US005807267A
`5,807,267
`(114) Patent Number:
`United States Patent 55
`Bryarsetal.
`[45] Date of Patent:
`Sep. 15, 1998
`
`
`[54] HEART PULSE MONITOR
`
`[75]
`
`Inventors: John D. Bryars, Encinitas; David
`Cavanaugh, San Diego, both of Calif.
`
`[73] Assignee: Advanced Body Metrics Corporation,
`Rancho Santa Fe, Calif.
`
`5,243,992
`9/1993 Eckerle et al. oc 128/690
`5,301,154
`4/1994 Suga cecccccsssssssssssssesessseessseeessen 128/690
`
`5,431,170
`7/1995 Mathews.............
`... 128/666
`
`5,539,706
`... 128/690
`7/1996 Takenaka etal.
`...
`5,558,096
`9/1996 Palatnik 0. eens 128/687
`Primary Examiner—Robert L. Nasser
`Attorney, Agent, or Firm—G. Donald Weber,Jr.
`
`[21] Appl. No.: 512,712
`[22]
`Filed:
`Aug. 8, 1995
`
`Related U.S. Application Data
`
`References Cited
`
`ABSTRACT
`[57]
`The invention herein described is intended to provide the
`user with a reliable heart rate monitor that is a completely
`self contained unit and is capable of providing accurate
`readings while the wearer is moving about. The use of
`piezoelectric sensing elements eliminates the power drain
`[63] Continuation-in-part of Ser. No. 462,152, Jun. 5, 1995,
`caused by LEDs and similar devices. The sensing element
`whichis a continuation of Ser. No. 252,605, Jun. 1, 1994,
`mounting means disclosed herein is devised to greatly
`abandoned.
`[51] Tint, Cd ccc ccccssseeecesssseessssseneeesssseees A61B 5/00_reduce the noise introduced into the pulse signal by body
`
`[52] U.S. Ch. eeeeeeeceees 600/500; 600/477; 600/479;
`motion. The use of optical sensors in a staring mode and
`600/503
`optical sensors in a pulsed mode is also presented. The
`Field of Search oo...eceeeeees 128/672, 666,
`effects of noise are further reduced by employing digital
`128/687-690, 706, 707; 600/500-503; 5/476-475
`signal processing algorithms to find the heart pulse inter-
`mixed with noise signals and present the heart pulse rate in
`beats per minute on a display. The resulting device permits
`the visual monitoringofthe heart pulse rate in a human body
`in a consistent, error-free manner.
`
`[58]
`
`[56]
`
`5,000,188
`5,197,489
`
`U.S. PATENT DOCUMENTS
`3/1991 Kojima ceeccccccscssssssssssssseesseesseees 128/687
`3/1993) Comlan w.ecccccccssesesssseseseeeesees 128/690
`
`27 Claims, 4 Drawing Sheets
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`0001
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`U.S. Patent No.
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`Apple Inc.
`APL1026
`8,652,040
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`0001
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`Apple Inc.
`APL1026
`U.S. Patent No. 8,652,040
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`

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`U.S. Patent
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`Sep. 15, 1998
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`Sheet 1 of 4
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`5,807,267
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`FIG. 4\|
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`FIG. 7
`516
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`DISPLAY
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`517
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`FIG.5
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`U.S. Patent
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`Sep. 15, 1998
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`Sheet 2 of 4
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`5,807,267
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`MICROPROCESSOR
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`U.S. Patent
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`Sep. 15, 1998
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`Sheet 3 of 4
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`5,807,267
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`1121
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`N22e
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`1225
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`1201
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`|. ALLOCATE ARRAY MEMORY(128 LOCATIONS )
`2. COMPUTE LINESHAPE TABLE FOR INTERPOLATION
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`3..SET UP TIMING PARAMETERS
`4. ENABLE INTERRUPT
`
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`202
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`WAIT FOR A/D VALUE TO APPEAR IN BUFFER
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`1203
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`STORE NEW VALUE IN ARRAY,
`INCREMENT ARRAY COUNTER i
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`FIG. 12
`— 1204
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`i=64 OR 128?
