Method for fault diagnosis of a switching element and control unit

Abstract

The invention relates to a method for fault diagnosis of a switching element (100) comprising an electromagnet (101) and a switching element (104), and which is configured to close or open an electrical connection (109) by moving the switching element (104) by means of a magnetic field of the electromagnet (101). The electromagnet (101) is controlled by means of a control circuit (111), wherein the control circuit (111) comprises a limiting element (120) configured to limit a voltage applied to the electromagnet (101).The method comprises the following steps: (i) opening actuation of the switching element (100); (ii) performing a fault diagnosis of the switching element (100) using an evaluation criterion based on the time course of an electrical quantity characterizing the control circuit (111) in response to the opening actuation, wherein the course depends on the limiting element (120) and on an arrangement of the switching element (104) with respect to the electromagnet (101). The invention further relates to a corresponding control unit (130) and a computer program.

Description

TECHNICAL AREA

[0001] The present disclosure relates to methods and control devices for fault diagnosis of a switching element, for example a relay or a contactor, such as those used in battery management systems of electric vehicles. Specifically, methods and control devices are described for detecting whether the switching element has opened correctly or incorrectly. BACKGROUND OF THE INVENTION

[0002] 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.

[0003] Due to safety requirements, it is important that the contactors open cleanly to reliably disconnect the high-voltage battery from the system. Disconnecting the battery is necessary, for example, when the vehicle is switched off, in the event of an accident, or in certain fault conditions. This ensures that no dangerous voltage is present in such situations.

[0004] However, various faults can occur during the control and operation of contactors. In particular, the contactors can stick or weld together while closed, for example due to high currents, and consequently no longer open. Such a fault must be reliably detected and diagnosed.

[0005] A conventional method for contact state detection in contactors is the use of positively guided auxiliary contacts, which are electrically connected when the contactor closes. Determining the contact state of the load switching contact is relatively easy for the user because the auxiliary contacts are typically electrically isolated from both the coil circuit and the load circuit. However, such auxiliary contacts increase the cost of the contactor, require additional installation space, additional drive power, and necessitate consideration of reliability and service life.

[0006] Another conventional method is the direct measurement and evaluation of the electrical voltages in the contactor's load circuit. With the contact open, a large voltage difference across the load terminals is to be expected. Conversely, with the contact closed, the voltage difference should be very small. However, especially in applications with higher operating voltages on the load side, the measurement effort becomes considerable, particularly if galvanic isolation from the control and evaluation electronics is to be maintained. Furthermore, there are cases where a clear determination of the contact state by voltage measurement is difficult, for example, when decoupling capacitors are used on the load side. SUMMARY AND FORMS OF EXECUTION

[0007] It is therefore an objective of the present disclosure to provide a convenient and reliable method for diagnosing faults in a switching element, in particular for detecting whether the switching element is opening correctly and / or incorrectly, and especially for detecting a welded switching element.

[0008] 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.

[0009] 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 electromagnet is controlled by means of a control circuit, wherein the control circuit has a limiting element configured to limit a voltage applied to the electromagnet, in particular one generated by self-induction during switch-off.The method comprises the following steps: (a) opening the switching element; (b) performing a fault diagnosis of the switching element using an evaluation criterion based on the time course of an electrical quantity characterizing the control circuit in response to the opening action, the course depending on the limiting element and on the arrangement of the switching element relative to the electromagnet. The course can correlate with the arrangement of the switching element relative to the electromagnet. The arrangement can be a time-varying arrangement, i.e., a movement, or a static arrangement. The course can be discrete or continuous.

[0010] 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.

[0011] According to another aspect, a computer program is provided that includes instructions which, when executed by a computer, cause it to perform the procedure described above. In the context of this disclosure, a computer is defined, for example, as a device that processes data using programmable computational instructions. Computers can be embedded in everyday devices, such as the control units of motor vehicles.

[0012] According to another aspect, a storage medium is provided with a computer program, wherein the computer program includes instructions which, when the computer program is executed by a computer, cause it to carry out the procedure described above.

[0013] 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.

[0014] 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.

[0015] 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 have 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. The closing element may have an overtravel spring or contact spring which is configured to assist in closing the electrical connection by means of spring force.

