JP3710994B2 - Flow rate sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a flow rate sensor, and in particular, a support base that forms a flow path through which a fluid whose flow rate is to be detected is formed, a heater that heats the fluid, and two types of conductive members. And a thermocouple having a hot junction and a cold junction that are connection points of the conductive member, and a thermocouple that detects the flow velocity according to the temperature of the fluid respectively detected using the thermoelectromotive force between the two contacts About.
[0002]
[Prior art]
As an example of the flow velocity sensor described above, one shown in FIG. 4 is generally known. In the figure, a thin-walled portion 1a-1 formed thinly by anisotropic etching is provided at the center of the support base 1a. And while providing a flow path on this thin part 1a-1, it is made for a fluid to flow to the upper surface of the thin part 1a-1 in the arrow 1b direction.
[0003]
Also, in the figure, a thin-film microheater 1c is provided on the upper or lower surface of the thin-walled portion 1a-1, and a power supply wiring 1d for supplying a drive current is connected to the microheater 1c. A microheater driving circuit (not shown) that outputs a pulse voltage or current is connected to the power supply wiring 1d. Therefore, every time the microheater driving circuit outputs a pulse, the microheater 1c is energized and heated through the power supply wiring 1d. On the upper surface of the thin-walled portion 1a-1, upstream and downstream thermopile 1e, 1f are connected to the upstream and downstream sides of the microheater 1c, respectively. The upstream and downstream thermopiles 1e and 1f are intended to improve sensitivity by connecting a large number of thermocouples in series.
[0004]
As shown in the enlarged view of the thermopile in FIG. 5, the upstream side and downstream side thermopile 1e, 1f are two kinds of conductive members constituting a thermocouple, for example, aluminum (hereinafter referred to as Al) wiring 10 and P-type silicon. Wirings 20 (hereinafter referred to as P ^++- Si) are alternately connected in series. The hot contact So, which is a contact point between one end of the Al wiring 10 and the P ⁺⁺ -Si wiring 20, is connected to the thin-walled portion 1 a-1 having a low thermal conductivity, and the other end of the Al wiring 10 and the P ⁺⁺ -Si wiring. Cold junction Sr which is a contact with 20 is arranged in thick part 1a-2 which is support base 1a other than thin part 1a-1. The thick portion 1a-2 is formed thick so that its thermal conductivity is higher than that of the Al wiring 10 and the P ^++- Si wiring 20.
[0005]
Therefore, as shown in FIG. 4, the upstream and downstream side thermopile 1 e, 1 f includes a cold junction group consisting of a plurality of cold junctions Sr and a hot junction group Sog consisting of a plurality of hot junctions So. Srg is provided in each of the thick portions 1a-2. For this reason, when the microheater 1c starts heating and the upstream and downstream fluid rises above the ambient temperature, the heat is not conducted to the thin-walled portion 1a-1 having a low thermal conductivity, and most of the hot junction groups. Since it is conducted to Sog, the hot junction group Sog becomes substantially equal to the temperature of the fluid on the upstream and downstream sides of the microheater 1c. On the other hand, in the cold junction group Srg, even if the temperature of the fluid passing over the cold junction group Srg rises, the heat is thicker than the Al wiring 10 and the P ^++- Si wiring 20, and the thick portion 1a- Since it is transmitted to 2 and escapes, it is kept at almost the ambient temperature. As a result, the thermoelectromotive force according to the temperature of the fluid on the upstream and downstream sides of the microheater 1c is generated in the hot junction group Sog and the cold junction group Srg of the upstream and downstream thermopiles 1e and 1f, respectively.
[0006]
The operation of the flow velocity sensor configured as described above will be described below with reference to the time chart of FIG. 6 and the temperature distribution diagram of FIG. In the microheater 1c, a driving current flows simultaneously with the rise of the rectangular pulse output from the microheater driving circuit, and heating is performed for a predetermined time (FIG. 6A). As a result, when no fluid is flowing through the flow path, heat is transferred to the gas near the microheater 1c, and the upstream and downstream temperature distributions near the microheater 1c are symmetrical. That is, the hot junction groups Sog of both thermopile 1e and 1f rise to the same temperature t (FIG. 7). Therefore, the upstream and downstream thermopiles 1e and 1f output pulse voltages V1 and V2 that are thermoelectromotive forces having equal peak voltages (FIGS. 6B and 6C).
