WO2006067690A2

WO2006067690A2 - Device for measuring a user´s heart rate

Info

Publication number: WO2006067690A2
Application number: PCT/IB2005/054246
Authority: WO
Inventors: Olaf Such; Jens Muehlsteff; Josef Lauter; Robert Pinter
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N. V.
Priority date: 2004-12-22
Filing date: 2005-12-14
Publication date: 2006-06-29
Also published as: WO2006067690A3

Abstract

The present invention relates to a device and method for measuring a user's heart rate. In order to provide a reliable and inexpensive heart rate measuring technique, a device (1) and a method for measuring a user's heart rate is suggested in which the relative movement between a heart rate sensor (2) and the user is detected to generate a correction signal for removing motion artifacts caused by a user' s movement.

Description

Device for measuring a user' s heart rate

The present invention relates to a device for measuring a user' s heart rate. The present invention also relates to a portable sound reproducing device comprising such a measuring device. Furthermore, the invention relates to a method of measuring a user' s heart rate and to a computer program for measuring a user' s heart rate. During sports and fitness training it has become common to read heart rate signals in order to monitor the user's state of health. In most cases the heart rate signals derive from a chest strap. The signals are transmitted to a display device, such as a wristwatch by using a radio transmission technique. The use of such a chest strap is not very convenient to most users. As an alternative technique photoplethysmography (PPG) has been used to monitor the heart rate signal near a user' s ear. However, this technique is not very suitable for use by persons exercising an activity, for example a sports activity, because of the high level of motion artifacts. In other words the motion of the user causes corruption of the PPG signal. These artifacts lead to erroneous interpretation of the PPG signals and degrade the accuracy and reliability of PPG-based algorithms for the estimation of the heart rate. In US 2003/0233051 Al it is suggested to remove motion artifacts by using a correction signal generated by an accelerometer device. The use of an accelerometer is expensive and power consuming.

It is therefore an object of the present invention to provide a reliable and inexpensive heart rate measuring technique. This object is achieved according to the invention by a device for measuring a user's heart rate, comprising a user- wearable heart rate sensor for measuring a heart rate signal, a motion sensor for sensing a motion signal, the motion signal representing a relative movement between the heart rate sensor and the user, and a processing unit for removing at least partially artifacts caused by motion in the heart rate signal by using the motion signal. In other words the motion signal representing the relative movement is used as a correction signal. The object of the present invention is also achieved by a portable sound reproducing device, which comprises such a measuring device.

The object of the present invention is also achieved by a method of measuring a user's heart rate, comprising the steps of measuring a heart rate signal, sensing a motion signal, the motion signal representing a relative movement between the heart rate sensor and the user, and removing at least partially artifacts due to motion in the heart rate signal by using the motion signal.

The object of the present invention is finally achieved by a computer program for measuring a user' s heart rate, the computer program comprising computer instructions to remove at least partially artifacts due to motion in a heart rate signal by using a motion signal representing a relative movement between the heart rate sensor and the user, when the computer instructions are carried out in a computer. The technical effects necessary according to the invention can thus be realized on the basis of the instructions of the computer program in accordance with the invention. Such a computer program can be stored on a carrier or it can be available over the internet or another computer network. Prior to executing the computer program, it is loaded into a computer by reading the computer program from a carrier, for example by means of a CD-ROM player, or from the internet or another network, and storing it in the memory of the computer. The computer includes inter alia a central processor unit (CPU), a bus system, memory means, e.g. RAM or ROM etc. and input/output units. The instructions of the computer program can also be implemented as embedded software. In other words an embedded system is used, which is housed for example on a single microprocessor board with the computer program stored in a ROM. The invention is based on the idea of utilizing the relative movement between the heart rate sensor and the user to generate a correction signal. In contrast to the prior art solution, where only the movement or acceleration of the device itself is detected, the present invention uses a more accurate method for carrying out the correction. Because the user himself is used as a reference, the correction signal represents the real relative sensor-to-body motion in a very realistic way. Thus artifact robustness is increased leading to a very precise measuring of the heart rate even in cases of heavy user motion, e.g. during sports activity. The relative movement between the heart rate sensor and the user can be detected in a comparatively inexpensive way. If the device according to the invention is used in a headset or earphone or another nonmedical device, the user acceptance is enhanced compared to the chest strap known from the prior art.

