Patent Application
20240161565
The invention relates to a sensor element for testing a flat data carrier, in particular a banknote, having a spin resonance feature. The invention also relates to a test device having a sensor element of this type, and a method for testing with a sensor element of this type.
Data carriers, such as security documents or identity documents, but also other items of value, such as e.g. branded articles, are often provided with protective security elements which allow the authenticity of the data carriers to be verified and simultaneously offer protection against unauthorized reproduction. In the case of automatic authenticity testing, it is known to use security elements having spin resonance features for the protection of documents and other data carriers. To do this, the security elements are provided with substances which have a spin resonance signature.
Spin resonance features are generally based on a resonant energy absorption of a spin ensemble in an external magnetic field. The resonant energy absorption is physically based on the splitting of the energy states of the spin ensemble in the external magnetic field. Spins, the magnetic moments of which are oriented parallel to the external field, have a lower energy state than spins having an antiparallel magnetic moment. Resonant transitions between the energy levels can be excited through the irradiation of an alternating magnetic field of suitable strength which is oriented perpendicular to the external magnetic field referred to as the polarization field.
The resonant frequency or Larmor frequency fL required in order to excite the transitions is determined by the energy difference between the split levels,
2πfL=γBc,
where γ indicates the gyromagnetic ratio of the spins involved, and Bc designates the relevant characteristic magnetic field.
For authenticity assurance, not only nuclear spins but also electron spins are used as spin ensembles in security elements, correspondingly referred to in the case of resonant excitation as nuclear magnetic resonance (NMR) or electron spin resonance (ESR). Materials having a high electron spin density, in which the interaction of the spins with one another is no longer negligible, can also be used. These include ferromagnetic and ferrimagnetic materials, generally referred to in these cases as ferromagnetic resonance (FMR).
In NMR and ESR, the relevant characteristic magnetic field Bc is essentially determined by the external magnetic field B0, whereas, in FMR, the resonance condition also depends on internal fields present in the material which, together with the external magnetic field, form an effective field Beff which then represents the characteristic magnetic field Bc relevant to the calculation of the resonant frequency.
With conventional magnetic field strengths, the resonant frequency in NMR is typically in the MHz range, whereas, in ESR and FMR, it is typically significantly higher in the GHz range.
The aforementioned spin resonances can be measured, for example, in a continuous-wave (CW) method in which a signal source is operated at a fixed excitation frequency fL which corresponds to the expected Larmor frequency of the spin resonance feature, and in which a parallel time-dependent modulation field is superimposed on the polarization field. Some spin resonance effects can also be measured in a pulsed method in which the occupancy states are manipulated by resonant single pulses or pulse sequences of the high-frequency signal. Further special methods can also be used, such as e.g. the rapid scan method, in which, with a substantially increased high-frequency field B1, the polarization field B0 is traversed so quickly that no saturation of the spin states occurs.
Irrespective of the measurement method that is used, the signal-to-noise ratio of a spin resonance measurement is always dominated by the first link in the high-frequency chain, i.e. by the resonator. The high-frequency signal S which is tappable at the resonator connection and is characteristic of the spin resonance is, in particular, proportional to the fill factor η of the resonator, to the resonator quality Q and to the number of particles NS in the sensitive area,
S˜η*Q*NS,
where the fill factor η indicates the ratio of the magnetic field energy stored in the sample volume to the total magnetic field energy of the resonator stored in the space.
Against this background, the object of the invention is to indicate an improved device for testing data carriers having spin resonance features, said device allowing, in particular, testing on fast-running data carriers using the CW method and/or using a pulsed method and/or using a rapid-scan method.
The invention provides a sensor element for the testing, in particular the authenticity testing, of a flat data carrier having a spin resonance feature. The flat data carrier can, for example, be a banknote. The sensor element comprises a magnetic core having an air gap into which the flat data carrier is insertable for the test, an element for generating a static magnetic flux in the air gap, and a resonator for exciting the spin resonance feature of the data carrier under test.
