US 9,392,946 B1
`(10) Patent No.:
`a2) United States Patent
`Sarantoset al.
`(45) Date of Patent:
`Jul. 19, 2016
`
`
`US009392946B1
`
`(54) HEART RATE SENSOR WITH
`HIGH-ASPECT-RATIO PHOTODETECTOR
`ELEMENT
`.
`cae
`.
`(71) Applicant: Fitbit, Inc., San Francisco, CA (US)
`
`(72)
`
`Inventors: Chris H. Sarantos, San Francisco, CA
`(US); Peter W. Richards, San Francisco,
`CA (US
`(US)
`.
`va,
`.
`(73) Assignee: Fitbit, Inc., San Francisco, CA (US)
`
`(*) Notice:
`
`Subject to any disclaimer, the term ofthis
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 14/724,750
`
`(22)
`
`Filed:
`
`May28, 2015
`
`2006.01
`
`S008 Ol)
`
`(51)
`
`Int. Cl.
`A6IB 5/024
`
`AGIB 5/00
`(52) U.S. Cl.
`CPC oo. A6I1B 5/02427 (2013.01); A6IB 5/02438
`(2013.01); A61B 5/681 (2013.01)
`(58) Field of Classification Search
`CPC . A61B 5/02427; A61B 5/02438; A61B 5/681
`See applicationfile for complete search history.
`
`(56)
`
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`Primary Examiner — Bo J Peng
`(74) Attorney, Agent, or Firm — Weaver Austin Villeneuve
`& Sampson LLP
`
`ABSTRACT
`(57)
`Heart rate sensors including high-aspect-ratio photodetector
`elements are discussed herein. Such high-aspect-ratio photo-
`detector elements may provide improved signal-strength-to-
`power-consumption performance forheart rate sensors incor-
`porating suchphotodetector elements as comparedwithheart
`.
`.
`rate sensors incorporating, for example, square photodetector
`elements.
`
`30 Claims, 14 Drawing Sheets
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`102
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`108
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`128
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`212
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` 102z
`000000
`
`1
`
`APPLE 1014
`
`APPLE 1014
`
`1
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`

`

`US 9,392,946 B1
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`Action Regarding a Patent or Trademark,”filed Oct. 29, 2015, inU.S.
`District Court of Delaware [Re: U.S. Pat. No. 8,868,377, 8,920,332,
`and 9,089,760].
`
`
`
`4
`
`

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`U.S. Patent
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`Jul. 19, 2016
`
`Sheet 1 of 14
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`US 9,392,946 B1
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`102
`
`102
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`108
`
`128
`
`108
`
`128
`
`104
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`104
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`FIG. 1
`(Prior Art)
`
`102
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`5
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`

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

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`U.S. Patent
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`Jul. 19, 2016
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`Sheet 3 of 14
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`US 9,392,946 B1
`
`
`
`Imm
`
`2mm
`
`3mm
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`4mm
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`5mm
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`FIG. 4
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`
`7
`
`

`

`U.S. Patent
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`Jul. 19, 2016
`
`Sheet 4 of 14
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`US 9,392,946 B1
`
`508
`
`aee= 542
`
`3mm
`
`2mm
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`Imm
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`-Imm -2mm
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`> Omm
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`mm1
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`FIG. 5
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`(ap)Ayisuazyyj2913d0pazijewsioN
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`8
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`

`

`U.S. Patent
`
`Jul. 19, 2016
`
`Sheet 5 of 14
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`US 9,392,946 B1
`
`Optical AC Power Beam
`
`y(mm)
`
`9
`
`

`

`U.S. Patent
`
`Jul. 19, 2016
`
`Sheet 6 of 14
`
`US 9,392,946 B1
`
`1.65 mm
`
`1
`
`2
`
`3
`4
`5
`7
`6
`Photodiode AspectRatio (Y:X)
`
`8
`
`9
`
`10
`
`FIG. 7
`
`AC Power vs. Aspect Ratio, Close Edge
`2.60 mm
`geomeaENAANANANAETEEETTTTTTTSN,
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`FIG. 8
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`
`
`

