Thermal flow sensor and method for operating same
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- INNOVATIVE SENSOR TECH IST
- Filing Date
- 2024-07-01
- Publication Date
- 2026-06-17
Smart Images
Figure EP2024068494_20022025_PF_FP_ABST
Abstract
Description
[0001] Thermal flow sensor and method for operating the same
[0002] The invention relates to a method for operating a thermal flow sensor, wherein the thermal flow sensor has a heating element and at least one temperature sensor. Furthermore, the invention relates to a thermal flow sensor for detecting the flow of a fluid measuring medium.
[0003] Thermal flow sensors are known for determining the flow rate or flow velocity of a measuring medium or fluid, such as a gas, gas mixture, or liquid. These utilize the fact that a (flowing) measuring medium dissipates heat from a heated surface. Thermal flow sensors typically consist of several functional elements, usually at least one low-resistance heating element and one high-resistance resistance element that serves as a temperature sensor. Alternatively, thermal flow sensors are constructed with several low-resistance heating elements as a heater and temperature sensor.
[0004] Anemometric thermal flow sensors consist of at least one heating element, which is heated during flow measurement. As the measured medium flows around the heating element, heat is transferred into the measured medium, which changes with the flow velocity. By measuring the electrical parameters of the heating element, the flow velocity of the measured medium can be determined.
[0005] Such an anemometric thermal flow sensor is typically operated in one of the following two control modes:
[0006] In the "Constant-Current Anemometry" (CCA) control mode, a constant current is applied to the heating element. As the measuring medium flows around it, the resistance of the heating element changes, and thus the voltage drop across the heating element, which represents the measurement signal. The "Constant-Voltage Anemometry" (CVA) control mode works in a similar way, in which the heating element is subjected to a constant voltage.
[0007] With the "Constant-Temperature Anemometry (CTA)" control method, the heating element is kept at an average constant temperature. Relatively high flow velocities can be measured using this control method. Depending on the flow velocity, more or less heat is carried away by the flowing medium, and accordingly, more or less electrical power must be supplied to maintain a constant temperature. This supplied electrical power is a measure of the flow velocity of the medium. Sensors that utilize the anemometric principle can also consist of a single structure with a heating element and a temperature sensor element. If the two elements are sufficiently thermally decoupled, the temperature of the medium can be determined "by design."
[0008] Calorimetric thermal flow sensors determine the flow or flow rate of the fluid in a channel based on a temperature difference between at least one temperature sensor located downstream or upstream of a heating element. Often, two temperature sensors are used, with the first temperature sensor located downstream and the second temperature sensor located upstream of the heating element.
[0009] Calorimetric thermal flow sensors take advantage of the fact that the temperature difference is linear to the flow or flow rate up to a certain point. This process or method is described extensively in the relevant literature. With this process, the temperature of the medium is not easily determined. The temperature may be necessary, for example, for temperature compensation. In many cases, an additional temperature element is integrated which is thermally decoupled from the heating element. The disadvantage of this additional temperature sensor element is that it requires more space in the flow channel, which in some cases is not available. A second option is to switch off the heating element at periodic intervals for a defined time and to measure the temperature during this time window. The disadvantage here is that the response time of the sensor is significantly increased.
[0010] Based on this problem, the invention is based on the object of determining the temperature of a flowing medium by means of a thermal flow sensor without the need for an additional temperature sensor or a shutdown phase.
[0011] The object is achieved by a method according to claim 1 and by a thermal flow sensor according to claim 6.
[0012] With regard to the method according to the invention, it is provided that this is solved for operating a thermal flow sensor, wherein the thermal flow sensor has a heating element and at least one temperature sensor, comprising:
[0013] Detecting a first measured variable, which first measured variable relates to a temperature of a fluid measuring medium, by means of the at least one temperature sensor;
[0014] Calculating a second measured variable, which second measured variable relates to a flow of the fluid measuring medium, from the first measured variable based on first calibration data; detecting a third measured variable, wherein the third measured variable relates to the electrical resistance of the heating element, and adjusting the third measured variable by eliminating a component caused by the flow of the fluid measuring medium based on the first measured variable or the second measured variable; and
[0015] Calculating a fourth measured value, which fourth measured value refers to the temperature of the fluid measuring medium, based on the adjusted third measured value.
