| Respiratory monitoring is an important method to treat sleep respiratory diseases such as obstructive sleep apnea syndrome.However,the existing respiratory monitoring system is often large in equipment,complex in operation and inconvenient in use,and most of them can only measure the respiratory frequency,unable to get other further respiratory characteristics information.To meet the needs of portable and low-cost monitoring of human respiratory characteristics,a quartz crystal microbalance(QCM)respiratory sensor based on graphene oxide(GO)sensitive film is proposed in this paper,and the corresponding continuous monitoring system of human respiratory characteristics is designed and implemented.The QCM respiratory sensor,which is located between the nose and the upper lip,monitors the change of humidity caused by inhalation and exhalation through the change of resonance frequency of QCM,so as to reflect the respiratory characteristics of human body.The challenge of QCM breath sensor is the requirement of fast response / recovery speed for continuous monitoring.In this paper,the effects of the concentration and thickness of the GO sensitive film on the performance of the sensor are investigated.It is found that the response / recovery time of the sensor can be significantly reduced by reducing the thickness of the sensitive film at a specific concentration,but at the expense of the sensitivity of the sensor.Through the research of the sensing mechanism and the adjustment of the preparation process of GO sensitive film,the best thickness of GO sensitive film is obtained,and the performance of the sensor is optimized.The experimental results show that when the concentration of GO sensitive membrane is 2 mg / ml and the thickness is 320 nm,the response and recovery time of QCM sensor to respiration can be shortened to 0.4 s and 1.2 s respectively,which can meet the needs of continuous respiration monitoring.At the same time,the change of QCM resonance frequency caused by respiration is about 1 k Hz,which has a strong ability of anti environmental interference.The system consists of three parts: sensor,signal acquisition and processing circuit and software.The sensor is placed on the flexible circuit board and connected with the hardware system.The hardware consists of oscillation circuit,amplification and shaping circuit,and equal precision frequency meter based on FPGA.It can accurately measure the resonance frequency of the sensor with a resolution of 1 Hz.The software interface and the algorithm can recognize every breath,and determine the corresponding exhalation and inspiratory process,so as to obtain the respiratory characteristic information such as breath time,breath intensity,breath frequency and breath interval.In addition,the system can judge and alarm the abnormal conditions such as apnea and sensor probe falling off.The experimental results of system functional verification show that the respiratory rate of the system can be measured up to 27 breaths per minute,which is higher than that of normal adults(16-20 breaths per minute),and it can effectively identify the symptoms of apnea.The system with the advantages of simple structure,portable use,low cost,has practical application value,and can be used in the actual human respiration monitoring.In the future,it can be expanded into a multi-sensor array to detect the exhaled gas composition of human body and realize intelligent application with machine learning algorithm.It can be further flexible,miniaturized and wireless,and has a good application prospect in wearable home health care. |
