FI95418C - Sensor used to verify the authenticity of the security paper - Google Patents

Description

- 95418

Sensor used to verify the authenticity of the security paper

The present invention relates to the identification and acceptance or rejection of a watermark on a paper banknote or document. The watermark pattern must have a special feature, namely that it comprises two adjacent areas of characteristic shape, the thickness of which differs in opposite directions from the average thickness of the banknote in the watermark area, while 10 words, surface density (mass per unit area) and thickness are variable when again, the mass density is constant. This is in contrast to the usual form of a counterfeit watermark made by compressing a sheet to give it a variable thickness. In this case, the mass density and thickness change inversely, while the surface density remains constant. A true watermark is formed by "thickness modulation" in the papermaking process, so the pulp density of the paper remains constant.

If the paper banknote is provided with an internal tur-20 thread to ensure authenticity, this thread may also serve as a useful test item in a variation of the present invention. Such a safety wire may consist of metal, metallized plastic, plastic of similar material.

. 25 For some time, there has been a need for a rapid and reliable method of verifying the authenticity of banknotes and documents in connection with the verification of banknotes by national banks and, to a lesser extent, by, for example, banknote vending machines.

30 Attempts have been made to solve this problem. · * Using optical techniques, but modern copying techniques are capable of tricking most optical detection methods. Watermarking is still considered an adequate and reliable way to mark a genuine banknote, and mechanical thickness measurement has previously been used to test watermarks. However, this technique is not well suited to the rapid mechanical method and is not very useful if small, randomly distributed damage has occurred to the banknote. Moreover, the watermark thickness modulus-5 can be relatively simply mimicked, as described above.

SE Application Publication No. 355,428 discloses a measurement technique based on the fact that the capacitance of an air-insulated plate capacitor changes when, for example, 10 banknotes are inserted into the air gap between the electrode plates.

The thickness of the paper, or rather the surface density of the paper, is proportional to the observed capacitance. A specially designed capacitor is used, in which the second electrode has the same shape as, for example, the thickened part of the watermark 15 sought. The dynamic measurement of capacitance is performed by guiding the banknote through a capacitor. If the correct watermark passes the matched electrode, the capacitance increases sharply before the maximum and decreases sharply after this maximum, which maximum is reached just when uniformity occurs. A curve showing the change in capacitance (as a function of time or banknote position) should have a specific shape acceptable under a certain condition or else it will be rejected. The SE publication also refers to the possibility of performing such an analysis twice, first for a thickened pattern and then for a thinned pattern, which usually belong to the same watermark.

However, the above-mentioned capacitive sensor device has a few disadvantages or weaknesses: First, this device cannot see the difference between thin and 30 thick sheets of paper. The reason for this is that the measurement is dynamic in nature and only indicates a change in capacitance as the watermark passes through the sensor. For this reason, a signal indicating the absolute thickness of the paper does not occur, but only a signal indicating the changes in thickness. Thus, the paper

II

3 95418 cannot be examined as the banknote passes. This device also does not indicate double or multiple, multiple paper feeds at the same time.

Both electrodes of the capacitance are electrically adapted to be "floating" with respect to the ground, which brings with them problems of stability and the effect of external electromagnetic fields.

However, the main weakness of the known device is that the dynamic measuring principle used means that the sensor device can be guided by, for example, a hole in the watermark area, which can be interpreted as an acceptable watermark. It is thought that this must be the main reason why said sensor device has not gained widespread recognition or why most manufacturers of vending machines or banknote testing machines have not adopted it.

Furthermore, the structure of the prior art sensor device appears to be unnecessarily complicated and must be constructed as a duplicate device in order to test a normal watermark with both thinned and thickened parts.

By using the method and apparatus of the present invention, it is achieved that a genuine watermark becomes identifiable, while a forged, printed counterfeit mark produces an anomalous signal. In addition, it is provided that only a properly formed watermark produces an identification signal, while holes in the paper or other differently formed paper thickness modulations are easily detected. (Hole ai-30, for example, throws a capacitance measurement that deviates in both positive and negative directions when the hole edges are in the sensor region, as opposed to a prior art device that can only give a positive signal when the capacitance value changes.) or absolute 9 95418 lute measurement of quality. Such an absolute thickness measurement gives the device according to the invention the advantage that the presence of a double feed or possibly several overlapping banknotes is measured just like a correspondingly thicker paper, and the presence of such a case can thus be demonstrated in a simple manner. This is a feature that can be useful in many cases. In addition, a measurement comprising both the thick and thin parts of the watermark is performed quickly and simply. The Si-10 weathered security wire can also be identified.