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 at least the heart rate sensor and/or the motion sensor is adapted to be in contact with the user. Preferably the heart rate sensor and/or the motion sensor is adapted to be fixed to an ear of a user. In this case the heart rate sensor and/or the motion sensor is preferably implemented in a device for reproducing sound, e.g. an earphone, stereo headset or mobile phone headset etc. If all parts of the device for measuring the heart rate are implemented in an earphone or headset etc. a very high usability is provided. Alternatively, another part of the user's body can be used, e.g. the tip of a finger etc.

A contact with the user, e.g. with the user's skin, makes a motion detection possible based on measuring the contact pressure corresponding to the pressure between the heart rate sensor and the user. According to this embodiment of the invention the mechanical coupling between heart rate sensor and user is determined. In case an optical sensor is used for heart rate measurement, the mechanical coupling corresponds to the optical coupling between heart rate sensor and user. Thus with the determining of the mechanical coupling, information can be given about the optical coupling. Movements of the sensor relative to the user lead to changes in optical coupling. Motion artifacts in the heart rate signal will occur. If changes in the mechanical coupling can be detected, motion artifacts can be predicted and a artifact correction can be carried out in the processing unit to achieve an accurate heart rate signal, which is at least partially free of motion artifacts.

The contact pressure between the heart rate sensor and the user is measured either directly or indirectly. In the latter case the contact pressure the heart rate sensor and the user is detected by measuring the contact pressure between a separate contact device and the user and predicting the real contact pressure using that measuring signal. If the device for measuring the heart rate is used in a headphone etc. the sensors for measuring heart rate and motion can easily be used in the design of the headphone.

The relative movement between the heart rate sensor and the user can not only be detected by measuring the contact pressure. Other measuring techniques can be implemented as well, for example distance measurements with optical, acoustic, mechanical or other means.

The contact pressure between the heart rate sensor and the user can be detected using several methods, as described below. All described methods can be realized in a very power-saving way, e.g. employing low-current devices.

According to a preferred embodiment the motion sensor is adapted to measure the contact pressure based on a piezoelectric effect. Preferably the motion sensor comprises a piezoelectric contact device, e.g. a piezoelectric polymer foil or another pressure-sensitive material. When a mechanical pressure is applied to said contact device a voltage is produced between surfaces of the contact device. A small current may be produced as well. In other words the contact device produces an electrical output signal from a mechanical input signal. The output signal is amplified and used as a measure for the contact pressure. The relative movement is determined on the basis of this information. According to another preferred embodiment of the invention the motion sensor is adapted to measure the contact pressure based on a piezoresistive or resistive effect. Preferably the motion sensor comprises a piezoresistive contact device, e.g. a metal or semiconductor material. Alternatively, the motion sensor comprises a resistive contact device, e.g. a soft conductive material, for example the form of an earphone. When a mechanical pressure is applied to said contact devices the electrical resistivity of the contact devices changes. The changes in resistivity are determined using measurement of current or voltage. The changes in resistivity are a measure for the contact pressure and the relative movement is determined based upon this information. According to yet another preferred embodiment of the invention the motion sensor is adapted to measure the contact pressure based on a bioimpedance measuring technique. For this purpose, alternating current of effective value passes through the living tissue, e.g. the user's ear; bioimpedance can be measured indirectly by means of the voltage. Alternatively, a direct current can be applied. Preferably the motion sensor comprises at least one bioimpedance contact device, preferably in the foam of an electrode to be fixed t the user's skin. The level of bioimpedance depends on the electrical contact between the contact device and the user' s skin. Since, on the other hand, the electrical contact depends on the mechanical contact between contact device and skin, the bioimpedance gives a measure for the level of contact pressure between contact device and user.

