FOOD PROCESSING MACHINE WITH CAPACITIVE TOUCH DISPLAY AND METHOD

Abstract

Food processing machine, in particular in the form of a packaging machine (A, B) in the form of a thermoforming packaging machine (1) or in the form of a tray sealing machine (15), wherein the food processing machine has at least one control device (19, 25) configured to control at least one working unit arranged on the food processing machine, wherein the control device (19, 25) has a display device (20, 26) with at least one capacitive touch display (27) configured to visualize and influence operating parameters of the working unit, wherein the control device (19, 25) has at least one control program (37) for the working unit, wherein the control program (37) has a plausibility check function (33) for detecting an electrically conductive contaminant adhering at least temporarily to the capacitive touch display (27),wherein the plausibility check function (33) is configured to detect the electrically conductive contaminant on the touch display (27) based on a geometry-dependent, time-dependent and / or temperature-dependent electrical control measurement signal (32, 39, 41) generated by the touch display (27), and wherein the control program (37) is configured to detect an area spread (30) based on the geometry-dependent control measurement signal (32) and / or a position of an electrically conductive contaminant located on the touch display (27) based on the time-dependent control measurement signal (39) and to display this on the touch display (27).

Description

[0001] The present invention relates to a food processing machine according to claim 1 and according to independent claim 2, and to a method according to claim 7, which serves to detect an electrically conductive contaminant adhering at least temporarily to a capacitive touch display of a food processing machine.

[0002] Conventional packaging machines use both resistive and capacitive touch displays as input and display devices, which allow an operator to call up operating parameters for a packaging process and to visualize the operating parameters set for the packaging process.

[0003] Resistive touch displays are a long-established technology whose input surface features a pressure-sensitive film to register user input. This finger-pressure input is then converted into screen coordinates by touch controller software, from which control commands can be derived.

[0004] Resistive touch displays are particularly robust against dirt and grime. However, a disadvantage is that the control software typically only recognizes a single point as user input. Gestures, i.e., multi-point input, as commonly seen on smartphone displays, are not possible. Furthermore, the pressure-sensitive film of a resistive touch display is subject to increased wear and tear and can develop cracks with intensive use.

[0005] Capacitive touch displays use an electric field for command input, which is then converted into screen coordinates by the touch controller software. A user interface using a capacitive touch display offers a user experience similar to that found on other everyday devices, such as smartphones. Capacitive touch displays are also suitable for multi-touch and gesture input. In particular, multi-point input, such as zooming, can be performed intuitively. Furthermore, capacitive touch displays offer superior optical properties compared to resistive touch displays. They are also easier to clean, resulting in a longer lifespan. Finally, capacitive touch displays offer greater scalability in terms of screen size.

[0006] However, a disadvantage of capacitive touch displays is that they can be unintentionally controlled under certain circumstances due to electrically conductive contaminants adhering to them, which can cause machine malfunctions.

[0007] CH 711 497 A2 describes a food processing machine with a function selection device which uses a handwheel instead of a touchscreen to operate the menu of the function selection device displayed by means of a display surface.

[0008] US 2013 / 0 100 032 A1 describes the use of a touchscreen on an injection molding machine.

[0009] DE 10 2019 209 920 A1 discloses a vehicle, in particular an autonomous vehicle, with a display device arranged, for example, in the area of ​​a vehicle door, the display surface of which can be configured as a touchscreen so that persons outside the vehicle can make entries via the display surface, for example, to enter a password for a driver's license.

[0010] DE 10 2019 103 584 A1 describes a plausibility check on a touch display to find out whether an input performed on it matches a button of the touch display, on the basis of which it can be determined whether the input is correct or the result of a possibly contaminated display surface.

[0011] WO 2018 / 042027 A1 discloses a food processing machine with a control unit configured as a touchscreen.

[0012] The object of the invention is therefore to provide a food processing machine and a method that makes it easier to avoid, or at least reduce, malfunctions.

[0013] This problem is solved by means of a food processing machine according to claim 1, by means of a food processing machine according to claim 2, and by means of a method according to claim 7.

[0014] Advantageous further developments of the invention are given by the respective subject matter of the dependent claims.

[0015] The food processing machine according to the invention has a control unit designed to control at least one working unit arranged on the food processing machine, for example, a sealing station. This control unit comprises a display device with at least one capacitive touch display configured to visualize and influence operating parameters of the working unit, wherein the control unit has at least one control program for the working unit.

