JP3710995B2 - Flow rate sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a flow rate sensor, and in particular, includes a heater that heats a fluid whose flow rate is to be detected, and two types of conductive members, and a hot junction and a cold junction that are connection points of the conductive members. The present invention relates to a flow rate sensor that measures a flow rate according to a temperature of a fluid that is detected using a thermoelectromotive force between both contact points.
[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.
[0004]
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.
[0005]
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.
[0006]
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.
[0007]
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.
[0008]
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).
[0009]
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).
[0010]
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.
[0011]
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.
[0012]
[Problems to be solved by the invention]
By the way, there is a gas meter that intermittently takes in the gas flow rate measured by the above-described flow rate sensor, measures the gas flow rate by multiplying the measured flow rate by an intermittent time, and displays the integrated value. Some combustors that consume gas supplied through the gas meter cause pressure fluctuations in the supply gas pressure during use. For example, in the case of GHP (gas heat pump), the use causes a fluctuation of about 15 mmH ₂ O in the gas pressure at a frequency of 10 to 20 Hz.
[0013]
Such a GHP becomes a pulsating flow source that generates a pulsating flow in the gas flow path. When the GHP is installed and used in a specific consumer house in an apartment house or the like, a pressure fluctuation generated by the GHP is generated. Since an instantaneous gas flow occurs in the gas flow path, a flow velocity corresponding to a certain flow rate is measured. If this is mistaken as a gas flow rate due to gas consumption, and it may be integrated as a passing flow rate to obtain the integrated flow rate, the integrated value will be larger than the actual gas consumption, which is fatal for a meter. It becomes a problem on reliability.
[0014]
That is, when pressure fluctuation occurs in the electronic gas meter as shown in FIG. 8A in a state where no gas is consumed, the gas in the gas flow path moves in the upstream direction and the downstream direction due to the influence of the pressure fluctuation. Therefore, a pulsating flow of 10 to 20 Hz as shown in FIG. If intermittent measurement is performed in response to this, a passage flow rate as shown in FIG. 8C is obtained, and the passage flow rate Ta measured at the timing a is integrated.
[0015]
Further, when the pressure generated by the above-described GHP fluctuates on the upstream side of the gas meter at the consuming house consuming the gas, the passing flow rate in the gas flow path of the electronic gas meter at the consuming house is shown in FIG. Thus, the increase / decrease is repeated. Then, if a flow rate that repeatedly increases or decreases with respect to the actual flow rate is obtained by an electronic gas meter, as in recent years, a gas leak detection function and a gas leak alarm function linked to it or a gas supply The following problems arise when a security function such as a shut-off function is mounted on an electronic gas meter.
[0016]
In other words, although the gas leak judgment level is not actually exceeded, the flow rate exceeding the gas leak judgment level is obtained by the electronic gas meter due to the timing of the measurement, and the gas leak alarm and supply cutoff are erroneous. On the contrary, even though the gas leak judgment level is actually exceeded, the flow rate below the gas leak judgment level is obtained by the electronic gas meter due to the timing of measurement, and the gas leak There arises a problem that execution of an alarm or supply interruption is delayed.
[0017]
Accordingly, the present invention provides a flow rate sensor capable of measuring the average flow velocity from which the influence of the pulsating flow is removed by increasing the heat capacity of the hot junction of the thermocouple, focusing on the above problems. This is the issue.
[0018]
[Means for Solving the Problems]
The invention according to claim 1, which has been made to solve the above problem, is composed of a heater for heating a fluid whose flow rate is to be detected, and two types of conductive members, and is a connection point of the conductive members A thermocouple having a hot junction and a cold junction, and a flow rate sensor for measuring a flow velocity according to a temperature of a fluid detected using a thermoelectromotive force generated between the two contacts, wherein the thermocouple is provided around the hot junction. The present invention resides in a flow rate sensor comprising a heat capacity increasing structure formed by expanding a conductive member while keeping its thickness constant .
[0019]
According to invention of Claim 1, a heater heats the fluid which should detect a flow velocity. A thermocouple including two types of conductive members and having a hot junction and a cold junction as a connection point of the conductive members detects the temperature of the fluid heated by the heater. The heat capacity of the hot junction is increased by the heat capacity increasing structure. Therefore, the temperature of the hot junction is not easily raised or lowered by the heat capacity increasing structure, so even if the fluid temperature increases or decreases due to the pulsating flow, the thermocouple detects the temperature that suppresses the increase or decrease of the temperature. can do.
