摘要 |
Apparatus for scanning a product to detect metal in that product comprises a drive coil 4, for generating an electromagnetic field in the product, and a detection coil arranged to detect fluctuations in the magnetic field caused by the presence of a metallic particle in the product. A drive circuit for the drive coils comprises a plurality of switches 19 to 22 e.g. in an H-bridge arrangement, driven by a controller 16, which switches alternately connect the drive coil across a potential difference to cause the drive coil to be driven at an operating frequency determined by operation of the switches. Using a switching circuit to drive the drive coil greatly increases the number of frequencies at which the drive coil may be operated. The invention has particular application to the food industry. An arrangement in which the drive coil is sequentially operated at a plurality of different frequencies is also disclosed. |