WO2016102184A1

WO2016102184A1 - Method and sleep monitoring device for determining a body posture of a person in a bed

Info

Publication number: WO2016102184A1
Application number: PCT/EP2015/079041
Authority: WO
Inventors: Petronella Hendrika Zwartkruis-Pelgrim; Erik Gosuinus Petrus Schuijers
Original assignee: Koninklijke Philips NV
Current assignee: Koninklijke Philips NV
Priority date: 2014-12-23
Filing date: 2015-12-09
Publication date: 2016-06-30
Anticipated expiration: 2017-06-23

Abstract

A method for determining a body posture of a person (16) in a bed (18) is disclosed. The method comprises the steps of generating a sensor signal (20) in response to pressure caused by the person via a supporting layer (14) supporting the person in the bed, and determining at least one spectral parameter of the sensor signal. The body posture is classified based on the determined at least one spectral parameter.

Description

Method and sleep monitoring device for determining a body posture of a person in a bed

FIELD OF THE INVENTION

The present invention relates to a method for determining a body posture of a person in a bed. The present invention further relates to a sleep monitoring device.

BACKGROUND OF THE INVENTION

Parents are often worried about the posture of their baby in the bed. Sleeping in a prone position, i.e. lying on the belly has been linked to the incidence of SIDS. From the late 1980s mortality from sudden infant death syndrome (SIDS) has decreased substantially and that this has been attributed to the change in infant sleep position initially from a prone to a side and then to predominantly a supine position, i.e. lying on the back. International public health campaigns have been promoting the supine sleeping position for infants.

Although parents might put their baby to bed in supine position, the baby might roll over during the night resulting in a prone position. Current SIDS detection mattresses focus on detecting movement and breathing. It does not signal a potentially hazardous situation related to SIDS until breathing and/or body movement are no longer detected. This may happen some time after the baby has rolled over to a prone position. It does not give parents information in an early stage on a potentially hazardous situation related to SIDS. SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved method and an improved device by means of which a potentially hazardous situation related to SIDS can be determined in an early stage.

The object of the invention is achieved by a method for determining a body posture of a person in a bed according to the invention. The method for determining a body posture of a person in a bed comprises the steps of generating a sensor signal in response to pressure caused by the person via a supporting layer supporting the person in the bed, determining at least one spectral parameter of the sensor signal, and classifying the body posture based on the determined at least one spectral parameter. The object of the invention is further achieved by a sleep monitoring device for determining a body posture of a person in a bed according to the invention. The sleep monitoring device for determining a body posture of a person in a bed, comprises a sensor device for generating a sensor signal in response to pressure caused by the person via a supporting layer supporting the person in the bed, a spectral parameter determining device for determining at least one spectral parameter of the sensor signal, and a processing unit for classifying the body posture based on the spectral parameter.

Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that the claimed monitoring device has similar and/or identical preferred embodiments as the claimed method and vise versa and as defined in the dependent claims.

According to the invention, a sensor signal is generated via a sensor for receiving a mechanical signal in response to pressure by the person through the supporting layer, such as mattress. The sensed pressure originates from e.g. body movement, respiration and heartbeat. From the generated sensor signal a spectral parameter is determined. Using the spectral parameter, the body posture can be classified and a distinction can be made between prone position and supine or side position. Because sleeping in a prone position is known to increase the risk of SIDS, it is clear that the distinction of prone position out of all other sleeping positions is highly important to be aware of as soon as it occurs. By applying the method or the monitoring device according to the invention a posture of a person, e.g. a baby, in a bed related to a higher risk to SIDS can be signaled as soon as the baby rolls over to the prone position and the potentially hazardous situation occurs.

In a preferred embodiment a first harmonic frequency and a further harmonic frequency of the generated sensor signal is determined, and the spectral parameter is determined as a ratio between at least one parameter of the first harmonic frequency and at least one parameter of the further harmonic frequency, and wherein the body posture is classified based on the ratio. This is a possibility to determine the body position precisely with low technical effort.

In a preferred embodiment the at least one parameter of the first harmonic frequency and the at least one parameter of the further harmonic frequency are respective amplitudes of the harmonic frequencies. This is a possibility to determine the ratio of the frequencies with low technical effort.

In an advanced embodiment, a pitch of the sensor is detected, followed by using the detected pitch to determine the first harmonic frequency and the further harmonic frequency. The detection of the pitch, also known as fundamental frequency or base frequency, facilitates the determination of the first harmonic frequency and the further harmonic frequency. Consequently, the determination of the ratio between the first harmonic frequency and the further harmonic frequency is facilitated and the classification of the body position is facilitated.

