CONTACT MONITORING DEVICE

Description

[0001] The present disclosure relates to a contact monitoring device for monitoring an electrical contact, in particular for monitoring a switching operation of a switching contact, a functional state of an overcurrent fuse and / or a functional state of an overvoltage fuse.

[0002] In electrical automation, actuators can be switched with a switch, whereby it may be necessary to detect the actual switching of a switch contact. Such monitored feedback can be implemented, for example, using additional auxiliary contacts. Alternatively, contact force guidance can be implemented to prevent incorrect switching of the switch, which increases production costs. A faulty switch contact can, for example, have defective, particularly welded, buttons or degraded semiconductor layers, so that switching on or off cannot be achieved despite an applied switch-on signal or despite mechanical actuation.

[0003] Monitoring switch contacts is particularly necessary for machine and system control systems, where, for example, switching relays are used whose switching contacts are frequently switched. Such contacts are subject to increased wear. Relays can be designed so that the switching contacts can withstand, for example, several hundred thousand switching operations. The contact monitoring device makes it possible to monitor the condition of the contact and, at the first sign of increased wear, issue a message indicating the wear condition. Operating personnel can then replace the contact, for example, in the form of a relay, in a timely manner.

[0004] Such contact monitoring devices are known in principle; for example, document DE 10 2009 006 970 A1 describes a method and circuit for functional testing of a semiconductor device. However, they often operate on the basis of an oscillating circuit influenced by the switching contact. In one switching state, a low attenuation is applied to the oscillating circuit, and in the other switching state, a higher attenuation. This changes at least the amplitude of the oscillation, which can be measured, for example, via a voltage drop across a resistor.

[0005] DE 10 2018 114 425 A1 shows a concrete example of this type of contact monitoring device. The contact monitoring device is described using the example of monitoring a relay switch contact. A resonant circuit is fed with an excitation signal from a signal generator via a transformer. However, resonant circuits can exhibit temperature-dependent properties, which can lead to problems with the reliability of measuring the amplitude change.

[0006] A solution for a temperature-compensated oscillator circuit is known from JP 2012 211 855 A. There, a FET transistor is used to adjust the resonant frequency of the oscillator circuit, whose operating point is temperature-compensated by the use of a thermistor in the voltage divider for adjusting the operating point. The temperature-compensated oscillator is used in a RADAR sensor for a vehicle. The FET transistor of the oscillator is also used as a frequency-determining element, because it is the transistor capacitances that, together with the coil, form the oscillator circuit. When the operating point is adjusted, the transistor capacitances change, and thus also the resonant frequency.

[0007] However, known solutions for a contact monitoring circuit have the disadvantage that they cannot reliably distinguish between proper contact function and malfunction, or they require a complex design and can therefore be complex and expensive. Additionally, there is a problem with the temperature dependence of existing contact monitoring devices. Therefore, there is a need for an improved contact monitoring device that, in particular, improves the reliability of contact monitoring and requires only a few, cost-effective components. This was recognized within the scope of the invention.

[0008] This object is achieved by the features of independent claim 1. Advantageous embodiments are the subject of the dependent claims, the description and the accompanying figures.

[0009] The present disclosure is based on the finding that the above object can be achieved by a contact monitoring device according to claim 1, which is designed to generate a contact monitoring signal at the electrical contact by means of an oscillator circuit and a coupling element and damping element connected downstream of the oscillator circuit, particularly in a closed contact state of the electrical contact. In one embodiment, the oscillator circuit is not capable of oscillation with the electrical contact in an open contact state, so that the contact monitoring signal is not generated.

[0010] In another embodiment, the oscillator circuit is capable of oscillating even when the contact is open, but at a different resonant frequency. This can be achieved, for example, by adding another capacitance to the resonant circuit when the contact is switched, resulting in a different total capacitance in the LC resonant circuit. If the contact is defective, it no longer closes properly during the switching process, and either no resonant oscillation is performed, or the other resonant oscillation is no longer performed. This is detected and displayed accordingly.

[0011] In a general embodiment, with the electrical contact in the first contact state on the oscillator circuit, a first damping element is activated, which causes a first impedance curve in the oscillator circuit, and with the electrical contact in the second contact state on the oscillator circuit, the first damping element is deactivated and / or a second damping element is activated, which causes a second impedance curve of the oscillator circuit, wherein the oscillator circuit is designed to generate a resonant oscillation with the electrical contact in the first contact state and the first impedance curve and to output the excitation signal; and with the electrical contact in the second contact state and the second impedance curve, either not to generate a resonant oscillation and not to output an excitation signal or to generate a different resonant oscillation and to output a different excitation signal;The detector is configured to detect one excitation signal and the other excitation signal, or no excitation signal, to monitor the electrical contact. The other resonant oscillation is generated when the change in the contact state activates another frequency-determining element in the oscillator circuit.

[0012] Furthermore, a means for thermally stabilizing the oscillator circuit is provided in the oscillator circuit. The oscillator circuit comprises a resonant circuit with a feedback branch and an amplifier, the amplifier amplifying the signal fed back via the feedback branch.

[0013] The solution then provides for the amplifier to have at least one transistor whose gain is adjustable by its operating point, wherein the gain of the at least one transistor is stabilized by the thermal stabilization means. This is achieved by shifting the transistor's operating point by changing the thermal stabilization means when the temperature changes. This automatically keeps the transistor's gain constant, even when the temperature changes.

[0014] In a preferred variant, a thermistor, particularly an NTC or PTC resistor, is used as an element of a voltage divider for thermal stabilization. The NTC resistor corresponds to a thermistor, and the PTC resistor to a PTC thermistor. Alternatively, a semiconductor diode can be used as the thermistor, which is switched in the reverse direction. By changing the resistance of the thermistor, the operating point of the transistor is shifted, thus keeping the gain stable.

[0015] In another embodiment, a temperature sensor is used as a means for thermal stabilization, the signal of which is evaluated by a microcontroller which generates an associated correction signal which sets the operating point of the at least one transistor for thermal stabilization.

[0016] The microcontroller can generate the correction signal in the form of a PWM signal, corresponding to a pulse-width modulated signal, and an averager can be used to output the average value of the PWM signal depending on the duty cycle. The operating point is set using the averaged correction signal.

[0017] In a cost-effective variant, the mean value generator is designed in the form of a low-pass filter consisting of a resistor and a capacitor.

[0018] This form of thermal stabilization has the advantage that temperature compensation can be performed very precisely. However, the cost of this solution is higher.

[0019] The contact monitoring signal can, in particular, be a high-frequency auxiliary energy generated by the oscillator circuit and coupled into the load circuit with the electrical contact through the coupling element. A change in the impedance of the load circuit, in particular due to switching of the electrical contact, is fed back to the oscillator circuit via the coupling element and generates a signal change between the oscillator circuit and the coupling element, which can be used to detect switching of the electrical contact.

[0020] According to a first aspect, the disclosure relates to a contact monitoring device for monitoring an electrical contact having a first contact state and a second contact state. In particular, the electrical contact can be a switching contact that is closed in the first contact state and open in the second contact state. For example, the electrical contact can be low-resistance in the first contact state and high-resistance in the second contact state.

[0021] The contact monitoring device comprises an oscillator circuit configured to generate an excitation signal, and a coupling element with a signal input and a signal output. The oscillator circuit is connected upstream of the signal input to apply the excitation signal to the coupling element, and the coupling element is configured to convert the excitation signal into a contact monitoring signal and output the contact monitoring signal at the signal output for applying the excitation signal to the electrical contact.

[0022] With the electrical contact in the first contact state, a first impedance value is set on the oscillator circuit, and with the electrical contact in the second contact state, a second impedance value is set on the oscillator circuit. The oscillator circuit is configured to generate a resonant oscillation with the electrical contact in the first contact state and with the first impedance value and to output a version of the excitation signal, for example, an excitation signal with a modified amplitude, in particular a damped one.

