JPH1062220A - Thermal air-flowemeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enlarge a flow velocity range and reduce output noises when a flow rate is measured and enhance resistance reliability to dust, by making a temperature of a central temperature-measuring resistance body constant, and detecting a flow rate, a direction of a fluid to be measured from a temperature difference of an upstream and a downstream temperature-measuring resistance bodies. SOLUTION: A measuring elements 1 has an upstream, a downstream temperature-measuring resistance bodies 5, 7 and a central temperature-measuring resistance body 6 arranged side by side to an air flow 10 to be measured. The resistance bodies are surrounded adjacently by heat-generating resistance bodies 4a, 4b. A heating current is fed to the resistance bodies 4a, 4b from terminal electrodes 9(2), 9(9), and a temperature of the resistance body 6 is set to be higher by a constant value than a temperature of an air temperature-measuring resistance body 8 arranged at a leading end of a flow path for the air flow 10. When the air flow 10 is zero, the resistance bodies 5, 7 are the same in temperature as the resistance body 6, without generating a temperature difference. However, when the air flow 10 flows in a forward direction,. the upstream resistance body 4a is more efficiently cooled by the air flow 10 than the downstream resistance body 4b, and therefore the temperature of the resistance body 5 is lower than that of the resistance body 7. If the air flow runs in an opposite direction, the temperature of the resistance body 5 becomes higher than that of the resistance body 7. A flow rate of the air flow can be detected from a temperature difference. and a direction of the air flow can be detected from which of the resistance bodies 5 and 7 is higher in temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal air flow meter, and more particularly to a thermal air flow meter suitable for measuring an intake air amount of an internal combustion engine.

[0002]

2. Description of the Related Art Conventionally, as an air flow meter which is provided in an electronically controlled fuel injection device of an internal combustion engine of an automobile or the like and measures an intake air amount, a thermal air flow meter has become mainstream since it can directly detect a mass air amount. I have. Among them, an air flow meter manufactured by a semiconductor micromachining technology has attracted particular attention because it can reduce cost and can be driven with low power. Such a thermal air flow meter using a conventional semiconductor substrate is disclosed in, for example,
0-142268 and JP-A-7-174600
There is a technique described in Japanese Patent Publication No. According to the technology disclosed in the above publication, the manufacturing cost is reduced to some extent.

[0003]

However, the above-mentioned prior art has a narrow measurement range when measuring the flow rate.
There were problems such as high noise and insufficient dust resistance reliability. Hereinafter, this will be described in detail.

FIG. 11 is a plan view of a measuring element of a conventional thermal air flow meter, which is FIG. 2 described in JP-A-60-142268, which will be described with reference to FIG.
In the figure, reference numeral 1 denotes a measuring element of a thermal air flow meter, and two bridges 24a made of an electric insulating film bridging cavities (23a, 23b, 23c) formed by anisotropic etching of a semiconductor substrate such as silicon. 24b, the upstream side of the air flow is the bridge 24a, and the downstream side is the bridge 24b. The heating resistors 4 are arranged with an open cavity 23c between the two bridges 24a and 24b, and these bridges 24a and 24b are respectively connected to the temperature measuring resistors 5 and 7 on the sides of the heating resistor 4. And an air temperature measuring resistor 8 for measuring the air temperature is provided in a part of the electrical insulating film on the upstream side of the cavity 23a. Further, since the cavities 23a, 23b and 23c are anisotropically etched on the semiconductor substrate using the openings of the electric insulating film, they are continuous and integrated under the bridges 24a and 24b of the electric insulating film. .

[0005] In such an air flow meter, the heating resistor 4 is driven to be heated to a temperature higher than the air temperature determined by the air temperature measuring resistor 8 by a certain temperature. The air flow rate is measured from the temperature difference generated between the upstream temperature measuring resistor 5 and the downstream temperature measuring resistor 7 of the flow channel by utilizing the heat transport effect of air.

