DETECTION OF THE SWITCHING STATE OF A SWITCHING ELEMENT

Description

[0001] The invention relates to a circuit arrangement and a method for detecting a switching state of a switching element, in particular within a power supply path.

[0002] Switching elements within the scope of the invention are known to be used to establish or disconnect an electrically conductive connection between two electrically conductive terminals or contacts by means of a switching path running between them, depending on the switching state. These switching elements can conventionally be configured as ordinary on / off switches, normally open (NO) switches, normally closed (NC) switches, or changeover switches. An ordinary on / off switch requires an external force or control only for the switching action itself, i.e., for establishing or disconnecting an electrically conductive connection between the two electrically conductive terminals or contacts. Normally open (NO) and normally closed (NC) contacts are defined by their respective rest and operating positions. In the rest position of the normally open contact, it is in its closing switching state (in English, "normally closed" - "NC"), i.e.,An electrically conductive connection is established between the two terminals or contacts, and it must be continuously energized, i.e., actuated, to assume the interrupting switching state, i.e., to open the electrically conductive connection between the terminals or contacts. A normally open (NO) contact, on the other hand, is in its rest position in its interrupting switching state (i.e., the electrically conductive connection between the two terminals or contacts is broken), and it must be continuously energized, i.e., actuated, to assume the closing switching state, i.e., to close the electrically conductive connection between the terminals or contacts. The closing switching state of a normally open contact is thus its operating position.

[0003] A changeover switch, also known as a changeover or toggle switch, has, in addition to the two terminals or contacts described above, at least one further terminal or contact, so that one electrically conductive connection is opened and another is closed simultaneously. A changeover switch thus establishes an electrically conductive connection between two terminals or contacts in every position. One of the terminals or contacts is the common terminal (or "COM") and, when switching, is connected to either the other terminal or to a different terminal or contact.

[0004] Normally, open and closed contacts switch monostablely without any special precautions, meaning that of two or more possible states, only one is stable (rest position). The other state (operating position) is maintained only as long as an external force acts upon it, preventing it from returning to its rest position. The return of a monostable switching element to its rest state can also be delayed by a time constant. A changeover switch, on the other hand, can be designed as a monostable switching element or as a stable switching element, where the external force is only required for the switching itself. If the third or any subsequent terminal is not used in the latter case, the changeover switch can also be used as an ordinary on / off switch. Monostable switching elements can, for example, be designed as momentary switches, i.e.,Switches are activated by pressing and automatically return to their initial position after being released, often with the aid of a mechanical spring. Other examples include contactors and relays. Contactors are conventionally defined as electrically or electromagnetically operated switches for high electrical loads, while relays are electrically operated, remotely controlled switches, typically with two switching positions, which are activated via a control circuit and can switch other circuits. Unlike contactors, which have only one or more normally open and normally closed contacts, relays can also have changeover contacts (changeover switches).

[0005] Switching elements can also be positively guided, in which case a normally open (NO) and a normally closed (NC) contact are mechanically connected in such a way that the NO and NO contacts can never be closed simultaneously. This means, for example, that a normally closed contact welded shut by overload—that is, one that does not open when the coil is de-energized—results in no normally open contact closing. In this case, a normally closed contact can therefore also be used to diagnose or monitor the state of the normally closed contact. This makes positively guided switching elements an ideal standard component for safety-critical applications, particularly, for example, as the ideal basic building block of a safety switching device. With integrated diagnostics, such diagnostics or monitoring can also be performed during operation.

[0006] However, for the use of generally more cost-effective and less complex non-force-guided switching elements in safety-critical applications, especially within a safety switching device, further measures are necessary.

[0007] It is known, for example, that "simple" switching elements, such as simple relays, can be used instead of positively driven switching elements in the field of safety technology, through the use of changeover switches, whereby the monitoring of such non-positively driven switching elements can be carried out, for example, on a microcontroller basis.

[0008] Examples include EP 1 869 687 B1 and EP 3 575 900 B1.

