WO2013042070A1

WO2013042070A1 - Heart rate monitor for measuring a heart rate of a user

Info

Publication number: WO2013042070A1
Application number: PCT/IB2012/055011
Authority: WO
Inventors: Peter Tjin Sjoe Kong Tsang; Cristian Nicolae Presura
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2011-09-22
Filing date: 2012-09-21
Publication date: 2013-03-28

Abstract

The present invention relates to Heart rate monitor (10) for measuring a heart rate of a user, including at least one artificial light source (14) for emitting light (16) into skin (18) of the user, a first sensor (22) for sensing light (16, 40) reflected through the skin (18) of the user and for generating a first sensor-signal in response to sensed light (16, 40), and an optical high-pass filter (44) for filtering out infrared light (48), wherein the optical high-pass filter (44) is arranged in an optical path (46) before the first sensor (22) for filtering out infrared light (48) from light (40) travelling to the first sensor (22).

Description

Heart rate monitor for measuring a heart rate of a user

FIELD OF THE INVENTION

The present invention relates to a heart rate monitor for measuring a heart rate of a user and a method for measuring a heart rate of a user. BACKGROUND OF THE INVENTION

The principle of optical heart rate monitors rely on an artificial light source that emits light into skin of a user. The emitted light is scattered within the skin, where it is absorbed partially by blood. Reflected light exits the skin and is captured by a sensor. The amount of the signal on the sensor is an indication of the blood volume. When the blood stream pulsates, the blood volume in the skin changes. Thus, the signal on the sensor changes directly in response to the pulsation. Hence, the sensor measures directly a pulse of the user in the skin and can thus determine the actual heart rate of the user at the moment.

US 5,807,267 describes the usage of optical sensors with artificial light sources, wherein an optical band-pass filter is used as to enhance a measurement quality. The optical band-pass filter is designed to filter out light with wavelengths greater and smaller than wavelengths emitted by the corresponding LED.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a heart rate monitor and a corresponding method, wherein a measurement quality is enhanced. A further object of the invention is to save electrical power within the heart rate monitor.

In a first aspect of the present invention a heart rate monitor for measuring a heart rate of a user is presented, including at least one artificial light source for emitting light into skin of the user, a first sensor for sensing light reflected through the skin of the user and for generating a first sensor-signal in response to sensed light, and an optical high-pass filter for filtering out infrared light, wherein the optical high-pass filter is arranged in an optical path before the first sensor for filtering out infrared light from light travelling to the first sensor. In a second aspect of the present invention a heart rate monitoring method for measuring a heart rate of a user is presented, said method comprising emitting light into skin of the user, sensing light reflected through the skin of the user, generating a first sensor- signal in response to sensed light, and filtering out infrared light before sensing the light reflected through the skin.

The invention is based on the idea to filter out infrared light before measuring light reflecting through the skin of the user as to enhance measurement quality. Experiments of the applicant have shown that infrared light is responsible for most artifacts in sensor- signals. Those artifacts may for example arise through moving the heart rate monitor between locations of different ambient lightings, as for example from shadow into the direct sun. By filtering out infrared light nearly all artifacts caused by the ambient light can be avoided or can be at least decreased substantially. Additionally, a saturation of the sensor is avoided.

The optical filter is preferably designed as an edge filter with high-pass characteristics, which is filtering out at least the infrared spectrum of light. Such filters are providing the advantage of being easy to design and to produce. Hence, the optical high-pass filter can be included into the heart rate monitor very cost-effectively compared for example to band-pass filters. Through this, an increase in measurement quality and measurement robustness is achieved, since artifacts through ambient light are at least mostly avoided.

It is preferred, if the heart rate monitor comprises a processing unit for receiving the sensor-signal and for determining the pulse and/or heart rate of the user in response to the sensor-signal.

Preferred embodiments of the invention are defined in the dependent claims.