[0016] The method according to the invention offers the advantage that the thermal flow sensor can be used to determine the temperature of the fluid measuring medium very precisely, which is not influenced by the heating element. Regular operation of the thermal flow sensor does not need to be interrupted to determine the fourth measured variable, i.e., the temperature of the fluid measuring medium.
[0017] The resistance value of the heating element, the third measured value, can be determined, for example, by measuring the value of a voltage applied to the heating element. A change in the temperature of the heating element creates a change in resistance in the heating element, which changes its resistance value. The voltage can be measured, for example, using a voltage divider.
[0018] One embodiment of the method provides that the third measured variable is adjusted based on second calibration data or a heating element-specific function. In particular, the heating element-specific function is dependent on the flow of the fluid measuring medium and has an offset that depends on the temperature of the heating element.
[0019] It has been shown that the voltage drop across the heating element depends on both the flow velocity and the temperature of the measuring medium. The relative dependence on the flow velocity is almost independent of the temperature of the measuring medium. Expressed as a formula, this dependence is:
[0020] UHE(T, = ^0 ff set (T) + U( / )
[0021] T denotes the medium temperature, f the flow of the fluid being measured. Uoffset denotes the temperature-dependent part of the voltage, with U(f) representing the flow-dependent voltage component.
[0022] One embodiment of the method provides that the fourth measured variable is used to perform temperature compensation for the first measured variable, and optionally also for the second measured variable. The first measured variable is also used by the thermal flow sensor for its regular operation, as it provides a measure of the flow velocity (e.g., the flow velocity).
[0023] Mass flow, volume flow, etc.). Temperature compensation of this measured variable can further improve the measurement accuracy of the thermal flow sensor. The temperature-compensated first measured variable can, in turn, be used to calculate the second measured variable, which also makes it temperature-compensated.
[0024] To refine the temperature compensation, it is advantageous to repeat all process steps at least once using the temperature-compensated first measured variable instead of the first measured variable. This increases the accuracy of the fourth measured variable obtained at the end of the process. With each repetition, or iteration, the accuracy can be further increased.
[0025] With regard to the thermal flow sensor according to the invention, it is provided that it serves to detect a flow of a fluid measuring medium and comprises the following:
[0026] A heating element, wherein the heating element is designed for locally heating the fluid measuring medium, and wherein the heating element consists of a material with a finite temperature coefficient of electrical resistance;
[0027] At least one temperature sensor, wherein the at least one temperature sensor is designed to detect a first measured variable relating to a temperature of the fluid measuring medium;
[0028] A control / evaluation unit for operating the thermal flow sensor, wherein the control / evaluation unit is designed to apply an electrical voltage to the heating element, and wherein the control / evaluation unit is further designed to: i. detect a first measured variable by means of the at least one temperature sensor, which first measured variable relates to a temperature of a fluid measuring medium, ii. calculate a second measured variable from the first measured variable based on first calibration data, which second measured variable relates to a flow of the fluid measuring medium, iii. detect a third measured variable, wherein the third measured variable relates to a voltage applied to the heating element, iv. correct the third measured variable by eliminating a component caused by the flow of the fluid measuring medium on the basis of the first measured variable or the second measured variable; and v.to calculate a fourth measured value based on the adjusted third measured value, which fourth measured value refers to the temperature of the fluid measuring medium.
[0029] The method is applicable to virtually any type of thermal flow sensor. A thermal flow sensor according to the invention requires, in addition to a heating element, at least one temperature sensor, with the aid of which a value regarding the temperature of the measuring medium can be initially determined. However, the number of temperature sensors can be arbitrarily high. For example, the thermal flow sensor can have two or four temperature sensors. The types of heating element and temperature sensors used, as well as the materials used (e.g., substrate material), can also be selected as desired to suit the application of the thermal flow sensor.
[0030] According to one embodiment of the thermal flow sensor, the control / evaluation unit is designed to perform the cleaning of the third measured variable on the basis of second calibration data or a heating element-specific function.
[0031] The heating element-specific function is advantageously dependent on the flow of the fluid being measured. The function also has an offset that depends on the temperature of the heating element.
[0032] According to one embodiment of the thermal flow sensor, the control / evaluation unit is configured to perform temperature compensation of the first measured variable using the fourth measured variable. In particular, to refine the temperature compensation, the control / evaluation unit is configured to repeat all method steps at least once using the temperature-compensated first measured variable instead of the first measured variable. Each iteration can further increase the accuracy of the measured value of the medium temperature.