[0016] In the context of this disclosure, a control circuit is defined, for example, as a circuit by means of which the switching element, in particular the electromagnet of the switching element, is controlled. The control circuit may be configured to provide and / or control a voltage applied to the coil of the electromagnet and / or a current flowing through the coil of the electromagnet. The control circuit may be galvanically isolated from a load circuit that has the electrical connection to be closed or opened. The control circuit may be a low-voltage circuit, in particular with a voltage < 60 V DC. The load circuit may be a high-voltage circuit, in particular with a voltage > 60 V DC.

[0017] In the context of this disclosure, an electrical quantity characterizing the control circuit is defined, for example, as a function that depends on one or more electrical quantities characterizing one or more components of the control circuit. For example, the electrical quantity characterizing the control circuit may depend on a voltage applied between two points of the control circuit and / or on the current flowing through a region of the control circuit. The voltage may be a coil voltage applied to a coil of the electromagnet. The current may be a coil current flowing through the coil of the electromagnet.

[0018] In the context of this disclosure, a limiting element is defined, for example, as an electrical or electronic component configured to limit a voltage applied to the electromagnet, particularly during and / or after the opening actuation of the switching element. The voltage may be applied to a coil of the electromagnet. The limiting element may be configured to limit the inductive voltage generated when the coil is switched off, in particular a negative overvoltage. The limiting element may be connected in parallel to the electromagnet, in particular to the coil of the electromagnet, in the control circuit. The limiting element may include a suppressor diode and / or a resistor.

[0019] The described method and the corresponding control unit can be advantageous for reliably diagnosing the opening of the switching element or the disconnection of the electrical connection. Because the behavior of the electrical quantity characterizing the control circuit depends on the arrangement of the switching element relative to the electromagnet, an analysis of this behavior using a suitable evaluation criterion can detect expected or unusual movement of the switching element. 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 stuck closed. Furthermore, an unusual position of the switching element can be diagnosed, such as if the switching element is stuck in the closed state even without being actuated, for example, because the switching element is welded shut.The relationship between the electrical quantity characterizing the control circuit and the switching element 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 and thus also affects electrical quantities characterizing the control circuit, such as coil current or coil voltage.

[0020] A diagnostic check during the opening actuation of the switching element can be advantageous because a non-opening switching element is particularly problematic from a safety perspective compared to, for example, a switching element that is stuck in the open position. For instance, uncontrolled current flows can occur if the contact surfaces of the switching element are welded together, which in extreme cases can endanger human lives and cause fires. By performing a diagnostic check during the opening actuation, a properly functioning switching element can be identified at the end of a driving cycle.

[0021] The fact that the current waveform depends on the limiting element can be advantageous when choosing the evaluation criterion. For example, if the limiting element is a suppressor diode, the coil current drops very rapidly after the opening action. It can already be close to zero before the switching element even begins to open. If, on the other hand, the limiting element is a resistor, the opening action can also be read from the coil current waveform. Furthermore, the waveform may depend, for example, on the breakdown voltage of the suppressor diode or on the resistance value of the limiting resistor, which may need to be considered for the evaluation criterion. Such dependencies can be used advantageously for fault diagnosis.

[0022] Furthermore, this method can be advantageous because the diagnostics are performed on the coil side within the control circuit, which is typically operated at a low voltage. In contrast, load-side diagnostics, as practiced in the prior art, are more demanding and complex in many applications due to the generally higher voltages involved, partly because galvanic isolation between the load and coil sides must be maintained. Finally, additional effort can be avoided if readily available measured variables are used for the electrical parameters characterizing the control circuit, such as the coil voltage or the coil driver current.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] According to one embodiment, the electromagnet comprises a coil, and the electrical quantity characterizing the control circuit correlates with a voltage applied to the coil and / or a current flowing through the coil. According to one further embodiment, the current and / or voltage across the coil can be determined by means of 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.

[0027] According to one embodiment, the evaluation criterion is based on the time course of a voltage applied to the coil in response to the opening action. Such an embodiment can be advantageous because inductive effects in the voltage signal can be particularly pronounced. This can be the case especially when the electrical connection is broken and / or if the limiting element has a diode, because then the current is typically low or zero, and correspondingly, inductive effects in the current signal are only weakly pronounced. According to one embodiment, the voltage applied to 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 or controlled, but results, in particular, from energy stored in the coil, which dissipates during freewheeling.

