SENSOR ELEMENT AND DEVICE FOR AUTHENTICITY VERIFICATION OF A DATA CARRIER WITH A SPIN RESONANCE FEDERATION

Description

[0001] The invention relates to a sensor element for the authentication of a planar data carrier, in particular a banknote, with a spin resonance feature, and to a testing device with such a sensor element.

[0002] Data carriers, such as valuables or identification documents, but also other valuables like branded goods, are often equipped with security features to ensure their authenticity and protect against unauthorized reproduction. It is known to use security features with spin resonance characteristics for automated authentication of documents and other data carriers. These security features are coated with substances that exhibit a spin resonance signature. Spin resonance signatures suitable for authentication include, in particular, nuclear magnetic resonance (NMR), electron spin resonance (ESR), and ferromagnetic resonance (FMR) effects.

[0003] When checking banknotes, three different magnetic fields are typically generated within the measuring range of, for example, a banknote processing machine to detect spin resonance signatures. Specifically, these are a quasi-static polarization field B0, which runs parallel to the axial direction (z-direction) of the air gap of a magnetic circuit. A second magnetic field is formed by a modulation field Bmod, which also runs parallel to the z-axis and typically has a frequency fmod in the kHz range. To excite transitions between the split spin energy levels of the spin resonance signature substances, an excitation field B1 is provided, which typically has frequencies above 1 GHz and is polarized perpendicular to the B0 direction.

[0004] To generate the polarization field B0, a magnetic circuit is often used, which directs the magnetic flux of a permanent magnet or a coil to the air gap where the spin resonance signatures are detected. To generate the modulation field Bmod, a single cylindrical coil or a pair of cylindrical coils in a Helmholtz configuration is usually employed. This means that two cylindrical coils with a coil radius r are positioned parallel to each other on the same axis at a distance r and are traversed by the same current in the same direction. The cylindrical coils are often wound around the magnetic circuit near the air gap, but can also be located directly within the air gap. A resonator, usually also located within the air gap, is typically used to generate the excitation field B1.

[0005] For authentication, a test object is placed in the air gap of the magnetic circuit and checked for the presence of a spin resonance signature. The measured spin resonance signal strength increases with both the field strength of the polarization field B0 and the field strength of the modulation field Bmod, so it is advantageous to maximize both field strengths. The field strength of the polarization field B0 is approximately inversely proportional to the height of the air gap and can therefore be increased by reducing the height of the air gap. An increase in the field strength of the modulation field can be achieved, for example, by using modulation coils with as many turns as possible.

[0006] In practice, these requirements lead to a conflict of objectives: If a cylindrical coil or a pair of cylindrical coils is arranged in the air gap to generate the modulation field B mod, the size of the cylindrical coils increases with the number of turns. Accordingly, either the air gap height must be increased or the maximum number of turns of the coil must be limited, so that either the field strength of the polarization field B 0 or the field strength of the modulation field B mod, and thus in both cases the strength of the spin resonance signal, is limited.

[0007] On the other hand, if the cylindrical coils are not arranged in the air gap, but wound around the magnetic circuit, the modulation field B mod induces increased eddy currents there, which in turn bring several disadvantages.

[0008] Firstly, the magnetic field generated by the eddy currents has a polarity opposite to that of its cause, according to Lenz's law. The eddy currents therefore weaken the modulation field and lead to signal reduction. Furthermore, sensor operation, especially in high-speed banknote processing machines, requires short measurement times and thus high modulation frequencies. However, since the strength of the eddy currents increases with the modulation frequency, sensor operation on high-speed machines becomes more difficult. Finally, the energy dissipation associated with the eddy currents in the magnetic core leads to a temperature increase and thus to a change in magnetic resistance, which in turn leads to a drift in the flux density in the air gap and ultimately to an undesirable drift in the useful signal.

