A sensor for acquiring at least one characteristic of a fluid medium in a measurement space

Abstract

A sensor (10) is proposed for capturing at least one characteristic of a fluid medium (12) in at least one measurement space (14). The sensor (10) includes a sensor element (18) with at least one heatable membrane (20) and an electrical measurement bridge (48). The membrane (20) is coupled to the electrical measurement bridge (48). The sensor (10) is further configured to capture an electrical resistance of the measurement bridge (48). The sensor (10) is further configured to capture a pressure of the fluid medium (12) based on the captured electrical resistance of the electrical measurement bridge (48).

Description

[Technical field]

[0001] The present invention relates to a sensor for a fluid medium. [Background technology]

[0002] From the prior art, many sensors, sensor elements and methods are known for capturing at least one characteristic of a fluid medium in a measurement space. This characteristic can essentially be any characteristic of a gaseous or liquid fluid medium, and one or more characteristics can be captured. In the following, the invention is described, without limiting further embodiments and applications, in particular with reference to a sensor for capturing a gas, in particular the H2 fraction in a measurement gas.

[0003] Sensors of the type described herein are applied in many fields, for example in automotive technology, process technology, chemistry and machine building, inter alia for determining gas concentrations. For example, the determination of hydrogen concentration, for example in air-hydrogen mixtures, plays a major role in the application of hydrogen fuel cell systems. Safety-related applications should also be mentioned here. Air-hydrogen mixtures become flammable at a hydrogen fraction of approximately 4%. Sensors for capturing hydrogen can be used, for example, in hydrogen fuel cell vehicles, for example to detect hydrogen escaping due to damage or defects and to activate warning signals and / or protective measures by coupling to corresponding systems. Therefore, several hydrogen sensors are required per fuel cell vehicle, which are either installed in the exhaust system (exhaust) or work under atmospheric conditions (ambient).

[0004] Many measurement principles can be used for such hydrogen sensors, including, among others, the following measurement principles: thermal conduction, catalytic pellistor, electrochemical cell, semiconducting metal oxide, chemiresistor and field effect transistor.

[0005] From DE102005058830A1 and DE102005058832A1 respectively a sensor is known for capturing one component of a gaseous fluid containing several components. The sensor comprises a housing defining a measurement space, in which a measuring tip with a heatable membrane is accommodated. The principle of the heated hydrogen sensor is based on the different thermal conductivities of the gas components involved. The asymmetry in the thermal system that is thereby generated is converted into a voltage signal.

[0006] Despite the advantages of sensors known from the prior art for capturing at least one characteristic of a fluid medium, these sensors still contain room for improvement. In current vehicles, the pressure in the exhaust system is not detected. This means that it is not possible to recognize if the exhaust system is blocked (e.g. by something that was intentionally or unintentionally introduced into it). A pressure build-up in the exhaust system could cause harm to the pre-existing components. In the case of the aforementioned sensors, the membrane must be very thin and therefore breaks at an overpressure of about 1 to 2 bar. Summary of the Invention

[0007] Therefore, within the scope of the present invention, a sensor element for capturing at least one characteristic of a fluid medium in a measurement space is proposed which avoids, at least to a large extent, the disadvantages of known sensor elements for capturing at least one characteristic of a fluid medium in a measurement space and, in particular, is designed to capture in addition to a characteristic of the fluid medium, such as the H2 concentration, also the pressure of the fluid medium.

[0008] The sensor according to the invention for capturing at least one property of a fluid medium in at least one measurement space comprises a sensor element with at least one heatable membrane and an electrical measurement bridge. The membrane is coupled to the electrical measurement bridge. The sensor is further configured to capture an electrical resistance of the measurement bridge. The sensor is further configured to capture a pressure of the fluid medium based on the captured electrical resistance of the electrical measurement bridge.

[0009] Within the scope of the present invention, a sensor element is essentially any device capable of capturing at least one property of a fluid medium and generating at least one measurement signal, for example an electrical measurement signal, for example a voltage or current, which corresponds to the captured property. The property can be, for example, a physical and / or chemical property. A combination of properties can also be captured. The sensor element can be designed, inter alia, to capture at least one property of a gas, in particular the H2 proportion in the measured gas. Other properties and / or combinations of properties can also be captured.

[0010] The sensor element may in particular be adapted for use in hydrogen fuel cell vehicles. The measurement space may in principle be any open or closed space in which a fluid medium, in particular a measurement gas, is contained and / or through which a fluid medium, in particular a measurement gas, is passed.

[0011] Capturing the electrical resistance can be done, for example, by so-called self-sensing. A membrane, within the framework of the present invention, can be understood to mean a thin structure having a planar extent that is large compared to its thickness, such as a skin or film.