[0009] According to EP 1 869 687 B1, a safety switching device for the safe disconnection of an electrical load, with at least one input for connecting a signaling device, is provided for by an evaluation and control unit to control a switching element in order to interrupt the power supply path to the load. The evaluation and control unit is further configured to perform functional tests at defined intervals to verify the switching function of the switching element, wherein the input for connecting the signaling device is also configured as an input for supplying a supply voltage required for the operation of the switching element.

[0010] According to EP 3 575 900 B1, in a very special embodiment, an intelligent safety relay is provided with two processing chips, each comprising a plurality of connections to control the safety relay to remain in an output standby state after receiving a sampling signal, and with two relay coils, each having a normally closed contact, each of which is connected to a diagnostic signal generation circuit, each having a normally open contact, each of which is connected to a relay output end, and each having a common contact, both common contacts being connected to each other and to a common diagnostic signal detection circuit, each relay coil being controlled by a logic circuit to be switched on or off, so that the respective common contact is electrically connected to the corresponding normally closed or normally open contact;wherein the electrical connections between each common contact and corresponding normally open contact connect the relay output ends to each other;

[0011] In summary, according to the two aforementioned documents, a diagnostic test of changeover relays is implemented using their NC contacts (NC: "normally closed"). Specifically, it is determined whether a test signal fed to the changeover relay via the NC contacts can be read back, whereby each switching element of each changeover switch of the relay is sequentially activated. Subsequent evaluation can then be performed by a microcontroller. Therefore, the NC path of a changeover relay is used for contact monitoring, and a microcontroller is used for control and evaluation.

[0012] DE 10 2010 060 323 A1 also describes a switching arrangement for monitoring the function of a safety circuit, wherein the switching arrangement consists exclusively of relays with non-force-guided contacts and a monitoring unit in the form of a microcontroller.

[0013] US 9,989,594 B2, on the other hand, describes a monitoring circuit for monitoring the switching state of an electrical switching contact, which has a monitoring unit and a transformer, wherein the secondary windings of the transformer together with the switching contact form a monitoring circuit.

[0014] Against the background of the aforementioned prior art, one object of the invention is to realize a new solution for monitoring the switching state of switching elements, which is particularly feasible with discrete components, i.e., with individual electrical circuit elements that can be arranged in their own housing with their own external connections and with only one functional unit instead of integrated circuits (ICs) that combine several electronic components in one housing. In particular, the invention should enable such monitoring even with non-force-guided switching elements, preferably simple normally open contacts, and should expediently be suitable for use in safety-critical applications, especially in safety-related switching devices. Furthermore, it is also desirable that load monitoring can be applied.

[0015] The solution according to the invention is provided by the objects having the features according to the appended independent claims. Advantageous embodiments are the subject of the respective appended claims.

[0016] Accordingly, the invention proposes a method for detecting the switching state of a switching element within a power supply path. This method involves a switching element, arranged between two terminals of a power supply path and possessing two terminals, each electrically connected to one terminal of the power supply path, which can be controlled to selectively assume a first switching state in which a switching path running between the two terminals is closed to close the power supply path between the two terminals, or a second switching state in which the switching path running between the two terminals is open to interrupt the power supply path. The switching element can thus create a resistance value between these two terminals that is either high or low depending on the switching state.

[0017] Furthermore, it is provided that an auxiliary circuit is formed by including the power supply path running at least between the terminal ends with a switching element arranged in between, and that this circuit is supplied with an auxiliary voltage. According to the invention, a voltage applied between the two terminal ends at the switching element is then tapped off and an output signal corresponding to the tapped voltage is output, which is finally evaluated with regard to the current switching state.

[0018] The method therefore offers the advantage that, in principle, a comprehensive recording of the respective switching state, even of non-forced switching elements, can be ensured, thus also enabling their functional testing. Furthermore, since a higher voltage signal and corresponding output signal are generally present in the open switching state than in the closed switching state, it is also possible to easily verify, even when using a normally open contact, whether the "NO path" formed between the two terminals is indeed high-impedance in its off state (i.e., its normally open state). The auxiliary voltage also allows for a wide variety of tests, making the method ideal for safety-critical applications, particularly in safety-related switching devices.

