Measuring system and method for characterizing a multilayer structure with layer-by-layer different ohmic properties, sensor module for a measuring system, manufacturing system for a multilayer structure with layer-by-layer different ohmic properties

Abstract

Measuring system (1) for characterizing a multi-layer structure with layer-by-layer different ohmic properties, in particular a battery electrode (B), with a sensor module (2), an electrical switching device (3) and measuring electronics, wherein the sensor module (2) comprises more than two planar coils (4, 5, 6, 7, 8) which are arranged in a coil stack and wherein the electrical switching device (3) is contacted with the planar coils (4, 5, 6, 7, 8) and is designed to connect the planar coils (4, 5, 6, 7, 8) to the measuring electronics in such a way that the planar coils (4, 5, 6, 7, 8) can be switched between at least one measuring configuration and a reference configuration, wherein in the measuring configuration the planar coils (4, 5, 6, 7, 8) at least partially form an optionally inductive or capacitive measuring sensor, the detection range of which is outside the sensor module (2),to determine a property of a low-resistance layer (N) and / or high-resistance layer (H) of the structure and, in the reference configuration, the planar coils (4, 5, 6, 7, 8) at least partially form an optionally inductive or capacitive reference sensor whose detection range extends substantially within the sensor module (2) in order to determine a sensor module property.

Description

DESCRIPTION OF THE INVENTIONThe invention relates to a measurement system and to a method for characterizing a multilayer structure having ohmic properties that differ in layers. The invention also relates to a sensor module for a measurement system and to a production system for producing a multilayer structure having ohmic properties that differ in layers.Multilayer structures having different ohmic properties are used in various industries. In the automotive industry, they can be present, for example, as battery electrodes in which a metal layer, which is typically formed by a copper or aluminum foil, is provided with a graphite-containing layer. The metal layer has a lower ohmic resistance than the graphite-containing layer. The metal layer can thus be regarded as a low-resistance layer and the graphite-containing layer as a high-resistance layer.In order to enable good functionality of the multilayer structure using the above example of the battery electrode, it must be ensured that, in particular, the above-mentioned constituents have the required properties. Quality assurance measures are required for this purpose, with which the stated properties can be determined and in order subsequently, if appropriate, to adapt a process parameter of a production process in the production of the multilayer structure or to exclude a checked structure from further processing or delivery in the case of inadequate quality. In particular, it is desired to be able to characterize the multilayer structure not only at points, but ideally over the entire surface. Particularly relevant in this case are the properties of the high-resistance layer, which is usually applied to the low-resistance layer and must be present in a desired layer thickness and layer density.One possibility for being able to perform a non-destructive characterization of the multilayer structure is offered by measurement systems based on X-ray technology. In this case, depending on the transmission or refraction behavior of X-rays with which the multilayer structure is exposed, it is possible to infer the layer properties. However, it is disadvantageous that such measurement systems are technically very complex and are susceptible to influences from the production environment.Another possibility is offered by optical measurement systems, of which in particular those which operate according to the confocal principle have become established in industrial measurement technology. Although such optical measurement systems have a high accuracy, they can only be used to capture information about the surface of an optically visible layer of the structure to be characterized at points. It is indeed possible to adjust such an optical measurement system by means of a manipulator with respect to the multilayer structure, such as a battery electrode. However, in this case too, despite the high technical complexity, it is not possible to check the structure over the entire surface, in particular if the structure to be characterized is in motion, as is customary, for example, in the coating of battery electrodes.In addition, the multilayer structures can also have relevant properties which, on the basis of principle, cannot be detected either by X-ray or optically. The example of the battery electrode can be the electrical properties of the high-resistance graphite-containing layer, which should move within predetermined tolerance limits, in particular after the so-called calendering process, so that the desired quality of the multilayer structure can be achieved. The electrical properties are dependent not only on the layer thickness, but also on the layer density.From YIN, Wuliang et al.: "Simultaneous measurement of distance and thickness of a thin metal plate with an electromagnetic sensor using a simplified model" (ISSN 0018-9456) a non-destructive method is known in which changes of an inductance in a non-magnetic metal plate can be detected by placing an air coil next to the metal plate. YIN, Xiaokang et al.: "A combined inductive and capacitive non-destructive evaluation technique using a single spiral coil sensor" (ISSN 1558-1748) likewise describes a non-destructive method for characterizing a multilayer structure, wherein inductive and capacitive measurement methods are evaluated in combination. For this purpose, a sensor is used which has a coil etched in a PCB with a single winding.The document DE 10 2021 213 302 B3 discloses a sensor chip having at least two magnetic field sensors which measure perpendicularly to the chip plane and are arranged adjacent to one another on the sensor chip, wherein a planar coil which is configured to generate a magnetic field directed perpendicularly to the chip plane is arranged on at least one of the magnetic field sensors.JP H06-82 506 U discloses a method for measuring the thickness of a coating or a thin film applied to an electrical conductor. For this purpose, two laser distance sensors are directed toward a layer to be measured, and an electromagnetic distance sensor is arranged between the laser distance sensors. A distance between the electromagnetic distance sensor is measured and a difference from an average is determined to determine the thickness of the layer.JP 2003-114 102 A discloses a method for measuring the thickness of an insulating layer applied to an electrically conductive substrate. An eddy current sensor is used to measure a first distance between the eddy current sensor and the electrically conductive layer. In addition, a distance sensor is used to measure a second distance between the distance sensor and the insulating layer. The layer thickness results from the difference between the first and the second distance.EP 4 053 495 B1 proposes a measuring device for measuring properties of a sheet metal which is processed in a rolling mill, wherein the measuring device comprises an inspection coil set which has a transmitting coil and a receiving coil. The transmitting coil is configured to apply a time-varying magnetic field to the sheet, and the receiving coil is configured to detect a magnetic field transient generated by the sheet in response to the applied time-varying magnetic field. The properties of the sheet metal can be derived from the magnetic field transient. The measuring device further comprises a correction coil set for detecting a spatial deviation of the sheet from a reference plane, each correction coil being connected to a capacitor to form a corresponding resonant circuit having a resonant frequency. The correction coils are designed to be tuned to the corresponding resonant frequency, wherein a shift of the resonant frequency is recognizable in the presence of the metal sheet. The spatial deviation can be derived from the shifts of the resonant frequencies.US 2006 / 0 167 650 A1 discloses a method for testing a connection pattern which is formed by depositing a metal on a substrate. The substrate has a connection pattern groove formed on its surface. The method comprises selectively measuring a thickness of a portion over the substrate of a metal film formed on the substrate, the portion over the substrate being made of the metal deposited substantially above the same surface as the surface of the substrate on which a connection pattern groove is formed. The method further includes evaluating how successfully the connection pattern groove is filled with the metal based on a film thickness value obtained by the selective measurement.US 2009 / 0 242 200 A1 discloses an apparatus and a method for testing a test object or in the vicinity of a further object, both made of electrically conductive material, wherein the apparatus comprises a transmitter / receiver arrangement for generating an electromagnetic field in the test object and for measuring a signal. The signal is indicative of a transient eddy current generated by the electromagnetic field in the test object. The apparatus also comprises a magnetic shielding device which provides for an at least partially magnetic shielding for the transmitter / receiver arrangement in a direction other than a measurement direction.EP 3 660 462 B1 discloses a device for measuring a physical parameter, which comprises a series circuit of at least one first electrical impedance element and one second electrical impedance element. The first electrical impedance element has a first terminal and a second terminal, wherein an impedance value of at least the first electrical impedance element depends on the physical parameter. The second electrical impedance member has a first terminal