[0016] According to the invention, the control program includes a plausibility check function for detecting electrically conductive contamination that is at least temporarily adhering to the capacitive touch display. This plausibility check function makes it possible to identify, based on implausible input values ​​detected by the touch display, for example, due to their measurement geometry, transit time, and / or measurement temperature, an electrically conductive contamination present on the user interface of the touch display. The plausibility check function can therefore be effectively used to monitor the cleanliness of the display and can also prompt the operator to clean the touch display by detecting contamination adhering to it.

[0017] Electrically conductive contaminants, especially electrically conductive liquids, can form deposits on the capacitive touchscreen, potentially causing malfunctions in the food processing machine. Examples include marinades, sauces, product juices (especially blood), and / or food components if they adhere to the display's user interface. Operators may come into contact with such liquids in the loading area of ​​the food processing machine, particularly when manually placing products into packaging trays. In such cases, product liquid adhering to the operator's hands could be transferred to the surface of the capacitive touchscreen when the operator interacts with it.

[0018] It is conceivable that the food processing machine is designed as a packaging machine, in particular in the form of a thermoforming packaging machine or a tray sealing machine. According to one embodiment, the food processing machine can be designed as a dough processing machine or a food slicing machine. Alternatively, the food processing machine can be configured as a labeler for a packaging machine. The capacitive touch display according to the invention can be used on the respective machine types mentioned above to achieve improved operation.

[0019] According to the invention, the plausibility control function is configured to detect the electrically conductive liquid on the touch display based on a geometry-dependent, time-dependent, and / or temperature-dependent electrical control signal generated by the touch display. Control signals indicating an obviously incorrect input geometry, input duration, and / or input temperature with regard to an acceptable operator input can be detected by the plausibility control function and filtered out as erroneous inputs in order to prevent them from affecting the operation of the food processing machine.

[0020] According to one embodiment of the invention, the control program of the plausibility check function maintains at least one geometry-related, time-related, and / or temperature-related control signal threshold for comparison with the detected geometry-dependent, time-dependent, and / or temperature-dependent electrical control signal. When performing the comparison, the presence of an electrically conductive contaminant can be detected if the corresponding control signal threshold is reached or exceeded, at least temporarily, or if there is a deviation in magnitude from the control signal threshold. Those control signals by which contaminants of the display can be measured can be disregarded during operation of the food processing machine.They serve to detect contamination, but are not used by the control unit to control the food processing machine, for example during the packaging process of the packaging machine.

[0021] Preferably, the plausibility control function is continuously activated during a work process taking place on the food processing machine, for example during a packaging process taking place on the packaging machine, so to speak running continuously in its control program during the operation of the food processing machine to check the display cleanliness.

[0022] It would be conceivable that the geometry-related, time-related, and / or temperature-related control signal threshold could be changed via the capacitive touchscreen on the food processing machine to adapt to different product types, particularly the liquids released by these products. Specifically, predetermined control signal thresholds for the plausibility check function could be automatically accessed on the packaging machine, depending on the product type being packaged. The product type could be detected by a sensor device installed on the packaging machine, particularly a camera system, and used by the control program to set the control signal thresholds.

[0023] According to one variant, a sensor system used for the plausibility control function to detect the geometry-dependent, time-dependent and / or temperature-dependent electrical control measurement signals is formed by a sensor system already integrated in the capacitive touch display.

[0024] One option involves using a capacitive touch display as a multi-touch screen. This allows for multi-touch input and / or gesture input to control the food processing machine.

[0025] According to the invention, the control program is configured to detect the area spread of an electrically conductive contaminant located on the touchscreen, based on a geometry-dependent, implausible control signal, and / or to detect its position based on a time-dependent, implausible control signal, and to display this information on the touchscreen. This makes it easier for the operator to clean the touchscreen accordingly. The area spread can be determined, for example, with respect to a large number of simultaneously activated, adjacent screen coordinates. If an area spread reaches a predetermined threshold, i.e., a predetermined area measurement, it can be deduced that an electrically conductive contaminant on the display is responsible for this measurement signal.The position of the contamination on the user interface can be derived, for example, with reference to an electrically conductive frame geometry of the touch display, for example, if the control measurement signal lasts for a predetermined measurement time.

[0026] In particular, the inventive plausibility control function allows the following contamination scenarios, conceivable in practice, to occur and be detected on the capacitive touch display of the food processing machine, especially on the capacitive touch display of a packaging machine, which can be used, among other things, to locate and display the contamination responsible for it.