These and other advantages are obtained by a method of accepting a watermarked banknote or document whose watermark pattern comprises two adjacent areas of a characteristic shape with a local pin-15 density (mass per unit area) well above or corresponding to the average surface density of the main banknote. in a watermark area characterized by the method of placing the watermark or characteristic portions of the banknote or document in a position corresponding to a two-part, dual-active capacitive sensor device comprising a flat metal plate on one side of a common capacitor that can be connected to ground; the sensor device on the other side of the capacitor is divided into two metal plates, both of which are located in the same plane and the shape of which is adapted to the shape of the adjacent region or of the characteristic parts of each of said characteristic shapes and which are electrically separated from each other, however, so that the distance separating them is insignificant compared to other dimensions of the surface of said two plates, whereby the set symmetry characteristic of the dual output signal * from the sensor is disturbed in a predetermined manner when the correct watermark is consistent with said two sensor plates. the signal processing apparatus connected to the turing device is monitored, which method is also apparent from claim 1 below.

Other advantages are achieved using the method and apparatus set out in the other claims.

5 In some cases, there may be relatively large variations in the thickness of the paper, randomly distributed over the area of the banknote. In that case, it may be advantageous to use only a part of the watermark instead of the whole watermark in order to obtain greater security against the effect of these measurements of random variations in thickness. It is possible to select the "characteristic part" of the watermark, taking into account that this part contains both thickened and thinned areas of the watermark. This part of the watermark should obviously not be made too small, because the characteristic features of the watermark then disappear and, in addition, the measurement signal (capacitance) is too small.

A "two-part double-active capacitive sensor" is mainly intended to denote a plate-type capacitor with air insulation and a metal electrode plate on the other side of the capacitor, cut into two parts and using these two parts in exactly the same way to measure their and capacitance-25 between a one-piece common electrode plate on the other side of the capacitor. This differs quite clearly, for example, from the case disclosed in the aforementioned SE Application No. 355,428, where a two-part capacitor plate is present, but only one central part is active in the sense of "capacitance measurement", while the other outer part is used to control electric field lines, i.e. .it is so · [so-called "protective ring".

The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 shows a portion of a banknote containing a putative genuine watermark, Figure 95 shows an upper dual capacitor plate constructed in accordance with the present invention to indicate a putative watermark, Figure 3 shows in a side view of a complete two-part capacitor according to the invention with an upper and a lower plate, Fig. 4 shows an example of an electrical signal processing circuit according to the invention including said two-part capacitor, Fig. 5 shows one particular form of an output signal from a part of the signal processing circuit of Fig. 6 shows another example of an electrical signal processing circuit according to the invention, and Fig. 7 shows one form of output signals obtained from parts of the signal processing circuit of Fig. 6.

Figure 1 shows a part of a banknote 1 comprising an authentic watermark 2a, 2b with a special pictorial shape, in this case two concentric circular areas 2a and 2b. In general, of course, the watermark may have a more complex shape, but a circular shape has been chosen here for simplicity.

The watermark is formed in the papermaking process and comprises one thick region 2a having a thickness of T + ΔΤ and another thinned region 2b having a thickness of T + ΔΤ, the average thickness of the paper around the watermark being T. The local pulp density is essentially constant throughout the paper, made so that it is homogeneous. Thus, the local surface density 30, i. mass per unit area, increases in the thick area 2a, while the local surface density is small in the area 2b.

It should be noted that, in contrast to the present, paper having a printed pattern of the same shape has a variable pulp density and a constant surface density.

li 7 95418

It is known empirically that a printed (i.e., counterfeit) stamp, despite a variation in thickness of the correct nature, provides a virtually constant capacitance when passed between two condensate plates due to the constant surface density. Otherwise, a genuine watermark with a variable surface density will give a variable capacitance value that is proportional to the surface density and easily measurable.

Figure 2 shows a two-part electrode plate of a capacitor. The board may comprise, for example, a printed circuit board 3 made of fiberglass having an etched pattern on a metal, preferably copper, the shape of which is adapted to the pattern shown in Fig. 1. The diameter of the inner circular region 6 of copper 15 is substantially the same as that of the region 2a. The outer ring 4 of copper has essentially the same dimensions as the region 2b. The circular area 6 and the annular area 4 are separated by a small gap 5.