[0021]
Further, according to the first aspect of the present invention, it is noted that the heat capacity of the hot junction increases when the mass around the hot junction is enlarged and the mass is increased, and the thickness of the conductive member around the hot junction is constant. The structure for increasing the heat capacity is formed by enlarging while maintaining the temperature. According to the above configuration, the heat capacity can be increased without increasing the thickness around the hot junction, and the thermal capacity of the hot junction can be easily increased.
[0022]
According to a second aspect of the invention, the flow sensor according to claim 1, by extending the conductive member of the heater side of the hot junction to the heater side, the conductive member near the hot junction, the thickness It exists in the flow rate sensor characterized by expanding, keeping constant.
[0023]
According to the second aspect of the present invention, the conductive member on the heater side is extended to the heater side from the hot junction, thereby expanding the conductive member around the hot junction while keeping the thickness constant. By the way, the sensitivity of the thermocouple is reduced by the increase in the heat capacity of the hot junction. Therefore, it is necessary to design a flow rate sensor that increases the heat capacity of the hot junction so that the pulsating flow can be suppressed to the maximum within a range in which a certain level of sensitivity can be maintained. According to the above configuration, since the heat capacity of the hot junction depends on the extension of the conductive member, the heat capacity of the hot junction can be easily adjusted by adjusting the length of the extension. It will be easy.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments 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.
[0027]
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, an upstream thermopile 1e disposed on the upstream side and the downstream side of the fluid with the microheater 1c interposed therebetween, and a downstream side. A side thermopile 1f is provided.
[0028]
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.
[0029]
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.
[0030]
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. Further, the P ^++- Si wiring 20 on the microheater 1c side from the hot junction So is extended to the microheater 1c side with a constant thickness. Specifically, it is extended as shown in the enlarged view of the thermopile in FIG. For this reason, the P ^++- Si wiring 20 around the hot junction So is expanded and the mass is increased by the extension, and as a result, the thermopile has a heat capacity increasing structure that increases the heat capacity of the hot junction So. .
[0031]
The output of the flow velocity sensor configured as described above will be described below with reference to FIG. First, when the microheater 1c is driven and heated, the temperature of the fluid flowing above the hot junction So rises from the ambient temperature. At this time, if a pulsating flow is generated, the temperature also increases or decreases accordingly (see 100 in FIG. 3). As the temperature rises, the temperature of the hot junction So also rises. At this time, since the heat capacity of the hot junction So is increased, the temperature of the hot junction So itself is difficult to rise and fall, so even if the temperature of the fluid increases or decreases according to the occurrence of the pulsating flow, The temperature does not change, and the temperature is suppressed from increasing or decreasing (see 200 in FIG. 3). Therefore, the flow velocity sensor can detect the flow velocity from which the influence of the pulsating flow is removed.
[0032]
In the embodiment described above, by extending the one in which P ⁺⁺ -Si wiring 20 of the conductive member constituting the thermocouple had been enlarged hot junction So near P ⁺⁺ -Si wiring 20 For example, even if the other Al wiring 10 is extended to the microheater 1c side, a heat capacity increasing structure for increasing the heat capacity of the hot junction So can be provided, and the flow velocity from which the influence of the pulsating flow is removed is detected. Can do. Further, the same effect can be obtained by extending both the wirings 10 or 20 to the micro heater 1c side.
[0033]
Further, with the same configuration, for example, by forming the thickness of the Al wiring 10 or the P ^++- Si wiring 20 around the hot junction So higher than other portions, the conductive member around the hot junction So is expanded. It is also conceivable to increase the heat capacity of the hot junction So. However, as described above, if the thickness of the Al wiring 10 or the P ^++- Si wiring 20 is increased only in the vicinity of the hot junction So, the number of manufacturing steps increases. Therefore, as described above, the Al wiring 10 or the P ^++- Si wiring The structure can be simplified and the cost can be reduced by forming the heat capacity increasing structure by enlarging it while keeping the thickness of 20 constant.