[0012] The capture can in particular refer to a measurement in which a property of a fluid medium is quantitatively determined. For example, the proportion of at least one target gas component, for example H2, of a gas, for example air, in the measurement space is quantitatively determined. The proportion of the at least one target gas component can for example be a partial pressure and / or a percentage of the target gas component. The proportion of the target gas component can also for example be a plurality of proportions of one target gas component. The target gas component can in principle be any gas whose proportion in the gas can be captured. Preferably, the target gas component can be hydrogen. The gas can in principle be any gas, in particular a gas mixture. Preferably, the gas can be air, in particular in the automotive field. The measurement space can for example be a gas cell of a sensor. In principle, the measurement space can be a space in which a fluid medium or a gas is present and thus in contact with a membrane.

[0013] The sensor may have a housing having at least one opening through which the sensor element can contact the fluid medium. Within the scope of the present invention, a housing is essentially any part or group of parts that can completely or partially surround the sensor element and / or seal it against the outside and provide mechanical stability to the sensor element. In particular, the housing can surround at least one internal space. The housing can, for example, at least partially surround the internal space and at least partially delimit the internal space with respect to its surroundings. The housing can in particular be made completely or partially from at least one of the following materials: semiconductor material, plastic, metal.

[0014] The housing may be a cap wafer, which within the scope of the present invention may be any wafer that at least partially defines an interior space, and which may comprise at least one material selected from the group consisting of silicon, silicon oxide, silicon nitride, and silicon carbide.

[0015] The sensor may be configured to vary an electrical heating voltage applied to the membrane, the sensor being configured to capture a property of the fluid medium when a first heating voltage is applied to the membrane, and the sensor being configured to capture a pressure of the fluid medium when a second heating voltage is applied to the membrane, the second heating voltage being smaller or larger than the first heating voltage, and the heating voltage being varied accordingly to determine the two different measurement quantities.

[0016] The first heating voltage may be selected such that the temperature of the membrane increases, and the second heating voltage is selected such that the temperature of the membrane remains substantially constant. For thermal conductivity measurements, the measuring bridge is supplied with a heating voltage high enough that the membrane heats up. For pressure measurements, a very small voltage is supplied to the membrane, so that the membrane does not heat up, but at the same time the resistance increase can be measured.

[0017] The electrical measuring bridge can have electrical resistors, which are arranged on the membrane in such a way that the electrical resistance of the resistor is proportional to the pressure of the fluid medium. The acquisition of the pressure of the fluid medium based on the acquired electrical resistance of the electrical measuring bridge is based on the knowledge that the membrane is deformed, in particular curved, when there is a pressure difference between its upper and lower sides. The resistor on the membrane then becomes longer and therefore the resistance becomes larger. Correspondingly, the pressure acting on the membrane is proportional to the deformation of the resistor and therefore to the resistance.

[0018] The sensor may be configured to generate a correction value for the trapped property of the fluid medium based on the trapped pressure of the fluid medium, which may increase the measurement accuracy of the sensor with respect to the concentration of the gas to be measured.

[0019] The sensor may further include a temperature sensor element configured to capture a temperature of the sensor element. The sensor may be configured to capture a property of the fluid medium based on the temperature of the membrane, thereby ensuring that the thermal conductivity of the fluid medium is captured.

[0020] The sensor may be configured to output an abnormality signal if the captured pressure exceeds or reaches a predefined threshold value, thereby, for example, signaling to a control system of a fuel cell vehicle that the pressure in the exhaust system is too high, for example due to clogging. At the same time, the sensor may report, in the sense of a self-diagnosis, that the measuring membrane was subjected to an impermissibly high pressure before it cracked and failed.

[0021] The sensor may further include an electrical heating element for heating the membrane, thereby ensuring that the temperature of the membrane can be controlled or regulated. The sensor may be configured to capture the H2 fraction in the measurement gas.

[0022] Information about the H2 concentration to be measured in the supplied measuring gas is provided by a sensor element in the form of a thermal conductivity sensor element. The thermal conductivity of a gas is, in a first approximation, inversely proportional to the square root of the mass of the gas molecules, so that a gas containing light atoms, for example H2 or He atoms, has a significantly higher thermal conductivity than air, which essentially consists of N2 and O2 molecules. The higher the measured thermal conductivity, the greater the proportion of light molecules. The measurement of the thermal conductivity is based on the fact that a membrane is cooled by the gas to be measured. The higher the thermal conductivity of the gas to be measured, the stronger the membrane is cooled. By measuring the temperature of the membrane, the thermal conductivity of the gas and indirectly the H2 content of the gas can thus be inferred.