[0009] US 5149946 A relates to a method for authenticating an object by electron spin resonance spectroscopy. In the method, at least one authentication substance is added to the object, which has an EPR peak whose width is not greater than about 1.5 gauss. The spectroscopy is performed in a static field with an amplitude such that the substance has a resonance frequency in the radio frequency range. The width of the substance's EPR peak is used as a distinguishing criterion. Preferably, a low-frequency collinear modulation field is superimposed on the static field, the amplitude of which is significantly larger than the EPR width of the substance.Under an electromagnetic radio frequency field that is perpendicular to the static field and the modulation field and has an amplitude not less than the peak width, paramagnetic materials with a peak width greater than that of the authenticating substance are not detected. A device for carrying out the method and a security paper usable with the method are also disclosed.

[0010] Based on this, the invention aims to provide an improved device for the authentication of data carriers with spin resonance features, which in particular enables a high signal strength and also allows the testing of rapidly moving data carriers.

[0011] This problem is solved by the features of the independent claims. Further developments of the invention are the subject of the dependent claims.

[0012] The invention provides a sensor element for testing, in particular for authentication, a planar data carrier with a spin resonance feature. The planar data carrier can, for example, be a banknote. The sensor element contains a magnetic core with an air gap into which the planar data carrier can be inserted for authentication. The air gap defines an axial direction extending between the adjacent surfaces of the magnetic core.

[0013] The sensor element further includes a polarization device for generating a static magnetic flux in the air gap, a modulation device for generating a time-varying magnetic modulation field in the air gap, and a resonator for exciting the spin resonance feature of the data carrier under test.

[0014] The resonator is specifically designed to detect spin resonance signals of the spin resonance feature. Furthermore, the resonator can receive a response signal from the spin resonance feature and output it to a detector.

[0015] The spin resonances can be determined, for example, using a continuous wave (CW) method, a pulsed method, or a rapid scan method.

[0016] A special feature of the sensor element is that the modulation device is formed by at least one planar coil arranged in the air gap, which has one or more turns around the axial direction of the air gap in one plane.

[0017] The modulation device designed according to the invention overcomes the problems described above. A planar coil with a sufficiently high number of turns can easily be arranged directly in the air gap of the magnetic core, since planar coils are significantly smaller, particularly in the axial direction, than comparable cylindrical coils. Due to the positioning of the at least one planar coil in the air gap, the induction of eddy currents in the magnetic core is suppressed or largely avoided. Finally, the modulation field generated by a planar coil is, due to its geometry, very easily accessible for banknote verification, whereas with a single cylindrical coil, the area of ​​strongest field lies within the coil body enclosed by the turns, which is inaccessible to a banknote sample during automated measurement.

[0018] The aforementioned plane with the one or more windings is advantageously perpendicular to the axial direction of the air gap. This advantageously aligns the modulation field parallel to the polarization field.

[0019] In a preferred embodiment, the at least one planar coil has a helical conductor path with a minimum inner radius and a maximum outer radius. The outer contour of the conductor path of the at least one planar coil is advantageously circular, elliptical, rectangular, or polygonal.

[0020] In an advantageous embodiment, the spacing between successive turns of the at least one planar coil is constant, thus enabling a particularly simple coil design. In an alternative, equally advantageous embodiment, the spacing between successive turns varies; in particular, it can increase or decrease strictly monotonically. The variation in spacing is advantageously chosen such that the field distribution generated by the planar coil in the air gap, especially at a measurement position within the air gap, is particularly homogeneous.

[0021] The number of turns of the at least one planar coil is advantageously between 2 and 100 (inclusive), preferably between 10 and 30 (inclusive). With a number of turns in this range, both the strength and the distribution of the modulation field in the air gap can be optimized for typical conductor track dimensions.

[0022] In an advantageously simple design, the modulation device contains only a single planar coil. However, several planar coils can also be provided in the modulation device; for example, several planar coils can be stacked to increase the strength of the generated modulation field. If the modulation device contains more than one planar coil, these planar coils are, according to an advantageous design, identically constructed, i.e., they have, in particular, the same geometry and number of turns.

