DE102013114424A1 - Thermal flow sensor - Google Patents

Abstract

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

Description

The invention relates to a thermal flow sensor, and a method for correcting and / or checking a determined volume flow.

Thermal flow sensors are well known in the art. Such sensors determine the volume flow of a medium or fluid (i.e., a liquid or gas) in a fluidic channel by utilizing the heat transfer of a flowing medium. Thus, for example, anemometric flow sensors are known, which are constructed such that they have at least one heating element to which a constant heating power is essentially supplied. To determine the volume flow of the resulting cooling effect is exploited, which exerts the medium on the flow around the heating element. The greater the flow velocity and thus the volume flow of the fluid (with the same channel cross-section), the greater the amount of heat conducted away from the flow sensor by heat transfer and convection via the medium, and the greater the power supplied to the heating element.

Furthermore, calorimetric flow sensors are known which provide for the arrangement of two temperature sensors, one upstream and one downstream, around a heating element, so as to evaluate the temperature difference arising due to the flow. Calorimetric flow sensors which merely have a temperature sensor arranged around the heating element are also common.

Both types of flow sensors have the problem that the measured values of the volumetric flow determined in this way may be omitted. may also have artifacts and / or measurement errors of the mass flow. Cause of such artifacts and / or measurement errors may be, for example, forming fluid dynamic boundary layers, which have no flow in the vicinity of the pipe wall. The boundary layers build up in the area of the pipe wall and then tear off again. The process of construction and demolition is little determined and manifests itself as a substantially periodic signal change of the measurement signal.

It is therefore an object of the invention to optimize the measured value determination.

The object is achieved by a thermal flow sensor and a method for determining a volume flow in a fluidic channel.

With regard to the flow sensor, the object is achieved by a thermal flow sensor for determining a volume flow in a fluidic channel through which a medium flows, wherein the channel in a first region a first channel cross-section and in a second region a second channel cross-section of the first Has at least a first heating element and in the second region at least a second heating element is arranged, wherein a control and evaluation unit is provided, the first heating element by means of a first signal and the second heating element by means of a second signal acted upon, wherein the control and evaluation unit determined at least on the basis of the first signal and / or the second signal, the volume flow in the fluidic channel and wherein the control and evaluation unit sets the first signal and the second signal in a relationship to the determined volume flow verifiable and / or correct.

According to the invention, use is made of different flow velocities in the two regions given different channel cross sections. The different flow velocities lead to different signals at the two heating elements, which are each arranged in the first and second region. However, the two signals of the heating elements are subject to a strong correlation. Thus, both signals have the same zero point both in the case where the flow sensor is designed according to the anemometric principle and according to the calorimetric principle. Furthermore, in the case that the two signals of the heating elements are set the same, the ratio of the two signals is independent of the heating power and thus also of temperature influences. In this way, for example, measurement artifacts can be determined, since a disturbance in the correlation of the two signals gives an indication of an event (measurement artifacts, parasitic measurement effects, etc.) which is due to the volume flow changed in the channel. The various evaluation options of the at least two signals of the two heating elements thus also allow the detection and filtering or Rausrechen of parasitic measuring effects. Thus, fluid-dynamic boundary layers, as described above, occur in both regions with different channel cross-sections, but with different behavior. Thus, it can be checked on the basis of the first and second signals, whether an expected correlation exists and in the event that there is no expected correlation, detect an artifact.

An advantageous embodiment provides that the control and evaluation unit for correcting and / or checking a correlation measure from the determined first signal and the second signal and compares the determined correlation measure with already stored correlation measures.

An advantageous embodiment provides that the control and evaluation unit uses the quotient of the first and second signal as the correlation measure.

In addition to the quotient, it is also conceivable to use the sum, the product or even the difference of the two signals. In order to carry out a check, a table is provided in which, for example, the quotient which serves as the correlation measure is stored. This stored or deposited correlation measure is typically recorded or determined before the actual operation of the flow sensor for a specific medium and then stored in the table. In this way, the volume flow determined by the thermal flow sensor can be compared with the volume flow stored for a specific quotient or a specific correlation measure. If these two correlation measures agree, it can be assumed that the volumetric flow is measured correctly. In the event that the two correlation measures do not match (or approximately agree), it can be assumed that there is an incorrect measurement.

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

An advantageous embodiment provides that the control and evaluation unit for correcting and / or checking the determined volume flow compares the first signal and the second signal with each other and in the case 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, which is located in the region with the smaller channel cross-section, detects a malfunction of the thermal flow sensor.