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`205——|
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`1206
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`1207
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`NORMALIZE THE ARRAY
`(MAX=1.0, MIN=-I.0)
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`COMPUTE FFT (l28 POINT)
`COMPUTE MAGNITUDE
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`NORMALIZE SPECTRUM (MAX=1.0)
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`FIND PEAK IN SPECTRUM
`(TRACKING OFF: SEARCH 30-210 BEATS PER MINUTE)
`(TRACKING ON: SEARCH PREVIOUS HEARTRATE +10 BEATS PER MINUTE)
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`1208
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`INTERPOLATE PEAK WITH LINE SHAPE FUNCTION
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`1209
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`STORE HEART RATE VALUE
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`i210
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`DISPLAY HEART RATE
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`i2t2
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`Y
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`0004
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`0004
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`U.S. Patent FLNNIWYHFdSLVIG
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`NOMLNGYLSIGAINFINOFHYI
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`SONO2DFSMFWILG]
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`IWLSAFidWVs
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`Sep. 15, 1998
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`Sheet 4 of 4
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`5,807,267
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`ICNLINDVW
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`0005
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`0005
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`5,807,267
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`1
`HEART PULSE MONITOR
`
`This is a Continuation-In-Part of application Ser. No.
`08/462,152, pending filed on Jun. 5, 1995, which is a
`Continuation of application Ser. No. 08/252,605, filed Jun.
`1, 1994 now abandoned.
`BACKGROUND
`1. Field of the Invention
`
`This invention relates to heart pulse monitors, in general,
`and, more particularly, to a heart pulse monitor that is worn
`by the user, for example at the wrist, and is capable of
`accurate measurement and display of the user’s heart pulse
`rate (and variation thereof) during physical exercise or other
`activity.
`2. Prior Art
`
`Today,there is a substantial interest in physical well-being
`and exercising. In conjunction therewith, electronic equip-
`ment is employed in health care institutions and by persons
`during athletic training for monitoring of a person’s heart
`pulse activity. One important measurement parameter is the
`rate of occurrence of heart pulsations. In healthy persons, the
`pulse rate is substantially uniform throughout the duration of
`an activity. However, the rate may vary with changes in the
`person’s activity when the heart may be called upon to pump
`at a higher or a lower rate. The rate of pulse change during
`increasing or decreasing activity is directly related to a
`person’s physical condition. Thus,it will be appreciated that
`a device providing an accurate measurementof pulse rate is
`most useful during athletic training as well as for the
`detection and treatment of disease. Similarly, heart rate
`monitoring can be used to guide cardiovascular intensity to
`achieve maximum fitness results within an aerobic exercise
`regimen.
`One device useful for the measurementof heart pulse rate
`is an electronic unit worn on the wrist. In the past, this has
`necessitated the use of complex electronic equipment. That
`is, the accurate measurementof an active person’s pulserate
`at
`the wrist
`is a complex process due to the artifacts
`produced by body motion. These artifacts are concurrent
`with the heart pulse and are detected by the heart pulse
`sensor as noise. In many cases,
`this noise can produce
`signals of sufficient amplitude to completely mask the heart
`pulse signal which is to be measured. In order to mitigate the
`effects of these body artifacts, it is necessary to filter out and
`electrically cancel as much of the noise signals occurring in
`the heart pulse frequency band as possible while retaining
`the desired pulse signal. This problem must be dealt with
`effectively over a considerable signal-to-noise ratio range.
`In some extreme cases the signal-to-noise ratio (SNR)
`will become negative even with very effective cancellation
`techniques. When it is no longer possible to reliably detect
`the heart pulserate, it is essential that no attempt is made to
`display a heart rate reading because of the high probability
`of introducing inaccuracies. It is better to store and display
`the last good reading until the severe noise condition is
`concluded and an accurate reading can be made.