The width of the gap 5 may be, for example, 0.1 mm, the diameters of the inner circular region 6 and the outer surrounding region 4 being 10.0 mm and 14.3 mm, respectively. (These diameters give for the two part of the same area, which can be convenient, but not necessary.) 3 Print 25 shows a circuit board 3 occurs again, »*« and the copper areas 4 and 6 form one side of the two-part capacitor which is seen page-view. On the opposite side of the capacitor there is one common copper electrode 30 on the fiberglass plate 8. The conductors are shown schematically by reference numerals 9, 10 and 11, but these should nevertheless be made as short as possible. The distance d between the capacitor plates is suitably selected in relation to the maximum permissible paper thickness, for example the distance d is about 0.2 mm.

8 95418

An example of a very suitable signal processing circuit recognizing the correct watermark is shown in Fig. 4. The two-part capacitors formed by the region 4 and the common electrode 7 and the region 6 and the common electrode 7, respectively, are represented in Fig. 4 by the capacitors C4 and C6, respectively. Suitable resistors R4 and R6, together with said capacitors, are components which determine the durations T4 and T6 of the unstable states 10 of each of the respective so-called monostable multivibrators 12 and 13, which further define the time constants, which multi-vibrators are further connected to each other. The output signal Uut, which may have been obtained from the second multivibrator, varies as shown in Fig. 5. This signal is a typical square wave signal in which changes between two constant voltage levels are rapid. The times during which the signal remains at both levels between changes are T4 and T6, respectively.

The values of the parameters, i.e. by suitable selection of the regions 4 and 6 of the electrodes and the resistors R4 and R6, T4 and T6 can be given, for example, an equal duration when a paper of uniform thickness is applied to the capacitors without a watermark. In this case, the output signal Uut will be a symmetrical square wave signal, and T4 is equal to T6. As soon as the capacitance value of the two capacitors C4 and C6 changes, in each of the different directions, a clear deviation of the symmetry of the square wave signal is obtained, for example in the form shown in Fig. 5, where T4 and T5 are different.

As long as Uut is symmetrical, its average 30 is located halfway between the two voltage levels, for example 0 volts. The asymmetric signal, due to the imbalance between the capacitance values C4 and C6, gives a deviating average, which in the case of a correct watermark applied to the correct and corresponding position of the sensor is a specific maximum value of 35.

Il 9 95418

One simple means for obtaining such an average is a low-pass filter formed in Fig. 4 by a resistor Ri and a capacitor Cx. Voltage U, *. is thus a direct voltage representing the average of the voltage Uut. A genuine ve-5 stamp can be identified by measuring the UDC if the areas 4 and 6 of the capacitor plates are suitably constructed according to the shape of the watermark or according to the characteristic feature of the watermark.

It is very difficult to obtain the correct DC voltage UDC 10 by any means other than that the correct watermark is consistent with the pattern formed by the electrode plates 4 and 6. The certainty is based precisely on the fact that the maximum imbalance between capacitances, which is a prerequisite for acceptance, is only achieved in the case of such 15 uniformities.

In order to achieve a high degree of safety against the undesired effect of external electric fields (noise) and to avoid crosstalk between two successive capacitance measurements (plates 4 and 6 alternately), it is preferable that the capacitor input of each monostable multivibrator is connected to an internal transistor for the entire part of the stable period. This achieves: 25 a) that the subcapacitor which is not currently being measured is grounded, so that only the field lines emanating from the currently active plate pass through the paper and come to the common plate 7. This gives the least possible crosstalk between the two measurements, because the second subcapacitor is held at the · '' ki potential while the other is charged and inverted; (b) that the static electricity in the paper is conducted to the ground because the banknote is in constant contact with areas of the ground on both sides of the paper.

10 95418

Another example of a well-suited signal processing circuit is shown in Figure 6. Here, the monostable multivibrators 16 and 17 are connected in parallel after the square pulse oscillator 14, which taps both multi-5 tivibrators simultaneously. Here again, the duration of the unstable voltage level of each multivibrator 16 and 17 is determined by the capacitors C4 and C6 connected to the multivibrators. The outputs of the multivibrators, each connected to the clock / logic circuit 15, are powered by two choke pulses which are similar, i. are symmetrically temporally when the capacitors C4 and C6 are insulated with a paper of uniform thickness, but differ in time symmetry when the surface densities have different values. Examples of the waveforms of the signals Uut4 15 and Uut6 are shown in Figure 7. A number of imbalances are shown here with different pulse durations. The time difference 2 T is measured by the clock / logic circuit 15, which then compares this value with a desired value corresponding to compliance with the correct watermark.