[0023] In an advantageous embodiment of the invention, the modulation device comprises two planar coils arranged axially spaced apart on the top and bottom surfaces of the air gap, respectively. The two planar coils are arranged and coordinated such that they generate a homogeneous modulation field between them. Preferably, each planar coil has a conductor track with the same minimum inner radius and the same maximum outer radius, and is axially spaced at a distance of preferably ±10% or substantially twice the inner radius of each coil. They are preferably traversed by a current of the same magnitude flowing in the same direction.

[0024] The at least one planar coil is advantageously mounted on a coil carrier, in particular a printed circuit board. This enables highly reproducible and cost-effective manufacturing of the coils. If, according to an advantageous embodiment, the resonator is also mounted on a printed circuit board, for example in the form of a stripline resonator, the resonator can advantageously be arranged together with the at least one planar coil on different printed circuit board layers of the same component, resulting in a particularly simple and compact design.

[0025] In another, equally advantageous embodiment, at least one planar coil is formed on a ferromagnetic support, for example, in the form of a conductor track on an electrically insulating layer, such as an oxide layer or a polymer layer, on a soft iron plate, or on a ferrite or powder core. The coil can also be formed by a enamelled wire on iron or a ferrite substrate or powder core. The planar coil is preferably electrically insulated from the ferromagnetic support. The use of a ferromagnetic support allows for a further reduction of the air gap height due to the increased magnetic flux density.

[0026] In an advantageous embodiment, at least one planar coil is connected to a capacitor, forming a resonant circuit. This increases the current flowing through the planar coil and consequently the strength of the modulation field, resulting in a stronger spin resonance signal during authentication. As explained above, the modulation field can also be increased by using several stacked planar coils in the modulation device. It has proven particularly effective for the modulation device to include at least one double planar coil, in which the windings of a planar coil are arranged coaxially on both the top and bottom surfaces of a coil former.

[0027] The element for generating a static magnetic flux is advantageously formed by a permanent magnet. Compared to a design where the static component of the magnetic flux is also generated by an electromagnet, this reduces energy consumption and heat dissipation. However, in other configurations, it can also be advantageous to generate not only the time-varying component but also the static component of the magnetic flux in the air gap using electromagnets. This allows for particularly easy handling after the sensor element is switched off, especially during the assembly, disassembly, and transport of the test device.

[0028] The resonator is advantageously arranged in the air gap of the magnetic core. A planar surface resonator, in particular a stripline resonator, is advantageously used for this purpose.

[0029] The sensor element advantageously further comprises a ramp coil for generating a magnetic field in the air gap that varies slowly over time relative to the modulation field. The ramp coil is preferably wound around the magnetic core. The ramp coil serves in particular to generate a ramp function of the polarization field and / or to compensate for any drifts in the strength of the polarization field.

[0030] The modulation coil mentioned is preferably designed to generate a magnetic field in the air gap that varies at a frequency between approximately 1 kHz and approximately 1 MHz. The ramp coil mentioned is preferably designed to generate a magnetic field in the air gap that varies at a frequency below approximately 1 kHz. The resonator is preferably designed to excite and detect spin resonance signals with a frequency between approximately 1 MHz and 100 GHz.

[0031] The spin resonance feature of the data carrier to be tested is advantageously a nuclear spin resonance feature, an electron spin resonance feature, or a ferromagnetic or ferrimagnetic resonance feature.

[0032] In the case of nuclear magnetic resonance features, the resonator is preferably resonant in a frequency range between 1 MHz and 70 MHz. A frequency range between 2 MHz and 50 MHz is particularly preferred.

[0033] In the case of electron spin resonance features or ferromagnetic or ferrimagnetic resonance features, the resonator is preferably resonant in a frequency range between 1 GHz and 100 GHz. A frequency range between 5 GHz and 85 GHz, and especially between 15 GHz and 50 GHz, is particularly preferred.