`During monitoring sequencesit is important that the user
`be able to receive accurate updates of heart pulse rate
`frequently. It has been demonstrated that this should occur
`no more frequently than every five seconds with an update
`every ten seconds seeming to be optimal. This is important
`to the user since even in situations where violent physical
`activity is creating body artifact noise in excess of what can
`be tolerated by the sensing system, a relatively short period
`of reduced movementis sufficient to provide an updated,
`accurate read-out of pulse rate.
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`Several devices have been proposed for providing a
`wrist-watch type of heart pulse monitor. U.S. Pat. No.
`4,120,269 (Prinz) describes one type of such device, viz. a
`digital plethysmography which customarily utilizes an infra-
`red light transducer.
`USS. Pat. No. 4,059,118 (Stupay) describes a device which
`uses an actuator pin pressing against a piezoelectric crystal.
`The known devices tend to have several shortcomings.
`Those devices using optical transducers, such as the digital
`plethysmographies, consume substantial power in the light
`emitting elements. Thus, use with a battery is not effective.
`Devices using piezoelectric transducers typically devote
`little attention to the substantial noise problemsthat attend
`the use of such transducers in this application.
`When such a pulse rate monitor is mounted on the
`wearer’s wrist, the pulse signal is, to a significant extent,
`masked by the concurrent noise signals generated due to
`body motions. The mechanical transducer responds both to
`pressure from the wearer’s pulse beat and to motion from
`walking, arm swinging and the like, and doesnotdistinguish
`between them. However, this response is noise insofar as
`pulse measurement
`is concerned. As the Stupay patent
`teaches, “the patient must remain quiet to avoid noise input”
`during the period in whichthe pulse rate is being measured.
`Also,
`if the piezoelectric transducer is not mounted
`directly overthe artery of the user, the pulse signal measured
`by the device will be significantly reduced in amplitude.
`Thus, the signal is even more likely to be masked bynoise.
`Typically, noise signals may be as high as 1.0 volt, while the
`pulse signal may be approximately 0.1 volt. Consequently,
`prior art wrist watch pulse rate monitors employing piezo-
`electric transducers have been inaccurate because of this
`unfavorable signal-to-noiseratio.
`US. Pat. No. 4,224,948 (Cramer) teaches that when a
`piezoelectric sensor is used,“the watch must be worn on the
`volar surface of the wrist but lateral to the tendon chord
`
`bundles”, so as to obtain a pulse reading from the radial
`artery in the subpollex depression. Moreover, Cramer
`requires that “the sensors must be forced into the flesh of the
`wrist for a reading which situation may be uncomfortable.”
`USS. Pat. No. 4,409,983 (Albert) uses a complex arrange-
`mentof piezoelectric sensors to develop whatis described as
`relatively noise free signals presented to the input of a
`microprocessor. This apparatus employsa piezo sensor array
`which is operated by providing a bending force to one end
`of the sensor elements. This bending interaction is accom-
`plished by using small pins pushing against the ends of the
`sensors with coil springs being used to dampen the high
`frequency noise products. Algebraic analog signal summing
`is used to create relatively noise free signals at the input the
`microprocessor in the form of an electrical pulse string
`having the same rate as the heart pulse. While this embodi-
`ment may reduce the attendant body noise, there are other
`problems caused by the complexity of the sensor systems
`which tend to make this arrangement impractical for mass
`production.
`Inventions using optical sensors to detect the heart rate
`pulse at the radial artery on the wrist tend to have many of
`the same body motion related problemsas the piezo sensor
`systems. Added to the motion induced noise problemsis the
`introduction of noise artifacts that are caused by ambient
`light conditions. These noise sources can be anyelectrical or
`natural light sources, including the sun. However, optical
`sensors do not tend to detect body transmitted acoustical
`noise. An effective method of dealing with these noise
`sources is necessary in order to make accurate heart pulse
`
`0006
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`5,807,267
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`3
`4
`adjacent to the radial artery 6. The assembly 2 is connected
`rate readings while the body is in motion or exposed to
`to a wrist band 3 adapted to surround and engage the wrist
`changing lighting conditions.