[0023] In one embodiment, the electrical contact in the second contact state and the second impedance value prevent the excitation signal from being output.

[0024] Furthermore, the contact monitoring device comprises a detector which is designed to detect a version of the excitation signal and / or a change in the excitation signal in order to monitor the electrical contact.

[0025] The first impedance value can, in particular, be greater than the second impedance value. Furthermore, a damping element can be provided to adjust the first impedance value and / or the second impedance value, in particular depending on the respective contact state of the electrical contact, at the signal input.

[0026] The electrical contact can be electrically conductive in the first contact state and electrically non-conductive in the second contact state. In particular, the contact monitoring device can be configured not to continuously apply the contact monitoring signal to the electrical contact in order to prevent a permanent coupling of a contact monitoring signal, in particular a high-frequency one, via the electrical contact into a load circuit, which in particular conducts low-frequency signals. Accordingly, the advantage of improved EMC properties of the contact monitoring device can be achieved.

[0027] The electrical contact can be a power switching contact of an electromechanical relay, a semiconductor relay, a contactor, a reed contact, or a mechanical switch. The electrical contact can, in particular, be configured to have a first ohmic resistance in a first, in particular closed, contact state and a second ohmic resistance in a second, in particular open, contact state, wherein the first ohmic resistance is smaller than the second ohmic resistance. In particular, the electrical contact has a low resistance in the closed contact state and a high resistance in the open contact state.

[0028] The electrical contact can be a fuse, in particular a melting fuse, which is electrically conductive or low-resistance in a first contact state and electrically non-conductive or high-resistance in a second contact state in which the fuse has blown.

[0029] Furthermore, the first contact state can be a first switching state of the electrical contact, in particular of an electrical switching element, and the second contact state can be a second switching state of the electrical contact, in particular of the electrical switching element.

[0030] In one embodiment, the contact monitoring device is configured to apply the contact monitoring signal to the electrical contact as a differential signal, wherein the differential signal is a current signal that causes a small voltage drop at a low contact resistance, in particular at the electrical contact in the first contact state. This achieves the particular advantage that the contact monitoring signal in the electrical contact has a low amplitude and, accordingly, only small high-frequency signal components are superimposed on a useful signal flowing through the electrical contact. Accordingly, the EMC properties of the contact monitoring device can be improved.

[0031] In one embodiment, the oscillator circuit is designed to generate the excitation signal in the high-frequency range, preferably at a frequency in a range of 1 MHz to 100 MHz, more preferably at a frequency of 50 MHz. This provides the particular advantage that frequency separation of the contact monitoring signal and the useful signal can be achieved using cost-effective frequency filter elements. The oscillator circuit can be a high-frequency generator, which is connected to the electrical contact, for example, in parallel, via the coupling element and the damping element.

[0032] In one embodiment, the oscillator circuit comprises a self-oscillating oscillator, which in particular has a variable resonance frequency and is designed to generate an electromagnetic oscillation with the electrical contact in a first contact state, in particular a low-resistance, closed contact state.

[0033] In one embodiment, the damping element is configured to adjust the first impedance value at the signal input with the second contact state. Accordingly, the oscillator circuit can be configured to generate the excitation signal with the electrical contact open, or not to generate the excitation signal with the electrical contact closed.

[0034] By monitoring the electrical contact of a relay using the contact monitoring device, switching information relating to the transition of the electrical contact between a first contact state, in which the electrical contact is electrically non-conductive, and a second contact state, in which the electrical contact is electrically conductive, can be generated when the electrical contact is subjected to the switch-on signal. This ensures that the relay switches with the switch-on signal and, accordingly, that there is no fault in the electrical contact and / or a magnetic system of the relay. Accordingly, if the switch-on signal is applied to the relay and the electrical contact does not switch, a malfunction of the relay can be detected. Furthermore, switching off of the relay when the switch-on signal drops can also be monitored.

[0035] The contact monitoring device offers the advantage of enabling clear detection of the contact state of the relay switching contact from a functional safety perspective, particularly according to the IEC 61508 standard. Accordingly, increased diagnostic coverage can be achieved.

[0036] The detector can be configured, in particular, to convert a change in the excitation signal or a change in the contact monitoring signal into an output signal, which can be a time-discrete and / or amplitude-discrete signal, in particular a binary signal and / or a digital signal. A first output signal amplitude can be assigned to the first contact state, or a closed electrical contact, and a second output signal amplitude can be assigned to the second contact state, or an open electrical contact. The detector can be configured to adapt the output signal or its version to a change in the excitation signal and / or the contact monitoring signal instantly or with the shortest possible time delay.A change in the excitation signal can in particular be a change between the generation of the excitation signal on the basis of a satisfied resonance condition of the oscillator circuit and the non-generation of the excitation signal on the basis of an unfulfilled resonance condition of the oscillator circuit.

[0037] In one embodiment, the formation of the resonant oscillation is prevented in the second contact state, thereby preventing the output of the excitation signal.

[0038] In one embodiment, the oscillator circuit is configured to use the first impedance value to generate the excitation signal at an excitation frequency that corresponds to a resonant frequency of the oscillator circuit, wherein the oscillator circuit is further configured to change a signal amplitude of the excitation signal based on an impedance value at the signal input. The oscillator circuit can be configured to change from a non-oscillatory state to an oscillatory state with a change in the impedance value at the signal input, wherein the signal amplitude of the excitation signal can also be continuously changed with a continuous change in the impedance value. The oscillator circuit is further configured to transition from the non-oscillatory state to the oscillatory state when an impedance limit value at the signal input is exceeded.

[0039] In one embodiment, the contact monitoring device comprises a damping element which is connected downstream of the signal output and coupled to the electrical contact, for example connected upstream or downstream, wherein the damping element is designed to provide an impedance with the electrical contact in the second contact state in order to prevent an impedance of the electrical contact from acting in the second contact state, wherein the action of the damping element dampens the signal input of the coupling element, the oscillator circuit is not capable of oscillation and an output of the excitation signal is prevented.

[0040] In one embodiment, the coupling element and the oscillator circuit are configured to form an oscillating circuit, wherein the damping element is configured to damp the oscillating circuit in the second contact state in order to reduce a signal amplitude of the excitation signal.

[0041] In one embodiment, the damping element is coupled to the electrical contact, for example, electrically connected in parallel, wherein the damping element is bridged with low impedance by the electrical contact in the first contact state, and the damping element's attenuation, coupled to the signal input by the coupling element, is reduced. The oscillator circuit is configured to generate the excitation signal with an increased signal amplitude using the reduced attenuation.

[0042] In one embodiment, the oscillator circuit has a detector output and is configured to provide the excitation signal or a version of the excitation signal to the detector via the detector output.

[0043] In one embodiment, the detector is configured to convert the excitation signal into an output signal and to output the output signal in at least two different signal states, wherein a first signal state of the at least two signal states indicates a closed electrical contact and a second signal state of the at least two signal states indicates an open electrical contact.

[0044] In one embodiment, the detector is configured to output the output signal in the first signal state when an oscillating excitation signal is present at the detector output and to output the output signal in the second signal state when a non-oscillating excitation signal is present at the detector output, a signal amplitude of the excitation signal is below an amplitude limit value and / or the excitation signal is not present at the detector output.

[0045] In one embodiment, the coupling element is designed to galvanically isolate the oscillator circuit and / or the detector from the electrical contact.

[0046] In one embodiment, the coupling element is designed as a direct galvanic connection, for example as a cable connection, of the contact to the oscillator circuit, so that a change in the electrical contact can act directly on the impedance of the oscillator circuit.