The sensor element preferably further comprises a modulation device for generating a modulated magnetic field parallel to the static magnetic flux in the air gap. This modulation can be performed, for example, at a frequency between around 1 kHz and around 1 MHz. This enables a spin resonance measurement having a good signal-to-noise ratio.
As a special characteristic, the resonator is formed by a strip line resonator which is arranged in the air gap of the magnetic core and which has a flat carrier with an upper side and an opposing lower side, and a conducting structure having a characteristic length l which is arranged on the upper side of the carrier.
The flat carrier preferably has a ground return on the upper side and/or the lower side. The ground return is preferably designed as a ground plane. The ground plane is preferably formed on the lower side of the carrier, thus enabling a more reliable measurement of the spin resonance signal, even in the presence of external interference signals.
In preferred designs, the strip line resonator forms a λ/4 fundamental mode, a λ/2 fundamental mode or a λ fundamental mode. This means that the spatial extension of the field corresponds to one quarter or one half of the entire wavelength λ of the injected high-frequency signal. A resonator having a λ/4 fundamental mode is advantageous if a small structural shape or size of the resonator is desired. The small size is accompanied by a spatially small B1 distribution with simultaneously high spatial resolution and a low number of measured particles NS. Conversely, a resonator having a λ fundamental mode has a greater extension of the B1 field, i.e. a lower spatial resolution, but a high number of measured particles NS. A resonator having a λ/2 fundamental mode is a compromise between the two previously mentioned types.
As a conducting structure, the strip line resonator appropriately comprises a rectangular structure, an annular structure having a circular or elliptical outer contour, a disc-shaped structure, a ring-sector structure having circular or elliptical outer edges, or a structure having a triangular or polygonal shape. Rectangular, annular and disc-shaped structures are particularly preferred, since a defined resonance mode and therefore also a defined B1 distribution can be excited in a simple manner within these structures. Conversely, alternative geometries can be advantageous with specific printed circuit board designs due to their small size. Combinations of the aforementioned elements can also be used as a conducting structure.
The strip line resonator with its characteristic length l is advantageously configured for testing the spin resonance feature of the data carrier in the spatial fundamental mode of the excitation field. This allows the strip line resonator to be particularly small in size. It has proven to be particularly advantageous if the strip line resonator with its characteristic length l is designed and configured for testing the spin resonance feature of the data carrier in a higher-order spatial mode of the excitation field, in particular the second-order or third-order mode of the excitation field. This enables a measurement in a larger sample volume and therefore an increase in the number NS of particles in the sensitive area.
In one advantageous embodiment, the strip line resonator has a quality Q between 50 and 400, in particular between 80 and 300.
The strip line resonator advantageously contains at least one signal conductor, to which the conducting structure is coupled. In one preferred embodiment, the strip line resonator comprises precisely one signal conductor and is operated in reflection mode. In a further preferred embodiment, the strip line resonator comprises precisely two signal conductors and is operated in transmission mode. The signal conductor(s) is/are preferably arranged on the upper side of the carrier, thereby simplifying the manufacture of the strip line resonator. In this case, the coupling of the signal conductor(s) is preferably implemented capacitively or via an inset feed. With the inset feed, the signal conductor and the resonator are, in particular, located on the same plane. However, the coupling point between the signal conductor and the resonator is preferably not located at the edge of the resonator, but inside the metallized copper area of the resonator. Unwanted signal reflections between the signal conductor and the resonator can be suppressed with a procedure of this type.
An arrangement of one or more signal conductors on the upper side of the carrier is also possible. In this case, an aperture coupling, for example, or a coaxial coupling can be used to couple the conducting structure to the signal conductor(s). The arrangement on the lower side enables low parasitic scattering losses and a signal coupling which is independent from the loading of the strip line resonator with a test item.
