Development of a Bluetooth 4.0 PPG Sensor for use in Heart Rate Variability Analysis

Mike BlakeZovandia

Development of a Bluetooth 4.0 PPG Sensor for use in Heart Rate Variability

Analysis

Development of a Bluetooth 4.0 PPG Sensor for use in HRV Analysis

Background – What is Heart Rate Variability?

Mike Blake - Zovandia

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• Even under steady state conditions there is significant variation in the time between heart beats.• HRV parameters are obtained by performing statistical, spectral, nonlinear and entropy calculations on a series of RR intervals• Heart rate variability has been found to reveal useful information concerning the subjects health and well-being.•Much of the raw data used in HRV analysis has historically been obtained in a lab using an ECG, with the HRV parameter calculations being performed on a PC• The processing power of the current generation of smartphones allows for realtime calculation of HRV


Background (2)

Parameter CategorySDNN Statisti

calRMSSD Statisti

calSDSD Statisti

calSDANN Statisti

calTINN Statisti

calpNNx Statisti

calVLF SpectralLF SpectralHF SpectralLF/HF SpectralSD1 Nonlinea

rSD2 Nonlinea

rApEn EntropySampEn EntropyFuzzEn Entropy



• Contrary to original expectations, the majority of applications of HRV are on subjects who are at rest (steady state), such as:

• Assessment of recovery from Chronic Fatigue Syndrome• Recovery from competition or intensive training • Monitoring trainee stock market traders• Stress management• Daily assessment of executive well being

• These users sought a sensor that was less cumbersome and more convenient for their use, such as a fingertip or earlobe sensor.

• An extensive search in early 2014 revealed no availability of such a sensor so the decision was taken to develop a photoplysmography (PPG) sensor.


Design Specifications

Item Priority Design Criterion

Requirement J ustification

1 Price to end user <$60 once volum e exceeds 2500 pcs Accessibility

2 As accurate as a quality chest strap when user is inactive User confidence

3 M otion compensation Allows user to move around

4 Lightweight (<20g) and sm all (40x25x20mm m ax)

Comfortable for long periods

5 Short development cycle Time to market

Background (3)


Major Functional Components


AFE 4490PPG analog front end

Courtesy TI

BL600BLE enabled microcontroller

Courtesy Laird

Courtesy Bosch

BMA222 Three axis accelerometer


Hardware


Double sided (2 layer) PCBDimensions 19 x 34 x 9 mmBattery CR1632

Solidworks model of enclosure

Prototype sensor. Weight 17g


Signal Processing


Raw Input From ADC

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40500

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ADC Co

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Filtered ADC Input

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40500

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1st Derivative of Input Data

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d/dt


Optical Component selection


Four Photo detectors and six LED’s from 5 different manufactures were selected based on datasheet parameters

10 different PD-LED combinations were tested.

Fingertip Earlobe1 e c 1300 9502 c c 1100 5003 a c 3500 24004 a b 2200 8005 e b 1400 4006 e a <400 <4007 b a <400 <4008 c d <400 <4009 b d <400 <40010 d d <400 <400

LED Photodetector Figure of M eritTest Num ber

TakeawaysThe data sheet parameters did not give any useful information relative to PPG performance.The variation in performance can be significant


The Big Challenge:The sensor delivered reliable data…….sometimes!

Sensor positioning on the earlobe was critical

Value of inverted signal is sent to the buzzer as a frequency so user can position sensor to maximize SNR



Testing• Equipment: Two Android handsets both running HRVxtPro V1.2

• Settings: Sample array size – 60; Ectopic beat removal – Disabled

• Parameters analyzed/saved: HR; RR; RMSSD; SDNN; LF; HF; SD1; SD2

• Reference sensor: Polar H7 Bluetooth Smart chest strap.

• Subject: 39 year old female

• Test site: 4th finger left hand, finger prone on tabletop

• Clip force: 2N

• Test Duration: 6 minutes


Results – RR Intervals

Development of a Bluetooth 4.0 PPG Sensor for use in HRV Analysis Mike Blake - Zovandia

PPG Sensor vs Polar H7 Chest Strap - RR Intervals (m S)

y = 0.9994x - 0.3271R2 = 0.9972

n=400

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550 600 650 700 750 800 850 900 950Polar H7

PPG Sensor

Results -SDNN


Polar H7 and PPG Sensor - SDNN

y = 0.9424x + 1.3217R2 = 0.9869

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PPG Sensor

Polar H7 and PPG Sensor - SDNN

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Sam ple Num ber

SDNN

Results - RMSSD


Polar H7 vs PPG Sensor - RM SSD

y = 0.9924x + 0.3978R 2 = 0.9796

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12 13 14 15 16 17 18 19Polar H7

PPG Sensor

Polar H7 and PPG sensor - RM SSD

02468101214161820

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RMSS

D

Results - LF


Polar H7 and PPG Sensor - LF

0102030405060708090100

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Sam ple Num ber

LF

Polar H7 and PPG Sensor - LF

y = 0.9884x + 1.0291R2 = 0.9882

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70 75 80 85 90 95Polar H7

PPG Sensor

Results - HF


Polar H7 and PPG Sensor - HF

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Sam ple Num ber

HF

Polar H7 and PPG Sensor - HF

y = 0.9884x + 0.1318R 2 = 0.9882

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Polar H7

PPG Sensor

Results – SD1


Polar H7 and PPG Sensor - SD1

0246810121416

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SD1


y = 0.9958x + 0.2292R 2 = 0.9685

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Polar H7

PPG Sensor

Results – SD2



01020304050607080

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SD2


y = 0.9686x + 1.2007R 2 = 0.9853

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Polar H7

PPG Sensor


Remaining work•Test multiple sensors subjects and test sites to confirm consistency of performance•Rigorously evaluate the limit of the motion mitigation algorithm•Find a partner to commercialize the product


Conclusions• The sensor is capable of delivering data that is usable for HRV analysis.• The sensor is small enough and light enough (17g) to be used on the fingertip or earlobe.• The components, when purchased in volume, should ensure the price target can be met.


Thank You


Development of a Bluetooth 4.0 PPG Sensor for use in Heart Rate Variability Analysis

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