[0047] In one embodiment, the detector is configured to detect a switch-on signal for switching the electrical contact and a change in the excitation signal, and to output a binary error signal indicating whether the electrical contact is switched according to the switch-on signal. This provides the advantage that the detector can detect whether both the switch-on signal and the excitation signal carry identical logical information about the contact state of the electrical contact. The switch-on signal can be provided to the detector, in particular, via a switching control terminal.

[0048] The detector can further be configured to detect a first piece of logic information when the excitation signal or a resonant oscillation is present at the detector input, and to detect a second piece of logic information when the activation signal is present at the switching control terminal. Furthermore, the detector can be configured to compare the first piece of logic information with the second piece of logic information.

[0049] The detector can be configured to generate an output signal based on the comparison of the first logical information with the second logical information, wherein the output signal indicates a match or a divergence of the first logical information with respect to the second logical information. The first logical information can, in particular, be binary switching information relating to the electrical contact, which is intended to trigger switching of the electrical contact, and the second logical information can, in particular, be binary test information describing the contact state of the electrical contact. In particular, the binary test information can provide information relating to the electrical conductivity of the electrical contact.

[0050] Four different signal combinations can be realized using the binary switching information and the binary test information (see Table 1). Table 1: Possible combinations of switching information and test information index Switching information Test information Condition / Description 0: Open contact 0: electrical contact open 1: Close contact 1: electrical contact closed 1 0 0 Proper operation, electrical contact open 2 0 1 error condition, 3 1 0 Error condition 4 1 1 Proper operation, electrical contact closed

[0051] Depending on the signal combinations, the detector can be configured to generate an output signal with a signal level representing a logical 0 in cases 1 and 4, and an output signal with a signal level representing a logical 1 in cases 2 and 3. Alternatively, the different signal combinations could be assigned to corresponding amplitude values ​​or output signal patterns of the output signal. An output signal pattern can be realized, for example, by a unique sequence of signal pulses, in particular signal pulses with different pulse lengths.

[0052] In one embodiment, the detector is configured to detect the excitation signal within a predetermined time interval upon receiving the switch-on signal in order to determine whether the electrical contact is switched according to the switch-on signal within the predetermined time interval.

[0053] In one embodiment, the contact monitoring device comprises an optical display element which is designed to indicate the detection of the switch-on signal and a corresponding detected switching of the electrical contact or a deviation from the expected switch-on signal.

[0054] In one embodiment, the coupling element comprises a transformer having a first inductance and a second inductance, wherein the first inductance is inductively coupled to the second inductance in order to convert the excitation signal into the contact monitoring signal.

[0055] In one embodiment, the coupling element comprises a transformer comprising a first air-core coil and a second air-core coil, which are inductively coupled to one another. The first air-core coil and the second air-core coil can be cylindrical coils or annular coils. Furthermore, a coil core, in particular a ferrite core or a magnetic core, can be arranged within the first air-core coil and / or within the second air-core coil. In one embodiment, a magnetic core can be arranged partially in the first air-core coil and / or in the second air-core coil. The coil core can be formed from flux-conducting ferrite plates or ferrite foils.

[0056] The transformer with the first inductance and the second inductance, which are each designed in particular as air-core coils, achieves the advantage, for example, that the coils can be manufactured using printed circuit board technology, thus reducing the number of discrete electronic components required during manufacture of the contact monitoring device. Furthermore, capacitive elements of the contact monitoring device can also be designed as printed circuit board capacitors to further reduce the number of discrete electronic components.

[0057] By designing the transformer's inductors as planar coils, the advantage of cost-effective production of the contact monitoring device can be achieved, in particular, since the need to intersperse the inductors with ferrite plates is eliminated. The transformer can be designed, in particular, to achieve a coupling factor of greater than 20% with respect to coupling the excitation signal via the first inductance into the second inductance in order to efficiently convert the excitation signal into the contact monitoring signal.

[0058] In one embodiment, the contact monitoring device comprises an impedance circuit which is connected downstream of the signal output and coupled to the damping element, for example upstream, wherein the impedance circuit is designed to compensate for a stray inductance and / or a stray capacitance of the coupling element in the case of a contact monitoring signal having a predetermined frequency.

[0059] In one embodiment, the impedance circuit is designed to minimize a total capacitance and / or a total inductance across the galvanic isolation of the coupling element.

[0060] In one embodiment, the contact monitoring device comprises a first filter element and a second filter element, wherein the electrical contact has a first output terminal, downstream of which the first filter element is connected, and a second output terminal, downstream of which the second filter element is connected, in order to attenuate high-frequency signals which are applied to the electrical contact and / or to an electrical load and / or voltage source coupled to the electrical contact by means of the coupling element.

[0061] The filter elements achieve the particular advantage of functionally isolating the contact monitoring device from an external circuit of the electrical contact, which is connected, for example, to a first output terminal and a second output terminal. The first filter element and / or the second filter element can each comprise an inductor or a ferrite, which is designed to be low-impedance for low-frequency AC signals and high-impedance for high-frequency AC signals. This can achieve the particular advantage that the filter elements can efficiently conduct low-frequency signals with high current intensities.

[0062] The contact monitoring signal can preferably have a frequency of 100 kHz to 100 MHz. The filter elements are high-impedance for the contact monitoring signal in this frequency range, so that the contact monitoring signal is not present at the output terminals or is present only with a reduced signal amplitude.

[0063] The filter elements further achieve the advantage that a low-resistance bridging of the electrical contact cannot be misinterpreted as an electrical contact in the first contact state by the contact monitoring device. In particular, the real part of an impedance value of the respective filter element, given a contact monitoring signal with a predetermined frequency, can reach the order of magnitude of the resistance value of an ohmic resistor of the damping element. This makes it possible to achieve the particular advantage that, with the electrical contact in the second contact state and an external low-resistance bridging of the electrical contact, a damping value corresponding to the damping value provided by the damping element can be achieved, and oscillation of the oscillator circuit can be prevented.

[0064] In one embodiment, the first filter element and / or the second filter element have an ohmic impedance component at the oscillator frequency, which is designed to prevent detection of an external short circuit of the first output terminal with the second output terminal as a closing of the electrical contact.

[0065] In one embodiment, the damping element comprises a capacitance formed by a printed circuit board capacitor.

[0066] In one embodiment, the contact monitoring device comprises a supply voltage terminal to which a supply voltage can be applied, wherein the oscillator circuit and / or the detector can be supplied with electrical energy by means of the supply voltage terminal. Furthermore, the contact monitoring device can have a switching control terminal to which a switch-on signal can be applied in order to trigger the electrical contact.

[0067] In one embodiment, the detector and / or the oscillator circuit can be supplied with the activation signal via the switching control terminal, wherein the detector and / or the oscillator circuit are configured to be supplied with electrical energy by the activation signal. The activation signal can, in particular, be a DC voltage signal suitable for supplying the detector and / or the oscillator circuit with electrical energy. In one embodiment, the activation signal can be used to charge a buffer capacitor configured to supply the detector and / or the oscillator circuit with a constant voltage and / or a constant current at least for a predetermined time interval.

[0068] In one embodiment, a rectifier may be connected between the switching control terminal and the buffer capacitor, which rectifier is designed to convert an alternating current-type switching signal into a supply voltage for the detector and / or the oscillator circuit.

[0069] In one embodiment, the buffer capacitor is configured to continue supplying the detector and / or the oscillator circuit with electrical energy for a predetermined time interval upon the loss of the activation signal, in order to detect an opening of the electrical contact corresponding to the loss of the activation signal and to generate a corresponding output signal. The detector can, in particular, be configured to output status information regarding a contact fault or a proper switching of the electrical contact with respect to the activation signal before the energy supply to the detector is interrupted with the output signal.

