| 摘要 |
A metal detector (20) has a metallic enclosure (21) with an entrance aperture (30) and an exit aperture (31). Arranged inside the enclosure (21) is a coil system with a transmitter coil and a first and a second receiver coil bounding a detection zone (28) which extends between the entrance and exit apertures (30, 31) and through which objects under inspection (2) travel. Relative to the travel direction (13), the first and second receiver coils are arranged on opposite sides of the transmitter coil. The first and second receiver coils are connected in series with each other, but their windings are wired with the opposite sense of rotation relative to each other. When the transmitter coil is energized by an alternating electric current, a metal contamination in an object under inspection (2) generates a detection signal in the otherwise balanced receiver coils. To cancel an undesirable sensitivity to metallic objects outside of the detection zone (28), the metal detector (20) has a first cancelling means (26) arranged at the entrance aperture (30) to cancel the undesirable influence of metallic objects beyond a first distance (d) upstream of the coil system and/or a second cancelling means (27) arranged at the exit aperture (31) to cancel the undesirable influence beyond a second distance (e) downstream of the coil system. The first and second cancelling means (26, 27) are different from each other and, consequently, the first and second distances (d, e) are likewise different from each other. |
