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Abstract

A document verification device detects the presence of a metal security thread embedded in a document such as currency paper. The device includes a sensor pad arrangement and corresponding signal processing electronics. The arrangement includes a single central sensing pad flanked by an array of twenty-seven pairs of outer pads. Each pair of outer pads in the array is electrically connected together. Every pad is made of conductive material. The width dimension of each outer pad in the array is greater than the width of the security thread. Each outer pad in the array is preferably angled to improve the reliability of detection of the security thread. The electronics generates a square wave oscillator signal that is applied to the outer array pads in a pattern that includes two adjacent pairs of array pads having the positive voltage level portion of the square wave signal applied thereto, and the following two adjacent pairs of array pads having the negative voltage level portion of the square wave signal applied thereto. This pattern is sequenced in time over the entire array. The document to be verified is transported with respect to the sensor pad arrangement such that the wide edge of the document is the leading edge. As the thread passes over the array, the pattern is sequenced throughout the pads fast enough such that, at a point in time, the thread will bridge the central sensing pad with a pair of outer two pads first having the positive voltage level portion of the square wave signal applied thereto, and then later the thread will bridge the central sensing pad with the same pair of outer two pads now having the negative voltage level portion of the square wave signal applied thereto. Since the thread is metallic, it will capacitively couple the square wave signal into the central sensing pad. The electronics senses the signal coupled to the sensing pad and interprets a valid security thread as being present in the currency from certain characteristics of the sensed signal.

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First Claim