[0019] In a preferred embodiment, the method further includes a step of selectively opening or disconnecting the auxiliary circuit in series with the switching element and evaluating the output signal corresponding to the tapped voltage with respect to a change in level. This procedure allows, in particular, a test to verify the existence of an actually open switching state, since in this case, opening and disconnecting the auxiliary circuit must result in a corresponding change in the output signal.

[0020] To selectively open or close the auxiliary circuit, for example the primary side of an optocoupler, whose secondary side is arranged in series with the switching element as part of the auxiliary circuit, can be controlled by a logic device and the output signal corresponding to the tapped voltage can be evaluated by the logic device with regard to a level change.

[0021] Depending on the specific evaluation, the invention provides for checking the switching element to be tested for functionality or a fault. Consequently, it is possible to detect whether the switching element is functioning correctly or if it is faulty.

[0022] Accordingly, the invention further provides a circuit arrangement for detecting the switching state of a switching element within a power supply path, with which the aforementioned method can also be expediently implemented. The circuit arrangement comprises the switching element and the power supply path, wherein the power supply path, particularly for supplying load current, is arranged between two potential terminals, and the switching element has two connections and is arranged in the power supply path between two terminal ends of the power supply path, each connection being electrically connected to one of the terminal ends. The switching element is further configured to assume, in particular by being controlled by a voltage supplied from outside the circuit arrangement, either a closed or an open switching state.wherein in the closing switching state a switching path running between the two terminals is closed for closing the power supply path between the terminal ends and in the interrupting switching state the switching path running between the two terminals is open for interrupting the power supply path between the terminal ends, according to the invention the circuit arrangement is further characterized by a diagnostic circuit which comprises an auxiliary circuit formed by including the power supply path running at least between the terminal ends with a switching element arranged in between, wherein the diagnostic circuit is at least configured to supply the auxiliary circuit with an auxiliary voltage, to tap off a voltage applied between the two terminal ends at the switching element,for outputting a signal corresponding to the tapped voltage and for evaluating the output signal in relation to the current switching state.

[0023] In addition to the advantages already mentioned regarding the method, the circuit arrangement also offers the advantage that it can basically be built using discrete components.

[0024] In a suitable embodiment, the auxiliary circuit of the diagnostic circuit further includes a resistor arranged in series with the switching element, in particular a resistor whose resistance value is many times lower than the resistance value caused by the switching element in the open switching state between the two terminals. This allows the detection and, in particular, the associated testing to be carried out in the simplest way even in the closed switching state, since in such a diagnostic case the auxiliary voltage can also drop across this resistor. Furthermore, a desired or required level difference between the voltage signals in the open and closed switching states can also be easily influenced by means of such a resistor.

[0025] In a suitable embodiment, it can alternatively or additionally be provided that the diagnostic circuit for outputting the output signal corresponding to the tapped voltage includes an operational amplifier, in particular an operational amplifier used as a differential amplifier, with one non-inverting and one inverting input, wherein each of the inputs is electrically connected to one of the two terminals. Furthermore, in a preferred practical embodiment, each of the inputs of the operational amplifier is electrically connected to its respective terminal via a resistor. This allows for high-impedance decoupling.

[0026] Particularly for carrying out a large number of tests, the diagnostic circuit expediently includes a logic device which is galvanically decoupled to provide the auxiliary voltage for supplying the auxiliary circuit and for evaluating the corresponding output signal.

[0027] For a galvanically decoupled connection to evaluate the corresponding output signal, a first optocoupler and a transformer to provide the electrical auxiliary potential can preferably be included.

[0028] In a particularly preferred embodiment, the diagnostic circuit can additionally or alternatively comprise a second optocoupler, the primary side of which can be controlled by the logic device and the secondary side of which is arranged in series with the switching element as part of the auxiliary circuit. In particular, such a component allows the auxiliary voltage to be activated and deactivated with extreme flexibility, and enables a corresponding dynamic adjustment of the evaluation signal depending on the diagnostic procedure being performed.

[0029] Further advantages and features of the invention will become apparent from the following description of a preferred embodiment with reference to the accompanying drawing, in which the following are shown: Fig. 1 shows a simplified circuit diagram of a preferred embodiment of a circuit arrangement according to the invention, and Fig. 2 shows two exemplary signal waveforms for evaluation with respect to an assumed position; as inFig. 1 depicted current switching state.