[0007]
Now, when the fluid flows in the direction of the arrow 1b while the microheater 1c is energized, the upstream side is cooled by the fluid, and the temperature is lowered by ΔTd according to the flow rate (FIG. 7). On the other hand, on the downstream side, heat conduction is promoted from the microheater 1c using a gas flow as a medium, and the temperature is increased by ΔTu according to the flow rate (FIG. 7). As a result, the upstream thermopile 1e outputs a pulse voltage V1 having a peak voltage lowered by the fluid, and the downstream thermopile 1f outputs a pulse voltage V2 having a peak voltage raised by the fluid (FIG. 6D). (E)). As the flow velocity increases, the temperature drop ΔTd and the temperature rise ΔTu described above also increase accordingly, so that the difference between the peak voltages of the pulse voltages V1 and V2 becomes an output corresponding to the flow velocity.
[0008]
There is also known a flow rate sensor that continuously drives the microheater 1c to energize it and outputs the voltage difference between the upstream and downstream thermopiles 1e and 1f according to the flow rate.
[0009]
[Problems to be solved by the invention]
By the way, there has been a demand for improving the sensitivity of the flow rate sensor. In order to meet such a demand, in the conventional flow rate sensor as described above, the sensitivity of the thermopile 1e, 1f is established by connecting a larger number of thermocouples in series to form the upstream and downstream thermopiles 1e, 1f. To improve the sensitivity of the flow velocity sensor. However, due to the space of the support base 1a, there is a limit to the number of thermocouples connected in series, that is, there is a limit to improving the sensitivity. Thus, in order to further improve the sensitivity, it is conceivable to amplify the pulse voltages V1 and V2 and the peak difference (V1−V2) with an amplifier circuit, but there arises a problem that the circuit scale and power consumption increase.
[0010]
Therefore, the present invention pays attention to the above problems, and an object of the present invention is to provide a flow rate sensor with improved sensitivity without increasing the circuit scale and power consumption.
[0011]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, the invention according to claim 1 is characterized in that at least a part of a support base that forms a flow path through which a fluid whose flow rate is to be detected is formed, a heater for heating the fluid, It is composed of a conductive member and is equipped with a thermocouple having a hot junction and a cold junction that is a connection point of the conductive member, and the thermocouple detects each using a thermoelectromotive force between the two contacts. In the flow rate sensor that detects the flow rate according to the temperature of the fluid , a protruding portion is provided on a mounting surface on which the heater and the thermocouple of the support base are mounted, and the cold junction of the hot junction and the cold junction is provided. The flow velocity sensor is characterized in that only the protrusion is mounted on the protruding portion .
[0012]
According to the first aspect of the present invention, the protruding portion is provided on the mounting surface on which the heater and the thermocouple of the support base are mounted, and only the cold junction is mounted on the protruding portion among the hot junction and the cold junction. . Therefore, when the heater heats the fluid whose flow rate is to be detected, the heat from the heater is transmitted to the cold junction through the support base on which the heater is provided, but the heater through the support base by the protruding portion. Therefore, heat conduction from the heater to the cold junction is prevented, so that the cold junction absorbs heat conducted from the heater through the support base, and the temperature of the cold junction does not increase. For this reason, since the temperature difference between the hot junction and the cold junction increases, the thermoelectromotive force of the thermocouple corresponding to the temperature difference can be amplified. That is, the output voltage of the thermocouple can be amplified without amplifying the output voltage that is the thermoelectromotive force of the thermocouple by the amplifier circuit.
[0013]
The invention according to claim 2 resides in the flow velocity sensor according to claim 1, wherein the protruding portion is formed of the same member as the support base .
[0014]
According to the invention described in claim 2, the protruding portion is formed of the same member as the support base. Therefore, it is not necessary to use another material simply by protruding the support base.
[0015]
A third aspect of the present invention resides in the flow velocity sensor according to the first or second aspect , wherein a member that covers a flow path side of only the cold junction is provided among the hot junction and the cold junction of the thermocouple .
[0016]
According to invention of Claim 3 , the member which covers the flow path side only of a cold junction among the hot junction and the cold junction of a thermocouple was provided . Accordingly, when the heater heats the fluid whose flow rate is to be detected, the heat from the heater is transmitted to the cold junction through the fluid. However, the member covering the flow path side of only the cold junction causes the heater to the cold junction via the fluid. Therefore, the cold junction absorbs the heat conducted through the fluid from the heater and the temperature of the cold junction does not rise. For this reason, since the temperature difference between a hot junction and a cold junction increases, the thermoelectromotive force of the thermocouple according to the temperature difference can be further amplified.