An advantageous embodiment provides that the control and evaluation unit for correcting and / or checking the determined volume flow compares the two signals with each other and in the case that the signal of the heating element, which is located in the region with the smaller channel cross-section, is smaller when the signal of the heating element located in the region with the larger channel cross-section detects a malfunction of the thermal flow sensor.

With regard to the method, the object is achieved by a method for determining a volume flow in a fluidic channel, wherein for correcting and / or checking the determined volume flow at least a first signal, which is used for controlling or controlling a arranged in a first channel section first heating element , and a second signal used to control a second heating element disposed in a second channel portion is related, the first and second channel portions having different channel cross-sections.

An advantageous embodiment of the method provides that for the correction and / or checking a correlation measure from the first signal and the second signal is formed and that this correlation measure is compared with already stored correlation measures.

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

mit:

k₁, k₂

und n als fluidische Koeffizienten des ersten und zweiten Bereiches, die durch Kalibration ermittelt werden;

U₁ und U₂

als erstes Signal und als zweites Signal;

v₁ und v₂

als Strömungsgeschwindigkeiten des Mediums im ersten und zweiten Bereich.

An advantageous embodiment of the method provides that the quotient corresponds to the following formula:

With:

k ₁ , k ₂

and n as fluidic coefficients of the first and second regions, which are determined by calibration;

U ₁ and U ₂

as the first signal and as the second signal;

v ₁ and v ₂

as flow velocities of the medium in the first and second region.

The invention will be explained in more detail with reference to the following drawings. It shows:

1a) a schematic representation of a first embodiment of the thermal flow sensor according to the invention, which operates on the anemometric principle;

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

2 Several schematic representations of further embodiment possibilities of the thermal flow sensor according to the invention.

1 shows a schematic representation of two embodiments of the thermal flow sensor according to the invention 1 , In both embodiments, the flow sensor sees 1 a fluidic channel 2 in front of which a first area 4 and a second area 5 having.

The two areas 4 . 5 are designed such that they have different diameters and thus different channel cross sections A ₁ , A ₂ , whereby a medium 3 passing through the canal 2 flows, a different flow velocity v ₁ , v ₂ in these areas 4 . 5 gets.

In 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 at this point.

The two heating elements 6 . 7 are each via a line with a control and evaluation 10 connected. The control and evaluation unit 10 acts on the first heating element 6 with a first signal U ₁ and the second heating element 7 with a second signal U ₂ and thus determines the volume flow 11 according to the anemometric principle, in which the heating power required to maintain a certain temperature is determined.

The design in 1b) provides that in addition to each heating element at least one 8a respectively. 9a , preferably two 8a . 8b respectively. 9a . 9b , Temperature sensor elements are arranged. The control and evaluation unit 10 speaks about the signals 12 and 13 the temperature sensors 8th . 9 and thus determined according to the calorimetric principle, the flow rate 11 , This is done through the channel 2 flowing medium 3 by at least one heating element 6 . 7 heated and the resulting temperature difference through the temperature sensors 8a . 8b . 9a . 9b measured.

In both ways can thus be the flow rate 11 ggfl. also determine the mass flow. The invention is detached from the way in which the control and evaluation unit 10 the volume flow 11 and / or mass flow determined.

According to the invention determines the control and evaluation 10 not just the volume flow 11 through the channel 2 but also checks and / or corrects it. For this the control and evaluation unit sets 10 the first signal U ₁ and the second signal U ₂ in a relationship, ie a correlation measure is determined.

mit:

U

als Spannungswert des ersten bzw. zweiten Signals;

U₀

als Offsetwert, d.h. U₀ repräsentiert die konstante Temperaturdifferenz zwischen dem Heizelement und dem Medium;

k

und n als fluidische Konstanten, und

v

als Strömungsgeschwindigkeit.

In general, the relationship between flow and heat transfer in the medium to be measured 3 described by King's Law. Algebraic transformations and simplifications lead to the following formula:

With:

U

as a voltage value of the first and second signals, respectively;

U ₀

as offset value, ie U ₀ represents the constant temperature difference between the heating element and the medium;

k

and n as fluidic constants, and

v

as flow velocity.

wobei U₁ dem ersten Signal und U₂ dem zweiten Signal entspricht.By means of this relationship, the first signal U ₁ and second signal U ₂ can be described as follows:

where U ₁ corresponds to the first signal and U ₂ corresponds to the second signal.

Due to the different channel cross sections A ₁ , A ₂ result at the same volume flow 11 different flow velocities v ₁ , v ₂ in the respective areas 4 . 5 and thus also different signals for U ₁ and U ₂ . However, both signals U ₁ , U ₂ are subject to a strong correlation. Both in anemometer and calorimeter operation both signals U ₁ , U _{2 have} the same zero point.

