Quality assurance in the production of stacks from solid oxide cells

Abstract

The invention relates to a solution for quality assurance in the production of stacks (1) from solid oxide cells SOxC (21, 22, ... 2 n ), which are joined together in a specially designed oven (7) in a joining process under the influence of temperature and force using a solder inserted between them to form the stack (1). Each stack (1) is monitored throughout the entire joining process for undesirable deformations occurring in its geometry by continuously repeating a measurement cycle at a plurality of points distributed on the outer surfaces of the stack, at several SOxC (21, 22, ... 2). n ) of the stack (1) measuring points arranged in pairs and spaced apart from each other (51, 52, ... 5 n ) the impedance is measured using multichannel impedance spectroscopy (EIS). From the measurements at the measuring points (51, 52, ... 5 nThe cell geometry of the cells with measuring points (51, 52, ... 5) is determined by means of a program application processed by a control and processing unit SVE of the furnace (7) in each of the measured impedances in the individual measurement cycles. n ) equipped SOxC (21, 22, ... 2 n ) descriptive geometric quantities are calculated, which are used to control the joining process and / or to derive quality statements about the generated stack (1).

Description

[0001] The invention relates to a solution for ensuring quality in the production of stacks of solid oxide SOxC cells. More specifically, it relates to stacks of high-temperature SOxC. Such stacks are manufactured in a special joining process under the influence of temperature and force on several stacked solid oxide cells. Using a solder, typically a glass solder, that is introduced between the cells and melted during the joining process. With the aid of the invention, the effects of the force applied during the joining process and the associated deformation phenomena occurring on the cells can be taken into account with regard to the service life of the cells or the stack assembled from them. The invention relates to a method and a system suitably designed for carrying out this method.

[0002] As already explained, the joining process used to manufacture high-temperature SOxC stacks requires the application of force to the cells forming the stack. The force required for the joining process is applied using at least one push rod, via which the force is applied to a central region of the top and / or bottom cell of the stack. Studies have shown that such a force not only ensures a secure connection of the cells using the solder applied, but can also lead to undesirable deformation of the cells. Such deformations, which occur in the cells of the stack and thus also in relation to the stack geometry, can in turn lead to degradation phenomena, i.e., premature aging of the cells, during later use of the produced stacks.This ultimately has a detrimental impact on the service life of the stacks. In the worst case, such undesirable deformations of the cell and stack geometry can even lead to such a reduction in quality that a stack either fails to function at all or is classified as scrap due to poor performance parameters and is therefore unusable.

[0003] It is known from the state of the art to use impedance spectroscopy to monitor and control the manufacturing process for SOxC solid oxide cell stacks and to evaluate the quality of intermediate products or stacks. For example, DE 10 2021 134 334 A1 discloses designing the joining process as an impedance-controlled process and efficiently controlling the duration of individual process phases through repeated impedance measurements while maintaining high quality.

[0004] The process for manufacturing stacks of solid oxide cells described in DE 10 2006 056 986 A1 mentions the use of impedance spectroscopy for quality assessment. Accordingly, after an initial stage of the joining process known as the pre-sealing process, in which a partially finished stack, referred to as a pseudostack, is first created, an intermediate inspection, for example, using impedance spectroscopy, is carried out. If none of the cells in the pseudostack are defective, or at least all cells are functional, the additional connections required to form the finished stack are created.

[0005] Furthermore, due to the inevitability of force being applied when joining stacks, deformations of the cells and stack cannot be completely avoided. However, manufacturers should strive to keep such deformations as low as possible, or at least take them into account appropriately in their quality management.

[0006] The object of the invention is therefore to provide a solution for quality assurance in the production of stacks of solid oxide cells produced in a joining process under the influence of temperature and force, with the aid of which the quality requirements of the users of such stacks can be appropriately met, despite any deformations that may occur in the cells and the stack geometry during the joining process. For this purpose, a method and a system suitable for implementing this method are to be specified.

[0007] The problem is solved by a method having the features of patent claim 1. A system that solves the problem and is suitable for carrying out the method is characterized by the first claim.

[0008] Also according to the method proposed to solve the problem, the production of stacks of solid oxide SOxC cells, as already known from the prior art, takes place in a furnace designed for this purpose in a joining process under the influence of temperature and force using a solder, preferably a glass solder, introduced between the cells. The force is applied by means of at least one push rod exerting pressure on a middle, or central, region of the uppermost and / or lowermost SOxC of the stack. The SOxCs of a stack are typically cuboid-shaped, with a relatively low height extending in the stacking direction of the stack and a square or rectangular cross-sectional area with edge lengths (circumference lengths) that are many times greater than the height.