[0070] Further embodiments are explained with reference to the accompanying figures. They show: Fig. 1 shows the contact monitoring device in one embodiment; Fig. 2 shows the contact monitoring device in one embodiment; Fig. 3 shows the impedance at the signal input in one embodiment; Fig. 4 shows the contact monitoring device in one embodiment; Fig. 5 shows the contact monitoring device in one embodiment; Fig. 6 shows the contact monitoring device in one embodiment; Fig. 7 shows a temperature-compensated contact monitoring device in principle; Fig. 8 shows a frequency-determining network of the contact monitoring device in principle; Fig. 9 shows two impedance curves of the frequency-determining network of the contact monitoring device; Fig. 10 shows the temperature-compensated contact monitoring device in a first embodiment; Fig. 11 shows the temperature-compensated contact monitoring device in a second embodiment; and Fig. 12 shows the temperature-compensated contact monitoring device in a third embodiment.

[0071] Fig. 1 shows a schematic representation of the contact monitoring device 100 for monitoring electrical contact 101 with at least two contact states according to one embodiment. The contact monitoring device 100 has an oscillator circuit 103 for generating an excitation signal and a coupling element 105 with a signal input 107 and a signal output 109.

[0072] The oscillator circuit 103 is connected upstream of the signal input 107 to apply the excitation signal to the coupling element 105, and the coupling element 105 is configured to convert the excitation signal into a contact monitoring signal and output the contact monitoring signal at the signal output 109 for applying the electrical contact 101. Furthermore, the contact monitoring device 100 comprises a damping element 111, which is connected downstream of the signal output 109 and upstream of the electrical contact 101.

[0073] With the electrical contact 101 in the first contact state, a first impedance value is set on the oscillator circuit 103, and with the electrical contact 101 in the second contact state, a second impedance value is set on the oscillator circuit 103.

[0074] The contact monitoring device comprises a damping element 111, which is connected downstream of the signal output 109 and upstream of the electrical contact 101. The damping element 111 is configured to provide an impedance with the electrical contact 101 in the second contact state in order to prevent an impedance effect of the electrical contact 101 in the second contact state. The damping element 111's action dampens the signal input 107 of the coupling element 105, prevents the oscillator circuit 103 from oscillating, and prevents the output of the excitation signal. The first impedance value is greater than the second impedance value.

[0075] The oscillator circuit 103 is configured to generate a resonant oscillation with the electrical contact 101 in the first contact state and at the first impedance value and to output the excitation signal, and to suppress the output of the excitation signal with the electrical contact 101 in the second contact state and at the second impedance value. Furthermore, the contact monitoring device 100 comprises a detector 113 configured to detect the excitation signal in order to monitor a change in the contact state of the electrical contact 101.

[0076] The damping element 111 comprises a resistor 129, which is designed to change or load an impedance value at the signal input 107 in such a way that an oscillating circuit, which is formed from the preferably inductive imaginary part of the coupling element 105 and the preferably capacitive imaginary part of the oscillator circuit 103, experiences a high damping with which the oscillator circuit 103 is not capable of oscillating and accordingly the contact monitoring signal is not present at the electrical contact 101.

[0077] Furthermore, the damping element 111 comprises a capacitor 131, which is designed to generate a high first impedance for a particularly low-frequency useful signal applied to the electrical contact 101 via the output terminals 125-1, 125-2, in order to minimize or prevent leakage currents through the damping element 111. Furthermore, the capacitor 131 is designed to generate a low second impedance for the contact monitoring signal in order to realize spectral separation of the contact monitoring signal and the useful signal.

[0078] Capacitor 131 has, in particular, a high dielectric strength, for example, greater than or equal to 30 kV / mm, wherein capacitance 131 is preferably implemented as a printed circuit board capacitor. This allows for the particular advantage of reducing the number of required discrete components, thus enabling more cost-effective production. Furthermore, this allows for the advantage of reducing the installation space of contact monitoring device 100.

[0079] In one embodiment, the circuit board capacitance 131 comprises conductor surfaces arranged on different layers of an at least two-layer circuit board. This can achieve, for example, the advantage that, when the contact monitoring device 100 is used in safety-relevant areas, a malfunction of the contact monitoring device 100 due to a faulty capacitance 131 can be reduced or eliminated. In particular, a malfunction due to a short circuit between different layers of a circuit board can be eliminated during a fault analysis, thus further achieving the advantage that the contact monitoring device 100 can be manufactured at reduced manufacturing costs.

[0080] When the electrical contact 101 closes, i.e., when the electrical contact 101 changes from the second contact state to the first contact state, the damping element 111 is bridged with low resistance due to the parallel connection of the damping element 111 and the electrical contact 101 and accordingly acts neither for a useful signal applied to the electrical contact 101 nor for the contact monitoring signal. By bridging the damping element 111, a transformed attenuation at the signal input 107 that can be generated by means of the resistor 129 of the damping element 111 is reduced, so that the resonant circuit has only low ohmic losses. Furthermore, the resonant circuit is designed to generate a resonant oscillation with the reduced attenuation.

[0081] Oscillator circuit 103 can be constructed as a Colpitts oscillator, which includes capacitors 139-1, 139-2 and a transistor 141. Transistor 141 is configured to amplify the excitation signal. Oscillator circuit 103 further includes a resistor 143, via which the excitation signal can be passed to detector 113. Inductance 119-1, which can be configured as a coil, can form an element of oscillator circuit 103.

[0082] The detector 113 may include a capacitor 145 configured to separate a DC component of the excitation signal. Furthermore, the detector 113 includes at least two diodes 147-1, 147-2 and a further capacitor 153, wherein the diodes 147-1, 147-2 and the capacitor 145 are configured to convert the excitation signal into a rectified output signal. Furthermore, the detector 113 includes a resistor 149 and a transistor 151, wherein the resistor 149 turns on the transistor 151 when an oscillation generated by the oscillator circuit 103 is present as an excitation signal at the detector output 115.Furthermore, the detector 113 comprises a pull-up resistor 155, wherein the detector 113 is designed to provide the output signal via the pull-up resistor 155, wherein the output signal is in particular an inverted signal which outputs a logical 0 when the electrical contact 101 is closed, or in the first contact state, and outputs a logical 1 when the electrical contact 101 is open, or in the second contact state.

[0083] In one embodiment, the detector 113 comprises an output signal terminal 161 and is configured to invert the output signal to output a logic 1 at the output signal terminal 161 with the electrical contact 101 in the first contact state and to output a logic 0 at the output signal terminal 161 with the electrical contact 101 in the second contact state.

[0084] Furthermore, the contact monitoring device 100 comprises a supply voltage terminal 157 to which a supply voltage can be applied, wherein the oscillator circuit 103 and / or the detector 113 can be supplied with electrical energy by means of the supply voltage terminal 157. The supply voltage terminal 157 can comprise a smoothing capacitor 159, which is designed to smooth a supply voltage. Furthermore, the smoothing capacitor 159 can be designed as a buffer capacitor, which is designed to provide electrical energy to the oscillator circuit 103 and / or the detector 113 for a predetermined time interval, in particular even if an external energy supply via the supply voltage terminal 157 fails.

[0085] The coupling element 105 comprises a transformer 117 which is designed to inductively couple the oscillator circuit 103 to the impedance circuit 121 and the damping element 111.

[0086] The impedance circuit 121 includes the capacitors 135-1, 135-2, which are designed to compensate for a stray inductance, in particular a stray inductance of the transformer 117 at a predetermined frequency of the excitation signal generated by the oscillator circuit 103. The capacitors 135-1, 135-2 achieve, in particular, the advantage of minimizing the total capacitance of the galvanic isolation by means of the coupling element 105 and / or the advantage of a symmetrical structure. The capacitors 135-1, 135-2 can, in particular, be formed by capacitors having identical capacitance values.

[0087] Furthermore, the impedance circuit 121 comprises the capacitors 137-1, 137-2 and the inductance 133. In one embodiment, only one of the capacitors 137-1, 137-2 and / or only one of the capacitors 135-1, 135-2 may be provided.