The resonator is preferably resonant in a frequency range between 1 GHz and 100 GHz. A frequency range between 5 GHz and 85 GHz, in particular between 15 GHz and 50 GHz, is particularly preferred. Compared with lower frequencies, this enables a higher spectral resolution and a stronger measurement signal.
The carrier of the strip line resonator is advantageously formed by a printed circuit board, thus allowing reproducible and low-cost manufacture. However, it is also advantageous, in particular for reducing dielectric losses in the carrier material, to use carriers based on ceramic, Teflon or hydrocarbons.
In one advantageous embodiment, the conducting structure of the strip line resonator is formed by a silver layer or by a conductor, for example made of copper, which is provided with a silver coating and a suitable protective layer. Due to the better conductivity of silver compared with copper, this enables a higher resonator quality Q. Since the current density is higher in the peripheral area of the signal conductor, the resonator quality can already be increased by a silver coating.
In a further advantageous embodiment, the conducting structure of the strip line resonator has a low surface roughness. A surface roughness Rq of less than 2.9 μm, particularly preferably less than 0.9 μm, in particular less than 0.4 μm, is preferred. The reduction in the surface roughness enables a high resonator quality Q.
In one advantageous development of the invention, the strip line resonator is arranged parallel in the air gap and distanced from a metallized shielding element in order to reduce radiation losses of the resonator. The shielding element advantageously has a carrier, for example a printed circuit board, which is provided with a metallization. This is advantageous from a manufacturing perspective. Furthermore, the printed circuit board can then also comprise the device for generating the modulation field, for example by means of planar coils. The strip line resonator and the shielding element appropriately form a narrow slot in which a transport path for the data carrier under test runs.
The spin resonance feature of the data carrier under test can be a nuclear spin resonance feature, an electron spin resonance feature or a ferromagnetic or ferrimagnetic resonance feature. The sensor element is particularly advantageously designed and configured for testing an electron spin resonance feature or a ferromagnetic or ferrimagnetic resonance feature, since particularly high resonant frequencies occur in said features.
The element for generating a static magnetic flux is advantageously formed by a permanent magnet. Compared with a structure in which the static component of the magnetic flux is generated by an electromagnet, this reduces energy consumption and waste heat. However, it can also be advantageous in other designs to generate not only a time-varying component, i.e. the modulation field, but also the static component of the magnetic flux in the air gap by means of electromagnets. Following the deactivation of the sensor element, this enables a particularly simple handling, particularly in the assembly, dismantling, and transport of the test device.
The invention also contains a test device for the testing, in particular the authenticity testing, of flat data carriers, in particular banknotes, having a sensor element of the type described. The test device also has a transport device which inserts the flat data carriers under test along a transport path into the air gap of the magnetic core or feeds them through the air gap of the magnetic core.
The transport device is particularly advantageously designed and configured for a fast-running transport, for example at a speed of 1 to 12 m/s of the flat data carriers under test, along the transport path.
The transport device is appropriately designed and configured to feed the flat data carriers under test along a transport path at a short distance above the upper side of the strip line resonator. A short distance here is, in particular, a distance of several tenths of millimeters, preferably less than 0.5 mm. In this way, the flat data carriers, and therefore the spin resonance feature, traverse an area having a particularly strong excitation field above the strip line resonator, as a result of which a high fill factor η is achieved.
In a further development of the invention, the sensor element of the test device is equipped with a shielding element of the aforementioned type, i.e. the strip line resonator is arranged in the air gap parallel to and distanced from a metallized shielding element in order to reduce radiation losses of the resonator. In a design of this type, the transport device of the test device is designed and configured to feed the flat data carriers under test along a transport path in a slot between the strip line resonator and the shielding element. This design enables a particularly high resonator quality Q.
In one advantageous design, the test device represents a set-up for a pulsed measurement and for this purpose has, in particular, a signal source for a pulsed activation of the strip line resonator. Compared with a CW application, a pulsed application offers the advantage that the decay behavior and the spectrum of a spin resonance feature can be measured more quickly.
