Affiliations: [a] NRC Aerospace, 1200 Montreal Rd, Building M-14,
Ottawa, ON K1A 0R6, Canada. Tel.: +1 613 991 2132; Fax: +1 613 952 7136;
E-mail: catalin.mandache@nrc-cnrc.gc.ca
| Université Laval, Canada | Tohoku University, Japan
Abstract: In recent years, solid-state devices made their way in the
development of electromagnetic non-destructive evaluation (NDE) probes. This
fact was evidenced especially for pulsed eddy current, where magneto-resistive
and Hall effect devices are used as sensing elements. Their low frequency range
and small surface area are suitable to improve the detection of buried and
small discontinuities. Although their properties are expected to enhance
detectability over simple induction coils, this was still to be proven or
demonstrated in a comparative study. This work compares the sensing
capabilities of an induction coil to those of two solid-state devices:
giant-magneto-resistive (GMR) and Hall effect sensors. All of them are used as
detectors in pulsed eddy current probes that have the same excitation
mechanism, a ring-type copper coil driven by a constant amplitude square
waveform. While the excitation part of the probe is fixed, the sensing
components are inter-changeable. Although both induction coils and solid-state
sensors output a voltage value as an indication of the magnetic field they are
detecting, the voltage for pick-up coils is directly proportional to the rate
of change of the magnetic flux. For solid-state sensors the output is in direct
relation to the detected magnetic field. Under this study, all three sensing
elements are used to detect the driving coil's magnetic output (magnetic field
or flux) in air, on planes perpendicular and parallel to the face of the driver
coil. The results obtained by all three sensors are quantitatively compared.
Then the sensing devices are inserted in the inner space of the driving coil
and, subsequently, used for detection of artificially made defects. Finally,
the results are compared in terms of magnetic field sensitivity and inspection
performance.
Keywords: Magnetic sensing, pulsed eddy current, non-destructive evaluation