[0088] The inductance 133, together with the capacitors 137-1, 137-2, forms an LC element configured to respond to electrical signals corresponding to an electrical line with invertible impedance. An inductance value of the inductance 133 and a capacitance value of the capacitors 137-1, 137-2 can be selected such that, with the electrical contact 101 in the first contact state, the LC element has a high impedance value in order to switch the signal output 109 of the coupling element 105 to a high impedance and correspondingly unloaded state. As a result, an impedance value at the signal input 107 can have a high real component, which can prevent the generation of the excitation signal by the oscillator circuit 103.

[0089] Fig. 2 shows a schematic representation of the contact monitoring device 100 for monitoring an electrical contact 101 with at least two contact states according to one embodiment. The contact monitoring device 100 is integrated into a module base 203, wherein the electrical contact 101 is part of a switching module 201. The module base 203 comprises three switching contact connections 205-1, 205-2, 205-3, which are designed in particular as socket plugs. Furthermore, the switching module 201 comprises corresponding plug contacts 207-1, 207-2, 207-3, which can be inserted into the switching contact connections 205-1, 205-2, 205-3 and electrically connected thereto in order to connect the electrical contact 101 to the damping element 111, in particular to connect it electrically in parallel with the damping element 111.

[0090] The electrical contact 101 can, in particular, be a three-pole switch, which is designed to electrically connect the plug contacts 207-3 and 207-2 in a first contact state and to electrically connect the plug contacts 207-1 and 207-2 in a second contact state. The contact monitoring device 100 is designed to detect an opening and a closing of the electrical connection between the plug contact 207-3 and the plug contact 207-2. The plug contact 207-1 can be forwarded to another contact monitoring device 100 via a through connection 209 in order to also monitor the electrical connection between the plug contacts 207-1 and 207-2.Furthermore, the contact monitoring device 100 can be designed to apply a further contact monitoring signal to the plug contacts 207-1 and 207-2 via the plug sockets 205-1 and 205-2 in order to monitor the electrical connection between the plug contacts 207-1 and 207-2.

[0091] The contact monitoring device 100 has a filter module 211, which includes the filter element 123-2. The filter element 123-2 can be formed by a single inductor, which is particularly designed as an RF ferrite. By omitting an additional filter element, the advantage of reduced manufacturing costs can be achieved with reliable operation of the contact monitoring device 100.

[0092] The capacitance 131 of the damping element 111 can be formed as a printed circuit board capacitor in order to have increased dielectric strength. Furthermore, at least one of the capacitances 137-1, 137-2, for example, the capacitance 137-1, is formed as a high-voltage-resistant printed circuit board capacitor. The capacitances 137-1, 137-2 can preferably each have a capacitance value in the pF range. More preferably, the capacitances 137-1, 137-2 each have a capacitance value of less than 100 pF. Furthermore, the further capacitances 135-1, 135-2 can also have a capacitance value in the pF range or each have a capacitance value of less than 100 pF. With low capacitance values, the advantage can be achieved in particular that, for example, in AC applications, low, in particular negligible leakage currents flow via the impedance circuit 121 and / or the damping element 111.

[0093] The inductance 133 can be formed as a planar coil in the form of a spiral conductor track.

[0094] In one embodiment, the detector 113 can be configured to provide an output signal at the output signal terminal 161 only when a switching error of the electrical contact 101 is present. The detector 113 can be configured to detect a switching error of the electrical contact 101 based on a logical deviation between the excitation signal and the activation signal used to trigger the electrical contact 101.

[0095] The switching module base 203 can further comprise a first switching control terminal 215-1 and / or a second switching control terminal 215-2, wherein the activation signal can be applied to the switching control terminals 215-1, 215-2 in order to trigger the electrical contact 101 in the switching module 201. The rectifier 213 is connected upstream of the first switching control terminal 215-1 and / or the second switching control terminal 215-2 and is configured to convert the activation signal into a rectified activation signal. The switching module base 203 further comprises a first switching signal output 217-1 and / or a second switching signal output 217-2, wherein the rectifier 213 is configured to provide the rectified activation signal at the first switching signal output 217-1 and / or a second switching signal output 217-2.

[0096] The switching module 201 comprises a first switching connector 219-1 and / or a second switching connector 219-2. The first switching connector 219-1 is electrically connectable to the first switching signal output 217-1 when the switching module 201 is plugged into the switching module socket 203, and / or the second switching connector 219-2 is electrically connectable to the second switching signal output 217-2 when the switching module 201 is plugged into the switching module socket 203, in order to apply the activation signal to the switching element.

[0097] In one embodiment, the turn-on signal may be supplied to the detector 113, wherein the detector 113 is configured to evaluate the turn-on signal with respect to the excitation signal in order to detect a switching and / or a contact state of the electrical contact 101 as a function of the turn-on signal and to provide a corresponding output signal at the output signal terminal 161.

[0098] The capacitors 135-1, 135-2, 137-1, 137-2, 131, 139-1, 139-2, in particular the coupling capacitors, can be formed individually or as a subset by conductor surfaces arranged on different layers of an at least two-layer printed circuit board. A plastic material of the printed circuit board can be arranged between the metallic conductor surfaces, so that a capacitor is formed as a printed circuit board capacitor.

[0099] Furthermore, the contact monitoring device 100 comprises a supply voltage connection 157 to which a supply voltage can be applied, wherein the oscillator circuit 103 and / or the detector 113 can be supplied with electrical energy by means of the supply voltage connection 157. The supply voltage connection 157 can in particular be formed by a supply terminal.

[0100] The contact monitoring device 100 further comprises a first load terminal 221-1 and a second load terminal 221-2, each of which is connected downstream of the filter module 211. The first load terminal 221-1 is electrically connected to the third switching contact terminal 205-3, and the second load terminal 221-2 is electrically connected to the second switching contact terminal 205-2. In particular, the first load terminal 221-1 is connectable to a load voltage supply 223 and is configured to apply a load voltage signal to the electrical contact 101 via the third switching contact terminal 205-3 and the third plug contact 207-3.

[0101] The second load terminal 221-2 is electrically connectable to an electrical load 225, wherein, with the electrical contact 101 in the first contact state, the load voltage signal is applied to the electrical load 225. Furthermore, the load terminals 221-1, 221-2 can be short-circuited with a line bridge 227. This can, for example, generate a test case for the contact monitoring device 100 to verify reliable detection of the contact state of the electrical contact 101 even with a low-resistance bridging of the electrical contact 101, in particular in the event of an external short circuit.

[0102] The coupling element 105 comprises a transformer 117, which is designed to inductively couple the oscillator circuit 103 to the impedance circuit 121 and the damping element 111. The transformer 117 has a first inductance 119-1, in particular a first planar coil, and a second inductance 119-2, in particular a second planar coil, wherein the first inductance 119-1 is inductively coupled to the second inductance 119-2 in order to convert the excitation signal into the contact monitoring signal. The transformer 117 is designed to galvanically isolate the oscillator circuit 103 from the impedance circuit 121, the damping element 111, and / or the electrical contact 101. The galvanic isolation is indicated by the separating line 229.

[0103] Fig. 3shows the frequency-dependent impedance at the signal input in one embodiment. An inductance acts at the signal input, which is formed in particular by the main inductance of the transformer of the coupling element. The inductance can, for example, have an inductance value of 200 nH. A capacitance, which is formed in particular by the capacitances of the oscillator circuit, also acts at the signal input. The capacitance can, for example, have a capacitance value of 50 pF. Furthermore, an ohmic resistor acts at the signal input, which is formed in particular by an impedance on the signal output side transformed by means of the transformer. With the electrical contact in the first contact state, ie with a closed, electrically conductive electrical contact, a first resistor with a resistance value of, for example, 4 kΩ can act at the signal input.With the electrical contact in the second contact state, ie with an open, electrically insulating electrical contact, a second resistor with a resistance value of, for example, 1 kΩ can act at the signal input.