In an embodiment the artificial light source is a light emitting diode. In this embodiment a light emitting diode (LED) for providing artificial light is used. The LED is having the advantage of emitting a spectrum of light with high frequency and none or only small amounts of infrared light. Accordingly, sensing the light of a LED is not affected by using the optical high-pass filter. Additionally, the LED is providing high energy efficiency. Further, LEDs are available for emitting different colors of visible and invisible light. They can be configured for example to emit red, green, blue and ultraviolet light. By using an optical high-pass filter, it is possible to use different LEDs for emitting light with high frequencies as the artificial light source without restrictions. Further, it is possible to use a plurality of LEDs for different light spectra simultaneously in one heart rate monitor or at least in a production series of heart rate monitors without the need for a different setup of the optical high-pass filter. Therefore, a freedom of design regarding the artificial light source is given, which can be used to optimally adapt the artificial light source for its application as to obtain optimal measurement quality. Hence, the usage of the optical high-pass filter in combination with the LED leads to further cost advantages and to a flexibility in design.

In a further embodiment the sensor is a photodetector-diode. In this embodiment a photodetector-diode is used for sensing the reflected light. This is

advantageous, since photodetector-diodes are easily available and therefore cost effective. Additionally, they are small in their spatial extension which leads to further freedom in design.

In a further embodiment the optical high-pass filter is integrated into the photodetector-diode. In this embodiment the photodetector-diode and the optical high-pass filter are assembled into one part. This is advantageous, since photodetector-diodes are typically comprising a light-permeable housing or housing part for protecting the actual diode. This housing can be made from a material comprising optical high-pass filtering characteristics. Alternatively, a photodetector-diode can be coated with an optical high-pass filtering material or finish. Hence, the heart rate monitor can be manufactured even more cost-effective and further constructing space is saved in the heart rate monitor.

In a yet further embodiment the heart rate monitor includes a housing having a contact part for making physical contact with the skin of the user, wherein the first sensor is arranged in a center area of the contact part. In this embodiment the heart rate monitor and its components are preferably arranged within the housing. This may be the housing of a watch for wearing the heart rate monitor at a wrist or arm of the user. The housing comprises a contact part for making physical contact with the skin of the user. This may be a bottom part of the housing, e.g. of the watch, which is making contact to the skin while it is used by the user in a proper way. Since the first sensor is arranged in a center area of the contact part it is arranged relatively distant from edges of the contact part. Therefore, the first sensor is effectively shadowed with respect to ambient light as to further avoid artifacts caused by the ambient light. Hence, the amount of infrared light travelling to the sensor is smaller compared to the amount of infrared light travelling to border areas at the edges of the housing. Therefore, the efficiency of the optical high-pass filter can be adapted to the amount of ambient light expected to be reflected to the first sensor. Hence, the usage of a very efficient optical high-pass filter allows the usage of a very compact housing for the heart rate monitor, since the first sensor must not be shadowed entirely with respect to ambient light. In summery, the quality of measurements of the first sensor can be enhanced through dimensioning the housing and the efficiency of the optical high -pass filter. In a further embodiment the heart rate monitor includes a second sensor adapted to sense ambient light reflected through the skin of the user and for generating a second sensor-signal in response to sensed ambient light. In this embodiment the heart rate monitor comprises two sensors for sensing reflected light through the skin of the user. Hence, it is possible to obtain a pulse and/or heart rate of the user in response to the first and/or the second sensor-signal of the different sensors. Therefore, it can be decided if the intensity of ambient light is sufficient for providing measurements of the desired measurement quality. The second sensor-signal can then be used to support or to replace the first sensor-signal as to ensure the measurement quality. Herein, the optical high -pass filter has the advantage of making the first sensor robust against ambient light. Hence, if the amount of ambient light is too high and the second sensor is driven to saturation, the first sensor-signal can be used, since it is not affected by the ambient light. Therefore, a sufficient measurement quality can be guaranteed independently of the intensity of the ambient light. Thereby, the measurement quality can be increased further.

In a further embodiment the heart rate monitor includes the housing having the contact part for making physical contact with the skin of the user, wherein the second sensor is arranged in a border area of the contact part. In this embodiment the second sensor is arranged preferably near at least one edge of the contact part. This leads to a high intensity of ambient light reflected through the skin with respect to the second sensor, since shadowing of the second sensor by the contact part is mostly avoided. Hence, ambient light can travel through the skin of the user to the second sensor without being blocked by the contact part respectively the housing. It is advantageous that already a small amount of ambient light can generate a sufficient measurement signal with the second sensor wherein the measurement signal quality is further enhanced.