`1. The band3is also fastened to a digital wrist watch 4 (see
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1). As described hereinafter,
`the watch product 4
`contains a microprocessor and a display 5. The wrist watch
`product 4 is disposed on top of the wrist in FIG. 1. The
`display 5, for example a liquid crystal display (LCD), is
`shownin side view only.
`Referring now to FIG. 2, there is shown the underside of
`the wrist 1 with the sensor assembly 2 positioned over the
`radial artery 6 (shownin dashedoutline). Aclasp mechanism
`7 of suitable configuration is used to secure the band 3.
`In this embodimentof the invention,it is desirable to have
`the sensor assembly 2 directly over the radial artery 6 of the
`wrist (or as close thereto as possible), in order to maximize
`the signal-to-noise ratio. The sensor assembly 2 is,
`preferrably, positioned over the radial artery 6 in the sub-
`pollex depression parallel to the tendon cord bundles on the
`volar surface of the wrist in order to obtain a reliable pulse
`reading. If the device is used to monitor pulses in a different
`artery, e.g. one in the human or animal user’s leg, the sensor
`assembly 2 should be as close as possible to a position
`directly over that artery.
`FIG. 3 shows a side view of a piezo electric sensor
`assembly 302 placed overthe radial artery 6 in the subpollex
`depression. FIG. 4 showsa plan view of the sensor assembly
`302 from the underside. Concurrent reference is made to
`
`FIG. 1 show one embodimentof the pulse monitor placed
`on the user’s wrist.
`
`FIG. 2 showsthe pulse monitor of FIG. 1 and indicates the
`placement of the sensor assembly overtheartery.
`FIG. 3 showsplacement of a piezo sensor overthe radial
`artery at the wrist.
`FIG. 4 showsthe configuration of the sensor assembly and
`its placement relative to the radial artery.
`FIG. 5 is a simplified schematic of the heart rate monitor
`using piezo sensors.
`FIG. 6 shows the construction of the optical sensor
`assembly and placement thereof over the radialartery.
`FIG. 7 shows additional detail of the optical sensor
`relative to the radial artery.
`FIG. 8 shows a simplified schematic diagram of one
`embodiment of the optical pulse rate monitor system.
`FIG. 9 showsa simplified schematic diagram of another
`embodiment of the optical pulse rate monitor system.
`FIG. 10 show another embodimentof the invention using
`optical sensors.
`FIG. 11 shows one embodiment of the layout and func-
`tions contained in the liquid crystal display.
`FIG. 12 shows a flow diagram of the basic software/
`processor functions on the heart rate monitor.
`FIG. 13 a graphical respresentation of a normalized
`sample vs time chart.
`FIG. 14 is a graphical representation of a frequency
`distribution of signal.
`
`SUMMARYOF THE INSTANT INVENTION
`
`Apulse rate monitor for sensing a pulse wave produced at
`an arterial pulse source of the wearer. Piezo pressure sensors
`or optical sensors outside the surface of the skin produce an
`electrical signal upon detecting the presence of a pulse in the
`artery. When using the piezo sensors, a second (background)
`sensor is employed. Signals from this background sensorare
`digitally subtracted from the primary pulse sensor thus
`significantly reducing the effects of body motion signals.
`Optical sensor configurations typically use only a single
`sensor. Signals from the sensors are amplified and passed
`through appropriate filters to reduce the bandwidth of the
`input circuitry to pass only the signals of interest. The
`filtered signals are converted to digital signals in a micro-
`processor. These signals are then digitally processed to
`produce an output on the display in pulses per minute. The
`unit may be worn in the same manneras a wrist watch and
`powered from a small battery. Some embodiments of this
`device may haveall the time keeping capability of a digital
`wrist watch.
`
`DESCRIPTION OF PREFERRED EMBODIMENT
`
`Referring now to FIG. 1, there is showna plan view of the
`simplest contemplated embodimentof the apparatus 100 of
`the present invention. In this embodiment, the apparatus is
`configured like a wrist watch. In fact, the apparatus 100 can
`be combined in a single unit with a wrist watch.
