Sensor and installation of sensorized railway tracks
Patent Information
- Application Number
- ES2023733871T
- Authority / Receiving Office
- ES · ES
- Patent Type
- Patents
- Current Assignee / Owner
- Priority Date
- 2022-06-21
- Filing Date
- 2023-06-06
- Publication Date
- 2026-09-17
- Estimated Expiration
- 2043-06-06
AI Technical Summary
Existing sensors for detecting rail vehicle wheels are complex in design and often suffer from interference and frequency-dependent fluctuations, making reliable detection challenging.
A sensor with a series resonant circuit, a DC power source, a converter, and a monitoring device that generates detection signals based on DC voltage or current values, decoupled from AC voltage, and adjusts frequency to optimize detection using a frequency generator.
The sensor provides reliable wheel detection with minimal interference, allowing for a simple design and robust operation by using DC-based measurement decoupled from AC frequency variations.
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Abstract
Description
[0001] The invention relates to sensors and railway track systems with sensors.
[0002] Sensors for detecting the wheels of rail vehicles are well known in railway technology, for example to detect when a rail vehicle enters or leaves a given section of a railway line and to report the respective section as clear or occupied.
[0003] European patent EP 1 479 587 B1 describes a circuit arrangement for calibrating inductive sensors. In this circuit arrangement, a resonant circuit is tuned to a specific frequency and amplitude by adding RC combinations. The resonant circuit is designed as a parallel resonant circuit.
[0004] German patent application DE 199 15 597 A1 presents a coil system consisting of at least two partial coils which, due to their opposing partial windings, are intended to suppress common-mode interference coming from outside.
[0005] German patent DE 10 221 577 B3 describes a further coil system consisting of at least two sub-coils. The coils are driven by current pulses and are not designed as a resonant circuit, thus being frequency-independent.
[0006] European patent application EP 1 538 058 A1 describes a method for compensating temperature-dependent fluctuations in the transmitting power of a transmitting coil of a rail contact by means of a transmitting coil supplied with a voltage.
[0007] The invention is based on the objective of providing a sensor that can reliably detect the approach of objects, in particular magnetizable objects, for example ferrous objects such as wheels of rail vehicles, and yet is simple in design.
[0008] This problem is solved according to the invention by a sensor with the features according to claim 1. Advantageous embodiments of the sensor according to the invention are specified in the dependent claims.
[0009] After that, the sensor includes a series resonant circuit comprising an inductor, a capacitor, and a series resistor connected electrically in series; a DC power source that is both current- and voltage-limited and outputs a predetermined maximum current, provided the output voltage required to drive this maximum current does not exceed a predetermined maximum voltage, and otherwise outputs the maximum voltage; a converter that is fed on the input side by the DC power source and applies an AC voltage to the series resonant circuit on the output side, the frequency of which is set such that the DC power source – in detection-free sensor operation – feeds the maximum current at maximum voltage into the converter; and a monitoring device that generates a detection signal when the output voltage of the DC power source falls below the maximum voltage by a predetermined amount, and / or generates a warning signal.when the output current of the DC power source falls below the maximum current by a predetermined amount.
[0010] A key advantage of the sensor according to the invention is that the detection signal is generated based on measured values acquired on the DC voltage side of the converter or the output side of the DC source. The measured value acquisition is thus decoupled from the AC voltage side and the series resonant circuit, so that the measurement has no, or at least no significant, influence on the series resonant circuit or its resonant frequency. The generation of the detection signal and / or the warning signal always occurs at the same frequency and depends solely on DC current or DC voltage values.
[0011] If the optimal frequency of the alternating voltage for the respective application area of the sensor is known, the converter can generate the alternating voltage using this known constant frequency.
[0012] However, the optimal frequency is usually not always precisely known, and can at least change over time. For this reason, it is considered advantageous if the sensor includes a frequency generator that drives the converter and thus specifies the frequency of the alternating voltage. The operating frequency of the frequency generator preferably corresponds to the frequency of the alternating voltage that the converter is to generate. To drive the converter, the frequency generator preferably produces a sine wave or a square wave signal whose frequency corresponds to the frequency of the alternating voltage to be generated for the series resonant circuit.
