在UHF頻段之表面聲波式無線射頻識別感測系統

Abstract

目前全球各主要識別系統中無線射頻辨識系統(RFID system)為市場正積極發展之系統，其因非接觸式之特性使得在應用方面適合做為監測系統之使用。無線射頻識別系統乃由一查詢器與應答器構成，其工作原理為，由查詢器發出詢問訊號，經由應答器反射訊號回查詢器以讀得標籤之編碼。應答器的製作可藉由表面聲波元件所構成之標籤與天線結合，可組成不需另供給能量之被動式無線射頻辨識晶片。 本文利用耦合模型理論來模擬分析使用於UHF頻段之表面聲波式無線射頻辨識標籤之頻率響應，再配合快速傅立業逆轉換，可得到識別標籤在時間域上之編碼。由模擬之結果，以探討壓電基板材料、交指叉電極與反射電極對數、交叉長度…等設計參數於頻率響應以及時間域訊號之影響，再考慮將識別標籤外接負載之邊界條件變化，由外接負載之變化可得到其與感測器結合之訊號模擬。最後以微機電製程技術(MEMS)，實際製作出以128°Y-X LiNbO3為壓電基板、中心頻率約為433MHz、具不同設計參數之表面聲波式無線射頻辨識標籤。並藉由實驗量測驗證數值分析結果，作為最佳化設計表面聲波式無線射頻辨識標籤之依據，說明耦合模型理論於分析表面聲波式無線射頻辨識標籤之適用性。值得一提的是，在實驗部分如設計可工作在不同頻率，可具有不同獨立編碼之識別標籤。 綜言之，本文以耦合模型理論分析聲波式無線射頻辨識標籤之頻率響應，可作為未來設計聲波式無線射頻辨識標籤之工具，並首先提出雙頻率聲波式無線射頻辨識標籤之概念，可降低製造成本、增加負載資訊，期望未來聲波式無線射頻辨識標籤能廣泛地應用在監測上。

Among the various identification systems that are currently in development, Radio Frequency Identification (RFID) systems show the most potential due to their wireless sensing capabilities, making them highly suited for remote monitoring and measurement systems. A RFID system consists primarily of a reader unit and transponder. The interrogation signal emitted from the reader unit is reflected by the transponder, and the information on the transponder is embedded into this reflected signal. This reflected signal is picked up by the reader unit, reading the information on the transponder. By combining an antenna with a surface acoustic wave (SAW) device, a passive RFID transponder that do not require any external power source could be achieved. In this thesis Abbott’s Coupling Of Mode (COM) theory is used to simulate and analyze the frequency response of the UHF SAW based RFID tag. Furthermore, inverse fast Fourier transform is applied in order to obtain the time domain response by which the coded information is encoded. Using the result of simulation, parameters such as substrate, finger pairs of the InterDigital Transducer (IDT), and apertures in frequency and time domain were discussed. By considering the influences of an external load (sensor) on the boundary conditions of the tag, we can simulate the results of varying load. Finally, different designs of SAW based RFID tags with central frequency of 433MHz were fabricated on the 128°Y-X LiNbO3 using Micro-Electro Mechanical Systems (MEMS) techniques. The experimental result and simulation results are compared and the COM theory proof to be suitable for analysis of SAW based RFID tag. Using these results, guiding rules for the designing of SAW based RFID tags are proposed and a single tag capable of carrying multiple independent codes is designed and fabricated. In conclusion, this thesis utilized the COM theory to analyze SAW based RFID tag, and designed a novel dual band RFID tag. This dual band RFID tag can reduce cost and increase the amount of information carried by a single tag. Hopefully, this SAW based RFID tag will be widely employed in monitoring and measurement applications.