and a second terminal, and wherein the second terminal of the first electrical impedance member is connected to the first terminal of the second electrical impedance member and forms a first node. The apparatus further comprises a signal generator configured to apply a first amplitude modulated signal to the first terminal of the first electrical impedance member and a second amplitude modulated signal to the second terminal of the second electrical impedance member. The device further comprises an evaluation device configured to receive a first measurement signal from the first node and to determine information about the at least one physical parameter depending on the first measurement signal.US 2013 / 0 328 555 A1 describes a pulsed eddy current sensor for measuring a lifting distance over a surface of a metallic substrate (a metallic surface). The pulsed eddy current sensor comprises a primary excitation coil which is supplied with voltage pulses in order to generate primary magnetic fields. Interaction of the primary magnetic fields with the metallic substrate produces secondary magnetic fields. The eddy current sensor also includes secondary acquisition probes, each of the secondary acquisition probes being disposed at a different vertical distance from the metal surface. The secondary acquisition probes serve to measure the primary and secondary magnetic fields and to generate a difference signal representative of the lift-off distance.In summary, the measurement systems in question for characterizing multilayer structures having different ohmic properties can either be integrated into production processes only with great effort or offer only inadequate information about the structure to be characterized. At this point, a high integration outlay means in particular that the above-mentioned measurement methods must be installed in each case on two sides of the structure to be tested, for example therefore above and below an electrode to be measured. In addition, the measurement systems in question are, on principle, limited to measurement variables which do not allow extensive characterization of multilayer structures.It is therefore the object of the invention to propose means for characterizing multilayer structures having different ohmic properties, which are associated with a permanently high accuracy, can be integrated with little effort into the production of such structures, in particular battery electrodes, and make possible a comprehensive conclusion about the layer properties.The object is achieved by means of a measuring system according to claim 1, a sensor module according to claim 14, a manufacturing system according to claim 15 and a method according to claim 16.The measuring system according to the invention is suitable for characterizing a multilayer structure having ohmic properties that differ in layers. In particular, the structure can be a battery electrode. The measuring system has a sensor module, an electrical switching device and measuring electronics. The sensor module comprises more than two planar coils arranged in a coil stack. The electrical switching device is contacted with the planar coils and is configured to connect the planar coils to the measurement electronics in such a way that the planar coils can be switched between at least one measurement configuration and a reference configuration. In the measurement configuration, the planar coils at least partially form an optionally inductive or capacitive measurement sensor, the detection range of which extends outside the sensor module in order to determine a property of a low-resistance and / or high-resistance layer of the structure. In the reference configuration, the planar coils at least partially form an optional inductive or capacitive reference sensor, the sensing range of which extends substantially within the sensor module to determine a sensor module characteristic.One finding on which the invention is based is that the use of planar coils makes it possible, on the one hand, to design an electromagnetic sensor which can be operated as an inductive as well as capacitive sensor in order to be able to comprehensively characterize a multilayer structure having layer-by-layer different ohmic properties, it being possible for said structure to be arranged on only one side of the structure. As will be explained in detail further below, in particular a layer thickness of the high-resistance layer can be determined with high precision. Furthermore, the use of planar coils enables a reversal of the direction of action of such an electromagnetic sensor, so that its inductive or capacitive principle of action can also serve for ascertaining a sensor module property. As required, this allows the accuracy in the determination of the layer properties, in particular the layer thickness of the high-resistance layer, to be substantially increased.Within the scope of the invention, the planar coils can each be considered as sensor elements having a plurality of turns, which each extend in a coil plane. These properties of a planar coil make it possible to make it compact and flat, which makes it particularly suitable for applications in which the available installation space is limited. Furthermore, the planar coils are available comparatively favorably, so that in particular a plurality of sensor modules can be arranged in the manner of a sensor array in order to enable full-area characterization of the structure. In particular, it is conceivable to arrange such a sensor array above a moving structure, in particular a battery electrode, and to configure the sensor array, for example, as a line array which extends at a distance transversely to a direction of movement of the moving structure. As a result, in the measurement configuration, a full-surface quality test of the structure can be carried out without contact.A coil stack can be considered within the scope of the invention as a spatial arrangement of planar coils in which at least three planar coils are arranged parallel to one another, in particular spaced apart from one another, with respect to their respective coil planes. In particular, planar coils are arranged in such a way that their respective windings or their extension regions overlap in a viewing direction orthogonal to the coil planes. In particular, the planar coils each have a central axis which runs substantially orthogonally to their respective coil plane. Preferably, the at least three planar coils are arranged coaxially with respect to their central axes. In particular, the planar coils of the coil stack are of identical construction.Within the scope of the invention, the switching device can be considered an electrical and / or electronic component or an arrangement thereof, which is electrically contacted with the planar coils, in particular with the winding ends of the planar coils, and with the measurement electronics and is configured to establish, disconnect or change a signal and / or energy connection between the measurement electronics and at least a part of the planar coils, in particular their winding ends, as required. For example, the switching device may comprise an analog multiplexer and / or a field programmable analog array.Within the scope of the invention, the measurement electronics can likewise be regarded as an electrical and / or electronic component or an arrangement thereof, which supplies the sensor module with electrical energy and / or signals, in particular so that a measurement sensor can be formed in the measurement configuration and a reference sensor can be formed in the reference configuration. Furthermore, the electronic measurement system serves to process the electrical measurement and reference variables that can be detected by means of the sensor module and to output a measurement result. In particular, the measurement electronics are configured to output a layer property, in particular the layer thickness of the high-resistance layer, or a measurement variable dependent thereon.The configuration of the planar coils and their arrangement in the coil stack make it possible to operate the planar coils as inductive sensor elements and / or as capacitive sensor elements both in the measurement configuration and in the reference configuration. In particular, at least one of the planar coils can be designed as a screen element in order to generate, as required, an active direction reversal between the measurement configuration and the reference configuration.In particular, the switching device and the measurement electronics are configured to cooperate in such a way that one or more planar coils of the sensor module for the measurement configuration can be configured in each case optionally as an inductive transmitting coil, inductive receiving coil, capacitive measuring electrode or as a shielding element. Accordingly, the switching device and the measuring electronics are preferably designed such that one or more planar coils of the sensor module for the reference configuration can each be designed selectively as an inductive reference transmitting coil, inductive reference receiving coil, capacitive reference measuring electrode or as a shielding element.As mentioned above, in the measurement configuration, the sensor module forms a sensing region extending outside the sensor module. In particular, the detection region can be defined by a magnetic field and / or an electric field. In a multilayer structure arranged in the sensing region, the sensing region serves to interact with the high-resistance and / or low-resistance layer of the structure. The planar coils can be contacted with the measurement electronics by means of the switching device in such a way that some of the planar coils are supplied with electrical energy in order to serve as an active sensor element and another part of the planar coils serves as a passive sensor element in order to receive an inductive and / or capacitive measurement signal.In the measurement configuration, in which the sensor module forms an inductive measurement sensor, the detection region is