In a further embodiment the heart rate monitor includes a controlling unit for receiving the first sensor-signal and for controlling an intensity of emitted light from the artificial light source in response to the first sensor-signal. In this embodiment control of the intensity of emitted light from the artificial light source is provided. Thereby, the

measurement quality can be enhanced if needed or electrical energy can be saved, if the intensity of the emitted light can be decreased without loss of measurement quality. If a higher quality of the first sensor signal is needed for sufficient measuring, an increase of emitted light can be indicated.

In a further embodiment the controlling unit is adapted to determine a quality of the first sensor-signal and to control the intensity of the emitted light from the artificial light source in response to the quality. In this embodiment the controlling unit is analyzing the first sensor-signal for its quality. If the quality is more than sufficient and a pulse and/or a heart rate can be determined from the sensor-signal with sufficient certainty, the intensity of the emitted light is preferably decreased. By decreasing the emitted light from the artificial light source power consuming of the artificial light source is also decreased. Thereby, electrical energy is saved. If the controlling unit determines that the quality of the sensor- signal is insufficient it is intended to increase the intensity of the emitted light from the artificial light source in order to enhance the measurement quality.

Additionally, it is preferred, if a signal-to-noise ratio of the sensor-signal is determined as the signal quality. In order to determine the signal-to-noise ratio an analysis of frequencies within the sensor-signal is preferably provided. Thereby, an amplitude for typical noise frequencies can be set into relationship with an amplitude of the measured pulse. This can be done for example by using a Fast Fourier Transformation, since pulses and heart rates are typically within a frequency range of 50 Hz to 220 Hz. By contrast frequencies of noise signals are typically much higher. It is further preferred to use the value of the signal-to-noise ratio as a control parameter for a continuous increase or decrease of the intensity of the emitted light. An advantage of this embodiment is that the relationship between the sensor- signal quality and the intensity of the emitted light of the artificial light source is almost a step function. More particularly, if a pulse is present, it can be calculated easily with a very small error. Hence, the intensity of the emitted light can be decreased very much, thereby preserving the measurement quality.

In an embodiment the heart rate monitor includes a controlling unit for receiving the second sensor-signal and for controlling the intensity of emitted light from the artificial light source in response to the second sensor-signal. In this embodiment the controlling unit is adapted to determine an intensity of ambient light reflected through the skin of the user to the second sensor based on the second sensor-signal. Further, it is adapted to control the intensity of the emitted light from the artificial light source in response to the intensity of ambient light. Depending on the intensity of the ambient light reflected through the skin, it can be easily determined if the intensity of the emitted light from the artificial light source can be decreased as to use the ambient light exclusively or additionally for measuring purposes. Hence, it is advantageous that the intensity of the emitted light from the artificial light source can be decreased further since the measurement is supported

additionally by ambient light in its whole spectrum (including the infrared spectrum), wherein an overall measurement quality is enhanced. In a further embodiment the controlling unit is adapted to compare the intensity of the ambient light with a first threshold and to turn off the artificial light source, if the intensity of the ambient light exceeds the first threshold. In this embodiment the intensity of the emitted light from the artificial light source is decreased to zero intensity. This is made, if the ambient light reflected through the skin of the user is providing an intensity high enough to perform the measurement on its own. Hence, it is advantageous that the artificial light source can be turned off completely without affecting the measurement quality, wherein the highest power saving possible regarding the artificial light source is realized.

In a further embodiment the controlling unit is adapted to compare the intensity of the ambient light with a second threshold and to turn on the artificial light source, if the intensity of the ambient light exceeds the second threshold. In this embodiment a second threshold is intended. The second threshold is preferably of a higher value than the first threshold. If the second threshold is exceeded by the intensity of the ambient light, it is intended to use only emitted light from the artificial light source for measuring purposes because too much ambient light can lead to a saturation of the corresponding sensor. Hence, the measurement quality is preserved in view of to much ambient light, since the first sensor is protected from saturation by the optical high-pass filter.