[0104] The resistance acting at the signal input can vary according to the contact state such that with a first resistance the oscillator circuit is capable of oscillation and can generate the excitation signal with a resonance signal amplitude and with the second resistance the oscillator circuit is not capable of oscillation and cannot generate the excitation signal.

[0105] The oscillator circuit can be configured to transition from a non-oscillatory state to an oscillatory state with a resistance threshold value 301 greater than 2.5 kΩ. The contact monitoring device 111 is configured, by appropriately dimensioning the electrical components, to separate a first impedance curve 303 and a second impedance curve 305 by the largest possible amplitude difference. The amplitude difference can be scaled with an impedance difference at the signal input based on the first contact state and the second contact state. With the first impedance curve 303, the oscillator circuit is in a non-oscillatory state, and the excitation signal cannot be generated. With the second impedance curve 305, the oscillator circuit is in an oscillatory state, and the excitation signal can be generated.

[0106] Fig. 4shows a schematic representation of the contact monitoring device 100 in one embodiment. The expected impedance in ohms for the expected oscillator frequency is indicated between the respective elements. Above the schematic representation of the contact monitoring device 100, the impedance for the electrical contact 101 in the first contact state, or for the closed electrical contact 101, is indicated.

[0107] Furthermore, below the schematic representation of the contact monitoring device 100, the impedance for the electrical contact 101 in the second contact state, or for the open electrical contact 101, is indicated. The load 225 can typically have an impedance in the range of mΩ to ∞Ω.

[0108] A ferrite with an ohmic impedance component in the Ω range is connected downstream of contact 101.

[0109] The damping element 111 and / or the filter element are configured to provide a defined impedance between the damping element 111 and the impedance circuit 121, independent of any external circuitry of the contact monitoring device 100, in particular independent of an external load 225. This provides the advantage that, when an electrical contact is open, the impedance value or an impedance range is predetermined at this point.

[0110] In particular, it can be prevented that an external, or load-side, short circuit leads to an impedance at the oscillator circuit 103, even when the electrical contact 101 is open, which allows the generation of an oscillation and, accordingly, the generation of the excitation signal. Accordingly, it can be prevented that an external short circuit is misinterpreted as a closed electrical contact 101.

[0111] The impedance circuit 121 can be configured to transform a small impedance on the switch contact side to a large impedance on the coupling element side with the electrical contact closed. Furthermore, the impedance circuit 121 can be configured to transform a large impedance on the switch contact side to a small impedance on the coupling element side with the electrical contact open. The specified impedance ranges correspond to the magnitude of the impedance at the respective point in the circuit relative to a high-frequency range.

[0112] Fig. 5shows a schematic representation of the contact monitoring device 100 in one embodiment. The electrical contact 101 is an overcurrent and / or overvoltage protection device, in particular a fuse. The expected impedance between the respective elements is indicated in ohms. Above the schematic representation of the contact monitoring device 100, the impedance for the electrical contact 101 in the first contact state, or for the fuse that has not been triggered, is indicated.

[0113] Furthermore, the impedance for the electrical contact 101 in the second contact state, or for a blown fuse, is indicated below the schematic representation of the contact monitoring device 100. The load 225 can have an impedance in the range of mΩ to ∞Ω.

[0114] If the electrical contact 101 is closed, or conductive, the oscillator circuit 103 cannot oscillate. If the electrical contact 101 is open, or the fuse has blown, the oscillator circuit can oscillate. This makes it possible, in particular, to monitor the status of a (fusible) fuse. The filter element can, for example, have an ohmic resistance in the kOhm range.

[0115] Fig. 6 shows a schematic representation of the contact monitoring device 100 in one embodiment. In contrast to the example in Fig. 5 In the embodiment shown, a damping element 611 is provided which is connected downstream of the contact 101, which can be designed as a switch.

[0116] The damping element 611 comprises a circuit with the ferrite 401, which causes signal attenuation.

[0117] An LC series resonant circuit with an inductance 603 and a capacitor 605 is connected downstream of the ferrite 401. The LC series resonant circuit is designed to provide a low impedance at the oscillation frequency, which allows the contact monitoring signal at the terminals to be minimized or reduced.

[0118] In one embodiment, the inductance 603 may also be omitted.

[0119] In one embodiment, the damping element 611 may be integrated into the filter module 211.

[0120] The damping element 611 corresponds in function to the damping element 111. In addition, the damping element 611 can have a filter characteristic due to the LC resonant circuit and therefore forms a filter in one embodiment.

[0121] In the following, exemplary embodiments of a temperature-compensated contact monitoring device are explained. Fig. 7A basic block diagram of the contact monitoring device 100 according to the invention with temperature compensation is shown. The contact monitoring device 100 includes an oscillator circuit consisting of an amplifier RV and a frequency-selective network FSN. A damping element DEL acts on the frequency-selective network FSN. The effective damping is determined by the state of a contact 101 to be monitored. The amplifier RV can have a gain dependent on the temperature ϑ and the operating point k of the amplifier. Typically, the temperature influence is such that a change in temperature changes the transistor currents, resulting in a changed gain. This presents a problem, as will be explained in more detail below.

[0122] First, the function of the contact monitoring device 100 is explained again in principle, see. Fig. 8 . In Fig. 8is a circuit diagram of the frequency selective network FSN from Fig. 7A state (e.g., contact closure yes / no) of the contact 101 to be monitored can influence the attenuation of the attenuation element DEL of the frequency-selective network FSN. The attenuation of the frequency-selective network FSN is represented by the attenuation elements or effective resistors 112 and 122, each of which has a different resistance value. The state of the contact 101 to be monitored influences the position of the illustrated switch 102, which, depending on the switch position, connects one of the two attenuation resistors 112 and 122 to the rest of the frequency-selective network FSN. The frequency-selective network FSN is additionally formed by a parallel connection of an inductance 105 and a capacitor 103.The oscillator circuit can be designed for an operating point k such that the gain V of the amplifier RV compensates for a small attenuation 112 for a first state, and an oscillating input signal a can be observed at input 114 or an oscillating output signal V*a at output 115. At the same time, the oscillator circuit can be designed such that a larger attenuation 122 is effective when the switching contact 102 assumes a second state. This then has the effect that no oscillating output signal V*a is present at input 114 or output 115.

[0123] As is well known, an oscillating circuit must satisfy the oscillation condition for stable oscillation to occur. It is illustrated by the following formula: The loop gain must be greater than or equal to 1 for stable oscillation. The phase shift of the feedback loop must be an integer multiple of 360° at this frequency.

[0124] This is precisely what the contact monitoring device exploits. In one switching state of the switching contact, the oscillation condition is met, while in the other switching state of the switching contact, the oscillating circuit is damped to such an extent that stable oscillation is not achieved (the oscillator's gain is not large enough to compensate for the losses due to the damping).

[0125] This issue is now being addressed in connection with the Fig. 9 clarified. In Fig. 9The impedance curve (frequency response) of a parallel resonant circuit is shown. If the switching contact to be monitored is switched to the low-damping state, the oscillator of the contact monitoring device oscillates at the resonant frequency of approximately 50.2 MHz with the impedance curve as specified in impedance frequency response 305, see point 305a. If, however, the switching contact is switched to the high-damping state, the oscillator of the contact monitoring device follows the impedance curve as specified in impedance frequency response 303, see point 303a. In this state with strong damping 303, stable oscillation does not occur. The oscillation limit 301 depends on the specific circuit and the amplifier element and should, if possible, lie in the middle of the two curves so that the oscillator reliably oscillates or reliably does not oscillate, depending on the contact state.

[0126] This desired behavior of the oscillator circuit can be disrupted by temperature changes. The amplifier RV, in particular, exhibits temperature-dependent behavior.

