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GMR array uniform eddy current probe for defect detection

Postolache, O. ; Ribeiro, A. L. ; Ramos, H.

Measurement: Journal of the International Measurement Confederation Vol. 46, Nº 1, pp. 4369 - 4378, December, 2013.

ISSN (print): 0263-2241
ISSN (online): 0263-2241

Journal Impact Factor: 0,662 (in 2008)

Digital Object Identifier: 10.1016/j.measurement.2013.06.050

Abstract
The usage of eddy current probes (ECP) with a single magnetic field sensor represents a
common solution for defect detection in conductive specimens but it is a time consuming
procedure that requires huge amount of scanning steps when large surface specimens are
to be inspected. In order to speed-up the nondestructive testing procedure, eddy current
probes including a single excitation coil and an array of sensing coils present a good solution.
The solution investigated in this paper replaces the sensing coils for giant magnetoresistors
(GMRs), due to their high sensitivity and frequency broadband response. Thus,
the ECP excitation coil can be driven at lower frequencies than the traditional ones allowing
defects to be detected in thicker structures.
In this work an optimized uniform eddy current probe architecture including two planar
excitation coils, a rectangular magnetic field biasing coil and a GMR magnetometer sensor
array is presented. An ac current is applied to the planar spiral rectangular coil of the probe,
while a set of GMR magnetometer sensors detects the induced magnetic field in the specimens
under test. The rectangular coil provides the DC uniform magnetic field, assuring
appropriate biasing of the GMR magnetometers of the probe, setting-up the functioning
point on the linear region and at the same branch of the GMR static characteristics. The differences
on the images obtained for the same specimen for each GMR are reduced if all sensors
are biased on the same working point. Elements of the automated measurement
system used to inspect the plate under test using the proposed eddy current probe, including
a validation procedure based on a 2D template matching algorithm and the corresponding
experimental results are included in the paper.