in particular designed in such a way as to interact with the low-resistance layer of the structure, in particular a metallic layer of a battery electrode. In the measurement configuration, in which the sensor module forms a capacitive measurement sensor, the detection region is in particular designed in such a way as to interact with the high-resistance layer of the structure.As also mentioned above, the sensor module in the reference configuration also forms a sensing region. This can likewise be defined by a magnetic field and / or an electric field and extends substantially in the sensor module. In particular, in the reference configuration, at least one of the planar coils is connected to the measurement electronics by means of the switching device in such a way that it forms an electrical shield with respect to the sensor environment. In particular, this can be an external planar coil within the coil stack. The remaining planar coils can be contacted with the measurement electronics by means of the switching device in such a way that a part of the planar coils is supplied with electrical energy in order to serve as an active sensor element and another part of the planar coils serves as a passive sensor element in order to receive an inductive and / or capacitive reference measurement signal which makes it possible to draw a conclusion about a sensor module property.The active direction reversal of the sensor module that can be achieved according to the invention is surprising in particular with regard to the reference configuration in which the planar coils at least partially form a capacitive reference sensor. High-impedance sensor elements usually function as field conductors and distort capacitive measurement results. However, as studies by the applicant have shown, this negative effect is almost negligible for a capacitive reference measurement because of the naturally flat design of planar coils. As a result, it is possible, for example, to determine a distance between two external planar coils of the coil stack on the basis of a capacitive reference measured variable. This is a sensor module property that can be taken into account in the determination of the layer property.A particular advantage can be achieved with the invention if the sensor module is designed in the manner of a multilayer printed circuit board, in which the planar coils are each formed by a substantially planar, spiral-shaped copper track, preferably having a thickness of at most 10 micrometers, and the planar coils are separated from one another in pairs by means of an electrically insulating carrier material.In the advantageous development described above, the sensor module and its sensor elements are designed as components with a high degree of functional integration. This has the disadvantage that other components, for example temperature sensors, cannot be integrated into the sensor module without changing its compact design. At the same time, the carrier material is susceptible to temperature fluctuations and moisture, as a result of which its dimensions and thus also the relative arrangement of the planar coils with respect to one another change. However, by means of the above-described reversal of the direction of action, it is possible in the reference configuration in particular to determine a relative position between at least two of the at least three planar coils with respect to one another and to take it into account for the evaluation of the measurement signals in the measurement configuration. In this way, in particular a sensor drift can be compensated which can be caused by the above-mentioned temperature fluctuations and moisture. The sensor drift can be regarded as a systematic or random deviation of a measurement variable in the measurement configuration. Consequently, no further sensors, in particular temperature sensors or humidity sensors, are required in order to enable a high accuracy of the measurement system.In an advantageous development, for the purpose of forming the inductive measurement sensor, at least one planar coil in the manner of a transmitting coil can be supplied with a high-frequency alternating voltage by means of the measurement electronics. At least one of the other planar coils, preferably two other planar coils, are each provided in the manner of a receiving coil in order to record an inductive measurement variable in cooperation with the measurement electronics. The measuring electronics are configured to determine a distance between the sensor module and the low-impedance layer of the structure as a function of the inductive measured variable.In the measurement configuration in which the sensor module forms an inductive measurement sensor, the mode of operation of an eddy current sensor can be utilized, in which an alternating magnetic field generated by means of a transmitting coil interacts with a low-impedance layer of the structure and this interaction or a change thereof can be detected by means of at least one receiving coil. In particular, the planar coil configurable as a transmitting coil is arranged in a coil stack between at least two other planar coils, of which at least one, in particular both, can be configured as receiving coils. The measurement electronics are configured to determine a distance between the sensor module and the low-impedance layer of the structure as a function of the inductive measurement variable.The inductive measurement signal can be an alternating voltage induced in the receiving coil or a difference between the alternating voltages of two receiving coils. An amplitude of such an inductive measurement signal is dependent, in particular at frequencies above 1 megahertz, on the distance between the sensor module and the surface of the low-resistance layer, in particular a distance between the coil plane of the transmitting coil and the surface of the metallic substrate of a battery electrode. At low frequencies, the eddy currents penetrate the μm thin metal layer of the electrode, wherein here electrode thicknesses result in dependent amplitudes and phase shifts as disturbing. The flat design of planar coils makes it possible in particular to reduce disruptive thickness influences as a result of a frequency-dependent penetration of the alternating magnetic field into the low-resistance layer, with the result that high accuracy in the distance measurement can be achieved. At the same time, the high-resistance layer, in particular a graphite-containing layer, generates no or only a negligibly small extent of eddy currents, so that the latter can be penetrated and in the process has no influences on the distance measurement with respect to the low-resistance layer.In an advantageous development, for the purpose of forming the capacitive measurement sensor, at least one preferably external planar coil of the coil stack, in the manner of a measurement electrode, can be supplied with a low-frequency alternating voltage by means of the measurement electronics and is provided for the purpose of detecting a capacitive measurement variable in cooperation with the measurement electronics. At least one planar coil adjacent to the measuring electrode is provided in the manner of a shield electrode to shield the detection region from the sensor module. The measurement electronics are configured to determine a distance between the sensor module and the high-resistance layer of the structure as a function of the capacitive measured variable.In a measurement configuration in which the sensor module forms a capacitive measurement sensor, a planar coil serves as a capacitive measurement electrode, in particular in that only one winding end is connected to the measurement electronics by means of the switching device and the respective other winding end is connected to high impedance or is open. Expediently, in this measurement configuration, an external planar coil of the coil stack is formed in the manner of a measurement electrode and serves to form an electric field which interacts with the high-resistance layer of the structure, in particular the graphite layer of the battery electrode. Another planar coil of the coil stack, which is in particular adjacent thereto, can form the shield electrode in the above-mentioned manner, in particular by the shield electrode likewise being connected at only one winding end with respect to the potential of the measurement electrode to another reference potential, in particular to ground.In a simple form, the capacitive measurement signal can be an AC voltage which can be detected by means of the measurement electronics, in particular by means of a measurement circuit of the measurement electronics, and which is dependent on the interaction of the electric field with the high-impedance layer. An amplitude of such a capacitive measurement signal is dependent, in particular at frequencies below 50 kilohertz, on the distance between the sensor module and the surface of the high-resistance layer, in particular between the coil plane of the measurement electrode and the surface of a graphite-containing layer of a battery electrode.In an advantageous development, the measurement electronics are configured to determine a layer thickness of the high-resistance layer as a function of the difference between the capacitive measurement variable and the inductive measurement variable.The advantageous development described above is based on the finding that the successive performance of an inductive and capacitive measurement in the measurement configuration enables an accurate determination of the layer thickness of the high-resistance layer. In particular, this is possible by initially conclusions being drawn as a function of the inductive measurement variable about a distance between the sensor module and the surface of the low-resistance layer and subsequently as a function of the inductive measurement variable about a distance between the sensor module and the surface of the high-resistance layer. The layer thickness of the high-resistance layer can then be determined by determining a difference