All described methods can be used for carrying out a heart rate measuring at a single location on the user, e.g. on one side of the user's ear, i.e. on the side of an earphone. For obtaining a larger number of motion signals, the number of motion sensors can be increased. In a further embodiment of the invention motion sensors are positioned on both sides of a user's ear. In yet another embodiment motion sensors are positioned near both ears. The motion signals obtained from the two locations can be correlated to reduce errors and to improve heart rate detection. If the motion sensors are positioned near both ears e.g. the bioimpedance is measured between the ears of the user, providing a current through the user' s head. All these different techniques can be employed alternatively as well as in addition to each other. In cases where two or more different measuring techniques are used at the same time, a high quality correction of the heart rate signal can be carried out because of the large number of motion signals at hand.

The contact pressure between the heart rate sensor and the user can also be measured employing other known measuring techniques.

According to a further preferred embodiment of the invention the portable device comprises a sensor unit using conductive electrodes for measuring a galvanic skin response and/or a sensor unit for measuring a temperature of the user's skin. These additional measurements can be used to obtain an index for stress. The signals obtained are correlated with the motion signals to provide a better correction of the heart rate signals. If the motion signals are detected based upon the bioimpedance measuring technique, the same electrodes can be used for both bioimpedance measuring and measuring of the galvanic skin response. According to still another preferred embodiment of the invention the heart rate sensor is an optical sensor. Preferably the heart rate sensor is a photoplethysmographic sensor. Instead of a transmission technique employing parts of the heart rate sensor on both sides of a user's body part, other techniques can be used as well, such as a so called reflection technique (based upon diffusion within the tissue), where e.g. both transmitter and receiver are positioned on the same side of the ear. Alternatively, transmission and reflection techniques can be employed in a combined approach.

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 shows a schematic block diagram of a portable device for measuring a user's heart rate, and

Figs. 2-5 show sectional views of different embodiments of the invention in a working position near a user's ear.

A device 1 for measuring a user' s heart rate according to the invention is illustrated in Fig. 1. The device 1 comprises a heart rate sensor 2 for measuring a heart rate signal, a motion sensor 3 for measuring a motion signal, the motion signal representing a relative movement between the heart rate sensor 2 and the user (not shown), and a processing unit 4 for removing at least partially artifacts due to motion in the heart rate signal by the use of the motion signal. Heart rate sensor 2, motion sensor 3 and processing unit 4 are integrated in an earphone 5, e.g. an earphone of a portable radio, CD-player or MP3-player or the like. The earphone design allows the heart rate sensor 2 and the motion sensor 3 to be in contact wih the user's ear. The heart rate sensor (optical PPG sensor) 2 is adapted to measure the user' s heart rate based on photoplethysmography (PPG) utilizing a transmission technique. A measurement of the subcutaneous blood flow in the ear is carried out. For this purpose the heart rate sensor 2 comprises a light emitting diode (LED) 6 as a light source and a photodetector 7 as a receiver. The optical signal is corrected in the processing unit 4 by combining the optical signals and the motion signals. In other words a data fusion is carried out. For this purpose a computer program for signal correction is provided. In the processing unit 4. The processing unit 4 is connectable to a display device 8 for displaying the user's heart rate. The communication link 9 is preferably established using a radio communication technique. In the present embodiment the display device 8 is a monitor implemented in a wrist-worn device.

In Fig. 2 an embodiment according to the invention is shown. An LED 6 is positioned in mechanical contact with the surface of a user's ear 10. On the opposite side of the ear 10 a photodetector 7 is positioned in mechanical contact with the surface of the ear 10. The light is scattered through the earlobe or cartilage of the ear 10, where it is submitted to modifications due to reflection, refraction and absorption, depending on the blood content. After propagation of the light through the tissue the light is picked up by the photodetector 7.

