Method and apparatus for measuring the degree of cable...

Electricity: measuring and testing – Magnetic – Plural tests

Reexamination Certificate

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C324S232000, C324S242000

Reexamination Certificate

active

06172501

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of and an apparatus for detecting a corrosion degree of a cable made of ferromagnetic materials (e.g., steel) used for, for example, a suspension bridge, a skew bridge.
The term “cable” includes not only a so-called cable made of ferromagnetic materials but also cable-shaped members such as a rope, strand, code, line, wire, thread, string, stick, rod, pole, staff, club, bar, shaft and the like all of which are made of ferromagnetic materials, irrespective of diameters thereof and shapes of the cross section thereof, and further includes a cable comprising line members which are neither twisted nor braided together but just bundled together as well as a twisted cable, throughout the specification and the appended claims.
2. Background Art
A cable made of a metal (particularly, steel) used for a suspension bridge, a skew bridge, and the like is exposed to the weather because it is outdoors. Further, it is affected by salt water in the neighborhood of the seashore, and is affected by sulfurous acid in an industrial area. Therefore, the cable is not immune to corrosion.
The corrosion degree of the cable has been generally evaluated as follows.
With respect to a cable having a diameter of 60 [mm] or less, the damage of the cable has been detected by a magnetic leakage flux testing. In this method, the local damage such as breaking can be detected, but the entire corrosion cannot be evaluated.
With respect to a cable having a diameter exceeding 60 [mm], the appearance of the cable has been viewed, to evaluate the corrosion degree of the cable. In this method, the precision in the evaluation of the corrosion degree of the cable is low.
There is an apparatus for evaluating the corrosion degree of a cable utilizing a so-called total flux method. The apparatus is forced to be increased in size, and its magnetized state varies depending on the measuring environment, whereby the precision may, in some cases, be decreased.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method and an apparatus in which the corrosion degree of a cable can be evaluated without making the apparatus relatively large in size and with relatively high precision.
A method of measuring a corrosion degree of a cable according to the present invention comprises the steps of magnetizing an object cable of measurement whose corrosion degree is to be measured, detecting strength of a magnetic field (or magnetic field intensity) for magnetizing the object cable of measurement, detecting an amount of (or quantity of)(or total) magnetic flux passing through the magnetized object cable of measurement, calculating a value relating to a cross-sectional area of the object cable of measurement on the basis of the detected magnetic field strength, the detected amount of magnetic flux, and permeability of the object cable of measurement, and measuring a degree of corrosion of the object cable of measurement on the basis of comparison between the calculated value relating to the cross-sectional area of the object cable of measurement and a value relating to a cross-sectional area of a reference cable.
The present invention presupposes that the object cable of measurement is a ferromagnetic material. The object cable of measurement is magnetized. Let H be the strength of the magnetic field (or magnetic field intensity) for magnetizing the object cable of measurement. Let (be the amount of (or total or main) magnetic flux passing through the magnetized object cable of measurement, and B be the magnetic flux density thereof.
Letting &mgr; be the permeability of the object cable of measurement, and A be the cross-sectional area of the object cable of measurement, the following equation holds:
B=&mgr;H=&PHgr;/A
  (1)
The following equation is obtained by modifying the above equation (1):
A=&PHgr;/&mgr;H
  (2)
If the permeability &mgr; and the magnetic field strength H are considered to be constant, the cross-sectional area A of the object cable of measurement is proportional to the amount of magnetic flux &PHgr;. The degree of corrosion of the object cable of measurement appears in the cross-sectional area A. If there is no corrosion, the cross-sectional area A is generally kept constant. If the corrosion advances, the cross-sectional area A is decreased (a corroded portion can be regarded as a non-magnetic material). By making a comparison between the cross-sectional area of the reference cable and the cross-sectional area of the object cable of measurement, the degree of corrosion of the object cable of measurement can be evaluated.
Examples of the reference cable include an object cable of measurement itself, and a cable of the same material and construction as those of the object cable of measurement. When a new cable is laid or used, the cross-sectional area thereof is measured (this cross-sectional area is taken as an initial cross-sectional area). When a certain time period (for example, one year, five years, ten years, or twenty years) has elapsed since the cable was laid or used, the cross-sectional area of the cable which has been laid or used is measured. If the measured cross-sectional area is compared with the initial cross-sectional area, the degree of corrosion caused in the elapsed time period can be evaluated. When corrosion is checked with respect to a cable which was already provided in the past, a new cable of the same material, construction and size as those of the past cable may be produced as a reference cable.
The use of the value relating to the cross-sectional area is sufficient to evaluate the corrosion degree. The value relating to the cross-sectional area may be the cross-sectional area A itself. Alternatively, it may be a value proportional to the cross-sectional area, the detected amount of magnetic flux &PHgr;, a value proportional to the amount of magnetic flux &PHgr;, and the like.
It is not always necessary to assume that the magnetic field strength H and the permeability &mgr; are fixed to be a constant. According to the equation (2), if the permeability &mgr;, the magnetic field strength H, and the amount of magnetic flux &PHgr; are determined, a value relating to the cross-sectional area A is found. Consequently, the magnetic field strength H and the amount of magnetic flux &PHgr; may be measured. With respect to a cable of the same material as that of the object cable of measurement (the cross-sectional area of which has been known), the permeability &mgr; can be previously obtained in correspondence with the magnetic field strength H.
A ferromagnetic material generally has magnetic hysteresis characteristics. A method of magnetizing the object cable of measurement (the direction of current, and the directions in which the current is increased and decreased) is fixedly predetermined. Measurements are made in a region where the amount of magnetic flux is uniquely determined or almost uniquely determined with respect to the magnetic field strength. For example, the magnetic field strength and the amount of magnetic flux may be measured in a region where the magnetic field is saturated.
The magnetization of the object cable of measurement can be realized by winding a magnetizing coil around the cable and causing current to flow through the magnetizing coil.
The present invention is also applicable to a case where the object cable of measurement is coated or covered with a non-magnetic material (concrete, synthetic resin), etc.. The method according to the present invention is rather effective in a case where the object cable of measurement is coated or covered because the state of the cable therein cannot be viewed by eyes.
An apparatus for realizing a method of measuring the corrosion degree of a cable according to the present invention is constructed as follows.
Specifically, an apparatus for measuring the corrosion degree according to the present invention comprises a magnetizing coil for magnetizing an object ca

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