between the two distances.It is within the scope of the invention that the switching device is configured to set the measurement configuration first for forming the inductive measurement sensor and then for forming the capacitive measurement sensor, or vice versa. As a result, the planar coils of one and the same sensor module can be operated as at least two types of measurement sensors in order to characterize the multilayer structure with high precision.In an advantageous development, for forming the capacitive reference sensor, at least one preferably external planar coil of the coil stack in the manner of a shield electrode is provided for shielding the detection range of the capacitive measurement sensor from a sensor environment. A planar coil adjacent thereto can be supplied with a low-frequency alternating voltage by means of the measuring electronics in the manner of a reference measuring electrode and is provided for the purpose of detecting a capacitive reference measured variable in cooperation with the measuring electronics. The measuring electronics are configured to determine a relative position between the reference measuring electrode and another planar coil of the coil stack as a function of the capacitive reference measured variable.The development described above relates to an aspect of the reversal of the active direction already described above. By connecting a planar coil, in particular an external planar coil, as a shield electrode, another planar coil adjacent thereto can serve as an active sensor element in order to ascertain a sensor module property, in particular a distance with respect to another planar coil of the coil stack. This is particularly advantageous since the dimensions of the sensor module can fluctuate as a result of temperature fluctuations and moisture and this can affect the accuracy in the measurement configuration. In a simple embodiment, the ascertained sensor module property can be matched at least to a limit value in order to be able to ascertain whether the distance between two of its planar coils is in a tolerable state.Preferably, for the reference configuration, in which the planar coils at least partially form a capacitive reference sensor, the same planar coils are connected to the measurement electronics as those which serve for the configuration of the measurement electrode and the shield electrode in the measurement configuration. In contrast to this, however, the functions of the planar coils mentioned are interchanged. In other words, the planar coil of the capacitive measurement sensor serving as the measuring electrode functions as the shield electrode of the capacitive reference sensor, and the planar coil of the capacitive measurement sensor serving as the shield electrode functions as the reference measurement electrode of the capacitive reference sensor.The capacitive reference measurement signal can be in a simple form an alternating voltage which can be detected by means of the measurement electronics, in particular by means of a measurement circuit of the measurement electronics, and which is dependent on the interaction of the electric field which can be generated by means of the reference measurement electrode with one of the other planar coils.The electronic measuring system is preferably configured to determine a correction value as a function of the relative position between the reference measuring electrode and the other planar coil of the coil stack and to determine the layer thickness of the high-resistance layer as a function of the correction value.The advantageous development described above is not restricted to the way in which the correction value is taken into account in the determination of the layer thickness of the high-resistance layer. In a simple embodiment, a look-up table can be stored in the measuring electronics or data processing unit connected thereto, which look-up table specifies the correction value as a function of the capacitive reference measured variable and can be calculated indirectly or directly with the determined layer thickness. It is also within the scope of the advantageous development that, in addition or as an alternative to a look-up table, a mathematical model is stored which, for example, specifies an analytical and / or numerical and / or statistical and / or experimental relationship between the correction value and the determined capacitive measured variable and by means of which, in particular, the layer thickness of the high-resistance layer can be determined.In an advantageous development, to form an inductive reference sensor, at least one planar coil is short-circuited by means of the switching device of the coil stack and another planar coil can be supplied with a high-frequency alternating voltage by means of the measurement electronics in the manner of a reference transmitting coil. At least one planar coil lying between the short-circuited planar coil and the reference transmitting coil is provided in the manner of a reference receiving coil in order to record an inductive reference measured variable in cooperation with the measuring electronics. The measurement electronics are configured to determine a property of the interposed planar coil, in particular its functionality, as a function of the inductive reference measurement variable.An advantage of the above-described development is that a self-check of the sensor module can be carried out in that one of the planar coils defines a test object by means of a short circuit of its winding ends, which test object can interact with the alternating magnetic field of another planar coil which serves as a reference transmission coil and in this case in particular eddy currents can be generated in the short-circuited planar coil. The intermediate planar coil serves as a reference receiving coil and serves to sense such interactions. In particular, when specifying an amplitude and frequency of the alternating voltage with which the transmitting coil can be excited, it is possible to check by means of the electronic measurement system whether the inductive reference measured variable, in particular its amplitude and / or frequency, exceeds or falls below a specified limit value. As a result, for example, the functionality of said intermediate planar coil can be determined.In an advantageous development, the sensor module is mounted so as to be adjustable along the stacking axis. For this purpose, a displacement-controlled actuator can be provided, by means of which the sensor module is adjustably mounted. By means of an adjustment, the detection range of the inductive and / or capacitive measurement sensor can be adjusted, in particular in the measurement configuration, whereby overall a large measurement range of the measurement system is obtained.In an advantageous development, the sensor module comprises more than three planar coils which are arranged in the coil stack. In particular, these planar coils can be contacted with the measurement electronics by means of the switching device and, as described above, form, if necessary, an inductive measurement sensor or a capacitive measurement sensor or an inductive reference measurement sensor or a capacitive reference measurement sensor. The same explanations apply to the embodiments of the measuring system according to the invention explained above or to one of its advantageous developments accordingly.An advantage associated with the configuration of a sensor module comprising more than three planar coils is associated with the fact that planar coils which are not configured as a transmitting coil, receiving coil, measuring electrode, reference transmitting coil, reference receiving coil, reference measuring electrode can be used as shielding elements. This is advantageous in particular since, in the reference configuration, the sensor module property can be ascertained in the presence of the structure to be characterized, in particular a battery electrode, without this influencing the reference measurement or vice versa.In one conceivable embodiment, the sensor module has in particular five planar coils which are arranged in the coil stack.In the measurement configuration in which the sensor module has five planar coils, two external planar coils can be contacted with the measurement electronics in each case in the manner of an electrical shield in order to form an inductive measurement sensor, and the three remaining planar coils can be connected, in the manner described above, as a transmitting coil or two receiving coils.In the measurement configuration in which the sensor module has five planar coils, two external planar coils can be contacted with the measurement electronics in each case in the manner of a measurement electrode in order to form a capacitive measurement sensor, and at least their respectively adjacent planar coils can be contacted with the measurement electronics in the manner described above in each case in the manner of an electrical shield.In the reference configuration, in which the sensor module for planar coils has, one of the external planar coils can be contacted with the measurement electronics in the manner of a reference transmission coil and another external planar coil can be short-circuited at the winding ends in order to form the inductive reference sensor. The interposed planar coils can each form a reference receiver coil, which is contacted with the measurement electronics in particular in a temporally consecutive manner in order to each record an inductive reference measurement variable, so that a property of the planar coils, preferably their functional capabilities, can be determined in each case.In the reference configuration, in which the sensor module for planar coils has, for forming the capacitive reference sensor, two external planar coils can each be contacted with the measuring electronics in the manner of an electrical screen and at least their respectively adjacent planar coils can be contacted with the measuring electronics, in particular in a temporally consecutive manner as reference measuring