Method for fault diagnosis of a switch element and control device

Abstract

The invention relates to a method for fault diagnosis of a switch element (100) which has an electromagnet (101) and a switching piece (104) and which is designed to close or disconnect an electrical connection (109), in which the switching piece (104) is moved by means of a magnetic field of the electromagnet (101), wherein the method comprises the following steps: actuating the switch element (100) using a control signal (111) which influences the magnetic field of the electromagnet (101); and carrying out a fault diagnosis of the switch element (100) by means of an evaluation criterion, which is based on a profile of the control signal (111), which correlates with an arrangement of the switching piece (104) in relation to the electromagnet (101). The invention also relates to a corresponding control device (130) and computer program.

Description

[0001] DESCRIPTION

[0002] DESIGNATION

[0003] Method for fault diagnosis of a switching element and control unit

[0004] TECHNICAL AREA

[0005] The present disclosure relates to methods and control devices for fault diagnosis of a switching element, in particular a power contactor, such as those used, for example, in battery management systems of electric vehicles. Specifically, methods and control devices are described for detecting whether the switching element is opening or closing correctly or incorrectly.

[0006] BACKGROUND OF THE INVENTION

[0007] Modern electric vehicles and plug-in hybrids are equipped with high-voltage batteries. To connect or disconnect the battery from the system depending on the operating mode, contactors are installed at the positive and negative terminals of the battery. These contactors can be located, for example, in a battery management system or in a high-voltage unit with charging and / or conversion electronics.

[0008] Due to safety requirements, it is important that the contactors open cleanly and can, for example, disconnect the high-voltage battery from the system. Disconnecting the battery may be necessary when the vehicle is switched off, in the event of a crash or accident, and / or in certain fault conditions.

[0009] This ensures that no dangerous voltage is present in such situations.

[0010] However, contactors themselves can also have defects; in particular, they can weld together while closed, for example due to high currents, and consequently no longer open. This defect must be detected and properly diagnosed.

[0011] SUMMARY AND FORMS OF EXECUTION

[0012] It is therefore an objective of the present disclosure to provide a reliable method for diagnosing faults in a switching element, in particular for detecting proper and / or faulty opening or closing of the switching element.

[0013] This task is solved by a method for fault diagnosis of a switching element, by a control unit, and by a computer program according to the independent patent claims. Advantageous embodiments and further developments are described in the respective dependent claims, the following description, and the drawings.

[0014] Thus, according to a first aspect, a method for fault diagnosis of a switching element, in particular a contactor or power contactor, is provided. The switching element comprises an electromagnet and a switching element, in particular an armature, and is configured to close or open an electrical connection by means of a magnetic field of the electromagnet moving the switching element. The method comprises the following steps: (a) controlling the switching element, in particular the electromagnet or a coil of the electromagnet, using a control signal that influences or determines the magnetic field of the electromagnet; (b) performing a fault diagnosis of the switching element using an evaluation criterion based on a waveform of the control signal that correlates with an arrangement of the switching element relative to the electromagnet. The arrangement can be a time-varying arrangement, i.e.,It can be a movement or a static arrangement. The process can be discrete or continuous.

[0015] According to another aspect, a control unit is provided which is configured to carry out the previously described procedure. The control unit can be a battery management system, or a battery management system can incorporate the control unit.

[0016] According to another aspect, a computer program is provided which includes commands that, when executed by a computer, cause it to carry out the procedure described above.

[0017] In the context of this disclosure, the switching element is defined, for example, as an electrically or electromagnetically actuated switch. The switching may be mechanical. The switch may be designed, in particular, for switching high electrical powers, such as those encountered, for example, when charging the high-voltage batteries of electric vehicles and / or when driving electric motors using such high-voltage batteries. The switching element may have two or more switching positions, in particular a switching position in which the connection is open and another switching position in which the connection is closed. The switching element may, for example, be a contactor, in particular a power contactor, or a relay. The switching element may be a solenoid actuator. The power contactor may be configured for switching the aforementioned high powers.The switching element can be arranged at an electrical connection to the positive terminal of a battery, particularly between the battery and a load. Alternatively, the switching element can be arranged at an electrical connection to the negative terminal of the battery, particularly between the battery and the load. Switching elements can be arranged at either connection.

