Sensor, sensor system and method

Abstract

The invention relates to a sensor for determining two measured variables relating to a gas. The sensor has at least a first sensor element and a second sensor element and also two control connections. The first sensor element and the second sensor element are connected in series. The sensor elements are arranged between the control connections. The first sensor element has a first electronic equivalent circuit diagram, the first electronic equivalent circuit diagram comprising a parallel connection containing a first resistance and a first capacitance. The second sensor element has a second electronic equivalent circuit diagram, the second electronic equivalent circuit diagram comprising a second capacitance.

Description

[0001] Description

[0002] title

[0003] SENSOR, SENSOR SYSTEM AND METHOD

[0004] The invention relates to a sensor, a sensor system, a method for operating the sensor and evaluation electronics for reading the sensor.

[0005] State of the art

[0006] Gas sensor modules are known from the prior art in which the resistance of a gas sensor chip and the capacitance of a humidity sensor are measured via separate signal inputs of an evaluation electronics unit, for example, a sensor ASIC. The gas sensor chip and the humidity sensor are connected to the evaluation electronics unit by means of separate bonding wires. Furthermore, a pressure sensor can be included in the gas sensor module, which can be, for example, a piezoresistive pressure sensor or a capacitive pressure sensor, and which is also connected to the evaluation electronics unit via separate bonding wires. In particular, capacitive pressure sensors are easy to implement and integrate into existing chip geometries. In particular, it can be provided that the gas sensor module has a semiconductor chip, wherein the semiconductor chip contains both the gas sensor or several gas sensors as well as the humidity sensor and, if necessary, also the pressure sensor.

[0007] Disclosure of the invention

[0008] One object of the invention is to provide an improved sensor, for example, configured as a gas sensor module. A further object of the invention is to provide an improved sensor system with such a sensor, an improved method for operating the sensor, and improved evaluation electronics for reading the sensor. These objects are achieved by the subject matter of the independent patent claims. Advantageous further developments are specified in the dependent patent claims.

[0009] According to a first aspect, the invention relates to a sensor for determining two measured variables of a gas. The gas can also be referred to as atmosphere. The sensor has at least a first sensor element and a second sensor element as well as two control connections. The first sensor element and the second sensor element are connected in series. The sensor elements are arranged between the control connections. The first sensor element has a first electronic equivalent circuit, wherein the first electronic equivalent circuit comprises a parallel circuit of a first resistor and a first capacitor. The second sensor element has a second electronic equivalent circuit, wherein the second electronic equivalent circuit comprises a second capacitor.

[0010] The measured variables can, in particular, include the identification of a gas type. Furthermore, the measured variables can include concentrations of specific gases, pressures, and / or humidities.

[0011] The sensor may further comprise a heating element that is in thermal contact with the first sensor element. Alternatively or additionally, the sensor may comprise a further heating element that is in thermal contact with the second sensor element. Furthermore, heating element control connections may be provided, with which the heating element and / or the further heating element can be controlled.

[0012] In particular, no further control connection is provided between the sensor elements, i.e., between the first sensor element and the second sensor element. An electronic signal introduced via one of the control connections thus passes through the first sensor element and the second sensor element and then the other control connection. The two sensor elements are therefore accessible in a common measurement via the shared control connection. This eliminates the need or shortens the length of individual wires between the sensor elements and an evaluation unit. To read out the first sensor element and the second sensor element, an asymmetry can be used between the first sensor element with a parallel first resistor and first capacitance and the second sensor element with a second capacitance, so that the properties of both sensor elements can be reconstructed.

[0013] The first sensor element and the second sensor element can each be designed as individual components and connected to one another. The first sensor element can, in particular, comprise a gas sensor with which a type of gas can be determined. In this case, it is generally necessary to determine both a resistance and a capacitance of a first material used for the first sensor element. This can be done using the parallel connection of the first resistance and the first capacitance already described for the first equivalent circuit. The second sensor element can then be a humidity sensor or a pressure sensor, with a capacitance of a second material being evaluated as the second capacitance in order to determine the humidity or the pressure.It can also be provided that, in addition to the second sensor element, a third sensor element is connected in series, that the third equivalent circuit includes a third capacitance, and that a humidity sensor is provided as the second or third sensor element and a pressure sensor as the third or second sensor element. It can further be provided that the second sensor element also includes a gas sensor, which can also be used to determine a type of gas. The first sensor element and the second sensor element can then be different in order to be able to determine different types of gas.

