Thermal flow sensor for determining a gas or the composition of a gas mixture as well as its flow velocity

Abstract

A thermal flow sensor for determining a gas or the composition of a gas mixture as well as its flow velocity, comprising: a substrate onto which at least a first dielectric layer is applied; at least one heating structure that is applied onto the first dielectric layer and serves to heat the gas or the gas mixture; at least a first temperature sensor element that is applied onto the first dielectric layer at a distance from the heating structure and captures the temperature of the gas or gas mixture heated at the heating structure; a control device that controls the heating structure in a first operating mode in such a way that the heating structure shows a predetermined temperature, and controls the heating structure in a second operating mode in such a way that a power input into the heating structure corresponds to a predetermined power; and an evaluation unit which determines at least one physical characteristic of the gas present or the gas mixture on the basis of the operating modes and determines the gas present or the composition of the gas mixture as well as its flow velocity on the basis of this physical characteristic.

Claims

1. A thermal flow sensor configured for determining a gas or the composition of a gas mixture as well as its flow velocity, comprising: a substrate onto which at least one dielectric layer is applied; at least one heating structure that is applied onto said at least one dielectric layer and serves to heat the gas or the gas mixture; at least one temperature sensor element that is applied onto said at least one dielectric layer at a distance from said at least one heating structure and captures the temperature of the gas or gas mixture heated at said at least one heating structure; a control device that controls said at least one heating structure in a first operating mode in such a way that said at least one heating structure shows a predetermined temperature, and controls said at least one heating structure in a second operating mode in such a way that a power input into said at least one heating structure corresponds to a predetermined power; and an evaluation unit which determines at least one physical characteristic of the gas present or the gas mixture on the basis of said operating modes and determines the gas present or the composition of the gas mixture as well as its flow velocity on the basis of this physical characteristic.

2. The thermal flow sensor according to claim 1, wherein: said evaluation unit identifies the concentration of the composition of the gas mixture; and the individual components of the gas mixture is necessarily communicated to said evaluation unit in order to determine the concentration of the composition of the gas mixture.

3. The thermal flow sensor according to claim 1, wherein: said substrate has a recess at least in a first area, so that said at least one first dielectric layer forms a membrane at least in said area on said substrate; and said at least one heating structure in said first area is arranged on said at least one dielectric layer formed into said membrane.

4. The thermal flow sensor according to claim 1, said at least one temperature sensor further comprises: a second temperature sensor element which is applied to said at least one dielectric layer.

5. The thermal flow sensor according to claim 4, wherein: said at least one heating structure is arranged along a flow direction of the gas or the gas mixture between said at least one first and said second temperature sensor elements.

6. The thermal flow sensor according to claim 5, wherein: said second temperature sensor element is made of a material that has a temperature coefficient of resistance in the range of 1 000-11 000 ppm/Kelvin.

7. The thermal flow sensor according to claim 1, wherein: said at least one heating structure and said at least one first temperature sensor element are each made of a material that has a temperature coefficient of resistance in the range of 1 000-11 000 ppm/Kelvin.

8. The thermal flow sensor according to claim 1, wherein: said control device keeps the relation between the predetermined power and the predetermined temperature constant in a third operating mode.

9. The thermal flow sensor according to claim 8, wherein: said control device controls said at least one heating structure with an excitation signal and said evaluation unit captures the temperature of the gas flowing past said first temperature sensor element and a second temperature sensor element by means of at least one response signal and a second response signal.

10. The thermal flow sensor according to claim 9, wherein: said evaluation unit configured to determine the flow velocity compares said first and/or said second response signal with first reference values or with second reference values.

11. The thermal flow sensor according to claim 9, wherein: said excitation signal represents an AC voltage signal.

12. The thermal flow sensor according to claim 11, wherein: said evaluation unit determines a phase shift between the excitation signal and said first and/or said second response signal.

13. The thermal flow sensor according to claim 12, wherein: said evaluation unit conducts a verification of the determination of the gas or the composition of the gas mixture as well as its flow velocity on the basis of the phase shift.

14. The thermal flow sensor according to claim 12, wherein: said evaluation unit determines a further physical characteristic of the gas present or the gas mixture based on the phase shift.

15. The thermal flow sensor according to claim 1, wherein: said thermal flow sensor comprises a second dielectric layer; said first dielectric layer and/or a second dielectric layer show a layer thickness of less than 100 microns.

16. The thermal flow sensor according to claim 15, wherein: said first and said second dielectric layers are of the same material.

17. The thermal flow sensor according to claim 16, wherein: said first and the second dielectric layers are made of a polymer.

18. A flow meter with a thermal flow sensor, comprising: a thermal flow sensor configured for determining a gas or the composition of a gas mixture as well as its flow velocity, comprising: a substrate onto which at least one dielectric layer is applied; at least one heating structure that is applied onto said at least one dielectric layer and serves to heat the gas or the gas mixture; at least one temperature sensor element that is applied onto said at least one dielectric layer at a distance from said at least one heating structure and captures the temperature of the gas or gas mixture heated at said at least one heating structure; a control device that controls said at least one heating structure in a first operating mode in such a way that said at least one heating structure shows a predetermined temperature, and controls said at least one heating structure in a second operating mode in such a way that a power input into said at least one heating structure corresponds to a predetermined power; and an evaluation unit which determines at least one physical characteristic of the gas present or the gas mixture on the basis of said operating modes and determines the gas present or the composition of the gas mixture as well as its flow velocity on the basis of this physical characteristic.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is further illustrated referring to the following drawings. Illustrated are:

(2) FIG. 1: a cross-section of an embodiment of the flow sensor according to the invention,

(3) FIG. 2: an electrical diagram of the control device and a voltage divider that serves to capture a response signal,

(4) FIG. 3: a first measuring curve comprising the first reference values and

(5) FIG. 4: a second measuring curve comprising the second reference values.