[0009] According to the method, the stack is continuously monitored throughout the entire joining process for any undesirable deformations occurring in its geometry. This is achieved by measuring the impedance at a number of measuring points distributed along the stack's outer sides using multi-channel impedance spectroscopy (EIS) in a measurement cycle that is continuously repeated during the joining process. The aforementioned measuring points are distributed along the outer sides of the stack in such a way that they are arranged in pairs and spaced apart from each other on several SOxCs of the stack.

[0010] In this respect, an even number of measuring points are arranged on an SOxC of the stack to be provided with measuring points, whereby the measuring points of a pair of measuring points are preferably (but not necessarily) formed on opposite outer sides (on opposite sides of the respective cell on the circumference of the stack). However, particularly in the case of elongated cells, a pair of measuring points can also be arranged at the ends (each measuring point at one end) of an outer longitudinal side - and thus spaced from each other. Through the continuously repeated impedance measurement, a temporal progression of the impedance, which inevitably changes during the joining process, is recorded for each of the measuring points of the stack, starting from a starting value measured for the impedance during the first measurement process.

[0011] The impedances measured at the measuring points during the individual measuring cycles are further processed by a control and processing unit of the furnace (hereinafter referred to as SVE) according to a physical-mathematical model. During the programmatic processing of the measured values, the SVE calculates geometric variables that describe the cell geometry of the SOxC of the stack provided with a measuring point and thus ultimately also the stack geometry as a whole. The calculated geometric variables are used directly to control the joining process to minimize unwanted deformation of the cells and thus of the stack and / or, based on their progression during the joining process, to derive quality statements for each produced stack.

[0012] The proposed method assumes, as experiments have shown, that there are relationships between the changes in the geometry of SOxCs (cells of the stack) that inevitably occur during the joining process and the similarly changing impedance of the cells forming the stack. These relationships can be described by a physical-mathematical model. In the best case, corresponding changes in impedance with respect to the individual measurement points defined on the cells of the stack should show approximately the same temporal progression, primarily caused by shrinkage processes.

[0013] If this is not the case, it can be attributed to changes in geometric variables describing the SOxC, which, when viewed in relation to one another, reveal undesirable deformations of the cells provided with measurement points (SOxC of the stack) and thus also of the stack geometry as a whole. These are not geometric changes that typically occur during the joining process, such as shrinkage, but rather geometric changes that cause the stack to deform to some extent, for example, causing the stack to become tilted in the stacking direction.The knowledge gained in this respect regarding the course of the changing geometry of the cells and the stack, or rather the calculated values, can be used in different ways, depending on the implementation of the process and the equipment of the system, with the SVE and the furnace which serves to temper the stack during the joining process and has a push rod for applying force to the stack.

[0014] In accordance with a lower demand on the production equipment or the system, the process is implemented using geometric variables calculated after each measurement process, which describe the cell geometry and thus also the stack geometry, as well as the temporal progressions resulting from these during the joining process, are used to classify stacks manufactured using the process into different quality classes. Stacks for which very severe, uneven deformations were detected during the joining process are assigned to a lower quality class. This lower quality can, for example, have a negative impact on the service life of a stack. Furthermore, such stacks are expected to be less efficient in practical use due to rising operating costs caused by the lower quality.

[0015] Therefore, if the stacks are sold with a corresponding quality class, they can only command a lower purchase price due to their potentially shorter service life and typically lower efficiency. In contrast, stacks that deform very evenly during the joining process, namely almost exclusively due to the unavoidable shrinkage processes, are assigned a higher quality class, which can typically be linked to an expected long service life, higher efficiency, and—in the event of a sale—a higher selling price.

[0016] In connection with this form of implementation, or rather, the design of the method, it should be noted that for a reliable classification of the stacks, the actual course of the geometric parameters describing the cell geometry recorded during the joining process should be used, and not just their actual state at the end of the joining process. This is due to the fact that deformations of a cell that occur during the joining process, possibly even significant temporary ones, may be compensated for in other phases of the process, but can still cause damage that impairs the service life of the cell and thus the stack.

