Next Generation Biometric Sensing in Wearable Devices C O L I N T O M P K I N S D I R E C T O R O F A P P L I C AT I O N S E N G I N E E R I N G S I L I C O N L A B S C O L I N.T O M P K I N S @ S I L A B S. C O M
Wearables & 3 rd Wave of Computing DESKTOP COMPUTERS LAPTOPS TABLETS HANDSETS INTERNET OF THINGS CONNECTED HOME SMART CITIES WEARABLES
Next Generation Wearable Sensing MOTION ENVIRONMENT BIOMETRICS Today s Wearables Accelerometer Magnetometer GPS Ambient Light Heart Rate Next Gen Wearables Gyroscope/ Accelerometer+ Barometer UV Index Temperature Heart Rate++ Skin Temp Other Biometrics Other Biometrics include SpO2, Stress, Recovery, Hydration High Performance optical sensing, EDA and ECG sensing
Next Gen Motion Sensing Next generation motion sensing Repetition counting Exercise identification Form and technique Coaching and guidance
Next Gen Environmental Sensing Barometer Combined with motion or GPS location tracking Ambient Temperature Sensing and Skin Temperature Sensing Difficult to implement UV Index Sensing Recommend diffusor & cumulative UV exposure sensing UV transparent overlay
Photoplethysmography Basics LED to Photodiode ~1.5 to 5 mm Green LED Light is modulated by the blood flow causing capillary diameter changes The green light is greatly attenuated by tissue Green light works well Red and IR works for finger tip but not at wrist Subdermal Tissue 100 us pulses repeated at 25 Hz Skin Green LED Sensor PD Attenuated and Modulated light Optical Blocking is critical
Photoplethysmography Examples Stationary Subject Modulation is 0.5% RMS in this case Very Clear, Low noise heart rate Signal ~75 BPM Modulation HR Period Exercising Subject Many motion artifacts Needs special software and accelerometer signal to extract HR signal Sample # (100 Hz) Sample # (100 Hz)
Heart Rate Sensor System Design Optical hardware design Photodiode to LED spacing Overlay Modules solve many system challenges Discrete PD + 2 LED s ~5.06 x 4.26 mm HRM Module 3.7 x 7.0 x 1.1 mm SKIN Glass Overlay Lens Photodiode Clear Encapsulant Substrate Blocking Plastic Clear Encapsulant Lens LED
Improved PPG Overlay and Fit Blocking Overlay Highest performance due to zero crosstalk Minimize overlay thickness Minimize module to overlay distance Band and fit considerations Sensor protrusion Fit and secure band
SNR [db] High Performance PPG Design 120 100 80 60 40 20 0 Optical Sensor ADC SNR 0 50 100 150 200 250 Integration Time [μs] RAM & MCU memory and MIPS Clock stability target <1% More important with FIFO/sample buffer Avoids high MCU power for synchronizing Synchronization between sensors Accelerometer interface supports data synchronization Higher performance optical sensors Multiple LED s, spacing from sensor to LED to make power/performance tradeoffs Noise mitigation becomes more critical: ambient light cancellation and motion artifact cancellation
System Power Considerations Power HRM Signal Capture Signal Processing Wireless Transaction Sleep Techniques to extend battery life High performance sensor system + good optical design enables more signal for less light/power Shorter LED duration, integration time while maintaining high SNR results at lower power Sample buffer reduces CPU bus transaction times Wireless radio update interval as long as practical Time
PPG+ Enables New Biometrics Heart Rate Variability (HRV) Hydration SpO2 Recovery rate Sleep Quality P u l s e O x i m e t r y
ECG & Bio-impedance Electrocardiogram (ECG) Sensing heart s electrical signals Chest strap most common tool for this today Used for heartbeat waveform analysis Bio-Impedance or tissue impedance sensing Most common method to analyze body fat Can be used for hydration measurements Alternate method of sensing heart rate
Heart Rate Measurement Algorithm Considerations Motion cadence may equal heart rate Algorithm uses multiple factors to determine HR Algorithm affects accuracy of biometrics & dictates power consumption of HRM system
Biometric Performance Optimization Tradeoffs for performance versus power consumption and cost of solution Market segments have different requirements Factors affecting performance Sensor/photodetector selection, optical design LED current dominates power consumption Algorithm performance variations and tuning vs particular activity Automatic motion detection as input to HRM algorithm Reducing noise critical to performance Band design very important to reduce motion noise Ambient light noise Accelerometer to cancel motion noise
Next Gen Wearable Wearable 3.0 Motion Tracking Guidance and Coaching Accurate Heart Rate Monitoring Accurate Calorie Estimate Stress Hydration Training effect & fitness Level UV Index & Comfort Level
Time for Questions