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

A magnetic detector comprising:

• means for 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 said magnetic field, said object having a surface;
• a high-pass filter for extracting a signal resulting from the magnetically defective portion and which is contained in the signal that is output by said first magnetic sensor;
• an output amplifier for amplifying a signal output by said high-pass filter and for outputting the amplified signal as a defect signal;
• a second magnetic sensor located near said first magnetic sensor, for detecting a component of said magnetic field which is parallel to the surface of said object;
• and
• amplifier control means for controlling an amplification factor of said output amplifier in accordance with a signal output by said second magnetic sensor.

A magnetic detector comprising:

• a magnetizer for 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 said magnetic field generated by said magnetizer, said object having a surface;
• said first magnetic sensor outputting a first output signal that is indicative of the leakage flux of the magnetically defective portion of the object;
• a high-pass filter for extracting a signal resulting from the magnetically defective portion and which contained in the signal that is output by said first magnetic sensor;
• a second magnetic sensor located near said first magnetic sensor, for detecting a component of said magnetic field which is parallel to the surface of said object;
• an output amplifier coupled to receive a signal output by the high pass filter and for outputting an amplified signal as a defect signal;
• and
• magnetizer control means for controlling intensity of an said magnetic field generated by said magnetizer, in accordance with a signal output by the second magnetic sensor.

A magnetic detector comprising:

• means for 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 said magnetic field and for producing a signal corresponding thereto, said object having a surface;
• a low-pass filter for extracting a low-frequency signal component contained in said signal output by the first magnetic sensor, said low-frequency signal component being indicative of a floating flux which crosses said first magnetic sensor and which is a result of the object moving through said magnetic field;
• an amplifier for amplifying the low-frequency signal component extracted by said low-pass filter;
• a compensating coil excited by the amplified low-frequency signal component output by the amplifier, for generating a magnetic flux which cancels out said floating flux crossing said first magnetic sensor, said compensating coil being located near said first magnetic sensor;
• a high-pass filter for extracting a signal resulting from the magnetically defective portion and which is contained in the signal that is output by said first magnetic sensor;
• an output amplifier for amplifying a signal output by said high-pass filter and for outputting the amplified signal as a defect signal;
• a second magnetic sensor located near said first magnetic sensor, for detecting a component of said magnetic field which is parallel to the surface of said object;
• and
• amplifier control means for controlling an amplification factor of said output amplifier in accordance with a signal output by the second magnetic sensor.

A magnetic detector comprising:

• a magnetic for 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 said magnetic field generated by a magnetizer and for producing a signal corresponding thereto, said object having a surface;
• a low-pass filter for extracting a low-frequency signal component contained in said signal output by the first magnetic sensor, said low-frequency signal component being indicative of a floating flux which crosses said first magnetic sensor and which is a result of the object moving through said magnetic field;
• an amplifier for amplifying the low-frequency signal component extracted by said low-pass filter;
• a compensating coil excited by the amplified low-frequency signal component output by the amplifier, for generating a magnetic flux which cancels out said floating flux crossing said first magnetic sensor, said compensating coil being located near said first magnetic sensor;
• a high-past filter for extracting a signal resulting from the magnetically defective portion and which is contained in the signal output by said first magnetic sensor;
• an output amplifier for amplifying a signal output by said high-pass filter and for outputting the amplified signal as a defect signal;
• a second magnetic sensor located near said first magnetic sensor, for detecting a component of said magnetic field which is parallel to the surface of said object;
• and
• magnetizer control means for controlling the intensity of the magnetic field generated by said magnetizer, in accordance with a signal output by the second magnetic sensor.

A magnetic detector comprising:

• means for generating a magnetic field;
• a magnetic sensor for detecting a leakage flux resulting from a magnetically defective portion of an object moving in and relative to said magnetic field and for producing a signal corresponding thereto;
• a low-pass filter for extracting a low-frequency signal component contained in said signal output by the magnetic sensor, said low-frequency signal component being indicative of a floating flux which crosses said magnetic sensor and which is a result of the object moving through said magnetic field;
• an amplifier for amplifying the low-frequency signal component extracted by said low-pass filter;
• a compensating coil excited by the amplified low-frequency signal component output by the amplifier, for generating a magnetic flux which cancels out said floating flux crossing said 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 said magnetic sensor;
• an output amplifier for amplifying a signal output by said high-pass filter and for outputting the amplified signal as a defect signal;
• and
• amplifier control means for controlling an amplification factor of said output amplifier in accordance with a signal output by said magnetic sensor.

A magnetic detector comprising:

• a magnetizer for generating a magnetic field;
• a magnetic sensor for detecting a leakage flux resulting from a magnetically defective portion of an object moving in and relative to said magnetic field generated by said magnetizer and for producing a signal corresponding thereto, said object having a surface;
• a low-pass filter for extracting a low-frequency signal component contained in said signal output by the magnetic sensor, said low-frequency signal component being indicative of a floating flux which crosses said magnetic sensor and which is a result of the object moving through said magnetic field;
• an amplifier for amplifying the low-frequency signal component extracted by said low-pass filter;
• a compensating coil excited by the amplified low-frequency signal component output by the amplifier, for generating a magnetic flux which cancels out said floating flux crossing said 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 said magnetic sensor;
• an output amplifier for amplifying a signal output by said high-pass filter and for outputting the amplified signal as a defect signal;
• and
• magnetizer control means for controlling the intensity of the magnetic field generated by said magnetizer, in accordance with a signal output by the magnetic sensor.

