Investigation of Bending Stress Effect on the Diagonal and Off-Diagonal Impedances for GMI Sensor Implementation

Electrical current measurement by a giant magnetoimpedance (GMI) sensor may require the sensitive element (amorphous wire) to be aligned with the magnetic field produced by the conductor (toroidal configuration), which involves a bending stress. In this paper, a first study is made to investigate the impact of bending stress on a GMI sensor. For practical GMI sensor implementation, the offset and intrinsic sensitivity are crucial parameters. This is why these quantities have been evaluated in both diagonal and off-diagonal configurations. A Co-rich amorphous wire (from Unitika Ltd.) of 100 μm diameter and 15 cm length was used. The off-diagonal voltage was measured through a pick-up coil. The wire was bent over a cylinder with a radius of 2.5 cm. The excitation frequency was 800 kHz. In the diagonal configuration, the sensitivity at a given bias field Hbias was decreased by 50% in the bent position compared to the straight position. The offset also decreased by 25%. In the off-diagonal configuration, the sensitivity around zero decreased by about 28%. The offset stayed quasi-null during the test. These results showed that the off-diagonal configuration seems to be best suited for an application as a current sensor that involves bending. The reversibility and repeatability of the bending effect have been evaluated under ten successive bending stresses. It was shown that the effect is quite reversible and repetitive.