A disturbance in the correlation of both signals U ₁ , U ₂ always indicates an event that is not due to a changed volume flow 11 The various evaluation options of at least two signals U ₁ , U ₂ further allow the detection and filtering out or Rausrechnen these parasitic measurement effects.

According to the invention can thus be determined by determining a correlation measure, for example. A ratio formation of the first signal U ₁ and second signal U ₂ a statement as to whether possibly parasitic measuring effects at the determined volume flow 11 present or not.

mit:

k₁

und n als fluidische Konstanten des ersten Bereiches;

k₂

und n als fluidische Konstanten des zweiten Bereiches;

v₁

als Strömungsgeschwindigkeit im ersten Bereich; und

v₂

als Strömungsgeschwindigkeit im zweiten Bereich.

Thus, for example, results from ratio formation or formation of the quotient of the two signals:

With:

k ₁

and n as fluidic constants of the first region;

k ₂

and n as fluidic constants of the second region;

v ₁

as flow velocity in the first area; and

v ₂

as flow velocity in the second area.

This ratio is independent of the temperature of the medium 3 and can be used to check (plausibility check) of the volumetric flow to be determined 11 or mass flow are used. Furthermore, this ratio (or another correlation measure) can be used to correct the volumetric flow to be determined 11 or mass flow, since the first signal U ₁ and the second signal U _{2 are} in a fixed correlation to each other, since the channel cross-sections A ₁ , A ₂ in the first region 4 and the second area 5 always the same.

For correction and / or verification of the determined volume flow 11 accesses the control and evaluation unit 10 on already deposited correlation measures, which are stored, for example, in a table. The already stored correlation measures were typically before the actual start-up of the flow sensor 1 determined and stored in the table. The stored correlation measures must then be in the actual operation of the flow sensor 1 correspond to the currently determined correlation measures, since the structure, ie the channel cross-sections A ₁ , A ₂ do not change, it should be noted that the already determined correlation measures on a specific medium 3 Respectively.

In the event that the currently determined correlation measures do not agree with the stored correlation measures, a fault or incorrect measurement of the flow sensor 1 are assumed, the corresponding from the control and evaluation 10 is signaled.

The inventive structure of a flow sensor 1 with two different channel cross sections A ₁ , A ₂ , further offers the possibility that the control and evaluation unit 10 for correcting and / or checking the determined volume flow 11 compares the first signal U ₁ and the second signal U ₂ with each other and in the case that the signal U ₁ or U _{2 of} the heating element 6 or 7 that is in the field 4 or 5 with the larger channel cross-section A ₁ or A ₂ is greater than the signal of the heating element 7 or 6 that is in the field 5 or 4 is located with the smaller channel cross-section A ₂ or A ₁ , a malfunction of the thermal flow sensor 1 detected. The malfunction will then be handled by the control and evaluation unit 10 again signaled accordingly.

Alternatively, the control and evaluation unit 10 for correcting and / or checking the determined volume flow 11 compare the two signals U ₁ , U _{2 in} such a way that in the case that the signal U ₁ or U _{2 of} the heating element 6 or 7 that is in the field 4 or 5 with the smaller channel cross section A ₁ or A _2, is smaller than the signal U ₂ or U _{1 of} the heating element 7 or 6 that is in the field 5 or 4 is located with the larger channel cross-section A ₂ or A ₁ , a malfunction of the thermal flow sensor 1 detected and signaled accordingly.

2 shows by way of example several schematic representations a) -c) of further embodiment possibilities of the thermal flow sensor according to the invention 1 ,

2a) shows an embodiment that the first heating element 6 and the associated temperature sensors 8a and 8b , on the "bottom" 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 has.

2 B) shows an embodiment dispenses with any temperature sensors and the only two heating element 6 and 7 are provided. From the two heating elements 6 and 7 is the first heating element 6 of the first area 4 arranged on a channel side and the second heating element 7 , the second area 5 , on the opposite side of the canal.

2c) shows an embodiment in which the first heating element 6 and the associated temperature sensor 8th on opposite channel sides of 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 , where the two heating elements 6 and 7 again face each other.