`FIG. 2 is taken from a vantage point looking toward the
`inside of wrist 1 of a user of the device. A sensor assembly
`2 containing either optical or piezo sensors is positioned
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`FIGS. 3 and 4. In this embodiment, the piezo sensors 307A
`and 307Bconsist of small ceramic blocks (chips) approxi-
`mately one eighth inch square, which have been processed
`to have piezo electrical properties. The sensors 307A and
`307B will produce an electrical signal only when pressure is
`being applied thereto or released therefrom.
`The piezo sensor 307A is connected to the electronic
`control and detection package 313. Attached to the inner
`surface of the piezo sensor 307A, i.e.
`the surface facing
`toward the wrist, is a small button 310. Button 310 is formed
`of hard plastic and extends to the outer surface of the sensor
`assembly 302. The button 310 and the chip 307A are
`encapsulated in a cover layer 311 which is composedofsoft
`silicone or a similar rubber compoundthat canflex easily,is
`comfortable to the wearer and provides protection to the
`sensors from moisture, body fluids, residue and dirt. Button
`310 with piezo sensor 307Ais adapted to be placed adjacent
`to the radial artery 6 to detect the heart rate pulse directly
`therefrom. The soft material of cover layer 311 does not
`inhibit the action of button 310 pressing against the piezo
`sensor 307. Typically, this arrangementis sufficient to detect
`the pulse in the artery 6. However, background noise can be
`a problem.
`in a preferred embodiment, a pair of piezo
`However,
`sensors 307A and 307B are utilized to provide the back-
`ground masking technique. In this preferred embodiment,
`piezo sensors 307A and 307B are bonded to a common
`backing plate 308 which can be formed of hard plastic, for
`example. The backing plate 308 is connected or attached to
`band 3. In addition, wires 314 connected to each of the
`sensors pass through the backing plate 308 and are available
`to be connected to the wrist electronic package 313. Thus,
`sensor 307A is provided as described supra. In addition,
`sensor 307Bis attached to a bridge assembly 312, typically,
`fabricated of hard plastic. The bridge 312 is placed along the
`sameaxis as the radial artery. However, bridge 312 straddles
`the radial artery 6 in such a mannerthat sensor 307B does
`not detect the pulse in the artery. Rather, only noise from
`local body motion will be detected by sensor 307B. Thesoft
`cover layer 311 is placed over the entire assemblage.
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`As described supra, the amplified and filtered outputs of
`piezo sensors 307A and 307B are subtracted electronically
`in the electronic package 313. This operation minimizes the
`effects of random body noise in masking the heart pulse
`signal generated by the pulsing action of the radial artery 6.
`Theeffect of the above described sensor configurationis that
`there is a significant reduction in the noise content of the
`processed signal.
`Referring now to FIG. 5, there is shown a schematic
`diagram of the major components of the heart rate monitor
`using piezo sensors. The piezo sensors 307A and 307B are
`disposed over different portions of the radial artery as
`depicted in FIG. 4. Electrical signals resulting from
`mechanical pressure being applied to the surfaces of the
`piezo transducers 307A and 307B are amplified in pre-
`amplifiers 513A and 513B. The amplified signals are then
`passed through switched capacitor low-passfilters 514A and
`514B whichare, preferably, set to pass frequencies between
`0.5 to 4 Hz. Thefilter assemblies also amplify the signals to
`a level of approximately one volt for a typical heart pulse
`signal from the piezo sensors.
`The outputs of the filters 514A and 514B are supplied to
`an analog-to-digital (A/D) converter 515 where the com-
`bined signals are converted into a digital word. Typically, 12
`bit words are the minimum acceptable; however, 16-bit
`words are preferred to assure accurate pulse rate readings.
`The digital words are supplied to the microprocessor 516
`where they are temporarily stored. The microprocessor 516
`also operates to digitally subtract the background signal
`produced by sensor 307A from the primary signal produced
`by sensor 307B. The resultant signal leaves only the differ-
`ence signal which will normally display a stronger heart
`pulse reading than the signal found in the background noise
`reading. The display 517 is connected to the microprocessor
`516 and provides the readout of the heart rate along with
`signal amplitude and time of day information.