In a preferred embodiment, the at least on spectral parameter is a signal-to- noise ratio of at least one frequency of the generated sensor signal. This is a possibility to further determine the body posture with low technical effort.

Advantageously the detected pitch is used to comb filter the sensor signal, followed by determining the signal-to-noise ratio based on the signal power before pitch detection and the noise power of the comb filtered sensor signal and classifying the body posture additionally based on the signal-to-noise ratio.

Comb filtering the sensor signal removes the harmonic frequencies and therefore provides a signal from which the noise power of the sensed signal can be estimated. From the sensor signal the signal power is known. The signal-to-noise ratio, which is the ratio between the signal power and the noise ratio, is, in combination with the ratio between the amplitudes of the first harmonic frequency and the further harmonic frequency, used to classify the sleeping posture. A high signal-to-noise ratio in combination with a high ratio between the amplitudes of the first harmonic frequency and the further harmonic frequency, which is indicative to a supine or side posture, classifies as "supine posture", while a low signal-to-noise ratio in combination with a high ratio between the first harmonic frequency and the further harmonic frequency, which is indicative to a supine or side posture, classifies as "side posture".

By classifying the prone position and the two side positions it is possible to determine what the preferred sleeping posture of the person in the bed is. It enables a user to derive the number of movements per sleeping period as well and can be used to further provide insight into sleeping behaviour and sleep quality.

A low ratio classifies as "prone position", while a high ration classifies as "supine or side position". Prone position is an important risk indicator for SIDS.

In a preferred embodiment, the first harmonic frequency is a second harmonic frequency of the generated sensor signal and the further harmonic frequency is a fundamental frequency of the generated sensor signal. This is a possibility to determine the ratio between the harmonic frequencies with high reliability and low technical effort, since the fundamental frequency and the second harmonic frequency have the largest amplitude difference. In a further preferred embodiment, the ratio between the first harmonic frequency and the further harmonic frequency is determined on the basis of an amplitude of the first harmonic frequency and the further harmonic frequency. This is a possibility to further reduce the technical effort for determining the ratio, since the amplitude can be determined with low technical effort and the ratio can be defined reproducible, since the amplitude is a well-defined parameter.

In a further preferred embodiment, the generated sensor signal is notch filtered to determine the amplitude of the first harmonic frequency and the further harmonic frequency. The notch filter used for notch filtering the generated sensor signal comprises to band pass frequencies, one having a frequency of the first harmonic frequency and one having a frequency at the further harmonic frequency so that the respective harmonics passes the notch filter and the respective other frequencies in the generated sensor signal are attenuated. This is a possibility to precisely determine the amplitude of the first harmonic frequency and the further harmonic frequency in order to determine the ratio.

In a preferred embodiment, the pitch is used to notch filter the sensor signal.

This is a possibility to adapt the notch filter to the respective first harmonic frequency and the second harmonic frequency.

In a further preferred embodiment, the sensor signal is based on a mechanical signal received from the person through the supporting layer. This is a possibility to determine the sensor signal with low technical effort and in a comfortable way for the person in the bed.

In a preferred embodiment of the sleep monitoring device the spectral parameter determining device comprises a frequency determining device for determining a first harmonic frequency and a further harmonic frequency of the generated sensor signal and for determining a ratio of at least one parameter of the first harmonic frequency and at least one parameter the further harmonic frequency as the spectral parameter. This is a possibility to determine the body posture precisely with low technical effort.

In a preferred embodiment, the sensor device comprises a piezoelectric sensor. This is a possibility to generate the sensor signal in response to the pressure caused by the person via the supporting layer with low technical effort and high precision, since the piezoelectric sensors can determine high mechanical frequencies.

In a further preferred embodiment, the sensor device comprises a plurality of sensors associated to the supporting layer, wherein the sensors are spatially separated from each other. This is a possibility to further improve the reliability of the body posture determination, since the pressure is determined at different positions and the distance between the sensors and the person in the bed can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the following drawings

Fig. 1 shows a schematic sleep monitoring device for determining a body posture of a person in a bed;

Fig. 2 shows a detailed block diagram of an embodiment of the monitoring device for determining the body posture of the person in the bed; and