[0018] In the context of this disclosure, an electromagnet is defined, for example, as a component capable of generating a magnetic field due to a current flowing through it. The electromagnet may be or comprise a coil. The electromagnet may have a core or magnetic core capable of guiding, amplifying, and / or increasing the inductance of the electromagnet. The core may comprise or consist of a soft magnetic material, such as a ferromagnetic material, for example, iron.

[0019] In the context of the present disclosure, the switching element is defined, for example, as a movable element by whose movement the electrical connection can be opened or closed. The switching element may have an armature which is moved by the magnetic field of the electromagnet. The armature, like the magnetic core, may also be made of or consist of a soft magnetic material. The switching element may further comprise an electrically conductive closing element by means of which the electrical connection is opened or closed. The closing element may be mechanically connected to the armature, in particular rigidly connected.

[0020] In the context of this disclosure, a control signal is defined, for example, as a signal used to control the electromagnet, in particular a coil of the electromagnet. The control signal can comprise a control current signal and / or a control voltage signal. The control current signal can flow through the coil. The control voltage signal can be applied to the coil. The control voltage signal can be predetermined, and the control current signal can be dependent on the predetermined control voltage signal, or vice versa. The control signal can be configured to move the switching element, in particular the armature of the switching element, for example, by means of a magnetic field of the electromagnet influenced by the control signal.

[0021] The described method and the corresponding control unit can be advantageous for reliably diagnosing the opening and / or closing of the switching element or the disconnection and / or connection of the electrical link. Specifically, if the control signal's behavior correlates with the switching element's position relative to the electromagnet, an expected or unusual movement of the switching element can be detected by analyzing the signal's behavior using a suitable evaluation criterion. For example, a lack of movement or insufficient movement can be detected if the switching element is stuck in a position, such as stuck open or closed. Furthermore, an unusual position of the switching element can be diagnosed, such as if the switching element is stuck in the closed position even without control input, for example, because the switching element is welded. The relationship between the control signal and the switching element's arrangement, or...Switching element movement is caused by the fact that the arrangement or movement of the switching element, in particular an armature of the switching element, affects the magnetic circuit and / or the inductance of the electromagnet.

[0022] In other words, the present disclosure, in one embodiment, takes advantage of the fact that a contactor is essentially a solenoid actuator, i.e., it has a magnetic circuit consisting of an iron core and a coil. The iron core is divided into a fixed stop and a movable armature. The electrical contact is connected to the armature. When the coil is not energized, the contactor is open. When the coil is energized, a magnetic field is generated. The magnetic flux in the magnetic circuit generates a force in the gap between the armature and the stop, causing the armature to move toward the stop. This closes the electrical contact, which is kept closed by continued energization.

[0023] The movement of the armature affects the magnetic circuit and the inductance. This results in an effect on the current signal and, when the contactor is switched off, on the voltage. These effects can be measured and detected using suitable algorithms and provide information about the armature's position and movement, thus indicating the correct functioning of the contactor. If corresponding characteristics are not found in the current and voltage, it can be assumed that the contactor is not operating correctly. This analysis can preferably be combined with the behavior of other voltages or signals present in the system to support the diagnosis. In this way, fault conditions can be additionally or alternatively detected or confirmed by monitoring and comparing the behavior of measured voltages before and after the contactors under various operating conditions.

[0024] According to one embodiment, the switching element is controlled in the direction of opening or closing. According to another embodiment, the shape of the control signal correlates with a movement of the switching element relative to the electromagnet.