`

`U.S. Patent
`
`Jul. 19, 2016
`
`Sheet 7 of 14
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`US 9,392,946 B1
`
` 908on-_ 3:
`
`Yi W294
`
`FIG. 11
`
`FIG. 12
`
`11
`
`

`

`U.S. Patent
`
`US 9,392,946 B1
`
`FIG. 13
`
`FIG. 14
`
`FIG. 15
`
`FIG. 16
`
`12
`
`
`
`

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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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`FIG. 17
`
`FIG. 18
`
`13
`
`
`

`

`US 9,392,946 B1
`
`U.S. Patent 1.5
`
`2
`Annular Width (w) in mm
`
`FIG. 20
`
`14
`
`

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`U.S. Patent
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`Jul. 19, 2016
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`Sheet 11 of 14
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`US 9,392,946 B1
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`2112 -----"
`
`15
`
`

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`U.S. Patent
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`Jul. 19, 2016
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`Sheet 12 of 14
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`US 9,392,946 B1
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`2226
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`FIG. 24
`
`16
`
`

`

`U.S. Patent
`
`Jul. 19, 2016
`
`
`
`FIG. 25
`
`LY
`
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`
`
`17
`
`
`
`

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`U.S. Patent
`
`Jul. 19, 2016
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`Sheet 14 of 14
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`US 9,392,946 B1
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`
`
`
`
`
`2706
`
`2768
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`
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`
`FIG. 27
`
`18
`
`

