(12) Unlted States Patent
`(10) Patent No.:
`US 9,392,946 B1
`
`Sarantos et al.
`(45) Date of Patent:
`Jul. 19, 2016
`
`US009392946B1
`
`(54) HEART RATE SENSOR WITH
`HIGH-ASPECT—RATIO PHOTODETECTOR
`ELEMENT
`.
`_
`_
`.
`(71) App11cant: Fltblt, Inc., San Franc1sco, CA (US)
`
`(72)
`
`Inventors: Chris H. Sarantos, San Francisco, CA
`(US); Peter W. Richards, San Francisco,
`CA (US)
`.
`_
`_
`.
`(73) Ass1gnee: Fltblt, Inc., San Franc1sco, CA (US)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 14/724,750
`
`(22)
`
`Filed:
`
`May 23, 2015
`
`(51)
`
`2006.01
`E2006 01;
`
`Int. Cl.
`A613 5/024
`A613 5/00
`(52) U-S- 0-
`CPC ......... A613 5/02427 (2013.01); A613 5/02438
`(2013.01);A6IB 5/681(2013.01)
`(58) Field of Classification Search
`CPC . A61B 5/02427; A61B 5/02438; A61B 5/681
`See application file for complete search history.
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`Primary Exammer * BO J Peng
`(74) Attorney, Agent, or Firm 7 Weaver Austin Vi11eneuve
`& Sampson LLP
`
`ABSTRACT
`(57)
`Heart rate sensors including high-aspect-ratio photodetector
`elements are discussed herein. Such high-aspect-ratio photo-
`detector elements may prov1de 1mproved s1gna1-strength-to-
`power-consumption performance for heart rate sensors incor-
`.
`.
`porating such photodetector elements as compared with heart
`rate sensors 1ncorporat1ng, for example, square photodetector
`elements-
`
`30 Claims, 14 Drawing Sheets
`
`102
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`104
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`212
`
` 102
`01101101]
`
`1
`
`APPLE 1014
`
`APPLE 1014
`
`1
`
`

`

`US 9,392,946 B1
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`
`
`4
`
`

`

`U.S. Patent
`
`Jul. 19, 2016
`
`Sheet 1 of 14
`
`US 9,392,946 B1
`
`102
`
`102
`
`128
`
`128
`
`104
`
`112
`
`212
`
`102
`
` flflflflflfl
`
`FIG. 1
`
`(Prior Art)
`
` flflflflflfl
`
`FIG. 2
`
`104
`
`102
`
`5
`
`

`

`US. Patent
`
`Jul. 19, 2016
`
`Sheet 2 of 14
`
`US 9,392,946 B1
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`—————————-350
`
`,------ 348
`
`346
`
`344
`
`FIG. 3
`
`6
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`