[0025] According to one embodiment, the switching element is a contactor, in particular a power contactor. A contactor can be designed to protect electrical components connected to the electrical connection that the contactor closes or opens, for example, to protect high-voltage components of a motor vehicle, such as an electric motor, an inverter, an air conditioner, or a high-voltage battery. A contactor can be double-break, particularly unlike a relay. Finally, the contactor can have an arc-quenching chamber to counteract the formation of arcs at the switching contacts.

[0026] According to one embodiment, the switching element is used in a battery management system, in particular for disconnecting an electrical connection to an associated battery, especially a vehicle battery. In the context of the present disclosure, a battery management system or BMS is defined, for example, as a component connected to the battery that performs at least one of the following functions: monitoring, regulating, and protecting the battery and / or components connected to the battery. The battery management system may include the control unit.

[0027] In the context of the present disclosure, a battery is defined, for example, as a storage device for electrical energy, particularly on an electrochemical basis. In one embodiment, the battery is an accumulator, i.e., a rechargeable battery. The battery can be a motor vehicle battery, in particular a high-voltage battery of a motor vehicle.

[0028] According to one embodiment, the electromagnet comprises a coil, and the control signal includes a voltage applied to the coil and / or a current flowing through the coil. In another embodiment, the voltage applied to the coil is predetermined, for example, as a constant voltage or as a voltage pulse, and the evaluation criterion is based on the current signal. According to yet another embodiment, the current and / or voltage across the coil can be determined using a measuring device. Such an embodiment can be advantageous because inductive effects generated by the arrangement or movement of the switching element particularly affect the coil voltage and / or coil current.

[0029] According to one embodiment, the evaluation criterion is based on the voltage curve across the coil when the switching element is activated to break the electrical connection. Alternatively or additionally, the voltage can also be examined when the switching element is activated to close the electrical connection. Such an embodiment can be advantageous because inductive effects in the voltage signal can be particularly pronounced. This can be especially true when the electrical connection is broken, because the current is then typically low or zero, and correspondingly, inductive effects in the current signal are only weakly pronounced. According to one embodiment, the voltage across the coil is, for example, specified as a constant voltage value until the coil is activated to break the connection. After that, the voltage is no longer specified.not controlled, but results in particular from energy stored in the coil, which dissipates during freewheeling.

[0030] According to one embodiment, the evaluation criterion is based on the current flowing through the coil when the switching element is activated to close the electrical connection. Alternatively or additionally, the current can also be examined when the switching element is activated to open the electrical connection. However, as already explained in connection with the previous embodiment, the current signal shows only weakly pronounced inductive effects when the electrical connection is opened. In contrast, the inductive effects in the current signal are clearly visible when the connection is closed.

[0031] According to one embodiment, the evaluation criterion is based on a

[0032] Gradient of the control signal's behavior. Evaluating the gradient or derivative of the control signal, especially the derivative with respect to time, can be advantageous because characteristics of the behavior, such as slope or extrema, can be analyzed particularly easily using the gradient.

[0033] According to one embodiment, the evaluation criterion is based on a sign change of the gradient, in particular on two successive sign changes of the gradient. The sign change of the gradient can correlate with or be caused by a movement of the switching element, especially the armature. The sign change can correlate with a closing movement of the switching element. More precisely, the first sign change can correlate with the beginning of the closing movement and the second sign change with the end of the closing movement. For example, the sign change(s) can relate to a current signal of a control current of the switching element. The sign change(s) can be more pronounced the larger the air gap to be overcome by the switching element. The air gap to be overcome can correlate with a time interval between the sign changes.The sign change(s) can occur following or during an increasing current waveform, with the increasing current waveform being indicative of a controlled closing of the switching element.

[0034] According to one embodiment, a change in sign, and in particular two consecutive changes in sign, is indicative of a functioning switching element, especially a functioning closing process. Conversely, the absence of a change in sign, and in particular two consecutive changes in sign, is indicative of a faulty switching element, especially a faulty closing process. This can be particularly true when the switching element is controlled in the direction of closing the electrical connection.