20 (The oscillator 14 can be synchronized to an external process, if desired, for example when the banknote is fed to a test area with capacitor plates. This is shown in Fig. 6 by reference numeral 18.). 25 The latter measurement method is fast (between 10-100 ps) thanks to the digital measurement of time differences. In this case, however, a certain amount of crosstalk must be accepted because both capacitances are measured simultaneously and because capacitors 4 and 6 si-30 are located close to each other and because they have a common counter electrode 7.

A common feature of both said measuring circuits, which operate only when the multivibrators are "in-phase or in-phase", is that the crosstalk between said two capacitors does not retain very many frequencies other than the change frequency itself. Thus, the stability of the stop trigger points of the capacitance-controlled multivibrators is ensured. (Conversely, if the two multi-5 vibrators oscillate freely relative to each other, i.e. at different frequencies, there is a risk that the charge curve of the second capacitor will be superimposed, for example, by a somewhat higher frequency, causing uncertainty / instability at the stop trigger point.) the following:

The banknote to be inspected is automatically transferred to the air gap between the electrode plates of the two-part capacitor. In order to obtain the best possible correspondence between the possibly correct watermark and the condenser-15 accompanying pattern, one of the known techniques can be used. For example, a plurality of similar capacitors may be sequentially spaced apart, with one of these capacitors achieving the necessary best possible matching with the watermark position range varying with the banknote in question. On the other hand, the banknote can be moved laterally relative to the capacitor plates according to a predetermined transition pattern, which guarantees uniformity if the watermark is present. 25 see Such techniques are known, as mentioned above, and are not part of the present invention.

At the time when the edge of the banknote reaches the actual region of the capacitor, a small disturbance of the capacitance balance occurs in the opposite direction to the disturbance produced by the right watermark, provided that the sensor electrode plates have a preferred geometric shape. When the paper of uniform thickness has completely entered the region of the shaped electrode plates, the capacitances C4 and C6 have changed considerably due to the dielectric constant of the paper, but the symmetry 12 95418 is maintained. In the circuit variant shown in Fig. 4, the frequency of the square wave signal Uut decreases, but the DC signal UDC does not change because the average Uut is the same.

In the variation shown in Fig. 6, the pulse width of the unstable 5 plane changes, but equally for both signals. Thus, the clock / logic circuit 15 does not detect any time difference.

If a counterfeit stamp of the now printed type enters the capacitor region, the shape is correct, but as mentioned above, the dielectric constant is almost the same in both the thick and thin regions, so that the required degree of asymmetry of the capacitance values is not achieved, i. stamp not accepted.

When the correct watermark hits the capacitor region, 15 the correct imbalance of the square wave signal Uut is generated, and with it the correct DC voltage UDC. This correct DC voltage is then released to allow the rest of the machine to pass through, while the discarded banknote is pushed out of the second outlet in a manner known per se. This 20 refers to a variation of Figure 4. Correspondingly, there is a correct time difference 2 T between the two unstable levels obtained from the outputs of the multifibrators of Figure 6, which is interpreted by the clock / logic circuit as the presence of a correct watermark.

25 It should be noted that banknotes with a few wrinkles or small tears do not cause problems with the operation of the device, as such defects only have a negligible effect on the capacitance.

It was mentioned above that it might be advantageous to use only one characteristic part of the watermark for the measurements.

In practice, it is preferred to use a portion of the watermark that comprises areas of approximately equal size in the thinned and thickened fields, although this is not necessary.

13,95418 It should be emphasized that the measurement method used in the present invention, which is in principle static in nature, offers several advantages. By "static nature" is meant that essentially 5 banknotes are stationary, and the actual capacitance is measured and not just the change in capacitance as the banknote passes through. The total capacitance depends, for example, on the thickness of the banknote. Thus, it is possible to derive the banknote thickness directly from the sum T4 + T6, see Figure 5. The obvious consequence is that this sum also indicates the presence of two or more paper banknotes on top of each other, so that the detection of double or multiple feed is also easily obtained.

Although the measurement is static in nature, it can be performed very quickly and adapted to the speed of normal automatic banknote processing. An ordinary banknote can be tested, for example, in less than 0.1 s, which includes the arrival of the banknote, settling in the test station and the determination of the capacitance, as well as an indication of acceptance or rejection.