`

`US. Patent
`
`Jul. 19, 2016
`
`Sheet 3 of 14
`
`US 9 392,946 B1
`
`408
`
`1mm
`
`
`3mm
`
`2mm
`
`>' 0mm
`
`-1mm
`
`-2mm
`
`-3mm
`0mm
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`U.S. Patent
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`Jul. 19, 2016
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`Sheet 4 of 14
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`US 9,392,946 B1
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`U S. Patent
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`Jul. 19, 2016
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`Sheet 5 of 14
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`US 9 392,946 B1
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`U.S. Patent
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`Jul. 19, 2016
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`Sheet 6 of 14
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`US 9,392,946 B1
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`US. Patent
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`Jul. 19, 2016
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`Sheet 7 of 14
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`US 9,392,946 B1
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`U.S. Patent
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`US 9,392,946 B1
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`U.S. Patent
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`Jul. 19, 2016
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`Sheet 9 of 14
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`US 9,392,946 B1
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`US. Patent
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`Jul. 19, 2016
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`Sheet 11 of 14
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`US. Patent
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`Jul. 19, 2016
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`Sheet 12 of 14
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`US. Patent
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`Jul. 19 2016
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`Sheet 13 of 14
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`U.S. Patent
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`JuL]9,2016
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`Sheet140f14
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`US 9,392,946 B1
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`US 9,392,946 B1
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`1
`HEART RATE SENSOR WITH
`HIGH-ASPECT—RATIO PHOTODETECTOR
`ELEMENT
`
`BACKGROUND
`
`Heart rate may be measured using any of a variety of
`different sensors, including, for example, electrode-based
`sensors, such as EKG sensors, and optical sensors, such as
`photoplethysmographic (PPG) sensors. PPG sensors typi-
`cally include a light source and a photodetector that are placed
`adjacent to a person’s skin. The light source and photodetec-
`tor are typically arranged so that light from the light source
`cannot reach the photodetector directly. However, when the
`PPG sensor is placed adjacent to a person’s skin, light from
`the light source may diffuse into the person’ s flesh and then be
`emitted back out of the person’s flesh such that the photode-
`tector can detect it. The amount of such light that is emitted
`from the person’s flesh may vary as a function of heart rate,
`since the amount of blood present in the flesh varies as a
`function of heart rate and the amount of light that is emitted
`from the person’s flesh, in turn, varies as a function of the
`amount of blood present.
`The assignee of this application, Fitbit, Inc., makes wear-
`able fitness monitoring devices, some of which, such as the
`Charge HRTM and the SurgeTM, incorporate PPG sensors that
`include two high-brightness, green light-emitting diodes
`(LEDs) that are spaced approximately 8 mm apart, as well as
`a 2 mm square photodetector element
`located midway
`between the LEDs. Various other companies that make wear-
`able fitness monitoring devices utilize a similar architecture.
`For example, the Basis PeakTM incorporates two green LEDs
`with a square photodetector element located midway between
`them, as does the Motorola Moto 360““.
`FIG. 1 depicts a simplified representation of a prior-art
`wristband-type wearable fitness monitor 100 that incorpo-
`rates a PPG sensor. The wearable fitness monitor 100, in this
`example,
`includes a housing 104 with two straps 102
`attached; the straps 102 may be used to fasten the housing 104
`to a person’s forearm, in much the same manner as a watch
`(indeed, many such devices may incorporate timekeeping
`functionality as well). The PPG sensor,
`in this example,
`includes two light sources 108, with a photodetector element
`112 interposed midway between them on a back face 128 of
`the housing 104;
`the photodetector element 112 in this
`example has a photosensitive area with a square aspect ratio.
`When the wearable fitness monitor 100 is worn by a person in
`a manner similar to a wristwatch, the back face 128 may be
`pressed against the person’s skin, allowing the light sources
`108 to illuminate the person’s skin. The photodetector ele-
`ment 112 may then measure the amount of that light that is
`then emanated back out of the person’s skin. Control logic
`(not pictured) within the housing 104 may then calculate the
`person’ s heart rate based on fluctuations in the amount oflight
`measured by the photodetector element 112.
`
`SUMMARY
`
`Details of one or more implementations of the subject
`matter described in this specification are set forth in the