[0014] According to a second aspect, the invention relates to an evaluation unit with a computing unit and at least two connections. The connections can be connected to the control connections of a sensor according to the invention. The evaluation unit is configured to excite the sensor via the connections at a predeterminable frequency, to determine a complex impedance, and to calculate the two measured variables from the complex impedance. The complex impedance can then contain a magnitude and a phase, with the two measured variables being calculated from the magnitude and phase of the complex impedance. As an alternative to magnitude and phase, an evaluation can also be performed using a real part and an imaginary part of the complex impedance.With this evaluation unit, in particular a sensor with the asymmetry in the circuit comprising the first sensor element and the second sensor element explained in connection with the first aspect can be read out and the properties of both sensor elements can be reconstructed.

[0015] The evaluation unit can in particular comprise an integrated circuit (IC), preferably an application-specific integrated circuit (ASIC). Alternatively, the evaluation unit can also comprise a microcontroller.

[0016] The evaluation unit may have heating signal output terminals that can be connected to the heating element control terminals.

[0017] According to a third aspect, the invention relates to a sensor system comprising the sensor according to the invention and the evaluation unit according to the invention. In particular, the sensor and the evaluation unit can be arranged within a common housing.

[0018] According to a fourth aspect, the invention relates to a method for operating a sensor according to the invention, in which the sensor is first excited at a predeterminable frequency. Subsequently, a complex impedance is determined. Subsequently, two measured variables are calculated from the complex impedance. The complex impedance can then contain a magnitude and a phase, wherein the two measured variables are calculated from the magnitude and phase of the complex impedance. As an alternative to magnitude and phase, an evaluation can also be carried out using a real part and an imaginary part of the complex impedance. With this method, in particular, a sensor with the asymmetry explained in connection with the first aspect in the circuit comprising the first sensor element and the second sensor element can be read out and the properties of both sensor elements can be reconstructed.

[0019] In one embodiment of the sensor, the first sensor element and the second sensor element are arranged on a common sensor chip. An electrical connection between the two sensor elements can also be implemented on the sensor chip, automatically resulting in the inventive series connection of the two sensor elements. This makes it possible to provide a smaller sensor, if necessary.

[0020] In one embodiment of the sensor, the second electronic equivalent circuit of the second sensor element comprises a second resistor. The second electronic equivalent circuit comprises a parallel circuit of the second resistor and the second capacitor. This allows other sensing methods to be used. It can also be provided that a third sensor element, if present, also has a third resistor in a third electronic equivalent circuit.

[0021] In one embodiment of the sensor, the first sensor element has a bridging resistor. The bridging resistor is arranged in parallel with the parallel circuit of the first resistor and first capacitor. This can increase readout accuracy in certain operating states or even make readout possible in the first place.

[0022] In one embodiment of the sensor, the second sensor element has a bridging capacitance. The bridging capacitance is arranged in parallel with the second capacitance. This can increase readout accuracy in certain operating states or even make readout possible in the first place.

[0023] In one embodiment of the sensor, it has a further first sensor element and a further second sensor element. The further first sensor element can be structurally identical to the first sensor element. The further second sensor element can be structurally identical to the second sensor element. The further first sensor element and the further second sensor element are also connected in series and connected via further control connections. The further control connections can be connected to further connections of the evaluation unit.