[0013] The monitoring device is preferably connected to the frequency generator and sets its operating frequency and thus the frequency of the alternating voltage. In this latter configuration, the monitoring device can advantageously determine the optimal operating frequency itself or at least readjust it if this seems appropriate.
[0014] It is considered particularly advantageous if the monitoring device is designed to perform a work frequency test regularly or irregularly, independently or in response to an external control command.
[0015] Regarding the operating frequency test, it is advantageous if the monitoring device performs the operating frequency test in detection-free sensor operation.
[0016] Regarding the execution of the operating frequency test, it is advantageous if the monitoring device leaves the operating frequency of the frequency generator, and thus the frequency of the alternating voltage to be generated for the series resonant circuit, unchanged when the DC source feeds the maximum current at maximum voltage into the converter.
[0017] If the monitoring device determines that the DC source is not supplying the maximum current or the maximum voltage to the converter, it is advantageous if, as part of the operating frequency test, it changes the operating frequency of the frequency generator and thus the frequency of the AC voltage to be generated for the series resonant circuit, until it determines that the DC source is supplying the maximum current at maximum voltage to the converter.
[0018] Regarding the direction of the frequency change of the operating frequency of the frequency generator, it is considered advantageous if the frequency is increased when the output voltage of the DC source is less than the maximum voltage, and reduced when the output current of the DC source is less than the maximum current.
[0019] Alternatively or additionally, it may be advantageously provided that, as part of the operating frequency test, the monitoring device initiates a frequency increase and a frequency reduction – with reference to the previously set operating frequency – and observes the effects of the frequency increase and the frequency reduction on the output voltage and the output current of the DC source.
[0020] The monitoring device preferably leaves the operating frequency unchanged if, with reference to the current operating frequency, the output current of the DC source decreases when the frequency increases and the output voltage of the DC source decreases when the frequency decreases.
[0021] If the behavior described above—that is, the output current decreases when the frequency increases and the output voltage decreases when the frequency decreases—is not observable, it is advantageous for the monitoring device to change the operating frequency during the operating frequency test until it determines that, relative to the respective changed operating frequency, the output current of the DC source decreases with a further frequency increase and the output voltage of the DC source decreases with a further frequency decrease. Once this is determined, it preferably uses the changed operating frequency as the new operating frequency for further sensor operation until the next operating frequency test.
[0022] The sensor is preferably a flange detector designed for detecting railway vehicle wheels traveling on a railway track and passing the sensor, and generates a wheel detection signal as the detection signal.
[0023] The invention also relates to a railway track system equipped with a sensor according to claim 1; the sensor is arranged in or attached to a railway track and serves to detect the wheels of railway vehicles traveling on the railway track and passing the sensor.
[0024] Regarding the advantages of the railway track system according to the invention and its advantageous embodiments, reference is made to the above explanations in connection with the sensor according to the invention and its advantageous embodiments. The monitoring device of the sensor preferably generates a wheel detection signal when the output voltage of the DC source falls below the maximum voltage by a predetermined amount.
[0025] The sensor's monitoring device preferably generates a warning signal when the output current of the DC source falls below the maximum current by a predetermined amount.
[0026] The sensor components are preferably arranged such that the series resonant circuit and the transducer are located close to a railway track. The DC power source and the monitoring device are preferably located further away from the railway track. The distance between the DC power source and the railway track, and the distance between the monitoring device and the railway track, are preferably at least 10 times greater than the distance between the series resonant circuit and the railway track, and the distance between the transducer and the railway track.
[0027] The invention also relates to a method for adjusting a sensor according to claim 1. According to the invention, it is provided that a maximum current and a maximum voltage are specified for the DC source and the frequency of the AC voltage applied to the series resonant circuit is adjusted such that the DC source feeds the maximum current into the series circuit at the maximum voltage.
