Research On Surface Acoustic Wave Tag Fruit Gas Sensor

SAW(Surface Acoustic Wave,SAW)gas sensor has the advantages of high sensitivity,miniaturization,low cost and so on,which has become a hot spot in the field of sensor research.This thesis combines SAW gas sensor and radio frequency identification technology,which has researched the 916.5 MHz SAW tag of the sensor design,production,test plan and test results under the background of the application of the fruit freshness.Designed the SAW electronic tag gas sensor,which consisted of SAW gas sensor and tag antenna.Proposed a testing program of the change of the reflected power of the tag in a gas environment for the sensitivity.Firstly,taking the change of resonant frequency as sensitivity parameters,SAW gas sensors that based on the 916.5 MHz are researched.Then selected a single terminal pair SAW resonators as SAW devices,and polyaniline material,multi-walled carbon nanotubes and graphene as a material sensitive film material,producing three different sensitive material of SAW gas sensors,tested the sensitivity to the smell of cherry tomatoes,bananas,apples and oranges.The results showed polyaniline sensor ' s low sensitivity to the smell of fruits and multi-walled carbon nanotube sensor ' s high sensitivity to cherry tomatoes,bananas and apples.Especially the sensitivity to cherry tomatoes was significantly higher than the other two fruits.The frequency offset of fresh,ripe,rotting cherry tomatoes was respectively 0.5 MHz,2.5 MHz and 3.0 MHz,which has good sensitivity and selectivity.Graphene sensors'sensitivity to the smell of orange is best,three sensitivity to different growth stages of orange are 0.1 MHz,0.2 MHz and 0.3 MHz.According to the results of the resonator frequency sensitivity test,the impedance sensitivity of multi-walled carbon nanotube sensor to the smell of cherry tomatoes,bananas and apples were researched.The results showed that the sensor's impedance sensitivity variation of different fruits was consistent with frequency sensitivity.The gas sensor had the largest impedance sensitivity of cherry tomatoes releasing gas.The sensor's impedance sensitivity to the smell of cherry tomatoes at 916.5 MHz in three stages was respectively 0.32,0.41 and 0.45.With the increase of cherry tomatoes maturity,impedance real part changes from 26.16 to 19.45.Nevertheless,the imaginary part had the opposite change tendency,which was from-160.93 to-123.41.In comparison,the imaginary part change was the main cause of the sensor which had large impedance sensitivity.Taking the multi-walled carbon nanotube sensor that the initial impedance was 39.9-j276.41 ? as impedance matching objects,this research designed a RFID antenna that the center frequency was 916.5 MHz.conducting and testing the production.Impedance at its resonant frequency was 44 + j227 ?,which broadly realized the conjugate match of the sensor impedance.Significant change was happened in the sensor impedance value at 916.5MHz,resulting in the change of power reflection coefficient ??.The ?? of theoretical calculation values of multi-walled carbon nanotube sensor for gas of different maturity cherry tomatoes were-3.58 dB,-4.32dB and-4.40 dB,which means that it's feasible to using the change of label reflected power as the parameter sensitivity.Integrated SAW multi-walled carbon nanotube gas sensor and tag antenna,produced a SAW tag gas sensors,and the sensitive nature of cherry tomatoes' gas were tested and analyzed.The amount of change in the reflected power was increasing as the growing season,respectively 0.07 dBm,0.36 dBm and 0.52 dBm,basically showing linear growth.The sensor are repeatability tested,and the results showed that the sensor has good repeatability.The results demonstrated that the developed SAW RFID fruit release gas sensor has advantage of high sensitivity,low cost,fast response,etc.,which is expected to achieve the recognition of fruit freshness.