Research On Tag Gas Sensor Based On RFID

RFID sensor network integrates the technology of radio frequency identification (RFID) with wireless sensor network (WSN). The RFID system can carry the information, which collected by sensor. The vigorous development of emerging WSN puts forward higher requirements on gas sensors. Therefore, the low-cost, high-sensitivity and wireless tag gas sensor has become a new research direction.The proposed tag gas sensor consists of tag antenna and gas sensing unit, and the sensing unit as the load of tag antenna. The conductive polymer is selected as the sensitive material to detect ammonia and fruits releasing gas (banana, orange and tomato). This thesis studied the design, fabrication and test of gas sensor.Two test solutions are proposed, which is based on power reflection coefficient and tag antenna return loss. A bow-tie tag antenna resonated at 915 MHz is designed and optimized by HFSS Ansoft Designer. The results show that this antenna has the advantages of miniaturization, broadband and high gain. So it can be applied to the tag sensor. On the premise of impedance matching between the antenna and sensor, the value of antenna load impedance is simulated and tested in two test solutions. Then the approximate range of impedance value can be determined, which provides the theoretical basis for regulating the impedance of gas sensor.In the test solution of reflection coefficient, the different ratio of P-toluene sulfonic acid (TSA) doped polyaniline (PANI) materials are prepared by one-step in situ polymerization method. PANI liquid is coated on interdigital electrodes forming sensor. It can realize the impedance matching between the antenna and the sensor at 915MHz. In the test solution of tag antenna return loss, the CNT and conductive ink mixed materials are coated on parallel electrode forming sensor, which achieves a good impedance matching at the whole test frequency range. Several gas sensors with impedance matching are produced.Gas sensing properties test system is built to test and analysis the impedance sensitivity to the target gas. In this system, the impedance is measured by impedance analyzer (Agilent E4991A). The results based on reflection coefficient show that the variation of the reflect power increases non-linearly with the increase of ammonia concentration. When the ammonia ranging from 6500 ppm to 9000 ppm, the sensitivity is linear approximately, and it is 3.22 dBm/500ppm. The sensor has a good sensitivity for banana releasing gas, and it can distinguish the mature banana. The rotten tomato can be detected. The mature orange could not be distinguished. The conclusions based on tag antenna return loss shows:the ammonia concentration can be detected by measure the variation of the tag sensor resonant frequency, when the ammonia ranging from 1000 ppm to 3000 ppm. In this range, sensitivity is 3.75 MHz/500ppm. The ripe banana, tomato and fresh orange can be detected with the same method. The ammonia concentration can be confirmed in the range of 500 ppm to 1000 ppm by measure the variation of tag sensor return loss at 915 MHz. In this range, the sensitivity is linear, and it is 2 dB/500ppm. The sensitivity of tomato releasing gas is also linear. It can distinguish the mature tomato. The ripe and rotten banana could not be distinguished, and the mature orange could not be confirmed. The research results can be expected to distinguish the fresh degree of fruits in radio frequency identification method.