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 first embodiment of a heart rate monitor according to the invention,

Fig. 2 shows artifacts caused by ambient light,

Fig. 3 shows the effect of an optical filter blocking out infrared light,

Fig. 4 shows a diagram describing a relationship between an intensity of light to a quality of a sensed signal,

Fig. 5 shows a second embodiment of a heart rate monitor according to the invention,

Fig. 6 shows a measurement signal sensed with and without light from the artificial light source, and

Fig. 7 shows a bottom view of a third embodiment of the heart rate monitor according to the invention. DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows schematically a heart rate monitor 10 which is arranged at a wrist 12 of a user. The heart rate monitor 10 comprises a LED 14 as an artificial light source. The LED 14 can emit artificial light 16 into skin 18 of the user. Within the skin 18 blood vessels 20 are arranged. The emitted artificial light 16 is partially absorbed by the blood within the blood vessels 20. Additionally, the artificial light 16 is scattered throughout the skin 18 as to be reflected back to a photodetector-diode 22 which is a first sensor. The photodetector-diode 22 is sensing the reflected light 16 through the skin 18 of the user and is generating a first sensor-signal thereof. The first sensor-signal is transmitted via a line 24 to a processing unit 26. The processing unit 26 is receiving the first sensor-signal and is determining a pulse and/or a heart rate of the user from the first sensor-signal. The information regarding the pulse and/or the heart rate is transmitted via a line 28 to displaying means 30. The displaying means 30 can be for example an LCD display for displaying the value of the pulse and/or the heart rate to the user.

The first sensor-signal is also transmitted from the line 24 to a controlling unit 32. In preferred embodiments the processing unit 26 and the controlling unit 32 are combined to one microcontroller. The controlling unit 32 is determining a needed intensity of emitted light from the artificial light source 14 and is controlling this intensity via a line 34 which is connected with the LED 14. Additionally, the heart rate monitor 10 comprises a battery 36 as a power source for the whole heart rate monitor 10. The battery 36 is powering the entire heart rate monitor 10 via a line 38.

Ambient light 40 is transmitted from an ambient light source 42. The ambient light 40 is emitted into the skin 18 of the user and therein reflected similar to the light 16.

Additionally, the heart rate monitor 10 comprises an optical high-pass filter 44. The optical high-pass filter 44 is arranged within an optical path 46. The optical path 46 is shown by the traveling directions of the ambient light 40. The optical filter 44 is an infrared blocking edge filter, which blocks out infrared light 48 from ambient light 40 with respect to the photodetector-diode 22. In other embodiments, the optical high-pass filter 44 is integrated into the photodetector-diode 22.

Fig. 2 shows a diagram 50 with an abscissa 52 for time and an ordinate 54 for a sensor-signal of a photodetector-diode in a heart rate monitor according to the heart rate monitor 10 of Fig.l but without the optical high-pass filter 44. Within the diagram 50 a curve 56 is shown describing the signal voltage over time. The heart rate monitor has been moved in a way wherein ambient light creates artifacts in the sensor-signal. As it can be seen within the curve 56 a plurality of steps 58 are created caused by movements and in particular by changes between shadowed areas and lighted areas affecting the photodetector-diode. It can be understood easily that this kind of artifact is leading to a loss of measurement quality.

Fig. 3 shows a further diagram 60. It comprises an abscissa 62 for time and an ordinate 64 for a voltage of a sensor-signal. Within the diagram 60 two curves 66 and 68 are shown. Curve 66 shows a sensor-signal created by a photodetector-diode in a heart rate monitor without an optical high-pass filter. The heart rate monitor has been moved in a way wherein ambient light creates artifacts in the sensor-signal. Therefore, steps 70 are arising caused by the movement. Curve 68 shows a first sensor signal of the photodetector-diode 22 of the heart rate monitor 10 over time. The heart rate monitor 10 has been moved in the same way as the heart rate monitor generating the curve 66. As a difference between those heart rate monitors, the heart rate monitor 10 corresponding to the curve 68 comprises the optical high-pass filter 44. As it can be seen, no steps 70 are arising and therefore the quality of the first sensor-signal is substantially increased. As an additional effect, the intensity of the artificial light source 14 can be decreased since a less dominant signal of the pulse is needed as to determine the actual heart rate and/or pulse.