[0028] The invention is explained in more detail below with reference to exemplary embodiments; these show, by way of example, Figure 1: Components of an embodiment of a sensor according to the invention; Figure 2: A section of a railway track system equipped with the sensor according to the invention. Figure 1 is equipped, and Figure 3 shows finding and setting the optimal operating frequency of the sensor according to the Figure 1 and 2 based on a frequency diagram.
[0029] For the sake of clarity, the same reference symbols are used in the figures for identical or comparable components.
[0030] The Figure 1 Figure 1 shows components of an embodiment of a sensor 1 according to the invention, which can also be referred to as an inductively operating proximity sensor. The sensor 1 is a flange detector used for wheel detection on a railway track 110 (see Figure 1). Figure 2 ) moving rail vehicle wheels passing sensor 120 (see Figure 2 ) is designed and can generate a wheel detection signal RDS as a detection signal.
[0031] The sensor 1 comprises a DC source 10 that is both current and voltage limited. In the exemplary embodiment according to Figure 1 a current source 11 which feeds a predetermined constant maximum current Imax into a downstream voltage limiter 12 of the DC source 10.
[0032] The voltage limiter 12 outputs the maximum current Imax of the current source 11 as the output current I of the DC source 10, provided that the voltage Us applied to the input side of the voltage limiter 12, i.e. the voltage applied to the voltage limiter 12 by the current source 11, is less than or at most as large as a specified maximum voltage Umax.
[0033] If the input voltage Us at the voltage limiter 12 reaches the maximum voltage Umax, the voltage limiter 12 outputs only this maximum voltage Umax as the output voltage U of the DC source 10, so that in such a case the output current I of the voltage limiter 12 may fall below the maximum current Imax of the current source 11.
[0034] In summary, the DC source 10 outputs the specified maximum current Imax as the output current I, provided that the output voltage U required to drive this maximum current does not exceed the specified maximum voltage Umax, and otherwise it outputs the maximum voltage Umax as the output voltage U.
[0035] The output current I or the output voltage U from the DC power source 10 or its voltage limiter 12 is fed into a downstream converter 20. Downstream of the converter 20 is a series resonant circuit 30, which is powered by the converter 20. The converter 20, which is supplied with DC current from the DC power source 10, applies an AC voltage Uw to the series resonant circuit 30 at its output.
[0036] The series resonant circuit 30 has a coil 31, a capacitor 32 and a series resistor 33, which are electrically connected in series.
[0037] The operating frequency of the converter 20, and thus the frequency f of the alternating voltage Uw applied by the converter 20 to the series resonant circuit 30, is determined in the exemplary embodiment according to Figure 1 The frequency is set by a frequency generator 40 using a frequency generator signal Sf via a clock line 41. The frequency generator signal Sf directly or indirectly controls internal switching elements 21 of the converter 20, which are located in the Figure 1 These figures are only schematically sketched. The converter 20 can contain, for example, semiconductor switches such as transistors or the like as switching elements 21, which can be connected, for example, in a half-bridge or a full-bridge circuit. In other words, the converter 20 can be implemented as a controllable inverter of any design.
[0038] The frequency generator 40, which can output a frequency generator signal Sf, for example a sine or square wave signal for controlling the converter 20, is controlled by a monitoring device 50 of the sensor 1 via a control line 51. The monitoring device 50 specifies the frequency of the frequency generator signal Sf to the frequency generator 40 and thus also determines the frequency f of the alternating voltage Uw applied by the converter 20 to the series resonant circuit 30.
[0039] The frequency f of the alternating voltage Uw applied by the converter 20 to the series resonant circuit 30 is preferably set by the monitoring device 50 such that the DC source 10 - in detection-free sensor operation - feeds the maximum current Imax at maximum voltage Umax into the converter 20; this will be explained in more detail below.
[0040] The monitoring device 50 is connected to the output of the DC source 10 via a measuring line 52 and a current and voltage sensor 60 and uses these to measure the output voltage U and the output current I of the DC source 10.