FIG. 12 is a view for explaining another conventional thermal air flow meter, which is disclosed in Japanese Patent Application Laid-Open No. 7-174600.
It is a top view of the measuring element of 3. The measuring element 1 is provided on the semiconductor substrate 2 with the electric insulating film 11 in the same manner as in the conventional example.
Is formed, a part of the electrical insulating film 11 is etched by a known photolithography technique, and further, the semiconductor substrate 2 in the etched portion is anisotropically etched to form the cavities 23d and 2d.
3e is formed. The cavities 23d and 23e form a continuous integral cavity under the bridge 24 of the electric insulating film, as in the above-described conventional example. In this conventional example, the heating resistor 4 and the temperature measuring resistor 5 are disposed on the bridge 24 in the vicinity of the upstream side of the air flow, and the air temperature measuring resistor 8 is located at Installed upstream.

In such an air flow meter, the heating resistor 4 is heated (indirectly heated) so that the temperature of the temperature measuring resistor 5 becomes higher than the air temperature detected by the air temperature measuring resistor 8 by a certain temperature. The air flow rate is measured from a heating current flowing through the heating resistor 4 that indirectly heats the temperature measuring resistor 5 that is cooled as the air flow rate increases.

In the measuring device shown in FIG. 11, the item (001) of the specification of Japanese Patent Application Laid-Open No. 7-174600 is referred to.
As described in 2), there is a problem in that the output change is small in a large air flow rate region, the linearity is deteriorated, and the measurable flow speed range is narrowed. FIG. 12 shows the improvement.
However, in this conventional example, there is a problem that the direction of air flow cannot be detected. Furthermore,
As a problem common to both the conventional examples of FIGS.
A large effective output noise due to a small detection effective area in contact with the airflow which is important in measuring the airflow (the area in contact with the airflow of the resistance temperature detectors 5 and 7 in this conventional example);
Further, the cavities 23a and 23a are provided on the surface of the measuring element in contact with the air flow.
3b, 23c, 23d, and 23e are open, and when used under severe conditions such as an automobile, dust and the like accumulate in the openings, and highly reliable measurement cannot be performed over a long period of time. .

Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art and to provide a thermal air flow meter having a wide flow velocity range for flow measurement, a small output noise, and a high dust-proof reliability. .

[0010]

In order to achieve the above object, a thermal air flow meter has a central temperature measuring resistor between a temperature measuring resistor upstream and a temperature measuring resistor downstream of a fluid to be measured. A measuring element having a side-by-side temperature measuring resistor group and a heat-generating resistor arranged in close proximity to and surrounding the temperature-measuring resistor group; and the heat-generating resistor so that the temperature of the central temperature measuring resistor becomes constant. Heating control means for controlling the heating of the fluid, and flow rate measuring means for detecting the flow direction and the flow rate of the fluid to be measured based on the temperature difference between the upstream temperature measuring resistor and the downstream temperature measuring resistor. It is a thing.

Another feature of the thermal air flow meter according to the present invention is that a temperature measuring resistor upstream and downstream of a fluid to be measured is provided on the electric insulating film of a semiconductor substrate whose upper surface is covered with the electric insulating film. A temperature measuring resistor group in which a central temperature measuring resistor is juxtaposed between the temperature measuring resistor, a heating resistor surrounding and surrounding the temperature measuring resistor group, A measuring element formed with an air temperature measuring resistor disposed at a position separated from the heating resistor, and a temperature difference between the central temperature measuring resistor and the air temperature measuring resistor is kept constant. Heating control means for controlling a heating current flowing through the heating resistor, and detecting a flow direction and a flow rate of the fluid to be measured based on a temperature difference between the upstream temperature measuring resistor and the downstream temperature measuring resistor. And a flow rate measuring means.

According to the present invention, the temperature measuring resistors on the upstream and downstream sides are closely surrounded by the heating resistors and disposed so as to detect a large temperature difference between the two temperature measuring resistors. The range and noise immunity are improved.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a plan view showing a measuring element according to a first embodiment of the present invention. 1 shows a measuring element 1 of a first embodiment for a thermal air flow meter. FIG. 2 is a diagram showing a cross section taken along line AA ′ of FIG.
1 and 2, a measuring element 1 includes a semiconductor substrate 2 made of silicon or the like, an electric insulating film 11a formed on the upper surface of the semiconductor substrate 2 to cover an upper surface of a cavity 3 described later, Insulating film 11a
The heating resistors 4a and 4b formed above, the temperature measuring resistors 5, 6, 7, an air temperature measuring resistor 8 for measuring the air temperature, and a signal for drawing out signals from each resistor to an external circuit. It is composed of a plurality of terminal electrodes 9 and an electric insulating film 11b formed so as to protect each resistor, the terminal electrodes 9, and the like.

