US-12626557-B2 - Sensor element and device for checking the authenticity of a data carrier having a spin resonance feature

US12626557B2US 12626557 B2US12626557 B2US 12626557B2US-12626557-B2

Abstract

A sensor element for checking the authenticity of a planar data carrier, in particular a banknote, has a spin resonance feature. The sensor element contains a magnetic core having an air gap, into which the planar data carrier can be inserted for authentication and which defines an axial direction extending between the adjoining surfaces of the magnetic core, 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 to be checked. The modulation device is formed by at least one planar coil arranged in the air gap, which planar coil has one or more turns about the axial direction of the air gap in one plane. The disclosure also relates to a checking device having such a sensor element.

Inventors

STEPHAN HUBER
Thomas Happ

Assignees

GIESECKE+DEVRIENT CURRENCY TECHNOLOGY GMBH

Dates

Publication Date: 20260512
Application Date: 20220322
Priority Date: 20210326

Claims (14)

1 . A sensor element for checking a planar data carrier having a spin resonance feature, with a magnetic core having an air gap into which the planar data carrier can be inserted for authenticity checking and which defines an axial direction extending between the adjacent surfaces of the magnetic core, 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 to be checked, wherein, the modulation device is formed by at least one planar coil arranged in the air gap, which has one or more turns about the axial direction of the air gap in one plane, wherein the at least one planar coil has a spiral-shaped conductor track layout with a minimum inner radius and a maximum outer radius.
2 . The sensor element according to claim 1 , wherein an outer contour of the conductor track layout of the at least one planar coil is circular, elliptical, rectangular or polygonal.
3 . The sensor element according to claim 1 , wherein a conductor track spacing (A) of successive turns is constant.
4 . The sensor element according to claim 1 , wherein the number of turns of the at least one planar coil is between 2 and 100 (inclusive in each case).
5 . The sensor element according to claim 1 , wherein the modulation device contains two planar coils axially spaced apart on the upper side and underside of the air gap.
6 . The sensor element according to claim 5 , wherein the two aforementioned planar coils are arranged and matched to each other in such a way that they generate a homogeneous field pattern of the modulation field between each other, such that the two aforementioned planar coils are each formed with a conductor track layout having the same minimum inner radius and the same maximum outer radius and are a distance apart in the axial direction which essentially corresponds to double the inner radius.
7 . The sensor element according to claim 1 , wherein the at least one planar coil is formed on a coil carrier, a printed circuit board, or on a ferromagnetic carrier.
8 . The sensor element according to claim 1 , wherein the at least one planar coil together with a capacitance forms a resonant oscillator circuit.
9 . The sensor element according to claim 1 , wherein the modulation device comprises at least one double planar coil, in which the coils of a planar coil are arranged coaxially on the upper side and underside of a coil carrier.
10 . The sensor element according to claim 1 , wherein the resonator is formed on a printed circuit board in the form of a planar surface resonator, a stripline resonator, and together with the at least one planar coil is arranged on different printed circuit board layers of the same component.
11 . A checking device for checking planar data carriers, having a sensor element according to claim 1 and having a transport device which inserts the planar data carriers to be checked into the air gap of the magnetic core along a transport path or guides them through the air gap of the magnetic core.
12 . A method for checking a planar data carrier having a spin resonance feature, by means of a sensor element according to claim 1 , wherein in the method: a planar data carrier to be checked is inserted into the air gap of the magnetic core of the aforementioned sensor element, the polarization device is used to generate a static magnetic flux in the air gap, the modulation device is used to generate a time-varying magnetic modulation field in the air gap, wherein a device is provided as the modulation device, which is formed by at least one planar coil arranged in the air gap, which planar coil has one or more turns about the axial direction of the air gap in one plane, and the resonator is used to excite the spin resonance feature of the data carrier to be checked.
13 . The method according to claim 12 , wherein the resonator is also used to record a response signal of the spin resonance feature generated by the excitation and output said signal to a detector.
14 . The method according to claim 12 , wherein the excitation of the spin resonance feature and/or the recording of the response signal of the spin resonance feature is/are carried out in a continuous wave method, in a pulsed method, or in a rapid scan method.

Description

BACKGROUND The invention relates to a sensor element for checking the authenticity of a planar data carrier, in particular a banknote, having a spin resonance feature, and to a checking device having such a sensor element. Data carriers, such as value or identification documents, but also other valuable objects, such as brand-name articles, are often provided with security elements that allow the data carriers to be authenticated and that also serve as protection against unauthorized reproduction. It is well known in the field of machine authentication to use security elements with spin resonance features to secure documents and other data carriers. The security elements are provided with substances that have a spin resonance signature. The spin resonance signatures that can be used for authenticity checking include, in particular, nuclear magnetic resonance (NMR) effects, electron spin resonance (ESR) effects, and ferromagnetic resonance (FMR) effects. In the process of checking banknotes, three different magnetic fields are usually generated in the measuring range of a banknote processing machine, for example, to detect the spin resonance signatures. This is specifically 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 f mod in the kHz range. For excitation of 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. To generate the polarization field B0, a magnetic circuit is often used that directs the magnetic flux of a permanent magnet or a coil to the air gap in which the detection of the spin resonance signatures takes place. To generate the modulation field Bmod, a single cylindrical coil or a cylindrical coil pair in a Helmholtz configuration is usually used, i.e. a cylindrical coil pair in a configuration in which two cylindrical coils with coil radius r are placed parallel to each other on the same axis at a distance r and through which the same current is allowed to flow in the same direction. The cylindrical coils are often wound around the magnetic circuit near the air gap, but can also be arranged directly in the air gap. For generating the excitation field B1, a resonator is typically used which is also usually arranged in the air gap. For the authentication check, a test specimen 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 that both field strengths are advantageously maximized. The field strength of the polarization field B0 is approximately indirectly proportional to the height of the air gap, which means it can be increased by reducing the overall 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. These requirements result in a conflict of objectives in practice: if a cylindrical coil or a cylindrical coil pair is arranged in the air gap for generating the modulation field Bmod, the overall size of the cylindrical coils increases as the number of turns increases. Accordingly, either the air gap height must be increased or the maximum number of turns of the coil must be limited, thus limiting either the field strength of the polarization field B0 or the field strength of the modulation field Bmod, and thus in both cases limiting the strength of the spin resonance signal. If, on the other hand, the cylindrical coils are not arranged in the air gap but are wound around the magnetic circuit, the modulation field Bmod induces more eddy currents there, which in turn introduce a number of disadvantages. On the one hand, the magnetic field generated by the eddy currents has a polarity opposite to its cause, according to Lenz's law. The eddy currents therefore weaken the modulation field and give rise to a signal reduction. Furthermore, the sensor operation requires short measuring times and therefore also high modulation frequencies, in particular for high-speed banknote processing machines. However, since the strength of 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 varying magnetic resistance, which in turn leads to a drift in the flux density in the air gap and thus ultimately to an unwanted drift in the useful signal. SUMMARY Based on this, the object of the invention is