measure a strength of the electromagnetic response signal (36) ; and

- a computing means (44) adapted to determine relative extrema in the strength of the electromagnetic response signal (36) .

19. The system (10) according to claim 18, wherein the computing means (44) is adapted to determine the respiratory cycle based on the intervals between the extrema.

20. The system (10) according to claim 19, wherein the computing means (44) is adapted to determine the resonance frequencies of the capacitor (12) based on the frequency of the electromagnetic sender signal (34) for which the strength of the electromagnetic response signal (36) reaches the relative maxima.

21. The system (10) according to claim 20, wherein the computing means (44) is adapted to determine the respiratory cycle based on the development of the resonance frequency over time.