[0034] The invention further comprises a testing device for the inspection, in particular for authentication, of planar data carriers, especially banknotes, with a sensor element of the type described. The testing device further includes a transport device that introduces the planar data carriers to be inspected along a transport path into the air gap of the magnetic core or guides them through the air gap of the magnetic core. The testing device can, in particular, be part of a banknote processing machine.

[0035] The invention also includes a method for testing a planar data carrier with a spin resonance feature, wherein the test is performed using a sensor element of the type described above. The planar data carrier to be tested can, for example, be a banknote. In the method, the planar data carrier to be tested is placed in the air gap of the magnetic core of said sensor element, and a static magnetic flux is generated in the air gap by the polarization device. A time-varying magnetic modulation field is generated in the air gap by the modulation device, wherein the modulation device is a device formed by at least one planar coil arranged in the air gap, which has one or more turns around the axial direction of the air gap in one plane. Furthermore, the spin resonance feature of the data carrier to be tested is excited by the resonator.

[0036] In an advantageous procedure, the resonator also records a response signal from the spin resonance feature generated by the excitation and outputs it to a detector. However, a response signal from the spin resonance feature can also be recorded using a separate detection device.

[0037] The excitation of the spin resonance feature and / or the recording of the response signal of the spin resonance feature is advantageously performed in a continuous wave (CW) technique, in a pulsed technique, or in a rapid scan technique.

[0038] When testing planar data carriers with a nuclear spin resonance feature, excitation by the resonator preferably occurs in a frequency range between 1 MHz and 70 MHz, particularly between 2 MHz and 50 MHz. When testing planar data carriers with an electron spin resonance feature or a ferromagnetic or ferrimagnetic resonance feature, excitation by the resonator preferably occurs in a frequency range between 1 GHz and 100 GHz, more preferably between 5 GHz and 85 GHz, and most preferably between 15 GHz and 50 GHz.

[0039] The evaluation of the test result is generally carried out by comparing the recorded response signal with a response signal expected for the spin resonance feature. If the expected response signal is recorded within a certain tolerance range, it can be concluded that the correct spin resonance feature is present and thus, for example, that the data carrier is authentic.

[0040] Further embodiments and advantages of the invention are explained below with reference to the figures, in the representation of which a scale and proportion-accurate reproduction has been omitted in order to increase clarity.

[0041] They show: Fig. 1 schematically shows a test device with a banknote test piece fed into it, Fig. 2 the sensor element of the Fig. 1The planar coil used is shown in (a) in plan view and in (b) in cross-section, Fig. 3 shows (a) to (d) some advantageous embodiments of planar coils according to the invention, Fig. 4 shows a sensor element according to another embodiment of the invention with a pair of planar coils in the air gap, Fig. 5 shows the analytically calculated field distribution of a pair of planar coils in comparison with a pair of cylindrical coils in Helmholtz configuration, and Fig. 6 shows experimental results of a comparison of planar coils with cylindrical coils wound around the magnetic core of the magnetic circuit outside the air gap.

[0042] The invention will now be explained using the example of verifying the authenticity of a banknote 10. Figure 1 Figure 1 schematically shows a test device 20 of a banknote processing system, of which only the part of the sensor element 26 that is particularly relevant for the present invention is shown in the figure.

[0043] The banknote test specimen 10 contains a spin resonance feature 12 to be tested in a feature area, the characteristic properties of which serve to verify the authenticity of the banknote. The test specimen is guided along a transport path 14 through the testing device 20, specifically through the air gap 24 of a magnetic circuit with a magnetic core 22.

[0044] The air gap 24 extends axially between the surfaces of the magnetic core 22 adjacent to the air gap. The axial direction of the air gap is also referred to as the z-axis, and the directions perpendicular to it define an xy-plane. In the xy-plane, the magnetic core 22 can be, for example, round, elliptical, square, rectangular, or polygonal. As explained above, three different magnetic fields are generated in the air gap 24 for the detection of signatures of the spin resonance feature 12: a quasi-static polarization field B0 parallel to the z-axis, a modulation field Bmod, also parallel to the z-axis, and an excitation field B1 perpendicular to it, which induces the energy transitions between the spin energy levels in the spin resonance feature 12.