[0017]
The invention according to claim 4 is composed of a heater for heating the fluid and two kinds of conductive members on a support base that forms a flow path through which a fluid whose flow rate is to be detected flows . A thermocouple having a hot junction and a cold junction as a connection point of the conductive member is mounted, and the thermocouple detects the flow velocity according to the temperature of the fluid detected using the thermoelectromotive force between the two contacts. In the flow rate sensor, a member that covers a flow path side of only the cold junction among the hot junction and the cold junction of the thermocouple is provided .
[0018]
According to invention of Claim 4 , the member which covers the flow path side only of a cold junction among the hot junction and the cold junction of a thermocouple was provided . Therefore, when the heater heats the fluid whose flow rate is to be detected, the heat from the heater is transmitted to the cold junction through the fluid, but the member that covers the flow path side of the cold junction alone causes the cold junction from the heater through the flow path. Therefore, the cold junction absorbs the heat conducted from the heater through the fluid, and the temperature of the cold junction does not increase. For this reason, since the temperature difference between the hot junction and the cold junction increases, the thermoelectromotive force of the thermocouple corresponding to the temperature difference can be amplified. That is, the output voltage of the thermocouple can be amplified without amplifying the output voltage that is the thermoelectromotive force of the thermocouple by the amplifier circuit.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
First Embodiment Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a sectional view showing an embodiment of a flow rate sensor of the present invention. As described above, the downstream side thermopile 1f is configured by connecting a plurality of thermocouples in series. However, in order to simplify the cross-sectional view, FIG. Sectional drawing when comprised with one thermocouple is shown. In this figure, the same reference numerals are given to the same parts as those in the prior art described with reference to FIG.
[0020]
In the figure, a support base 1a includes a silicon substrate (hereinafter referred to as Si substrate) 30 and a silicon tetranitride / 3 silicon dioxide layer (hereinafter referred to as Si ₃ N ₄ / SiO ₂ layer) formed on both surfaces of the Si substrate 30. ) 41 and 42. The support base 1a has a microheater 1c for heating a fluid whose flow rate is to be detected, and an upstream thermopile 1e and a downstream thermopile disposed on the upstream and downstream sides of the fluid with the microheater 1c interposed therebetween. 1f. The upstream and downstream thermopiles 1e and 1f are composed of two types of conductive members, that is, an aluminum wiring (hereinafter referred to as Al wiring) 10 and a P-type silicon wiring (hereinafter referred to as P ^++- Si), as in the conventional case. The wiring 20 is comprised. In these conductive members, only the hot junction So and the cold junction Sr are connected, and the other portions are insulated from each other by the Si ₃ N ₄ / SiO ₂ layer 41.
[0021]
Further, the support base 1a at the center portion where the microheater 1c and the hot contacts So of both the thermopile 1e and 1f are provided is formed by the anisotropic etching to the Si ₃ N ₄ / SiO ₂ layer 42 and the Si substrate 30. Is removed to form a thin wall. The thin part is the thin part 1a-1, and the support base 1a other than the thin part 1a-1 is the thick part 1a-2.
[0022]
Furthermore, the upper surface of the support base 1a is a flow path, and a fluid whose flow rate should be detected flows in the direction of the arrow 1b. And between this flow path and the cold junction Sr of both thermopile 1e, 1f, the 2nd thermal insulation member 1g-2 (= member which covers the flow path side of only a cold junction ) is provided, This 1st The two heat insulating members 1g-2 are thermally insulated from the flow path and the cold junction Sr between the two thermopiles 1e and 1f. When the second thermal insulation member 1g-2 is provided between the flow path and the cold junction Sr as described above, the second thermal insulation member is provided between the flow path and the cold junction group Srg as shown in FIG. 1g-2 will be provided.
[0023]
The heat transfer of the flow rate sensor configured as described above will be described below. First, when the microheater 1c is driven and heated, the temperature distribution of the upstream and downstream fluids near the microheater 1c becomes higher than the ambient temperature as described above with reference to FIG. Accordingly, the upstream and downstream thermopile 1e, 1f arranged on the upstream and downstream sides of the fluid with the microheater 1c interposed therebetween absorbs heat from the fluid heated by the microheater 1c, thereby The temperature rises according to the temperature of the fluid passing through the upper surface of the contact group Sog.