[0033] According to a first variant of the thermal flow sensor, it is provided that the thermal flow sensor has a single temperature sensor, wherein the first measured variable is an electrical voltage dropping across the temperature sensor.
[0034] According to a second variant of the thermal flow sensor, the thermal flow sensor comprises two temperature sensors, wherein the first measured variable is a differential value between the electrical voltage drop across the two temperature sensors. For both of these variants, it can be provided that the at least one temperature sensor is a resistance thermometer.
[0035] An alternative to this is that the two temperature sensors are thermocouples.
[0036] The invention is explained in more detail with reference to the following figures.
[0037] Fig. 1: an embodiment of a thermal flow sensor according to the invention;
[0038] Fig. 2: a schematic overview of an embodiment of the method according to the invention;
[0039] Fig. 3: a graph showing a dependence of the first measured variable on a measure of the flow of the medium;
[0040] Fig. 4: a graph showing a dependence of the third measured variable on a measure of the flow of the medium;
[0041] Fig. 5: a graph showing the adjusted third measured value over a temperature range; and
[0042] Fig. 6: a graph showing the accuracy of the fourth measured value over a temperature range.
[0043] Fig. 1 shows an embodiment of a thermal flow sensor ST according to the invention. The thermal flow sensor has a carrier element on which a first temperature sensor TS1, a second temperature sensor TS2 and a heating element HE are mounted on a measuring area. The two temperature sensors TS1 and TS2 are arranged symmetrically to the heating element HE along a flow direction of the medium (indicated here by the arrow). The first temperature sensor TS1 determines the temperature of the medium flowing past. The heating element HE heats the medium and the second temperature sensor TS2 determines the temperature of the heated medium. From the temperature difference, a value for the flow of the fluid measuring medium, e.g. a measure of the mass or volume flow, can then be determined.
[0044] This takes place here, for example, in a control / evaluation unit AE, which is brought into electrical contact with the thermal flow sensor and which applies an electrical voltage to the heating element HE to emit heat and records temperature values of the two temperature sensors TS1, TS2. The carrier element is, for example, a substrate made of a semiconductor material, a metallic material or a ceramic material. The temperature sensors TS1, TS2 and the heating element HE are in this case applied to the substrate using a thick-film or thin-film process, for example. Alternatively, the carrier element can be a printed circuit board. In this alternative, the temperature sensors TS1, TS2 and / or the heating element can be components that are connected to the printed circuit board.
[0045] To perform the process described below, the heating element must have a finite temperature coefficient of resistance (TCR). For example, the heating element HE is made of platinum and is a PT1000 resistor.
[0046] The measured values shown in Figs. 3 to 6 were recorded using a thermal flow sensor having two temperature sensors TS1, TS2 in the form of thermocouples. However, the method according to the invention is also suitable for other thermal flow sensor designs. Thus, the number (at least one temperature sensor) of temperature sensors is variable. The specific type of temperature sensor, e.g., thermocouples or resistance thermometers, is also variable, as is the precise design of the first measured variable. When using a single temperature sensor, the first measured variable can, for example, be a voltage drop across the temperature sensor.
[0047] Fig. 2 shows a flowchart of an exemplary embodiment of the method according to the invention. The method is used to determine the precise temperature of the fluid measuring medium, since neither the first temperature sensor TS1 nor the second temperature sensor TS2 can detect a measured value of the temperature of the fluid measuring medium that is unaffected by the heating element HE.
[0048] In a first process step 1, the control-Z evaluation unit AE records a first measured variable U T si-UTS2. The first measured variable UTSI-UTS2 refers to the temperature of the fluid measuring medium. In this specific application, the first measured variable UTSI-UTS2 is a difference between the voltages applied to the two temperature sensors. The faster the fluid measuring medium flows, the greater the temperature difference between the two temperature sensors TS1, TS2, or the greater the first measured variable UTSI-UTS2.
[0049] In process step 2, a second measured variable is calculated from the first measured variable UTSI-UTS2. The second measured variable is a measure of the flow velocity of the fluid being measured. The calculation is performed based on initial calibration data, which was created specifically for the thermal flow sensor or the type of thermal flow sensor and which describes a ratio of the first measured variable UTSI-UTS2 to the second measured variable.
[0050] In process step 3, a third measured variable UHE is measured using the heating element HE. The third measured variable refers to the voltage applied to the heating element HE.