[0045] In the exemplary embodiment, a magnetic circuit with the magnetic core 22 is used to generate the polarization field B 0, which directs the magnetic flux of a permanent magnet and / or a coil to the air gap 24. A special feature is the use of a planar coil 30 arranged in the air gap 24 for generating the modulation field B mod; this coil has several turns around the z-axis in the xy-plane.

[0046] The excitation field B1 is generated using a resonator, which for clarity is shown in Fig. 1 omitted.

[0047] The planar coil 30 is in Fig. 2(a) under supervision and in Fig. 2(b)The cross-section is shown in more detail. In contrast to a cylindrical coil with the same number of turns, the planar coil 30, with a sufficiently high number of turns, can easily be placed in a narrow air gap 24. Due to the arrangement of the planar coil 30 in the air gap 24, the induction of eddy currents in the magnetic core 22, with the disadvantages described above, is suppressed. Furthermore, the modulation field generated by the planar coil 30 is, due to its geometry, very readily accessible to the banknote test specimen 10.

[0048] The planar coil 30 is mounted on a printed circuit board 32 and features a spiral arrangement of the conductor tracks 34 between a minimum inner radius r1 and a maximum outer radius r2. In the exemplary embodiment, the planar coil 30 has five turns with a constant conductor track spacing Δ. A via 36 for the return path of the conductor is provided in the central region of the planar coil 30.

[0049] As explained above, the integration of the planar coil 30 onto a printed circuit board offers several advantages. In addition to reproducible and cost-effective manufacturing, the resonator for the excitation field B 1 can also be integrated onto a printed circuit board, allowing the planar coil 30 (or multiple planar coils) and the resonator to be compactly implemented on different board layers of the same component.

[0050] However, the planar coil 30 can also be mounted on other supports, for example ferromagnetic ones, and a planar coil with exposed windings is also an option.

[0051] In addition to the geometry of planar coils, the following can be considered: Fig. 2 The spiral conductor path shown with circular outer contour can also be used to advantage in other geometries. Figure 3Figures (a) to (d) show some further advantageous embodiments, where for the sake of clarity only the planar coil itself is shown without the coil support.

[0052] Specifically, the planar coil 30 of the Fig. 3(a) a spiral conductor track with elliptical outer contour 40 and the planar coil 30 of the Fig. 3(b) a spiral conductor path with a rectangular outer contour 42. Other polygonal outer contours, for example triangular, pentagonal, hexagonal or octagonal, are also possible.

[0053] Such a variation of the outer contours is advantageous in order to adapt the spatial B mod distribution to the distribution of the polarization field B 0. This allows a similar B 0 and B mod distribution to be achieved in the air gap, thus suppressing the formation of signal artifacts.

[0054] The exemplary embodiment of the Figure 3(c)This illustrates that the spacing Δ between successive turns and the width of the conductor tracks do not have to be constant. For the planar coil 30 of the Fig. 3(c)For example, the distance Δ decreases strictly monotonically from the inside to the outside, and the conductor tracks 34 are wider in the inner part 44 of the coil than in the outer part 46. Variations in the conductor track spacing Δ and the conductor track width make it possible to design the field distribution of the planar coil 30 in the air gap 24 to be particularly homogeneous. Thus, the B mod field generated by a planar coil has its maximum on the coil axis. With increasing radial distance from the coil axis, the field decreases. This field decrease can be compensated for by increasing the number of turns in the outer part of the coil, and the field distribution can be homogenized in the radial direction. By simultaneously reducing the conductor track width in the outer part of the coil, the turning density can be further increased. It is understood that the associated Fig. 3(c) The described variations can be combined with all geometries of the planar coils.