[0035] Such an embodiment can be advantageous because the described sign change, especially the two successive sign changes, is particularly pronounced in switching elements, such as contactors, due to the comparatively large air gap that must be overcome. Consequently, the sign change is easy and reliable to analyze. Such an evaluation criterion therefore allows for robust fault diagnosis of the switching element.

[0036] According to one embodiment, the evaluation criterion is based on a dip during an increase or decrease in the control signal. The dip can be characterized by the sign changes described in connection with the previous embodiment. However, a sign change of the gradient is not a mandatory requirement for a dip. The dip can be characterized by a temporary deviation from a continuous increase or decrease.

[0037] The dip can occur particularly during a rise in the control signal, where the rise is indicative of the switching element closing. For example, the dip can affect the current signal of a control current for the switching element. The dip can correlate with movement of the switching element. In particular, the greater the movement of the switching element and / or the wider the air gap that the switching element has to overcome, the more pronounced the dip can be. The shape of the dip can also depend on component deviations.

[0038] According to one embodiment, the indentation indicates a properly functioning switching element, particularly a proper closing process. Conversely, the absence of the indentation indicates a faulty switching element, particularly a faulty closing process. This can be especially true when the switching element is activated to close the electrical connection.

[0039] Such an embodiment can be advantageous because the described indentation is particularly pronounced in switching elements, especially contactors, due to the air gap that must be overcome. Consequently, the indentation is easy and reliable to analyze. The evaluation criterion thus allows for robust fault diagnosis of the switching element. According to one embodiment, performing the fault diagnosis includes detecting a faulty switching element and / or detecting a functioning switching element, particularly based on the evaluation criterion.

[0040] A method according to one of the preceding claims, wherein the switching element is moved over a distance of at least 0.1 mm, in particular at least 0.5 mm, and in particular at least 1 mm. In other words, the air gap to be overcome by the switching element when closing and / or opening the switching element is at least 0.1 mm wide. Such an embodiment can be advantageous because, with a distance of at least 0.1 mm, in particular at least 0.5 mm, and in particular at least 1 mm, inductive effects can be particularly large, and thus the evaluation criterion can be particularly robust and reliable. For example, the sign change or the dent described above can be particularly pronounced.

[0041] According to one embodiment, during fault diagnosis, a faulty switching element and / or a functioning switching element is detected based on a current and / or a voltage induced by the arrangement, particularly the movement, of the switching element. Such diagnoses can be advantageous because they are particularly robust and reliable.

[0042] According to one embodiment, during fault diagnosis, the evaluation criterion detects whether the switching element, for example a contactor, is stuck closed, particularly if it is welded shut. Such an embodiment can be advantageous because a welded switching element cannot be reopened, and therefore the electrical connection cannot be disconnected. This can be problematic if disconnecting the connection is necessary for safety reasons, for example, in the event of a short circuit. According to one embodiment, the switching element is controlled by a control pulse designed such that the electrical connection is not completely closed. The control pulse can be designed so that the switching element experiences only minimal movement or no movement at all. Minimal movement can be defined as movement over less than one-fifth of the air gap, particularly less than one-tenth.Alternatively, the control pulse can be designed such that the switching element undergoes significant movement, for example, more than one-tenth of the air gap, particularly more than one-fifth, without closing the connection. The air gap can be defined by the distance between the contacts of the electrical connection to be joined, especially when the switching element is fully open.

[0043] Such an embodiment can be advantageous because fault diagnosis is performed without closing the electrical connection. This makes welding of the switching element, particularly the contactor, less likely, or can even prevent it entirely with only slight movement. Furthermore, no electrical currents occur through the electrical connection, which might be undesirable under the respective operating conditions.

