大範圍超音波定位系統

室外定外系統已被發展多年，相當成熟，如全球定位系統(GPS)的服務被廣泛使用在任何地方，然而室內定位系統發展還未到達此水準。超音波裝置有眾多優點，如低成本，操作簡易等，但缺點則是超音波訊號在空氣中衰減得相當嚴重，為了改善此現象，我們將直接展頻序列(Direct Sequence Spread Spectrum : DSSS)應用於超音波測距，在發射端以偽雜訊碼(Pseudo Random Noise Code : PRN Code)和二元相位反轉調變(Binary Phase Shift Keying :BPSK)來調變訊號，其偽雜訊碼速率為4KHz，長度為1023 碼片(Chips)，載波頻率為40KHz。而在接收端，我們使用取樣頻率為320KHz 來截取超音波訊號。再使用複數解調變，相關運算，偵測真實峰值等，來計算超音波從發射端傳送訊號至接收端之飛行時間，再將其乘上當時的音速，其可得到發射端與接收端的距離。在本文採用到達時間差(Time Difference of Arrival : TDOA)演算法，進行定位運算。本系統採用四組超音波發射器與一組超音波接收器實現。由於使用了直接展頻序列的技術主要貢獻有兩部分，第一部分是高處理增益(High ProcessingGain)，即使強度低於背景雜訊的微弱超音波訊號仍可以被偵測，進而造成可測距距離長，定位範圍也可增廣。第二部分為具有良好的測距解析度，所以定位精準度提高。而此兩貢獻均由室內定位實驗結果得到佐證。

關鍵字

到達時間差定位演算法；室內定位系統；超音波訊號；偽雜訊碼；相關運算

並列摘要

Outdoor positioning systems have been matured for many years. Now days, people enjoy the service of GPS (Global Positioning System) almost everywhere at any time. However, the indoor positioning systems are still lacking. Ultrasonic devices own the advantages of low cost, simple operation etc. Unfortunately, the strength of sound wave decays quickly in the air. To overcome this disadvantage, we propose the direct sequence spread spectrum (DSSS) approach for range detection. The 4KHz rate pseudo random noise (PRN) code of 1023 chips are binary phase modulated to a 40KHz ultrasonic carrier as transmitting signal. At the receiving side, 320KHz sampler is used to collect all needed data for ranging. The complex number demodulation, correlation computation, peak detection, etc. will be applied to determine the travelling time of the signal from the transmitter to the receiver. The above value being multiplied with speed of sound will be the distance. Finally, TDOA (Time Difference of Arrival) model is used to get the position coordinate of the receiver when 4 ranging data are measured from 4 transmitters; respectively. The main contributions of spread spectrum technology are two. The first one is the high processing gain, so that the very weak ultrasonic signal can be detected even its power is less than that of environmental background noise. The second one is the fine ranging resolution, so that the positioning accuracy is improved. The proposed approach is confirmed by the indoor experiment results.

並列關鍵字

TDOA ； indoor positioning system ； Ultrasonic signal ； PRN code ； correlation

參考文獻

[12] 呂宜璋 “直接序列展頻之超音波測距系統” ，國立臺灣大學電機資訊學院電

Instrumentation and Measurement Technology Conference, Sorrento, Italy, April

[2] P. Misra and P. Enge, Global Positioning System Signals, Measurements, and

[3] D. Webster, “A Pulsed Ultrasonic Distance Measurement System based upon

[4] I. C. Lu, P. H. Jau, W. J. Cheng, Y .T. Chiang and F. R. Chang, “The Ultrasonic

國際替代計量

大範圍超音波定位系統

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