The pair of resistance temperature detectors 5 and 7 are
The heating resistor 4 (4a, 4b) is surrounded by the heating resistor 4 on the upstream side with respect to the air flow 10; the heating resistor 4a on the upstream side; The heating resistors 4b are arranged in the vicinity of the downstream heating resistors 4b. The resistance temperature detector 6 is disposed between the resistance temperature detectors 5 and 7. Therefore, from the upstream side of the airflow 10, the heating resistor 4
a, the temperature measuring resistor 5, the temperature measuring resistor 6, the temperature measuring resistor 7, and the heating resistor 4b are arranged adjacently in this order.

In other words, the measuring element 1 is connected to the airflow 10 as the fluid to be measured by the temperature measuring resistor 5 as the upstream temperature measuring resistor and the downstream temperature measuring resistor 5 located on the downstream side. A resistance thermometer group in which a resistance thermometer 6 as a central resistance thermometer is juxtaposed with a resistance thermometer 7 as a resistance thermometer
It can be said that there are (resistance temperature element 5, temperature resistance resistor 6, temperature resistance resistor 7), and the heating resistor 4 (heating resistor 4a, 4b) in which the temperature sensing resistor group is arranged in close proximity. . The number of the temperature measuring resistors 5, 6, 7 and the number of the heating resistors 4a and 4b are one.
It is not limited to individual items.

In addition, a portion of the electric insulating film 11a on which the temperature measuring resistors 5, 6, 7 and the heating resistors 4a and 4b are formed on the semiconductor substrate 2 is provided on the lower surface side of the semiconductor substrate 2 (the opening 3).
A cavity 3 is provided by drilling a hole from a) to the boundary surface of the electric insulating film 11a by anisotropic etching. Further, the heat generating resistors 4a and 4b which are arranged separately on the upstream side and the downstream side are connected in series and connected to two terminal electrodes 9 (,). Then, the heating resistor 4 (4a, 4a, 4a, 4b) is arranged so that the temperature of the temperature measuring resistor 6 disposed at the center becomes higher than the temperature of the air temperature measuring resistor 8 disposed at the end of the flow path of the air flow 10 by a certain temperature. In 4b), a heating (indirect heat) current is flowing from the terminal electrodes 9 (,).

On the other hand, the air flow rate and the direction of the air flow 10 are determined by the upstream portion 4a and the downstream portion 4b arranged close to the upstream portion 4a and the downstream portion 4b of the heating resistor 4.
Is detected by each temperature (each resistance value corresponding to the temperature) of the pair of temperature measuring resistors 5 and 7 for measuring the temperature. That is, when the airflow is zero, the temperature of the resistance temperature detectors 5 and 7 is substantially the same as the temperature of the resistance temperature element 6, and no temperature difference occurs. However, when the air flow 10 is in the forward flow direction shown in FIG. 1, the cooling effect of the air flow 10 is mainly larger in the upstream portion 4a of the heating resistor 4 than in the downstream portion 4b. The temperature of the resistance bulb 5 close to the upstream part 4a of the sensor element is changed to the resistance of the resistance bulb 7 close to the downstream part 4b.
Is lower than the temperature of.

On the other hand, when the air flow 10 is in the reverse flow direction opposite to that in FIG. 1, the temperature of the temperature measuring resistor 7 in the downstream portion 4b is lower than the temperature of the temperature measuring resistor 5 in the upstream portion 4a. . As described above, the air flow rate utilizes the fact that the temperature difference (resistance difference) between the above-mentioned resistance temperature detectors 5 and 7 is proportional to the air flow rate.
It can be measured from the magnitude of the temperature difference. Then, the direction of the air flow 10 can be detected from the magnitude comparison of the temperatures (resistance values) of the resistance temperature detectors 5 and 7.

