Patent Publication Number: US-2017347902-A1

Title: Optical vital signs sensor

Description:
FIELD OF THE INVENTION 
     The invention relates to an optical vital signs sensor and a method of operating an optical vital signs sensor. 
     BACKGROUND OF THE INVENTION 
     Optical heart rate sensors are well known to monitor or detect vital signs like a heart rate of a user. Such a heart rate sensor can be based on a photoplethysmographic PPG sensor and can be used to acquire a volumetric organ measurement. By means of pulse oximeters, changes in light absorption of a human skin is detected and based on these measurements a heart rate or other vital signs of a user can be determined. The PPG sensors comprise a light source like a light emitting diode LED which is emitting light into the skin of a user. The emitted light is scattered in the skin and is at least partially absorbed by the blood. Part of the light exits the skin and can be captured by a photodiode. The amount of light that is captured by the photo diode can be an indication of the blood volume inside the skin of a user. A PPG sensor can monitor the perfusion of blood in the dermis and subcutaneous tissue of the skin through an absorption measurement at a specific wave length. If the blood volume is changed due to the pulsating heart, the scattered light coming back from the skin of the user is also changing. Therefore, by monitoring the detected light signal by means of the photodiode, a pulse of a user in his skin and thus the heart rate can be determined. 
     US 2014/0275852 A1 discloses a wearable fitness monitoring device including a motion sensor and a PPG sensor. 
     US 2013/0261415 A1 discloses an optical vital signs sensor which operates in a transmissive mode and comprises several light emitting diodes and several photo diodes. The sensor comprises a processing unit configured to dynamically configure an operational geometry of the sensor by controlling the intensity of one or more of the light emitting diodes and the gain of at least one of the photo detectors. 
     US 2010/0081901 A1 discloses an optical vital signs sensor having optical light emitting elements and light detecting elements. The light detecting elements can be adaptive light detecting elements to selectively detect light of certain wavelengths or from certain regions of the tissue of the user. 
     WO 2014/184447 A1 relates to a portable pulse measuring device having several light emitting sources and a light detector for detecting the intensity of the reflective light. The light emitting sources are arranged in an asymmetrical configuration in relation to the light detectors. 
     US 2010/0113948 A discloses a PPG sensor arranged behind the ear of a user. 
       FIG. 1  shows a basic representation of an operational principle of an optical heart rate sensor. In  FIG. 1 , a heart rate sensor is arranged on an arm of a user. The heart rate sensor  100  comprises a light source  110  and a photo detector  120  arranged at a fixed distance. The light source  110  emits light onto or in the skin  1000  of a user. Some of the light is reflected and the reflected light can be detected by the photo detector  120 . The optical heart rate sensor can be embodied as a PPG sensor. Such a PPG sensor is however not easily adaptable to different operating scenarios. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an optical vital signs sensor which is able to efficiently detect vital signs of a user during different operating conditions. 
     According to an aspect of the invention, an optical vital signs sensor is provided which comprises a light source having at least one light unit each configured to generate light. The light of the light source is directed towards the skin of the user. The optical vital signs sensor furthermore comprises at least one photo detector unit having at least one photo diode configured to detect light which is indicative of a reflection of the light from the at least one light unit in or from a skin of a user. The sensor furthermore comprises an adjusting unit (which can be mechanical or optical) configured to mechanically adjust a position of one of the at least one light unit relative to a position of one of the at least one photo diode thereby adjusting the distance between an active light unit and an active photo diode. 
     According to a further aspect of the invention, the sensor furthermore comprises a control unit configured to control an operation of the adjusting unit based on a signal quality of an output signal of the photo detector analyzed by a signal analyzing unit in the control unit thereby further adjusting a distance between an active light unit and an active photo diode. 
     According to an aspect of the invention, an optical vital signs sensor comprising a light source having at least one light unit configured to generate light is provided. The light of the light source is directed towards a skin of a user. The optical vital signs sensor further comprises at least one photo detector unit having a plurality of photo diodes configured to detect light which is indicative of a transmission or reflection of the light from the at least one light unit in or from the skin of the user. The adjusting unit can comprise or be implemented as a control unit configured to control an operation of the light source and/or the photo detector unit based on a signal quality of an output of the photo detector as analyzed by a signal analyzing unit in the control unit. At least one light unit from among the at least one light unit and/or at least one photo diode from among the plurality of photo diodes is selected and activated thereby adjusting a distance between an active light unit and an active photo diode or adjusting a position where the light from the at least one light source enters the skin of the user. 