[0023] Further optional details and features of the invention become apparent from the following description of preferred exemplary embodiments which are illustrated diagrammatically in the drawings. [Brief description of the drawings]

[0024] [Figure 1] 1 is a schematic diagram of a sensor for capturing at least one characteristic of a fluid medium in at least one measurement space; [Diagram 2] FIG. 2 is a schematic diagram of the electrical circuit diagram of a sensor element of the sensor; [Diagram 3] FIG. [Figure 4] FIG. 4 is a further cross-sectional view of the sensor element; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] FIG. 1 shows a schematic diagram of a sensor 10 for capturing at least one property of a fluid medium 12 in at least one measurement space 14, in particular for capturing the H2 proportion in a measurement gas 16. The sensor 10 may be adapted in particular for use in hydrogen fuel cell vehicles. However, other applications are also possible. The sensor 10 may in particular include one or more further functional elements not shown in the figure, such as electrodes, electrode leads and contacts, multiple layers or other elements. Correspondingly, the sensor 10 may be mounted in the exhaust system of a hydrogen fuel cell vehicle (exhaust) or may work under atmospheric conditions (ambient). The measurement space may therefore be the exhaust system, the anode system or the interior space of a hydrogen fuel cell vehicle.

[0026] The sensor 10 comprises a sensor element 18. The sensor element 18 comprises at least one heatable membrane 20. A first electric heating element 22 is disposed on the membrane 20. An upper surface 24 of the membrane 20 is exposed to a first reference space 26, which is sealed with respect to the measurement space 14. The first reference space 26 is filled with a fluid medium of known composition. A lower surface 30 of the membrane 20 may be exposed to the fluid medium 12.

[0027] The sensor element 18 further comprises a reference membrane 32, on which a further electric heating element 28 is arranged. The upper side 36 of the reference membrane 32 is exposed to a second reference space 34, which is sealed with respect to the measurement space 14. The second reference space 34 is filled with a fluid medium of known composition. The lower side 42 of the reference membrane 32 is exposed to a third reference space 38 (which may be the same medium as in the reference space 34) and is isolated from the fluid medium 12, for example by a medium separator 44. It is expressly emphasized that the first heating element 22 and the second heating element 28 may be formed integrally. It is expressly emphasized that the reference membrane 32 is optional. The entire structure may be implemented, for example, in the form of a silicon chip wafer. The sensor 10 may further comprise a housing 40, in which the sensor element 18 is arranged.

[0028] 2 shows a schematic diagram of the electrical circuit diagram of the sensor element 18 of the sensor 10. The sensor element 18 is shown with a membrane 20 and a reference membrane 32. A first heating element 22 is supplied with a heating voltage U H can be applied (also to the heating element 28 for heating the reference membrane 32). H is provided by a voltage source 46. The membrane 20 and the reference membrane 32 are connected to an electrical measurement bridge 48, which is, for example, a Wheatstone bridge. For this purpose, the measurement bridge 48 is formed, for example, by four resistors 50, of which two resistors 50 are arranged on the membrane 20 and two resistors 50 on the reference membrane 32. At two points between the resistors 50 of the membrane 20 and the reference membrane 32, an electrical bridge voltage U B can be extracted.

[0029] 3 shows a cross-sectional view of the sensor element 18. The sensor 10 further comprises a temperature sensor element 52. The temperature sensor element 52 is configured to capture the temperature of the sensor element 18 or its silicon chip. For this purpose, the temperature sensor element 52 is disposed at the edge of or adjacent to the membrane 20. The sensor 10 is configured to capture a property of the fluid medium 12 based on the temperature of the membrane 20.

[0030] Fig. 4 shows a further cross-sectional view of the sensor element 18. Fig. 4 shows, inter alia, the sensor element 18 with a deformed membrane 20. The sensor 10 is further formed to capture the electrical resistance of the measuring bridge 48, i.e. the sensor 10 is further formed to capture the pressure of the fluid medium 12 on the basis of the captured electrical resistance of the electrical measuring bridge 48. As explained above, the electrical measuring bridge 48 has an electrical resistance 50. The resistance 50 is arranged on the membrane 20 such that the electrical resistance value of the resistance 50 is proportional to the pressure of the fluid medium 12, as will be explained in more detail below.

[0031] The sensor 10 is connected to the membrane 20 via an electrical heating voltage U H In particular, the sensor 10 applies a first heating voltage U to the membrane 20.H The sensor 10 is configured to capture the properties of the fluid medium 12 when a second heating voltage U is applied to the membrane 20. H is applied to capture the pressure of the fluid medium 12, and a second heating voltage U H is the first heating voltage U H Less than or greater than the first heating voltage U H is selected such that the temperature of the membrane 20 increases. H is selected such that the temperature of membrane 20 remains substantially constant or does not increase. Sensor 10 is further configured to generate a correction value for the trapped property of fluid medium 12 based on the trapped pressure of fluid medium 12. Sensor 10 is further configured to output an abnormal signal if the trapped pressure exceeds or reaches a predetermined threshold.