On the electric insulating film 11a on the cavity 3, the heating resistors 4a and 4b are formed so as to surround the temperature measuring resistors 5, 6, and 7, respectively. The area of 4 (4a, 4b) is wider (effective detection area) than the area of one heating resistor in the conventional example, so that a large air flow signal can be taken out and the dynamic range is wide. I can take it. In addition, since the output becomes an average with respect to the local turbulence of the air flow, in other words, the noise ratio (N / S) to the output becomes small, so that the configuration is strong against noise.

Further, as shown in FIG. 1, the air temperature measuring resistor 8 includes a temperature measuring resistor group (5, 6,
7) and is disposed at a position separated from the heating resistor 4 so as to protrude into the flow path of the airflow 10 and is heated by the heating resistor 4 regardless of whether the airflow 10 is a forward flow or a reverse flow. It is arranged at a position that is not affected by the air flow, and enables highly accurate measurement of the air flow rate.

FIG. 3 is a plan view showing a measuring element according to a second embodiment of the present invention. 5 shows a measuring element 1 of a second embodiment for a thermal air flow meter. The measuring element 1 of the second embodiment shown in FIG. 3 has substantially the same configuration as the first embodiment of FIG. The difference from the first embodiment is that a part of the heat generating resistor 4 is configured to extend between the resistors of the temperature measuring resistors 5, 6, and 7. With such a configuration, the heating resistors 4 are more densely arranged, so that there is an advantage that the heating of the temperature measuring resistors 5, 6, and 7 can be more effectively performed.

FIG. 4 is a view showing a cross section of a thermal air flow meter according to an embodiment of the present invention, on which the measuring element of FIG. 1 or 3 is mounted. For example, it is a cross-sectional view showing an embodiment of a thermal air flow meter mounted in an intake passage of an internal combustion engine such as an automobile.
As shown in the figure, the thermal air flow meter has a measuring element 1 and a support 1.
4 and an external circuit 15. Then, the measuring element 1 is arranged in the sub-passage 13 inside the intake passage 12. The external circuit 15 is electrically connected to the terminal electrode 9 of the measuring element 1 via the support 14. Here, normally, the intake air as the fluid to be measured flows in the direction indicated by the airflow 10, and the intake air flows in the opposite direction (backflow) to the airflow 10 depending on the conditions of the internal combustion engine.

FIG. 5 is an enlarged view showing the measuring element section of FIG. The measuring element section includes the measuring element 1 and the support 14. 6 and 7 are sectional views taken along the line BB ′ of FIG.
It is a figure which shows C 'cross section. As shown in FIG. 5 to FIG. 7, the measuring element 1 has an air flow
The external circuit 15 is fixed on the supporting body 14b so as to be directly exposed to the external circuit 15 and has a terminal electrode 16 and a signal processing circuit, and is a part of a signal processing circuit formed on an electrically insulating substrate such as alumina. Are also fixed on the support 14b.

The measuring element 1 and the external circuit 15 are electrically connected between the terminal electrode 9 and the terminal electrode 16 by wire bonding with a gold wire 17 or the like, and then the gold wire 17, the terminal electrodes 9, 16 and the external circuit 15 are connected. 15 to protect 15
Is sealed and protected. As shown in FIGS. 6 and 7, the measuring element 1 mounted as described above has the lower surface of the cavity 3 closed by the support 14 b and the upper surface of the cavity 3 closed by the electric insulating film 11, and Almost isolated. Therefore, if the present embodiment as described above is employed, there is no portion where the cavity 3 is open to the airflow 10 as in the conventional example, so that when measuring the airflow rate of the internal combustion engine of an automobile or the like, Dust and the like that cause a problem do not accumulate in the cavity or the opening, and highly reliable measurement can be performed.

In an internal combustion engine of an automobile or the like, the temperature of the intake passage 12 and the support 14 shown in FIG. 4 rises due to the heat of the internal combustion engine.
An error may occur in the measurement of the air flow rate and the temperature characteristics may be deteriorated. On the other hand, in the present embodiment, as shown in FIG. 5, the air temperature measuring resistor 8 is disposed at a position farthest from the support 14 and further protrudes from the support 14. Both sides are exposed to the airflow 10,
Since the heat is sufficiently dissipated, the configuration is excellent in the temperature characteristic which is hardly affected by the temperature rise of the intake passage 12 and the support 14. Further, FIG.
As shown in the section B ′, the support 1
4b has a streamlined tip shape, so that the airflow 10
Can flow even at a position reaching the measuring element 1 without turbulence in the air flow, so that measurement with less noise can be performed.