Fig. 4 shows a diagram 72 with an abscissa 74 for the intensity of the emitted light 16 and an ordinate 76 for a quality of the first sensor-signal. Within the diagram 72 a curve 78 is shown, describing the relationship between the intensity and the signal quality. As it can be seen, curve 78 is almost a step function. This means that a decrease of the intensity of emitted artificial light in direction of an arrow 80 can be made near to an interval 82 without a relevant loss of signal quality and therefore measurement quality. Hence, the LED 14 of Fig. 1 can be driven near an upper boundary of the interval 82 without a relevant loss of signal quality. Hence, a high measurement quality is given with a relatively low intensity of artificial light, wherein electrical energy from the battery 36 is saved.

Fig. 5 shows schematically a further embodiment of a heart rate monitor 84. The heart rate monitor 84 comprises the same components as the heart rate monitor of Fig. 1. Therefore, the same components are described with the same reference signs. Additionally, a second photodetector-diode 86 is provided. The second photodetector-diode 86 is adapted for sensing ambient light 40 and to generate a second sensor-signal. The second sensor-signal is transmitted via a line 88 to the processing unit 26. The heart rate monitor 84 comprises the first sensor 22 and the second photodetector-diode 86. Additionally, it comprises a housing 90 in which most of its components are arranged. The contact part 92 is making contact to the skin 18 of the user, since it is worn properly. The first sensor 22 is arranged in a center area the contact part 92. Hence, the first photodetector-diode 22 is shadowed with respect to the ambient light source 42. Additionally, the sensor 22 is working with the optical high-pass filter 44, whereby the first photodetector-diode 22 is receiving a smaller amount of ambient light then the second photodetector-diode 86.

By contrast, the second photodetector-diode 86 is arranged in a border area 98 of the contact part 92. The second photodetector-diode 86 is arranged near at least one edge 96 as to receive as much reflected ambient light 40 as possible. Since the processing unit 26 is now provided with two sources for two sensor-signals, the heart rate monitor 84 can be driven in different modes.

In a first mode the heart rate monitor 84 can measure the pulse and/or heart rate only with the artificial light source 14 and the photodetector-diode 22 without being affected by the ambient light 40. Additionally, the intensity of the emitted light 16 from the artificial light source 14 can be adjusted according to a quality as described above.

In a second mode, the heart rate monitor 84 can measure the pulse and/or heart rate based on ambient light 40 with the second photodetector-diode 86. If the processing unit 26 is determining that enough ambient light 40 is received by the second photodetector-diode 86, it can determine the pulse and/or the heart rate from the second sensor-signal or from a combined signal, combined from the first sensor-signal and the second sensor-signal.

The processing unit 26 is determining the possibility to switch from the first sensor-signal to the second sensor-signal by measuring the intensity of the ambient light 40. Additionally, the controlling unit 32 is receiving and comparing the intensity of the ambient light 40 to a predefined first threshold. If the intensity exceeds the first threshold the artificial light source 14 can be turned off entirely or at least be decreased in its intensity by the controlling unit 32. As a further feature the controlling unit 32 is also comparing the intensity of the ambient light 40 to a predefined second threshold. If the intensity exceeds the second threshold, which is higher than the first threshold, then the artificial light source 14 is turned on or at least increased in its intensity. Hence, it is avoided to analyze a sensor-signal generated by a saturated sensor 86, wherein the measurement quality is preserved. Since the first sensor 22 is shadowed and protected by the filter 44 a saturation is avoided. Hence, a measurement quality is assured, even if ambient light is saturating the sensor 86.