[0034]
Further, in the above-described embodiment, the P ^++- Si wiring 20 on the microheater 1c side is extended to the microheater 1c side to expand the conductive member around the hot junction So while keeping the thickness constant. However, for example, if the thickness is constant, the area around the hot junction So may be simply enlarged as shown in FIG. By the way, if the heat capacity of the hot junction So is increased too much, the temperature change due to the fluctuation of the gas flow rate itself due to the use of the combustor cannot be followed, and if the gas flow rate fluctuates frequently, the flow rate detection accuracy decreases. Resulting in. Therefore, even when the gas flow rate fluctuates frequently, it is necessary to increase the heat capacity of the hot junction So so that the pulsating flow can be suppressed to the maximum within a range where a certain degree of flow rate detection accuracy can be maintained. .
[0035]
Therefore, as shown in FIG. 2A, by extending the P ^++- Si wiring 20 on the micro heater 1c side to the micro heater 1c side, a heat capacity increasing structure for increasing the heat capacity of the hot junction So can be formed. Since the heat capacity of the hot junction So depends on the extension of the conductive member, the heat capacity of the hot junction can be easily adjusted by adjusting the length of the extension, and the flow velocity sensor can be designed easily. Can do.
[0036]
In addition, as shown in FIG. 4, the thermopile 1 e and 1 f are provided with a plurality of hot junctions So arranged vertically to the fluid flow direction 1 b. Therefore, as shown in FIG. 2B, if the vicinity of the hot junction So is simply enlarged, the interval between the hot junctions So must be enlarged, and accordingly, the thermopiles 1e and 1f themselves become larger. However, as shown in FIG. 2A, if the P ^++- Si wiring 20 on the microheater 1c side is extended to the microheater 1c side to increase the heat capacity of the hot junction So, the distance between the hot junctions So is increased. There is no need to change.
[0037]
Therefore, it is possible to increase the heat capacity while suppressing the size of the thermopiles 1e and 1f, and it is not necessary to reduce the number of hot junctions So due to the size limitation. Further, it is not necessary to increase the power consumption of the micro heater 1c in order to compensate for the sensitivity reduction of the thermopiles 1e and 1f caused by the decrease in the number of hot junctions So.
[0038]
【The invention's effect】
As described above, according to the first aspect of the present invention, the temperature of the hot junction is not easily increased or decreased by the heat capacity increasing structure, so even if the temperature of the fluid increases or decreases according to the pulsating flow, Since the thermocouple can detect the temperature at which the increase and decrease of the temperature is suppressed, it is possible to obtain a flow rate sensor that can measure the average flow velocity without the influence of the pulsating flow.
[0039]
In addition, according to the first aspect of the present invention, the heat capacity can be increased without increasing the thickness around the hot junction, and the thermal capacity of the hot junction can be easily increased. A flow rate sensor can be obtained.
[0040]
According to the invention described in claim 2 , since the heat capacity of the hot junction depends on the extension of the conductive member, the heat capacity of the hot junction can be easily adjusted by adjusting the length of the extension. Therefore, it is possible to obtain a flow rate sensor capable of reducing the cost.
[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 an enlarged top view of a thermocouple constituting the thermopile of FIG.
FIG. 3 is a diagram for explaining the output of the flow velocity sensor of FIG. 1;
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 a conventional flow velocity sensor.
6 is a time chart for explaining the operation of the flow rate sensor of FIG. 5;
7 is a graph showing temperature distribution on the upstream and downstream sides of the micro heater in FIG.
FIG. 8 is a diagram for explaining a problem of a conventional flow rate sensor.
[Explanation of symbols]
1c Micro heater (heater)
10 Al wiring (conductive member)
20P ^++- Si wiring (conductive member)
So Hot junction Sr Cold junction 1e Upstream thermopile (thermocouple)
1f Downstream thermopile (thermocouple)

Claims (2)

A heater for heating a fluid whose flow rate is to be detected, and a thermocouple having a hot junction and a cold junction, which are connection points of the conductive member, are provided between the two contacts. In the flow velocity sensor that measures the flow velocity according to the temperature of the fluid detected using the thermoelectromotive force generated in
The thermocouple includes a heat capacity increasing structure formed by expanding a conductive member around the hot junction while keeping the thickness constant . The flow rate sensor according to claim 1 ,
A flow rate sensor, wherein a conductive member on the heater side is extended from the hot junction to the heater side, thereby expanding the conductive member around the hot junction while maintaining a constant thickness.

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