`Referring now to FIG. 6, there is shown another embodi-
`ment of the invention.
`In this embodiment,
`the sensor
`apparatus utilizes optical devices to measure the heart pulse
`rate from the radial artery 6. The shape of the optical sensor
`assembly 620 is similar to that of the piezo sensor assembly
`302 shown in FIG. 3. In assembly 620, the sensor system
`consists of an infrared light emitting diode (LED) 618 and
`a suitable photo detector 619, such as a photo transistor or
`the like. The LED 618 andthe transistor 619 are encapsu-
`lated in a lens 621 made of a material which is relatively
`hard and preferably opaque to ambientlight, for example, a
`dense, black plastic material. An infrared (IR) source, i.e.
`LED 618,is preferred because IR iseasily reflected from the
`surface of the artery 6. In addition, IR lightis outside of the
`spectral peak of sunlight whereupon the chancesoferror are
`reduced, particularity when using the monitor outdoors. The
`spectral response of the photo transistor 619 must match that
`of the LED light source 618, of course This can be accom-
`plished by employing optical filters which pass only the
`desired IR light and absorb visible light from other sources
`if necessary.
`The optical sensor assembly 620 is mounted to the band
`3 in orderto be positioned along the axis of the radial artery
`6 as shown in FIG. 6.
`
`there is shown the relative
`Referring now to FIG. 7,
`positioning of the source 618 and sensor 619. The most
`efficient operation is obtained by directing the light from the
`LED source 618 and the look angle of the optical sensor 619
`at the same point on the artery 6. Sensor output data from the
`optical transistor 618 is amplified and processed in muchthe
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`same manner as with the piezo sensors. However, in the
`optical sensor system there is no separate background sensor
`shown.
`
`Referring now to FIG. 8, there is shown one embodiment
`of an optical sensor system. The LED 618 is connected to
`produce a constant light output which is focused on the
`radial artery 6. Pulsations of the radial artery (caused by the
`pumping action of the heart) cause the walls of the radial
`artery 6 to expand and contract at the heart rhythm rate.
`These pulsations cause variations (modulation)
`in the
`amountoflight being reflected from the surface of the artery
`6 to the photo sensitive surface of the photo transistor 619.
`The photo transistor 619 converts the changes in the
`received light level to a varying electrical signal which is
`supplied to and amplified by the photo pre-amplifier 813.
`The output of the photo preamplifier 813 is fed into thefilter
`814. Typically, filter 814 is a band pass, switched capacitor
`filter which is band limited to a frequency range of from 0.5
`to 4 Hz. The filtered signal is then supplied to the A/D
`converter 815 and transformed into a digital word. The
`larger the numberof bits in the digital word, the greater the
`resolution of the pulse capture. In the preferred embodiment,
`a minimum numberof bits is 12, with 15 to 16 bits being
`highly desirable. The digital word is processed into numeri-
`cal results by the microprocessor 816. The numerical results
`are displayed on the LCD 617 as heart beats per minute. A
`battery 801, typically a wrist watch-type battery (or a lithium
`battery), is used to provide the power source.
`Referring now to FIG. 9, there is shown another embodi-
`ment of the optical sensor design. In this embodiment, the
`LED 618 and the phototransistor 619 are arranged relative to
`each other and to the radial artery 6 as described supra. In
`addition, the conduction path of switching transistor 920 is
`connected in series with LED 618. The transistor 920 is
`
`connected to microprocessor 916 which controls the on/off
`timing of transistor switch 920. Transistor 920 selectively
`turns on LED 618. During the interval when LED 618 is
`turned on and providing illumination to the radial artery 6,
`photo transistor 619 detects the reflected light signal. This
`detected signal is amplified in pre-amplifier 913 and sup-
`plied to the A/D converter 915 where the signal is converted
`into a digital word and sent into microprocessor 916. In this
`embodiment, a low-passfilter is not required inasmuch as
`the sampling technique provides the filtering necessary to
`eliminate body motion noise from the signal.
