面向畜禽场景氨气检测的无源RFID传感标签研制及应用

Development and application of the passive RFID sensing tags for ammonia detection in livestock and poultry environment

氨气是畜禽场景中的主要有害气体之一,针对有源传感器不适于电路有线连接受限的畜禽场景问题,该研究基于高频电磁仿真软件(high frequency structure simulator,HFSS)设计了无源传感器仿真模型,选择聚酰亚胺(polyimide,PI)作为基板材料,采用丝网印刷技术研制了基于射频识别(radio frequency identification,RFID)原理的无源氨气传感标签。通过对氨气无源检测原理的解析,选择了具有高表面积的碳纳米管作为氨气敏感材料,推导了通过测量射频接收功率变化实现无源检测的数学模型;考虑谐振频率的动态调整,无源RFID传感标签采用开口间隙可调的裂环谐振器结构,通过分析传输系数的变化对RFID传感标签的检测过程进行模拟;搭建了用于实验室和畜禽场景氨气检测的射频测试系统,围绕功率反射系数、谐振频率、传输系数开展测试分析。试验结果表明,该标签检测效率易受到到二氧化碳、温湿度因素的影响,由于人工切割、基板变形、环境干扰等因素,实物标签的谐振频率与2.4 GHz的仿真谐振频率之间存在0.05 GHz左右的偏差,传感标签的灵敏度约为15 MHz·L/mg,最大阅读距离为24 cm,相比于商用氨气传感器,该传感标签在使用寿命、响应时间方面有明显优势。研究结果为畜禽场景的氨气无源检测提供了有效的理论和实践依据。

Ammonia is one of the most common toxic gases in livestock and poultry environment.Its high concentrations can pose a potential health threat to humans,plants,and animals.Traditional active detection methods increase energy consumption,heat buildup may affect detection system performance,and are not suitable for livestock and poultry environment where circuit wired connections are limited.With the gradual transition from traditional to smart agriculture,radio frequency identification(RFID)technology has been widely used to integrate device sensing and wireless communication,due to its lightweight,low-cost,and non-line-of-sight readability.In this study,a passive RFID sensor simulation model was designed by using the high-frequency structure simulator(HFSS)software.A split-ring resonator was employed to operate at a center frequency,which was adjusted according to the length of the open gap.The additional flexibility was provided rather than the closed-loop resonant structure.According to the HFSS simulation model,a physical RFID tags were fabricated by screenprinting technique based on polyethylene terephthalate and polyimide substrates,and carbon nanotubes with high surface area were selected as ammonia-sensitive materials.The surface morphology and nanostructure of carbon nanotube materials were characterized by scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The sensor resistance was measured due to the contact of ammonia molecules on the surface of the sensitive materials,mathematical model of passive detection was established to analyze the sensing mechanism.Once an ammonia molecule came into contact with the surface of a carbon nanotube,some electrons or holes were used to change the carrier concentration,thus leading to the varying resistance of the carbon nanotube.Antenna frequency or impedance mismatch was found during adsorption between ammonia and free carriers on the surface of carbon nanotubes,thereby affecting the backscattering signal domain of RFID,the detection process of RFID sensing tags was simulated by analyzing the variation of transmission coefficients.Furthermore,a radio-frequency test system for ammonia testing in laboratories and livestock environment was built.The transmission gain of the tag can be calculated by detecting the transmission coefficient offset and amplitude change.According to the tag power reflection coefficient,return loss and phase analysis,the resonant frequency of the tags varied by 270 MHz under 0-18 mg/L ammonia ambient.The detection efficiency of the tag was easily affected by carbon dioxide,temperature and humidity factors,due to manual cutting,substrate deformation,environmental interference and other factors.There was a deviation about 0.05 GHz between the resonance frequency of the physical tag and the simulated resonance frequency,the sensing tag's sensitivity was about 15 MHz·L/mg,and the maximum reading distance was 24 cm.The sensing tag has obvious advantages in terms of service life and response time compared to commercial ammonia sensors.The tag sensor can be expected to fully meet the passive detection needs of ammonia.This finding can provide a reliable theoretical and practical basis for the passive detection of ammonia from agricultural sources.Further research can also be conducted to select the sensitive materials or suppress the interferences of radio frequency links in the future.