[0041] The operation of sensor 1 utilizes the finding that the series resonant circuit 30, and in particular its resonant frequency, is susceptible to external influences. For example, if iron material, such as the iron material of a railway vehicle wheel 120, is brought near the coil 31, the resonant frequency of the series resonant circuit 30 changes, and the electrical power that can be fed into the series resonant circuit 30 decreases. This decrease—due to the current limiting by the current source 11—results in a drop in the output voltage U applied by the DC source 10 to the converter 20.
[0042] Accordingly, the monitoring device 50 generates the wheel detection signal RDS when the output voltage U of the DC source 10 falls below the specified maximum voltage Umax by a predetermined amount.
[0043] The Figure 2 shows a section of a railway track system 100, which is equipped with sensor 1 according to Figure 1 is equipped. Figure 2 Figure 1 shows the installation situation on railway track 110 as a cross-section through railway track 110. Above railway track 110 is a railway vehicle wheel 120 with its flange 121.
[0044] Since the sensor 1 is located in or attached to the railway track 110, it is suitable for detecting the wheel of the railway vehicle 120. For this purpose, the coil 31 is preferably arranged in the sensor housing 1a of the sensor 1 such that the magnetic field of the coil 31 runs predominantly vertically, so that the metal mass of the railway vehicle wheel 120 above it can be easily detected.
[0045] The monitoring device 50 of the sensor 1 generates the wheel detection signal RDS when the output voltage U of the DC source 10 falls below the maximum voltage Umax by a predetermined amount.
[0046] Since sensor 1 is mounted in the area of railway track 110, the resonant frequency of the series resonant circuit 30 is also influenced by the iron material of the track. If, in the event of a malfunction, sensor 1 is removed from railway track 110, for example because it falls off, is torn off, or is simply stolen, the influence of the iron of railway track 110 is eliminated, which also changes the resonant frequency of the series resonant circuit 30; this leads to a drop in the output current I of the DC source.
[0047] Accordingly, it is advantageous if the monitoring device 50 generates a warning signal WS when the output current I of the DC source 10 falls below the maximum current Imax by a predetermined amount. This warning signal indicates that the position of the sensor 1 has been changed or that another defect has occurred and that an inspection or repair should be carried out.
[0048] Sensor 1 according to Figure 1 and 2 is preferably operated as follows: Commissioning:
[0049] After installing the sensor 1, for example on railway track 110 according to Figure 2 , in detection-free operation without the influence of railway wheels to be detected, the monitoring device 50 first searches for the optimal operating frequency of the frequency generator signal Sf of the frequency generator 40 and thus the optimal alternating voltage Uw for the series resonant circuit 30.
[0050] As part of this search for the optimal operating frequency, the monitoring device 50 changes the operating frequency until it determines that, with respect to the respective applied operating frequency, the output current I of the DC source 10 decreases with further frequency increase and the output voltage U of the DC source 10 decreases with frequency reduction.
[0051] The Figure 3The diagram illustrates the search process in more detail. A first operating frequency f1, initially set by the monitoring device 50 during commissioning, can be seen. It is evident that the output voltage U is lower than the maximum voltage Umax. Accordingly, the monitoring device 50 will change the operating frequency, as shown in the diagram. Figure 3 shown, increase, and continue to do so until it finds the optimal operating frequency fopt, at which - i.e., with respect to this optimal operating frequency fopt - with further frequency increase the output current I of the DC source 10 decreases and with frequency reduction the output voltage U of the DC source 10 decreases.
[0052] Since the optimal operating frequency fopt determined in this way corresponds to the resonant frequency of the series resonant circuit 30, the magnitude of the impedance of the series resonant circuit 30 is minimal and the DC source 10 will feed the maximum current Imax at maximum voltage Umax into the converter 20. Operating frequency test:
[0053] The monitoring device 50 will preferably perform an operating frequency test regularly or irregularly, independently or in response to an external control command SB. The operating frequency test is preferably performed in detection-free sensor mode to avoid errors or delays in the operating frequency test due to the influence of interfering iron material from passing railway vehicle wheels 120.
