| 摘要 |
PURPOSE:To detect an abnormality with high accuracy irrespective of a fluctuation in a detected sheet thickness by a method wherein the flaw-detection frequency of ultrasonic waves is changed so as to obtain a resonance frequency according to the sheet thickness. CONSTITUTION:A personal computer 32 sets a flaw-detection reference frequency and a reference voltage according to coil information on an input sheet thickness or the like, and it sends them to a peak-frequency detection circuit 34 and a function generator 36. The generator 36 generates an electric signal at a frequency and a reference voltage which are set by a peak frequency detection circuit 34, and ultrasonic waves are incident on a steel plate 10 by the use of transmission probe 22A and they are received by a reception probe 22B. The received ultrasonic waves are input to a gate peak detector 42, and an abnormal particle is detected when the level of an extracted peak voltage is at a prescribed value or lower. At this time, a flaw-detection frequency shift circuit 44 shifts a flaw-detection frequency when an input peak voltage has a shape corresponding to a normal part. Then, the circuit 34 changes the flaw-detection frequency on the basis of the relationship between a sheet thickness (d)[m] and a resonance frequency (f)[Hz] when a sheet-thickness fluctuation width is at DELTAd, it finds a peak frequency at which a sound pressure becomes maximum, and it sends the peak frequency to the generator 36. Thereby, it is possible to obtain the resonance frequency according to the sheet thickness. |