[0027] An electrically conductive contaminant adhering to the touch display, particularly a liquid such as marinade, sauce, and / or product fluid (e.g., blood), can, according to a first contamination scenario, spread across the center of the touch display without coming into contact with an electrically conductive border, such as a metal frame. This contaminant, especially a liquid, only affects the operation of the food processing machine when it is touched by the operator, i.e., when it is electrically grounded.Therefore, as soon as the operator makes a point-like input with their finger within the area of ​​contamination, it is possible that the control unit of the food processing machine will register a touch event that is distorted by the surface area of ​​the contamination, meaning it does not receive the point-like input actually intended by the operator. It could thus be that, based on the geometric area of ​​the contamination on the touchscreen, the control unit registers several operator inputs simultaneously, as if a palm were being used for input. Precise operator input is therefore impossible. Alternatively, no operator input at all may be accepted because it is unclear what input is actually intended.

[0028] Another possible contamination scenario involves an electrically conductive contaminant, particularly a liquid, adhering to the touchscreen. Such a contaminant could be positioned in such a way that it is already grounded, meaning it has the potential to cause an incorrect input even without any additional operator intervention. This can occur, for example, if the electrically conductive contaminant, especially the liquid, is in contact with an electrically conductive border of the touchscreen and is grounded via the metal housing of the display device and the machine frame. This can trigger an incorrect input, at least temporarily. Furthermore, such incorrect inputs, which may last for several seconds, can block any input actually made by the operator in the contaminated area, preventing the display from responding to operator input there.

[0029] The erroneous inputs described above are not easily recognizable by the operator, thus creating a risk that the food processing machine will be operated in a somewhat uncontrolled manner. However, the plausibility control function according to the invention allows the erroneous inputs caused by the contamination scenarios described above to be reliably detected. The associated control measurement signals can therefore be evaluated as unintentional erroneous inputs and eliminated from operation of the food processing machine.

[0030] The invention provides that the capacitive touch display has a screen border surrounded by an electrically conductive frame, wherein the plausibility control function is configured to generate a geometry-dependent electrical control signal for an electrically conductive liquid spaced away from the frame and / or a time-dependent electrical control signal for an electrically conductive liquid in contact with the frame. Accordingly, the plausibility control function can easily detect both impurity or liquid accumulations isolated from the frame and those in contact with the frame, at least partially, including their position. These impurities can be grounded indirectly, i.e., via user input, or directly, i.e., via the frame itself, due to their electrical conductivity.

[0031] Preferably, the display device is located in a loading area of ​​the food processing machine, in particular a loading area of ​​the packaging machine, the dough processing machine, or the food slicing machine. Here, the operator comes into contact with the products, especially when manually placing them into packaging trays. Upon subsequent touching of the capacitive touch display, the operator may transfer any liquid adhering to their hand during the loading process to the user interface of the touch display. Using the plausibility control function according to the invention, such contamination, which has the potential to cause incorrect entries, can be easily detected.

[0032] It is useful if the control program is designed to prompt an operator to clean the touchscreen as soon as an electrically conductive contaminant is detected. This ensures that the touchscreen always has a clean user surface, thus preventing incorrect entries. This prompt can be visual and / or audible.

[0033] The invention further relates to a method on a food processing machine for detecting an electrically conductive contaminant that adheres at least temporarily to a capacitive touchscreen of the food processing machine by means of a plausibility check function of a control program installed on the food processing machine. Detecting such a contaminant on the touchscreen, for example a deposited electrically conductive liquid, offers the advantage that erroneous inputs, i.e., unintentional inputs, can be reduced, leading to improved operation of the food processing machine.

[0034] According to the invention, the plausibility control function detects electrically conductive contamination on the touch display based on a geometry-dependent, time-dependent, and / or temperature-dependent electrical control signal generated by the touch display. Using such control signals, the plausibility control function can reliably distinguish plausible from implausible control signals in order to detect unwanted electrically conductive contamination, particularly liquids, adhering to the touch display. Such contamination could lead to an incorrect input, i.e., incorrect control of the food processing machine, either directly through frame contact or indirectly through operator input.