Next, referring to FIG. 8, FIG. 9 and FIG.
A configuration and operation for detecting the flow direction and the flow rate of the fluid to be measured using the measuring element 1 of the present embodiment will be described. FIG.
FIG. 1 is a diagram showing a heating circuit of one embodiment of a thermal air flow meter according to the present invention. Heating control means for controlling the heating of the heating resistor 4, comprising resistors 4 (4 a, 4 b), 6, 8 of the measuring element 1 shown in FIG. 1 or 3 and an external circuit 15 for signal processing. As a heating circuit. FIG.
FIG. 1 is a diagram showing a measurement circuit of one embodiment of a thermal air flow meter according to the present invention. Measurement as flow rate measuring means for detecting the flow direction and flow rate of the fluid to be measured, comprising the resistors 4 (4a, 4b), 5, 7 of the measuring element 1 shown in FIG. The circuit is shown. In the figure, 18a and 18b are differential amplifiers, and 19 is a heating resistor 4
, A transistor for passing a heating (indirectly heated) current, a power supply 20, and resistors 21a, 21b, 21c, 21d, and 21e.

In FIG. 8, a bridge circuit composed of the resistance temperature detector 6, the air temperature resistance resistance 8, and the resistances 21b and 21c is connected to the temperature (resistance value) of the resistance temperature detector 6 located at the center of the measurement element 1. ) Is the air temperature measuring resistor 8 corresponding to the air temperature
Each resistance value is set so as to be higher by a certain value (for example, 150 ° C.) than the temperature (resistance value). In other words, when the temperature of the resistance bulb 6 is lower than the set value, a difference occurs between the potentials A and B at the middle point of the bridge circuit, and the transistor 19 is turned on by the output C of the differential amplifier 18a. Then, a heating current flows through the heating resistor 4. When the temperature of the resistance bulb 6 heated by the heating resistor 4 reaches the set value, the transistor 19 is turned off by the output C of the differential amplifier 18a, and the heating current is cut off. Thus, the temperature of the resistance bulb 6 is set to the set value.
Feedback heating control is performed so as to be (constant).

Next, the air flow rate and the direction are closely surrounded (arranged) by the upstream portion 4a and the downstream portion 4b of the heating resistor 4.
It is detected from the temperature (resistance value) of the measured resistance elements 5 and 7. In FIG. 9, the measuring circuit is an example of an output circuit that forms a bridge circuit with the resistance temperature detectors 5 and 7 and the resistances 21 d and 21 e and extracts a resistance value difference (ΔR) between the resistance temperature detectors 5 and 7. is there. The temperature measuring resistor 5 is set at a temperature (resistance value) corresponding to the upstream portion 4 a of the heating resistor 4.
Is a temperature (resistance value) corresponding to the downstream portion 4b. Here, the differential amplifier 18b has a potential D corresponding to the temperature of the upstream portion 4a and the temperature measuring resistor 5 and a potential D corresponding to the temperature of the downstream portion 4a.
b and a potential E corresponding to the temperature of the resistance bulb 7 are input, and F is output as the potential difference. And the potential difference F
Is measured from the absolute value of, and the direction of the air flow is detected from the positive or negative of the potential difference F.

Further, the operation for detecting the flow direction and the flow rate of the fluid to be measured will be described in detail. FIG. 10 is a diagram showing a cross section AA ′ of the measuring element of FIG. 1 and an operation principle.
The figure shows the heating resistor 4 (4a, 4b) and the temperature measuring resistors 5, 6,
7 schematically shows the temperature distribution of No. 7 when the air flow is a forward flow and a reverse flow. In the figure, as described above, the resistance bulb 6 is set to a certain reference temperature by the heating circuit of FIG. When the air flow is a forward flow, the temperature of the temperature measuring resistor 5 disposed close to the upstream portion 4a is lowered since the upstream portion 4a receives more heat by the air flow.
On the other hand, when the air flow is the reverse flow, the temperature of the resistance temperature detector 7 is lowered.