[0030] Shown at Fig. 1 Figure 1 is a simplified circuit diagram of a circuit arrangement according to the invention for detecting the switching state of a switching element SW3 within a power supply path. Accordingly, the circuit arrangement comprises the switching element SW3 and the power supply path, which, as shown, is arranged between two potential terminals 13 and 14. The power supply path can thus be used, in particular, for supplying load current, so that, for example, a working voltage is present or can be applied to potential terminal 14, and one or more loads are connected or can be connected to potential terminal 13. However, such loads are not shown in the figure for the sake of clarity.

[0031] The circuit arrangement is at Fig. 1 represented by a switching element SW3, which has two terminals 11, 12 and is arranged in the power supply path between two terminal ends 11', 12' of the power supply path, each of the terminals 11, 12 being electrically connected to one of the terminal ends 11', 12'. Fig. 1 The switching element SW3 is preferably configured as a normally closed switch, but can alternatively also be configured as a normally open switch or changeover switch. The switching element SW3 can be controlled in a manner known per se. In particular, however, within the scope of the invention, the control can be effected by means of a voltage supplied from outside the circuit arrangement. Since the type of control and / or a control circuit provided for this purpose is not part of the invention, the control of the switching element SW3 is described in more detail below. Fig. 1 Not shown for the sake of clarity.

[0032] The switching element SW3 is thus configured to assume either a closed or an open switching state, wherein in the closed switching state a switching path running between the two terminals 11, 12 is closed for closing the power supply path between the terminal ends 11', 12' and in the open switching state the switching path running between the two terminals 11, 12 is open for interrupting the power supply path between the terminal ends 11', 12'. This can therefore be done in a manner known per se.Essential to the invention is that the circuit arrangement includes a diagnostic circuit which comprises an auxiliary circuit formed by incorporating the power supply path running at least between the terminal ends 11', 12' with a switching element SW3 arranged between them, and the diagnostic circuit is configured to supply the auxiliary circuit with an auxiliary voltage, to tap a voltage applied to the switching element SW3 between the two terminal ends 11', 12', to output a corresponding output signal, and to evaluate the output signal AS with respect to the current switching state. The diagnostic circuit is thus, in particular, independent of the control signal.

[0033] If the switching element SW3 is thus designed as a closing switch, this circuit arrangement, i.e. in particular by means of the diagnostic circuit, can be used to check whether, in the switched-off state of the switching element, the "NO path" formed via the switching path between the terminals 11, 12 or the terminal ends 11', 12' is high-impedance and thus the power supply path is interrupted.

[0034] The switching element SW3 thus creates a resistance value between these two terminals 11', 12', which is either high or low depending on the switching state. If the auxiliary circuit, which includes the power supply path running at least between the terminals 11', 12' and has the switching element SW3 positioned between it, is supplied with an auxiliary voltage, and the voltage applied to the switching element SW3 between the two terminals 11', 12' is tapped, the corresponding output signal AS can be evaluated with respect to the current switching state, and thus the switching state of the switching element SW3 within the power supply path can be determined.

[0035] Fig. 2 This shows, as an example, a roughly sketched course of a stimulus signal generated to check the diagnostic circuit and the course of a corresponding output signal AS (diag. input) received for evaluation in the case of a switching element SW3 in the case of, as in the case of Fig. 1 The depicted, i.e., open, state. If no stimulus signal is applied (stimuli in OFF), the corresponding output signal AS is also at zero or nearly zero (diag. input in OFF). If the stimulus signal is switched on and the switching element SW3 is open and thus has a high impedance (stimuli in ON), a corresponding output signal AS is also present (diag. input in ON). Using such a stimulus signal, a corresponding dynamic change in the evaluation signal can therefore be triggered.