[0051] In process step 4, an estimate is made as to the extent to which the third measured variable UHE depends on the flow velocity, i.e. the second measured variable.
[0052] For this purpose, Fig. 3 shows the first measured variable UTSI-UTS2 (ordinate) as a function of the flow rate of the fluid measuring medium (abscissa) for different temperatures of the fluid measuring medium. Furthermore, Fig. 4 shows the third measured variable UHE (ordinate) as a function of the flow rate of the fluid measuring medium (abscissa) for different temperatures of the fluid measuring medium.
[0053] It can be seen that the first measured variable UTSI-UTS2, as already described, increases with increasing flow velocity. However, it is also clear that the temperature of the fluid measuring medium has only a minor influence on the first measured variable UTsi-UTS2.
[0054] Furthermore, it is shown that the third measured variable, UHE, is strongly dependent on both the temperature of the fluid measuring medium and the flow velocity of the fluid measuring medium. However, the relative flow dependence of the third measured variable, UHE, is almost temperature-independent.
[0055] With this knowledge, the following function, specific to the HE heating element, can be established:
[0056] UHE(T, = ^0 ff set (T) + U( / )
[0057] This function depends on the flow f of the fluid measuring medium and has an offset Uoffset, which in turn depends on the temperature T of the heating element HE. The two parts Uoffset and U(f) (the flow-dependent voltage component), from which the function is composed, have one or more parameters or factors that can be determined experimentally. In the case that the voltage of the heating element (i.e., the third measured variable) is measured at a voltage divider, Uoffset can be rewritten (where U o a measuring voltage): RHE is the resistance of the heating element HE, while R v the additional series resistance in the voltage divider.
[0058] The function is stored in the control / evaluation unit AE. Using this function, the control / evaluation unit AE generates a temperature compensation of the third measured variable UHE in process step 5, which is now stored as the temperature-compensated third measured variable U H E_korr is present. The compensated third measured value UHE can also be generated using second calibration data stored in the control / evaluation unit AE and specifically created for the heating element HE.
[0059] In Fig. 5, the temperature-compensated third measured variable U HE_korr (ordinate) is shown as a function of the temperature of the measuring medium. Temperature compensation was performed such that the first measured variable was determined at a medium temperature of 45 °C. For the graph shown in Fig. 5, several measurements were performed and recorded for each temperature point. A nearly linear dependence of the temperature-compensated third measured variable U is shown. H E_kor r on the temperature of the measuring medium.
[0060] In a process step 6, based on the temperature-compensated third measured variable U H E_korr, a fourth measured variable can be calculated. The fourth measured variable corresponds to the temperature of the fluid measuring medium. For this purpose, a look-up table or additional calibration data can be stored in the control / evaluation unit AE, which shows the dependence of the fourth measured variable on the temperature-compensated third measured variable U H E_korr shows.
[0061] Fig. 6 shows the error margin for each of these temperature points. It can be seen that the error margin in the range of 15 °C and 55 °C is a maximum of approximately ± 1 °C. The temperature-compensated third measured value U H E_korr, or the fourth measured value calculated from it, can thus be determined very precisely.
[0062] In an optional process step 7, the calculated fourth measured variable is used to perform temperature compensation for the first measured variable. As described above, the first measured variable is slightly dependent on the temperature of the fluid being measured. By knowing the temperature of the measured medium, this slight dependence can be removed, increasing the accuracy of the entire thermal flow sensor even during normal measurement operation (acquisition of the first measured variable to determine the flow velocity as the second measured variable).
[0063] The procedure can optionally be repeated. For this purpose, the temperature-compensated first measured value is used as the input for process step 2. In principle, a fixed number of repetitions or a recursive implementation would be possible.