[0028] According to one embodiment, the limiting element comprises at least one of the following electrical or electronic components: a diode, in particular a diode configured to conduct in reverse bias when a voltage applied to the diode exceeds a predetermined limit; a resistor. The predetermined limit can be a breakdown voltage. The diode can be a flyback diode or, preferably, a suppressor diode or a Zener diode. The suppressor diode can be bidirectional or unidirectional. A unidirectional suppressor diode can be used as a limiting element in combination with another diode connected in series in reverse bias. Likewise, a Zener diode can be used as a limiting element in combination with another diode connected in series in reverse bias.All these limiting elements can be advantageous because they protect components of the control circuit, especially the electromagnet, from excessive voltages. With suppressor diodes, the voltage peak can be smaller and the voltage drop faster compared to, for example, a resistor, especially a high-resistance resistor. However, resistors are typically less expensive compared to, for example, suppressor diodes.

[0029] According to one embodiment, the electromagnet has a coil and the limiting element is connected in parallel to the coil. This parallel connection can be advantageous for limiting voltages occurring across the coil using limiting elements such as resistors or suppressor diodes.

[0030] According to one embodiment, the electrical quantity characterizing the control circuit depends on a voltage across the limiting element and / or a voltage across the coil. Such an embodiment can be advantageous because the respective waveforms of these voltages are indicative of whether the switching element moves in response to the opening actuation. It has been found that fault diagnosis during the opening of the switching element is generally more challenging than during the closing of the switching element because the signals to be analyzed are generally less pronounced and the signal progression is faster. Under these circumstances, the voltage signal can be particularly suitable for fault diagnosis. For example, if the limiting element is a suppressor diode or a Zener diode, the current signal decays much faster than the voltage signal and is often already close to zero when the switching element moves.

[0031] According to one embodiment, the evaluation criterion is based on the investigation of deviations from an exponential curve, particularly with regard to the voltage across the limiting element and / or the coil voltage. The deviation can be in the form of a bump or a dent. Such an embodiment can be advantageous because an exponential voltage drop of the aforementioned quantities is to be expected in the case of a glued or welded switching element, whereas when the switching element is properly opened, the changing inductance of the electromagnet causes a deviation from exponential behavior.

[0032] According to one embodiment, the evaluation criterion takes into account a time derivative of the curve at at least one point in time, and in particular at several points in time. For example, deviations from an exponential curve can be detected using the time derivative. If the electrical quantity characterizing the control circuit is, for example, a coil voltage or a voltage across the limiting element, a gradient reversal can occur during the curve when the switching element is properly opened, and this can be used for the evaluation criterion. Evaluating the time derivative can be advantageous because characteristics of the curve, such as the slope or extrema, can be analyzed particularly easily using the derivative.

[0033] According to one embodiment, the evaluation criterion considers a time integral of the behavior. Such an embodiment can be particularly advantageous for opening actuation. It was previously mentioned that, compared to closing actuation of the switching element, the differences in behavior between a functioning and a faulty switching element can be smaller during opening actuation. By integrating values ​​over an extended period, a more precise fault analysis can be achieved compared, for example, to a point-in-time evaluation of the behavior or a time derivative of the behavior.

[0034] In one embodiment, the starting point of the time integral is defined by a change in the sign of the electromagnet's coil voltage in response to the opening action. In another embodiment, the endpoint of the time integral is defined by a fixed integration time or by the coil voltage falling below a threshold value.

[0035] According to one embodiment, the integration of the time integral is performed using an integration circuit. Such hardware-implemented integration can have the advantage over software-implemented integration of typically being faster and more reliable. Furthermore, less expensive analog-to-digital converters with lower temporal resolution can be used. Alternatively, the integration can, of course, be implemented using software.

[0036] According to one embodiment, the integration circuit integrates a voltage signal, which may be provided, for example, by a voltmeter connected in parallel to the coil and / or the limiting element in the control circuit.

[0037] According to one embodiment, integration start is implemented using a diode. For example, the diode can be configured and arranged such that only values ​​after a sign change of the quantity to be integrated, for example the coil voltage, contribute to the time integral.