[0036] For this purpose, the auxiliary circuit of the diagnostic circuit is preferred, as in Fig. 1 The circuit is shown with a resistor R7 arranged in series with the switching element SW3, in particular a resistor R7 whose resistance value is many times lower than the resistance value caused by the switching element SW3 in the open switching state between the two terminals 11, 12. In the open switching state of the switching element, a series circuit of resistor R7 and the high-impedance switching contact SW3 can therefore be formed by means of the diagnostic circuit, so that it can be checked whether the switching path is open in the open state, i.e., according to Fig. 1 In particular, the "NO path" has a high impedance when the switching element is in the off state. Consequently, a corresponding voltage applied to switching element SW3 can be tapped. As a result, the switching state of switching element SW3 can be detected within the power supply path. An advantage is that such a voltage divider can be formed without reference potential, i.e., independently of the operating voltage applied to potential terminals 13 and 14 and any connected loads.

[0037] To output the signal corresponding to the tapped voltage, it is preferable to use the following method: Fig. 1 As shown, an operational amplifier A3, in particular an operational amplifier used as a differential amplifier, with one non-inverting and one inverting input, is part of the diagnostic circuit, wherein each of the inputs is electrically connected to one of the two terminal ends 11', 12', in particular via a resistor R8, R9. The resistors R8, R9 thus provide decoupling, in particular high-impedance decoupling, and are also relevant for fault exclusion in the preferred execution of various functional tests, as will be shown below. The resistors R8, R9 should therefore preferably be in the range of 1 MΩ or greater.

[0038] In other words, in a preferred embodiment, not only can the switching state of the switching element SW3, particularly in the case of a non-forced switching element, be detected within the power supply path, but the switching element under test can also be checked for functionality or a fault. Thus, depending on the evaluation, it is possible to detect whether the switching element SW3 is functioning correctly, particularly in the case of a non-forced switching element, i.e., especially when monitoring the contact of a non-forced relay, or whether a fault has occurred. Consequently, the invention is particularly suitable for use in safety-critical applications, especially in safety-related switching devices.

[0039] To provide the auxiliary voltage for supplying the auxiliary circuit and to evaluate the corresponding output signal, as with Fig. 1 In particular, a galvanically decoupled logic device should be part of the diagnostic circuit.

[0040] In this context, it has proven advantageous to use an optocoupler for the galvanically isolated connection to evaluate the corresponding output signal, Fig. 1 designated OC4, and a transformer to provide the auxiliary electrical voltage, at Fig. 1 to be provided with a T3 designation. Additionally or alternatively, the diagnostic circuit can also include optocouplers, in the case of Fig. 1 Designated OC3, the circuit comprises a primary side that can be controlled by the logic device, and a secondary side that is arranged in series with the switching element SW3 as part of the auxiliary circuit. The optocoupler OC3 can be used, for example, to switch off the auxiliary voltage (e.g., as in the case of...). Fig. 2 (as shown) in the direction of switching element SW3, a malfunction of operational amplifier A3 can also be detected by provoking a change in state at its inputs. This type of dynamic evaluation signal can also be used, in particular, to rule out or detect a stuck-at fault (especially a malfunction of operational amplifier A3) in the diagnostic chain.

[0041] Preferred procedures for selectively opening or disconnecting the auxiliary circuit in series with the switching element SW3 in the further course to detect the switching state and evaluating the output signal corresponding to the tapped voltage with regard to a level change are described below.

[0042] As can be seen from these procedures, the primary side of the optocoupler OC3 is preferably controlled by the logic device Logik to selectively open or close the auxiliary circuit, and the output signal corresponding to the tapped voltage is also evaluated by the logic device Logik with regard to a level change.

[0043] With reference to Fig. 1 Let it be assumed that the auxiliary circuit of the diagnostic circuit is supplied with an auxiliary voltage via the transformer T3 and the circuit section U3, whereby this auxiliary voltage is independent of the potentials connected to the potential terminals 13, 14 and only serves the diagnostic circuit for its function.

[0044] For resistors R7, R8, and R9, specially designed MELF resistors ("MELF" - Metal Electrode Leadless Faces) are used. MELF (especially mini / micro-MELF) resistors are known to offer the advantage of better achieving specific characteristics such as pulse current capability, temperature stability, long-term stability, and voltage withstand capability, as well as precisely specified fault behavior (fuse resistor). With MELF resistors, a short circuit of the component in the event of a fault can therefore be essentially prevented.