In a further advantageous embodiment, the test device represents a set-up for a rapid-scan method and for this purpose has, in particular, a powerful signal source for activating the resonator, and a device for quickly traversing the polarization field strength. Compared with a CW application, a rapid-scan method offers the advantage that the spectrum of a spin resonance feature can be measured more quickly.
In contrast, a measurement in CW mode enables a particularly simple design of the electronics.
The test device can, in particular, be part of a banknote processing machine.
The invention also contains a method for the testing, in particular the authenticity testing, of a flat data carrier having a sensor element, in particular of the type described.
In the method, a flat data carrier having a spin resonance feature is tested by means of a sensor element. The following steps are carried out:
In the method, the strip line resonator is preferably operated in a higher-order spatial mode of the excitation field, in particular the second-order or third-order mode of the excitation field. In a similarly advantageous method variant, the test is carried out with pulsed high-frequency excitation.
The features of the advantageous embodiments of the sensor element according to the invention are also valid for the method according to the invention. Conversely, the features of the advantageous embodiments of the method according to the invention are also valid for the sensor element according to the invention.
On the whole, the use according to the invention of a strip line resonator in the air gap of the magnetic core offers a range of advantages:
In the case of strip line resonators, the quality Q is typically in the order of magnitude of 100, thus enabling very short rise times down to 100 ns, and even below 25 ns. The strip line resonator can therefore be quickly adjusted to the resonant frequency in the case of a measurement of fast-moving test items, and it further allows a pulsed measurement operation. The quality of the strip line resonator can also be further optimized, for example, via an additional shielding element, insofar as this is technically advantageous.
Further exemplary embodiments and advantages are explained below with reference to the figures, the presentation of which, for the sake of greater clarity, is not true-to-scale or proportional.
In the figures:
The invention will now be explained using the example of authenticity testing of banknotes. Quality control, for example, or a test for fitness for circulation can also be carried out with the method. For this purpose,
In one feature area, the banknote test item 10 has a spin resonance feature 12 under test, of which the characteristic properties serve to establish the authenticity of the banknote. For the authenticity test, the banknote test item 10 is fed along the transport path 14 through a sensor element 30 of the test device 20 which will be described more precisely below. The design according to the invention offers particular advantages in the testing of fast-running banknotes, wherein the banknote test items 10 are fed at high speed through the sensor element of the test device 20.
The test device 20 contains a signal source 22 and, in this example, a switch 23 for generating resonant single pulses or pulse sequences for the excitation of the spin resonance feature 12. The pulsed electrical excitation signal is fed via a duplexer 24 to a resonator 32 of the sensor element 30 which is located in the field of a polarization magnet 34, and generates a pulsed alternating magnetic field there. The response signal of the spin resonance feature 12 is captured by the resonator 32 in the pulse pauses and is fed via the duplexer 24 to a detector 26 and to an evaluation unit 28.
As a special characteristic, the resonator 32 is formed by a strip line resonator which is arranged in the air gap of a polarization magnet with magnetic feedback. To provide a more detailed explanation,
The strip line resonator 40 comprises a flat carrier 42 which, in the exemplary embodiment, is formed by a printed circuit board, but, in other designs, can also be formed, for example, by a ceramic material. The carrier 42 has an upper side 44-O facing the banknote test item 10 and an opposing lower side 44-U, wherein a conducting structure 46 having a characteristic length l is arranged on the upper side 44-O of the carrier, and a ground area 48 is arranged on the lower side 44-O as a ground return. For the sake of clarity, the signal conductor and the coupling of the conducting structure 46 are not shown in
If the conductor path wavelength λ of the injected high-frequency signal matches the dimension l of the conducting structure 46 in the test, a standing wave can form in the resonator, and the strip line resonator 40 is in resonance with the excitation frequency associated with the wavelength λ.