`accompanying drawings and the description below. Other
`features, aspects, and advantages will become apparent from
`the description, the drawings, and the claims. Note that the
`relative dimensions ofthe following figures may not be drawn
`to scale unless specifically indicated as being scaled draw-
`ings.
`
`2
`
`In some implementations, an apparatus having a light
`source and one or more discrete photodetector elements may
`be provided. Each photodetector element may have a first
`edge having a first length and may also have a first width in a
`direction perpendicular to the first edge. The apparatus may
`also include control logic, which may be communicatively
`connected with the light source and each photodetector ele-
`ment and configured to cause the light source to emit light,
`obtain one or more measured light intensity measurements
`from the one or more photodetector elements, and determine
`a heart rate measurement based, at least in part, on the one or
`more light intensity measurements. In such implementations,
`the ratio of the first length to the first width of each photode-
`tector may be substantially between 2:1 to 5:1.
`In some such implementations, the first edge of each pho-
`todetector element may be perpendicular or transverse to an
`axis radiating out from a center of the light source.
`In some implementations of the apparatus, the light source
`may include a plurality of light-emitting devices.
`In some such implementations, the plurality of light-emit-
`ting devices may include at least two light-emitting devices
`that predominantly emit light of different wavelengths. In
`some further or alternative such implementations, there may
`be a plurality ofphotodetector elements arranged in a pattern,
`and the plurality of light-emitting devices may be collocated
`at a center point of the pattern of photodetector elements. In
`some such implementations, each photodetector element in
`the pattern may be equidistant from the center of the light
`source and/or evenly spaced within the pattern.
`In some implementations of the apparatus, the ratio of the
`first length to the first width of each photodetector element
`may be substantially between 2:1 to 3.5: 1. In some other
`implementations of the apparatus, the ratio of the first length
`to the first width of each photodetector element may be sub-
`stantially between 3.5:1 to 5:1.
`In some implementations of the apparatus, each photode-
`tector element may have a first length between 1 mm and 5
`mm and a first width between 0.5 mm and 2 mm, with the ratio
`of the first length to the first width substantially between 2:1
`to 5:1, and each such photodetector element may be posi-
`tioned such that an edge of the photodetector element closest
`to the light source is between 1 mm and 4 mm from the light
`source.
`
`In some implementations of the apparatus, there may be a
`pattern of photodetector elements that includes three or four
`photodetector elements that are equidistantly spaced about
`the light source.
`In some implementations of the apparatus, the apparatus
`may also include a housing having a back face that includes
`one or more transparent window regions through which light
`may enter the apparatus. In such implementations, each pho-
`todetector element is positioned such that that photodetector
`element is overlapped by a corresponding one of the one or
`more transparent window regions, and the housing may be
`configured such that the back face is adjacent to the skin of a
`person wearing the apparatus when the apparatus is worn by
`that person.
`In some such implementations of the apparatus, the back
`face may include a thin window, and the window regions may
`be sub-regions of the window that are defined by the photo-
`detector elements. In some other or additional such imple-
`mentations, each photodetector element may be offset from
`the corresponding transparent window region by a corre-
`sponding gap in a direction normal to the photodetector ele-
`ment, and the gap may be free of optical light guides.
`In some implementations of the apparatus, each photode-
`tector element may, in addition to the first edge, have an
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`US 9,392,946 B1
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`3
`arcuate second edge opposite the first edge. The arcuate sec-
`ond edge may have a maximum distance from the first edge,
`when measured along a direction perpendicular to the first
`edge, that is equal to the first width.
`In some implementations, an apparatus may be provided
`that includes a first light source and a second light source, as
`well as a photodetector element interposed between the first
`light source and the second light source. The apparatus may
`also include control logic that is communicatively connected
`with the first and second light sources and the photodetector
`element and that is configured to cause the first light source
`and the second light source to emit light, obtain measured
`light intensity measurements from the photodetector element,
`and determine a heart rate measurement based, at least in part,
`on the light intensity measurements. In such implementa-
`tions, the photodetector element may be rectangular in shape,
`have a first edge with a first length, and have a second edge,
`perpendicular to the first edge, with a second length. Further-
`more, in such implementations, the ratio of the first length to
`the second length may be substantially between 2:1 to 5:1.
`In some such implementations, the first edge of each pho-
`todetector element may be perpendicular or transverse to an
`axis spanning between a center of the first light source and a