[0038] According to one embodiment, the time integral is representative of at least one of the following quantities: a switching energy; work done on the limiting element, particularly if it is a resistor, and / or on the resistance of the electromagnet's coil; an induced voltage integral; a magnetic flux that has flowed through a magnetic circuit of the switching element before the opening action. The switching energy can take into account energy stored in a restoring element, for example, a spring, of the switching element. It can also take into account energy stored in an overtravel spring of the switching element. Such an embodiment can be advantageous because these quantities can reflect the change in inductance due to the movement of the switching element.

[0039] According to one embodiment, the time integral is compared with a reference value for the evaluation criterion. In particular, it can be checked whether the integral is greater or less than the reference value. For example, proper switch opening can be assumed if the integral is greater than the reference value. A fault, such as welding of the switching element, can be assumed if the integral is less than the reference value. The reference value can be determined, for example, using a plurality of instances of the switching element in which the switching piece is either free or blocked.

[0040] According to one embodiment, the reference value depends on the current flowing through the coil before the opening action. Accordingly, it may be necessary for the coil current to assume a predetermined value before the opening action. Alternatively, coil current-dependent reference values ​​can be stored in a lookup table, a characteristic map, or a formula. The method then involves determining the coil current before the opening action and selecting the reference value based on this determination.

[0041] According to one embodiment, the coil resistance of the electromagnet and / or the voltage across the coil resistance are not considered for the evaluation criterion. For example, the proportion of the voltage drop across the coil resistance may be negligible for fault analysis. This can be the case when a small holding voltage of the switching element is combined with a comparatively high limiting voltage of the limiting element. Such an embodiment can be advantageous because properties of the coil resistance do not need to be determined, thereby saving, for example, measurement hardware (such as for the coil current) or at least computing power and time.

[0042] According to one embodiment, the evaluation criterion takes into account the coil resistance of the electromagnet and / or the voltage across the coil resistance. Such an embodiment can enable a particularly accurate and reliable fault evaluation of the switching element.

[0043] According to one embodiment, the resistance value of the coil is determined based on the current of a driver current of the control circuit, which is supplied to the control circuit before activation, particularly immediately before activation. The activation is preferably an opening activation. The driver current can be supplied by means of a coil driver. Such an embodiment can be advantageous because the coil resistance can be determined without measuring the current in the branch containing the coil.

[0044] According to one embodiment, the temperature dependence of the coil resistance is taken into account. This temperature dependence can be considered, for example, by measuring the coil current and voltage before the switching element is activated and then calculating the coil resistance based on these measurements. Such an embodiment can be advantageous if the switching element is to be operated over a wide temperature range, such as in automotive applications.

[0045] According to one embodiment, performing the fault diagnosis includes detecting a faulty switching element and / or detecting a proper switching element, in particular based on the evaluation criterion.

[0046] 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.

[0047] According to one embodiment, the evaluation criterion used during fault diagnosis 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] 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.

[0049] They show: Fig. 1 a switching element and a control unit for controlling the switching element according to an embodiment of the present disclosure; Fig. 2 a control circuit for controlling a switching element in order to carry out a fault diagnosis method according to an embodiment of the present disclosure; Fig. 3 different boundary elements for the in Fig. 2 control circuit shown; Fig. 4 and Fig. 5 exemplary waveforms of a coil current and a coil voltage during the opening actuation of a switching element for evaluation in a method according to an embodiment of the present disclosure; the waveform of Fig. Figure 4 illustrates a proper switching element, the course of Fig. 5 a closed clamping switching element.

[0050] 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. DETAILED DESCRIPTION OF EXAMPLES OF EXECUTION

[0051] Fig. Figure 1 shows a switching element 100, here a contactor, which has an electromagnet 101 and a switching piece 104 and which is designed to close or open an electrical connection by moving the switching piece 104 by means of a magnetic field of the electromagnet 101.

[0052] 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.

[0053] Adjacent to the electromagnet 101 is the switching element 104, which is movable along a predetermined direction with respect to the electromagnet 101, in particular the coil 102 of the electromagnet 101. 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.

[0054] 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.

[0055] The control unit 130 is configured to perform a fault diagnosis procedure for the switching element 100. The procedure comprises the following steps: (a) opening the switching element 100 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 time course of a control circuit (see reference numeral 111, Fig. 2) characteristic size in response to the opening actuation, the course being determined by a limiting element (see reference numeral 120, Fig. 2) and depends on an arrangement of the switching element 104 in relation to the electromagnet 101.