[0024]
On the other hand, the cold junction groups Srg of the upstream and downstream thermopiles 1e and 1f are also arranged on the upstream and downstream sides of the fluid sandwiching the microheater 1c, but the heat from the fluid passing through the upper surface of the cold junction group Srg is The cold junction group Srg is kept at the ambient temperature because the cold junction group Srg escapes through the thick portion 1a-2 having a high thermal conductivity. Moreover, since the flow path and the cold junction group Srg are thermally insulated by the second thermal insulation member 1g-2, heat conduction from the microheater 1c to the cold junction group Srg through the fluid is prevented. Has been. Therefore, the cold junction group Srg is not affected by the fluid heated by the microheater 1c, and the temperature does not rise. That is, the cold junction group Srg is further maintained at the ambient temperature by the second thermal insulation member 1g-2.
[0025]
For this reason, the temperature difference between the hot junction group Sog and the cold junction group Srg increases, and the thermoelectromotive forces of the upstream and downstream thermopiles 1e and 1f according to the temperature difference can be amplified. In other words, the output scale of the upstream and downstream thermopile 1e, 1f can be amplified without amplifying the output voltage, which is the thermoelectromotive force of the upstream or downstream thermopile 1e, 1f, by the amplifier circuit. The sensitivity can be improved without increasing the power consumption.
[0026]
Second embodiment By the way, in the first embodiment described above, the heat from the microheater 1c transmitted by the second heat insulating member 1g-2 through the fluid passing through the upper surface of the cold junction group Srg. In this case, the temperature of the cold junction group Srg is prevented from rising. In addition to the heat transferred through the fluid, the Si ₃ N ₄ / SiO provided with the micro heater 1c shown in FIG. It is also conceivable that the temperature of the cold junction group Srg rises due to the heat transmitted from the _second layer 41.
[0027]
Therefore, in FIG. 1, a Si ₃ N ₄ / SiO ₂ layer micro heater 1c is provided, Si ₃ N ₄ / SiO ₂ layer to electrically insulate the Al wiring 10 and the P ⁺⁺ -Si wiring 20 41 In the second embodiment, as shown in the sectional view of the flow velocity sensor in FIG. 3, the Si ₃ N ₄ / SiO ₂ layer 41 provided with the microheater 1c, the Al wiring 10 and A SiO ₂ layer 43 that electrically insulates the P ⁺⁺ -Si wiring 20 is formed separately, and further, a Si ₃ N ₄ / SiO ₂ layer constituting the support base 1a on which the microheater 1c is provided. 41 and the cold junction group Srg are provided with a first thermal insulation member 1g-1 (= protrusion) . By this first heat insulating member 1g-1, the Si ₃ N ₄ / SiO ₂ layer 41 constituting the support base 1a on which the micro heater 1c is provided and the cold junction group Srg are thermally insulated. .
[0028]
The heat transfer of the flow rate sensor in the second embodiment having the above-described configuration will be described below. First, when the microheater 1c is driven and heated, heat from the microheater 1c is conducted in the direction of the arrow 50 through the Si ₃ N ₄ / SiO ₂ layer 41 provided with the microheater 1c. As described in the above configuration, the Si ₃ N ₄ / SiO ₂ layer 41 and the cold junction group Srg are thermally insulated by the first thermal insulation member 1g-1, and the Si ₃ N ₄ / SiO ₂ layer 41 is Heat conduction from the microheater 1c through the Si ₃ N ₄ / SiO ₂ layer 41 to the cold junction group Srg is conducted to the cold junction group Srg. In other words, the temperature of the cold junction group Srg does not rise above the ambient temperature due to the heat conducted from the Si ₃ N ₄ / SiO ₂ layer 41. Accordingly, since the temperature difference between the hot junction group Sog and the cold junction group Srg is further increased, the sensitivity of the flow velocity sensor can be improved.