A magnetic detector comprising:

• a magnetizer for generating a magnetic field;
• a magnetic sensor for detecting a leakage flux resulting from a magnetically defective portion of an object moving in and relative to a magnetic field generated by said magnetizer;
• magnetizer control means for controlling the intensity of the said magnetic field generated by said magnetizer, in accordance with a signal output by said second magnetic sensor;
• a high-pass filter for extracting a signal resulting from the magnetically defective portion and which is contained in a signal output by said magnetic sensor;
• an output amplifier for amplifying a signal output by said high-pass filter and for outputting the amplified signal as a defect signal;
• and
• amplifier control means for controlling an amplification factor of said output amplifier in accordance with a signal output by said magnetic sensor.

A magnetic detector comprising:

• means for generating a magnetic field;
• a magnetic sensor for detecting a leakage flux resulting from a magnetically defective portion of an object moving in and relative to said magnetic field and for producing a signal corresponding thereto;
• a low-pass filter for extracting a low-frequency signal component contained in said signal output by the magnetic sensor, said low-frequency signal component being indicative of a floating flux which crosses the magnetic sensor and which is a result of the object moving through said magnetic field;
• an amplifier for amplifying the low-frequency signal component extracted by the low-pass filter;
• a compensating coil excited by the amplified low-frequency signal component output by the amplifier, for generating a magnetic flux which cancels out said floating flux crossing said magnetic sensor;
• a high-pass filter for extracting a signal resulting from the magnetically defective portion and which is contained in the signal that is output by said magnetic sensor;
• a speed detector for detecting a speed at which said object moves relative to said magnetic sensor;
• and
• frequency control means for controlling a pass frequency of said high-pass filter in accordance with the speed detected by said speed detector.

A magnetic detector comprising:

• means for generating a magnetic field;
• a magnetic sensor for detecting a leakage flux resulting from a magnetically defective portion of an object moving in and relative to said magnetic field and for producing a signal corresponding thereto;
• a low-pass filter for extracting a low-frequency signal component contained in said signal output by the magnetic sensor, said low-frequency signal component being indicative of a floating flux which crosses the magnetic sensor and which is a result of the object moving through said magnetic field;
• an amplifier for amplifying the low-frequency signal component extracted by the low-pass filter;
• a compensating coil excited by the amplified low-frequency signal component output by the amplifier, for generating a magnetic flux which cancels out said floating flux crossing said magnetic sensor;
• said means for generating said magnetic field comprising a magnetizer arranged such that a pair of magnetic poles oppose said object;
• said magnetic sensor being arranged on a line connecting said magnetic poles;
• a high-pass filter for extracting a signal resulting from the magnetically defective portion and which is contained in the signal that is output by said magnetic sensor;
• a speed detector for detecting a speed at which said object moves relative to said magnetic sensor;
• and
• frequency control means for controlling a pass frequency of said high-pass filter in accordance with the speed detected by said speed detector.

A magnetic detector comprising:

• means for 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 said magnetic field, said object having a surface;
• a high-pass filter for extracting a signal resulting from the magnetically defective portion and which is contained in the signal that is output by said first magnetic sensor;
• an output amplifier for amplifying a signal output by said high-pass filter and for outputting the amplified signal as a defect signal;
• a second magnetic sensor located near said first magnetic sensor, for detecting a component of said magnetic field which is parallel to the surface of said object;
• amplifier control means for controlling an amplification factor of said output amplifier in accordance with a signal output by said second magnetic sensor;
• a reference signal generating circuit for generating a reference signal corresponding to a reference amplification factor;
• and
• a division circuit connected between said second magnetic sensor and said output amplifier for detecting that component of said magnetic field which is parallel to the surface of said object, and for dividing a signal output by said second magnetic sensor by said reference signal for controlling amplification of said output amplifier.

A magnetic detector comprising:

• a magnetizer for 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 said magnetic field generated by said magnetizer, said object having a surface;
• said first magnetic sensor outputting a first output signal that is indicative of the magnetically defective portion of the object;
• a second magnetic sensor located near said first magnetic sensor, for detecting a component of said magnetic field which is parallel to the surface of said object;
• magnetizer control means for controlling an intensity of said magnetic field generated by said magnetizer, in accordance with a signal output by the second magnetic sensor;
• a high-pass filter for extracting a signal resulting from the magnetically defective portion of the object and which is contained in the first output signal output by said first magnetic sensor;
• an output amplifier for amplifying the first signal output by said first magnetic sensor and for outputting an amplified signal as a defect signal;
• and
• amplifier control means for controlling an amplification factor of said output amplifier in accordance with a signal output by said second magnetic sensor for detecting said component of said magnetic field which is parallel to the surface of said object.