That is, as shown in FIG.
By comparing the temperatures (resistance values) of the resistance temperature detectors 5 and 7 disposed in close proximity to a and 4b, for example, “resistance value of resistance temperature sensor 7−resistance value of resistance temperature sensor 5 = positive value” ΔT = forward flow ”and“ resistance value of resistance temperature detector 7−resistance value of resistance temperature detector 5 =
Negative ΔT = backflow ”. The direction of the air flow can be detected based on the sign. The air flow rate is
It can be measured from the absolute value of the temperature difference between the resistance temperature detectors 5 and 7. Therefore, the measurement circuit determines that the temperature difference (ΔT) between the resistance temperature detectors 5 and 7 is equal to the resistance value difference (ΔR),
(ΔR) is output.

By the way, as shown in FIG. 10, the upstream and downstream temperature measuring resistors 5 and 7 are sandwiched between the heating resistors 4a and 4b distributed to the upstream and downstream, respectively. With the configuration of the present invention in which the temperature of the resistor 6 is set to the reference temperature, the temperature difference ((Δ
T)) can be detected with a sufficiently large value. On the other hand, in the conventional example shown in FIG. 11, the central heating resistor is sandwiched between the left and right temperature measuring resistors, and the central heating resistor is set to the reference temperature. As in the temperature distribution of the conventional example shown in FIG. 10, both the temperatures of the right and left temperature measuring resistors are both lower than the central reference temperature, and a large temperature difference cannot be detected as compared with the present invention. In other words, it can be said that the configuration of the present invention can detect a large temperature difference as compared with the conventional example, so that the signal level becomes large, a wide dynamic range can be obtained, and the noise immunity is advantageous.

Next, a specific example of the measuring element of the thermal air flow meter according to the present invention will be described with reference to FIGS.
First, an electric insulating film 1 is formed on a semiconductor substrate 2 made of silicon.
As 1a, about 0.
A film of silicon dioxide, silicon nitride or the like having a thickness of 5 microns is formed. Further, as the resistors 4, 5, 6, 7, and 8, platinum having a thickness of about 0.2 micron is formed by a method such as sputtering. And by the known photolithography technology,
After a resist is formed in a predetermined shape, platinum is patterned by a method such as ion milling.

Next, after the terminal electrode 9 is formed by gold plating or the like, an electric insulator 11b having a thickness of about 0.5 μm is formed in the same manner as described above, using a portion other than the terminal electrode 9 as a protective film. Finally, the cavity 3 is formed by anisotropic etching from the back surface of the semiconductor substrate 2 using silicon dioxide or the like as a mask material, and cut into chips to obtain the measuring element 1.
Is obtained.

Here, the electric insulating film 11 on the cavity 3 has a detection effective area (about 0.2 mm × 1 m) of the conventional example shown in FIG.
m: specification (0028) of JP-A-7-174600
In the present embodiment shown in FIG.
The detection effective area was mx 1.5 mm, which was about 10 times the size of the conventional example. Further, since the heating resistor 4 is formed so as to surround the temperature measuring resistors 5, 6, and 7, the occupied area of the heating resistor can be increased. As a result, the dynamic range and the noise resistance of the air flow rate signals of the resistance temperature detectors 5 and 7 on the cavity 3 are greatly improved as compared with the conventional example.

Even when the cavity 3 is enlarged as described above,
The size of the measuring element 1 of this embodiment is about 2.5 mm × 5 mm
Then, a conventional example (about 3 mm × 3 mm: JP-A-7-17460)
This is only about 1.4 times that described in the specification (0028) of Japanese Patent Publication No. 0 (1999), and the reliability of dust resistance is improved because there is no opening exposed to the air flow 10 as in the conventional example.

[0036]

According to the present invention, a temperature measuring resistor group in which the temperature measuring resistor 6 is juxtaposed between the temperature measuring resistor 5 and the temperature measuring resistor 7 and the temperature measuring resistor group is closely surrounded. Heating resistor 4
A thermal air flow meter that measures the direction and flow rate of air flow from the temperature difference between the two resistance temperature detectors 5 and 7 to improve the dynamic range and noise immunity when measuring the air flow rate Can be provided.