DETAILED DISCUSSION IN CONJUNCTION WITH THE DRAWINGS

(6) FIG. 1 shows a cross-section of an embodiment of the flow sensor 1 according to the invention. The thermal flow sensor 1 features a substrate 3 onto which a first dielectric layer 6 is applied. The substrate 3 further features a recess 5 in a first area 4 so that the first dielectric layer generates a membrane 7 in the first area 4 on the substrate 3. A heating structure 8 is applied onto the membrane 7 in such a way that it is between a first and second temperature sensor element 9, 10 along the flow direction of the gas or the gas mixture 2 and serves to heat the gas or gas mixture 2. The two temperature sensor elements 9, 10 are also applied onto the first dielectric layers 6 and preferably arranged in such a way that they are in the first area 4. Those two temperature sensor elements 9, 10 serve to capture the temperature of the gas or gas mixture 2 heated at the heating structure 8. In order to protect the heating structure 8 and the two temperature sensor elements 9, 10, a second dielectric layer 16 is applied onto the first dielectric layer 6. In order to determine a gas or the composition of a gas mixture 2, the side of the second dielectric layer 18 that is pointing away from the substrate 3 is exposed to the flowing gas or gas mixture 2.

(7) The thermal flow sensor 1 further comprises a control device 11 that controls the heating structure 8 with an excitation signal 12 and controls it in a first operating mode in such a way that the heating structure has a predetermined temperature and controls the heating structure 8 in a second operating mode such a way that a power input into the heating structure 8 is mainly adjusted to a predetermined power. Thus, the control devices uses the heating structure to control the gas temperature of the gas or the gas mixture to the predetermined temperature in the first operating mode. In the second operating mode, the control device controls the power input into the heating structure mainly to a predetermined power that is typically constant at least in its average value. The excitation signal 13 is an AC voltage signal that has, for example, an excitation frequency of 1 Hz. It has been proven as advantageous if the temperature predetermined in the first operating mode shows a constant over-temperature of the gas and/or gas mixture compared to the surrounding temperature in the range of 120? C. and the power predetermined in the second operating mode has a peak-peak value of 20 mW (milliwatts). The excitation frequency, the predetermined temperature and the peak-peak value of the AC voltage signal may vary depending on the gas present and/or the gas mixture 2. In addition to a first and a second operating mode, the control device 11 keeps the relationship between the power input and the predetermined temperature mainly constant in a third operating mode.

(8) Furthermore, the control device 11 is designed as a switchable device so it can be switched from one operating mode to another.

(9) In addition to the control device 11, the thermal flow sensor 1 also has an evaluation unit 12 which determines at least the gas present or the composition of the gas mixture 2 as well as the flow velocity of the gas and/or the gas mixture by means of the different operating modes. For this purpose, the thermal flow sensor measures the thermal conductivity of the gas or the gas mixtures and identifies the gas present and/or the composition of the gas mixture on the basis of the thermal conductivity. For this purpose, the evaluation unit 12 captures the temperature of the gas and/or gas mixtures 2 passing the first temperature sensor element 9 by means of a first response signal 14 and the temperature of the gas and/or gas mixture 2 passing the second temperature sensor element 10 by means of a second response signal 15. In order to determine the flow velocity, the evaluation unit 12 either separately compares the first response signal 14 with a first reference value 16 or the second response signal 15 with a first reference value 16, or the first and second response signal 14, 15 together with the first reference values 16. In order to determine the gas present or the composition of the gas mixture 2, the evaluation unit 12 either compares the first response signal 14 with second reference values 17 or the second response signal 15 with the second reference values 17, or the first and the second response signal 14, 15 with the second reference values 17, with the first reference values 16 being different from the second reference values 17. In order to determine the composition of the gas mixtures 2 which is preferably a binary gas mixture, such as, for example, an mixture of argon an helium, the evaluation unit 12 has to know the individual components, in this case helium and argon in order to be able to determine the concentration and composition of the (binary) gas mixture.

(10) FIG. 2a) shows and electrical diagram of the control device 11 and FIG. 2b) a voltage divider that serves to capture the response signals 14, 15. The control device 11 in its most simple form is a voltage divider, with the ohmic heat resistance of the heating structure 8 being represented by R.sub.H and a series resistor R.sub.VH for the heat structure 8 being inserted before the heat resistor. By means of the total voltage for the heating structure UH it is thus possible to generate the excitation signal 13 which is, as mentioned above, an AC voltage signal with a peak-peak value of 20 mW. The excitation signal 13 to be used may either be a sine-wave voltage, a square-wave voltage or any other form of an AC voltage signal.

(11) FIG. 2b) shows a voltage divider to capture a response signal 14, 15. For simplicity's sake, FIG. 2b) only shows the voltage divider that serves to capture the first response signal 14 of the first temperature sensor element 9. The evaluation unit 12 therefore in its most simple case comprises at least one voltage divider for teach temperature sensor element 9, 10. In FIG. 2b), the ohmic resistance of the temperature sensor element is represented as R.sub.T. The voltage divider further comprises a series resistor R.sub.VT and is operated with a total voltage U.sub.T, with the total voltage U.sub.T being a direct current. The first response signal 14 is tapped above the resistor of the temperature sensor element R.sub.T.

(12) FIG. 3 shows a first measuring curve that comprises the first reference values 16 required for the determination of the flow velocity of the gas or the gas mixture 2.

(13) FIG. 4 shows a second measuring curve that allows the determination of the gas present or the composition of the gas mixture 2.