     According to a further aspect of the invention, the signal quality is analyzed by the signal analyzing unit regarding at least one of a signal-to-noise ratio, an amplitude of the DC component of the output signal, an amplitude of the AC component of the output signal and a ratio between the AC and DC component of the output signal of the photo detector. 
     According to an aspect of the invention, a computer program product comprising a computer readable memory storing computer program code means for operating the optical vital signs sensor is provided. 
     While known optical vital signs sensors like heart rate sensors have a predetermined and fixed distance between the light source and the photo detector, the aspects of the invention relate to an optical vital signs sensor which allows a variation of the distance between the light source and the photo detector. By providing a variation mechanism, the optical vital signs sensor according to an aspect of the invention allows an adaptation on different detecting situations like different users, different climatic conditions. It should be noted that a larger distance between the light source and the detector is advantageous as the AC portion of the output signal increases over the DC portion of the output signal. On the other hand, if the distance between the light source and the photo detector is too high, the signal-to-noise ratio will suffer. The signal-to-noise ratio as well as the battery life may suffer. The optical vital signs sensor according to an aspect of the invention allows an adaptation of the distance between the light source and the photo detector. This variation can be achieved by changing the distance mechanically between the light source and the photo detector. Alternatively, a plurality of light sources and/or photo detectors can be provided as a control unit can select at least one light source and at least one photo detector from among the plurality of light sources and/or photo detectors, which can be arranged at different positions in order to achieve the desired distance between the light source and the photo detector. Alternatively, a light guide can be provided between the light source and a photo diode placed above a skin of a user, wherein such a light source comprises several controllable outputs at different positions. The position of the output of the light out of the light guide with reference to the photo detector is varied. The controllable outputs can be implemented by a steerable optical reflection layer which can be a liquid crystal or an electrochromic layer. 
     It shall be understood that a preferred embodiment of the present invention can also be a combination of the dependent claims or above embodiments or aspects with respective independent claims. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following drawings: 
         FIG. 1  shows a basic representation of an operational principle of an optical vital signs sensor, 
         FIG. 2  shows a schematic block diagram of an optical vital signs sensor according to an aspect of the invention, 
         FIGS. 3 to 7  each show a basic representation of an optical vital signs sensor according to aspects of the invention, 
         FIGS. 8 and 9  each show a representation of an optical vital signs sensor according to aspects of the invention, and 
         FIG. 10  shows a schematic representation of an optical vital signs sensor according to an aspect of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     According to an aspect of the invention, an optical vital signs sensor is provided which is based on a photoplethysmograph PPG sensor. Such a PPG sensor is depicted in  FIG. 1 . A light source emits light onto or into a skin  1000  of a user and some of the light is reflected and the reflected light can be detected by a photo detector  120 . The output of the photo detector can be analyzed to determine a heart rate or other vital signs of a user. 
     The PPG sensor or optical vital signs sensor according to an aspect of the invention can be implemented as a wrist device like a smart watch. The optical vital signs sensor according to an aspect of the invention can also be implemented as a device which is for example worn behind the ear of a user or at any other parts of the user as long as it is ensured that the optical vital signs sensor is placed directly onto the skin of a user. 
       FIG. 2  shows a schematic block diagram of an optical vital signs sensor according to an aspect of the invention. The optical vital signs sensor  100  comprises a light source  110  and a photo detector  120 . The light source  110  emits light onto or in the skin  1000  of a user at a certain position and the photo detector  120  is able to detect reflected or transmitted light. The light source may comprise several light units  111 ,  112  which can for example be implemented as light emitting diodes LED. The photo detector  120  may comprise several photo diodes  121 ,  122  which serve to detect light as reflected or transmitted from the skin  1000  of a user. The optical vital signs sensor  100  furthermore comprises a control unit  140  which can control the operation of the light source  110  and/or the photo detector  120 . The control unit  140  can optionally receive the output of the photo detector  120 . The control unit  140  comprises a signal analysis unit  141  which serves for analyzing the output signal of the photo detector  120 . 
