The method using chirped ultrasonic wave to detect objects being near the boundary surface is presented. The flaws being near the surfaces of steel billets or the parts placed on a table are the examples of such object. In this method the discrimination of the objects and the measurement of the distances between the boundary surface to the objects can be performed, and the period of the modulated waves appearing on the envelope of the received signal corresponds to depth of the flaw and the modulation factor corresponds to the size of the object. The relation between the distance from the boundary to the object and the range of the sweep frequencies are discussed, thereby the guide line for the pulse width and the frequency deviation is derived. We carried out the two experiments in order to prove that this method is more useful than the current pulse echo method for detection of the objects being near the boundary surfaces. The one experiment is for the detection of the flaws in the billets, and the other is for the shape classification of the objects on the table.
The method proposed by this paper has the following practical advantages: (1) higher signal to noise ratio, (2) without the accurate time measurement, (3) without the precise data for the distance between the transducer and the boundary, (4) without the complicated signal processing.