[0045] To correctly detect the voltage that is present at the switching contact SW3 in the high-impedance state, i.e. between terminals 11 and 12, an operational amplifier is used as a differential amplifier A3.

[0046] To further enhance the dynamic nature of the diagnostic result, it is also provided, as shown above, to switch off the auxiliary voltage in the direction of the switching contact SW3. This switch-off is preferably implemented using the optocoupler OC3, e.g., an opto-FET, which is controlled via the logic unit.

[0047] The detection of the switching state of the switching element SW3, in particular including the associated diagnosis of the switching contact SW3, can then always be carried out according to the same procedure.

[0048] If the logic device, in its appropriate configuration, knows the control state, the diagnosis of the switching element SW3 is preferably carried out in the open state of the switching element SW3, i.e., in a state according to Fig. 1 The depicted normally closed contact is in its unactivated state.

[0049] In the unactivated state, the normally closed contact thus creates a high resistance between the two terminals 11 and 12, or between the terminals 11' and 12' of the power supply path. The auxiliary voltage is enabled by the logic circuit (OC3), which is galvanically isolated, and is now applied to the resulting series circuit consisting of resistor R7 and the high-resistance switching contact SW3. As previously described, the resistance value of resistor R7 is advantageously many times lower than the resistance value caused by the switching element SW3 in the open state, i.e., Rsw3_open >> R7. By tapping the voltage present between the two terminals 11' and 12' at the switching element SW3, the operational amplifier A3 can measure the auxiliary voltage across the switching contact SW3 and output a corresponding signal for evaluation.If almost the entire auxiliary voltage can be measured via the switching element SW3, then the switching element SW3 has high resistance, i.e., is open.

[0050] The corresponding output signal AS at the output of the differential amplifier A3 therefore assumes a high signal level, i.e., a HIGH level. The output signal AS is then conveniently passed on by the optocoupler OC4 to the logic unit for evaluation.

[0051] To rule out a stuck-at fault of operational amplifier A3, the logic device Logic preferentially deactivates optocoupler OC3 after this cycle, thus eliminating the measurable auxiliary voltage at switching element SW3. Consequently, operational amplifier A3 outputs a lower signal level, i.e., a LOW level, which is then passed to the logic device Logic via optocoupler OC4.

[0052] Does the resulting signal sequence of the measurement correspond to a pattern expected by the logic device (cf. Fig. 2 ) ensures that the switching contact is open and, in this example case, has also assumed a safe state.

[0053] Appropriately based on the in Fig. 1 In the illustrated embodiment, the following further tests can be carried out, for example, i.e., in particular with appropriately provoked dynamics of the output signal to be evaluated, hereinafter also referred to as the evaluation signal.

[0054] With regard to the switching element SW3, the following errors and their effects can be considered in particular.

[0055] If the switching element SW3 is "welded," i.e., permanently in the closed state, and thus the resulting resistance value is permanently low regardless of the control signal, then no or only a very small auxiliary voltage can be tapped across it. Consequently, the operational amplifier A3 would also produce an output signal AS with a lower signal level, i.e., a LOW level, regardless of whether a stimulus signal is applied or not.

[0056] Based on the circuit arrangement according to Fig. 1 In the event of a diagnosis, essentially the entire auxiliary voltage, preferably provided via the transformer T3 and the circuit section U3, would drop across OC3 or R7.

[0057] However, any voltage measured by operational amplifier A3 would permanently result in a low output signal AS, i.e., a permanently LOW signal. Since this signal contradicts the pattern expected by the logic, i.e., a correspondingly provoked dynamic change of the evaluation signal does not occur, an error response can be triggered.

[0058] Is the switching element SW3 "broken," meaning permanently in the open state, and therefore the resulting resistance value permanently high regardless of the control signal? To diagnose this fault, the diagnostic procedure should be performed when switching element SW3 is driven to assume the closed switching state. If, despite the application of an auxiliary voltage, a high-level output signal AS is present at the output of operational amplifier A3, it can be assumed that switching element SW3 is not responding as expected. Therefore, if the auxiliary voltage is measured at switching element SW3, it can be assumed that the normally closed contact is not closed, contrary to expectations. However, for a Stop Category 0 device (Safe State OFF), this is the safe state. Nevertheless, a fault response / fault message can still be triggered.