     Optionally, the optical vital signs sensor  100  can comprise an adjustable light guide  130  which serves to guide the light from the light source  110  onto the skin  1000  of a user. The light guide  130  can be arranged between the light source  110  and the skin  1000  of the user. 
     Furthermore, optionally, the optical vital signs sensor  100  can comprise an adjusting unit  150  which serves to adjust the distance between the light source  110  or one of the light units  111 ,  112  and the photo detector or the photo diodes  121 ,  122 . The adjusting unit  150  can be controlled by the control unit  140  or can be adjusted by a user. 
     In addition, optionally, the optical vital signs sensor  100  comprises a display  160 , by means of which an (adjustable) feedback can be provided. In particular, the feedback to the user can be implemented by a green or red light or by a transitioning from red to green or from red via yellow to green. This feedback serves to enable the user to adjust the optimum distance for the detector. In addition or alternatively, the feedback can be visual, audible or haptic or a combination of these. The feedback can be provided via the display  160  or via other feedback means like a loudspeaker, etc. The function of the feedback is to improve the adjustment of the sensor for optimum performance. 
       FIG. 3  shows a basic representation of an optical vital signs sensor according to aspects of the invention. According to this aspect of the invention, the optical vital signs sensor  100  is implemented as a wrist device like a smart watch. The optical vital signs sensor  100  comprises two light units  111 ,  112  which can be implemented as light emitting diodes LED. Between the first and second light unit  111 ,  112 , the photo detector  120  with a plurality of photo diodes  121 - 12   n  each at a different position is provided. Optionally, a first and second light emitting diode  111 ,  112  and the photo diodes  121 - 12   n  are arranged in a line. 
     Optionally, all photo diodes  121 - 12   n  are activated and the signal analyzing unit  141  can receive the signals from all of the photo diodes  121 - 12   n  and can determine which signal has the best signal quality. The signal quality may be determined based on the signal to noise ratio or by the AC or DC amplitude or by the ratio between the AC and DC component. The control unit  140  may select one or a plurality of the photo diodes  121 - 12   n  which provides the best signal quality and may deactivate the other photo diodes  121 - 12   n.  The control unit  140  may activate all of the photo diodes either concurrently or subsequently in order to detect the signal quality of all of the photo diodes  121 - 2   n  and to decide which one has the best signal quality. The decision on the best signal quality may be determined on the signal-to-noise ratio, the best AC or DC amplitude or the best ratio between the AC and DC amplitude. 
     As each of the photo diodes  121 - 2   n  has a different position, the distance between the at least one light emitting diode  111 ,  112  and the selected photo diode  121 - 12   n  is changed. The variation of the distance between the active light emitting diode  111 ,  112  and the active photo diode  121 - 12   n  is selected by the control unit  140  for example based on the results of the signal analysis unit  141 . According to this aspect of the invention, there is no mechanical change between the positions of the photo diode and the light emitting diode. In contrast, at least one of the photo diodes  121 - 12   n  is selected. Accordingly, redundant photo detectors can be provided. These can be used with three photo diodes or with pairs of three photo diodes and three light emitting diodes. The pair with the best signal or best signal quality can be selected in order to exclude an influence of black spots on the skin, namely spots where no signal is detected. 
     If the photo diodes are used concurrently, they can enhance or debilitate the signal. If the signal is to be enhanced, multiple photo diodes can be used simultaneously. 
     Optionally, the wave length of the first light emitting diode  111  may be different to the wavelength of the second light emitting diode. Although in  FIG. 3  only two light emitting diodes are shown, it should be noted that the number of light emitting diodes may also be a larger one. 
     According to this aspect of the invention, the control unit  140  may select the first or the second light emitting diode  111 ,  112  or may select both of them simultaneously. However, a selection of one of the light emitting diodes is preferred. In particular, the light emitting diode which is resulting in the best signal quality at the photo diodes is selected and the output of the light emitting diodes can be increased by increasing its current input. This is advantageous as the robustness of the system is increased. The robustness is in particular important in view of a motion between the skin of the user and the sensor. 
     The selection of the light emitting diode which is resulting in the best output signal of the photo diode is advantageous in view of the power consumption of the sensor as the power consumption of one light emitting diode will be less than the power consumption of a multiple of light emitting diodes. 
