Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Patent
1990-09-10
1992-06-16
Wieder, Kenneth A.
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
324663, 324671, 324677, 324678, 324686, 194206, 194213, G01R 2726
Patent
active
051227541
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns recognition and approval or rejection of a watermark in a paper note or a document. The pattern of the watermark must comprise a special feature, namely that it consists of two characteristically shaped neighbouring areas, whose thicknesses differ in being both thicker and thinner than the average thickness of the note in the watermark region, while the words, area density (mass per unit area) and thickness are variable quantities, while mass density is constant. This as opposed to a usual form of counterfeit watermark, which is made by pressing the sheet together in order to give a variable thickness. In this case mass density and thickness will vary in an inverse relationship, while area density stays constant. A genuine watermark is formed by "thickness modulation" during the paper production process, so that mass density of the paper stays constant.
If the paper note is equipped with an implanted security thread for verification of genuineness, this thread may also serve as a usable test object in a variant of the present invention. Such a security thread may consist of metal, metallized plastics, plastics of a similar material.
There has for quite some time existed a need of a fast and reliable method of verification of genuineness of banknotes and documents in connection with the banknote testing in national banks, and also in a smaller scale, for instance in banknote operated vending machines.
There has been made attempts to solve this problem by the use of optical techniques, but modern copying engineering is capable of fooling most of the optical detection methods. The watermark is still regarded to be an adequate and safe way of marking a genuine note, and a mechanical measurement of thickness has previously been used in testing watermarks. However, this technique is not well suited to a rapid machine procedure, and is not very useful when the note has small injuries distributed at random. Besides, the thickness modulation of a watermark may be initiated relatively simply as explained above.
However, Swedish laid-open publication No. 355,428 discloses a measuring technique which is based upon the fact that the capacitance of an air plate capacitor is changed when for instance a paper note is pushed into the air space between the electrode plates. The paper thickness, or rather the area density of the paper, is related to the capacitance that is sensed. A specially designed capacitor is used, in which one of the electrodes has the same shape as for example a thickened part of the sought watermark. A dynamic measurement of capacitance is made while the note is led through the capacitor. If a correct watermark passes the adjusted electrode, capacitance will increase abruptly before and decrease equally abruptly after a maximum which is reached just at coincidence. The graph showing the capacitance change (as a function of time or position of the note) should have a special appearance to be approved according to particular condition, or else rejected. The Swedish publication also hints at the possibility of making a double such analysis, first one for a thickened pattern, and thereafter one for a thinned pattern, which will usually belong to the same watermark.
The capacitive sensor device mentioned above suffers, however, from a few drawbacks or weaknesses:
Firstly, this device is unable to see the difference between thin and thick paper sheets. The reason for this is that the measurement has a dynamic character and only detects the change in capacitance as the watermark passes the sensor. A signal indicating absolute thickness of the paper will therefore not appear, only one indicating only one indicating changes of thickness. Thus paper quality cannot be investigated while the note is passing. Nor will a double or possibly multiple paper feeding, with a number of paper simultaneously, be detected by this device.
Electrically both the capacitor electrodes of the known sensor device are arranged "floating" relative to ground, which entails problems concerning stability
REFERENCES:
patent: 3764899 (1973-10-01), Peterson et al.
patent: 3815021 (1974-06-01), Kerr
patent: 4099118 (1978-07-01), Franklin et al.
patent: 4642555 (1987-02-01), Swartz et al.
Brown Glenn W.
Inter Marketing Oy
Wieder Kenneth A.
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