[0024] In one embodiment of the evaluation unit, it is configured to excite the sensor with a plurality of predeterminable frequencies via the connections, to determine an extreme value frequency, and to determine the two measured variables from the complex impedance at the extreme value frequency and from the extreme value frequency. The complex impedance can contain a magnitude and a phase, with the two measured variables being calculated from the magnitude and phase of the complex impedance. As an alternative to magnitude and phase, an evaluation can also be carried out using a real part and an imaginary part of the complex impedance. The extreme value frequency can be assigned to a maximum of the phase of the complex impedance, with the maximum being able to have a negative sign and a magnitude of the maximum then corresponding to a minimum. This enables simple readout of a sensor with a series circuit comprising a parallel circuit comprising a first resistor and a first capacitor with a second capacitor.

[0025] In one embodiment of the evaluation unit, it is configured to excite the sensor at multiple frequencies via the connections and to determine more than two measured variables based on the magnitudes and phases of the complex impedance at the frequencies. This can be particularly advantageous when more than two sensor elements are provided and also when the second sensor element comprises the second resistor.

[0026] Embodiments of the invention are explained with reference to the following drawings. The schematic drawing shows:

[0027] Fig. 1 shows a sensor system with a sensor and an evaluation unit;

[0028] Fig. 2 shows an equivalent circuit diagram of a sensor;

[0029] Fig. 3 an impedance spectrum;

[0030] Fig. 4 shows another sensor system;

[0031] Fig. 5 is a flowchart of another method for operating another sensor; and

[0032] Fig. 6 is an equivalent circuit diagram of another sensor. Fig. 1 shows a sensor system 1 with a sensor 10 and an evaluation unit 80. The sensor 10 is suitable for determining two measured variables of a gas and has at least a first sensor element 11 and a second sensor element 12 as well as two control connections 17. The first sensor element 11 and the second sensor element 12 are connected in series. The sensor elements 11, 12 are arranged between the control connections 17. The evaluation unit 80 has a computing unit 81 and at least two connections 82. The control connections 17 can be connected to the connections 82 via connections 2, wherein the connections 2 can be arranged outside the sensor system 1 (solid lines) or alternatively within the sensor system 1 (dashed lines).

[0033] In particular, no further control connection is provided between the sensor elements 11, 12, i.e. between the first sensor element 11 and the second sensor element 12. An electronic signal introduced via one of the control connections 17 thus passes through the first sensor element 11 and the second sensor element 12 and then the other control connection 17. The two sensor elements 11, 12 are therefore accessible in a common measurement via the common control connection 17. As a result, individual wires between the sensor elements 11, 12 and the evaluation unit 80 can be omitted or shortened. The first sensor element 11 can have a first material 14. The second sensor element 12 can have a second material 15. For certain sensor types (e.g.

[0034] Pressure sensor 15 can also have a more extensive measuring cell, e.g., a membrane with a mechanical coupling to a cavity. The first material 14 can be a first sensing material. The second material 15 can be a second sensing material.

[0035] The first sensor element 11 and the second sensor element 12 can each be designed as individual components and connected to one another. The first sensor element 11 can, in particular, comprise a gas sensor with which a type of gas can be determined. In this case, it is generally necessary to determine both a resistance and a capacitance of the first material 14 used for the first sensor element 11. The second sensor element 12 can then be a humidity sensor or a pressure sensor in which a capacitance of the second material 15 is read out. It can also be provided that, in addition to the second sensor element 12, a third sensor element (not shown here) is connected in series, and that a humidity sensor is provided as the second sensor element 12 and a pressure sensor as the third sensor element, or vice versa.However, the second sensor element 12 can also comprise another gas sensor with which a type of another gas can be determined.

[0036] The evaluation unit 80 is configured to excite the sensor 10 with a predeterminable frequency via the connections 82, to determine a complex impedance, and to calculate the two measured variables from the complex impedance. The complex impedance can then contain a magnitude and a phase, wherein the two measured variables are calculated from the magnitude and phase of the complex impedance. As an alternative to magnitude and phase, an evaluation can also be carried out using a real part and an imaginary part of the complex impedance. The evaluation unit 80 and sensor 10 can be arranged in a common housing 3. Alternatively, however, it can also be provided that the evaluation unit 80 and sensor 10 are arranged in different housings or components. These method steps can also be carried out for several predeterminable frequencies, for example as part of a frequency scan.

