A total-field magnetometer system for location and identification of compact ferrous objects

A self-contained person-portable magnetometer system which accurately locates, identifies, and estimates orientation of compact ferrous objects on-line in near real time is described. It consists of a cart-mounted cesium magnetometer, optical encoder, microcontroller, interface, and laptop computer. The instrument guides the operator to collect simultaneous magnetic field and position data in a horizontal plane above an object. Custom algorithms estimate location and dipole moment, which is used to classify the object. Data collection takes 6-13 min, location and moment estimation 5 s, and classification 30 s. Experiments on ferrous spheroids are used to quantify error in estimation of location and dipole moment, error in classification, and relative effects of error sources. RMS error for location vector components was 0.019-0.045 m compared to the average precision of 0.003-0.005 m. Precision in estimation of the magnitude of the dipole moment was 0.51-8.21% of the dipole moment, and the accuracy was 24.5±11.4%. If an experimental design set were used with the present experimental precision, the misclassification error should be between 2 and 5%. The misclassification rate using a theoretical design set was 28%. For correctly classified objects, angle estimates were generally within 5-10° of the actual values