     The control unit  140  is adapted to activate or deactivate the light emitting diodes  111 ,  112  and the photo diodes  121 - 12   n  to reduce the power consumption of the optical vital signs sensor. 
       FIG. 4  shows a basic representation of an optical vital signs sensor according to aspects of the invention. In this aspect of the invention, an alternative to the solution of  FIG. 3  is depicted. Here, only a single light emitting diode  111  is provided while several photo diodes  121 - 12   n  at different positions are provided. The light emitting diode  111  is arranged between the photo diodes  121 - 12   n.  Optionally, the positions of the photo diodes  121 - 12   n  and the light emitting device  111  are arranged in a line. As mentioned in the previous aspect of the invention, the control unit  140  may control the operation of the light emitting diode  111  as well as the photo diodes  121 - 12   n  to achieve an output signal with a good signal quality while still taking care of the power consumption of the sensor. 
       FIG. 5  shows a basic representation of an optical vital signs sensor according to aspects of the invention. According to this aspect of the invention, two light emitting diodes  111 ,  112  are provided which are arranged next to or adjacent to each other. The arrangement of the photo diodes  121 - 2   n  corresponds to the aspect of the invention according to  FIG. 4 . The operation of the control unit  140  corresponds to the operation of the control unit  140  according to  FIG. 2 . 
       FIG. 6  shows a basic representation of an optical vital signs sensor according to aspects of the invention. The optical vital signs sensor  100  according to the aspect of the invention of  FIG. 6  substantially corresponds to the optical vital signs sensor according to  FIG. 5  but the photo detector  120  comprises four arms  120   a,    120   b,    120   c,    120   d  each of several photo diodes, wherein the four arms are arranged in form of a cross, with the first and second light emitting diode  111 ,  112  being arranged at the centre of the cross. The operation of the control unit  140  according to this aspect corresponds to the operation of the control unit  140  according to the aspect of  FIG. 2 . 
       FIG. 7  shows a basic representation of an optical vital signs sensor according to aspects of the invention. The optical vital signs sensor  100  according to this aspect of the invention is based on the optical vital signs sensor according to  FIG. 6 . However, the photo detector  120  comprises six arms  120   a - 120   h  each having several photo diodes, wherein the arms  120   a - 120   h  are arranged around the first and second light emitting diode  111 ,  112  which are arranged in the centre of the arms. 
     The operation of the control unit according to the aspect of  FIG. 7  corresponds to the operation of the control unit  140  according to the aspect of  FIG. 2 . 
       FIG. 8  shows a representation of an optical vital signs sensor according to aspects of the invention. The optical vital signs sensor  100  according to this aspect of the invention comprises a light source with at least one light emitting diode  111 ,  112  as well as a photo detector with at least one photo diode  121 . The photo detector  120  with the at least one photo diode  121  can be coupled to an adjusting unit  150  which can adjust the distance between the at least one light emitting diode  111 ,  112  and the at least one photo diode  121 . The adjusting unit  150  is preferably a mechanical adjusting unit. Optionally, the adjusting unit  150  comprises an adjusting ring  151  which is coupled to the at least one photo diode  121 . By turning the ring  151  either electrically or mechanically, the at least one photo diode  121  is rotated. In  FIG. 8 , three different positions  121   a,    121   b,    121   c  of the photo diode  121  are depicted. It should be noted that more positions are possible. Thus, by turning the ring  151 , the distance between the at least one light emitting diode  111 ,  112  and the at least one photo diode  121  can be adjusted. The signal analysis unit  141  analyses the output signal of the photo detector  120  during the activation of the adjusting unit to determine a position of the at least one photo diode  121  at which the best signal quality for example in turns of signal-to-noise ratio, AC or DC amplitude or the ratio between AC and DC of the output signal is achieved. 
       FIG. 9  shows a representation of an optical vital signs sensor according to aspects of the invention. According to this aspect of the invention, the optical vital signs sensor  100  comprises three different photo diodes  121 - 23  and at least one light emitting diode  111 ,  112  which are arranged on a ring  151  of an adjustment unit  150 . With the adjusting unit  150 , the position of the at least one light emitting diode  111 ,  112  can be adjusted relative to the positions of the at least one photo diode  121 ,  123 . Thus, the distance between the photo diode and the light emitting diodes can be adjusted. 