[0035] The invention is explained in more detail with reference to exemplary embodiments according to the following figures. Specifically, the figures show: Fig. 1. A perspective view of a packaging machine configured as a thermoforming packaging machine. Fig. 2 a perspective view of a packaging machine configured as a tray sealing machine, Fig. 3 a schematic representation of a contaminated capacitive touch display, Fig. 4 another schematic representation of a contaminated capacitive touch display, and Fig. 5 another schematic representation of a contaminated capacitive touch display.

[0036] Identical components are consistently labelled with the same reference symbols in the figures.

[0037] Fig. Figure 1 shows a perspective view of a food processing machine, specifically an intermittently operating packaging machine A configured as a thermoforming packaging machine 1. This thermoforming packaging machine 1 has a forming station 2, a sealing station 3, a cross-cutting unit 4, and a longitudinal cutting unit 5, arranged in that order along a transport direction R on a machine frame 6. At the inlet side of the machine frame 6 is a feed roller 7 from which a bottom film 8 is unwound. Furthermore, the thermoforming packaging machine 1 has a transport chain 11 that grips the bottom film 8 and transports it along the transport direction R for each main work cycle. In particular, transport chains or clamping chains 11 are arranged on both sides.

[0038] In the illustrated embodiment, the forming station 2 is designed as a thermoforming station in which cavities are formed in the base film 8 by thermoforming, for example, using compressed air and / or vacuum. The forming station 2 can be configured such that several cavities are formed side by side in the direction perpendicular to the transport direction R. A filling section or insertion area 12 is provided downstream of the forming station in the transport direction R, in which the cavities formed in the base film 8 are filled with products. The filling of the cavities can be carried out manually by an operator.

[0039] The sealing station 3 has a hermetically sealable chamber 3a in which the atmosphere in the recesses can be evacuated and / or replaced by gas purging with a replacement gas or with a gas mixture before sealing with the top film 10 dispensed from a top film receiver 9.

[0040] The cross-cutting device 4 can be designed as a punch that cuts the bottom film 8 and the top film 10 in a direction transverse to the transport direction R between adjacent troughs. The cross-cutting device 4 operates in such a way that the bottom film 8 is not cut across its entire width, but at least in one edge area remains intact. This allows for controlled onward transport through the transport chain 11.

[0041] The longitudinal cutting device 5 can be designed as a knife arrangement with which the lower film 8 and the upper film 10 are cut between adjacent troughs and at the lateral edge of the lower film 8 in the transport direction R, so that individual packages are present behind the longitudinal cutting device 5.

[0042] The right and left transport chains 11 of the thermoforming packaging machine 1, which grip the bottom film 8 on both sides, are each guided in a chain guide 13. These chain guides 13 are each protected on the outside by a side panel 14 of the thermoforming packaging machine 1 and may be attached to the side panel 14. The side panel 14 can be a sheet metal part.

[0043] The thermoforming packaging machine 1 also has a control unit 19. Its function is to control and monitor the processes taking place in the thermoforming packaging machine 1. A display device 20 serves to visualize and / or influence the process sequences in the thermoforming packaging machine 1 for or by an operator.

[0044] Fig. Figure 2 shows another food processing machine, configured as a further packaging machine B in the form of a tray sealing machine 15. Such a tray sealing machine 15 is also known in the industry as a "tray sealer". The tray sealing machine 15 comprises a feed conveyor 16, a sealing station 17, and a discharge conveyor 18, arranged in this order in the transport direction R on a machine frame 21. The feed conveyor 16 is positioned in a loading area 24. During operation of the tray sealing machine 15, an operator can stand in the loading area 24 and fill the trays S with products. Furthermore, the tray sealing machine 15 has a gripper device 22, which is configured to pick up the trays S filled with a product from the feed conveyor 16 and transport them into the sealing station 17 for a sealing process.The trays S transported into the sealing station 17 are sealed with a top film 23 and then transported by means of the gripper device 22 to the discharge conveyor 18.

[0045] The tray sealing machine 15 also has a control unit 25. Its function is to control and monitor the processes taking place in the tray sealing machine 15. A display device 26 serves to visualize and / or influence the process sequences in the tray sealing machine 15 for or by an operator.

[0046] Fig. Figure 3 schematically shows the display device 20 of the thermoforming packaging machine 1 and the display device 26 of the tray sealing machine 15. The display device 20, 26 is in the form of a capacitive touch display 27, which is specifically designed as a multi-touch screen. The display device 20, 26 has a screen border 28, which is surrounded by an electrically conductive frame 29. The frame 29 can be part of a metal housing of the control unit 19, 25.

