Close-Range Electromagnetic Tracking of Pulse Induction Search Coils for Subsurface Sensing

In this article, we investigate a specific search coil tracking problem arising from the use of electromagnetic induction (EMI) sensors to classify buried low metal objects. In the proposed approach, a moving search coil is used as a magnetic field beacon, two triaxial tunnel magnetoresistance (TMR) sensors are used as anchors, and the extended Kalman filter (EKF) is used to estimate five degrees of freedom information on coil's relative pose. A specific feature of the analyzed application is the pulsed beacon field and the coil's size comparable to the tracking distance. To address the resulting near-field effects and improve the tracking accuracy, we include a rigorous analytical multiloop model of the search coil into the EKF estimation algorithm and introduce a novel sensor calibration scheme. The system is experimentally validated in realistic dynamic conditions using a handheld pulse induction metal detector as a beacon, and a laboratory-grade optical tracking system with declared sub-millimeter accuracy as a source of ground truth data. For a near-horizontal zigzag scan pattern within a volume of ~40 × 50 × 5 cm, our system yields mean absolute errors <; 3 mm for the coil's x- and y-coordinates, <; 2 mm for z-coordinates, and <; 0.5 for the pitch and roll angles, making it a viable solution for pose-referenced EMI sensing.