LIST OF REFERENCE NUMBERS

1

 Flow Sensor

2

 channel

3

 medium

4

 First area

5

 Second area

6

 First heating element

7

 Second heating element

8th

 Temperature sensor or temperature sensors in the first area

9

 Temperature sensor or temperature sensors in the second area

10

 Control and evaluation unit

11

 volume flow

12

 Signal or signals for temperature sensor (s) in the first range

13

 Signal or signals for temperature sensor (s) in the second area

v ⇀ ₁

 Flow velocity in the first area

v ⇀ ₂

 Flow velocity in the second area

A ₁

 First channel cross-section

A ₂

 Second channel cross section

U ₁

 First signal

U ₂

 Second signal

Claims (10)

Thermal flow sensor ( 1 ) for determining a volume flow ( 11 ) in a fluidic channel ( 2 ) through which a medium ( 3 ) flows, the channel ( 2 ) in a first area ( 4 ) has a first channel cross-section (A ₁ ) and in a second region ( 5 ) has a second channel cross-section (A ₂ ), which of the first channel cross-section (A ₂ ) 4 ), wherein in the first area ( 4 ) at least a first heating element ( 6 ) and in the second area ( 5 ) at least one second heating element ( 7 ), wherein a control and evaluation unit ( 10 ) is provided, which the first heating element ( 6 ) by means of a first signal (U ₁ ) and the second heating element ( 7 ) acted upon by a second signal (U ₂ ), wherein the control and evaluation unit ( 10 ) at least on the basis of the first signal (U ₁ ) and / or the second signal (U ₂ ) the volume flow ( 11 ) in the fluidic channel ( 2 ) and wherein the control and evaluation unit ( 10 ) the first signal (U ₁ ) and the second signal (U ₂ ) in a relationship to the determined volume flow ( 11 ) and / or correct. Thermal flow sensor according to claim 1, wherein the control and evaluation unit ( 10 ) for correcting and / or checking a correlation measure from the first signal (U ₁ ) and the second signal (U ₂ ) and compares the determined correlation measure with already stored correlation measures. Thermal flow sensor according to claim 1 or 2, wherein the control and evaluation unit ( 10 ) used as correlation measure the quotient of the first signal (U ₁ ) and second signal (U ₂ ). Thermal flow sensor according to claim 3, wherein the correlation between the first signal (U ₁ ) and the second signal (U ₂ ) is substantially constant, so that in case of a change in the correlation measure a malfunction of the thermal flow sensor ( 1 ) is detectable. Thermal flow sensor according to at least one of claims 1 to 4, wherein the control and evaluation unit ( 10 ) for correcting and / or checking the determined volume flow ( 11 ) compares the first signal (U ₁ ) and the second signal (U ₂ ) with each other and in the case that the signal (U ₁ , U ₂ ) of the heating element ( 6 . 7 ), which is in the region with the larger channel cross-section (A ₁ , A ₂ ), is greater than the signal (U ₂ , U ₁ ) of the heating element ( 7 . 6 ), which is located in the region with the smaller channel cross-section (A ₂ , A ₁ ), a malfunction of the thermal flow sensor ( 1 ) detected. Thermal flow sensor according to at least one of claims 1 to 4, wherein the control and evaluation unit ( 10 ) for correcting and / or checking the determined volume flow ( 11 ) compares the two signals (U ₁ , U ₂ ) with each other and in the case that the signal (U ₁ , U ₂ ) of the heating element ( 6 . 7 ), which is in the region with the smaller channel cross-section (A ₁ , A ₂ ), is smaller than the signal ( 7 . 6 ) of the heating element, which is located in the region with the larger channel cross-section (A ₂ , A ₁ ), a malfunction of the thermal flow sensor ( 1 ) detected. Method for determining a volume flow ( 11 ) in a fluidic channel ( 2 ), whereby for correcting and / or checking the determined volume flow ( 11 ) at least one first signal (U ₁ ), which for controlling a in a first channel section ( 4 ) arranged first heating element ( 6 ), and a second signal (U ₂ ), which is used to control a signal in a second channel section (U ₂ ). 5 ) arranged second heating element ( 7 ), the first and second channel sections ( 4 . 5 ) have different channel cross-sections (A ₁ , A ₂ ). The method of claim 7, wherein for correcting and / or checking a correlation measure from the first signal (U ₁ ) and the second signal (U ₂ ) is formed and the correlation measure is compared with already stored correlation measures. Method according to claim 8, wherein the quotient is used for correction and / or verification Correlation between the first signal (U ₁ ) and the second signal (U ₂ ) is used. The method of claim 9, wherein the quotient corresponds to the formula:

where k1, k2 and n fluidic coefficients of the first and second regions to be determined by calibration, U ₁ and U ₂ correspond to the first signal and the second signal and v ₁ and v ₂ correspond to the flow velocities of the medium in the first and second regions.

Images