[0127] This is because the amplifier's amplification factor RV is temperature-dependent. This is especially true for transistor amplifiers, because in semiconductor materials, the intrinsic conductivity and thus the transistor currents change with temperature. This can disrupt the desired behavior of the oscillator circuit, even leading to malfunctions. Oscillating signals occur at output 115 in one temperature range with both effective attenuations 112 and 122, and / or oscillating signals do not occur at output 115 in another temperature range with both effective attenuations 112 and 122, making it impossible for the contact monitoring device to reliably distinguish between the states.

[0128] In a further development of the invention, the temperature dependence of the oscillator circuit is compensated by shifting the operating point of the amplifier RV as a function of the temperature, i.e. k -> k(ϑ), so that the gain of the amplifier RV has a negligible or no temperature dependence.

[0129] In Fig. 9 is the impedance frequency response of the frequency selective network FSN from Fig. 8 Depending on whether the damping resistor 112 or 122 is active, different impedance curves 303 or 305 result. The description of the impedance curves has already been explained in connection with the description of the Fig. 3explained. At the resonant frequency at point 305a, in the case of impedance curve 305, a resonant oscillation occurs, and the oscillating signal appears at input 114 and output 115. At the resonant frequency at point 303a, in the case of impedance curve 303, no resonant oscillation occurs, and no oscillating signal appears at input 114 and output 115.

[0130] In Fig. 10A more detailed circuit diagram of an exemplary embodiment of the temperature-compensated oscillator circuit according to the invention is shown. The same reference numerals denote the same components as in the previous figures. The amplifier RV is formed here by a junction field-effect transistor (JFET) 141 in gate connection, resistors 144, 146, and 143, and capacitor 142, which are used to adjust the operating point of transistor 141. The frequency-selective network FSN consists of inductance 105, capacitive element 139, formed by capacitors 139-1 and 139-2, and damping element DEL, consisting of effective resistors 112, 122, and contact 101 to be monitored, the state of which determines the effect of one or the other damping resistor 112, 122. Capacitor 159 stabilizes the operating voltage.The circuit can be designed such that for a first effective damping via the damping resistor 112 an oscillating signal can be observed at the output 115 and for an effective damping via the damping resistor 122 this oscillating signal is not present.

[0131] According to the invention, the operating point of transistor 141 is influenced by a voltage divider consisting of a thermistor 146 and resistor 144 such that the gain of transistor 141 is (almost) independent of temperature or can be neglected. A temperature-dependent resistor can be used as a thermistor. There is the so-called thermistor. This is, for example, a PTC resistor (Positive Temperature Coefficient Resistor), whose resistance increases with rising temperature. In addition, there is the so-called thermistor, NTC resistor (Negative Temperature Coefficient Resistor), whose resistance decreases with rising temperature. In addition, other components with temperature-dependent behavior, such as semiconductor diodes, can be used as thermistors in the reverse direction (not shown).

[0132] The operating point voltage U ap results from the supply voltage Vcc and the temperature-dependent divider ratio of the resistors 146 and 144. The operating point voltage U ap is calculated using the following formula: U ap = V cc * R144 / R146 ϑ + R144 .

[0133] Here, U ap refers to the operating point voltage at point 114, Vcc to the supply voltage and ϑ to the temperature.

[0134] By appropriately designing resistors 144 and 146, the operating point voltage rises (or falls) with temperature just enough to compensate for the effects of the temperature-dependent gain with the operating point-dependent gain. In other words, a temperature-dependent bias voltage is applied to the gate of transistor 141 just enough to make the gain factor V as independent of temperature as possible. This allows for a largely temperature-independent gain, and thus a temperature-independent oscillation limit, to be achieved. Capacitor 142 is optional and can be used, for example, to smooth the operating point voltage. The optimal combination of resistors 144, 146, and capacitor 142 can be determined experimentally, for example.

[0135] The Fig. 11shows a different type of temperature compensation. In this variant, the operating point of transistor 141 is also stabilized. This solution corresponds to a digital implementation of the operating point stabilization. The operating point voltage U ap is generated digitally and provided directly with temperature compensation. This requires measuring the temperature ϑ. Therefore, a temperature sensor is required. This is located in Fig. 11designated by reference number 161. Reference number 200 denotes a microcontroller. The temperature sensor 161 is connected to it. A temperature operating point table 245 is stored in the microcontroller 200. The measured temperature value indexes the temperature operating point table 245. The information for an associated pulse-width modulated signal is taken from the temperature operating point table 245. The microcontroller 200 generates the associated pulse-width modulated signal and outputs it via one of its general-purpose outputs GPIO (General Purpose Input / Output). It is fed into point 114 at the contact monitoring device 100. The pulse width of the signal is modulated depending on the temperature. In order to produce the temperature-compensated operating point voltage U ap, the pulse / pause-dependent mean value of the signal must be calculated. This occurs in the Fig. 11The circuit shown includes resistor 148 and capacitor 142, which together form a low-pass filter. The low-pass filter is connected between feed point 114 and transistor 141.

[0136] This solution can compensate for other effects besides temperature, such as aging, component tolerances, supply voltage fluctuations, etc. This solution can also be used to calibrate the manufactured product after the manufacturing process. Component tolerances and manufacturing deviations may require product calibration.

[0137] In Fig. 12 is the circuit diagram of the temperature compensation circuit together with the one used in the embodiment of Fig. 1The circuit diagram shown is shown, in which temperature compensation is used with thermistor 146. The temperature-dependent voltage divider consisting of NTC resistor 146 and resistor 144 provides the gate voltage with U ap = V cc * R144 / (R146(ϑ)+R144), which is smoothed with the help of capacitor 142. The temperature-dependent gate voltage ensures, through appropriate dimensioning of the components, that the gain of transistor 141 is as temperature-independent as possible.

[0138] It should be understood that the proposed method and associated devices can be implemented in various forms of hardware, software, firmware, special-purpose processors, or a combination thereof. In a preferred variant, microcontrollers with integrated RAM memory and integrated I / O interfaces are used. Special-purpose processors may include application-specific integrated circuits (ASICs), reduced instruction set computers (RISCs), and / or field-programmable gate arrays (FPGAs). Preferably, the proposed method and device are implemented as a combination of hardware and software. The software is preferably installed as an application program on a program storage device.Typically, it is a machine based on a computer platform that includes hardware such as one or more central processing units (CPUs), random access memory (RAM), and one or more input / output (I / O) interfaces. An operating system is also typically installed on the computer platform. The various processes and functions described here may be part of the application program or a portion executed by the operating system. List of reference symbols

[0139] 100Contact monitoring device 101Electrical contact 102Switching contact 103Oscillator circuit 105Coupling element 107Signal input 109Signal output 111Attenuator 112First effective resistance 113Detector 114Feed point 115Detector output 117Transformer 119-1First inductance 119-2Second inductance 121Impedance circuit 122Second effective resistance 123-1First filter element 123-2Second filter element 125-1First output terminal 125-2Second output terminal 127Switching element 129Resistance 131Capacitance 133Inductance 135-1Capacitance 135-2Capacitance 137-1Capacitance 137-2Capacitance 139Capacitive element 139-1Capacitance 139-2Capacitance 141Transistor 142Capacitor 143Resistance 144Resistance 145Capacitance 146Thermistor 147-1Diode 147-2Diode 149Resistance 151Transistor 153Capacitance 155Pull-up resistor 157Supply voltage terminal 159Smoothing capacitor 161Output signal terminal 201Switching module 203Switching module base 205-1Switching contact connection 205-2Switching contact connection 205-3Switching contact connection 207-1Plug-in contact 207-2Plug-in contact 207-3Plug-in contact 209Feed-through connection 211Filter module 213Rectifier 215-1First switching control connection 215-2Second switching control connection 217-1First switching signal output 217-2Second switching signal output 219-1First switching connection plug 219-2Second switching connection plug 221-1First load connection 221-2Second load connection 223Load voltage supply 225Electrical load 227Cable bridge 229Separation line 240Microcontroller 245Temperature operating point table 301Resistance limit 303First impedance curve 303aFirst resonant frequency point 305Second impedance curve 305aSecond resonant frequency point 401Ferrite 611Damping element 601Inductance 603Capacitance DEL Damping element FSN Frequency-selective network RV Resonance amplifier