`center of the second light source.
`In some other or additional such implementations, the
`apparatus may include a housing having a back face that
`includes a transparent window region that overlaps the pho-
`todetector element and two further window regions that are
`each associated with a different one of the first light source
`and the second light source and that allow light from the
`associated light source to pass through the back face. In such
`implementations, the first light source and the second light
`source may be the only light sources in the apparatus config-
`ured to emit light through the back face, and the housing may
`be configured such that the back face is adjacent to the skin of
`a person wearing the apparatus when the apparatus is worn by
`that person.
`In some implementations ofthe apparatus, the photodetec-
`tor element may be equidistant from the first light source and
`the second light source.
`In some implementations, an apparatus may be provided
`that includes a light source and one or more photodetectors,
`each photodetector having a photosensitive area. In such
`implementations, at least 90% of the photosensitive area of
`the photodetector is defined by a first dimension along a first
`axis and a second dimension along a second axis perpendicu-
`lar to the first axis. The apparatus may also, in such imple-
`mentations, include control logic that is communicatively
`connected with the light source and each photodetector and
`that is configured to cause the light source to emit light, obtain
`one or more measured light intensity measurements from the
`one or more photodetectors, and determine a heart rate mea-
`surement based, at least in part, on the one or more light
`intensity measurements. In such implementations, the ratio of
`the first dimension to the second dimension may be substan-
`tially between 2:1 to 5:1.
`In some implementations of the apparatus, the light source
`may include a plurality of light-emitting devices. In some
`such implementations, the plurality of light-emitting devices
`may include at least two light-emitting devices that predomi-
`nantly emit light of different wavelengths. In some further or
`alternative such implementations, there may be a plurality of
`photodetector elements arranged in a pattern, and the plural-
`ity of light-emitting devices may be collocated at a center
`point of the pattern of photodetector elements. In some such
`implementations, each photodetector element in the pattern
`
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`may be equidistant from the center of the light source and/or
`evenly spaced within the pattern.
`
`In some implementations of the apparatus, the ratio of the
`first length to the first width of each photodetector element
`may be substantially between 2:1 to 3.5: 1. In some other
`implementations of the apparatus, the ratio of the first length
`to the first width of each photodetector element may be sub-
`stantially between 3.5:1 to 5:1.
`
`In some implementations of the apparatus, each photode-
`tector element may have a first length between 1 mm and 5
`mm and a first width between 0.5 mm and 2 mm, with the ratio
`of the first length to the first width substantially between 2:1
`to 5:1, and each such photodetector element may be posi-
`tioned such that an edge of the photodetector element closest
`to the light source is between 1 mm and 4 mm from the light
`source.
`
`In some implementations of the apparatus, there may be a
`pattern of photodetector elements that includes three or four
`photodetector elements that are equidistantly spaced about
`the light source.
`
`In some implementations of the apparatus, the apparatus
`may also include a housing having a back face that includes
`one or more transparent window regions through which light
`may enter the apparatus. In such implementations, each pho-
`todetector element is positioned such that that photodetector
`element is overlapped by a corresponding one of the one or
`more transparent window regions, and the housing may be
`configured such that the back face is adjacent to the skin of a
`person wearing the apparatus when the apparatus is worn by
`that person.
`In some such implementations of the apparatus, the back
`face may include a thin window, and the window regions may
`be sub-regions of the window that are defined by the photo-
`detector elements. In some other or additional such imple-
`mentations, each photodetector element may be offset from
`the corresponding transparent window region by a corre-
`sponding gap in a direction normal to the photodetector ele-
`ment, and the gap may be free of optical light guides.
`In some implementations, an apparatus may be provided
`that includes a light source and at least one photodetector
`element. The apparatus may also include control logic that is
`communicatively connected with the light source and the
`photodetector element and that is configured to cause the light
`source to emit light, obtain at least one measured light inten-
`sity measurement from the at least one photodetector ele-
`ment, and determine a heart rate measurement based, at least
`in part, on the at least one light intensity measurement. In such
`implementations, the at least one photodetector element may
`subtend an angle at the center of the light source of substan-
`tially at least: 2~arctan
`
`radians, where rl. is a measurement of a distance from the
`center ofthe light source to the photodetector element, at least
`80% of the photodetector element covers an annular region
`centered on the center ofthe light source and defined by rl. and
`r0, and r0 is greater than rl. by not more than 2 millimeters.
`In some