electrodes, in order to determine the distance from one another, in particular by averaging two distances determined in a temporally consecutive manner.The switching device is preferably designed to contact a part of the planar coils from the coil stack to form the measurement sensor and / or to form the reference sensor with the measurement electronics. This is advantageous in particular if the sensor module comprises more than three planar coils, so that only some of them serve to form the respective sensors. In this way, in particular planar coils can be selected, as a result of which the detection range can be adjusted as required in the measurement configuration and / or the reference configuration, in particular displaced spatially along the stack axis.Such a spatial adjustment of the detection range is particularly advantageous for the configuration of the inductive measurement sensor. This is because, although the inductive measurement sensor with only three planar coils is in principle capable of forming a detection region with an extent of up to 10 mm parallel to the stack axis, its sensitivity reduces with increasing distance from the sensor module. Although this can be counteracted by amplifying the applied alternating voltage at the transmitting coil, the signal-to-noise ratio of the inductive measured variable typically also changes as a result. Therefore, the switching device can be configured to contact at least two different planar coils of the sensor module in a temporally consecutive manner in the manner of a transmitting coil and at least one other planar coil in the manner of a receiving coil with the measurement electronics. As a result of the displacement of the detection range that can be achieved as a result, the desired sensitivity can be achieved already at a distance of at least 10 micrometers between the two selected transmitting coils.A comparable advantage can be achieved for the configuration of the capacitive measurement sensor. In this case, the switching device can be configured to contact at least two different planar coil pairs of the sensor module with the measurement electronics in a temporally consecutive manner in the manner of a measurement electrode and an electrical shield, as a result of which the detection range can likewise be adjusted along the stack axis.The switching device is preferably configured to electrically connect at least two planar coils in series as required. This is conceivable in particular if more than three planar coils are arranged in the coil stack. One advantage which results is that the number of turns of one planar coil can be extended by the number of turns of at least one other planar coil. In this way, it is possible in particular to set the total number of turns of a transmitting coil and / or receiving coil of an inductive measurement sensor and / or of a reference transmitting coil and / or reference receiving coil of an inductive reference sensor in stages.It is advantageous if the switching device is designed to form two inductive measurement sensors in chronological succession in the measurement configuration, each of which comprises at least one transmitting coil whose total number of turns differs. This embodiment of the measurement configuration is based on the finding that low numbers of turns of a planar coil are advantageous in order to generate a high-frequency alternating magnetic field. While frequencies above 1 megahertz are advantageous in order to determine the distance between the sensor module and the surface of the low-resistance layer in the manner already described above, other properties can be determined below 1 megahertz at a comparatively higher number of turns, in particular those which can only be detected as a result of a higher penetration depth of the alternating magnetic field. It is likewise conceivable for at least two planar coils to be connected in series in order to form a receiving coil.In particular, it is conceivable for the switching device to be configured to electrically connect in series at least two planar coils of the coil stack which are separated from one another by at least one other planar coil. This results in particular in advantages if the two planar coils connected in series are contacted with the measurement electronics as a coherent receiving coil and the at least one other planar coil lying between them is intended to serve as the transmitting coil of an inductive measurement sensor. As a result, a parasitic capacitance of the series-connected planar coils can be reduced to a tolerable minimum and the resonant frequency can be changed as required.In a sensor module having more than three planar coils, the switching device is preferably configured to switch at least a part of the planar coils in an advantageous sequence. The sequence is determined here according to the position of the planar coils to be connected in the coil stack, wherein in particular a first group of planar coils and a second group of planar coils are separated from one another by at least one planar coil. The planar coils of the first and second groups may be arranged symmetrically with respect to the intermediate planar coil of the coil stack and are connected such that the planar coils of the same position in the first and second groups are connected in series to form coil pairs and these coil pairs are also connected in series to one another. Investigations have shown that parasitic capacitances can be effectively reduced as a result.In an advantageous development, for the purpose of forming the capacitive measurement sensor, an externally located planar coil of the coil stack, in the manner of a measurement electrode, can be supplied with alternating voltage in two low-frequency regions by means of the measurement electronics. This planar coil is provided in cooperation with the measuring electronics to record a first capacitive measurement variable in a first low-frequency range and to record a second capacitive measurement variable in the second low-frequency range. The measuring electronics is configured to determine a sheet resistance of the high-resistance layer as a function of an amplitude of the first measurement variable and a phase position of the second measurement variable. Expediently, a planar coil adjacent thereto can serve as a screen electrode.The above-described development is based on the discovery by the applicant that it is possible by means of the measuring system according to the invention to infer, in addition to the layer thickness of the high-resistance layer, further layer properties. In particular, this includes the determination of the sheet resistance of the high-resistance layer, since this permits a conclusion to be drawn as to its moisture and density.The latter are particularly relevant with the example of the battery electrode, since their production comprises a quality-relevant drying step and calendering, according to which the moisture or density must be present as desired.The ability to ascertain the moisture and the density of the high-resistance layer is based on the fact that they interact with the ohmic properties of the high-resistance layer and can be ascertained at different frequencies. In particular, at comparatively low frequencies, the resistive change of the high-resistance layer has a negligible influence on the phase position of a capacitive measurement variable. As a result, the distance between the sensor module and the surface of the high-resistance layer can be determined. At comparatively higher frequencies, in particular above 500 kilohertz, the phase position changes, however, as a function of the ohmic resistance, as a result of which this can be determined. Taking into account the layer thickness, which can be determined in the manner already described as a function of the inductive measured variable, the sheet resistance can be determined.For the purpose of conceptual delimitation, the frequencies of the first and second low-frequency ranges are both below a high-frequency range likewise mentioned above. In particular, a high frequency range is above about 1 megahertz.As mentioned above, the object of the invention is also achieved by a sensor module according to claim 14. With regard to the advantages and possible embodiments of the sensor module that can be achieved as a result, the statements regarding the measurement system according to the invention and its advantageous refinements apply accordingly.As likewise mentioned, the object of the invention is also achieved by a production system for a multilayer structure having ohmic properties that differ in layers, using a measurement system according to the invention or an advantageous refinement thereof.In particular, it is a manufacturing system for a battery electrode, which can have a low-resistance metallic layer and a high-resistance graphite layer applied thereon. Such a production plant can comprise, for example, conveying rollers or a comparable conveying means, by means of which the low-resistance layer is conveyed and in the process is provided with the high-resistance layer. The sensor module of the measurement system is arranged here such that the structure to be characterized at least partially enters its detection range. The measuring system preferably has a plurality of sensor modules which are arranged in the manner of a sensor array. The sensor modules are preferably arranged as a line array which extends substantially transversely to the conveying axis of the structure. The explanations regarding the measuring system according to the invention and its advantageous refinements also apply accordingly.If the production plant is used for producing a battery electrode, it is advantageous if at least one sensor module is arranged behind a calender and / or behind a drying device with respect to the conveying direction of the battery electrode.The object of the invention is also achieved by a method for characterizing a multilayer structure having ohmic properties that differ in layers, having the following method steps: A) providing a sensor module, a