[0056] Fig. Figure 2 shows a control circuit 111 for controlling, for example, the switching element 100. Fig. 1. The control circuit 111 comprises a coil driver 113 and a measuring device 115 for determining the coil driver current Ic_d. The following elements are connected in parallel in the control circuit 111: a measuring device 114 for determining the coil voltage Uc; a limiting element 120, in this case a bidirectional suppressor diode 121; and finally an electromechanical drive with winding 116 or a coil 102 of an electromagnet 101, which is configured to move a switching element 104 that can open and close an electrical connection 109 by means of a closing element 106 (see Figure 11). Fig. 1) In the branch of the parallel circuit with the electromechanical drive 116, the coil current Ic can be measured using a measuring device. The coil has an inductance Lc and a temperature-dependent resistance Rc. The working connection 109 is part of a high-voltage load circuit 112, while the control circuit 111 is a low-voltage circuit that is galvanically isolated from the load circuit 112.

[0057] Fig. Figure 3 shows various limiting elements 120, which replace the bidirectional suppressor diode 121. Fig. 2 can be used, from left to right: a combination of a unidirectional suppressor diode 121 and a diode 124; a combination of a Zener diode and a diode 124; a resistor 126 with resistance value R; a freewheeling diode 125.

[0058] The Fig. 4 and Fig. Figure 5 shows the curves of various electrical quantities characterizing the control circuit 111, in particular the coil current Ic and the coil voltage Uc (see Figure 5). Fig. 2), plotted against time t. A suppressor diode was used as the limiting element. The course of Fig. Figure 4 illustrates the proper opening of the switching element, the course of Fig. 5 a closing clamping switching element. In comparison of the Fig. 4 and Fig. 5 is in Fig. 4 in the voltage signal Uc a bump-shaped deviation from the exponential curve is recognizable, which indicates a proper opening movement 143 of the switching element.

[0059] The proposed diagnosis of the contact state after switching off a DC relay or contactor coil 116 refers to the typical control of such a coil 116 by a coil driver 113, which provides either a predetermined voltage or a predetermined current and switches this voltage or current on and off to switch the relay / contactor on and off. As already mentioned, a bidirectional suppressor diode 121, also called a TVS diode, is provided to limit the inductive negative overvoltage that occurs when the coil 116 is switched off. A unidirectional TVS diode 122 or a Zener diode 123 with an additional blocking diode 124 has a comparable limiting effect. The following explanations apply to these types of limiting elements 120, in contrast to limiting elements 120 such as the connection with a resistor 126 or the connection with a freewheeling diode 125.

[0060] The aforementioned basic control concept and the aforementioned limiting elements are in Fig. 2 and Fig. Figure 3 shows the coil circuit 111, which is often located at a low-voltage level, e.g., 12 V DC, while the load circuit 112 is galvanically isolated at a high-voltage level, e.g., 400 V DC or AC. For the function of the coil driver 113 and for functional diagnostics, the coil circuit 111 often includes a coil current measurement Ic_d (see reference numeral 115) and a coil voltage measurement Uc (see reference numeral 114). During operation, the relay coil 116 carries the coil current Ic and is electrically characterized by the ohmic coil resistance Rc and the coil inductance Lc, where Rc and Lc vary depending on the operating state of the relay / contactor due to heating and magnetic saturation.

[0061] Especially in safety-relevant applications, a challenge lies in diagnosing the safe open state of the switching contact 107 on the load side after the switching element 100, here a relay or a contactor, has been switched off, i.e. to be able to reliably detect, for example, a stuck or welded contact 107 or a blocked armature 105.

[0062] The approach described here is based on a special dynamic effect when relays and contactors are switched off. This effect is commonly called armature reaction. Fig. 4 and Fig. Figure 5 illustrates the effect of the coil voltage Uc and coil current Ic over time when a relay or contactor coil is switched off, using the example of a DC contactor for high-voltage vehicle battery applications with a 30V suppressor-diode circuit. When an inductor with a suppressor-diode circuit D is switched off, the coil current Ic and the coil voltage Uc do not drop to zero abruptly, but rather exhibit a typical curve for this type of circuit. The coil voltage Uc first jumps to a negative value in the range of the suppressor diode's breakdown voltage for a few milliseconds before approaching zero with an approximate exponential function. The coil current Ic drops to zero during the diode breakdown time.