[0017] According to a preferred design of the method, the information obtained from the EIS regarding the temporal progression of the geometric conditions on the stack during the joining process is used to directly influence the joining process and thereby keep the extent of undesired deformations as low as possible. In this design of the method, the geometric variables calculated after each of the cyclical measuring processes throughout the entire joining process, which describe the geometry of the cells provided with the measuring points and thus also of the stack, are incorporated as actual variables into a control function for adjusting the alignment of the push rod. In this case, the angle assumed by the push rod with respect to an imaginary straight line running perpendicular through the stack in the stacking direction is slightly corrected in accordance with the detected undesired deformations.

[0018] During the joining process, any undesirable deformations detected are immediately responded to and continuously counteracted by continuously correcting the angle of the push rod relative to the vertical in the stacking direction of the stack. This ensures that stacks manufactured according to this implementation of the process deform very evenly throughout the entire joining process, largely only in a manner caused by the shrinkage that occurs.

[0019] Nevertheless, even with this process design, undesirable changes in the cell geometry almost inevitably occur, at least briefly, which can impact the service life of a particular cell even if they are essentially counteracted immediately by correcting the alignment of the push rod. Therefore, according to a similarly possible implementation of the process, both of the previously described process designs can also be implemented cumulatively. This means that, on the one hand, the alignment of the push rod is continuously corrected during the joining process; on the other hand, the resulting stacks are classified according to their quality after completion of the joining process, taking into account any undesirable deformations of their cells or stack geometry that may occur during the joining process.

[0020] As stated above, to carry out the method, several measuring points are distributed across the outer surfaces of the stack and its constituent SOxCs, and arranged in pairs on the relevant cells. To obtain the most accurate information about stack deformation, the corresponding measuring points should, of course, be distributed approximately evenly across the outer surfaces of the stack. As long as this is ensured, it is simply a matter of the physical-mathematical model stored in the program application processed by the control unit to correctly calculate the geometric quantities required for a well-founded statement about the temporal progression of the stack geometry using the impedances measured at these measuring points and the resulting temporal progression of the impedance for these measuring points.

[0021] The model used for the calculation can be obtained through suitable experiments. For the practical implementation of the method, it has proven particularly advantageous to arrange at least two equally spaced, preferably opposite, measuring points on several SOxCs (i.e., if appropriate, or preferably, on all SOxCs) of the stack. These measuring points are each connected to a channel of an impedance measuring device used for impedance spectroscopy (EIS).It has proven particularly useful to arrange at least four measuring points on each regularly cuboid-shaped SOxC of the stack, each provided with measuring points, in such a way that at least two measuring points are arranged on opposite circumferential sides of the SOxC and at least two further measuring points are arranged on the two other circumferential sides of the respective SOxC, which extend orthogonally to them and are also opposite one another. If one considers the stacking direction of the stack as the Z-axis in an imaginary coordinate system placed in the stack, one obtains information for the individual SOxCs provided with measuring points regarding the deformations occurring on them during the joining process along the X-axis and along the Y-axis with respect to the Z-direction.

[0022] In the context of conducting tests to determine the physical-mathematical model to be used for the respective procedure, it has proven advantageous to appropriately limit the spectrum of frequencies used for the EIS. It has been shown that the correlation between the change in the geometric dimensions of the stack and the measured impedances is particularly evident in certain frequency ranges of the EIS measurement spectrum.To achieve high efficiency, particularly in connection with the control aimed at continuously correcting the alignment of the push rod, it is possible to shorten the duration of the individual measurement processes by limiting the frequency range to a specific frequency window, thereby performing a larger number of measurement processes and calculations of changes to the stack geometry throughout the entire joining process. The frequency range to be advantageously selected in each case depends, among other things, on the respective type of stack cells and on the basis of the relevant test prior to the implementation of the process and the configuration, i.e., it can be determined based on the system setup.

[0023] A system suitable for implementing the method, i.e., a system for a joining process involving temperature and force for producing a stack of solid oxide cells (SOxC), comprises a furnace for tempering the stack, at least one push rod exerting pressure on a central region of the uppermost and / or lowermost SOxC cells of the stack located in the furnace during the joining process, and a control and processing device (SEV). The system is also equipped with a multi-channel impedance measuring device for impedance spectroscopy (EIS). During the joining process, several measuring channels of this impedance measuring device are connected to the stack.The connection of these multiple measuring channels to the stack is such that each of the measuring channels is connected to one of several measuring points distributed on the outer sides of the stack on several SOxCs of the stack, each arranged in pairs and spaced apart from each other.

