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A highly accurate binary frequency shift-keyed ͑BFSK͒ ultrasonic distance measurement system ͑UDMS͒ for use in isothermal air is described. This article presents an efficient algorithm which combines both the time-of-flight ͑TOF͒ method and the phase-shift method. The proposed method can obtain larger range measurement than the phase-shift method and also get higher accuracy compared with the TOF method. A single-chip microcomputer-based BFSK signal generator and phase detector was designed to record and compute the TOF, two phase shifts, and the resulting distance, which were then sent to either an LCD to display or a PC to calibrate. Experiments were done in air using BFSK with the frequencies of 40 and 41 kHz. Distance resolution of 0.05% of the wavelength corresponding to the frequency of 40 kHz was obtained. The range accuracy was found to be within Ϯ0.05 mm at a range of over 6000 mm. The main advantages of this UDMS system are high resolution, low cost, narrow bandwidth requirement, and ease of implementation.

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