[0063] If, as in the case of a relay or contactor, a movable armature 105 is present that is spring-loaded in the opening direction, then the exponential function of the coil voltage decay curve is significantly deformed at the opening moment of the armature 105. This is due to the dramatic increase in the magnetic resistance of the iron core due to the opening of the working air gap. The mechanical energy stored by the pre-loaded return spring 108 is released and is partially visible as an additional electrical voltage across the coil. Fig. 4) In the case of a mechanically blocked armature 105, e.g. if the switching contact 107 connected to the armature 105 is welded shut, the course of Uc is actually almost a smooth exponential function ( Fig. 5).

[0064] In the contact state detection approach described here, the integral of the induced voltage is calculated directly after the switch-off moment. This voltage is the sum of the external coil voltage Uc and the voltage drop across the internal ohmic resistance URc. Physically, the induced voltage integral is a measure of the magnetic flux Φ that flowed through the magnetic circuit of the relay or contactor before the switch-off and depends essentially only on the coil holding current and, of course, the armature position.

[0065] This induced voltage integral Ψ is then compared with a previously stored value for Ψ, the reference value. If the measured induced voltage integral Ψ is greater than the reference value, then the armature 105 has opened, and consequently, so has the connected switching contact 107. The reference value is determined in advance by measurements of the free armature 105 and the blocked armature 105 on a sufficient number of units of the respective type series. If there is too much variation between units, the induced voltage integral Ψ for the normally free armature 105 can also be determined for each individual unit before use, and the reference value can then be set by subtracting a suitable amount.

[0066] An important aspect for the correct functioning of this approach is that the induced voltage integral Ψ, and thus also the reference value, is approximately linearly dependent on the coil current flowing before switch-off, in addition to magnetic saturation effects. Therefore, it must either be ensured that the coil current always has a fixed value within small error limits before switch-off, to which the reference value is then referenced. Or, as is also taken into account in the following embodiments, the coil current must have been measured immediately before the switch-off process. Then, a stored function, characteristic curve, or table of values ​​is used that defines the reference value as a function of the coil current.

[0067] Furthermore, it must generally be considered that the induced voltage after the switch-off moment occurs not only at the limiting element 120 (Uc) but also at the ohmic resistance of the coil (URc). Although the significantly larger component of the induced voltage integral Ψ occurs at the limiting element 120, and the armature feedback hump typically only appears after the coil current has dropped to zero, neglecting the ohmic coil resistance Rc would nevertheless contribute a non-negligible error, depending on the coil design. This is because the ohmic coil resistance Rc is dependent on the coil temperature by approximately 0.4% / K, which can vary considerably depending on the ambient temperature and operating conditions, e.g., between -40°C and 180°C.

[0068] The following are detailed examples of various implementations: The first embodiment relates to the case where coil voltage Uc and coil current Ic are taken from the coil circuit 111 according to Fig. 2 are available and the induced voltage integral is calculated as a comparison value.

[0069] In this first embodiment, the induced voltage integral Ψ is determined as the integral of the electrical voltage generated by the inductance L of the coil, which drops across the limiting element 120 and the ohmic coil resistance Rc immediately after the coil is switched off, with the coil voltage Uc(t) and the coil current Ic(t) available as input measurements. The contact state determination process can be programmed, for example, on microcontroller hardware, but can also be implemented as an analog computer using an operational amplifier (op-amp).

[0070] The induced voltage integral Ψ is obtained as the integral of the measured electrical voltage Uc(t) across the limiting element 120 and the electrical voltage URc(t) across the ohmic resistance of the coil. Both voltages are added as positive quantities. The starting point of the integration is defined by the jump of the coil voltage Uc(t) into the negative range. The endpoint of the integration is defined either by an integration time fixed for this type of relay or contactor, or by the point at which the input measured quantity Uc(t) has sufficiently approached zero. ψ=∫(Uc(t)+URc(t))dt

[0071] The voltage drop URc across the ohmic resistance of the coil is calculated by multiplying the measured coil current Ic(t) by the coil resistance Rc: U Rc (t) = I c (t) · R cThe ohmic coil resistance Rc cannot be assumed to be constant and stored as a fixed value due to its strong dependence on the operating temperature, but is calculated from the coil voltage Uc_h and the coil current Ic_h in the holding state of the relay or contactor, immediately before the coil is switched off: Rc=Uc_hIc_h

[0072] To compare the determined induced voltage integral Ψ with the reference value, the latter is first determined by applying the coil current Ic_h, which flowed through the coil immediately before switching off, to a stored characteristic curve or table of values. Finally, the comparison between the determined induced voltage integral Ψ and the reference value results in the output "Contact is open" if Ψ is greater than the reference value, and otherwise in the statement "Contact is still closed".