[0055] With suitable design of the conductor tracks 34, a through-hole for conductor track return can also be omitted. The embodiment of the Fig. 3(d) Figure 30 illustrates a planar coil in which the windings are circular with a constant radius and the conductor return 48 is arranged in the same plane as the windings of the planar coil, thus eliminating the need for vias. This allows the coil to be advantageously implemented on a single PCB layer. Of course, this design can also have a different outer contour geometry instead of the circular one, and variations in conductor spacing and conductor width are possible.

[0056] A planar coil 30 arranged in the air gap 24 of the magnetic core advantageously has between 10 and 30 turns, since it has been shown that with such numbers of turns, both the strength and the distribution of the modulation field in the air gap 24 can be optimized for typical conductor track dimensions. As shown from Fig. 1 As can be seen, due to the planar design of the coil 30, only a minimum of installation space is used in the air gap, so that the air gap for a high modulation field B mod does not have to be unnecessarily enlarged and thus the field strength of the polarization field B 0 does not have to be reduced.

[0057] Figure 4Figure 1 shows a further embodiment of a sensor element 50 according to the invention, in which a pair of planar coils is provided in the air gap. Specifically, the sensor element 50 comprises a polarization magnet 52 with magnetic return, a permanent magnet 54, and a magnetic core 56 with an air gap 58 through which the transport path of the banknote test specimens runs.

[0058] In the air gap 58, in addition to a resonator 66 for generating the excitation field B 1, a modulation device 60 for generating the magnetic modulation field B mod is arranged. In the exemplary embodiment, this modulation device consists of a pair of planar coils 62, 64, which are arranged axially spaced apart on the upper and lower sides of the air gap 58, respectively. The minimum inner radius r 1 and the maximum outer radius r 2 of the conductor track of the planar coils 62, 64 are matched to the axial coil spacing such that the most homogeneous field distribution possible is achieved in the air gap between the two planar coils 62, 64. For this purpose, the axial spacing of the coils can, for example, be twice the inner radius r 1 of the two coils.

[0059] Figures 5 and 6 Illustrate, using comparative calculations or comparative tests, the superior properties of modulation devices according to the invention with planar coils compared to designs with conventional cylindrical coils.

[0060] First, the diagram shows 70 of the Fig. 5 The analytically calculated field distribution of a pair of planar coils (curve 72) is compared with a corresponding pair of cylindrical coils in a Helmholtz configuration (curve 74). The vertical lines mark the area of ​​the freely accessible air gap 76.

[0061] For the calculation, it was assumed that there was an unobstructed distance of d = 5 mm between each pair of coils ( Fig. 4) for the transport path of the banknote test specimens 10. It was further assumed that each individual coil has ten turns and is implemented with a conductor track 0.5 mm wide. The distance between successive turns is 0.25 mm. A value of r1 = 0.375 mm was assumed for the inner radius r1 of the planar coils; the influence of the respective coil supports was neglected. With these assumptions, the length of the two cylindrical coils is 7.25 mm, and the outer radius r2 of the two planar coils is also 7.25 mm.

[0062] Diagram 70 shows the field strength B mod in the z-direction calculated with these assumptions as a function of the axial position coordinate z. The relevant part of the air gap between lines 76 is also shown at the values ​​z = ± 2.5 mm corresponding to d = 5 mm. As can be seen directly from the diagram, the planar coil pair according to the invention (curve 72) generates a modulation field B mod of a significantly higher field strength in the air gap than the conventional cylindrical coil pair (curve 74).

[0063] In addition to generating a higher modulation field, the planar coil pair according to the invention also requires significantly less installation space in the air gap than the corresponding cylindrical coil pair. The two planar coils together have an axial extension in the z-direction of only 1 mm, while the two cylindrical coils together have a z-extension of 14.5 mm. Considering the required unobstructed gap area of ​​d = 5 mm for banknote transport, the magnetic circuit using a cylindrical coil pair must therefore have an air gap of at least 19.5 mm. In contrast, an air gap of only 6 mm is sufficient when using the planar coil pair according to the invention. This narrower air gap is associated with a significantly stronger quasi-static polarization field B₀, which also contributes to a considerably higher signal strength in spin resonance measurements.