`While LED 618is in the off condition, the ambientlight
`present at the photo sensitive surface of photo transistor 619
`is detected. This signal is amplified by preamplifier 913 and
`digitized by the A/D converter 915. This digital signal is
`supplied to microprocessor 916 where the “ambient” signal
`is algebraically subtracted from the “detected” signal gen-
`erated by the reflected light level from the radial artery 6.
`Thus, the effect of ambient light noise is removed from the
`output.
`Another feature of this embodiment of the invention is
`
`that much less power is required to operate the LED 618.
`This effectively reduces the power duty factor for the LED
`by a factor of a hundred or more, thus having the effect of
`considerably extending the life of battery 901 in the wrist
`worn instrument. In addition, performance of the unit is
`improved by maintaining a low dischargerate.
`Referring now to FIG. 10, there is shown another embodi-
`mentof the instant invention using optical sensor technology
`along with signal sampling and background noise cancella-
`tion techniques. The principal differences between this
`embodiment of the invention and the embodiment shown
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`5,807,267
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`7
`and described relative to FIG. 9 are in the method of
`capturing and processing the signals obtained from the wrist
`prior to being converted into digital values.
`Microprocessor 1001 generates the timing pulses T1
`which selectively tum on MOSFET 1002 which passes a
`current from +V through R,. This current activates light
`emitting diode (LED) 1003 for a predetermined period. In
`one example, a pulse duration of approximately 2 millisec-
`onds with a pulse repetition rate between 100 and 200 Hz
`wasused. Shorter “ON”time duration would have the effect
`of reducing the average power required to operate the
`electronics and would be used in battery powered applica-
`tions. Light emitting diode 1003 is selected for a spectral
`output which optimizes the signal-to-noise ratio of the pulse
`being detected by photodetector 1004. The photo detector
`1004 can be either a photo-transistor or a photodiode. Either
`device will function adequately in this application, although
`a photo-transistor may be more stable and produce noise-
`free signals. Optical filter 1005 is located adjacent to pho-
`todetector 1004. The filter 1005 possesses optical band pass
`characteristics compatible with the spectral output of the
`LED 1003. The filter 1005 is used to limit the input light
`spectrum supplied to photodetector 1004 by transmitting
`only light from the LED (and any ambient light present
`within the band pass spectrum). This reduces, and in many
`cases eliminates, the effects of ambient light present at the
`wrist during heart rate monitoring. All light wave frequen-
`cies outside the band passlimits of the optical filter 1005 are
`rejected. Similarly, the design of the photo preamplifier 1006
`determines whethera photo diodeor phototransistor is to be
`used. In some applications where the full gain capability of
`the photo transistor can berealized, it is possible to eliminate
`the photo pre-amplifier 1006 and pass the output signal from
`the detector directly into the sample and hold circuits 1007A
`and 1007B.
`
`Sample and hold circuits 1007A and 1007B are used to
`capture and hold a sample of the output signal from the
`photo detector 1004 and photo preamplifier 1006. This
`provides a signal averaging function. Timing pulse T2 is
`approximately 0.3 milliseconds in duration and occurs
`approximately 1.7 milliseconds after timing pulse T1 turns
`the LED 1003 “ON”. The 1.7 millisecond sampling delay
`time duration is required for
`the comparatively slow
`responding photo preamplifier 1006 to rise to full amplitude
`and produce a stable light “N” signal.
`Timing pulse T3 is set to occur approximately 0.5 milli-
`seconds secondpriorto timing pulse T2 and producesa light
`“OFF” background signal used to determine the intensity of
`any ambient light present in the detected signal. The back-
`groundsignal is output from sample and hold circuit 1007A.