[0054] As part of the operating frequency test, the monitoring device 50 will leave the current operating frequency of the frequency generator 40 unchanged if the DC source 10 feeds the maximum current Imax at maximum voltage Umax into the converter 20, i.e., the optimal operating point has already been reached.
[0055] If the maximum voltage Umax or the maximum current Imax is not reached, the monitoring device 50 changes the operating frequency until it determines that the DC source 10 is feeding the maximum current Imax into the converter 20 at the maximum voltage Umax.
[0056] Alternatively or additionally, the monitoring device 50 can initiate a frequency increase and a frequency reduction as part of the operating frequency test - with reference to the previously set (i.e. the current) operating frequency - and observe the effects of the frequency increase and the frequency reduction on the output voltage U of the DC source 10 and the output current I of the DC source 10.
[0057] In the latter variant, the monitoring device 50 will leave the currently set operating frequency unchanged if - with reference to this respective current operating frequency - the output current I of the DC source 10 decreases when the frequency increases and the output voltage U of the DC source 10 decreases when the frequency decreases.
[0058] Otherwise, the monitoring device 50 will change the operating frequency as part of the operating frequency test until it determines that – with reference to the last set operating frequency – the output current I of the DC source 10 decreases with further frequency increase and the output voltage U of the DC source 10 decreases with further frequency reduction; as soon as it determines this, it will continue to use the last set operating frequency as the new optimal operating frequency fopt for further sensor operation until the next operating frequency test.
[0059] The sensor 1 can be configured as a multi-channel system, e.g., as a dual system with two sensors mounted one behind the other in the longitudinal direction of the rail, which are used to detect the direction of a train due to their signal overlap. In this configuration, the individual sensors 1 preferably have series resonant circuits 30 with different resonant frequencies in order to avoid interference caused by beat frequencies on the signal voltages.
[0060] In connection with the Figures 1 to 3In the illustrated embodiments, the frequency generator 40 can be adjusted by the monitoring device 50 with respect to its operating frequency in order to find and set the optimal operating frequency fopt. If the optimal operating frequency fopt is already known for a given application, for example because it has been measured or simulated, the converter 20 can always be operated at the same constant operating frequency, and the control signal from the monitoring device 50 and the control line 51 between the monitoring device 50 and the frequency generator 40 can be omitted. In such a case, a simple oscillator resonant circuit that defines the operating frequency can be used instead of the frequency generator 40.
[0061] The exemplary embodiments of sensor 1 described above may – but do not necessarily – exhibit one or more of the following features, advantages or properties listed in bullet points: The series resonant circuit 30 is preferably not operated as a self-oscillating oscillator, but is preferably driven at its resonant frequency via a converter 20. The converter 20 is preferably designed such that its own power consumption is negligible compared to that of the series resonant circuit 30, so that the measured quantities M for U and I essentially represent the power consumption of the series resonant circuit 30. The frequency generator 40 can be a simple square wave generator. The converter 20 can comprise a simple bridge circuit made of transistors or similar components that are digitally switched on and off. A capacitor is preferably located at the current / voltage supply to the converter 20 to smooth the supplied current I or voltage U.The levels of the measured variables M for the output voltage U and the output current I, which occur during operation and are present at the input of the converter 20, are preferably transmitted via a measuring line 52 to the monitoring device 50, which may, for example, include a microprocessor. This device evaluates the voltage and current levels and outputs the detection signal or the warning signal for further processing. In the case of frequency adjustment to the optimal resonant frequency, the frequency of the frequency generator 40 is preferably detuned via a control signal so that—as described—the values Imax and Umax are set at the input of the converter 20. The sensor 1 can advantageously be designed such that only a few components, such as the converter 20 and the series resonant circuit 30, are mounted directly on the rail, because the mechanical stress on the components is greatest at the rail.The other components, namely the monitoring device 50, the frequency generator 40, and the DC power source 10, are preferably located at a greater distance, e.g., next to the track. The series resonant circuit 30 can