| 摘要 |
PURPOSE:To reduce the influence of the temperature of a cast billet and variation in lift-off by extracting the signal component of an edge part from a detection signal, and setting a decision level according to the signal level of this signal component. CONSTITUTION:The impedance variation of an eddy current sensor 14 is detected by a signal detecting circuit 20 and the output AS of a band-pass filter 26 is full-wave rectified by a full-wave rectifying circuit 27. The output signal BS of this circuit is supplied to an envelope detecting circuit 40 to detect its envelope, which is inputted to a decision level setting circuit 50. When a signal waveform by the edge of the envelope detection output CS is detected by an edge detecting circuit, a timer 55 sends a switching signal and a switching circuit 53 is switched to the side of a peak holding circuit 51 only for a specific period. Consequently, the decision level set by the peak holding circuit 51 is supplied to a comparing circuit 60 in said period. |
