Thermal flow sensor and method for determining a volume flow rate

Abstract

Thermal flow sensor (1) for determining a volume flow rate (11) in a fluidic channel (2) through which a medium (3) flows, wherein the channel (2) has a first channel cross-section (A1) in a first region (4) and a second channel cross-section (A2) in a second region (5), which differs from the first channel cross-section (4), wherein at least one first heating element (6) is arranged in the first region (4) and at least one second heating element (7) is arranged in the second region (5), wherein a control and evaluation unit (10) is provided which supplies the first heating element (6) by means of a first signal (U1) and the second heating element (7) by means of a second signal (U2), wherein the control and evaluation unit (10) determines the volume flow rate (11) in the fluidic channel (2) at least on the basis of the first signal (U1) and / or the second signal (U2), and wherein the control and evaluation unit (10) (U1) and the second signal (U2) are relatedto check and / or correct the determined volume flow rate (11).

Description

[0001] The invention relates to a thermal flow sensor and a method for correcting and / or verifying a determined volume flow.

[0002] Thermal flow sensors are well-known in the art. These sensors determine the volumetric flow rate of a medium or fluid (i.e., a liquid or gas) in a fluidic channel by utilizing the heat transfer of the flowing medium. For example, anemometric flow sensors are known that are designed to include at least one heating element to which a substantially constant heating power is supplied. The resulting cooling effect exerted by the medium on the heating element is used to determine the volumetric flow rate. The greater the flow velocity and thus the volumetric flow rate of the fluid (for the same channel cross-section), the greater the amount of heat carried away from the flow sensor per unit time by heat transfer and convection via the medium, and therefore the greater the power supplied to the heating element must be.

[0003] Furthermore, calorimetric flow sensors are known that employ the arrangement of two temperature sensors, one upstream and one downstream, around a heating element in order to evaluate the temperature difference resulting from the flow. Calorimetric flow sensors that only have one temperature sensor arranged around the heating element are also common.

[0004] Both types of flow sensors suffer from the problem that the measured volumetric flow rates, and potentially also the mass flow rates, obtained in this way can exhibit artifacts and / or measurement errors. Such artifacts and / or measurement errors can be caused, for example, by the formation of fluid dynamic boundary layers that exhibit no flow near the pipe wall. These boundary layers build up in the region of the pipe wall and then dissipate. The process of buildup and dissipation is not very predictable and manifests itself as an essentially periodic change in the measurement signal.

[0005] Therefore, the purpose of the invention is to optimize the measurement process.

[0006] The problem is solved by a thermal flow sensor and a method for determining a volume flow in a fluidic channel.

[0007] With regard to the flow sensor, the problem is solved by a thermal flow sensor for determining a volumetric flow rate in a fluidic channel through which a medium flows, wherein the channel has a first channel cross-section in a first region and a second channel cross-section in a second region, which differs from the first channel cross-section, wherein at least one first heating element is arranged in the first region and at least one second heating element is arranged in the second region, wherein a control and evaluation unit is provided which supplies the first heating element by means of a first signal and the second heating element by means of a second signal, wherein the control and evaluation unit determines the volumetric flow rate in the fluidic channel at least on the basis of the first signal and / or the second signal, and wherein the control and evaluation unit relates the first signal and the second signal to each other.to check and / or correct the determined volume flow rate.

[0008] The invention exploits the fact that different channel cross-sections result in different flow velocities in the two regions. These different flow velocities lead to different signals at the two heating elements, which are located in the first and second regions, respectively. However, the two signals from the heating elements are strongly correlated. Thus, both signals have the same zero point regardless of whether the flow sensor is based on the anemometric or calorimetric principle. Furthermore, if the two signals from the heating elements are set to the same value, the ratio between the two signals is independent of the heating power and therefore also independent of temperature influences. In this way, measurement artifacts can be detected, since a disruption in the correlation of the two signals can indicate an event (measurement artifacts, parasitic measurement effects, etc.).This is due to the change in volume flow within the channel. The various evaluation options for the at least two signals from the two heating elements thus also allow for the detection and filtering out or calculation of parasitic measurement effects. As described earlier, fluid dynamic boundary layers occur in both areas with different channel cross-sections, but with different behavior. Therefore, the first and second signals can be used to check whether an expected correlation exists and, if no expected correlation exists, to detect an artifact.

[0009] An advantageous design provides that the control and evaluation unit determines a correlation measure from the first signal and the second signal for correction and / or verification and compares the determined correlation measure with previously stored correlation measures.

[0010] An advantageous design provides that the control and evaluation unit uses the quotient of the first and second signals as a correlation measure.

