Research On Chipless RFID Humidity Sensors

Humidity is closely related to human everyday life.In order to create a better living en-vironment and production conditions,the sense of humidity has evolved into real-time moni-toring and automatic regulation,and the application of humidity sensors is becoming more and more widespread.At present,the common humidity sensors in the market need to be connected with complex computing circuits,leading to high detection costs.The chipless RFID humidity sensor has been widely concerned because of its lower cost,lower radiated power and longer storage life.While monitoring the humidity changes,the chipless RFID humidity sensor can also use the RFID technology to track and locate the target,which inte-grates the functions of chipless RFID tag and sensor.Therefore,the chipless RFID humidity sensors have important research significance and good application prospects.With the support of the National Natural Science Foundation of China under Grant61372008 and Guangdong Provincial Science and Technology Planning Projects under Grant2014A010103014,2015B010101006,this dissertation has studied the designing principles of chipless RFID humidity sensors and the reader antenna.The main works are summarized as follows:(1)A chipless RFID humidity sensor based on square ring resonators is proposed.The designed chipless sensor consists of three sets of nested concentric square rings with cham-ferings on the main diagonal and splits on the vice diagonal.The outer two resonators are in-troduced to encode 6-bit sensor ID through using frequency shift encoding method,while the innermost scatterer acts as the sensing unit.Polyvinyl alcohol(PVA)film is deposited on the surface of the sensing unit,making it sensitive to the humidity.The sensor has a compact size of 15×15×0.8 mm~3 and it operates in the band of 4–7.5 GHz.Moreover,the sensor pos-sesses depolarizing characteristic,improving its detection robustness in harsh environments.Finally,the feasibility of the chipless RFID humidity sensor working in humid environment is verified by experiments.The sensitivity of the sensor is 3.65 MHz/%RH at a humidity range of 58%–78%.(2)A chipless RFID humidity sensor based on electric-LC resonators is investigated.In order to improve the performance of the sensor,a Z-gap resonator is introduced as a sensing unit,which has good resonance characteristics and can improve the sensitivity of the sensor as the relative permittivity of the material changes.The Z-gap resonator obtains depolarizing characteristic by loading a complete metal floor.Three nested concentric square rings with chamferings on the main diagonal are adopted as coding units,thus the sensor can encode 9bits of identity information.The Z-gap resonator is embedded in the coding unit to form the proposed sensor with the dimensions of 20×20×0.8 mm~3.Finally,the reliability of the chipless RFID humidity sensor is verified by experiments.The sensitivity of the sensor is 5.9MHz/%RH at a humidity range of 50%–80%.(3)A dual polarized UWB antenna fed by CPW is presented.The proposed antenna is composed of two orthogonally distributed UWB slot antenna elements.The measured results of the proposed antenna indicate that the impedance bandwidth of the antenna can cover the whole UWB of 3.1–10.6 GHz,and the isolation between the two ports is greater than 25 dB.Besides,the antenna exhibits omnidirectional radiation characteristics in the operating fre-quency band.The dual-polarized antenna has the advantages of low profile,low cost,and easy integration with small mobile devices.Finally,the working feasibility of the du-al-polarized antenna in the chipless RFID reading system is verified.