[0059] With regard to changes in the characteristic values ​​of R7, the following errors and their effects can be considered in particular.

[0060] A short circuit is essentially prevented by a MELF design. Increasing, e.g., doubling the resistance of R7 has no negative effects, at least as long as the resistance caused by the switching element SW3 in the open switching state is many times greater than the resistance of R7, i.e., Rsw3_open >> R7.

[0061] Consequently, even a reduction, e.g. halving the resistance value of R7, has no negative effects.

[0062] If R7 exhibits an idle behavior, no auxiliary voltage would be measurable at the switching element SW3, thus enabling the detection of such a fault.

[0063] With regard to a change in the characteristic values ​​of R8 and R9, which serve for the high-impedance decoupling of the operational amplifier A3, the following errors and their effects can be considered in particular.

[0064] A short circuit is essentially ruled out by a MELF design.

[0065] An increase, e.g. doubling, of one or both resistance values ​​generally has no negative effects.

[0066] A reduction, e.g. halving one or both resistance values, generally has no negative effects.

[0067] If an idle behavior is present, no auxiliary voltage would be measurable at the switching element SW3, so the presence of such a fault can therefore be detected.

[0068] With regard to the OC3 optocoupler, the following errors and their effects can be considered in particular.

[0069] If this component is permanently conductive, the auxiliary voltage towards switching element SW3 can no longer be switched off. Via operational amplifier A3, a voltage would be continuously measured across switching element SW3 in the open switching state. This results in an expected output pattern being faulty, which can then be detected, particularly by the logic device, and trigger an error response.

[0070] If the optocoupler OC3 is permanently high-impedance, i.e., open, no auxiliary voltage can be measured by the operational amplifier A3 at the switching element SW3. This results in an expected output pattern being faulty. This fault can also be detected, particularly by the logic device, and trigger an error response.

[0071] With regard to the OC4 optocoupler, the following errors and their effects can be considered in particular.

[0072] If the optocoupler OC4 is permanently conductive, the output signal of the operational amplifier A3 is permanently detected as a HIGH signal, which in turn results in an expected output pattern being incorrect. The presence of such a fault can therefore also be detected and subsequently trigger an error response.

[0073] If the optocoupler OC4 is permanently high-impedance, i.e., open, the output signal of the operational amplifier A3 is permanently detected as a LOW signal, which in turn results in an expected output pattern being incorrect. The presence of such a fault can therefore also be detected and subsequently trigger an error response.

[0074] With regard to the transformer T3, the following errors and their effects can be considered in particular.

[0075] If there is a break in the primary and / or secondary winding, no voltage can be transmitted. In the event of such a fault, the output signal of operational amplifier A3 would be permanently recognized as a LOW signal, which in turn would result in an incorrect expected output pattern and could consequently trigger an error response.

[0076] If there is a short circuit in the primary and / or secondary winding, no voltage can be transmitted. In the event of such a fault, the output signal of operational amplifier A3 would be permanently recognized as a LOW signal, which in turn would result in an expected output pattern being incorrect and could consequently trigger a fault response.

[0077] The inventive method and circuit arrangement are therefore also highly suitable for use with more cost-effective and less complex non-force-guided switching elements, i.e., in particular for contact monitoring of a non-force-guided relay, in safety-related applications. With appropriate use, the Safe Failure Fraction (SFF) can be increased through various diagnostic measures, and thus the hardware fault tolerance (HFT) to be implemented can also be reduced. For a quantification of the aforementioned values ​​SFF and HFT, reference can be made in particular to IEC EN 61508-2 in the version applicable on the filing date, especially to Tables 2 and 3 therein.