exhibit a high oscillation amplitude, thus absorbing a high amount of energy. This, in turn, leads to a correspondingly high interference immunity of the sensor 1, since the effect of coupled interference energy depends on the resonant circuit amplitude, i.e., the energy contained in the series resonant circuit 30. A self-compensating arrangement with oppositely wound partial coils in series is therefore unnecessary. The series resonant circuit 30 is preferably not operated as a feedback-controlled, self-oscillating oscillator, but rather driven at its resonant frequency via the converter 20 using the frequency generator 40. Precise adjustment of the optimal resonant frequency is possible without having to directly measure the oscillation amplitude.The series resonant circuit 30 itself therefore does not need to have a signal output for level measurement. When a metal wheel passes over it, the resonant circuit amplitude decreases due to eddy current losses in the metal and its field distortion of the alternating magnetic field. For a simple evaluation, only the power consumption of the series resonant circuit 30 can be considered: The series resonant circuit 30 is preferably supplied or driven with a fixed, upper-limited amount of power. The series resonant circuit 30 has the lowest impedance at its resonant frequency. At the resonant frequency, the series resonant circuit 30 therefore consumes the maximum possible power. If the input frequency f rises above the resonant frequency, the input current I decreases, while the applied voltage U remains constant.If the input frequency f drops below the resonant frequency, the input current I remains constant, while the applied voltage U decreases. Frequency tuning can be performed by measuring the voltage U and the current I flowing into the converter 20 during a frequency sweep: When sweeping from low to high frequencies, the maximum possible current (Imax of the current source 11) will initially flow into the series resonant circuit 30 at a low applied voltage. As the frequency f increases, the applied voltage U increases and, with a constant current flow Imax, reaches its maximum value Umax at resonance. As the frequency f continues to increase, the current I decreases, and the applied voltage U remains constant at the maximum voltage Umax (due to the voltage limiter 21). The resonant frequency is therefore the frequency f at which the current I and the voltage U are at their maximum.This establishes the resonant frequency for the series resonant circuit 30 in the specific installation situation. This frequency f can be stored and used as the operating frequency during operation. Since it involves a direct current and a direct voltage, these values are easy to measure without having to interfere with the actual series resonant circuit 30. This allows for a very compact design, minimizing potential interference. Signal distortion caused by the power draw of a measurement signal that directly measures the amplitude voltage can also be avoided. At the operating point of the resonant frequency, the voltage U applied to the transducer 20 decreases compared to the quiescent value when a wheel passes over it, due to the damping effect of the iron mass. This voltage reduction can be used for detection and evaluated or further processed in a subsequent unit.A check of the resonant frequency in the quiescent state, which may have changed due to mechanical or thermal influences, is preferably carried out by briefly changing the instantaneous operating frequency by a defined amount, frequency change Δf. A frequency change Δf below the operating frequency causes a reduction in the supply voltage level by a specific value. A frequency change Δf above the operating frequency causes a reduction in the current level by a specific value. If the deviation values on both sides are as expected, then the set operating frequency is also the resonant frequency.
Claims
1. Sensor (1), comprising - a series resonant circuit (30), which has a coil (31), a capacitor (32) and a series resistor (33) that are electrically connected in series, - a direct current source (10) that is both current-limited and also voltage-limited and outputs a specified maximum current (Imax) as the output current (I), provided the output voltage (U) required to drive this maximum current (Imax) does not exceed a specified maximum voltage (Umax), and otherwise outputs the maximum voltage (Umax), - a converter (20) which is fed on the input side by the direct current source (10) and on the output side applies an alternating voltage (Uw) to the series resonant circuit (30), the frequency (f) of said alternating voltage being set such that - in detection-free sensor operation - the direct current source (10) feeds the maximum current (Imax) to the converter (20) at the maximum voltage (Umax), and - a monitoring device (50) which generates a detection signal (RDS) if the output voltage (U) of the direct current source (10) falls below the maximum voltage (Umax) by a specified amount, and / or generates a warning signal (WS) if the output current (I) of the direct current source (10) falls below the maximum current (Imax) by a specified amount.
