Method and apparatus for detecting small magnetizable particles and

Abstract

A pair of Hall sensors are symmetrically positioned between the poles of a horseshoe magnet. The Hall sensors are mounted in a plane flush with the ends of the magnet faces, and are located equidistant from the centerline of the magnet. A small magnetic object in the field of the magnet, preferably located on the magnet axis, has an induced dipole moment which is detected by the Hall sensors. The small magnetic object dipole field is such that the flux at each Hall sensor is in opposite directions, and by differentially sensing the outputs of the Hall sensors, a signal proportional to the magnetic object is derived. Leakage flux from the magnet as well as the flux of the earth's magnetic field, are in the same direction at both Hall sensors, and their effects are zeroed out by the differential sensing of the Hall sensors' outputs. When the poles of the magnet are drawn across the surface of a non magnetic sheet of conductive material, eddy currents are induced in the sheet by the moving magnetic field. Due to the symmetry of the magnet/sensor arrangement, in a uniform sheet these currents generate identical magnetic fields which result in sensor outputs that cancel. However, a crack or other anomaly in the sheet results in unequal signals at the two sensors, and an output signal proportional to the anomaly occurs.