The strip line resonator 50 shown in
l=n*λ/4,
where n represents a mode factor which, in this embodiment, must be an odd whole number greater than or equal to 1. Depending on the effective permittivity of the set-up, the length l can, for example, be 14.9 mm for a resonant frequency of 3 GHz, wherein, in this example, an effective permittivity of 2.78 has been chosen.
The strip line resonator 60 shown in
l=n*λ/2,
with a mode factor n which, in this case, represents a whole number greater than or equal to 1. The length l can, for example, be 29.9 mm for a resonant frequency of 3 GHz and an effective permittivity of 2.78. An effective permittivity of 2.02 and a resonant frequency of 20 GHz produce a characteristic length l of 5.3 mm.
The strip line resonator 70 shown in
l=2πr=n*λ,
with a mode factor n which represents a whole number greater than or equal to 1. With a resonant frequency of 3 GHz, for example, the conductor path is 1.8 mm wide and has a circumference of 59.7 mm.
The strip line resonators are designed, in particular, so that their resonant frequency f corresponds to the expected Larmor frequency fL of the spin resonance feature under the predefined measurement conditions. In the case of the resonator shown in
l=n*c/(2*f*√εeff)−Δl,
where εeff represents the effective permittivity of the set-up, c the speed of light, and Δl a length correction known per se for taking account of electrical stray areas at the edge of the resonator. If the expected Larmor frequency fL of the spin resonance feature is chosen in the indicated relationship for f, the characteristic length l of the conducting structure 62 suitable for the test is obtained.
The geometric shapes of the conducting structures 52, 62, 72 shown in
A capacitive coupling and a coupling via an inset feed are illustrated as coupling mechanisms in
The high fill factor of the strip line resonator that is advantageous for the application in the case of banknote testing is obtained, in particular, from the spatial distribution of the B1 field which is particularly well adapted to the geometry of a banknote or other flat data carrier, and which is strongest on the upper side of the resonator and decreases substantially as the distance from the resonator increases (see
It is also clear from the representation shown in
In this respect,
The associated characteristic of the respectively generated B1 field (curve 84 or 94) is shown schematically in each case below the strip line resonators 80, 90. As is immediately evident, the surface area of a test item 10 covered with the resonator 80 shown in
The quality Q of a strip line resonator according to the invention is typically around 100 and is therefore significantly below the typical quality of a cavity resonator. According to current understanding, the lower quality is based above all on the fact that the electromagnetic oscillations of the strip line resonators are present primarily in the conducting structure openly applied to the carrier. Metallic losses occur here in the material of the conducting structure (for example copper), dielectric losses in the carrier material (for example FR4 materials), and also radiation losses due to antenna effects which result in each case in attenuation and therefore reduced resonator quality.
However, it has surprisingly become clear that a quality in the range from around Q=50 to around Q=400 for spin resonance measurements on fast-running data carriers does not represent a disadvantage, but, on the contrary, is even associated with a plurality of advantages.
On one hand, as already explained in detail above, a lower quality for measurements on fast-moving samples and for pulsed methods is essentially advantageous, since a high resonator quality is always associated with a long rise time of the resonator. However, the quality must not be too low, since signal intensity rises proportionally with quality. The optimum quality of a resonator for measurement on fast-moving samples is therefore obtained from a careful balancing of the two factors of signal intensity and time constant, and it has become evident that quality in the order of magnitude of 100 represents a good compromise between the two opposing requirements for the purpose according to the invention.
In addition, the quality of the strip line resonator can be optimized within the framework of the required measurement dynamics by a plurality of measures. Metallic losses can be reduced through the choice of material for the conducting structure, for example by forming the conductor from silver rather than copper. A silver coating of the conductor is often sufficient at the high operating frequencies due to the skin effect. Quality can furthermore also be increased by using conductor structures having a low surface roughness. Suitable materials can be used for the carrier of the strip line resonator, for example based on ceramic, Teflon or hydrocarbons, in order to reduce dielectric losses.