[0029]
The first thermal insulating member 1g-1 may be formed of another material having a lower thermal conductivity than that of the Si ₃ N ₄ / SiO ₂ layer 41. For example, the Si ₃ N constituting the support base 1a may be used. ₄ / by the SiO ₂ layer 41 protruding cause may be formed by Si ₃ N ₄ / the same material and the SiO ₂ layer 41. In this case, the heat conduction path from the microheater 1c to the cold junction group Srg becomes longer by the protruding portion of the Si ₃ N ₄ / SiO ₂ layer 41, and from the microheater 1c through the support base 1a. Heat transmitted to the cold junction county Srg can be blocked.
[0030]
For this reason, it is possible to make it difficult to transfer heat from the micro heater 1c to the cold junction group Srg. In other words, the support base 1a and the cold junction group Srg can be thermally insulated simply by protruding the Si ₃ N ₄ / SiO ₂ layer 41 constituting the support base 1a. The first heat insulating member 1g-1 can be formed without using a separate material simply by projecting, and the configuration is simplified and the cost can be reduced.
[0031]
In the first to second embodiments described above, the second thermal insulation member 1g-2 or the first thermal insulation member 1g-1 with respect to the cold junction groups Srg of both the upstream and downstream thermopiles 1e, 1f. However, for example, if it is not necessary to improve sensitivity so much, it may be provided for one of the cold junction groups Srg of the upstream side and downstream side thermopile 1e, 1f.
[0032]
【The invention's effect】
As described above, according to the first and fourth aspects of the invention, the output voltage of the thermocouple can be amplified without amplifying the output voltage that is the thermoelectromotive force of the thermocouple by the amplifier circuit. A flow rate sensor with improved sensitivity can be obtained without increasing the circuit scale and power consumption.
[0033]
According to the second aspect of the present invention, since it is not necessary to use another material simply by projecting the support base , it is possible to obtain a flow rate sensor that can be simplified in structure and reduced in cost .
[0034]
According to the invention described in claim 3, since the temperature difference between the hot junction and the cold junction increases, the thermoelectromotive force of the thermocouple corresponding to the temperature difference can be further amplified, so that the sensitivity is further increased. It is possible to obtain a flow rate sensor that is improved.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing an embodiment of a flow rate sensor of the present invention.
FIG. 2 is a plan view of the flow velocity sensor of FIG.
FIG. 3 is a cross-sectional view showing an embodiment of a flow rate sensor of the present invention.
FIG. 4 is a diagram illustrating an example of a conventional flow velocity sensor.
FIG. 5 is an enlarged top view of a thermopile constituting the flow velocity sensor of FIG. 4;
6 is a time chart for explaining the operation of the flow rate sensor of FIG. 4;
7 is a graph showing temperature distribution on the upstream and downstream sides of the micro heater in FIG.
[Explanation of symbols]
1a Support base 1c Micro heater (heater)
1e Upstream thermopile (thermocouple)
1f Downstream thermopile (thermocouple)
10 Al wiring (conductive member)
20P ^++- Si wiring (conductive member)
So Hot junction Sr Cold junction 1g-2 Second thermal insulation member (second thermal conduction blocking means)
1g-1 1st heat insulation member (1st heat conduction prevention means)

Claims (4)

At least a part of the support base that forms the flow path through which the fluid whose flow rate should be detected flows is composed of a heater for heating the fluid and two types of conductive members, and at the connection point of the conductive members. at a flow rate sensor in hot junction and mounting a thermocouple having a cold junction, the thermocouple detects the flow rate by the temperature of the fluid detected respectively by using the thermoelectromotive force between the two contact points,
A protruding portion is provided on a mounting surface on which the heater and the thermocouple of the support base are mounted, and of the hot junction and the cold junction, only the cold junction is mounted on the protruding portion. A flow rate sensor. The flow rate sensor according to claim 1, wherein the protruding portion is formed of the same member as the support base . The flow velocity sensor according to claim 1 or 2 , wherein a member that covers a flow path side of only the cold junction is provided among a hot junction and a cold junction of the thermocouple . At least a part of the support base that forms the flow path through which the fluid whose flow rate should be detected flows is composed of a heater for heating the fluid and two types of conductive members, and at the connection point of the conductive members. at a flow rate sensor in hot junction and mounting a thermocouple having a cold junction, the thermocouple detects the flow rate by the temperature of the fluid detected respectively by using the thermoelectromotive force between the two contact points,
A flow rate sensor comprising a member that covers a flow path side of only the cold junction among the hot junction and the cold junction of the thermocouple.

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