無線射頻辨識系統應用及測試之穩健設計

Abstract

隨著無線射頻辨識(RFID)技術的普遍，RFID網絡系統日益受到各方的關注，如何達到爲讀取器安排最佳的配置，提高讀取器收集資 訊的效率，是一個重要的研究方向，本論文透過穩健實驗設計，找出 影響無線射頻辨識讀取率及讀取範圍的主要因素和因子的最佳配置 條件，並設計一套RFID 天線配置工具，提供使用者快速且穩健的結 果。 首先，為了掌握無線射頻技術在讀寫資料上受影響的成因及程 度，本文以實驗設計方式對RFID 的表現及影響因子作一統計分析， 以田口方法出發，進行RFID 影響因子的實驗，以找出影響RFID 系 統表現的關鍵因素，並以所找出的顯著因子為基礎，應用反應曲面法 進行另一組實驗，分別測試915MHz 及2.45GHz RFID 天線的讀取距 離與範圍，以建構出穩健的模型來估計RFID 在各情境下的讀取空間。 本研究亦針對RFID 讀取器的讀取範圍及電場分布形狀做一實際 的量測，將空間切割成等大小的立方格後，一格一格地描繪出 915MHz 及2.45GHz RFID 的天線的電場形狀，打破傳統以圓型來模擬的方式，以較接近量測結果的橢圓形來模擬天線，藉由長、短軸的 彈性調整使適用度更加提升，相較於傳統所提供的方式更具實用性。 接下來進行RFID 天線佈置的軟體設計以提供使用者一個完整的 RFID 配置方案，在使用者輸入空間範圍及一些必需的條件之後，根 據先前的模型及對天線的假設，以最有效率的方式擺放RFID 讀取器 (天線)，在使用者所給定的限制之內，提供涵蓋率最高且花費最少的 天線擺放組合，並且將參考擺設圖示於使用者介面中，本研究亦針對 RFID 系統中容易發生的讀取器干擾問題作一討論，提出了簡單的群 聚方法，將可能干擾的讀取器分在不同的時間發送訊號，以避免干擾 發生而導致系統效率低落。最後，進行模擬及實際測試來驗證先前所 提出的運算的正確性及模型及適當性。

Recently, there has been wide interest in RFID (Radio Frequency Identification). With increasing applications of RFID technology, reliability requirements for deployment of RFID readers have become more critical. For a robust deployment of RFID network, a systematic solution was proposed in this study. Firstly, the performance of RFID was estimated statistically. Referring to Taguchi method, an experimental design was designed for finding out the factors which affect the reading rate of RFID significantly. The content in the chest (the tag attachment) and the direction of tags were obtained to be important causes for practical RFID applications. Another experiment for RFID interrogation zone was performed by applying Response Surface Method. Based on the significant factors and adding a new factor “tag density”, interrogation range models for 915MHz and 2.45GHz were constructed respectively. Hence, the size and shape of interrogation zone for various scenarios could be quickly estimated by the constructed models. Secondly, the interrogation zone was measured in practice. The measured point was set in the center of a cube and the cubes spread uniformly on each layer. By moving the tag cube by cube, layer by layer and assuming the performance of the tag is homogenous in each grid, the result was plotted and used to describe the shape of a RFID antenna’s interrogation zone. To simplify the reader deployment problem, instead of the traditional 2D circle-like shape assumption, the ellipse/ellipsoid-like shape was taken to represent the signal range of an RFID antenna in this study. It is more flexible and fitting for practicability than the traditional way. Thirdly, a deployment tool was designed for users to obtain a recommended layout. For a given area, solutions of number and placement of RFID readers are computed fast and robustly. Under input constraints, the system suggests a proper deployment for users to reach a high reading rate and ensure a complete coverage. In addition, two simplified mechanisms were provided for users to avoid reader collision which may result in low efficiency of a RFID network. Finally, the practical implementation and simulations were performed to test the practicability of the deployment tool and further to confirm the robustness and adequacy of the proposed models.