[0064] 1 , 2, ..., 7 process steps
[0065] AE Control / Evaluation Unit HE Heating Element
[0066] MB measuring range
[0067] ST thermal flow sensor
[0068] TS1, TS2 temperature sensors
[0069] UHE third measure UHE_corr adjusted third measure
[0070] UTS1-UTS2 first measured value, differential voltage
[0071] Temperature sensors
Claims
Patent claims 1. A method for operating a thermal flow sensor (ST), wherein the thermal flow sensor (ST) has a heating element (HE) and at least one temperature sensor (TS1, TS2), comprising: Detecting a first measured variable (UTSI-UTS2), which first measured variable (UTSI-UTS2) relates to a temperature of a fluid measuring medium, by means of the at least one temperature sensor (TS1, TS2); Calculating a second measured variable, which second measured variable relates to a flow of the fluid measuring medium, from the first measured variable (UTSI-UTS2) on the basis of first calibration data; Detecting a third measured variable (UHE), wherein the third measured variable (UHE) relates to an electrical resistance of the heating element (HE), and adjusting the third measured variable (UHE) by eliminating a portion caused by the flow of the fluid measuring medium on the basis of the first measured variable (U T SI-U TS2) or the second measured value; and Calculating a fourth measured variable, which fourth measured variable refers to the temperature of the fluid measuring medium, based on the adjusted third measured variable (U H E_corr).
2. The method according to claim 1, wherein the third measured variable (UHE) is adjusted on the basis of second calibration data or a heating element-specific function.
3. The method according to claim 2, wherein the heating element-specific function is dependent on the flow of the fluid measuring medium and has an offset which is dependent on the temperature of the heating element (HE).
4. Method according to one of the preceding claims, wherein the fourth measured variable is used to perform a temperature compensation of the first measured variable (U T SI-U T S2), and optionally also the second measured variable.
5. The method according to claim 4, wherein, to refine the temperature compensation, all method steps are carried out using the temperature-compensated first measured variable instead of the first measured variable (U T SI-U T S2) must be repeated at least once.
6. Thermal flow sensor (ST) for detecting a flow of a fluid measuring medium, comprising: A heating element (HE), wherein the heating element (HE) is designed for locally heating the fluid measuring medium, and wherein the heating element (HE) consists of a material with a finite temperature coefficient of electrical resistance; At least one temperature sensor (TS1, TS2), wherein the at least one temperature sensor (TS1, TS2) is designed to detect a first measured variable (UTSI-UTS2) with respect to a temperature of the fluid measuring medium; A control / evaluation unit (AE) for operating the thermal flow sensor (ST), wherein the control / evaluation unit is designed to apply an electrical voltage to the heating element (HE), and wherein the control / evaluation unit (AE) is further designed to: i. detect the first measured variable (UTSI-UTS2) by means of the at least one temperature sensor (TS1, TS2), ii. calculate a second measured variable from the first measured variable (UTSI-UTS2) on the basis of first calibration data, which second measured variable relates to a flow of the fluid measuring medium, iii. detect a third measured variable (UHE), wherein the third measured variable (UHE) relates to a voltage applied to the heating element (HE), iv. calculate the third measured variable (UHE) by eliminating a portion caused by the flow of the fluid measuring medium on the basis of the first measured variable (U T SI-U TS2) or the second measured variable; and v. to calculate a fourth measured variable based on the adjusted third measured variable, which fourth measured variable relates to the temperature of the fluid measuring medium.
7. Thermal flow sensor (ST) according to claim 6, wherein the control / evaluation unit is designed to perform the cleaning of the third measured variable (UHE) on the basis of second calibration data or a heating element-specific function.
8. Thermal flow sensor (ST) according to claim 7, wherein the heating element-specific function is dependent on the flow of the fluid measuring medium and has an offset which is dependent on the temperature of the heating element (HE).
9. Thermal flow sensor (ST) according to one of claims 6 to 8, wherein the control / evaluation unit is designed to carry out a temperature compensation of the first measured variable (U T SI-U TS2), and optionally also the second measured variable.
10. Thermal flow sensor (ST) according to claim 9, wherein the control-evaluation unit for refining the temperature compensation is designed to repeat all method steps at least once using the temperature-compensated first measured variable instead of the first measured variable (UTSI-UTS2).
11. Thermal flow sensor (ST) according to one of claims 6 to 10, wherein the thermal flow sensor (ST) comprises a single temperature sensor, wherein the first measured variable (UTSI-UTS2) is an electrical voltage drop across the temperature sensor.
12. Thermal flow sensor (ST) according to one of claims 6 to 10, wherein the thermal flow sensor (ST) has two temperature sensors (TS1, TS2), wherein the first measured variable (UTSI-UTS2) is a difference value between the electrical voltage drop across the two temperature sensors (TS1, TS2).
13. Thermal flow sensor (ST) according to one of claims 6 to 12, wherein the at least one temperature sensor is a resistance thermometer.
14. Thermal flow sensor according to claim 12, wherein the two temperature sensors (TS1, TS2) are thermocouples.