Study On Wearable Anti-noise Blood Oxygen Saturation Monitoring Technology

Oxygen, as an important substance for the metabolism of human, is exchanged withcarbon dioxide in lung and dissolved into blood. Then it is bound on hemoglobin and sentto all over the body. Saturation of blood oxygen, being one of the most important parameterfor respiratory and circulatory system, reflects the oxygen cycle condition. Non-invasiveoxygen saturation measurement is harmless to body. It contains two kinds of methods:transmission-type and reflection-type measurement technology. The method oftransmission-type measurement detects light go through the human body, the signal isstrong enough to be detected, so it is used widely in clinical application. Whereas themethod of reflection-type measurement detects light reflect from the human body, so theprobe can be set on any place of body. This characteristic makes the reflect-type moreprospective and become the focus of study in the field of Biomedical Engineering.In order to realize real-time oxygen saturation monitoring in ambulant environments, awearable oximeter based on reflection-type technology was developed. Considering theability of anti-motion artifact, saving power consumption and minimizing in shape for thisdevice, MSP430F1611was chosen as the MCU to control LED, signal collection and datatransmission. Firstly, light-frequency converter was used as the light sensor to substitutedphotodiode. It converted light signal to frequency signal directly, which can avoid the effectof environments noise in the process of transmission. Secondly, accurate timing controlreduced the duration of lightening the LED, lowered the power consumption greatly.Integrated electronic components were used in this design, which simplified the electriccircuit and reduced the instrument size. The device works as follows: the D/A module inMSP430F1611outputs voltage signal to control constant current source to drive the LED,then the light goes through tissue and reflects, which is captured and transformed tofrequency signal by light-frequency converter. After that, the frequency signal is collectedand processed by MCU and sent out by Bluetooth module. Oxygen saturation monitoring in ambulant environments can be easily affected bymotion-artifact. In order to solve this problem, a new anti-motion artifact algorithm basedon adaptive filter was developed. By analyzing the characteristics of motion artifact mixedin photoplethysmography signal recorded in ambulant environments, the envelopes of redand inferred light PPG signal were calculated. And then these envelopes were used to createan artifact-related signal which can be used as the reference signal to remove motionartifact by adaptive filter. Adaptive filter overcomes the defect in traditional signal processmethod that frequency-overlapped noise can’t be eliminated, and reduces the effect ofmotion artifact during monitoring successfully.To evaluate the performance of this algorithm, PPG signal was collected by the deviceand processed by this proposed algorithm, and then it was compared with DST method. Theresult shows that this proposed algorithm can eliminate motion artifact efficiently and costless calculating time than DST method.The oximeter is proved by the experiments that it can obtain person’s PPG signal withaccurate, stable and time-saving characteristics. Meanwhile the algorithm is also proved tobe effective in eliminating the motion artifact in ambulant environments, and thencalculating the accurate result, realizing real-time monitoring in ambulant environments.