| 摘要 |
A device for validating the authenticity of a document uses changes in capacitance due to the presence of an electrically conductive security thread extending across the document. The device uses a drive arrangement for moving of a document such as a bank note, past a sensing arrangement which senses changes in capacitance due to movement of a security thread therepast. A frequency generator produces a high frequency time varying oscillator signal and provides this signal to an elongate oscillator electrode extending across the path of movement. A lead elongate measuring electrode is electrically conductive and positioned in front of the oscillator electrode and extends across said path and a trailing elongate measuring electrode is positioned behind the oscillator electrode and extends across the path. The signals from the measuring and the oscillator electrode are provided to a signal processing arrangement. The signal processing arrangement processes the signals relative to detect a phase shift between the signals caused by a conductive security thread passing by the electrodes. Sensing of the phase shift reduces sensitivity to variations caused by wobble of the bank note and changes in separation distance of the bank note from the sensor. |
