Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Reexamination Certificate
2002-08-26
2004-08-10
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
C324S235000
Reexamination Certificate
active
06774627
ABSTRACT:
TECHNICAL FIELD
This invention relates to leak magnetism detection sensors for use in magnetic flaw detection systems, and particularly, it relates to a leak magnetism detection sensor for the online magnetic flaw detection system that is suitable for use in leakage flux sensing for detecting surface flaws and inclusions of a steel strip by sensing the leakage flux arising from the internal and surface of the ferromagnetic object under test.
BACKGROUND ART
The flaw detection technique using leakage flux is a method for detecting defect by generating a magnetic field along the traveling direction of the target object and then sensing the leakage flux arising from internal and surface defects of the object. As sensors for detecting leakage flux, there are semiconductor-type magnetic sensors such as magnetic diodes, magneto-resistance devices and Hall devices, and the coil-type ones such as planar coils and induction coil sensors that hold coils wound on ferrite.
Among them, the magnetic diode has the advantages of being high in sensitivity and small in size. On the other hand, it has the disadvantages of having poor temperature characteristics, large inherit noise and low mechanical strength.
The coil-type sensors have simple structures and good temperature characteristics, while they have a disadvantage of being low in sensitivity.
Meanwhile, the Hall device, which was a low-sensitivity semiconductor magnetic sensor, has come to have improved sensitivity and temperature characteristics, thus being widely adopted as a leak magnetism detection sensor for use in the flaw detection using leakage flux.
Tin plates, which are employed in food cans, are strongly worked during the production of two-piece cans (DI cans). Thus non-metallic inclusions inside the material (hereafter, simply referred to as inclusions) cause cracks during working. The target for sensing is a volume of 0.5×10
−3
mm
3
, assuming an elliptic region of, approximately, 1.0 mm long, 0.1 mm wide and 0.01 mm thick. Thus there are such problems that as many as around 1000 semiconductor magnetic sensors are needed to perform an online flaw detection over the full width of the target and that a huge number of signal processing circuits for as many as 1000 channels must be prepared.
An invention similar to this invention is disclosed in Japanese Utility Model Laid-Open Publication No. Hei. 7-38956. In that disclosure, a ferromagnetic material is directly attached to the opposite side of the magnetism sensing face of a coil sensor where a coil is formed on the magnetism sensing surface, in order to enhance the sensor sensitivity by gathering leakage flux in the ferromagnetic material and rendering most of the gathered magnetic flux intersect the magnetism sensing face of the coil sensor. However, it does not refer to the expansion of detection coverage of the sensor.
Japanese Patent Laid-Open Publication No. Hei. 4-296648 has also disclosed a ferromagnetic jig installed near a magnetic sensor. This jig is a magnetic shield prepared for reducing magnetic flux around the magnetic sensor so that the high density of magnetic flux in the surrounding space bypasses the magnetic sensor and thereby the magnetic sensor may not saturate. This jig is expected to improve sensor sensitivity to some extent but does not aim to expand the detection coverage.
SUMMARY OF THE INVENTION
This invention has been made to solve those conventional problems and aims to reduce the number of sensors and signal processing circuits by expanding the detection coverage of each leak magnetism detection sensor.
In order to solve the problems mentioned above, according to this invention, a leak magnetism detection sensor, which is used in a magnetic flaw detection system that generates a magnetic field along a traveling direction of a target strip and detects online leakage flux arising from internal and surface defects of the target strip with a flaw detection head equipped with a number of magnetism sensing devices arrayed across the width of the target strip to provide signals indicating the existence of defects, has a soft-magnetic material that is installed on an opposite side of the magnetism sensing face of a magnetism sensing devices and is larger than the magnetism sensing face.
Further, each of the magnetism sensing devices is located apart from the soft-magnetic material and has another soft-magnetic material kept contact with the opposite side of the magnetism sensing face of the magnetism sensing device.
Still further, the magnetism sensing device has another soft-magnetic material kept contact with its magnetism sensing face.
Yet further, the magnetism sensing device is a Hall device.
In addition, the present invention provides a method of detecting online flaws in strips, using a leak magnetism detection sensor comprising a number of magnetism sensing devices arrayed across a width of a target strip for detecting leakage flux arising from internal and surface defects thereof and a soft-magnetic material that is installed on an opposite side of a magnetism sensing face of the magnetism sensing devices and is larger than the magnetism sensing face.
FIG. 1
shows the leak magnetism detection sensor of the present invention (for example, a semiconductor sensor). In this leak magnetism detection sensor, a soft-magnetic material
14
that has a permeability much higher than that of air and is larger than a magnetism sensing face
12
A of a magnetism sensing device (for example, a Hall device) of a semiconductor magnetic sensor
12
is installed on the opposite side of this magnetism sensing face
12
A, namely, on the-other side of the sensor facing the target strip, apart from this magnetism sensing face
12
A at a predetermined distance, namely apart from the magnetism sensing device. Then the soft-magnetic material
14
attracts leakage flux F arising from an inclusion
10
A in a steel strip
10
. As a result, the coverage of each sensor becomes larger and its sensitivity is enhanced because more of the magnetic flux F is concentrated to intersect the magnetism sensing face
12
A in the direction perpendicular to both magnetism sensing face
12
A and strip
10
.
As shown in
FIG. 2
, the semiconductor magnetic sensor
12
shown in
FIG. 1
may have a magnetism sensing device
12
C such as a Hall device on the surface of a soft-magnetic material
12
B such as ferrite. Namely, the soft-magnetic material
12
B is kept contact with the opposite side of the magnetism sensing face of the magnetism sensing device
12
C. Seen from the target strip
10
, the semiconductor magnetic sensor
12
holding the magnetism sensing device
12
C on the side facing the strip
10
and the soft-magnetic material
14
are installed in this order. Then since further more of the leakage flux F is concentrated to intersect the magnetism sensing face
12
A at right angles as shown in
FIG. 3
, the detection coverage of each sensor is expanded and its sensitivity is enhanced in this further preferable example. Reference numeral
16
in
FIG. 2
denotes a supporting plate for sensor mounting.
Higher sensitivity is provided by the following mechanism. When the magnetism sensing device
12
C such as a Hall device is mounted on the soft-magnetic material
12
B such as ferrite as shown in
FIG. 2
, more of the leakage flux F gathered by the aforementioned soft-magnetic material
14
is further gathered in the magnetism sensing face
12
A by the soft-magnetic material
12
B used in the semiconductor magnetic sensor
12
. As a result, more of the leak flux F comes to intersect the magnetism sensing face
12
A in the direction normal thereto.
As shown in
FIG. 4
, the semiconductor sensor may have the magnetism sensing device
12
C such as a Hall device on the surface of the soft-magnetic material
12
B such as ferrite and may sandwich this magnetism sensing device
12
C such as a Hall device between soft-magnetic materials
12
B and
12
D such as ferrite by installing a soft-magnetic material
12
D like ferrite adhered to the magnetism sensing face
12
A of the magne
Tomura Yasuo
Tsuruoka Shigetoshi
Unzaki Hideaki
Yokota Hiroyuki
Kawasaki Steel Corporation
Kinder Darrell
Patidar Jay
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