switching device and measurement electronics, wherein the sensor module comprises more than two planar coils which are arranged in a coil stack, and wherein the electrical switching device is contacted with the planar coils and is configured to connect the planar coils to the measurement electronics; B) driving the switching device such that the planar coils are brought into a measurement configuration, wherein the planar coils at least partially form an inductive and / or capacitive measurement sensor, the detection range of which extends substantially outside the sensor module and a property of a low-impedance and / or high-impedance layer of the structure is detected in the process; C) Driving the switching device, so that the planar coils are brought into a reference configuration, wherein the planar coils at least partially form a capacitive and / or inductive reference sensor, the detection range of which extends substantially within the sensor and a property of the sensor module is detected in the process.The method according to the invention is preferably carried out with a measurement system according to the invention or an advantageous development thereof. In particular, the measuring system according to the invention or an advantageous development thereof is suitable for carrying out the method according to the invention. With regard to the advantages achievable by the method and its possible embodiments, the statements regarding the measurement system according to the invention and its advantageous refinements apply accordingly.Advantages of the invention will be explained below with reference to exemplary embodiments and the figures.They show FIG. 1 shows a schematic side view a) of a first measurement system for characterizing a multilayer structure having five planar coils and a schematic plan view b) of a planar coil; FIG. 2 shows a schematic illustration a) of a measurement configuration in which the sensor module of the measurement system forms an inductive measurement sensor and a schematic illustration b) of a measurement configuration in which the sensor module of the measurement system forms a capacitive measurement sensor; FIG. 3 shows a schematic illustration a) of a reference configuration in which the sensor module of the measurement system forms an inductive reference sensor and a schematic illustration b) of a measurement configuration in which the sensor module of the measurement system forms a capacitive reference sensor; FIG. 4 shows a schematic side view of a second measurement system; FIG. 5 shows a schematic side view a) of the sensor module of the second measurement system for ascertaining a sheet resistance, measured variable profiles in Cartesian coordinates in view b) and as a pointer diagram in view c).Battery electrodes typically represent structures with layer-by-layer different ohmic properties. In this case, a metallic component which serves as a current collector is typically present as copper or aluminum foil and is provided on the surface with a graphite layer. The metallic component typically has a higher electrical conductivity than the graphite layer having a comparatively lower electrical conductivity. In relation to each other, this is therefore a low-resistance layer or a high-resistance layer of the battery electrode.The quality of the battery electrode is highly relevant to the quality of the battery cell in which it is used, wherein in particular the electrical properties play an important role. These depend in particular on the properties of the high-resistance layer. Means are explained below with which in particular the layer thickness and further properties of the high-resistance layer can be determined with high precision and with a low integration outlay during its production.FIG. 1 shows in view a) a measurement system 1, which comprises a sensor module 2, an electrical switching device 3 and measurement electronics (not shown). The sensor module 2 is designed as a printed circuit board (PCB), in which five planar coils 4, 5, 6, 7, 8 are arranged in a coil stack and define the sensor elements of the sensor module 2. The planar coils 4, 5, 6, 7, 8 are each present as a substantially planar, spiral-shaped copper track and are separated from one another in pairs by means of an electrically insulating carrier material 9. A plan view of such a planar coil 4, 5, 6, 7, 8 is shown by way of example with reference to planar coil 4 in view b) of FIG. 1. The planar coils 4, 5, 6, 7, 8 are substantially identical in construction and are present concentrically with respect to a common stacking axis which, according to view a) of FIG. 1, lies in the image plane.The planar coils 4, 5, 6, 7, 8 each have two winding ends K 1 and K 2, via which they are electrically contacted to the switching device 3, respectively. In the exemplary embodiment shown in FIG. 1, the switching device 3 is designed as a field programmable analog array. This is an integrated circuit which enables analog signals to be processed with a compact construction. For this purpose, control signals can be input via the control inputs A0, A1, A2, A3, as a function of which the analog inputs, which in the exemplary embodiment shown here are in contact with the respective winding ends K1, K2 of the planar coils 4, 5, 6, 7, 8, can be connected to one another as required and can be connected through to the analog outputs Gnd, E1, M, E2, S1, S2, Gnd, C1, C2, NC, NC, NC, to which the measurement electronics (not shown) are connected. This is explained with reference to examples in FIGS. 2 to 7 for better clarity.It is relevant that the switching device 3 is configured to switch at least a part of the planar coils 4, 5, 6, 7, 8 between at least one measurement configuration and a reference configuration. In the measurement configuration, the planar coils 4, 5, 6, 7, 8 at least partially form an inductive or capacitive measurement sensor, the detection range of which extends outside the sensor module 2. As a result, a property of a low-resistance layer N or high-resistance layer H of a structure to be characterized, for example a battery electrode B, can be determined. In the reference configuration, at least three of the planar coils 4, 5, 6, 7, 8 form an inductive and / or capacitive reference sensor, the detection range of which extends substantially within the sensor module 2 in order to determine a sensor module property.Although the sensor module 2 shown in view a) of FIG. 1 comprises five planar coils 4, 5, 6, 7, 8, it would be sufficient for realizing the measurement configuration and the reference configuration if only three planar coils, for example the planar coils 5, 6, 7, were present in the coil stack. Advantageous functions can be advantageously mapped with more than three planar coils in the aforementioned configurations, which are explained below by way of example.According to view a) of FIG. 2, the switching device 3 is set to contact the planar coils 4, 5, 6, 7, 8 with the measurement electronics for a measurement configuration, so that the planar coils 4, 5, 6, 7, 8 form an inductive measurement sensor. In this case, the planar coil 6 is contacted at its winding ends K 1, K 2 with the connections S 1 and S 2, respectively, in the manner of a transmitting coil, via which the measuring electronics can apply a high-frequency alternating voltage above 1 megahertz to the planar coil 6. The planar coils 5 and 7 are each contacted with the connections M and E2 and M and E1 respectively in the manner of a receiving coil with their respective winding ends K1, K2, via which the electronic measurement system can record an inductive measurement variable.As a result of the high-frequency alternating voltage, the planar coil 6 serving as transmitting coil generates an alternating magnetic field which induces eddy currents in the low-impedance layer N of a battery electrode B. An interaction between the alternating magnetic field and the eddy currents can be detected by the planar coils 7 and 8 serving as receiving coils and by the electronic measuring system. In a simple case, the inductive measured variable can be a measurable alternating voltage or variable dependent thereon, the amplitude of which indirectly indicates the distance d 1 between the sensor module 1, in particular the planar coil 6, and the surface of the low-impedance layer N. The high-resistance layer H does not generate eddy currents, so that these do not influence the inductive measured variable.In the exemplary embodiment shown here, the planar coils 4, 8 are firstly contacted with their respective winding ends K 1, K 2 to a reference potential Gnd of the measurement electronics or are connected to the connections C 2 and NC of the measurement electronics with a high impedance, so that they likewise have a negligible influence on the inductive measurement variable. Alternatively, the winding ends of the planar coils 4 and 9 could also be open instead of the connections C 2 and NC, respectively, in order to achieve a comparable effect.According to view b) of FIG. 2, the switching device 3 is set in such a way as to contact the planar coils 4, 5, 6, 7, 8 with the measurement electronics for a measurement configuration in such a way that a part of the planar coils 4, 5, 6, 7, 8 forms a capacitive measurement sensor. In this case, the planar coil 4 is in contact with the connection C 2 by one of its winding ends K 2 in the manner of a measuring electrode and is open by the other winding end K 2. By applying a low-frequency alternating voltage with respect to its environment, in particular below 50 kilohertz, the planar coil 4 forms a capacitive sensor element. An alternating field formed thereby interacts with the high-resistance layer H, that is to say the graphite layer of the battery electrode B. This can be determined by the measuring electronics at terminal C 2 as a capacitive measured variable, which can in particular likewise be present as a measurable alternating voltage or a variable dependent thereon, the amplitude of which indirectly indicates the distance d 2 between the sensor