`

`US 9,392,946 B1
`
`1
`HEART RATE SENSOR WITH
`HIGH-ASPECT-RATIO PHOTODETECTOR
`ELEMENT
`
`BACKGROUND
`
`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 havinga first length and mayalso have a first width ina
`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-
`Heart rate may be measured using any of a variety of
`ment and configured to cause the light source to emitlight,
`different sensors, including, for example, electrode-based
`obtain one or more measured light intensity measurements
`sensors, such as EKG sensors, and optical sensors, such as
`from the one or more photodetector elements, and determine
`photoplethysmographic (PPG) sensors. PPG sensors typi-
`a heart rate measurementbased, at least in part, on the one or
`cally includealight source and a photodetector that are placed
`morelight intensity measurements. In such implementations,
`adjacentto a person’s skin. The light source and photodetec-
`the ratio of thefirst length to thefirst width of each photode-
`tor are typically arranged so that light from the light source
`tector may be substantially between 2:1 to 5:1.
`cannot reach the photodetector directly. However, when the
`In some such implementations,the first edge of each pho-
`PPGsensoris placed adjacent to a person’s skin, light from
`todetector element may be perpendicular or transverse to an
`the light source maydiffuse into the person’s flesh and then be
`axis radiating out from a centerofthe light source.
`emitted back out of the person’s flesh such that the photode-
`In some implementationsof the apparatus, the light source
`tector can detect it. The amount of such lightthat is emitted
`may include a plurality of light-emitting devices.
`from the person’s flesh may vary as a function of heart rate,
`In some such implementations, the plurality of light-emit-
`since the amount of blood present in the flesh varies as a
`ting devices may includeat least two light-emitting devices
`function of heart rate and the amountof light that is emitted
`that predominantly emit light of different wavelengths. In
`somefurther or alternative such implementations, there may
`from the person’s flesh, in turn, varies as a function of the
`bea plurality ofphotodetector elements arrangedin a pattern,
`amountof blood present.
`and the plurality of light-emitting devices may be collocated
`The assignee ofthis application, Fitbit, Inc., makes wear-
`at a center point of the pattern of photodetector elements. In
`able fitness monitoring devices, some of which, such as the
`some such implementations, each photodetector element in
`Charge HR™and the Surge™, incorporate PPG sensorsthat
`the pattern may be equidistant from the center of the light
`include two high-brightness, green light-emitting diodes
`source and/or evenly spaced within the pattern.
`(LEDs)that are spaced approximately 8 mm apart, as well as
`In some implementations of the apparatus, the ratio of the
`a 2 mm square photodetector element
`located midway
`first length to the first width of each photodetector element
`between the LEDs. Various other companies that make wear-
`may be substantially between 2:1 to 3.5:1. In some other
`able fitness monitoring devices utilize a similar architecture.
`implementations of the apparatus, the ratio of thefirst length
`For example, the Basis Peak™incorporates two green LEDs
`to the first width of each photodetector element may be sub-
`witha square photodetector element located midway between
`stantially between 3.5:1 to 5:1.
`them, as does the Motorola Moto 360™.
`In some implementations of the apparatus, each photode-
`FIG. 1 depicts a simplified representation of a prior-art
`tector element may havea first length between 1 mm and 5
`wristband-type wearable fitness monitor 100 that incorpo-
`mm anda first width between 0.5 mm and 2 mm,with the ratio
`rates a PPG sensor. The wearable fitness monitor 100, in this
`of thefirst length to the first width substantially between 2:1
`example,
`includes a housing 104 with two straps 102
`to 5:1, and each such photodetector element may be posi-
`attached; the straps 102 may be used to fasten the housing 104
`tioned suchthat an edge of the photodetector element closest
`to a person’s forearm, in much the same manneras a watch
`to the light source is between 1 mm and 4 mm from the light
`(indeed, many such devices may incorporate timekeeping
`source.
`functionality as well). The PPG sensor,
`in this example,
`includes twolight 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 aspectratio.
`Whenthe wearable fitness monitor 100 is worn by a person in
`a mannersimilar 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 amountofthat 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 amountoflight
`measured by the photodetector element 112.
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`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 dimensionsofthe following figures may not be drawn
`to scale unless specifically indicated as being scaled draw-
`ings.
`
`60
`
`65
`
`19
`
`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
`mayalso include a housing having a back face that includes
`one or more transparent window regions through whichlight
`mayenter the apparatus. In such implementations, each pho-
`todetector elementis 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 adjacentto the skin ofa
`person wearing the apparatus whenthe 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 normalto the photodetector ele-
`ment, and the gap maybefree of optical light guides.
`In some implementations of the apparatus, each photode-
`tector element may, in addition to the first edge, have an
`
`19
`
`