[0037] The evaluation unit 80 can, in particular, comprise an integrated circuit (IC), preferably an application-specific integrated circuit (ASIC). Alternatively, the evaluation unit 80 can also comprise a microcontroller. This can be implemented within the computing unit 81.

[0038] Fig. 2 shows an electronic equivalent circuit 20 of the sensor 10 of Fig. 1. The first sensor element 11 has a first electronic equivalent circuit 21. The second sensor element 12 has a second electronic equivalent circuit 22. The first electronic equivalent circuit 21 comprises a parallel circuit of a first resistor 23 and a first capacitor 24. The second electronic equivalent circuit 22 comprises a second capacitor 25. The reading of the first sensor element 11 to determine the resistance and capacitance of the first material 14 can be carried out via the parallel circuit of the first resistor 23 and the first capacitance 24 already described for the first electronic equivalent circuit 21. The reading of the second sensor element 12 can be carried out via the second capacitance 25 described for the second electronic equivalent circuit 22 in order to determine the humidity or the pressure.

[0039] To read the first sensor element 11 and the second sensor element 12, the asymmetry of the first sensor element 11 with a parallel first resistor 23 and first capacitance 24 and of the second sensor element 12 with a second capacitance 25 can be used, so that the properties of both sensor elements 11, 12 can be reconstructed. With the evaluation unit 80, in particular, the sensor 10 can be read with the asymmetry explained in connection with Fig. 2 in the circuit comprising the first sensor element 11 and the second sensor element 12, and the properties of both sensor elements 11, 12 can be reconstructed.

[0040] Fig. 3 shows an impedance spectrum 100 in which a phase 102 is plotted against a frequency 101, which phase characteristic 103 is plotted, and an impedance magnitude 104 is plotted against an impedance magnitude 105. The impedance spectrum 100 can be calculated theoretically for the sensor 10 or determined experimentally. There can be an extreme value frequency 106 in the impedance spectrum 100 at which the phase 102 has a maximum value, which can be referred to as a phase maximum 107. In particular, the phase 102 can be plotted entirely in the negative, so that, including the negative sign, a phase maximum 107 results, which is a minimum in terms of magnitude. The impedance magnitude 104 can also be determined for the extreme value frequency 106, which can be referred to as the extreme value frequency magnitude 108.Using the extreme value frequency 106, the phase maximum 107, and the extreme value frequency magnitude 108, a reconstruction of the first resistance 23, the first capacitance 24, and the second capacitance 25 can then be performed using a stored characteristic map. It can be provided that more measured values ​​are temporarily stored, but these are primarily necessary for locating the extreme value frequency 106. However, once the extreme value frequency 106 has been identified, all other measured values ​​at other frequencies can be discarded. This enables an efficient method for reading the sensor 10.

[0041] It can be specified that measured values ​​at other frequencies are not discarded but are also used to determine the measured variables. This can achieve a more accurate measurement.

[0042] In one embodiment of the sensor 10, the first sensor element 11 and the second sensor element 12 are arranged on a common sensor chip 18, as shown in Fig. 1. Alternatively, however, it can also be provided that the first sensor element 11 and the second sensor element 12 are each arranged on a separate sensor chip.

[0043] Under certain circumstances, a gas atmosphere present during a measurement may place the first resistor 23, the first capacitance 24, and / or the second capacitance 25 into unfavorable measuring ranges. These can be remedied by modifications, so that there is no disadvantage compared to a measurement with two separate sensor elements.

[0044] In one embodiment of the sensor 10, the first sensor element 11 has a bridging resistor 26, as shown in Fig. 2. The bridging resistor 26 is arranged in parallel with the parallel circuit comprising the first resistor 23 and the first capacitor 24, with the connecting lines being shown in dashed lines to clarify that the bridging resistor 26 does not have to be used in all embodiments. The bridging resistor 26 can be particularly advantageous when the first resistor 23 is large compared to the impedance of the first capacitor 24 at the extreme value frequency 106. In this case, a current provided by the control terminals 17 in the first sensor element 11 would flow essentially, or possibly exclusively, through the first capacitor 24.Effectively, the two sensor elements 11, 12 would be connected in series to form two capacitors 24, 25, which, as an electronic equivalent circuit, would again correspond to one capacitor. A distinction would then no longer be possible. The bridging resistor 26 at least makes it possible to distinguish between the first capacitor 24 and the second capacitor 25.

