AMR gradiometer for mine detection and sensing

We used anisotropic magnetoresistors (AMR's) to build an advanced mine detector. The sensing head involves gradiometric pair of AMR sensors and a continuous-wave driven excitation coil, so the gradiometer is capable of detection ferromagnetic materials as well as diamagnetic metals. The sensors are specially arranged to suppress the large AC-excitation field, so the mine detector senses both DC and AC field gradient response of the object of interest. Being limited by the sensor noise of 214 pTrms/√Hz at 1 kHz, we were able to detect an 50x50x1.5 mm3 aluminum test object in a 20 cm-depth using a 300 μT, 1 kHz excitation field.