2. Sensor (1) according to claim 1, wherein - the sensor (1) has a frequency generator (40) which drives the converter (20) and thus specifies the frequency (f) of the alternating voltage (Uw), and - the monitoring device (50) is connected to the frequency generator (40) and sets its operating frequency and thus the frequency (f) of the alternating voltage (Uw).
3. Sensor (1) according to one of the preceding claims, wherein the monitoring device (50) is configured to carry out an operating frequency check, regularly or irregularly, independently or in response to an external control command (SB).
4. Sensor (1) according to claim 3, wherein the monitoring device (50) is configured to carry out the operating frequency check in detection-free sensor operation.
5. Sensor (1) according to one of the preceding claims 3 to 4, wherein in the context of the operating frequency check, the monitoring device (50) leaves the operating frequency of the frequency generator (40) unchanged if the direct current source (10) feeds the maximum current (Imax) into the converter (20) at the maximum voltage (Umax).
6. Sensor (1) according to one of the preceding claims 3 to 5, wherein in the context of the operating frequency check, the monitoring device (50) changes the operating frequency until it ascertains that the direct current source (10) feeds the maximum current (Imax) into the converter (20) at the maximum voltage (Umax).
7. Sensor (1) according to one of the preceding claims 3 to 6, wherein in the context of the operating frequency check, the monitoring device (50) initiates - relative to the respective previously set operating frequency - a frequency increase and a frequency reduction and observes the effects of the frequency increase and the frequency reduction on the output voltage (U) of the direct current source (10) and the output current (I) of the direct current source (10).
8. Sensor (1) according to claim 7, wherein the monitoring device (50) leaves the operating frequency unchanged if - relative to the respective current operating frequency - on a frequency increase, the output current (I) of the direct current source (10) falls and, on a frequency reduction, the output voltage (U) of the direct current source (10) falls.
9. Sensor (1) according to one of the preceding claims 7 to 8, wherein in the context of the operating frequency check, the monitoring device (50) changes the operating frequency, specifically until it ascertains that - relative to the respective changed operating frequency - on a further frequency increase, the output current (I) of the direct current source (10) falls and, on a further frequency reduction, the output voltage (U) of the direct current source (10) falls, and - as soon as it ascertains this - it continues using the changed operating frequency as the new operating frequency for the further sensor operation until the respective next operating frequency check.
10. Sensor (1) according to one of the preceding claims, wherein the sensor (1) is a flange detector which is configured for wheel detection of railway vehicle wheels (120) travelling on a railway rail (110) and passing the sensor (1) and, as the detection signal (RDS), generates a wheel detection signal.
11. Railway track system (100), wherein - it is equipped with a sensor (1) according to one of the preceding claims, - wherein the sensor (1) is arranged in the region of a railway rail (110) or is mounted thereon and serves for wheel detection of railway vehicle wheels (120) travelling on the railway rail (110) and passing the sensor (1).
12. Railway track system (100) according to claim 11, wherein the monitoring device (50) of the sensor (1) generates a wheel detection signal if the output voltage (U) of the direct current source (10) falls below the maximum voltage (Umax) by a specified amount.
13. Railway track system (100) according to claim 11 or 12, wherein the monitoring device (50) of the sensor (1) generates a warning signal (WS) if the output current (I) of the direct current source (10) falls below the maximum current (Imax) by a specified amount.
14. Method for setting a sensor (1) according to one of claims 1 - 10, wherein - for the direct current source (10), the maximum current (Imax) and the maximum voltage (Umax) are specified and - the frequency (f) of the alternating voltage (Uw) applied to the series resonant circuit (30) is set such that the direct current source (10) feeds the maximum current (Imax) into the series circuit at the maximum voltage (Umax).
15. Method according to claim 14, wherein - in the context of an operating frequency check, the operating frequency of a frequency generator (40) and thus the frequency (f) of the alternating voltage (Uw) applied to the series resonant circuit (30) is adjusted until it is established that, on a frequency increase, the output current (I) of the direct current source (10) falls and, on a frequency reduction, the output voltage (U) of the direct current source (10) falls, and - the adjusted operating frequency is further used as the new operating frequency for the further operation of the sensor (1).