[0045] Even in the event that the first sensor element 11 is completely blocking, i.e. that an impedance of the first resistor 23 and the first capacitance 24 becomes similar to or greater than a measuring range of the evaluation unit 80, it can be achieved by means of the bridging resistor 26 that at least the second capacitance 25 remains determinable.

[0046] If the first resistance 23 becomes very small due to the gases to be measured, in particular smaller than the impedance of the first capacitance 24 at the extreme frequency 106, the current essentially flows past the first capacitance 24, and it can no longer be measured. This disadvantage would also exist with a single sensor element. The measurement of the first resistance 23 and the second capacitance 25 remains possible.

[0047] In one embodiment of the sensor 10, the second sensor element 12 has a bridging capacitance 27, as shown in Fig. 2. The bridging capacitance 27 is arranged parallel to the second capacitance 25, wherein the connecting lines are shown in dashed lines to clarify that the bridging capacitance 27 does not have to be used in all embodiments. If the second capacitance 25 becomes so small due to the gases to be measured that an impedance of the second capacitance 25 at the extreme value frequency 106 becomes comparable to or greater than the measuring range of the absolute value of the impedance of the evaluation unit 80, a blocking component is present. The first resistor 23 and the first capacitance 24 can then no longer be measured. The bridging capacitance 27 makes the first resistance 23 and the first capacitance 24 determinable again.

[0048] The bridging resistor 26 and / or the bridging capacitance 27 can be accommodated outside the sensor elements 11, 12. The bridging resistor 26 and / or the bridging capacitance 27 can, however, also be integrated on the chips of the sensor elements 11, 12 or the common chip 18. Fig. 4 shows a further embodiment of a sensor system 1, in which a sensor 10 is connected to an evaluation unit 80, as explained in connection with Fig. 1, and for which the electronic equivalent circuit 20 is identical to Fig. 2. The values ​​of the capacitances and resistances are not necessarily the same; identical should refer to the circuit arrangement. Furthermore, the sensor system 1 has a further sensor 30, which includes a further first sensor element 31 and a further second sensor element 32.The further first sensor element 31 and the further second sensor element 32 are connected in series and connected to further terminals 83 of the evaluation unit via further evaluation terminals 37, analogous to the sensor 10. A further electronic equivalent circuit 40 of the further sensor 30 is identical to the electronic equivalent circuit 20 of the sensor 10, so that the further first sensor element 31 contains a further first electronic equivalent circuit 41 with a parallel circuit of a further first resistor 43 and a further first capacitor 44, and the further second sensor element 32 contains a further second electronic equivalent circuit 42 with a further second capacitor 45. The further sensor 30 can then be read out analogously to the sensor 10.

[0049] The elements explained in connection with Fig. 2, such as the bridging resistor 26 or the bridging capacitor 27, can also be provided in the sensor 10. Analog components can also be provided for the additional sensor 30. Furthermore, the sensor system can also comprise more than one additional sensor 30.

[0050] Fig. 5 shows a flowchart 120 of a method for operating a sensor 10, which can be carried out, for example, by the computing unit 81 of the evaluation unit 80. In a first method step 121, the sensor 10 is excited with a predeterminable frequency. In a second method step 122, a complex impedance is determined. In a third method step 123, two measured variables are calculated from the complex impedance. In the third method step 123, the two measured variables can be calculated, in particular, using a model stored for the sensor 10. The measured variables can then be output if necessary. The complex impedance can contain a magnitude and a phase, with the two measured variables being calculated from the magnitude and phase of the complex impedance. As an alternative to magnitude and phase, an evaluation can also be carried out using a real part and an imaginary part of the complex impedance.