Also, since there is no opening to the air flow 10, the reliability of dust resistance is improved, and the air temperature measuring resistor 8 is projected into the air flow. Accordingly, there is an effect that a thermal air flowmeter with improved temperature characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a plan view showing a measuring element of a first embodiment according to the present invention.

FIG. 2 is a diagram showing a cross section taken along line A-A 'of FIG.

FIG. 3 is a plan view showing a measuring element according to a second embodiment of the present invention.

FIG. 4 is a diagram showing a cross section of a thermal air flow meter according to an embodiment of the present invention, on which the measuring element of FIG. 1 or 3 is mounted.

FIG. 5 is an enlarged view of a measuring element unit in FIG. 4;

6 is a diagram showing a cross section taken along line B-B 'of FIG.

FIG. 7 is a view showing a cross section taken along line C-C ′ of FIG. 5;

FIG. 8 is a diagram showing a heating circuit of a thermal air flow meter according to an embodiment of the present invention.

FIG. 9 is a diagram showing a measurement circuit of the thermal air flow meter of one embodiment according to the present invention.

FIG. 10 is a diagram showing an AA ′ cross section and an operation principle of the measuring element of FIG. 1;

FIG. 11 is a plan view illustrating a measuring element of a conventional thermal air flow meter.

FIG. 12 is a plan view illustrating a measuring element of another conventional thermal air flow meter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Measurement element, 2 ... Semiconductor substrate, 3 ... Cavity, 4,4a,
4b: heating resistor, 5, 6, 7 ... temperature measuring resistor, 8: air temperature measuring resistor, 9, 16 ... terminal electrode, 10: air flow,
11, 11a, 11b: electric insulating film, 12: intake main passage, 13: sub passage, 14, 14a, 14b: support, 1
5: external circuit, 17: gold wire, 18a, 18b: differential amplifier, 19: transistor, 20: power supply, 21a, 21
b, 21c, 21d, 21e... resistors, 23a, 23b,
23c, 23d, 23e ... hollow, 24, 24a, 24b
…bridge

Claims (4)

[Claims] 1. A temperature measuring resistor group in which a central temperature measuring resistor is juxtaposed between a temperature measuring resistor on an upstream side and a temperature measuring resistor on a downstream side of a fluid to be measured, and a group of the temperature measuring resistors. A measuring element having a heating resistor disposed in close proximity to the heating element, heating control means for controlling heating of the heating resistor so that the temperature of the central temperature measuring resistor becomes a constant value, and the upstream temperature measuring resistor A thermal air flow meter, comprising: flow rate measuring means for detecting a flow direction and a flow rate of the fluid to be measured based on a temperature difference between a body and the downstream resistance temperature detector. 2. The heating element according to claim 1, wherein the measuring element has an air temperature measuring resistor disposed at a position separated from the temperature measuring resistor group and the heating resistor. A thermal air flowmeter, wherein a heating current flowing through the heating resistor is controlled so that the temperature of the central temperature measuring resistor becomes higher than the temperature of the air temperature measuring resistor by a constant value. 3. A central temperature measuring resistor is provided between an upstream temperature measuring resistor and a downstream temperature measuring resistor of a fluid to be measured, on the electric insulating film of a semiconductor substrate having an upper surface covered with an electric insulating film. A group of juxtaposed temperature measuring resistors, a heating resistor arranged so as to closely surround the temperature measuring resistor group, and an air temperature sensor arranged at a position separated from the temperature measuring resistor group and the heating resistor. A heating element for forming a heating resistor; and a heating control for controlling a heating current flowing through the heating resistor so as to keep a temperature difference between the central temperature measuring resistor and the air temperature measuring resistor constant. Means, and a flow rate measuring means for detecting a flow direction and a flow rate of the fluid to be measured based on a temperature difference between the upstream temperature measuring resistor and the downstream temperature measuring resistor. Type air flow meter. 4. The temperature measuring resistor group and the heating resistor according to claim 3, wherein the semiconductor substrate has a cavity penetrating from the upper surface to the lower surface covered with the electrical insulating film. A thermal air flowmeter formed at a position of the electric insulating film on the cavity.