Claims

1. Circuit arrangement for detecting a switching state of a switching element (SW3) within a power supply path, comprising the switching element (SW3) and the power supply path, wherein the power supply path is arranged for supplying load current between two potential terminals (13, 14), and wherein the switching element (SW3) has two terminals (11, 12), is arranged in the power supply path between two terminal ends (11', 12') of the power supply path, wherein each of the terminals (11, 12) is electrically connected to one of the terminal ends (11', 12'), respectively, and is configured to assume, in particular by being actuated by a voltage to be supplied from outside the circuit arrangement, a selectable switching state of either closing or interrupting, wherein in the closing switching state, a switching path extending between the two terminals (11, 12) is closed to close the power supply path between the terminal ends (11', 12'), and in the interrupting switching state, the switching path extending between the two terminals (11, 12) is opened to interrupt the power supply path between the terminal ends (11', 12'), a diagnostic circuit comprising an auxiliary circuit formed by incorporating the power supply path extending at least between the terminal ends (11', 12') with a switching element (SW3) arranged therebetween, and wherein the diagnostic circuit is further configured to supply the auxiliary circuit with an auxiliary voltage, for tapping a voltage present at the switching element (SW3) between the two connection terminals (11', 12'), for outputting an output signal corresponding to the tapped voltage, and for evaluating the output signal with regard to the respective switching state.

2. Circuit arrangement according to claim 1, wherein the auxiliary circuit of the diagnostic circuit comprises a resistor (R7) arranged in series with the switching element (SW3), in particular a resistor (R7) whose resistance value is a multiple of times lower than a resistance value produced by the switching element (SW3) in the interrupting switching state between the two terminals (11, 12).

3. Circuit arrangement according to claim 1 or 2, wherein the diagnostic circuit for outputting the output signal corresponding to the tapped voltage includes an operational amplifier (A3), in particular an operational amplifier used as a differential amplifier, having a non-inverting and an inverting input, wherein each of the inputs is electrically connected, in particular via a respective resistor (R8, R9), to one of the two terminal ends (11', 12').

4. Circuit arrangement according to claim 1, 2 or 3, wherein the diagnostic circuit comprises a logic device which is galvanically decoupled and connected for the purpose of supplying the auxiliary voltage to the auxiliary circuit and for evaluating the corresponding output signal.

5. Circuit arrangement according to claim 4, wherein for the galvanically decoupled connection a first optocoupler (OC4) is included for evaluating the corresponding output signal, and a transformer (T3) is included for supplying the auxiliary voltage.

6. Circuit arrangement according to claim 4 or 5, wherein the diagnostic circuit further comprises a second optocoupler (OC3), the primary side of which can be driven by the logic device, and the secondary side of which is arranged in series with the switching element (SW3) as a component of the auxiliary circuit.

7. Method for detecting a switching state of a switching element (SW3) within a power supply path, in particular using a circuit arrangement according to one of claims 1 to 6, comprising the following steps: Controlling a switching element (SW3), which is arranged between two terminal ends (11', 12') of a power supply path for supplying load current and has two terminals (11, 12), each of which is electrically connected to a respective terminal end (11', 12') of the power supply path, for selectively assuming a first switching state, in which a switching path extending between the two terminals (11, 12) is closed to close the power supply path between the two terminal ends (11', 12'), or to assume a second switching state in which the switching path extending between the two terminals (11, 12) is open to interrupt the power supply path, wherein the switching element (SW3) provides a resistance value between these two terminal ends (11', 12') which, depending on the switching state, is either high-impedance or low-impedance, supplying an auxiliary circuit formed by incorporating the power supply path extending at least between the terminal ends (11', 12') with a switching element (SW3) arranged therebetween, tapping a voltage present at the switching element (SW3) between the two terminals ends (11', 12'), outputting a voltage corresponding to the tapped voltage, and evaluating the output signal with respect to the current switching state.

8. Method according to claim 7, wherein, in a further step, the auxiliary circuit n series with the switching element (SW3) is selectively opened or disconnected i, and the output signal corresponding thereupon to the tapped voltage is evaluated with regard to a change in level.

9. The method according to claim 8, wherein, for selectively opening or closing the auxiliary circuit, the primary side of an optocoupler (OC3), the secondary side of which is arranged in series with the switching element (SW3) as part of the auxiliary circuit, is driven by a logic device, and the output signal corresponding thereupon to the tapped voltage is compared with a level change from the logic device.

10. Method according to claim 7, 8 or 9, wherein, depending on the evaluation, a functional switching element (SW3) or a fault is detected.