[0051] The excitation of the sensor 10 in the first method step 121 can take place with the extreme value frequency 106 explained in connection with Fig. 3. For example, it can be assumed that the extreme value frequency 106 determined in a previous measurement changes only slightly over time if it is assumed that gas concentrations at the sensor 10 also change only slowly. In particular, an identical time scale can be relevant for both changes. Therefore, the extreme value frequency 106 can continue to be used in a subsequent measurement. It can therefore be assumed that subsequent impedance spectra will have a similar shape with nearby extreme values ​​or phase maxima. The assumption of a slowly changing extreme value frequency can also be used by using the previous extreme value frequency as the starting point for the search for the new extreme value frequency, for example in the context of a frequency scan.

[0052] For the method of Fig. 5 and also for the evaluation unit 80, it may be provided to first determine the extreme value frequency 106. This can be done, for example, by exciting the sensor 10 with several predeterminable frequencies via the terminals 82 and determining the extreme value frequency 106 therefrom. The two measured variables are then determined from the magnitude 104 and the phase 102 of the complex impedance at the extreme value frequency 106 and the extreme value frequency 106, as explained in connection with Fig. 3.

[0053] Various methods can be used to locate the extreme frequency 106. For example, a fixed frequency pattern, such as frequency steps, can be used, and the extreme value of this rasterization can be considered close enough to the true extreme frequency 106, while accepting a limited rasterization. Alternatively or additionally, an iterative method can be selected, which can be based, for example, on an interval halving method or on common methods from numerical optimization. In this combination, for example, a coarse rasterization can be used first, followed by a finer rasterization within a narrower range.

[0054] Fig. 6 shows a further embodiment of a sensor 10, in which the second electronic equivalent circuit 22 of the second sensor element 12 comprises a second resistor 28. The second electronic equivalent circuit 22 comprises a parallel circuit comprising the second resistor 28 and the second capacitor 25. Furthermore, the sensor 10 optionally has a third sensor element 13 with a third material 16, which is connected in series with the second sensor element 12 and the first sensor element 11. Thus, if necessary, three sensor elements 11, 12, 13 (or even more sensor elements, not shown here, however) can be controlled via the control connections 17. The third sensor element 11 has a third resistor 54 and a third capacitor 55. A third electronic equivalent circuit 53 of the third sensor element 13 has a parallel circuit comprising the third resistor 54 and the third capacitor 55.Such a sensor 10 is more general than the sensor explained in connection with Figs. 1 and 2 and can, for example, include a gas sensor, a humidity sensor and a pressure sensor. The method explained in connection with Fig. 5 can also be used here in principle and in particular comprise that in a first method step 121 the sensor 10 is excited with a predeterminable frequency, in a second method step 122 a complex impedance with a magnitude and a phase is determined and in a third method step 123 two measured variables are calculated from the magnitude and phase of the complex impedance. The measured variables can then also be output if necessary. In this case, it can be provided that in the first method step 121 the sensor 10 is excited with several frequencies via the connections 82 and that more than two measured variables are determined based on the magnitudes and phases of the complex impedance at the frequencies.This can also be done using a model stored for the sensor 10. In particular, for the sensor 10 shown in Fig. 6, the measured variables can be determined independently of the extreme value frequency. Furthermore, for the sensor 10 of Fig. 6, similar to Fig. 2, bridging resistors 26 and / or bridging capacitors 27 can be provided for the individual sensor elements 11, 12, 13. The third material 16 can be a third sensing material.

[0055] In principle, two or three or even more fully functional RC elements can be provided for the respective sensor elements 11, 12, 13. Reconstruction of completely arbitrary resistances 23, 28, 54 or capacitances 24, 25, 55 would then no longer be possible. However, the specific materials 14, 15, 16 used and the geometries of a receptor layer, and possibly also their operating temperatures, can limit the variety of measured values. For the first sensor element 11 and the second sensor element 12, for example, the first resistance 23 can always be significantly smaller than the second resistance 28, with the second resistance 28 being virtually constant.The second capacitance 25 can be variable and a ratio of the first resistance 23 to the second capacitance 25 within a defined, predetermined range, so that an asymmetry is again established between the first electronic equivalent circuit 21 and the second electronic equivalent circuit 22. The third electronic equivalent circuit 53 can also exhibit a further asymmetry.

