US 4,383,218 AGranted

Eddy current flow detection including compensation for system variables such as lift-off

AssigneeThe Boeing Company

InventorsKarl A. Hansen, Iver G. Hendrickson

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Abstract

Disclosed is an eddy current inspection system for nondestructive detection of subsurface flaws in regions of multilayer conductive structure that surround a fastener which joins the conductive layers. The eddy currents are induced by an aperiodic drive current having a relatively long rise time and a relatively last fall time that is supplied to a drive coil and a pick-up coil that is mounted with the drive coil supplies a signal representative of the induced eddy currents. Compensation is provided for variation in lift-off (spacing between the surface of the structure being inspected and the surface of the drive coil and pick-up) by utilizing parametric relationships relative to the maximum amplitude of inspection signals supplied when subsurface flaws exist and inspection signals supplied when lift-off is present. Because similar parametric relationships exist relative to other system characteristics that affect the electromagnetic environment of the inspection media, compensation can also be provided for variation in the height of fasteners (protrusion), fastener edge margin, and gaps or spacing between the successive conductive layers of the multilayer structure. By utilizing a microprocessor or other sequential digital signal processing apparatus, values that define the parametric relationships utilized in compensating for any particular system characteristic can be stored in memory for the inspection of a relatively large number of structural configurations.

Claims8

Dependent claims

An eddy current inspection system for detecting the presence of subsurface regions within a body wherein the region to be detected is of a different electrical conductivity than portions of said body that surrounds said region to be detected, said eddy current inspection system comprising:

drive coil means positionable on the surface of said body to be inspected, said drive coil means being configured for inducing a magnetic field that is directed inwardly into an inspected region of said body in response to an applied drive signal;
electrical signal supply means interconnected with said drive coil for supplying said drive signal, said electrical signal supply means including means for supplying said drive signal as a current pulse having a relatively long rise time and a substantially shorter fall time;
magnetic flux detection means positionable on said surface of said body being inspected at a location proximate to said drive coil means, said magnetic flux detection means for supplying an inspection signal representative of the eddy currents produced in said body subsequent to that portion of said drive current exhibiting said short fall time;
and
analysis and display means for indicating the presence of a subsurface region of different continuity, said analysis and display means connected for receiving said inspection signal supplied by said magnetic detection means, said analysis and display means including signal compensation means for compensating each said inspection signal for variations in a selected inspection system characteristic that affects the electromagnetic environment of said eddy current inspection system, said signal compensation means including means for compensating said inspection signal on the basis of first and second parametric relationships, said first parametric relationship being determined by the maximum amplitude of inspection signals supplied by said magnetic flux detection means when a subsurface region's of different electrical conductivity is present, said second parametric relationship being determined by the maximum amplitude of inspection signals supplied by said magnetic flux detection means when variations in said selected inspection system characteristic are known to be present.

2 more elements

A method for detecting subsurface flaws within a conductive body comprising the steps of:

inducing a slowly rising magnetic field into a region of said conductive body that is to be examined for said subsurface flaws with the lines of said magnetic flux extending inwardly into said conductive body;
rapidly collapsing said induced magnetic field to generate substantial eddy currents which circulate through said region of said conductive body being examined;
monitoring the magnetic field variations caused by said eddy currents to supply a detection signal representative of the magnetic field variations caused by said eddy currents;
determining a first difference signal by comparing said detection signal with a first reference signal which results when said induced magnetic field is introduced into a flaw-free conductive body having substantially the same geometry as said region of said conductive body being examined;
determining a second difference signal by comparing said first reference signal with a second reference signal which results when said induced magnetic field is introduced into a flaw-free conductive body having substantially the same geometry as said region of said conductive body being examined and said method is conducted to introduce a known variation in a parameter known to affect the electromagnetic environment that determines said magnetic field variations caused by said eddy currents;
determining a third difference signal by comparing said first reference signal with a third reference signal which results when said induced magnetic field is introduced into a conductive body having substantially the same geometry as said region of said conductive body being examined and having a known subsurface flaw;
and
combining said first, second and third difference signals to produce a flaw indication that is compensated for variations in said inspection system characteristic that may be present.

5 more elements

Dependencies are inferred from the claim text.

Facts

Application: 06/192,251Filed 30 Sep 1980 · continuation in part
Priority: 29 Dec 1978
Granted: 10 May 1983
Term ends: Not estimatedThe source records don’t support an estimate.

Classification6 codes

G01V 3/08
Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation: operating with…
G01N 27/023
By investigating impedance: where the material is placed in the field of a coil
G01N 27/9046
Using eddy currents: by analysing electrical signals
G01N 27/9053
By analysing electrical signals: compensating for probe to workpiece spacing