`

`US 9,392,946 B1
`
`4
`may be equidistant from the centerofthe light source and/or
`evenly spaced within the pattern.
`
`3
`arcuate second edge opposite the first edge. The arcuate sec-
`ond edge may have a maximum distance from thefirst edge,
`when measured along a direction perpendicular to the first
`In some implementations of the apparatus, the ratio of the
`edge, that is equal to thefirst width.
`first length to the first width of each photodetector element
`In some implementations, an apparatus may be provided
`may be substantially between 2:1 to 3.5:1. In some other
`that includesafirst light source and a secondlight source, as
`implementations of the apparatus, the ratio of thefirst length
`well as a photodetector element interposed between the first
`to the first width of each photodetector element may be sub-
`light source and the second light source. The apparatus may
`stantially between 3.5:1 to 5:1.
`also include controllogic that is communicatively connected
`In some implementations of the apparatus, each photode-
`with the first and second light sources and the photodetector
`tector element may havea first length between 1 mm and 5
`element andthat is configured to causethefirst light source
`mm anda first width between 0.5 mm and 2 mm,with the ratio
`and the second light source to emit light, obtain measured
`of thefirst length to the first width substantially between 2:1
`light intensity measurements from the photodetector element,
`to 5:1, and each such photodetector element may be posi-
`and determinea heart rate measurementbased,at leastin part,
`tioned suchthat an edge of the photodetector element closest
`on the light intensity measurements. In such implementa-
`to the light source is between 1 mm and 4 mm from the light
`tions, the photodetector element may be rectangularin shape,
`source.
`have a first edge with a first length, and have a second edge,
`In some implementations of the apparatus, there may be a
`perpendicular to the first edge, with a second length. Further-
`pattern of photodetector elements that includes three or four
`more, in such implementations, the ratio ofthe first length to
`photodetector elements that are equidistantly spaced about
`the second length may be substantially between 2:1 to 5:1.
`the light source.
`In some such implementations, thefirst edge of each pho-
`todetector element may be perpendicular or transverse to an
`In some implementations of the apparatus, the apparatus
`axis spanning between a centerofthefirst light source and a
`mayalso include a housing having a back face that includes
`center of the secondlight source.
`one or more transparent window regions through whichlight
`In some other or additional such implementations, the
`mayenter the apparatus. In such implementations, each pho-
`apparatus may include a housing having a back face that
`todetector elementis positioned such that that photodetector
`includes a transparent window region that overlaps the pho-
`element is overlapped by a corresponding one of the one or
`todetector element and two further window regionsthat are
`more transparent window regions, and the housing may be
`each associated with a different one of the first light source
`configured such that the back face is adjacentto the skin ofa
`and the second light source and that allow light from the
`person wearing the apparatus whenthe apparatus is worn by
`associated light source to pass through the back face. In such
`that person.
`implementations, the first light source and the second light
`In some such implementations of the apparatus, the back
`source maybethe only light sourcesin the apparatus config-
`face may include a thin window,and the window regions may
`ured to emit light through the back face, and the housing may
`be sub-regions of the window that are defined by the photo-
`be configured such thatthe back face is adjacentto the skin of
`detector elements. In some other or additional such imple-
`a person wearing the apparatus whenthe apparatus is worn by
`mentations, each photodetector element may be offset from
`that person.
`the corresponding transparent window region by a corre-
`In some implementationsofthe apparatus, the photodetec-
`sponding gap in a direction normalto the photodetector ele-
`tor element may be equidistant from thefirst light source and
`ment, and the gap maybefree of optical light guides.
`the second light source.
`In some implementations, an apparatus may be provided
`In some implementations, an apparatus may be provided
`that includes a light source and at least one photodetector
`that includes a light source and one or more photodetectors,
`element. The apparatus mayalso include control logic that is
`each photodetector having a photosensitive area. In such
`implementations, at least 90% of the photosensitive area of
`communicatively connected with the light source and the
`
`the photodetector is defined byafirst dimension alongafirst photodetector elementand that is configuredto causethe light
`axis and a second dimension along a second axis perpendicu-
`source to emit light, obtain at least one measuredlight inten-
`lar to the first axis. The apparatus may also, in such imple-
`sity measurement from the at least one photodetector ele-
`ment, and determinea heart rate measurementbased,at least
`mentations, include control logic that is communicatively
`connected with the light source and each photodetector and
`in part, on the at least one light intensity measurement. In such
`that is configuredto cause the light source to emit light, obtain
`implementations, the at least one photodetector element may
`one or more measured light intensity measurements from the
`subtend an angle at the center of the light source of substan-
`one or more photodetectors, and determine a heart rate mea-
`tially at least: 2-arctan
`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 maybe substan-
`tially between 2:1 to 5:1.
`In some implementationsof the apparatus, the light source
`may include a plurality of light-emitting devices. In some
`such implementations,the plurality of light-emitting devices
`mayincludeat least two light-emitting devices that predomi-
`nantly emit light of different wavelengths. In somefurther 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
`
`radians, where r, is a measurement of a distance from the
`center ofthe light sourceto the photodetector element, at least
`80% of the photodetector element covers an annular region
`centered onthe centerofthe light source and defined by r, and
`r,, and r, is greater than r, by not more than 2 millimeters.
`In some such implementations, the angle at t