LED 6 and photodetector 7 are each placed in a support ring 11, 12 made of a soft material for a better mechanical contact to the ear 10. Instead of the form of a ring the support member can also take another form, e.g. the form of a sheet or the like. The photodetector 7 and its support ring 12 is fixed directly to a carrier 13, made e.g. from a plastic material. The carrier 13 is preferably part of the earphone 5. The LED 6 and its support ring 11 are fixed to another part of the carrier 13 situated on the opposite side of the ear 10 by means of an intermediate pressure layer 14. In other words the LED 6 and its support ring 11 are in direct mechanical contact with the pressure layer 14. The pressure layer 14 consists of a piezoelectric polymer foil. If there is a relative movement between the LED 6 and the ear 10, e.g. because of a sports activity of the user, a mechanical pressure is applied to the pressure layer 14 and a voltage is produced, approximately proportional to the pressure resting on the layer 14 and representing the user's movement. This pressure also directly influences the optical coupling of the device and the underlying motion additionally causes blood volume changes in the tissue.

The resulting voltage signal is detected by an adequate measuring device (not shown) preferably used in the carrier 13 and transmitted to the processing unit 4, amplified and used for correction of the heart rate signal. For this purpose the processing unit 4 comprises processing means for processing input data from the photodetector 7 as well as from the pressure layer 14. In Fig. 3 another embodiment according to the invention is shown. In this embodiment an intermediate pressure layer 14 is provided on both sides of the ear 10. In other words both the LED 6 and its support ring 11 and the photodetector 7 and its support ring 12 are fixed to the carrier 13 by means of the pressure layer 14. Since the components 6, 7 of the heart rate sensor 2 are located on both sides of the ear 10, it is advantageous to detect changes of the contact pressure between these components 6, 7 and the ear 10 in the same way, i.e. on both sides of the ear 10. In this case there are two motion signals to be fed into the processing unit 4 for correlation with the heart rate signal.

In Fig. 4 a further embodiment according to the invention is shown. In this embodiment support rings 15 surrounding the LED 6 and the photodetector 7 are adapted to take over the function of the pressure layer. In this case the support rings 15 are made of a resistive material, e.g. the foam of the earphones. In this case a movement of the user results in a mechanical pressure applied to the support ring 15, leading to a change in electrical resistivity of the support ring 15. This resistivity change is determined using a measurement of voltage by an adequate measuring device (not shown) preferably implemented in the carrier 13, transmitted to the processing unit 4, amplified and used for correction of the heart rate signal.

In Fig. 5 another embodiment according to the invention is shown. In this embodiment the support ring 16 of the LED 6 comprises at least two electrodes 17 to be in contact with the user's ear 10. The electrodes 17 are preferably made of conductive plastics or rubber. They can also be formed of metal or conductive foam, to integrate well with the design of the earphone 5.

The electrodes 17 are supplied with an alternating current in a way that the current passes through the ear 10. The bioimpedance is measured indirectly by voltage. For current supply and voltage measuring adequate devices (not shown) are provided, preferably used in the carrier 13. The resulting signal is transmitted to the processing unit 4, amplified and used for correction of the heart rate signal. The support ring 12 of the photodetector 7 serves as a support member only.

In Fig. 6 still another embodiment according to the invention is shown. In this embodiment not only the support ring 16 of the LED 6 but also the support ring 18 of the photodetector 7 comprises bioimpedance electrodes 17. Thus bioimpedance measurements can be carried out between electrodes 17 on the same side of the ear 10 and furthermore between electrodes 17 on different sides of the ear 10, i.e.leading a current flow from one side of the ear 10 to the other side.