[0047] In Fig. 3. An electrically conductive contaminant, in this case a liquid F, for example a marinade, is present on the capacitive touch display 27. The liquid F may, for example, have been picked up by the operator on his hands in the insertion area 12, 24 and accidentally transferred to the capacitive touch display 27.

[0048] The liquid F represents a contamination on the capacitive touch display 27, which can trigger a malfunction in the packaging machine A, B. According to Fig. 3 the liquid F is positioned on the capacitive touch display 27, spaced away from the frame 28.

[0049] A spreading of 30 of the liquid F is according to Fig. 3 is many times larger than an area 31 that would result from an ordinary touch if the operator made a touch input with his finger on the capacitive touch display 27.

[0050] The liquid F on the capacitive touch display 27 can trigger an erroneous input during the operation of the packaging machine A, B if it is touched by the operator, i.e., electrically grounded. Therefore, as soon as the operator makes a finger input within the area 30, for example, across area 31, it is possible that the control unit 19, 25 of the packaging machine A, B would register a touch event that is distorted by the area 30 of the liquid F, i.e., it would not receive the actual point input intended by the operator on area 31. It could therefore be that the control unit 19, 25, corresponding to the geometric area 30 of the liquid F on the touch display 27, receives several operator inputs simultaneously, as if a palm were being used for input.

[0051] Fig. Figure 3 shows a control measurement signal 32 resulting from the spread 30 of the liquid F, i.e., a geometry-dependent signal, which is checked for plausibility with regard to a touch input performed by a finger using the control device 19, 25. For this purpose, the control device 19, 25 has a plausibility control function 33.

[0052] The plausibility control function 33 is configured to detect the electrically conductive liquid F on the touch display 27 based on the geometry-dependent electrical control measurement signal 32 generated by the touch display 27, and in particular to display its spread 30 to the operator, for example by means of a visual image 34, to show him that the touch display 27 is contaminated. If necessary, the detection of the conductive liquid F may also trigger a separate prompt 35, in particular an acoustic signal and / or a cleaning symbol 36 displayed on the touch display 27, if the plausibility control function 33 detects a contaminated touch display 27.

[0053] For the detection of the liquid F on the touch display 27, a control program 37 of the control device 19, 25 of the plausibility control function 33 can maintain at least one geometry-related control measurement signal threshold value 38 for a comparison with the detected geometry-dependent electrical control measurement signal 32, wherein when performing the comparison, the electrically conductive liquid F is detectable upon reaching or exceeding the control measurement signal threshold value 38.

[0054] Fig. Figure 4 shows another contamination scenario of the capacitive touch display 27. As in Fig. As shown in Figure 4, it is conceivable that the electrically conductive liquid F on the touch display 27 is spread in contact with the conductive frame 29, so that the liquid F is already grounded, meaning that it has the potential for an incorrect input even without additional intervention from the operator. This means that an incorrect input, at least temporarily, can be triggered even without operator intervention, which in Fig. 4 is represented by a time-dependent control measurement signal 39.

[0055] For the detection of the liquid F on the touch display 27, the control program 37 of the control device 19, 25 of the plausibility control function 33 can maintain at least one time-related control measurement signal threshold value 40 for a comparison with the detected time-dependent electrical control measurement signal 39, wherein when the comparison is carried out, the electrically conductive liquid F can be detected upon reaching or exceeding the control measurement signal threshold value 40.

[0056] Fig. Figure 5 shows a variant for detecting the liquid F, whereby the temperature of the liquid F, measured by means of the touch display 27, is available in the form of a resulting, i.e., temperature-dependent, control signal 41 of the control device 19, 25. The temperature-dependent control signal 41 is checked for plausibility with regard to the operator's normal finger temperature by means of the plausibility check function 33.For the detection of the liquid F on the touch display 27, the control program 37 of the control device 19, 25 of the plausibility control function 33 can maintain at least one temperature-related control measurement signal threshold value 42 for comparison with the detected temperature-dependent electrical control measurement signal 41, wherein, when performing the comparison, the electrically conductive liquid F can be detected if there is a deviation in magnitude of the temperature-dependent electrical control measurement signal 41 from the control measurement signal threshold value 42.

[0057] This in connection with the Fig. 3, Fig. 4 to Fig. The capacitive touch display described in section 5, including its display functions for detecting an electrically conductive contaminant, can also be used in applications other than those described in the Fig. 1 and Fig.2 food processing machines shown, for example on a dough processing machine, a food slicing machine or on a labeler for a packaging machine.

