| Summary: | 碩士 === 國立臺灣大學 === 電機工程學研究所 === 100 === To get the distance information is an interesting and important topic. Laser rangefinders and ultrasonic rangefinders are two typical instruments for this purpose. In general, the advantages of laser rangefinder include long range, fast and accurate. However, the prices of laser rangefinders are more expensive than those of ultrasonic rangefinders. The advantages of ultrasonic rangefinders are simple, small and cheap. They have been widely used in cars or robots to detect obstacles within close range. Other applications are oil level measurement, medical imaging and so on.
Traditionally, a short ultrasonic burst is transmitted from the transmitter in ultrasonic sensor. When there is an object in the path, some of transmitted ultrasonic wave is reflected. The receiver then can detect this echo. The signal’s time of flight (TOF) from the sending terminal to the receiving terminal being multiplied by the speed of the ultrasonic in the air, the distance between them can be calculated.
This article discusses a ranging algorithm based on direct sequence spread spectrum (DSSS) approach, which contains the ideas of pseudo-random noise code (PRN code) and binary phase shift keying (BPSK). The time of flight is obtained by the peak finding of the correlation results between the echoed and transmitted signals. The process gain makes the peak appears even if the echoed signals are buried in the background noises. In order to improve the signal-to-noise ratio (SNR), we use I-Q channel demodulation and complex variable demodulation method. The algorithm is verified by software simulations.
For the designed ultrasonic ranging system, we use sound pressure level (SPL) to represent sound propagation amplitude in the air. The SPL of transmission signal is 120 dB, and the SPL of background noise 50 dB. The frequency of the ultrasonic wave (i.e. carrier frequency) is 40 KHz. The carrier is modulated by BPSK with PRN code of 1023 chips. The chipping rate of PRN code is 4 KHz. In order to compute the correlation operation, the sampling rate used is 320 KHz. The simulation results show that the maximum distance estimated is approximately 20 meters in the two-way ranging scheme.
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