Claims

1. A contact monitoring device (100) for monitoring an electrical contact (101) with a first contact state and a second contact state, wherein the contact monitoring device (100) has the following features: an oscillator circuit (103) which is configured to generate an excitation signal, a coupler (105) with a signal input (107) and a signal output (109), wherein the oscillator circuit (103) is connected upstream of the signal input (107) in order to apply the excitation signal to the coupler (105), wherein the coupler (105) is configured to convert the excitation signal into a contact monitoring signal and to output the contact monitoring signal at the signal output (109) for applying the electrical contact (101), wherein with the electrical contact (101) in the first contact state, a first impedance value is set on the oscillator circuit (103), and with the electrical contact (101) in the second contact state, a second impedance value is set on the oscillator circuit (103), and a detector (113) which is configured to detect a version of the excitation signal in order to monitor the electrical contact, characterized in that the oscillator circuit (103) is configured to generate a resonant oscillation with the electrical contact (101) in the first contact state and the first impedance value and to output the excitation signal, wherein the oscillator circuit (103) has a means for thermally stabilizing the oscillator circuit (103), wherein the oscillator circuit (103) has an oscillating circuit with a feedback branch and an amplifier (RV), where the amplifier (RV) amplifies the signal fed back via the feedback branch, where the amplifier (RV) has at least one transistor (141) whose operating point is adjustable, where the amplification of the at least one transistor (141) is stabilized by the means for thermal stabilization by adjusting the operating point.

2. The contact monitoring device (100) according to claim 1, wherein the contact monitoring device (100) is configured to either prevent the formation of the resonance oscillation and thus the generation of the excitation signal in the second contact state or to generate another resonance oscillation and output another excitation signal, and wherein the detector (113) is configured to detect a version of the first excitation signal and a version of the other excitation signal or a version of the first excitation signal and the absence of the first excitation signal in order to detect the electrical contact state.

3. The contact monitoring device (100) according to claim 1 or 2, wherein the operating point of the transistor (141) can be adjusted via a voltage divider, wherein a thermistor (146), in particular an NTC or PTC resistor, or another hot conductor or cold conductor is used as an element of the voltage divider as a means for thermal stabilization.

4. The contact monitoring device (100) according to one of claims 1 to 3, wherein a temperature sensor (161) is used as a means for thermal stabilization, the signal of which is evaluated by a microcontroller (240) which generates an associated correction signal which sets the operating point of the at least one transistor (141) for thermal stabilization.

5. The contact monitoring device (100) according to claim 4, wherein the microcontroller (240) generates the correction signal in the form of a PWM signal, corresponding to a pulse width modulated signal, and an averager averages the PWM signal and supplies it to the transistor (141), optionally wherein the averager is designed in the form of a low-pass filter consisting of a resistor (148) and a capacitor (142).

6. The contact monitoring device (100) according to one of the preceding claims, wherein the oscillator circuit (103) is configured to generate the excitation signal with the first impedance value at an excitation frequency which corresponds to a resonance frequency of the oscillator circuit (103), and wherein the oscillator circuit (103) is further configured to change a signal amplitude of the excitation signal based on an impedance value at the signal input (107).

7. The contact monitoring device (100) according to one of the preceding claims, with a attenuator (111, 611) which is connected downstream of the signal output (109) and coupled to the electrical contact (101), wherein the attenuator (111, 611) is configured to provide an impedance with the electrical contact (101) in the second contact state in order to prevent an impedance of the electrical contact (101) from acting in the second contact state, wherein the signal input (107) of the coupler (105) is dampened with the action of the attenuator (111, 611), the oscillator circuit (103) is not capable of oscillation and an output of the excitation signal is prevented.

8. The contact monitoring device (100) according to claim 7, wherein the coupler (105) and the oscillator circuit (103) are configured to form an oscillating circuit, and wherein the attenuator (111) is configured to dampen the oscillating circuit in the second contact state in order to reduce a signal amplitude of the excitation signal, and / or wherein the electrical contact (101) in the first contact state bridges the attenuator (111, 611) with low resistance and a damping of the attenuator (111, 611) coupled to the signal input (107) by means of the coupler is reduced, and wherein the oscillator circuit (103) is configured to generate the excitation signal with an increased signal amplitude with the reduced damping, and / or wherein the attenuator (111, 611) comprises a capacitance which is formed by a circuit board capacitor, and / or further comprising an impedance circuit (121) which is connected downstream of the signal output (109) and coupled to the attenuator (111, 611), wherein the impedance circuit (121) is configured to compensate for a stray inductance and / or a stray capacitance of the coupler (105) in the case of a contact monitoring signal with a predetermined frequency, optionally wherein the impedance circuit (121) is configured to reduce a total capacitance and / or a total inductance across the galvanic isolation of the coupler (105).

9. The contact monitoring device (100) according to one of the preceding claims, wherein the oscillator circuit (103) has a detector output (115) and is configured to provide the excitation signal to the detector (113) via the detector output (115).

10. The contact monitoring device (100) according to one of the preceding claims, wherein the detector (113) is configured to convert the excitation signal into an output signal and to output the output signal in at least two different signal states, wherein a first signal state of the at least two signal states indicates a closed electrical contact (101) and a second signal state of the at least two signal states indicates an open electrical contact (101), optionally wherein the detector (113) is configured to output the output signal in the first signal state when an oscillating excitation signal is present at the detector output (115) and to output the output signal in the second signal state when a non-oscillating excitation signal is present at the detector output (115) or a signal amplitude of the excitation signal is below an amplitude limit value.

11. The contact monitoring device (100) according to one of the preceding claims, wherein the coupler (105) is configured to galvanicaly isolate the oscillator circuit (103) and / or the detector (113) from the electrical contact (101).

12. The contact monitoring device (100) according to one of the preceding claims, wherein the detector (113) is configured to detect a switch-on signal for switching the electrical contact (101) and a change in the excitation signal, and wherein the detector (113) is further configured to output a binary error signal which indicates whether the electrical contact (101) is switched in accordance with the switch-on signal, optionally wherein the detector (113) is configured to detect the excitation signal within a predetermined time interval when receiving the switch-on signal in order to determine whether the electrical contact (101) is switched in accordance with the switch-on signal within the predetermined time interval, and / or further comprising an optical display element which is configured to display the detection of the switch-on signal and a corresponding detected switching of the electrical contact (101) or a deviation from the expected switch-on signal.

13. The contact monitoring device (100) according to one of the preceding claims, wherein the coupler (105) comprises a transformer (117) having a first inductance (119-1) and a second inductance (119-2), wherein the first inductance (119-1) is inductively coupled to the second inductance (119-2) in order to convert the excitation signal into the contact monitoring signal.

14. The contact monitoring device (100) according to one of the preceding claims, with a first filter element (123-1) and a second filter element (123-2), wherein the electrical contact (101) has a first output connection (125-1), which is followed by the first filter element (123-1), and wherein the electrical contact (101) has a second output connection (125-2), which is followed by the second filter element (123-2) in order to attenuate high-frequency signals which are present at the switching contact (101) and / or at an electrical load and / or voltage source coupled to the electrical contact (101) by means of the coupler (105).

15. The contact monitoring device (100) according to claim 14, wherein the first filter element (123-1) and / or the second filter element (123-2) have an ohmic impedance component at the oscillator frequency, which is configured to prevent detection of an external short circuit of the first output terminal (125-1) with the second output terminal (125-2) as a closing of the electrical contact (101).