| 摘要 |
A metal detector (420) has a metallic enclosure (421) with an entrance aperture (430) and an exit aperture (431) whose cross-sectional areas differ from each other. Arranged inside the enclosure (421) is a coil system with at least one transmitter coil (423) and at least one first and at least one second receiver coil (424, 425). The entrance and exit apertures (430, 431) and the coil system (423, 424, 425) enclose a detection zone (428) through which objects under inspection move along a travel path entering the metal detector (420) through the entrance aperture (430) and leaving the metal detector (420) through the exit aperture (431). The detection zone (428) is asymmetric and its cross-sectional profile varies along the travel path from the entrance aperture (430) to the exit aperture (431), wherein the coils (423, 424, 425) differ in size from each other. The first and second receiver coils (424, 425) are connected in series with each other, but their windings are wired with the opposite sense of rotation relative to each other. The at least one transmitter coil (423), when energized by an alternating electric current, generates a primary electromagnetic field which, in turn, induces a first voltage in the at least one first receiver coil (424) and a second voltage in the at least one second receiver coil (425). In spite of the asymmetry of the metal detector (420), the first and second voltages cancel each other when there is no metal present in the objects moving through the detection zone. |