[0056] In general, an impedance spectrum can be recorded with a given number j of measured values, and k elements of individual resistances (23, 28, 54) and capacitances (24, 25, 55) can be reconstructed from the more than two sensor elements 11, 12, 13 in order to draw conclusions about the concentrations of a number n of gases and vapors. In this case, it can be provided that the step of determining the tuples of capacitances and associated resistances is skipped, and the n concentrations are determined directly from a given number j of measured values ​​using a characteristic map and a model.

[0057] Another method for increasing the asymmetry of the sensor elements 11, 12 can be for the first sensor element 11 and the second sensor element 12 to be operated at different operating temperatures. For this purpose, a heating element (not shown in the figures) can be used to heat the first sensor element 11 or the second sensor element 12. Although the invention has been described in detail by means of the preferred embodiments, the invention is not limited to the disclosed examples, and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.

Claims

Claims 1. Sensor (10) for determining two measured variables of a gas, comprising at least a first sensor element (11) and a second sensor element (12) as well as two control connections (17), wherein the first sensor element (11) and the second sensor element (12) are connected in series, wherein the sensor elements (11, 12) are arranged between the control connections (17), wherein the first sensor element (11) has a first electronic equivalent circuit (21), wherein the first electronic equivalent circuit (21) comprises a parallel circuit of a first resistor (23) and a first capacitor (24), wherein the second sensor element (12) has a second electronic equivalent circuit (22), wherein the second electronic equivalent circuit (22) comprises a second capacitor (25).

2. Sensor (10) according to claim 1, wherein the first sensor element (11) and the second sensor element (12) are arranged on a common sensor chip (18).

3. Sensor (10) according to claim 1 or 2, wherein the second electronic equivalent circuit (22) of the second sensor element (12) comprises a second resistor (28) and wherein the second electronic equivalent circuit (22) comprises a parallel circuit of the second resistor (28) and the second capacitor (25).

4. Sensor (10) according to one of claims 1 to 3, wherein the first sensor element (11) has a bridging resistor (26), wherein the bridging resistor (26) is arranged in parallel with the parallel circuit of the first resistor (23) and the first capacitor (24).

5. Sensor (10) according to one of claims 1 to 4, wherein the second sensor element (12) has a bridging capacitance (27), wherein the bridging capacitance (27) is arranged in parallel with the second capacitance (25).

6. Evaluation unit (80) with a computing unit (81) and at least two connections (82), wherein the connections (82) can be connected to the control connections (17) of a sensor (10) according to one of claims 1 to 5, wherein the evaluation unit (80) is set up to excite the sensor (10) with a predeterminable frequency via the connections (82), to determine a complex impedance and to calculate the two measured variables from the complex impedance.

7. Evaluation unit (80) according to claim 6, wherein the evaluation unit (80) is configured to excite the sensor (10) with a plurality of predeterminable frequencies via the terminals (82), to determine an extreme value frequency (106) and to determine the two measured variables from a magnitude (104) and a phase (102) of the complex impedance at the extreme value frequency (106) and the extreme value frequency.

8. Evaluation unit (80) according to claim 6 or 7, wherein the evaluation unit (80) is configured to excite the sensor (10) with a plurality of frequencies via the terminals (82) and to determine more than two measured variables based on the magnitudes and phases of the complex impedance at the frequencies.

9. Sensor system (1) with a sensor (10) according to one of claims 1 to 5 and an evaluation unit (80) according to one of claims 6 to 8.

10. A method for operating a sensor (10) according to one of claims 1 to 5, comprising the steps: - Exciting the sensor (10) with a predeterminable frequency; determining a complex impedance; and calculating two measured variables from the complex impedance.

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