[0011] Besides the quotient, it is also conceivable to use the sum, product, or difference of the two signals. A table is provided for verification purposes, containing, for example, the quotient that serves as a correlation measure. This stored correlation measure is typically recorded or determined for a specific medium before the flow sensor is actually operated and then entered into the table. In this way, the volumetric flow rate determined by the thermal flow sensor can be compared with the volumetric flow rate stored for a specific quotient or correlation measure. If these two correlation measures match, the volumetric flow rate measurement can be considered error-free. If the two correlation measures do not match (or only nearly match), a measurement error can be assumed.

[0012] In particular, the design provides that the correlation measure between the first signal and the second signal is essentially constant, so that in the event of a change in the correlation measure, a malfunction of the thermal flow sensor can be detected.

[0013] An advantageous embodiment provides that the control and evaluation unit compares the first signal and the second signal with each other to correct and / or verify the determined volume flow rate and, in the event that the signal of the heating element located in the area with the larger channel cross-section is greater than the signal of the heating element located in the area with the smaller channel cross-section, detects a malfunction of the thermal flow sensor.

[0014] An advantageous embodiment provides that the control and evaluation unit compares the two signals to correct and / or verify the determined volume flow rate and, in the event that the signal of the heating element located in the area with the smaller channel cross-section is smaller than the signal of the heating element located in the area with the larger channel cross-section, detects a malfunction of the thermal flow sensor.

[0015] With regard to the method, the problem is solved by a method for determining a volume flow rate in a fluidic channel, wherein, for the correction and / or verification of the determined volume flow rate, at least a first signal, which is used to control or actuate a first heating element arranged in a first channel section, and a second signal, which is used to control or actuate a second heating element arranged in a second channel section, are related, wherein the first and the second channel section have different channel cross-sections.

[0016] An advantageous embodiment of the method provides that, for correction and / or verification, a correlation measure is formed from the first signal and the second signal, and that this correlation measure is compared with previously stored correlation measures.

[0017] In particular, the embodiment of the method provides that the quotient is used as a correlation measure between the first signal and the second signal for correction and / or verification.

[0018] An advantageous embodiment of the method provides that the quotient corresponds to the following formula: with: k1, k2 and n as fluidic coefficients of the first and second domains, which are determined by calibration; U1 and U2 as the first signal and as the second signal; v1 and v2 are the flow velocities of the medium in the first and second regions.

[0019] The invention is explained in more detail with reference to the following drawings. They show:

[0020] Fig. 1a): a schematic representation of a first embodiment of the thermal flow sensor according to the invention, which operates according to the anemometric principle;

[0021] Fig. 1b): a schematic representation of a second embodiment of the thermal flow sensor according to the invention, which operates according to the calorimetric principle; and

[0022] Fig. 2: Several schematic representations of further embodiment possibilities of the thermal flow sensor according to the invention.

[0023] Fig. Figure 1 shows a schematic representation of two embodiments of the thermal flow sensor according to the invention. 1 In both configurations, the flow sensor 1 a fluidic channel 2 before, which is a first area 4 and a second area 5 exhibits.

[0024] The two areas 4 , 5 are designed in such a way that they have different diameters and thus different channel cross-sections A1, A2, whereby a medium 3 , which through the canal 2flows, with different flow velocities v1, v2 in these areas 4 , 5 receives.

[0025] In Fig. 1a) is in the first area 4 a first heating element 6 and in the second area 5 a second heating element 7 arranged. The heating elements 6 , 7 are designed to heat the medium 3 serve this purpose.

[0026] The two heating elements 6 , 7 are each connected via a line to a control and evaluation unit 10 connected. The control and evaluation unit 10 The first heating element is activated 6 with a first signal U1 and the second heating element 7 with a second signal U2 and thus determines the volume flow 11according to the anemometric principle, in which the heating power required to maintain a certain temperature is determined.

[0027] The design in Fig. 1b) provides that at least one additional heating element must be installed around each heating element. 8a or 9a , preferably two 8a , 8b or 9a , 9b , temperature sensor elements are arranged. The control and evaluation unit 10 talks about the signals 12 and 13 the temperature sensors 8 , 9 and thus determines the volume flow rate according to the calorimetric principle. 11 . For this purpose, the channel 2 flowing medium 3 through at least one heating element 6 , 7 heated and the resulting temperature difference is measured by the temperature sensors 8a , 8b , 9a , 9b measured.

[0028] The volume flow can therefore be determined in both ways. 11 The mass flow rate can also be determined, if necessary. The invention is independent of the method used for the control and evaluation unit. 10 the volume flow 11 and / or mass flow determined.

[0029] According to the invention, the control and evaluation unit determines 10 not only the volume flow 11 through the canal 2 , but also checks and / or corrects it. For this purpose, the control and evaluation unit uses 10 The first signal U1 and the second signal U2 are related, i.e., a correlation measure is determined.