[0032] The operation or function of the sensor 10 according to the invention is described below. A first heating element 22 is supplied with a first electrical heating voltage U H is applied. The measuring bridge 48 is then supplied with a voltage high enough that the membrane 20 heats up. In this way, the reference membrane 32 can also be heated by the second heating element 28. In this case, different temperatures arise in the area of ​​the membrane 20 and the reference membrane 32 due to the different thermal conductivities of the gases in contact with the membrane 20 and the reference membrane 32. The temperature of the sensor element 18 or its silicon chip is captured by the temperature sensor element 52. This temperature corresponds to the ambient temperature in a first approximation, unless the sensor element 18 is additionally heated. A bridge signal, which represents the temperature difference between the measuring membrane 20 and the reference membrane 32, is taken off as shown in FIG. 2. The measurement of the thermal conductivity is based on the membrane 20 being cooled by the fluid medium 12 to be measured. The higher the thermal conductivity of the fluid medium 12 to be measured, the more the membrane 20 is cooled. By measuring the temperature of the membrane 20, the thermal conductivity of the fluid medium and indirectly the H2 content of the fluid medium can thus be inferred.

[0033] To capture the pressure of the fluid medium 12, the sensor 10 detects a heating voltage U HThe first heating element 22 is supplied with a second electrical heating voltage U H is applied. A very small voltage is then applied to the measuring bridge 48, so that the membrane 20 does not heat up. If a higher pressure is applied to the lower surface 30 of the membrane 20 due to the fluid medium 12 than to the upper surface 24 of the membrane 20, the membrane 20 will bend as shown in FIG. 4. The resistor 50 on the membrane 20 will become longer and therefore have a higher resistance. The second heating voltage U H When a pressure difference between the lower surface 30 and the upper surface 24 of the membrane 20 is applied, a resistance increase can be measured. The greater the pressure difference between the lower surface 30 and the upper surface 24 of the membrane 20, the greater the resistance of the resistor 50. For H2 measurements, the pressure ratio can be taken into account as a correction factor to improve the sensor accuracy.

[0034] The invention can be demonstrated by analyzing the signal of a sensor based on the thermal conductivity principle: if such a sensor transmits a signal upon application of pressure, then there must be a corresponding pressure recognition (unless another pressure sensor is installed).

Claims

1. A sensor (10) comprising a sensor element (18) with at least one heatable membrane (20) and an electrical measurement bridge (48) for capturing at least one characteristic of a fluid medium (12) in at least one measurement space (14), said membrane (20) being coupled to said electrical measurement bridge (48), said sensor (10) being further formed for capturing an electrical resistance of said electrical measurement bridge (48), said sensor (10) being further formed for capturing a pressure of said fluid medium (12) based on said captured electrical resistance of said electrical measurement bridge (48), a pressure sensor configured to vary an electrical heating voltage (UH) applied to said membrane (20), configured to capture said properties of said fluid medium (12) when a first heating voltage (UH) is applied to said membrane (20), configured to capture said pressure of said fluid medium (12) when a second heating voltage (UH) is applied to said membrane (20), said second heating voltage (UH) being smaller or larger than said first heating voltage (UH); The sensor (10), wherein the first heating voltage (UH) is selected such that the temperature of the membrane (20) increases and the second heating voltage (UH) is selected such that the temperature of the membrane (20) remains substantially constant.

2. 2. The sensor (10) of claim 1, wherein the electrical measurement bridge (48) has an electrical resistance (50) disposed on the membrane (20) such that the electrical resistance value of the electrical resistance (50) is proportional to the pressure of the fluid medium (12).

3. The sensor (10) of claim 1 or 2, configured to generate a correction value for the trapped property of the fluid medium (12) based on the trapped pressure of the fluid medium (12).

4. The sensor (10) of any one of claims 1 to 3, further comprising a temperature sensor element (52), said temperature sensor element (52) configured to capture a temperature of said sensor element (18).

5. The sensor (10) of claim 4 configured to capture the property of the fluid medium (12) based on a temperature of the membrane (20).

6. The sensor (10) of any one of claims 1 to 5, wherein the sensor (10) is configured to output an abnormal signal when the captured pressure exceeds or reaches a predetermined threshold.

7. The sensor (10) of any one of claims 1 to 6, further comprising an electric heating element (22) for heating the membrane (20).

8. H in the measurement gas ２ The sensor (10) of any one of claims 1 to 7, configured to capture a ratio.

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