[0064] Diagram 80 of the Fig. 6 contains experimental results of a comparison of planar coils according to the invention in the air gap with conventional cylindrical coils that are wound outside the air gap around the magnetic core of a magnetic circuit.

[0065] For the comparative measurement, a paper sample was loaded with a marker substance possessing a spin resonance signature and placed on a stripline resonator. The resonator was located within the air gap of a soft iron magnetic circuit. Two series of measurements were performed using this setup.

[0066] In the first series of measurements, a spiral planar coil 30 of the in Fig. 2The type shown, with constant spacing of the conductor tracks, is used as a modulation coil. The planar coil 30 is implemented on a printed circuit board and has a total of 25 turns with an inner radius r1 = 0.375 mm and an outer radius r2 = 7.25 mm. The total thickness of the circuit board thus manufactured is 0.75 mm. The planar coil 30 was mounted directly in the air gap of the magnetic circuit and was positioned 2.8 mm from the resonator along the z-axis.

[0067] In the second series of measurements, a conventional cylindrical coil was used as the modulation coil. The cylindrical coil, with a radius of 22.2 mm, was wound with 72 turns around the magnetic core of the magnetic circuit at the edge of the air gap. The distance along the z-axis between the center of the coil and the stripline resonator was 7.1 mm.

[0068] Within each measurement series, the spin resonance signature of the marker was recorded at different modulation frequencies f mod, and the signal maximum of each measurement was extracted. For better comparability, the measured values ​​obtained in this way were normalized to the combined maximum of both measurement series to obtain a normalized signal strength S n. This normalized signal strength S n is plotted in diagram 80 as a function of the modulation frequency f mod and shows the frequency dependence of the signal strength when using a planar coil according to the invention (curve 82) or when using a conventional cylindrical coil (curve 84).

[0069] As can be seen from diagram 80, the signal strength of the spin resonance signal decreases for both designs with increasing modulation frequency f mod. According to current understanding, this is primarily due to eddy currents induced by the modulation field B mod in the soft iron of the magnetic circuit, which, according to Lenz's law, weaken the generating modulation field. However, due to its size and position, the cylindrical coil induces eddy currents to a significantly greater extent than the planar coil according to the invention in the air gap. For this reason, the signal strength S n for the cylindrical coil (curve 84) decreases much more rapidly with increasing modulation frequency f mod than for the planar coil according to the invention (curve 82). A spin resonance sensor element for use in a high-speed banknote processing machine requires high modulation frequencies to reduce or avoid motion artifacts.

[0070] At such a high modulation frequency, the operation of a cylindrical coil wound around the magnetic core is inefficient due to the eddy currents and does not allow for a sufficiently high signal strength of the spin resonance signal for a reliable authenticity check. Reference symbol list

[0071] 10 Banknote test piece 12 Spin resonance feature 14 Transport path 20 Test device 22 Magnetic core 24 Air gap 30 Planar coil 32 Printed circuit board 34 Conductor traces 36 Via 40 Elliptical outer contour 42 Rectangular outer contour 44 Inner coil part 46 Outer coil part 48 Conductor trace return 50 Sensor element 52 Polarizing magnet 54 Permanent magnet 56 Magnetic core 58 Air gap 60 Modulation device 62, 64 Planar coils 66 Resonator 70 Diagram 72 Field distribution of a pair of planar coils 74 Field distribution of a pair of cylindrical coils 80 Diagram 82 Frequency dependence of planar coil 84 Frequency dependence of cylindrical coil

Claims

1. Sensor element (26) for the checking of a planar data carrier (10), in particular a banknote, with a spin resonance feature (12), with - a magnetic core (22) with an air gap (24), into which the planar data carrier (10) can be introduced for authenticity checking, and which defines an axial direction extending between the adjacent surfaces of the magnetic core (22), - a polarization device for generating a static magnetic flux in the air gap (24), - a modulation device (30) for generating a time-varying magnetic modulation field in the air gap (24), and - a resonator for the excitation of the spin resonance feature (12) of the data carrier (10) to be checked, characterized in that - the modulation device is formed by at least one planar coil (30) arranged in the air gap, which in one plane has one or more turns around the axial direction of the air gap.