Fig. 3 shows a spectrogram of a sensor signal for determining the body posture of the person in the bed.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a schematic diagram of a sleep monitoring device for determining a body posture of a person in a bed, wherein the device is generally denoted by 10. The monitoring device 10 comprises a sensor device 12, which is associated to a support layer 14 which is supporting a person 16 lying in a bed 18. The person 16 is a baby lying in the bed 18, wherein the monitoring device 10 is configured to determine a body posture of the baby 16 in order to detect a potentially hazardous situation related to SIDS at an early stage. The baby 16 is lying on the support layer 14, which is preferably formed as a mattress. The sensor device 12 comprises a mechanical sensor, which generates an electrical sensor signal 20 on the basis of a mechanical signal received from the person 16 through the support layer 14. The sensor device 12 preferably comprises one or more piezoelectric sensors in order to transform the mechanical signal to the electrical sensor signal 20. The sensors are preferably spatially separated in order to determine the mechanical signal with high precision for different positions on the support layer 14. The sensor device 12 is connected to a control unit 22, wherein the sensor signal 20 is provided to the control unit 22 in order to evaluate the sensor signal 20 and to determine the body posture of the person 16 on the support layer 14. The control unit 22 provides a corresponding indication signal 24 to an indication device 26, wherein the signal 24 may be an optical or an acoustical signal.

The mechanical signal received from the person 16 through the support layer 14 which is received by the sensor device 12 and transformed to the electrical sensor signal 20 is mainly based on the cardiac activity of the person 16, wherein the body posture of the person 16 with respect to the support layer 14 influences the mechanical signal and the electrical sensor signal 20 so that the body posture of the person 16 can be determined by an analysis of the electrical sensor signal 20. The characteristic of the electrical sensor signal 20 is dependent on the body posture, since the signal of the cardiac activity bridges more body mass in a side position than in a prone or supine position so that the electrical sensor signal 20 is different in a side position than in a prone or supine position of the person 16. The monitoring device 10 makes use of the fact that a transfer function (attenuation as a function from frequency) from the heart signal through the body which reaches the mattress depends on the body posture.

In one embodiment, two features of the electrical sensor signal 20 are derived by means of the control unit 22, wherein a first feature is a signal-to-noise ratio of the fundamental frequency and also the harmonics of the heart rate signal, wherein a high signal- to-noise ratio corresponds to a supine position and a low signal-to-noise ratio corresponds to a side position of the person 16 on the support layer 14. The signal-to-noise ratio is determined based on the amplitudes of the fundamental frequency and the harmonics with respect to the noise of the electrical sensor signal 20.

A second feature is derived from the electrical sensor signal 20 corresponding to an amplitude of two different harmonics of the heart rate signal, wherein a low ratio of a higher harmonic to a lower harmonic or the fundamental frequency corresponds to a prone position of the person 16 on the support layer 14 and a high ratio corresponds to a supine position of the person 16 on the support layer 14. Hence, the control unit 22 can determine from the electrical sensor signal 20 the three positions of the person 16 on the support layer 14 corresponding to the prone, the supine and the side position. On the basis of the so determined or classified body position of the person 16 in the bed 18, the indication signal 24 is provided to the indication unit 26 e.g. as an alarm or the like.

In order to determine the two features of the electrical sensor signal 20, the control unit 22 comprises a frequency determining device which determines a first harmonic frequency and a further harmonic frequency from the electrical sensor signal 20 and determines the ratio between the determined harmonic frequencies and, further, comprises a processing unit for classifying the body posture on the basis of the ratio of the harmonic frequencies. In order to determine the respective features of the electrical sensor signal 20, the control unit 22 comprises a pitch detector, a comb filter and a notch filter as described in the following. Preferably the ratio is determined between amplitudes of the different harmonic frequencies. Fig. 2 shows a schematic block diagram of an embodiment of the control unit 22 for determining the position of the person 16 on the support layer 14. The control unit 22 receives the sensor signal 20 and a segmentation unit 28 segments the sensor signal 20 and provides the segmented signal to a pitch detector 30, wherein the pitch detector 30 estimates the pitch of each segment of the segmented signal e.g. by using an autocorrelation method. The pitch detector 30 determines a fundamental frequency f or a pitch, which is used in the following to control a series of filters. The so determined fundamental frequency f is provided to a comb filter 32, which removes the content of the fundamental frequency f and also higher harmonics 2f, 3f, 4f, etc. from the sensor signal 20. The comb filter determines a correspondingly resulting signal which primarily consists of the remaining noise. The power of the noise is determined in a noise estimator 34 and a signal-to-noise ratio is determined in a signal-to-noise ratio detector 36 which receives a signal power of the sensor signal 20 from a signal power estimator 38.

