| 摘要 |
<p>The invention relates to metallic object detector comprising a coil system with a transmitter coil for generating an electromagnetic field and a receiver coil which is arranged in such a way that an electromotive force is induced in the coil when the coil is affected by the field generated by a metallic object and the field generated directly by the transmitter coil. The invention relates particularly to the transmitter side and includes a feed-back circuit for keeping the amplitude of the electromagnetic field generated by the transmitter coil constant. The transmitter side comprises, specifically, an oscillator which generates a sinusoidal signal for the transmitter coil and a variable attenuation/amplification circuit which controls the amplitude of the sinusoidal signal. The attenuation/amplification circuit is controlled by a signal which is emitted by the coil system after the sinusoidal signal has been rectified and compared with a reference voltage. In one embodiment it is the signal emitted by the receiver coil due to the directly induced electromotive force in the coil which is fed back but in a second embodiment it is a signal emitted by the transmitter coil which is fed back.</p> |