[0073] The second embodiment concerns the case where coil voltage Uc and coil driver current Ic_d are taken from the coil circuit according to Fig. 2 are available and the induced voltage integral is calculated as a comparison value.

[0074] In this second embodiment, the measured quantity coil current Ic(t), i.e., the current in the coil discharge circuit, is not available. Only the measured quantities coil voltage Uc(t) and coil driver current Ic_d are available from the coil circuit, according to... Fig. 2 for evaluation. Therefore, in contrast to the first embodiment, only the electrical voltage Uc (t) that drops across the limiting element 120 immediately after the coil 102 is switched off is used in this second embodiment to determine the induced voltage integral Ψ.

[0075] This simplified approach to the induced voltage integral Ψ can be used when the component of the voltage drop across the ohmic coil resistance Rc during the integration is so small that it can be neglected compared to the useful signal, which results from the difference in the induced voltage integral of the free armature 105 and the blocked armature 105. This can be the case, in particular, when the design conditions of coil 102 and coil driver 113 allow the combination of a very low holding voltage with a correspondingly high TVS limiting voltage. For example, in Fig. 4 and Fig. Figure 5 shows such a configuration, where approximately 3V holding voltage and approximately 30V limiting voltage are combined. The portion of the induced voltage integral originating from the ohmic coil resistance is less than 1% of the total integral in this case. Simultaneously, the difference in the coil voltage integral between a free armature 105 and a blocked armature 105 in this example is in the range of 15-20%.

[0076] The contact state determination process for this second embodiment can be programmed on microcontroller hardware, for example, but can also be implemented as an analog computer using an operational amplifier (op-amp). The induced voltage integral Ψ is calculated as the integral of the measured electrical voltage Uc(t) across the limiting element 120. The starting point of the integration is defined by the jump of the coil voltage Uc(t) into the negative range. The endpoint of the integration is defined either by a fixed integration time for this type of relay or contactor, or by the point at which the input measurement Uc(t) has sufficiently approached zero.

[0077] To compare the determined induced voltage integral Ψ with the reference value, the latter is first determined by applying the coil current Ic_h, which flowed through the coil immediately before switching off, to a stored characteristic curve or table of values. Finally, the comparison between the determined induced voltage integral Ψ and the reference value results in the output "Contact is open" if Ψ is greater than the reference value, and otherwise in the statement "Contact is still closed".

[0078] An advantageous variant of the second embodiment relates to the case where the coil voltage Uc and coil driver current Ic_d are taken from the coil circuit according to Fig. One value is available, and the induced voltage integral is calculated as a comparison value. A hardware integrator is used.

[0079] In this advantageous variant of the second embodiment, integration over the negative time course of the coil voltage Uc(t) is performed in real time by means of an electronic circuit. The subsequent comparison of the induced voltage integral Ψ with the reference value to determine the contact state then takes place on a microcontroller control unit, which is advantageously also responsible for controlling the coil driver. The advantage of this variant is that, for the integration task, which must start precisely at the negative voltage step of Uc(t), no high-precision, high-resolution analog-to-digital conversion of Uc(t) is required. Instead, the integration result Ψ is read in as a DC voltage value by the control unit at the end of the integration time.The further value to be read in, Ic_d, as an operating parameter of the coil circuit, immediately before the coil is switched off, is also an electrical DC voltage quantity.