Fig. 6 shows a diagram 100 within an abscissa 102 for time and an ordinate 104 for a voltage of the photodetector-diodes22 and 86. Within the diagram 100 a curve 106 is shown which describes a combined sensor-signal combined of the first and the second sensor signal. Within a first interval 108 the curve 106 is based on the first sensor-signal from the first photodetector-diode 22. For a second interval 110 the same curve 106 is based on the second sensor-signal from the second photodetector-diode 86 and therefore produced by ambient light 40. In a final interval 112 the curve 106 is based on the first sensor-signal as in interval 108.

As it can be seen the signal-to-noise ratio is not changing substantially between the intervals 108 to 110. The major difference between the intervals is a different basic level of voltage. Hence, it can be understood easily that a heart rate measurement based on ambient light 40 as shown within the interval 110 can be made sufficiently, if the conditions are set right. Hence, electrical energy for the artificial light source 14 can be saved efficiently, wherein measurement quality is preserved.

Fig. 7 shows a bottom view of a heart rate monitor 114. It comprises a housing 116 with a contact part 118. In a center area 120 a plurality of LEDs 14 and a photodetector- diode with an optical high -pass filter 44 are arranged.

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. Heart rate monitor (10, 84, 114) for measuring a heart rate of a user, comprising:

at least one artificial light source (14) for emitting light (16) into skin (18) of the user,

- a first sensor (22) for sensing light (16, 40) reflected through the skin (18) of the user and for generating a first sensor-signal in response to sensed light (16, 40), and

an optical high-pass filter (44) for filtering out infrared light (48), wherein the optical high-pass filter (44) is arranged in an optical path (46) before the first sensor (22) for filtering out infrared light (48) from light (40) travelling to the first sensor (22).

2. Heart rate monitor (10, 84, 114) as claimed in claim 1, wherein the artificial light source (14) is a light emitting diode (14).

3. Heart rate monitor (10, 84, 114) as claimed in claim 1, wherein the sensor (22, 86) is a photodetector-diode (22, 86).

4. Heart rate monitor (10, 84, 114) as claimed in claim 3, wherein the optical high-pass filter (44) is integrated into the photodetector-diode (22, 86).

5. Heart rate monitor (10, 84, 114) according to claim 1, further comprising a housing (90, 116) having a contact part (118) for making physical contact with the skin (18) of the user, wherein the first sensor (22) is arranged in a center area (120) of the contact part (118).

6. Heart rate monitor (84) as claimed in claim 1, further comprising a second sensor (86) adapted to sense ambient light (40) reflected through the skin (18) of the user and for generating a second sensor-signal in response to sensed ambient light (16, 40) .

7. Heart rate monitor (84) as claimed in claim 6, further comprising the housing

(90) having the contact part (92) for making physical contact with the skin (18) of the user, wherein the second sensor (86) is arranged in a border area (98) of the contact part (86).

8. Heart rate monitor (10, 84) as claimed in claim 1, further comprising a controlling unit (32) for receiving the first sensor-signal and for controlling an intensity of emitted light (16) from the artificial light source (14) in response to the first sensor-signal.

9. Heart rate monitor (10, 84) as claimed in claim 8, wherein the controlling unit (32) is adapted to determine a quality of the first sensor-signal and to control the intensity of the emitted light (16) from the artificial light source (14) in response to the quality.

10. Heart rate monitor (84) as claimed in claim 6, further comprising a controlling unit (32) for receiving the second sensor-signal and for controlling the intensity of emitted light (16) from the artificial light source (14) in response to the second sensor-signal.

11. Heart rate monitor (84) as claimed in claim 10, wherein the controlling unit (32) is adapted to compare the intensity of the ambient light (40) with a first threshold and to turn off the artificial light source (14), if the intensity of the ambient light (40) exceeds the first threshold.

12. Heart rate monitor (84) as claimed in claim 12, wherein the controlling unit (32) is adapted to compare the intensity of the ambient light (40) with a second threshold and to turn on the artificial light source (14), if the intensity of the ambient light (40) exceeds the second threshold.

13. Heart rate monitoring method for measuring a heart rate of a user, comprising:

emitting light (16) into skin (18) of the user,

sensing light (16, 40) reflected through the skin (18) of the user, - generating a first sensor-signal in response to sensed light (16, 40), and

filtering out infrared light (48) before sensing the light reflected through the skin.