| 摘要 |
PROBLEM TO BE SOLVED: To detect the presence of a foreign object according to an acoustic signal even if, for example, a thin and long foreign object such as a nail where an inner sectional area of a tube does not change greatly exists in the tube. SOLUTION: An acoustic signal is transmitted from a transmitter 14 that is mounted to one edge T1 of a tube T into the tube T and at the same time an acoustic signal that is reflected in the tube T is received by the receiver 15 being mounted to one edge T1 of the tube T. A partial frequency is eliminated from the received acoustic signal by a filter 7. When a position that is closer to the side of the other edge T2 from one edge T1 of the tube T is inspected, the threshold of the filter 7 is set lower and the signal intensities before and after the acoustic signal passes the filter 7 for the signal based on the same reflection source. Every time when the threshold id changes in steps, an acoustic signal is transmitted and received. |
