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A linear magnetic stray flux array based on GMR gradiometers

: Kloster, A.; Kröning, M.; Smorodinsky, J.; Ustinov, V.

Dobmann, G. ; Japan Society of Applied Electromagnetics and Mechanics -JSAEM-:
Electromagnetic Nondestructive Evaluation VII : 12.-14. June 2002, Saarbrücken
Amsterdam: IOS Press, 2006 (Studies in Applied Electromagnetics and Mechanics 26)
ISBN: 1-58603-594-0
International Workshop on Electromagnetic Nondestructive Evaluation <8, 2002, Saarbrücken>
Conference Paper
Fraunhofer IZFP ()
giant magnetoresistance-sensor; magnetic field

The dramatic increase in storage density in magnetic media had a strong impact on the development of new sensor concepts for measurement of weak magnetic fields. Two prerequisites were miniaturization and the efficiency of the effect used. A number of magnetic field sensors based on various mechanisms has been developed and found applications in information storage technology , mechanical engineering and automotive industry. It is attractive to use such sensors for special applications in nondestructive testing (NDT) i.e. eddy-current testing with high penetration and on-line monitoring.
Compared to the most conventional sensors the sensitivity of such sensors based on giant magneto-resistance (GMR) effect is higher, so smaller pre-magnetization fields are necessary.
Based on a commercial GMR chip, we have built up an array with 16 gradiometer sensors arranged in a line with a pitch of 4 mm. A pre-magnetization device and multiplexer is integrated in a sensor head. A PC data acquisition board digitizes the output signals of the array. For spatial filtering a wavelet transformation algorithm based on Doubechies wavelet is used. The sensor array was tested on samples in different forms with artificial surface and sub-surface defects in various depths and widths as well as for natural defects. An outlook on the design of equipment including special sensor-on-chip technologies fit for industrial application is presented.