室內定位與姿態判定系統

Abstract

摘要 本研究主要目的是利用GPS室外定位原理，來發展出一套室內定位系統，並且具備三維姿態的即時量測。由於室內距離遠小於GPS系統中衛星與接收端的距離，因此本系統採用超音波訊號取代GPS系統中的射頻訊號(RF signal)，且由於藉由超音波傳輸時間所獲致的距離量測精度有限(約在1~2 cm)，因此在三維姿態的計算當中，本研究採用載波相位差的技術來提高其姿態判定的精度。 載波相位差的技術是利用多個接收器及其接收訊號的載波相位差來進行姿態判定。此技術的優點在於高精度，而主要挑戰在於：(1)整數波判定(integer wavelength ambiguity problem)；(2)訊號傳輸距離差轉換三維姿態的計算法則。文獻上可發現許多利用多根天線(multi-antenna)GPS系統進行三維姿態量測，惟在GPS系統中，衛星與接收裝置的距離相當遠，可假設衛星(發射端)與各個接收裝置(接收端)之路徑近似於平行，進而簡化訊號傳輸距離差與接收端姿態的數學關係。而在室內定位系統下，因為發射端與接收端距離不夠遠，若作同樣的平行假設，得到的姿態誤差會相當大。因此必須重新推導發射端與接收端的向量方程式，使其適用於室內環境下的姿態判定。 本研究採用Cricket系統[1]來進行超音波訊號的發射與接收。在實驗設備的硬體建構部分，目前已架構完成3個訊號發射器、4個訊號接收器，4個訊號接收器所接收的訊號分別透過RS232與ADC管道進入PC進行訊號分析與計算，並透過Labview的人機介面即時顯示接收訊號與分析所得之物體的空間位置與姿態判定。在姿態判定計算法則部分，本研究利用訊號傳輸時間來簡化整數波估算的問題，並重新推導Wahba姿態判定計算法則[8]，使其適用於室內定位系統。先前研究所發展的計算法則[17]，至少需要2個發射端與4個接收端，且4個接收端不可共平面。本研究所提出的計算法則，僅3個發射端與3個接收端，且3個接收端可共平面，因此系統架構更具彈性。 實驗結果顯示本系統可進行三維空間定位與姿態判定，在接收端被放置於一移動物體上，且彼此相距約100cm，姿態判定的精度約1.2度。

Abstract In this research, we developed an indoor GPS system which can be used to for the object 3D positioning and attitude determination. Since the space in an indoor environment is rather limited, the ultrasonic signal (system) is used in the indoor GPS system. Besides, the carrier phase technique is used to improve the accuracy of the attitude determination. Carrier phase technique uses multi-receivers to receive signals from beacons, and the phase differences of the received signals to calculate the attitude of the object. The advantage of employing this technique is the accuracy improvement. However, its challenges are the “integer wavelength ambiguity” and the algorithm that converts the difference of signal path to the attitude of the object. From literature review, one can find many algorithms for the attitude determination using multi-antenna GPS system. However, those algorithms cannot be directly applied to the indoor systems because those approaches simplify the problem by assuming the beacons and receivers are far apart, which is not true in an indoor environment. In this research, we used the Cricket system [1] to implement the receiving and sending of the ultrasonic signals. Currently, we have completed the experiment setup which includes: (1) 3 beacons and 4 receivers, (2) the received signals going through RS232 and ADC channels to the PC and being processed for the 3D positioning and attitude determinations, (3) a graphic user interface to show the received signal and the 3D position/ attitude of the object in real time. For the attitude determination algorithm, we used the time difference of the received signals to approximate the integer for the wavelength ambiguity problem, and modify the algorithm proposed by Wahba [8] for the use in an indoor environment. As compared to the previous work [17] which requires, at least, 2 beacons and 4 non-coplanar receivers, this algorithm requires, at least, 3 beacons, 3 receivers and, better off, they all can be coplanar. The preliminary results show that the proposed system can be used for the 3D positioning and attitude determination in an indoor environment. The accuracy of the measured attitude is 1.2 degree when the multi-receivers are placed approximately 100 cm apart on a moving object.