Study On LC Passive Wireless Sensors And Readout Optimization Methods

LC(inductor-capacitor)passive wireless sensors are an important research direction of the Internet of Things,which have a simple structure,no need of power supply and wireless detection,especially suitable for applications in confined and narrow space,rotating movable structures and harsh environment.However,most of the researches on LC passive wireless sensors focus on the LC passive wireless sensor nodes.In order to get practical application,the readout performances also need to be further optimized.In this dissertation,the development of LC passive wireless sensors and readout circuits at home and abroad are reviewed,LC passive wireless sensors and readout optimization methods are studied,in order to optimize the readout performances,the parasitic effects are analyzed to improve the readout accuracy,and the method to enhance the readout distance is further studied.The main research contents and innovations include:(1)The basic theories of LC passive wireless sensor design,readout method and coupling coefficient are analyzed.The methods of impedance parameter readout and S parameter readout are compared,and the two measurement results are proved to be interchangeable.Theoretically,the readout frequency of impedance parameter can accurately obtain the resonant frequency of LC sensor,while the readout frequency of S parameter will be affected by coupling coefficient k.This study provides a theoretical basis for the readout optimization methods of LC sensor.(2)A method to design and implement a low cost paper substrate LC humidity sensor is presented.The equivalent model of LC humidity sensor on paper substrate is established.The single spiral structure is designed,which can be processed in one step.Screen printing technology is adopted to realize rapid batch manufacturing.The effect of different structures on sensitivity and linearity of humidity sensor is studied in detail.The developed LC humidity sensor has a linearity of 0.997(linear fitting R-square = 0.997,ranging from 15%RH to 75%RH)and a sensitivity of-140 k Hz /%RH.The low-cost paper-based substrate LC humidity sensor can be used for humidity monitoring in food and drug packaging.(3)The parasitic effects of LC passive wireless sensing system are analyzed in depth,the calculation compensation method and three-step de-embedding method for impedance parameter readout are proposed,and the quasi-resonance method for S parameter readout is proposed,all of which improve the readout accuracy of the LC sensor.The influences of parasitic electric field coupling and parasitic shunt capacitor at the readout port are analyzed in detail,and the theoretical formulas of the readout frequency changing with the coupling coefficient are obtained.The theoretical and experimental results show that the parasitic electric field coupling only affects the readout frequency when the readout distance is very small,however,parasitic shunt capacitor at the readout port will make the readout frequency of both impedance parameter and S parameter vary with the coupling coefficient k,affecting the accuracy of the readout frequency.Based on the theoretical formula of impedance parameter readout frequency shift,the calculation compensation method and the three-step deembedding method are proposed,and the frequency measurement error of both methods after calibration is less than 0.1%.Based on the theoretical formula of S parameter readout frequency shift,a quasi resonance method is proposed,which reduces the parasitic electric field coupling,and the frequency measurement error after calibration is less than 1.1%.(4)A sweep-frequency impedance matching method for LC passive wireless sensing system is proposed,the sweep-frequency impedance matching circuit is realized,which does not affect the resonant frequency readout of LC sensor,and improves the readout distance.The sweep-frequency impedance matching method for LC passive wireless sensor system is studied deeply,and the scanning change of resonant frequency at the readout port is realized by voltage controlled varactor diode.Two types of matching network,RC type(resistorcapacitor)and L type(inductor-capacitor)are analyzed,optimized and verified systematically.RC type sweep-frequency impedance matching can extend the readout distance to 1.625 times as long as no matching,and L type sweep-frequency impedance matching can extend the readout distance to 2.25 times as long as no matching.This dissertation gives in-depth theoretical analysises and experimental researches on LC passive wireless sensing system from sensor to readout optimization,which provides feasible methods for the practical application of LC passive wireless sensing system.