A first notch band pass filter 40 and a second notch band pass filter 42 is connected to the pitch detector 30. The first notch band pass filter 40 determines a signal around the fundamental frequency f of the sensor signal 20 and the second notch band pass filter 42 determines a signal around a higher harmonic e.g. 2f while respectively attenuating all other frequencies. A first amplitude determining unit 44 and a second amplitude determining unit 46 respectively determine an amplitude of the respective signals received from the notch band pass filters 40, 42 and a ratio determining unit 48 determines a ratio of the amplitude of the fundamental frequency f and the second harmonic 2f. The signal-to- noise ratio detector 36 and the ratio determining unit 48 are connected to a signal processor 50, which classifies the position of the person 16 on the support layer 14 on the basis of the signal-to-noise ratio as a first feature of the sensor signal 20 and on the basis of the ratio of the fundamental frequency f and the second harmonic 2f as the second feature of the sensor signal 20. The so determined or classified position is evaluated and the indication signal 24 is provided to the indication unit 26 in order to indicate a potentially hazardous position of the person 16 on the support layer 14.

Hence, the different features can be extracted from the sensor signal 20 and the position of the person 16 can be determined on the basis of the two different features.

Fig. 3 shows a spectrogram of the sensor signal 20 during a deep sleep of the person 16. In a first time frame denoted by A, the person 16 is in a supine position, in a following time frame denoted by B, the person 16 is in a prone position and during a further time frame denoted by C, the person is in a right-side position. The three positions are determined as mentioned above, wherein for a high signal-to-noise ratio a supine position is determined, for a low signal-to-noise ratio a side position is determined and for a high ratio of the second harmonic 2f to the fundamental frequency f a prone position is determined and for a respective high ratio of the harmonic to the fundamental frequency a supine or a side position is determined.

The classifying of the position is possible, since the relative strength of the higher harmonics 2f, 3f, etc. compared to the fundamental frequency f is stronger when the person 16 is lying in the prone position than in the other positions.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other 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 element or other unit may fulfill 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.

Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. Method for determining a body posture of a person (16) in a bed (18) comprising the steps of:

generating a sensor signal (20) in response to pressure caused by the person via a supporting layer (14) supporting the person in the bed,

determining at least one spectral parameter of the sensor signal, and classifying the body posture based on the determined at least one spectral parameter.

2. Method according to claim 1, further comprising the step of:

determining a first harmonic frequency (2f) and a further harmonic frequency

(f) of the generated sensor signal,

wherein the spectral parameter is determined as a ratio between at least one parameter of the first harmonic frequency and at least one parameter of the further harmonic frequency.

3. Method according to claim 2, wherein the at least one parameter of the first harmonic frequency and the at least one parameter of the further harmonic frequency are respective amplitudes of the harmonic frequencies.

4. Method according to claim 2, further comprising the step of

detecting a pitch of the sensor signal and using the detected pitch to determine the first harmonic frequency and the further harmonic frequency.

5. Method according to claim 1, wherein the at least on spectral parameter is a signal-to-noise ratio of at least one frequency of the generated sensor signal.

6. Method according to claim 5, further comprising the steps of

using the detected pitch to comb filter the sensor signal,

determining the signal-to-noise ratio based on the signal power of the sensor signal and a noise power of the comb filtered sensor signal, and

classifying the body posture additionally based on the signal-to-noise ratio.

7. Method according to any of the preceding claims, wherein

- the first harmonic frequency is a second harmonic frequency of the generated sensor signal and the further harmonic frequency is a fundamental frequency of the generated sensor signal.

8. Method according to claim 4, wherein the generated sensor signal is notch filtered to determine the amplitude of the first harmonic frequency and the further harmonic frequency.

9. Method according to claim 8, wherein the pitch is used to notch filter the sensor signal.

10. Method according to any of the preceding claims, wherein the sensor signal is based on a mechanical signal received from the person through the supporting layer.

11. Sleep monitoring device (10) for determining a body posture of a person (16) in a bed (18), comprising:

a sensor device (12) for generating a sensor signal (20) in response to pressure caused by the person via a supporting layer (14) supporting the person in the bed,

a spectral parameter determining device (30) for determining at least one spectral parameter of the sensor signal, and

- a processing unit (50) for classifying the body posture based on the at least one spectral parameter of the sensor signal.

12. Sleep monitoring device as claimed in claim 11, wherein the spectral parameter determining device (30) comprises a frequency determining device (30) for determining a first harmonic frequency (2f) and a further harmonic frequency (f) of the generated sensor signal and for determining a ratio of at least one parameter of the first harmonic frequency and at least one parameter the further harmonic frequency as the spectral parameter.

13. Sleep monitoring device according to claim 11, wherein the sensor device comprises a piezoelectric sensor.

14. Sleep monitoring device according to claim 11, wherein the sensor device comprises a plurality of sensors associated to the supporting layer, wherein the sensors are spatially separated from each other.

15. Sleep monitoring device according to claim 11, wherein the frequency determining device comprises a comb filter (32) for determining a comb filtered sensor signal, and wherein the processing unit is configured to classify the body posture additionally based on a signal-to-noise ratio determined on the basis of the sensor signal and the comb filtered sensor signal.