The Development Of Wearable Photoplethysmography Sensor

Because of the rising threat of cardiovascular diseases(CVDs),the real-time monitoring of cardiovascular status is increasingly emphasized.The clinical research of photoplethysmography(PPG)indicates that PPG contains lots of cardiovascular physiological and pathological information,which has been the important means to realize the real-time monitoring of blood pressure(BP),vascular elasticity and other cardiovascular indicators.But unfortunately,the current PPG sensor is poor in portability,inconvenient and sensitive to motion artifacts,which are the main challenges to realize its application in daily life.To solve the above problems,a wearable PPG sensor system is designed in this thesis.The system is divided into two parts: an acquisition module and a system-board.For the acquisition module,two 630 nm red LEDs are employed as the optical emitter.The signal conditioning circuit is designed based on the AFE chip with maximum transimpedance gain of144 dB.And background light cancellation loop is utilized to cancel the DC photo-current.In addition,1% duty cycle control is adopted with the LED driver to reduce the LED current from 10 mA to 0.3 mA.The power consumption of acquisition module is about 2 mA and the size is8mm×10mm×2mm,which is suitable for the wearable systems.A power management circuit equipped with 90 mA battery is implemented on the system-board,and the control model with functions of signal sampling,BLE and control signal generating is designed based on Programmable embedded System-on-Chip.The system-board consumes 8mA during operation.The test result of the proposed sensor show that the best sensing positions are fingertips and finger pulp.Based on the proposed sensor,wealso can acquire PPG signals from toes,earlobes,forehead and other parts.In addition,the proposed sensor is wearable and can work for several hours continuously.To improve the anti-motion interference ability of the wearable PPG sensor,an adaptive filter is studied in this thesis.A motion signal acquisition circuit is designed to provide reference signal to NLMS adaptive filter.The performance of the NLMS adaptive filter is verified in MATLAB,and the result shows that NLMS adaptive filter can recover the PPG signals under mild movement.Finally,a real-time heart rate(HR)and BP monitoring system is implemented based on the proposed wearable PPG sensor and the noninvasive BP prediction model developed by our group.The user can wear the proposed PPG sensor in the form of the ring,the PPG signals can be transmitted to the mobile phone through BLE in real time.The value of HR and BP will be calculated in the software.An experiment involving 20 subjects has been conducted to validate the performance of the proposed BP monitoring system compared with OMRON standard BP monitoring device.The system reports a tolerant error(MAE±STD)of 4.05±3.22 mmHg for systolic blooad pressure(SBP),6.10±5.48 mmHg for diastolic blood pressure(DBP)and 1.45±1.10 bpm for HR.The result shows the potential of the wearable PPG sensor in the field of real time blood pressure monitoring.