[0024] The SEV is designed to - to control the impedance measuring device to measure the impedance at all measuring points arranged on the stack in a measuring cycle that is constantly repeated during this joining process, - to calculate from the measured values ​​of the impedance measuring device the corresponding geometric quantities describing the cell geometry of the SOxC provided with measuring points and thus the stack geometry, - to control the joining process using the calculated geometric parameters with a view to minimizing undesirable deformations of SOxC and thus of the stack as a whole and / or to derive quality statements about the produced stack from the temporal progression of these geometric parameters.

[0025] According to a first concrete embodiment of the system characterized above with regard to its basic structure, its SEM is further configured to assign a quality class in a manufactured stack in an automated process after completion of the joining process, depending on the temporal profiles determined for the geometric parameters based on its repeated calculations. The assignment of the respective quality class by the SEM is based on a scheme stored in the SEM. Just as described for the method regarding the relationship between the measured impedances and the geometric parameters of an SOxC, the aforementioned classification scheme is determined based on the results of corresponding tests.Such tests can, for example, include artificial aging processes, which can be used to identify relationships between deformations occurring on the SOxC of the stack and thus on the stack geometry during the joining process and the expected service life of a respective stack (determined in the course of artificial aging induced on individual stacks for test purposes).

[0026] According to a particularly preferred embodiment of the system, its SEV is configured, alternatively to the last-described embodiment or in addition, to realign the push rod, by controlling an actuator, with respect to the angle of its longitudinal axis to a vertical in the stacking direction of the stack after each measurement cycle carried out using the impedance measuring device based on the principle of impedance spectroscopy (EIS). The SEV is designed such that, to adjust the aforementioned angle, i.e., to align the push rod, it executes a control function into which the geometric variables calculated from the measured values ​​of a respective measurement cycle are incorporated by the SEV as actual values.

[0027] Some aspects of the invention will be explained below in the form of an exemplary embodiment using drawings. The drawings show in detail: Fig. 1: the schematic representation of a possible embodiment of the system according to the invention, Fig. 2: an example of possible deformations occurring on a stack during the joining process in a schematic representation, Fig. 3: a top view of a SOxC of the stack with measuring points arranged on it, Fig. 4: An example of impedances measured at different measuring points of a SOxC.

[0028] The Fig. Figure 1 shows a possible embodiment of the system according to the invention, including a roughly schematic representation of a stack 1 manufactured by means of the system in a joining process. Key components of the system are a furnace 7 for tempering a stack 1 located therein during the joining process, consisting of SOxC 21, 22, ... 2 arranged between an upper and a lower end plate. nand a push rod 3 with an actuator 6 for its actuation and alignment. During the joining process, pressure is exerted by the push rod 3 on a central region of the upper end plate and thus on the uppermost SOxC 21 of the stack 1, which is tempered in the furnace 7. To implement a defined heating regime, the furnace 7 has several heating elements 10 arranged to form heating zones and cooling tubes 11 arranged around the stack.

[0029] The system also includes an impedance measuring device (EIS) operating on the principle of impedance spectroscopy and a control and processing device (SEV), which are not shown in detail in the drawing. Regarding the impedance measuring device, the Fig. 2 rather only symbolically their individual, each with a measuring point 51, 52, ... arranged on the stack 1, or on a SOxC 5 n connected measuring channels 41, 42, ... 4 nIn a joining process lasting several hours or even several days, the stack 1 is joined by means of the push rod 3 under the influence of temperature according to a predetermined temperature sequence and with the continuous application of force.

[0030] During the entire joining process, stack 1 is monitored with regard to the changing geometry of its SOxC 21, 22, ... 2 n However, the monitoring is not carried out using optical sensors or the like, but rather by using impedance spectroscopy EIS and further processing of the measured values ​​obtained by this to geometric, the cell geometry of the measuring points 51, 52, ... 5 n SOxC 21, 22, ... 2 ndescriptive quantities. It should be noted that any undesired deformations of the stack 1 that may occur are typically so slight that they could hardly be detected by an optical system. In addition, the arrangement of such sensors or other sensors, such as strain gauges and the like, on the stack 1 is likely to be associated with a number of difficulties. This is especially true since very high temperatures prevail in the furnace 7 in which the joining process is carried out. Corresponding sensors would therefore also have to be highly temperature-resistant. Finally, it should also be pointed out that processes that occur in the volume of the cells (SOxC 21, 22, ... 2 n ) processes are essentially impossible to detect with sensors of the aforementioned type arranged on the outside of the stack.

