Research On Wireless Passive Ammonia Sensor And Extraction Of Resonance Frequency

Ammonia is the most widely distributed toxic gas in human daily life,which has certain harm to human body.Therefore,it is of great research value to detect the concentration of ammonia in air.In some special occasions,such as food packaging bags and other closed-loop environments,it is impossible to connect the wired circuit.Wireless passive ammonia sensor can play a role in detecting the concentration of ammonia.Usually,network analyzer is used to test these devices,but network analyzer is expensive and difficult to carry and can not be widely used,so it is also of great significance to develop low-cost portable signal reading circuit.In this thesis,a LC resonant wireless ammonia gas sensor is fabricated based on the theory of inductive coupling.The gas sensing film used in the device is determined by analyzing the principle of gas adsorption reaction of gas sensitive materials.The overall framework of the signal reading circuit is designed according to the test method of standing wave ratio,and the detection of the wireless ammonia gas sensor is completed.The performance of the data obtained is analyzed.The main contents of this thesis are as follows.1.The signal transmission principle of LC resonant wireless passive ammonia gas sensor and the principle of gas-film adsorption reaction are studied.The relationship between the size and structure of the sensor and its resonance frequency is analyzed.According to the physical and chemical characteristics of Fe3O4,the adsorption theory for ammonia molecules was formed.The good physical properties of carbon nanotube were used to reduce the working temperature of metal semiconductor oxides and improve the ammonia adsorption capacity of gas sensing films.2.According to the principle of inductance coupling,the whole frame of the signal reading circuit is determined.By introducing the theory of VSWR,the design concept and function of each module are analyzed.This thesis introduces the chip selection of main control module,the generation principle of sweep signal of signal source and the simulation design of filter circuit respectively.It also discusses the method of acquiring standing wave ratio,divides the signal by standing wave ratio bridge,calculates the reflection coefficient of input voltage by phase detection module,collects and calculates the output signal by analog-to-digital conversion(ADC)function of main control chip,and completes the signal acquisition and calculation.3.Because of the internal calculation of AD8302,the reflection coefficient of the input voltage is obtained,and on this basis,the software is compiled.The software mainly includes three parts: the control software of the system signal source,the setting of AD conversion and the calculation of standing wave ratio.The main program of the system is introduced,and the design idea and process are elaborated.The configuration of each part is described in blocks.According to the specific chip manuals,the address of the relevant register is found and the data is written into the register to control the output of signal source.The parameter configuration of AD acquisition is completed,the acquisition signal is calculated and the program is written.4.The output frequency range and precision of the linear sweep source of the signal reading circuit are tested to verify the dynamic testing ability of the circuit to the variable frequency.The signal reading circuit is used to detect the prepared ammonia gas sensor in different volume fraction of ammonia atmosphere.The repeatability and linearity of the sensor are analyzed according to the collected data,and the performance of different gas sensitive films is compared.