Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Reexamination Certificate
1999-12-22
2001-09-11
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
C324S240000, C324S229000, C324S260000
Reexamination Certificate
active
06288537
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an eddy current probe for detecting flaws and thicknesses in the surface and sub-surface region of a conductive material by inducing and measuring an eddy current on the surface of the material. The invention is particularly suitable for inspecting surface coatings, subsurface cracks and surface flaws in metallic and semi-conductive materials. The eddy current probe may also be used to measure the thickness of an insulating coating on a metallic or semi-conductive material.
BACKGROUND OF THE INVENTION
Eddy current probes are used to inspect the surfaces of metal and semiconductive objects. In one application, eddy current probes are applied to inspect the surfaces of metallic objects, such as rotor blades of steam and gas turbines. Eddy current probes provide a non-destructive test for inspecting the rotor blades.
Eddy current probes are applied to the surface (or near to the surface) of the metallic or semiconductive object being inspected. To induce a useful eddy current on a conductive surface, the probe applies an electromagnetic field to the surface to induce eddy currents. This electromagnetic field is generated by current in an inducing (driving) coil of the eddy current probe. The eddy currents induced on a surface have relatively low energy and are best detected by probes placed on (or at least near) the surface. A sense coil in the eddy current probe is placed against a surface so that the sense coil current is influenced by the surface eddy currents. The effects on the sense coil currents by the surface eddy currents are measured by processing circuits associated with the probe. To measure the thickness of an insulating coating, the eddy currents on a conductive or semi-conductive surface below the coating is measured. This eddy current measurement is indicative of the coupling between the eddy current probe and the having the coating. This coupling between the probe and part is a function of the distance between the probe coils and the conductive or semi-conductive surface below the insulating coating. Accordingly, the thickness of the coating may be determined from the eddy currents measured by the probe.
Many of the objects to be inspected with an eddy current probe have complex surface contours. For example, a rotor blade has a twisted airfoil column, a flange and a pine tree root. Applying an eddy current probe to the surface contours of a rotor blade or other object is difficult, especially with robot controlled inspection instrument. The tip of the eddy current probe may not conform to the surface of the object being inspected. If good contact is not established between the eddy current probe and the surface, or if there is an air gap between the probe and surface, the probe may be unable to accurately measure the eddy currents induced on the conductive or semi-conductive the surface. Similarly, the tip of the probe may be broken if it is too forceably driven onto the surface. Accordingly, there is a need for an eddy current probe that is sufficiently flexible to conform to odd surface shapes, and that is capable of withstanding impacts with a surface being inspected.
BRIEF SUMMARY OF THE INVENTION
A novel eddy current probe has been developed having a pair of conductive coils embedded in a foil strip and backed by a deformable nose at the sensory tip of the probe. The foil strip is wrapped over the deformable nose to provide a flexible backing to the coils in the strip. This flexibility allows the coils, strip and nose to conform to a surface being inspected and withstand an impact with that surface. Of the pair of conductive coils, the inducing coil (the driving coil) induces an eddy current on the surface of an object being inspected. The other coil (sense coil) carries a current induced by the magnetic flux from the eddy currents on the conductive or semi-conductive surface. The induced current in the sense coil is indicative of the surface eddy currents, and is used to measure the strength of those surface eddy currents on the object being inspected.
The foil strip of the eddy current probe sandwiches the inducing and sense coils between two flexible films that are laminated together. The coils may be arranged at the center of the foil strip, and connected to terminals at the ends of the strip by parallel tracks of wire extending laterally along the length of the films. At a center section of the foil, each of the coils may be arranged in a rectangular and spiraling coil. The coils may be nested together in the strip.
The foil strip with the embedded coils is wrapped over a flexible nose of the eddy current probe. The nose provides a soft base to support the film and coils. The nose may be formed of a silicon rubber or other deformable material. For the grinding applications, a rubber material, such as VITON™, that is more impervious to the effects of cutting fluid may be needed. The base of the nose is mounted on the probe housing. The outer surface of the nose may be a rounded ridge or have some other shape conforming to the intended surface to be measured. As the nose is pressed against a surface, it deforms and conforms to that surface. The nose is resilient and returns to its original shape when lifted from a surface. The foil strip is wrapped over the nose to fit smoothly over the outer surface of the nose. Accordingly, the nose provides a supporting surface for the foil and coils, and allows the foil to flex when in contact with another surface.
As the eddy current probe is positioned on the surface of an object being inspected, the sensory tip of the probe is pressed against the surface with sufficient force to deform the nose, and cause the foil and coils to conform to the contours of the surface. As the coil conforms to the surface, the coils are properly positioned against the surface to induce and measure an eddy current on the surface. By flattening the coils against the surface, the coils are inherently positioned to be immediately adjacent the surface and to eliminate air pockets between the coils and the surface. Because the inducing coil is at the surface, the current in that coil need be relatively modest to induce measurable eddy currents on the conductive or semi-conductive surface. Similarly, the proximity of the sensing coil to the surface allows for relatively large currents to be induced in that coil due to the eddy currents on the surface. Accordingly, the ability to conform the coils to and against a surface of the object being measured is an advantageous feature of the present eddy current probe.
Applying the foil strip and coils of the present eddy current probe directly against the surface is due, in part, to the deformable nose on the eddy current probe. As the probe moves into contact with the surface, the film is pressed firmly against the surface by the nose of the probe. As the nose deforms under the force between the nose and the surface, the film is flattened against the surface and thereby provides good contact between the film strip and surface. Accordingly, the deformable nose is helpful in causing the coils to conform to a surface and, thus, to providing good eddy current measurements.
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Lee Martin K.
Viertl John R. M.
General Electric Company
Metjahic Safet
Nixon & Vanderhye P.C.
Sundaram T. R.
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