[0044] As described above, detection can be performed during the actuation process in the rising current phase, i.e., during the closing actuation of the switching element, particularly the contactor. From a safety perspective, however, it is essential to ensure that the contactor has opened cleanly and is therefore not welded shut. During opening, the effect is difficult or impossible to detect due to the timing of the current, as it is typically already close to zero when the contactor moves. This can be resolved by applying a test pulse, as previously explained, which, for example, causes movement but does not fully close the contactor. The current waveform is then analyzed, revealing the differences between a welded and a properly functioning contactor. This allows for diagnosis after the contactor has intentionally opened at the end of a drive cycle.The test pulse is executed in such a way that the contactor does not complete a full movement and the high-voltage contacts do not close. This ensures that it does not get stuck in the closed position or weld shut during testing.

[0045] According to one embodiment, the evaluation criterion is based on the slope of the control signal and / or on a maximum value of the control signal. The slope and / or the maximum value can be compared with a respective threshold value, where the threshold value is representative of a properly functioning switching element. The evaluation criterion can be based on the build-up of the magnetic field and may not require any movement of the switching element.

[0046] Such an embodiment can be advantageous for detecting a closed-jamming switching element, particularly a welded contactor. If the switching element is closed-jamming and not in an open state, the switching element is arranged differently with respect to the electromagnet. In particular, with a closed-jamming switching element, a larger area of ​​the armature is located in the magnetic field compared to an open switching element. This increases the inductance of the electromagnet and consequently also the inductive effects on the control signal. Due to these inductive effects, the same control pulse, especially the same voltage pulse, results in a lower slope of the current signal and a lower maximum value of the current signal with a closed-jamming switching element compared to an open and properly functioning switching element.Similarly, an open stuck switching element can also be detected based on a slope and / or a maximum value of the control signal.

[0047] According to a further embodiment, the evaluation criterion additionally considers voltages measured before and / or after the switching element, particularly under one or more predefined operating conditions. This can be a voltage between a supply line to a load or battery and a return line from the load or battery. Such an embodiment can be advantageous because a closed-clamp switching element leads to voltage equalization, while an open-clamp switching element prevents this. The measured voltages can be used to validate a diagnostic result.

[0048] BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Further advantages and beneficial designs and further developments of the method, the control unit and the computer program result from the following exemplary embodiments shown in connection with the figures.

[0050] They show:

[0051] Figure 1 shows a switching element and a control unit for controlling the switching element according to an embodiment of the present disclosure; Figure 2 shows a control signal for use in a method for fault diagnosis of a switching element according to an embodiment of the present disclosure;

[0052] Figure 3 shows a further control signal for use in a method for fault diagnosis of a switching element according to an embodiment of the present disclosure.

[0053] Identical, similar, or similarly effective elements are marked with the same reference symbols in the figures. In some figures, individual reference symbols have been omitted for clarity. The figures and the relative sizes of the elements depicted within them are not to be considered to scale. Rather, individual elements may be exaggerated for better representation and / or comprehensibility.

[0054] DETAILED DESCRIPTION OF EXAMPLES OF EXECUTION

[0055] Figure 1 shows a switching element 100, here a contactor, which has a

[0056] comprising electromagnets 101 and a switching element 104, and which is configured to close or disconnect an electrical connection by moving the switching element 104 by means of a magnetic field of the electromagnet 101.

[0057] The electromagnet 101 has a coil 102 and a magnetic core 103 located at least partially inside the coil 102. A voltage can be applied to the coil 102 and a current can be passed through the coil 102 via control lines 110. The voltage signal at the control lines 110 and the current signal through the control lines 110 are implemented by a control unit 130.

[0058] Adjacent to the electromagnet 101 is the switching element 104, which is movable relative to the electromagnet 101 along a predetermined direction. The switching element 104 has an armature 105 and a closing element 106 attached to the armature 105 by means of a connecting piece. The closing element 106 is an electrical conductor or has one, which closes the electrical connection 109 by means of contacts 107 when the switching element 104 is in the corresponding position.