     This can either be performed mechanically by a user which can directly or indirectly turn the ring  151 . In addition or alternatively, the control unit  140  may control the turning of the ring in the aspect according to  FIG. 8  as well as in the aspect according to  FIG. 9 . According to this aspect of the invention, the signal analysis unit  141  can analyse the output signal of the photo detector during the activation of the adjusting unit  150 , i.e. the turning of the ring  151 . If the optimal signal quality is achieved, the adjusting by means of the adjusting unit  50  can be stopped. 
     According to an aspect of the invention, the optical vital signs sensor according to  FIG. 8  or  FIG. 9  can be combined with an optical vital signs sensor according to one of the  FIGS. 2 to 7 . Thus, the mechanical adjustment by means of the adjustment unit and the adjustment ring  151  can be combined with the arrangement of the light emitting diode  111 ,  112  and the photo diodes  121 - 12   n  of  FIG. 3 ,  FIG. 4 ,  FIG. 5 ,  FIG. 6  or  FIG. 7 . 
     By means of the mechanical adjustment of the light emitting diodes and the photo diodes, the length of the optical path between at least one light emitting diode and at least one photo diode can be adjusted. This can be performed for example according to the skin type or morphology of the user. 
     According to one aspect of the invention, only one pair of light emitting diodes and one pair of photo sensors can be used. Thus, a low complex and low cost arrangement can be achieved while still being able to adjust the length of the optical light path according to the skin type and morphology of the user. 
     According to an aspect of the invention, the arrangement of the light emitting diodes  111 ,  112  as well as the photo detectors  121 - 2   n  according to the embodiments of  FIGS. 8 and 9  can optionally correspond to the arrangements of the light emitting diodes and photo diodes according to the  FIG. 3, 4, 5, 6 or 7 . 
     According to an aspect of the invention, the adjustment of the length of the optical light path in an optical vital signs sensor according to the invention is performed by mechanically adjusting the relative or absolute positions of the light emitting diodes and the photo diodes. 
       FIG. 10  shows a schematic representation of an optical vital signs sensor according to an aspect of the invention. The optical vital signs sensor  100  according to this aspect of the invention comprises a light source  110  for example in form of a light emitting diode as well as a photo detector in form of a photo diode  120 . Between the light emitting diode  110  and the photo detector  120 , a light guide  130  can be arranged. The light guide  130  comprises a first end  130   a  and a second end  130   b.  The first end  130   a  is coupled or is directed towards the light source  110 , i.e. light from the light source  110  is coupled into the light guide  130 . The light guide  130  comprises several for example voltage dependent reflective elements  131 - 133  which can be activated by a supply voltage or control voltage. 
     The voltage dependent reflective elements  131 - 133  are arranged at different positions. During operation, the control unit  140  can activate one of the voltage dependent reflective elements  131 - 133  such that the light from the light source  110  travels through the light guide  130  and exits the light guide at one of the voltage dependent reflective elements  131 - 133 . As the voltage dependent reflective elements  131 - 133  are arranged at different positions, by controlling the voltage dependent reflective elements, a position where the light enters the skin  1000  of the user can be adjusted or the distance between the light leaving the light guide and the photo detector  120  can be adjusted. As in the other aspects of the invention, the signal analysis unit  141  can detect the output signal of the photo detector  120  while several of the voltage dependent reflective elements  131 - 133  are activated in order to determine that voltage dependent reflective elements at which the signal quality of the photo detector is at its best. 
     According to the aspects of the invention, the control unit  140  can control the effective distance between the active light source and the active photo detector or photo diode. 
     According to a further aspect of the invention, a mechanical adjusting unit may also be introduced according to the aspect of  FIG. 10 . Here, in addition to the operation of the light guide  130 , a mechanical adjusting unit may be provided to further mechanically adjust the position of the light guide  130 . 
     Other variations of the disclosed embodiment 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 in the indefinite article “a” or “an” does not exclude a plurality. 
     A single unit or device may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutual different dependent claims does not indicate that a combination of these measurements cannot be used to advantage. A computer program may be stored/distributed on a suitable medium such as an optical storage medium or a solid state medium, supplied together with or as a part of other hardware, but may also be distributed in other forms such as via the internet or other wired or wireless telecommunication systems. 
     Any reference signs in the claims should not be construed as limiting the scope.