module 1, in particular planar coil 4, and the surface of the high-resistance layer H.The planar coil 5, which is adjacent to the planar coil 4 in the coil stack, serves as an electrical shield with respect to the sensor module 2, so that the other planar coils 6, 7, 8 and other sensor components have no or at least constant influence on the capacitive measurement variable. For this purpose, the planar coil 5 is contacted at one winding end K 1 to a reference potential Gnd of the measurement electronics and the other winding end K 2 is open.By setting up the inductive measurement sensor according to view a) of FIG. 1 and the capacitive measurement sensor according to view b) of FIG. 1 in chronological succession, the distances d 1 and d 2 can be determined on one and the same battery electrode. At least with knowledge of the relative position of the planar coils 4, 5, 6, 7, 8, a thickness d3of the high-resistance layer can be deduced by forming the difference between the distances d1and d2.In view a) of FIG. 3, a reference configuration of the measuring system 1 is shown, in which the switching device 3 is set in such a way as to contact the planar coils 4, 5, 6, 7, 8 with the measuring electronics in such a way that a capacitive reference sensor is formed. As a result, it is at least possible to determine a distance between the planar coils 5 and 7.The capacitive reference sensor according to view a) of FIG. 3 is formed by connecting the external planar coils 4 and 8 to the measurement electronics in the manner of an electrical shield. For this purpose, one winding end is in each case present at the reference potential Gnd of the measurement electronics, while the respective other winding end is open. The planar coils 5 and 7 adjacent thereto serve as reference measuring coils, by means of which a distance d4 from one another can be determined successively in time. For this purpose, one of the planar coils 5 or 7 can first be formed via the terminal C 2 or C 1 as a reference measurement electrode, by means of which an electric field is generated, which extends substantially within the sensor module 2. The distance d4 can be deduced by means of the interaction of this alternating electric field with the respective other planar coil 7 or 5, via a measurable capacitive reference measured variable.Such a reference measurement is advantageous because a relative position of the planar coils 5 and 7 can be determined with the distance d 4 and, as a result, at least approximately also the positions of the other planar coils 4, 6, 8, in particular if the sensor module 2 is configured symmetrically with respect to the positions of its planar coils 4, 5, 6, 7, 8.It is an advantage that the capacitive reference measurement can be carried out in the presence of the battery electrode B to be characterized without having a disturbing influence on the result of the capacitive reference measurement.In view b) of FIG. 3, a reference configuration of the measuring system 1 is shown, in which the switching device 3 is set in such a way as to contact the planar coils 4, 5, 6, 7, 8 with the measuring electronics in such a way that an inductive reference sensor is formed. As a result, it is at least possible to determine a functionality of the planar coils 5, 6 and 7.The inductive reference sensor according to view b) of FIG. 3 is formed in that the planar coil 9 is short-circuited by means of the switching device of the coil stack and a high-frequency alternating voltage is applied to the planar coil 4 by means of the measurement electronics in the manner of a reference transmitting coil. The planar coils 5, 6, 7 situated between the short-circuited planar coil 9 and the reference transmitting coil 4 are each provided in the manner of a reference receiving coil in order to record an inductive reference measured variable in cooperation with the measurement electronics. For this purpose, the planar coils 5, 6, 7 are connected to the connections E1 and E2 successively in time at their respective winding ends K1, K2. Depending on the induced voltages in the planar coils 5, 6, 7, their functionality can be deduced.FIG. 4 shows a second measuring system 1, which has a sensor module 2, a switching device 3 and measurement electronics (not shown). As explained with reference to FIGS. 1 to 3, the sensor module is designed in the manner of a printed circuit board. As explained with reference to the embodiments according to FIGS. 1 to 3, it is likewise possible by means of the second measurement system 1 according to FIG. 4 to set up a measurement configuration and a reference configuration, so that substantially the same explanations apply. As can be seen from FIG. 4, however, the sensor module 2 has more than five planar coils which are arranged in a carrier material 9.By means of the switching device 3, it is possible to contact a part or all of the planar coils 4, 5, 6, 7, 8, 10, 11, 12, 13 with the measurement electronics for forming a measurement sensor or reference sensor from the coil stack. In this case, it is possible, for example, to select individual planar coils 4, 5, 6, 7, 8, 10, 11, 12, 13 from the coil stack and only use these for the formation of the measurement sensor or the reference sensor or else to connect them in series in order to influence the sensor module properties.In the example shown in FIG. 4, the planar coils 4, 5, 6 can be connected in series by means of the switching device 3 and can be contacted with the connection S 1 via the winding end K 1 of the planar coil 6 and with the measurement electronics via the winding end K 2 of the planar coil 4. By applying current to the series connection of the planar coils 4, 5, 6, an alternating magnetic field can be generated in the manner already described above, which however, as a result of the increased number of turns, can lie, for example, in a different frequency range than in the measuring system 1 according to FIGS. 1 to 3.In a manner not shown here, the switching device can be configured to contact at least two different planar coils of the sensor module in the manner of a transmitting coil and at least one other planar coil in the manner of a receiving coil with the measurement electronics in a temporally consecutive manner. As a result of the displacement of the detection range achievable as a result, the sensitivity of such a measurement sensor can be set to a desired sensitivity range. A comparable advantage can be achieved for the configuration of the capacitive measurement sensor. In this case, the switching device can be configured to contact at least two different planar coil pairs of the sensor module with the measurement electronics in a temporally consecutive manner in the manner of a measurement electrode and an electrical shield.As an alternative to the interconnection shown in FIG. 4, it can furthermore be advantageous if the planar coils 4, 5, 6, 11, 12, 13 are connected in series in order to reduce parasitic capacitances of the sensor module. In particular, it is advantageous if the planar coils are interconnected in pairs, so that at least one other planar coil is arranged between the planar coils of a series-connected planar coil pair. On the basis of the example shown in FIG. 4, a series connection is conceivable in which the planar coils are connected in series in the sequence 4, 13, 5, 12, 6, 11 and in each case one winding end of the planar coils 4 and 11 is contacted with the measurement electronics for detecting the inductive measurement variable.In addition, the sensor module shown in FIG. 4 can likewise be interconnected in such a way that an external planar coil of the coil stack, in the manner of a measuring electrode, is supplied with alternating voltage in two low-frequency regions by means of the measuring electronics. In a first low-frequency range, a first capacitive measured variable can be detected in the manner already described, with which, as already described, a distance between the sensor module, in particular the planar coil serving as the measuring electrode, and the high-impedance layer can be determined. In the second low-frequency range, which covers higher frequencies than the first low-frequency range, a second capacitive measurement variable can be detected.Studies by the applicant have shown that the first capacitive measured variable has an amplitude in the first low-frequency range which is dependent on the distance between the sensor module and the surface of the high-resistance layer and its phase position is independent of the ohmic properties of the high-resistance layer. In contrast to this, the second capacitive measured variable has a phase position in the comparatively higher, second low-frequency range, which phase position is dependent on the ohmic properties of the high-impedance layer. It is a realization that a sheet resistance of the high-resistance layer can be deduced taking into account the phase position of the second capacitive measured variable.The appendix of view a) of FIG. 5 illustrates by way of model that an inductive measurement variable can be ascertained first in order to ascertain a distance d 1 between the sensor module 2 and the surface of the low-resistance layer N. Two capacitive measured variables can then be ascertained in two different low-frequency regions in order to draw conclusions about the sheet resistance of the high-resistance layer.View b) of FIG. 5 shows the curves of the amplitude and phase position as a function of the frequency when the distance d 2 between the sensor module fluctuates (upper diagram Δd 2) or when the ohmic properties of the high-resistance layer fluctuate (lower diagram ΔR). It can be seen from the amplitude profile in diagram Δd2 that it is dependent on the distance d2in the first low-frequency range f1. In comparison, it can be seen from the phase profile in diagram ΔR that a change in the ohmic properties in the second low-frequency range leads to a change in the phase position of the capacitive measurement signal. View c) of FIG. 5 illustrates the state of affairs shown in view b) in a pointer diagram.