[0030] In general terms, the relationship between flow and heat transfer into the medium being measured is 3 described by King's Law. Through algebraic transformations and simplifications, the following formula is obtained: with: U as the voltage value of the first or second signal; U0 as an offset value, i.e., U0 represents the constant temperature difference between the heating element and the medium; k and n as fluidic constants, and v as flow velocity.

[0031] Using this relationship, the first signal U1 and the second signal U2 can be described as follows: where U1 corresponds to the first signal and U2 to the second signal.

[0032] Due to the different channel cross-sections A1, A2, the following results occur with the same volume flow rate. 11 different flow velocities v1, v2 in the respective areas 4 , 5 and therefore different signals for U1 and U2. However, both signals U1 and U2 are strongly correlated. In both anemometer and calorimeter operation, both signals U1 and U2 have the same zero point.

[0033] A disruption in the correlation between both signals U1 and U2 always indicates an event that is not due to a change in volume flow. 11 This is not due to the error itself, but possibly to measurement artifacts, parasitic measurement effects, etc. The various evaluation options for at least two signals U1, U2 also allow for the detection and filtering out or calculation of these parasitic measurement effects.

[0034] According to the invention, by determining a correlation measure, e.g. a ratio, of the first signal U1 and the second signal U2, a statement can be made as to whether there are possibly parasitic measurement effects in the determined volume flow. 11 present or not.

[0035] For example, by forming a ratio or quotient of the two signals, the following results: with: k1 and n as fluidic constants of the first domain; k2 and n as fluidic constants of the second domain; v1 as flow velocity in the first region; and v2 as flow velocity in the second area.

[0036] This ratio is independent of the temperature of the medium. 3 and can be used to verify (plausibility check) the volume flow rate to be determined 11 or mass flow rate. Furthermore, this ratio (or another correlation measure) can be used to correct the volume flow rate to be determined. 11 or mass flow, since the first signal U1 and the second signal U2 are in a fixed correlation to each other, because the channel cross-sections A1, A2 in the first area 4 and the second 5 are always the same.

[0037] To correct and / or verify the determined volume flow rate 11 The control and evaluation unit intervenes 10This refers to previously stored correlation measures, which are, for example, stored in a table. These previously stored correlation measures were typically configured before the actual commissioning of the flow sensor. 1 The data is determined and stored in the table. The stored correlation measures must then be used in the actual operation of the flow sensor. 1 correspond to the currently determined correlation measures, since the structure, i.e., the channel cross-sections A1, A2, does not change, whereby it should be noted that the previously determined correlation measures refer to a specific medium. 3 relate.

[0038] If the currently determined correlation measures do not match the stored correlation measures, this may indicate a malfunction or a measurement error of the flow sensor. 1 It is assumed that the control and evaluation unit will accordingly 10 is signaled.

[0039] The inventive design of a flow sensor 1 with two different channel cross-sections A1, A2, it also offers the possibility that the control and evaluation unit 10 for the correction and / or verification of the determined volume flow rate 11 compares the first signal U1 and the second signal U2 with each other and in the case that the signal U1 or U2 of the heating element 6 or 7 , which is located in that area 4 or 5 with the larger channel cross-section A1 or A2, is larger than the signal of the heating element. 7 or 6 , which is located in that area 5 or 4 with the smaller channel cross-section A2 or A1, a malfunction of the thermal flow sensor 1 The malfunction is detected. The control and evaluation unit then processes the fault. 10 This is signaled accordingly.

[0040] Alternatively, the control and evaluation unit can 10 for the correction and / or verification of the determined volume flow rate 11 compare the two signals U1, U2 in such a way that, in the case that the signal U1 or U2 of the heating element 6 or 7 , which is located in that area 4 or 5 with the smaller channel cross-section A1 or A2, is smaller than the signal U2 or U1 of the heating element. 7 or 6 , which is located in that area 5 or 4 with the larger channel cross-section A2 or A1, a malfunction of the thermal flow sensor 1 detected and signaled accordingly.

[0041] Fig. Figure 2 shows several schematic representations a)–c) of further embodiments of the thermal flow sensor according to the invention. 1 .

[0042] Fig. 2a) shows a design which includes the first heating element 6 and the associated temperature sensors 8a and 8b , on the "underside" of the first area 4 and the second heating element 7 and the associated temperature sensors 9a and 9b of the second area 5 on the "opposite" side.

[0043] Fig. 2b) shows a design that does without any temperature sensors and in which there are only two heating elements. 6 and 7 are provided. Of the two heating elements 6 and 7 is the first heating element 6 of the first area 4 arranged on one side of the channel and the second heating element 7 , of the second area 5 , on the opposite side of the canal.