`The modulated or changing light
`level signal from the
`sample and hold circuits 1007A and 1007B are supplied to
`pre-filter circuitry 1009A and 1009B via coupling capacitors
`1008A and 1008B, respectively. The direct current voltage
`components of the output signals from sample and hold
`circuits 1007A and 1007B are removed by capacitors 1008A
`and 1008B. The capacitors also remove any off-set voltage
`levels present at the output of the sample and hold circuits.
`The prefilters 1009A and 1009B are used to remove switch-
`ing transients and to smooth out step signals from the output
`of the sample and hold circuits. In one embodiment, the
`pre-filters were configured as second order, multiple feed-
`back band-passcircuits and provided a signal voltage gain of
`15. The prefilters used an undedicated operational amplifier
`housed in the switched capacitor filters integrated circuit
`packages 1010A and 1010B. The 3 dB cutoff frequency of
`the prefilters was set at 24 Hz. The filters also act as an
`
`10
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`15
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`20
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`25
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`30
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`35
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`40
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`45
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`50
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`60
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`65
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`8
`anti-aliasing filter for the switched capacitor filters, thus
`removing any unwanted harmonic signals. Typically, the
`switched capacitor filters 1010A and 1010B are 8th order
`band-pass filters with the band passset at 0.5 to 5 Hz. This
`band pass correspondsto a heart rate input range from 30 to
`240 beats per minute.
`The output of switched capacitorfilter 1010A contains the
`background or ambient light modulation information and is
`supplied to the inverting input of differential amplifier 1011.
`The output of switched capacitor filter 1010B contains the
`heart rate modulated signal and the ambient light modulated
`signal and is supplied to the non-inverting input of the
`differential amplifier 1011. The resultant output signal from
`differential amplifier 1011 is the heart pulse signal minus the
`ambient light signal. This design provides approximately 60
`dB of common moderejection or approximately a 1000:1
`rejection ratio. This rejection ratio has proved adequate for
`the test unit to be operated with the optical components
`exposed to light from incandescent and fluorescent lighting
`as well as sunlight. In cases wherethe circuitry was operated
`with the LED turned “OFF” all
`the time, virtually no
`interference from ambient
`lighting was observed at
`the
`output of differential amplifier 1011 while being operated in
`a normally illuminated room.
`Log amplifier 1012 is employed to extend the dynamic
`range of the analog pulse processing system. Signals from
`the sensors at the wrist will vary in amplitude from a few
`millivolts to a volt or more at the output of differential
`amplifier 1011. These signals which have nearly 60 dB of
`dynamic range, would be difficult to capture and processif
`presented directly to analog to digital converter (A/D) 1015.
`The log amplifier 1012 employed herein employs logarith-
`mic amplifier principles, and is designed to perform as a
`Log-n (rather that Log-10 or Log-e amplifier). This amplifier
`provides a relatively high gain to low level signals and
`compresses the output of higher amplitude signals. Thus,
`those signals introduced into the system as noise caused by
`body motion or ambient light artifacts do not exceed the
`input range of A/D converter 1015. The dynamic range of
`the log amplifier 1012 employed in the above described
`embodimentofthis invention is approximately 40 dB. Other
`gain ratios may be required to properly implementvariations
`of this same functional configuration.
`Microprocessor 1001 accepts and stores the digital output
`of the A/D converter 1015 and processes the data using the
`same algorithms as for the piezo and steady state light
`embodiments of the invention described supra. Pulse rate in
`beats per minute along with time is displayed on LCD 1017
`which is connected to the microprocessor 1001.
`FIG. 11 shows one proposed layout of the liquid crystal
`display 1017 (see FIGS. 9 and 10, for example). This layout
`is representative only and is not intended to be limitative.
`For example, the pulse rate is displayed at digits 1121 in the
`upperleft corner of the display. The heart-shaped logo 1122
`is also a functional element of the display. The heart logo
`1122 is composed of three segments 1122A, 1122B and
`1122C which indicate relative signal level from the sensor
`placed overthe artery. The graduated lines 1122B and 1122C
`at the left side of the heart logo 1122 are, in essence, a bar
`graph which indicates the level of the detected pulse signal
`from the sensor. This graphic display will provide the wearer
`with an analog indi