In addition both LED 6 and photodetector 7 and the support rings 16, 18 are provided with a pressure layer 14 for carrying out a piezoelectric measurement as described above. Alternatively pressure layers for a piezoresistive or a resistive measurement can be used. In this case both the bioimpedance signals and the voltage signals derived from the other motion sensors are transmitted to the processing unit 4, amplified and used for correction of the heart rate signal. The carrier 13 comprises an earclip 19 which serves as a fixation to hold the earphone parts in place. In another embodiment (not shown) the earclip is adapted to be in contact both with the ear 10 and the skull of the user. In this case additional impedance electrodes are positioned on the distant side of the earclip 19 to enable measurement of the impedance between ear 10 and skull. This additional bioimpedance signal is used as a further parameter for correction of the heart rate signal.

The heart rate sensor 2 can of course also be used to determine a further parameter, e.g. heart rate variability (HRV), a stress factor or breathing rate. The removing of artifacts according to the present invention can be applied in those cases as well. Generally, the present invention is not limited to heart rate sensors. The invention can be applied to other sensors for measuring vital parameters, e.g. to a sensor for direct determining of the breathing rate. The invention can be applied to any sensor for measuring any kind of parameter, if there are artifacts in the sensor signal thatare due to motion of the sensor relative to the user. 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 to be 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 heart rate measuring device

2 heart rate sensor

3 motion sensor

4 processing unit

5 earphone

6 LED

7 photodetector

8 display device

9 communication link

10 ear

11 support ring

12 support ring

13 carrier

14 pressure layer

15 support ring

16 support ring

17 electrode

18 support ring

19 earclip

Claims

CLAIMS:

1. A device (1) for measuring a user's heart rate, comprising

- a heart rate sensor (2) for measuring a heart rate signal,

- a motion sensor (3) for measuring a motion signal, the motion signal representing a relative movement between the heart rate sensor (2) and the user, and

- a processing unit (4) for removing at least partially artifacts due to motion in the heart rate signal by using the motion signal.

2. The device (1) as claimed in claim 1, wherein at least the heart rate sensor (2) and/or the motion sensor (3) is adapted to be in contact with the user.

3. The device (1) as claimed in claim 2, wherein at least the heart rate sensor (2) and/or the motion sensor (3) is adapted to be fixed to an ear (10) of the user.

4. The device (1) as claimed in claim 2, wherein the motion sensor (3) is adapted to measure a contact pressure corresponding to the pressure between the heart rate sensor (2) and the user.

5. The device (1) as claimed in claim 4, wherein the motion sensor (3) is adapted to measure the contact pressure based on a piezoelectric effect.

6. The device (1) as claimed in claim 4, wherein the motion sensor (3) comprises a piezoelectric contact device (14).

7. The device (1) as claimed in claim 4, wherein the motion sensor (3) is adapted to measure the contact pressure based on a piezoresistive or resistive effect.

8. The device (1) as claimed in claim 4, wherein the motion sensor (3) comprises a piezoresistive contact device (15) or a resistive contact device.

9. The device (1) as claimed in claim 4, wherein the motion sensor (3) is adapted to measure the contact pressure based on a bioimpedance measuring technique.

10. The device (1) as claimed in claim 4, characterized in that the motion sensor (3) comprises a bioimpedance contact device (17).

11. The device (1) as claimed in claim 1, further comprising a sensor unit for measuring a galvanic skin response and/or a sensor unit for measuring a temperature of the user's skin.

12. The device (1) as claimed in claim 1, wherein the heart rate sensor (2) is an optical sensor.

13. The device (1) as claimed in claim 12, wherein the heart rate sensor (2) is a photoplethysmographic sensor.

14. A portable device (5) for reproducing sound, comprising a device (1) as claimed in claim 1.

15. A method of measuring a user's heart rate, comprising the steps of

- measuring a heart rate signal,

- measuring a motion signal, the motion signal representing a relative movement between the heart rate sensor and the user, and

- removing at least partially artifacts due to motion in the heart rate signal by using the motion signal.

16. A computer program for measuring a user' s heart rate, the computer program comprising computer instructions to remove at least partially artifacts due to motion in a heart rate signal by using a motion signal representing a relative movement between the heart rate sensor and the user, when the computer instructions are carried out in a computer (4).