[0059] The electrical connection 109 is closed when the coil 102 is sufficiently energized by the control lines 110 to generate a magnetic field that moves the switching element towards the magnetic core 103. When no more current flows through the coil 102, the magnetic field disappears and the return element 108, in this case a return spring, causes the switching element 100 to open and the electrical connection 109 to be disconnected.

[0060] The control unit 130 is configured to perform a fault diagnosis procedure for the switching element 100. The procedure comprises the following steps: (a) actuating the switching element 100, in particular the coil 102, using a control signal that influences the magnetic field of the electromagnet 101; (b) performing a fault diagnosis of the switching element 100 using an evaluation criterion based on a curve of the control signal 111 that correlates with an arrangement of the switching piece 104 relative to the electromagnet 101. Figures 2 and 3 show various control signals 111 plotted against time, as used in the procedure described in connection with Figure 1. In the upper graph of these figures, the control signal 111 is plotted against time, and in the lower graph, the resulting stroke 115 of the switching piece 104 is plotted against time. The time scales are identical.

[0061] In the control signal 111 shown in Figure 2, constant voltage signals 112 are applied to the coil 102 of the electromagnet 101. An initial voltage is used to build up a magnetic field until this field is strong enough to move the switching element 100 against the resistance of the return element 108, thus closing the electrical connection 109 by means of the locking piece 106. The magnetic field is then further intensified until saturation is reached.

[0062] This process can be traced by the current signal 113 of the coil current, whereby an initial rise in the current signal 113 indicates the build-up of the magnetic field, a subsequent dip 114 in the current signal 113 indicates the closing movement of the switching element 104, and finally a second rise in the current signal 113 indicates the further build-up of the magnetic field. Saturation, or the complete build-up of the magnetic field, is ultimately indicated by a constant current flow.

[0063] The indentation 114 can be easily identified by two successive sign changes of the gradient or the time derivative of the current signal 113. It indicates the closing movement of the switching element 100. Therefore, it can serve as an evaluation criterion, whereby the indentation 114 indicates a functioning switching element 100 and the absence of the indentation 114 indicates a faulty switching element 100 when the switching element 100 is activated in a closing direction.

[0064] Subsequently, a second voltage, lower than the first, holds the switch in the closed position, causing the current to also drop to a lower, constant value. The magnetic field decreases accordingly. When the voltage is switched off, the current also stops flowing after a short transition period. The magnetic field disappears, and the switching element 100 is opened by the reset element 108, thereby also disconnecting the electrical connection 109.

[0065] In the control signal shown in Figure 3, a control pulse 120 in the form of a short voltage pulse 112 is used when the switching element is presumably open. This pulse is applied to the coil 102 by means of the control unit 130. The voltage pulse 112 can be provided following the control signal shown in Figure 2. With a properly functioning switching element 100, the voltage pulse 112 prevents the switching piece 104 from closing completely. In other words, a corresponding stroke 115 does not reach the maximum stroke 116 at which the electrical connection 109 is closed. If the switching element 100 is faulty, particularly if it is stuck closed, the switching piece remains at its maximum stroke 123, 116, as indicated by the dashed line.

[0066] The current signal 113 corresponding to the voltage pulse 112 differs depending on whether the switching element 100 opens and closes correctly or whether it is stuck closed, for example, because it is welded shut. The different arrangement of the switching element 104, in particular the armature 105, relative to the electromagnet 101 changes the inductance of the electromagnet 101. When the switching element 104 is stuck closed, the inductance is increased compared to a properly functioning switching element 100. The correspondingly stronger inductive effects result in the current signal 122 rising more slowly and reaching a lower maximum value when the switching element 100 is faulty compared to the current signal 121 when the switching element 100 is functioning correctly.A diagnosis using such a control pulse 120 is carried out at least once until the end of the respective driving cycle 124, preferably after the last opening of the switching element 100 during the driving cycle.

