Method and apparatus for magnetically detecting defects in an object with compensation for magnetic field shift by means of a compensating coil

• Main Authors: MATSUFUJI; YASUHIRO, INABA; MAMORU, IWANAGA; KENICHI, TAKEOSHI; ATSUHISA, ANDO; SEIGO, TAKENAKA; MASAKI, MAKI; HIROSHI
• Format: Patent
• Language: English
• Published: 30.04.1996
• Edition: 6
• Subjects: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIRCHEMICAL OR PHYSICAL PROPERTIES; MEASURING; PHYSICS; TESTING

PCT No. PCT/JP92/00191 Sec. 371 Date Feb. 1, 1993 Sec. 102(e) Date Feb. 1, 1993 PCT Filed Feb. 24, 1992 PCT Pub. No. WO92/21964 PCT Pub. Date Dec. 10, 1992.A magnetic detector includes a magnetizer generating a magnetic field; a first magnetic sensor for detecting a leakage flux resulting from a magnetically defective portion of an object moving in and relative to the magnetic field; a low-pass filter for extracting a low-frequency signal component contained in the signal output by the magnetic sensor, the low-frequency signal component being indicative of a floating flux which crosses the magnetic sensor and is a result of the object moving through the magnetic field; an amplifier for amplifying the extracted low-frequency signal component; a compensating coil excited by the amplified low-frequency signal component output by the amplifier, for generating a magnetic flux which cancels out the floating flux crossing the magnetic sensor; a high-pass filter for extracting a signal resulting from the magnetically defective portion and contained in the signal that is output by the first magnetic sensor; an output amplifier for amplifying a signal output by the high-pass filter to produce a defect signal; a second magnetic sensor located near the first magnetic sensor, for detecting a component of the magnetic field which is parallel to the surface of the object; an amplifier control circuit for controlling an amplification factor of the output amplifier in accordance with a signal from the second magnetic sensor; and a magnetizer control circuit for controlling the intensity of the magnetic field generated by the magnetizer, in accordance with a signal output by the second magnetic sensor.