A capacitive sensor of the type in question can also be used to identify an internal safety tag in the paper when the wire has a certain shape, possibly a straight line. Security thread. The dielectric constant is significantly higher than that of paper, which makes it possible to detect the wire by means of an electrode having an elongated and matched shape. The total paper thickness in this area is also greater than elsewhere. The capacitive sensor can thus be constructed to detect both the watermark and the security wire simultaneously.

*, The arrangement of two similar sensors in succession, one mirror-mirrored with respect to the other, makes it possible to detect a certain type of counterfeiting, namely the addition of mass to one side, 35 for example a piece of tape glued together.

14 95418

Since the electric field lines from the shaped electrodes 4 and 6 to the grounded common plate 7 are not perpendicular to the plates, i.e. the field is not homogeneous, the changes in capacitance are considerably different when the banknote is validly viewed on both sides in two corresponding measurements. The thickness of the paper actually fills a substantial part of the air gap, and the image of the field lines passing through the added pulp is substantially different depending on whether this pulp is closer to a grounded common plate 7 or shaped electrode plates 4 and 6.

Regarding the practical design of the device, note the following:

To minimize noise problems, a grounded common plate 7 or capacitor can be connected to the Faraday cage surrounding the device. Of course, the necessary openings must be fitted in the cage for the insertion and removal of the banknote. In order to make the effect of temperature fluctuations and external fields on both multivibrators equal and to avoid stray capacitances, it is preferred to use an integrated circuit in which two monostable multivibrators are built together, and possibly multivibrators can be formed as a piece with four amplifier functions. It is very important 25 to ensure that the asymmetry in the measurements • · is only caused by the capacitances to be measured and not by various external influences. The integrated circuit is preferably mounted on the same printed circuit board 3 as the sub-boards 4 and 6 to minimize the wiring capacitances.

30 As mentioned above, the quality of the paper can be checked. When the banknote enters the sensor, i.e. before the water-stamp is in the correct position, the voltage New in the circuit of Figure 4 can be used for detection. The acceptable paper quality corresponds to a specific amount T4 + T6, which can be determined and checked by any suitable device known per se.

Claims (16)