Claims

Measurement system (1) for characterizing a multilayer structure having ohmic properties that differ in layers, in particular a battery electrode (B), having a sensor module (2), an electrical switching device (3) and measurement electronics, wherein the sensor module (2) comprises more than two planar coils (4, 5, 6, 7, 8) which are arranged in a coil stack, and wherein the electrical switching device (3) is contacted with the planar coils (4, 5, 6, 7, 8) and is configured to connect the planar coils (4, 5, 6, 7, 8) to the measurement electronics in such a way that the planar coils (4, 5, 6, 7, 8) can be switched between at least one measurement configuration and a reference configuration, wherein, in the measurement configuration, the planar coils (4, 5, 6, 7, 8) at least partially form an optionally inductive or capacitive measurement sensor, the detection range of which, extends outside the sensor module (2), in order to determine a property of a low-resistance layer (N) and / or high-resistance layer (H) of the structure and, in the reference configuration, the planar coils (4, 5, 6, 7, 8) at least partially form an optionally inductive or capacitive reference sensor, the detection range of which extends substantially within the sensor module (2) in order to determine a sensor module property.Measuring system (1) according to Claim 1, in which, in order to form the inductive measuring sensor, at least one of the planar coils (4, 5, 6, 7, 8) can be supplied with a high-frequency alternating voltage by means of the measuring electronics in the manner of a transmitting coil, and at least one of the other planar coils (4, 5, 6, 7, 8), preferably two other planar coils (4, 5, 6, 7, 8), are each provided in the manner of a receiving coil, in order to record an inductive measurement variable in cooperation with the measuring electronics, and wherein the measuring electronics are configured to determine a distance (d1) between the sensor module (2) and the low-impedance layer (N) of the structure as a function of the inductive measurement variable.Measuring system (1) according to one of the preceding claims, in which, in order to form the capacitive measuring sensor, at least one preferably external planar coil (4, 5, 6, 7, 8) of the coil stack can be supplied with a low-frequency alternating voltage by means of the measuring electronics in the manner of a measuring electrode and is provided in order to record a capacitive measured variable in cooperation with the measuring electronics and at least one adjacent planar coil (4, 5, 6, 7, 8) is provided in the manner of a shield electrode in order to shield the recording region from the sensor module (2), and wherein the measuring electronics are configured in order to determine a distance (d2) between the sensor module (2) and the high-impedance layer (H) of the structure as a function of the capacitive measured variable.Measuring system (1) according to claims 2 and 3, wherein the measuring electronics are configured to determine a layer thickness (d3) of the high-resistance layer (H) as a function of the difference between the capacitive measured variable and the inductive measured variable, in particular distances (d1 and d2) respectively derived therefrom.Measuring system (1) according to one of the preceding claims, in which, in order to form the capacitive reference sensor, at least one preferably external planar coil (4, 5, 6, 7, 8) of the coil stack in the manner of a shield electrode is provided to shield the detection range of the capacitive measuring sensor from a sensor environment, and wherein a planar coil (4, 5, 6, 7, 8) adjacent thereto can be supplied with a low-frequency alternating voltage in the manner of a reference measuring electrode by means of the measuring electronics and is provided to detect a capacitive reference measurement variable in cooperation with the measuring electronics and the measuring electronics is configured to determine a relative position between the reference measuring electrode and another planar coil (4, 5, 6, 7, 8) of the coil stack as a function of the capacitive reference measurement variable.Measuring system (1) according to claims 4 and 5, wherein the measuring electronics are configured to determine a correction value depending on the relative position between the reference measuring electrode and the other planar coil (4, 5, 6, 7, 8) of the coil stack and to determine the layer thickness (d3) of the high-resistance layer (H) depending on the correction value.Measuring system (1) according to one of the preceding claims, in which, in order to form an inductive reference sensor, at least one planar coil (4, 5, 6, 7, 8) is short-circuited by means of the switching device (3) of the coil stack and another planar coil (4, 5, 6, 7, 8) can be supplied with a high-frequency alternating voltage by means of the measuring electronics in the manner of a reference transmitting coil, and at least one planar coil (4, 5, 6, 7, 8) lying between the short-circuited planar coil (4, 5, 6, 7, 8) and the reference transmitting coil (4, 5, 6, 7, 8) in the manner of a reference receiving coil is provided for the purpose of detecting an inductive reference measured variable in cooperation with the measuring electronics, and the measuring electronics are configured to determine a property of the intermediate planar coil (4, 5, 6, 7, 8), in particular its functionality, as a function of the inductive reference measured variable.Measuring system (1) according to one of the preceding claims, in which the sensor module (2) is mounted so as to be adjustable along the stacking axis.Measuring system (1) according to one of the preceding claims, in which the sensor module (2) comprises more than three planar coils (4, 5, 6, 7, 8) which are arranged in the coil stack and the switching device (3) is designed to contact at least some of the planar coils (4, 5, 6, 7, 8) from the coil stack with the measurement electronics for forming the measurement sensor and / or for forming the reference sensor.Measuring system (1) according to one of the preceding claims, in which the switching device (3) is configured to electrically connect at least two planar coils (4, 5, 6, 7, 8) of the coil stack in series, in particular at least two planar coils (4, 5, 6, 7, 8) which are separated from one another by at least one other planar coil (4, 5, 6, 7, 8).Measuring system (1) according to one of the preceding claims, in which the switching device (3) comprises an analog multiplexer and / or a field programmable analog array.Measuring system (1) according to one of the preceding claims, in which the sensor module (2) is designed in the manner of a multilayer printed circuit board, in which the planar coils (4, 5, 6, 7, 8) are each formed by a substantially planar, spiral-shaped copper track, preferably having a thickness of at most 10 micrometers, and the planar coils (4, 5, 6, 7, 8) are separated from one another in pairs by means of an electrically insulating carrier material (9).Measuring system (1) according to one of the preceding claims, in which, in order to form the capacitive measuring sensor, an preferably external planar coil (4, 5, 6, 7, 8) of the coil stack in the manner of a measuring electrode can be supplied with alternating voltage by means of the measuring electronics in two different low-frequency regions (f1, f2) and is provided, in cooperation with the measuring electronics, to record a first capacitive measurement variable in a first low-frequency region (f1) and to record a second capacitive measurement variable in the second low-frequency region (f2), and wherein the measuring electronics are configured to determine a sheet resistance of the high-resistance layer (H) as a function of an amplitude of the first measurement variable and a phase position of the second measurement variable and in particular to determine a moisture and / or density of the high-resistance layer (H).Sensor module (2) for a measurement system (1) according to one of the preceding claims, having more than two planar coils (4, 5, 6, 7, 8) which are arranged in a coil stack.Production installation for a multilayer structure having ohmic properties which differ in layers, in particular a battery electrode (B), having a measurement system (1) according to one of the preceding claims.Method for characterizing a multilayer structure having ohmic properties that differ in layers, having the following method steps: A) providing a sensor module (2), a switching device (3) and measurement electronics, wherein the sensor module (2) comprises more than two planar coils (4, 5, 6, 7, 8) that are arranged in a coil stack, and wherein the electrical switching device (3) is contacted with the planar coils (4, 5, 6, 7, 8) and is configured to connect the planar coils (4, 5, 6, 7, 8) to the measurement electronics; B) driving the switching device (3) so that the planar coils (4, 5, 6, 7, 8) are brought into a measurement configuration, wherein the three planar coils (4, 5, 6, 7, 8) form an inductive and / or capacitive measurement sensor, the detection range of which extends substantially outside the sensor module (2) and in the process detects a property of a low-resistance layer (N) and / or high-resistance layer (H) of the structure; C) driving the switching device (3) so that the planar coils (4, 5, 6, 7, 8) are brought into a reference configuration, wherein the three planar coils (4, 5, 6, 7, 8) form a capacitive and / or inductive reference sensor, the detection range of which extends substantially within the sensor module (2) and in the process detects a property of the sensor module (2).

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