[0031] The Fig. Figure 2 shows, by way of example and also in a highly schematic representation, possible configurations of a stack 1 and its cells (SOxC 21, 22, ... 2 n ) geometric deformations occurring during the joining process. The geometric conditions that occur at any time on the stack 1 according to the corresponding example are shown in the drawing without the furnace 7 surrounding the stack 1. As indicated in the drawing, the deformations on the cells (SOxC 21, 22, ... 2 n ) of the stack 1 and thus deformations occurring in the stack geometry as a whole due to misalignments of the push rod 3, i.e. deviations of its longitudinal axis 9 at an angle α from the perpendicular 8 of the stacking direction of the stack 1.

[0032] In the drawing, however, these deviations, or rather the angle α, are greatly exaggerated for clarity. In reality, these deviations are so small that they are barely perceptible by optical means, but they nevertheless lead to deformations of the SOxC 21, 22, ... 2 that are also barely perceptible visually, but which impair the quality and possibly the service life of the stack 1. n of the stack 1. Therefore, the stack 1 is monitored for the occurrence of such undesirable deformations throughout the entire joining process. This is done by continuously repeating measurement cycles on a plurality of sensors arranged on the stack 1, each with a channel 41, 42, ... 4 n an impedance measuring device operating according to the EIS principle connected to measuring points 51, 52, ... 5 nThe impedance is measured in each case, and geometric quantities expressing the deformations are calculated from the impedance measurements. Depending on the deformations determined, the alignment of the push rod 3 is continuously corrected using a control function and by actuating the actuator 6.

[0033] In the example shown, each SOxC 21, 22, ... 2 n on two opposite circumferential sides (outer sides) a pair of measuring points 51, 52, ... 5 n arranged each of which is connected to a channel 41, 42, ... 4 n an impedance measuring device (not shown) operating according to the EIS principle. The impedance measured values ​​obtained from these devices allow statements to be made about the deformations occurring in a particular cell in the X-direction. Preferably, Fig. 2 not shown - in addition, on the visible outer longitudinal side extending in the plane of the drawing and on the opposite (not visible) outer longitudinal side, also one each, with a channel 41, 42, ... 4 n measuring point 51, 52, ... 5 connected to the impedance measuring device n be arranged, with these measuring points 51, 52, ... 5 n which are located at a respective cell 21, 22, ... 2 n deformations occurring in the Y-direction can be determined.

[0034] The latter is suggested to be in the (in relation to the Fig. 2 not to scale) Fig. 3 in a plan view of a SOxC 21 of the stack 1 (otherwise not shown, as well as the furnace and push rod), wherein here the measuring points 51, 52 and the (measuring) channels 41, 42 connecting them to the impedance measuring device for detecting deformations in the Y direction are only indicated by a symbol and are not provided with a reference symbol.

[0035] The Fig. 4 shows examples of the different measuring points 51, 52, ... 5 n different impedances Z measured at a specific time during the joining process with respect to the X-direction. In the Fig. In the diagram shown in Figure 4, Z, unlike X, does not represent the corresponding direction in space, but rather denotes the impedance.

