FIG. 1 is a graph showing a conventional distance measuring method, which decides according the signal size whether a signal is arrived. One example of this type of method is a method of measuring distance using ultrasonic waves, which is disclosed in U.S. Pat. No. 5,197,019 (Delon-Martin, et al.). However, this technique has a problem in that it may be mal-operated in response to a large noise. That is, this technique uses a variable gain in order to maintain constantly the shape of signals. A time-domain signal is transformed into a frequency-domain signal, which is again transformed into a time-domain signal after being digital-processed, so that a high-performance computer is required, thereby lacking practicality. It can be seen from FIG. 1 that this method responds easily to noise. Therefore, with this conventional method, the size of signal cannot be amplified above the noise.
In order to solve these problems, the present inventor has proposed a distance measurement method and device using ultrasonic waves, which is disclosed in PCT application No. PCT/KR/2004/001917
As shown in FIG. 2, the above distance measuring method and apparatus is designed to measure a distance using an analogue-digital converter (AD), a digital signal processor (DSP), and a period measurement. That is, in the previous technique of the present inventor using ultrasonic waves to measure a distance, a received ultrasonic signal is adequately amplified, and a natural frequency is separated from the ultrasonic signal mixed with undesired signal and transformed into a digital signal, so that the arrived signal of initial pulse can be extracted, thereby enabling the calculation of target distance in a stable fashion, as compared with any other conventional one.