AMR gradiometer for mine detection

Anisotropic magnetoresistors (AMRs) are used to build an advanced mine detector. The sensing head involves a gradiometric pair of AMR sensors and a continuous-wave driven excitation coil, so the gradiometer is capable of detection of 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 responses of the object of interest. Being limited by the sensor and electronics noise of 268 pT/√Hz at 1 kHz, we were able to detect a 50 mm × 50 mm × 1.5 mm aluminum test object in a 20 cm depth using a 150 μT, 1-kHz excitation field.