Development Of A Chipless RFID Humidity Sensor Based On Metamaterials

This paper aims to intensively study chipless RFID and humidity sensing technology to effectively avoid the loss caused by humid packages.It was raised under the application background of low cost detection for packages' identity and the degree of being affected with damp,and a high performance and miniature chipless RFID humidity sensor based on metamaterials was designed to identify object and detect the ambient humidity.2.45 GHz ELC resonator was designed by radio simulation software HFSS.It was proven to have metamaterials' property which can realize miniaturization and high quality factor through extracting constitutive parameter in parameter estimation method.Three humidity sensors were made by forming different thickness film on ELC surface with prepared 5wt%,10wt% and 15wt% PVA solution in quantitative drops of coating.Humidity sensitive test system was built for testing in the range of 35%RH~88%RH.The result showed that three humidity sensors all had good sensitive properties,their resonant frequencies all move to low when humidity increased,and sensitive property in high humidity range was significantly higher than low.Bedsides,three thickness film humidity sensors' total frequency deviations respectively was 70 MHz,95MHz and 101 MHz.Moreover,low weight percentage PVA was more suitable for wide range humidity test,and high weight percentage PVA could obtain more high sensitivity during high humidity test.Chipless coding method based on frequency spectrum characteristics was systematic researched.Through the performance comparison of four resonators,the metamaterials U-ring resonator was chosen as the basic coding structure.Fitting formula for structure design was obtained after studying its main structure parameters' effect on resonant and phrase properties by software simulation.A coding structure based on frequency shift technology was designed,which was made of four nested U-ring resonators,by methods of formula and parameter optimization simulation.Tags with different coding states were tested,the result was exactly same with simulation,and obtained coding capacity of 256 in 2.4GHz~5.9GHz.A coding structure with three parallel located U-ring resonators based on the two-dimension hybrid coding technique with frequency shift and phase shift was designed in order to further improve the coding capacity.Simulation and test results showed that the hybrid coding technique obtained coding capacity of 9261 in 2.45GHz~5.45 GHz.It greatly improved coding capacity,but its decoding technology was more difficult,and more easily met wrong decoding.The frequency shift coding tag and the humidity sensor of 5wt% PVA were selected to integration by analyzing the result of united simulation for humidity sensor and two coding units.The humidity sensitive and coding tests were carried out in the range of 50%RH~80%RH.The result showed that the resonated frequency declined with the increase of relative humidity.Moreover,the totally frequency shift was 50 MHz in testing range,and coding part's corresponding resonated frequency didn't shift.The right decoding was obtained,even though coding unit's resonant frequency shows a little different from independent test as it was affected by the electromagnetic coupling with sensor's resonant structure.The research showed that the chipless RFID humidity sensor based on metamaterials had the advantages of small size,high sensitivity,simple test and so on.It realized the identification of target identity and real-time detection of the relative humidity of the target surrounding environment,which was expected to be applied in the field of intellectual packaging.