Radiation losses can finally be reduced, for example, by a shield, as illustrated in
A first strip line resonator has a rectangular conducting structure 110 having a width of 1.8 mm and a length of 29.9 mm. The resonator is implemented on a 0.81 mm thick printed circuit board having a relative permittivity of 3.55, resulting in an effective permittivity of 2.78 for the set-up. The first strip line resonator has a resonant frequency of around 3 GHz and is operated in the fundamental mode n=1. In the alignment, it has been assumed that the strip line resonator is centered in the x-y plane.
A second strip line resonator contains an annular structure having a conductor path width of 1.8 mm and a circumference of 59.7 mm in center of the conductor path as the conducting structure 120. The resonator is implemented on a printed circuit board with the same specifications as the resonator shown in
As is immediately evident from
The fill factor 11 of the resonators has been calculated on the basis of the calculated field distributions in order to quantify this accessibility. Along with the two resonators already described in connection with
A conventional dielectric resonator having a cylindrical geometry with an outer diameter of 17.9 mm, an inner diameter of 5.5 mm, a height of 10.6 mm and a relative permeability of 37 serves as a comparative example.
For the comparison, it has been assumed in each case that the resonators are loaded with a 100 μm thick banknote test item which, in the case of the strip line resonators, is arranged directly on the upper side of the resonators, and, in the case of the dielectric resonator, on the circular base area of the cylinder. The area content A50, which indicates the area inside a 50% contour of the B1 distribution, serves as a measure for the number NS of respectively measured particles. This area content has been determined twice, i.e. once (referred to below as A50, loc) for a B1 distribution which is normalized onto its local maximum in the plane of the banknote test item, and once (referred to below as A50, glob) for a B1 distribution which is normalized onto its global maximum within the total volume.
As shown in the table, the strip line resonators, when used in the measurement of spin resonance features of flat data carriers, have an at least 27 times higher fill factor η than the dielectric resonator used as a comparative example.
In addition, the calculated A50 area contents prove that the number NS of examined particles can be increased by increasing the resonator mode. The rectangular resonator with n=2 has almost double the A50 area of the rectangular resonator in the fundamental mode with n=1.
A comparison of the local and global A50 areas for each of the resonators reveals that the banknote test item in the strip line resonators is located in each case in an area with an almost maximum B field (A50, glob≈A50 , loc), whereas the banknote test item in the dielectric resonator is located only in a weak peripheral area of the generated field (A50, glob≈0).
Finally, for an experimental verification, a paper sample with a spin marker was loaded and the spin resonance spectrum of the marker in the paper sample was then recorded over a B0 sweep. The B0 field pointed parallel to the z-axis. The measurement was carried out with two different resonators.
In a first measurement, a strip line resonator according to the invention was used which had a conducting structure having a rectangular geometry with a length of 40 mm and a width of 2.3 mm. The conducting structure was arranged on a printed circuit board having a thickness of 0.5 mm and a relative permittivity of 2.33, and therefore in a set-up with an effective permittivity of 2.02. The strip line resonator had a resonant frequency of 2.6 GHz, a quality of Q=104 and was aligned so that it was located in the x-y plane perpendicular to the B0 direction.
In a second measurement, a conventional dielectric resonator was used as a comparative example, the parameters of which corresponded to the parameters described above for the simulation. The known dielectric resonator had a resonant frequency of 3.0 GHz and a quality of 800. Due to the geometry of the known dielectric resonator, only the stray components of the B1 field could be used to measure the paper sample. The known dielectric resonator was therefore aligned so that its cylinder axis pointed parallel to the z-axis. Due to the slightly higher resonant frequency, a spin resonance was expected to occur in the measurement with the dielectric resonator with a B0 field that was roughly 15% higher than with the strip line resonator according to the exemplary embodiment of the invention.
In the experiment, the paper sample was laid directly onto the upper side of the respective resonators in the x-y plane. The results of the two measurements are summarized in the diagram 130 shown in
The diagram in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2021 001 593.2 | Mar 2021 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/025115 | 3/22/2022 | WO |