[0044] Fig. 2c) shows a design in which the first heating element 6and the associated temperature sensor 8 on opposite sides of the canal in the first area 4 are arranged. The same applies to the second heating element. 7 and the associated temperature sensor 9 of the second area 5 , whereby the two heating elements 6 and 7 They are opposite each other. Reference symbol list 1 flow sensor 2-channel 3 Medium 4 First area 5 Second area 6 First heating element 7 Second heating element 8 temperature sensor(s) in the first area 9 temperature sensor(s) in the second area 10 Control and evaluation unit 11 Volume flow 12 Signal(s) for temperature sensor(s) in the first area 13 Signal(s) for temperature sensor(s) in the second area v⇀1 Flow velocity in the first area v⇀2 Flow velocity in the second area A1 First channel cross-section A2 Second channel cross-section U1 First Signal U2 Second Signal

Claims

[1] Thermal flow sensor ( 1 ) to determine a volume flow rate ( 11 ) in a fluidic channel ( 2 ), through which a medium ( 3 ) flows, whereby the channel ( 2 ) in a first area ( 4 ) a first channel cross-section (A1) and in a second area ( 5 ) a second channel cross-section (A2) that is separated from the first channel cross-section ( 4 ) differs, exhibits, with the first area ( 4 ) at least one first heating element ( 6 ) and in the second area ( 5 ) at least a second heating element ( 7 ) is arranged, wherein a control and evaluation unit ( 10 ) is provided for, which the first heating element ( 6 ) by means of a first signal (U1) and the second heating element ( 7 ) is actuated by means of a second signal (U2), whereby the control and evaluation unit ( 10) at least based on the first signal (U1) and / or the second signal (U2) the volume flow rate ( 11 ) in the fluidic channel ( 2 ) determined and wherein the control and evaluation unit ( 10 ) relates the first signal (U1) and the second signal (U2) to determine the volume flow rate ( 11 ) to check and / or correct. [2] Thermal flow sensor according to claim 1, wherein the control and evaluation unit ( 10 ) to correct and / or verify a correlation measure from the first signal (U1) and the second signal (U2) and compares the determined correlation measure with previously stored correlation measures. [3] Thermal flow sensor according to claim 1 or 2, wherein the control and evaluation unit ( 10 ) the quotient of the first signal (U1) and second signal (U2) is used as a correlation measure. [4] Thermal flow sensor according to claim 3, wherein the correlation measure between the first signal (U1) and the second signal (U2) is substantially constant, so that in the event of a change in the correlation measure a malfunction of the thermal flow sensor ( 1 ) is detectable. [5] Thermal flow sensor according to at least one of claims 1 to 4, wherein the control and evaluation unit ( 10 ) for correcting and / or verifying the determined volume flow rate ( 11 ) compares the first signal (U1) and the second signal (U2) and in the case that the signal (U1, U2) of the heating element ( 6 , 7 ), which is located in the area with the larger channel cross-section (A1, A2), is larger than the signal (U2, U1) of the heating element ( 7 , 6 ), which is located in the area with the smaller channel cross-section (A2, A1), a malfunction of the thermal flow sensor ( 1) detected. [6] Thermal flow sensor according to at least one of claims 1 to 4, wherein the control and evaluation unit ( 10 ) for correcting and / or verifying the determined volume flow rate ( 11 ) compares the two signals (U1, U2) with each other and in the case that the signal (U1, U2) of the heating element ( 6 , 7 ), which is located in the area with the smaller channel cross-section (A1, A2), is smaller than the signal ( 7 , 6 ) of the heating element located in the area with the larger channel cross-section (A2, A1), a malfunction of the thermal flow sensor ( 1 ) detected. [7] Methods for determining a volume flow rate ( 11 ) in a fluidic channel ( 2 ), whereby for the correction and / or verification of the determined volume flow rate ( 11 ) at least one first signal (U1) used to control a signal in a first channel section (4 ) arranged first heating element ( 6 ) is used, and a second signal (U2) is used to control a signal in a second channel section ( 5 ) arranged second heating element ( 7 ) is used, is related, where the first and second channel sections ( 4 , 5 ) have different channel cross-sections (A1, A2). [8] Method according to claim 7, wherein for correction and / or verification a correlation measure is formed from the first signal (U1) and the second signal (U2) and the correlation measure is compared with previously stored correlation measures. [9] Method according to claim 8, wherein the quotient is used as a correlation measure between the first signal (U1) and the second signal (U2) for correction and / or verification. [10] Method according to claim 9, wherein the quotient corresponds to the following formula: where k1, k2 and n are the fluidic coefficients of the first and second regions to be determined by calibration, U1 and U2 correspond to the first and second signals, and v1 and v2 correspond to the flow velocities of the medium in the first and second regions.