Claims

[1] Food processing machine, in particular as a packaging machine (A, B) in the form of a thermoforming packaging machine (1) or in the form of a tray sealing machine (15), wherein the food processing machine has at least one control device (19, 25) configured to control at least one working unit arranged on the food processing machine, wherein the control device (19, 25) has a display device (20, 26) with at least one capacitive touch display (27) configured to visualize and influence operating parameters of the working unit, wherein the control device (19, 25) has at least one control program (37) for the working unit, wherein the control program (37) has a plausibility check function (33) for detecting an electrically conductive contaminant adhering at least temporarily to the capacitive touch display (27),wherein the plausibility check function (33) is configured to detect the electrically conductive contaminant on the touch display (27) based on a geometry-dependent, time-dependent and / or temperature-dependent electrical control measurement signal (32, 39, 41) generated by the touch display (27), and wherein the control program (37) is configured to detect an area spread (30) based on the geometry-dependent control measurement signal (32) and / or a position of an electrically conductive contaminant located on the touch display (27) based on the time-dependent control measurement signal (39) and to display this on the touch display (27). [2] Food processing machine, in particular as a packaging machine (A, B) in the form of a thermoforming packaging machine (1) or in the form of a tray sealing machine (15), wherein the food processing machine has at least one control device (19, 25) configured to control at least one working unit arranged on the food processing machine, wherein the control device (19, 25) has a display device (20, 26) with at least one capacitive touch display (27) configured to visualize and influence operating parameters of the working unit, wherein the control device (19, 25) has at least one control program (37) for the working unit, wherein the control program (37) has a plausibility check function (33) for detecting an electrically conductive contaminant adhering at least temporarily to the capacitive touch display (27),wherein the plausibility control function (33) is configured to detect the electrically conductive contaminant on the touch display (27) based on a geometry-dependent, time-dependent and / or temperature-dependent electrical control measurement signal (32, 39, 41) generated by the touch display (27), and wherein the capacitive touch display (27) has a screen border (28) surrounded by an electrically conductive frame (29), wherein the plausibility control function (33) is configured to generate a geometry-dependent electrical control measurement signal (32) for an electrically conductive contaminant spaced away from the frame and / or to generate a time-dependent electrical control measurement signal (39) for an electrically conductive contaminant in contact with the frame (29). [3] Food processing machine according to claim 1 or 2, characterized by, that the control program (37) of the plausibility control function (33) maintains at least one geometry-related, time-related and / or temperature-related control measurement signal threshold value (38, 40, 42) for comparison with the detected geometry-dependent, time-dependent and / or temperature-dependent electrical control measurement signal (32, 39, 41), wherein when performing the comparison, the electrically conductive contamination is detectable upon reaching or exceeding the control measurement signal threshold value (38, 40) or upon a deviation in magnitude from the control measurement signal threshold value (42). [4] Food processing machine according to any of the preceding claims, characterized by , that the plausibility check function (33) can be activated by operator input on the touch display (27) and / or can be carried out automatically according to preset check intervals. [5] Food processing machine according to any one of the preceding claims, characterized by , that the display device (20, 26) is assigned to an insertion area (12, 24) of the food processing machine. [6] Food processing machine according to any one of the preceding claims, characterized by , that the control program (37) is designed to prompt an operator to clean the touch display (27) as soon as an electrically conductive contaminant is detected on it. [7] Method on a food processing machine for detecting an electrically conductive contaminant adhering at least temporarily to a capacitive touch display (27) of the food processing machine by means of a plausibility control function (33) of a control program (37) installed on the food processing machine, wherein the plausibility control function (33) detects the electrically conductive contaminant on the touch display (27) on the basis of a geometry-dependent, time-dependent and / or temperature-dependent electrical control measurement signal (32, 39, 41) generated by means of the touch display (27), and wherein • the control program (37) detects an area spread (30) based on the geometry-dependent control measurement signal (32) and / or a position of an electrically conductive contaminant located on the touch display (27) based on the time-dependent control measurement signal (39) and displays it on the touch display (27), and / or • the capacitive touch display (27) has a screen border (28) surrounded by an electrically conductive frame (29), wherein the plausibility control function (33) generates a geometry-dependent electrical control measurement signal (32) for an electrically conductive contaminant spaced away from the frame and / or generates a time-dependent electrical control measurement signal (39) for an electrically conductive contaminant contacting the frame (29).

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