使用eTag反射訊號的交通流感測

Abstract

由於汽車數量的迅速增加，交通擁堵變成一個很大的問題。有鑒於此，我們需要即時的交通資訊和交通管理系統來提高運輸效率。交通流感測和車輛偵測在交通監控和規劃中扮演著很重要的地位，在近年的研究中許多基於各式傳統感測器和圖像辨識技術的交通流感測方法被提出；然而，這些方法存在著許多的缺點，而隨著台灣車輛廣泛的使用無線射頻辨識標籤，使得無線射頻辨識技術越來越適合使用在這項應用中。因此，在本論文中，我們提出一種使用無線射頻辨識和都卜勒效應的車輛計數及速度估算系統。 我們使用指向性天線和軟體定義無線電裝置實現無線射頻辨識系統。透過將發射器和接收器的天線放在道路旁，我們的系統可以觸發鄰近的無線射頻辨識標籤並使標籤反射訊號進行回應；透過計算無線射頻辨識標籤的電子產品代碼數量來測量通過的車輛數量。此外，我們使用都卜勒效應估算車輛的速度。通過快速傅立葉變換，我們可以獲得標籤反射訊號的頻率位移，藉此來計算移動車輛的速度。實驗的結果顯示，即使車輛的速度達到時速六十公里，系統計算的車速平均誤差也小於時速五公里。此外，我們使用商用現成的無線射頻辨識標籤和eTag、不同類型的車輛及標籤不同的擺設位置來分析系統的效能。

Traffic congestion has been a major problem due to the rapid increase in the number of vehicles. Hence, real-time traffic information and traffic management is required to improve transportation efficiency. Traffic sensing and vehicle detection play an important role in traffic monitoring and planning. Several traffic sensing methods have been demonstrated in the recent studies, such as sensor-based and image-based approaches. However, these approaches have their shortages. Benefit from the wide deployment of RFID tags attached to vehicles in Taiwan, RFID technology becomes an attractive approach for this application. Therefore, in this work we develop a vehicle counting and speed estimation system using RFID and Doppler effect. We implement the RFID system using directional antennas and the USRP platform, which is a software defined radio. By putting the transmitter and the receiver antennas at roadside, our system can trigger neighboring RFID tags to response with a backscattered signal. We can measure the number of passing vehicles by counting the number of different electronic product code (EPC) of RFID tags. Furthermore, we estimate the speed of vehicles by leveraging the Doppler effect. By Fast Fourier Transform (FFT), we can obtain the frequency shift of received signals so as to measure the speed of a moving vehicle. Real-world experimental results show that the average error of the speed estimation is less than 5 km/hr even if the speed is up to 60 km/hr. In addition, we compare the system performance using commercial off-the-shelf (COTS) RFID tags and eTag, different type of cars and different attachment positions of eTag.