2. Sensor element (26) according to claim 1, characterized in that the at least one planar coil (30) has a spiral conductor track routing with a minimum inner radius (r1) and a maximum outer radius (r2).

3. Sensor element (26) according to claim 1 or 2, characterized in that the outer contour (40, 42) of the conductor track routing of the at least one planar coil (30) is circular, elliptical, rectangular or polygonal.

4. Sensor element (26) according to at least one of claims 1 to 3, characterized in that the conductor track spacing (Δ) of successive turns is constant.

5. Sensor element (26) according to at least one of claims 1 to 4, characterized in that the number of turns of the at least one planar coil (30) lies between 2 and 100 (inclusive in each case), preferably between 10 and 30 (inclusive in each case).

6. Sensor element (26) according to at least one of claims 1 to 5, characterized in that the modulation device (60) contains two planar coils (62, 64) which are arranged axially spaced on the top side and bottom side, respectively, of the air gap (58).

7. Sensor element (26) according to claim 6, characterized in that the two said planar coils (62, 64) are arranged and matched to one another such that they generate a homogeneous field profile of the modulation field between them, preferably, that the two said planar coils are each formed with a conductor track routing with the same minimum inner radius and the same maximum outer radius and have a spacing in the axial direction which essentially corresponds to twice the inner radius.

8. Sensor element (26) according to at least one of claims 1 to 7, characterized in that the at least one planar coil (30) is formed on a coil carrier, in particular a printed circuit board (32), or on a ferromagnetic carrier.

9. Sensor element (26) according to at least one of claims 1 to 8, characterized in that the at least one planar coil (30) forms a resonant circuit together with a capacitance.

10. Sensor element (26) according to at least one of claims 1 to 9, characterized in that the modulation device comprises at least one double planar coil, in which on the top side and bottom side of a coil carrier the turns of a respective planar coil are arranged coaxially.

11. Sensor element (26) according to at least one of claims 1 to 10, characterized in that the resonator is formed on a printed circuit board, preferably in the form of a planar surface resonator, in particular a stripline resonator, and is arranged together with the at least one planar coil on different printed circuit board layers of the same component.

12. Checking device (20) for the checking of planar data carriers, in particular banknotes, with a sensor element (26) according to any of claims 1 to 11 and with a transport device which introduces the planar data carriers to be checked along a transport path into the air gap of the magnetic core or guides them through the air gap of the magnetic core.

13. Method for the checking of a planar data carrier, in particular a banknote, with a spin resonance feature (12) by means of a sensor element (26) according to any of claims 1 to 11, wherein in the method - a planar data carrier to be checked is introduced into the air gap (24) of the magnetic core (22) of the said sensor element (26), - with the polarization device a static magnetic flux is generated in the air gap (24), - with the modulation device (30) a time-varying magnetic modulation field is generated in the air gap (24), wherein as the modulation device a device is provided which is formed by at least one planar coil (30) arranged in the air gap, which in one plane has one or more turns around the axial direction of the air gap, and - with the resonator the spin resonance feature (12) of the data carrier (10) to be checked is excited,14. Method according to claim 13, characterized in that with the resonator there is also recorded a response signal of the spin resonance feature (12) generated by the excitation and output to a detector.

15. Method according to claim 13 or 14, characterized in that the excitation of the spin resonance feature (12) and / or the recording of the response signal of the spin resonance feature (12) is carried out in a continuous-wave (CW) method, in a pulsed method, or in a rapid-scan method.