Claims

[1] Quality assurance procedures for the production of stacks (1) from solid oxide cells SOxC 21, 22, ... 2 n which are joined together to form the stack (1) in a furnace (7) designed for this purpose in a joining process under the influence of temperature and force by means of a solder introduced between them, wherein the force is applied by means of at least one push rod (3) exerting pressure on a central region of the uppermost and / or lowermost SOxC of the stack (1), characterized by that a respective stack (1) is monitored during the entire joining process with regard to undesired deformations occurring in its geometry by measuring a plurality of SOxCs (21, 22, ... 2) distributed on the outer sides of the stack in a measuring cycle that is constantly repeated during the joining process. n ) of the stack (1) each arranged in pairs and spaced from each other measuring points (51, 52, ... 5 n) using a multi-channel impedance spectroscopy EIS, the impedance was measured and thus for each of the measuring points (51, 52, ... 5 n ), starting from a value measured in the first measuring cycle at the respective measuring point (51, 52, ... 5 n ) for the impedance measured start value, a temporal progression of the impedance changing during the joining process is recorded and from the values measured at the measuring points (51, 52, ... 5 n ) in the individual measuring cycles, the cell geometry of the measuring points (51, 52, ... 5 n ) SOxC (21, 22, ... 2 n ) are calculated and that these calculated geometrical quantities are directly used to control the joining process to minimize undesirable deformations of SOxC (21, 22, ... 2 n) and thus of the stack (1) and / or from the temporal progressions determined for these geometrical quantities by repeated calculation during the joining process, quality statements for the stack (1) produced are derived. [2] Method according to claim 1, characterized by that at all SOxC (21, 22, ... 2 n ) of the stack (1) at least one pair of measuring points (51, 52, ... 5 n ) and each of these measuring points (51, 52, ... 5 n ) is connected exclusively to one channel of an impedance measuring device used for impedance spectroscopy EIS. [3] Method according to claim 1 or 2, characterized by that at a respective measuring point (51, 52, ... 5 n ) provided with cuboidal SOxC (21, 22, ... 2 n ) of the stack (1) at least four measuring points (51, 52, ... 5 n) and are connected to a respective channel of the impedance measuring device, wherein at least one pair of measuring points (51, 52, ... 5 n ) on opposite circumferential sides of the SOxC (21, 22, ... 2 n ) and at least one further pair on the two other, orthogonally extending and also mutually opposite circumferential sides of the respective SOxC (21, 22, ... 2 n ) is arranged. [4] Method according to one of claims 1 to 3, characterized by that the stack (1), depending on the temporal progression of the cell geometry calculated during the joining of the stack (1), with measuring points (51, 52, ... 5 n ) SOxC (21, 22, ... 2 n ) describing geometrical quantities, is assigned a quality class in an automated process according to a scheme stored for this purpose in the program application processed by the SEV. [5] Method according to one of claims 1 to 4, characterized by that the geometric variables calculated from the measured values for the impedance after a respective measuring cycle are included as actual values in a control function carried out by the SEV, according to which the orientation of the at least one push rod (3) with regard to the angle (α) between its longitudinal axis (9) and a perpendicular dropped in the stacking direction of the stack is controlled by controlling an actuator (6). [6] System for a joining process under temperature and force influence for the production of a stack (1) of solid oxide cells SOxC (21, 22, ... 2 n ), with a furnace (7), with at least one push rod (3) exerting pressure on a central region of the uppermost and / or lowermost SOxC of the stack (1) located in the furnace (7) during the joining process, and with a control and processing device SEV, characterized bythat the system is equipped with several measuring channels (41, 42, ... 4 n ) for impedance spectroscopy EIS, wherein during the joining process several of the measuring channels (41, 42, ... 4 n ) of the impedance measuring device are connected to the stack (1), namely one of the measuring channels (41, 42, ... 4 n ) with one of several distributed on the stack outer sides on several SOxC (21, 22, ... 2 n ) of the stack (1) each arranged in pairs and spaced from each other measuring points (51, 52, ... 5 n ), and that the SEV is designed to - to control the impedance measuring device to measure in a continuously repeating measuring cycle at all measuring points (51, 52, ... 5) arranged on the stack (1) n ) to measure the impedance, - from the measured values of the impedance measuring device, correlating with each other, the cell geometry of the measuring points (51, 52, ... 5 n ) SOxC (21, 22, ... 2 n ) to calculate descriptive geometric quantities, - using the calculated geometrical dimensions, the joining process with a view to minimising undesirable deformations of SOxC (21, 22, ... 2 n ) and thus of the stack (1) and / or to derive quality statements about the generated stack (1) from the temporal progression of these geometric quantities. [7] System according to claim 6, characterized by that the SEV is designed to assign a quality class to a manufactured stack (1) in an automated process, depending on the time courses determined for the geometric variables on the basis of its repeated calculation, after the completion of the joining process, in accordance with a scheme stored for this purpose in the SEV. [8] System according to claim 6 or 7, characterized by that the SEV is designed to align the at least one push rod (3) by controlling an actuator (6) after each measuring cycle, with regard to the angle (α) of its longitudinal axis (9) to a perpendicular (8) in the stacking direction of the stack (1), in accordance with a control function into which the geometric variables calculated from the measured values of the measuring cycle are included by the SEV as actual values.

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