[0067] The invention is not limited to the exemplary embodiments described therein. Rather, the invention encompasses every new feature as well as every combination of features, which in particular includes every combination of features in the exemplary embodiments and claims.

[0068] REFERENCE MARK

[0069] 100 switching elements

[0070] 101 Electromagnet

[0071] 102 coil

[0072] 103 Magnetic core

[0073] 104 Switch piece

[0074] 105 anchors

[0075] 106 Locking piece

[0076] 107 Contact / Work contact

[0077] 108 Reset element

[0078] 109 electrical connection / working connection

[0079] 110 Control line / coil connection

[0080] 111 Control signal

[0081] 112 Voltage signal

[0082] 113 Current signal

[0083] 114 dents

[0084] 115 stroke of the switching piece

[0085] 116 maximum stroke / air gap width

[0086] 120 control pulse

[0087] 121 Control pulse current with intact switching element

[0088] 122 Control pulse current with closed, jammed switching element

[0089] 123 stroke with the switching element clamped closed

[0090] 124 End of driving cycle

[0091] 130 control unit

Claims

PATENT CLAIMS 1. Method for fault diagnosis of a switching element (100) comprising an electromagnet (101) and a switching piece (104) and configured to close or open an electrical connection (109) by moving the switching piece (104) by means of a magnetic field of the electromagnet (101), the method comprising the following steps: Controlling the switching element (100) using a control signal (111) which influences the magnetic field of the electromagnet (101); Performing a fault diagnosis of the switching element (100) using an evaluation criterion based on a course of the control signal (111) that correlates with an arrangement of the switching piece (104) in relation to the electromagnet (101).

2. Method according to the preceding claim, wherein the electromagnet (101) has a coil (102) and wherein the control signal (111) comprises a voltage (112) applied to the coil (102) and / or a current (113) flowing through the coil (102).

3. Method according to the preceding claim, wherein the evaluation criterion is based on the course of the voltage (112) applied to the coil (102) when the switching element (100) is activated to disconnect the electrical connection (109).

4. Method according to claim 2 or 3, wherein the evaluation criterion is based on the course of the current (113) flowing through the coil (102) when the switching element (100) is activated to close the electrical connection (109).

5. Method according to one of the preceding claims, wherein the evaluation criterion is based on a gradient of the course of the control signal (111 ).

6. Method according to the preceding claim, wherein the evaluation criterion is based on a sign change of the gradient, in particular on two successive sign changes of the gradient.

7. Method according to one of the preceding claims, wherein the evaluation criterion is based on a dip (114) during a rise or during a fall in the course of the control signal (111).

8. Method according to one of the preceding claims, wherein the switching element (104) is moved over a distance of at least 0.1 mm, in particular at least 0.5 mm, in particular at least 1 mm.

9. Method according to one of the preceding claims, wherein, during fault diagnosis, a faulty switching element (100) and / or a proper switching element (100) is detected based on a current (113) induced by the arrangement of the switching element (104) or a voltage (112) induced by the arrangement of the switching element (104).

10. Method according to one of the preceding claims, wherein, when performing the fault diagnosis, it is detected by means of the evaluation criterion whether the switching element (100) is jammed closed, in particular welded shut.

11. Method according to one of the preceding claims, wherein the switching element (100) is controlled by means of a control pulse (120) which is designed such that the electrical connection (109) is not completely closed.

12. Method according to the preceding claim, wherein the evaluation criterion is based on a slope (111) of the control signal and / or on a maximum value of the control signal (111).

13. A method according to any one of the preceding claims, wherein the evaluation criterion additionally takes into account voltages measured before and / or after the switching element (100).

14. A control unit (130) configured to perform a method according to any one of the preceding claims.

15. Computer program comprising instructions which, when executed by a computer, cause the computer to perform a method according to any one of claims 1 to 13.

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