A method of approving a document, such as a banknote (1) with a watermark (2a, 2b), wherein the pattern of the watermark consists of two characteristic shaped, interconnected areas (2a, 2b) having a local area. density (mass per unit area) which is markedly higher, respectively, lower than the main average density of the banknote (1) in the region of the watermark, wherein the watermark or at least one characteristic section thereof is brought into a position corresponding to a two-part capacitive sensor device 6, 7) and the sensor device consists of a common metal plate (7) for forming one capacitor side and on the other capacitor side it is divided into two metal plates (4, 6), each of which lies in a common pipe and is electrically separated, however, with insignificant separation distance (5) compared to the other. The dimensions of the two plaques (4, 6) related to the area, and the change in capacitance due to the watermark is observed and compared to a change caused by a correct watermark, characterized in that the watermark or the aforementioned characteristic portion thereof is brought into a position corresponding to a double acting capacitive sensor device (4, 6, 7), where both plates (4, 6) lie in a common, fixed plane and the shape is suitable for each of the two characteristically contiguous areas (2a, 2b) or for the characteristic sections thereof, such that a preset symmetry property of the dual output from the sensor device is disturbed in a predetermined manner where a correct watermark coincides with both sensor plaques (4, 6), and the symmetry property is continuously monitored by a signal processing output. armor connected to the sensor device. Il 95418 2. A method according to claim 1, characterized in that the sensor device is arranged in such a way that the capacitances which correspond to the two metal plates (4, 6) change in opposite directions to a predetermined degree. acceptable water-stamp is present. Method according to Claim 1 or 2, further characterized in that the sensor capacities act on circuit means (12, 13) housed in the signal processing equipment, so as to generate a square pulse sequence with a symmetry directly related to the capacitance values. , wherein the pulse symmetry or asymmetry is detected by an averaging circuit (Rx, C *). 4. A method according to claim 1, further characterized in that two disposable multivibrators (12, 13) which are included in the circuit means and whose respective time constants for the duration of their unstable level are determined by the respective sensor capacitance inputs, respectively. the earth through an inner active circuit element during each stable period portion, whereby the momentarily inactive metal plates (4 or 6) on the second capacitor side are grounded and thereby conduct static electricity from the paper note. 5. A method according to claim 3 or 4, further characterized in that the paper thickness, including including any feeding of double or multiple paper banknotes, is determined on the basis of a complete time cycle of the square pulse sequence. Method according to claim 1 or 2, further characterized in that the sensor capacities (C4, C6) affect circuit means (16, 17) which are included in the signal processing equipment, so that they generate two square pulse sequences at separate outputs, have mutual symmetry that is directly dependent on the capacitance values, and that the time symmetry or asymmetry is detected by a clock / logic circuit (15). 95418 Apparatus for approving a document, such as a banknote (1) with a watermark (2a, 2b), wherein the pattern of the watermark consists of two characteristic shaped, adjacent areas (2a, 2b) having a local air density 5 (mass per unit area) which is marked higher and lower, respectively, than the main average density of the banknote (1) in the region of the watermark, the device comprising a suitable, bifurcated, capacitive sensor device (4, 6, 7) and a signal processing equipment such as connected to the sensor device, and the sensor device (4, 6, 7) consists of a common, metal plate (7) on one capacitor side and on the other capacitor side of two metal plates (4, 6) which lie in a common path and are electrically separated from each other, however, at a negligible separating distance (5) in comparison with the other dimensions referred to in the area of the two plates (4, 6), characterized in that The sensor device (4, 6, 7) is a dual active capacitive sensor device, that both plates (4, 6) lie in a common tab and are suitable for the shape of each of the two characteristic features. formed, adjacent areas (2a, 2b) or for the characteristic sections thereof, and that the signal processing equipment comprises circuit means (12, 13, R4, R6, Rx, Ci) for continuous monitoring of a preset symmetry property of the double the output of the sensor device (4, 6, 7). 8. Device according to claim 7, further characterized in that the common metal1 plate (7) is suitable to be connected to a grounded Faraday cage which encloses the whole device and leaves only necessary openings for insertion and removal of the banknote (1). . Device according to claim 7 or 8, further characterized in that the circuit means 35 comprises two interconnected single-multivibrators (12, 95418 13.) and each multivibrator has its time constant determined by appropriate coupling to the two-part sensor device and two parts (4, 7, respectively). 6, 7; C4 and C6, respectively), wherein the dual output from the sensor device is defined as the output (Uut) from one (13) of the multivibrators and this output can, after setting the physical parameters of the circuit means. , take the form of a symmetrical square signal where the sensor device detects a region without a watermark but disrupts its time in a predetermined manner in the presence of a correct watermark. 10. Device according to claim 9, further characterized in that the capacitance inputs of the multivibrators (12, 13) are suitable for short-circuiting the ground via an internal active circuit element during a stable period part. Device according to claim 9 or 10, further characterized in that the circuit means: further comprises a circuit (Rx, Cx) for determining the average value (UDC) of the output signal (Uut). Device according to claim 7 or 8, further characterized in that the circuit means comprises two single-use multivibrators (16, 17) which are connected in parallel, each multivibrator having its time constant determined by suitable connections to the two-part capacitive sensor device respectively. two parts (4, 7 and 6, 7; C4 and C6, respectively), and the multivibrators are suitable to be triggered synchronously through a square pulse oscillator (14) and to each feed an output signal Uut4, Uut6) into a clock / logic circuit (15), which is suitable for measuring the degree of time asymmetry or asymmetry between the two outputs. Device according to any of claims 7-12, further characterized in that the single-use multivibrators (12, 13 and 16, 17) are encapsulated in one and the same integrated circuit and mounted next to the sensor 95418 device, preferably on a common device. printed circuit board (3), comprising the two metal plates (4, 6). Device according to any one of claims 7-13, further characterized in that the two metal plates (4, 6) in the sensor device further have suitability for capacitive detection of an implanted security tree in the banknote, the security trees comprising a metal, metallized plastic, plastic. or a similar material. Device according to any of claims 7-13, further characterized in that the two metal plates (4, 6) are designed such that the sensor device, where the leading edge of the banknote (1) enters the sensor area, produces an equilibrium disturbance in the opposite direction of the disturbance produced by a correct watermark which brines gas in a position coinciding with the two metal plates (4, 6). Device according to any of claims 7-15,. further characterized by a further shape-appropriate capacitive sensor device arranged in series behind the first-mentioned sensor device, however, the capacitor plaques (4, 6 and 7, respectively) are reversed relative to the plaques of the first-mentioned sensor device, and wherein the suitable capacitor plaques (4, 6, 6) ) of the first sensor device is located on one side of the banknote and of the further sensor device on the other side of the banknote. IN!