[0080] The coil voltage Uc(t) coming from the coil circuit is first fed to a circuit consisting of diode D and resistor R1. This circuit ensures that only the negative portion of Uc(t) is present at R1, and thus at the input of the integrator, after the coil is switched off. Advantageously, a diode with a short forward recovery time and low forward voltage is used. The negative portion of Uc(t) is then fed to an inverting integrator amplifier, which essentially consists of an operational amplifier in combination with resistor R2 and capacitor C. The product of R2 and C represents the integration time constant and provides scaling for the output voltage of the integrator. Ψ=−1R2⋅C⋅∫Uc(t)dt

[0081] If the output voltage of the integrator, which is read by the control unit, is to be exactly the mathematical integral over the time course of Uc(t), then the value 1 must be chosen for the product R2*C. However, it can sometimes be advantageous to use this scaling factor other than 1 in order to optimally utilize a given voltage range of the A / D converter for reading Ψ. This scaling factor is then taken into account during the processing of Ψ or in the reference value on the control unit. The integrating amplifier also has a reset switch for shorting C to reset the accumulated output voltage to zero after reading and to prevent the output from drifting due to offset voltages and small voltage errors over time. The control unit will advantageously open the reset switch only at the same time as the command to the coil driver to switch off "Coil driver OFF". Examples of suitable reset switches include...A junction field-effect transistor (JFET) or analog switch is a possibility.

[0082] The subsequent comparison of Ψ with the reference value is carried out analogously to the procedure described in the second example.

[0083] 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. REFERENCE MARK 100 switching elements 101 Electromagnet 102 coil 103 Magnetic core 104 Switch piece 105 anchors 106 Locking piece 107 Contact (work contact) 108 Reset element 109 electrical connection (working connection) 110 Control line / coil connection 111 Control circuit / Low-voltage coil circuit 112 High-voltage load circuit 113 Coil drivers 114 Voltage meters 115 current meters 116 electromechanical drive with winding 120 boundary element 121 bidirectional suppressor diode 122 unidirectional suppressor diode 123 Zener diode 124 Diode 125 freewheeling diode 126 Resistance 130 control unit 143 Opening movement Uc coil voltage URc voltage across the coil's ohmic resistance IC coil current RC coil resistance Lc coil inductance Ic_d coil driver current U voltage I current t time

Claims

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), wherein the electromagnet (101) is controlled by means of a control circuit (111), wherein the control circuit (111) comprises a limiting element (120) configured to limit a voltage applied to the electromagnet (101), wherein the method comprises the following steps: - opening control of the switching element (100);- Performing a fault diagnosis of the switching element (100) by means of an evaluation criterion which is based on a time course of an electrical quantity characterizing the control circuit (111) in response to the opening actuation, wherein the course depends on the limiting element (120) and on an arrangement of the switching piece (104) in relation to the electromagnet (101). Method according to the preceding claim, wherein the limiting element (120) comprises at least one of the following electrical or electronic components: a diode (121, 122, 123) which is configured to conduct in a reverse direction when a voltage applied to the diode (121, 122, 123) exceeds a predetermined limit; a resistor (126). Method according to one of the preceding claims, wherein the electromagnet (101) has a coil (102) and the limiting element (120) is connected in parallel to the coil (102). Method according to one of the preceding claims, wherein the electrical quantity characterizing the control circuit (111) depends on a voltage across the limiting element (120). Method according to one of the preceding claims, wherein the evaluation criterion is based on the investigation of a deviation of the course from an exponential course. Method according to one of the preceding claims, wherein the evaluation criterion takes into account a temporal derivation of the course at at least one point in time. Method according to one of the preceding claims, wherein the evaluation criterion takes into account a temporal integral of the course. Method according to the preceding claim, wherein a starting point of the time integral is defined via a change in sign of a coil voltage (Uc) of the electromagnet (101) in response to the opening actuation. Method according to claim 7 or 8, wherein the time integral is representative of at least one of the following quantities: a switching energy; an induced voltage integral; a magnetic flux that has flowed through a magnetic circuit of the switching element before the opening actuation. Method according to one of claims 7 to 9, wherein the time integral is compared with a reference value for the evaluation criterion. Method according to the preceding claim, wherein the reference value depends on a current (Ic, Ic_d) flowing through the coil before the opening actuation. Method according to one of the preceding claims, wherein for the evaluation criterion a coil resistance (Rc) of the electromagnet (101) and / or a voltage (URc) across the coil resistance (Rc) is taken into account. Method according to one of the preceding claims, wherein a temperature dependence of the coil resistance (Rc) is taken into account. Control unit (130) which is configured to perform a method according to one of the preceding claims. 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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