Study On Passive Wireless Temperature And Humidity Sensors

The Internet of Things(IOT)is a product of internet and information technology development to a certain stage,it is a new emerging modern information industry.The IOT has great potential applications in the areas of intelligent transportation,environment monitoring,smart health care,public security,and defense etc.The sensor is one of the most core technologies for IOT.Micro-electro-mechanical system(MEMS)technology has been successfully applied in the physical sensing areas,which has yielded a range of small,rubustness and inexpensive sensors.Passive wireless sensor is desirable in many emerging applications in harsh environments,unaccessable places,as well as in rotating components.Inductor-Capacitor(LC)type passive wireless sensors with simple structure,stable and reliable performance,can be achieved with various materials,as well as non-contact interrogation and theoretically unlimited lifetime,are preferred over their counterparts.Temperature and humidity sensing are very important for IOT applications and sensors measurements.However,structure types of the LC passive wireless sensors are quite rare now days.Besides,there are few efforts to measure multiple parameters using a single LC-type sensor.For the above reasons,this dissertation proposes two kinds of LC passive wireless sensors.The main work and innovations of this dissertation are described as follows:(1)A detail introduction to the fundamentals of LC passive wireless sensing system is given,including the construction,sensing principle,characteristic of the electrical impedance spectroscopy and portable interrogator.Design rules are also presented,including type of the basic components,theoretical design methods for the LC tank and so on.(2)A linear self-calibration temperature sensing capacitor is proposed.The capacitive temperature sensing structure consists of two kinds of temperature sensitive capacitors,which are vertical movement temperature sensitive capacitor and lateral movement temperature sensitive capacitors.By using lateral capacitors to compensate the nonlinearity of vertical capacitor,high linearity and proper sensitivity characterization is achieved.The mechanical characteristics of the temperature sensing capacitor are theoretically analyzed with the thermo-elasticity model.Comparison between Comsol simulation model and theoretical model are also proposed.The temperature sensing capacitor is fabricated by the high-aspect-ratio MetalMUMPs process.The measured nonlinearity error of the temperature sensing capacitor is less than 6%,and its sensitivity is 19.4fF/?.(3)A novel LC passive wireless temperature sensor is proposed,which is constructed from the high linearity capacitive temperature sensor die directly attached with a PCB substrate with a planar embedded inductor.This structure simplifies the fabrication of thick inductor,which hardly can be achieved on CMOS substrate.Measurement results by using an implemented portable reader show that the sensitivity of the LC passive wireless temperature sensor is 37.7kHz/?,which is comparable with other LC types of passive wireless sensors(2kHz/??16.97kHz/?).(4)A mathematic model to characterize the humidity,temperature sensing behaviour of frequency-dependence graphene oxide(GO)films is established.Graphite oxide was synthesized by a modified Hummers method.The TEM,FTIR,XRD and XPS characterizations of the GO films are given.The GO electrical properties is studied by using alternating current complex impedance technique,relationship between changing leakage resistance and sensing capacitance with humidity,temperature and frequency can be given by the surface fit results.And this study may provide a fundamental theory for the applications of GO films based electrical sensing.(5)An integrated wireless passive sensor for remotely monitoring both temperature and relative humidity is proposed.The sensor consists of a coil and a capacitor to form a LC resonant circuit.The inductor is a single layer planar spiral copper inductor and the capacitor is fabricated by the Silicon-On-Glass(SOG)process which utilizes graphene oxide films as the sensing material.By monitoring the real portion magnitude maximum of the impedance and the resonant frequency of the sensor,it is possible to get the capacitance and resistance from which the temperature and humidity can be extracted.The results presented here show that the sensitivity of the passive wireless sensor is about-17.80kHz/%RH and-7.3?/%RH at 25? from 55%RH to 95%RH,and it is about-7.69kHz/? and-6.27?/? at 65%RH from 10? to 40?.In conclusion,this dissertation proposes two kinds of LC passive wireless sensors.The former provides a novel type of LC passive wireless temperature sensor with a high linearity capacitive temperature sensing structure.The other one gives a new approach to sensing multiple parameters using a single LC-type sensor.Besides,this dissertation also presents a comprehensive study of design rules for the LC passive wireless sensor,and this study may provide fundamental theory for further design of new types of LC passive wireless sensors.