JPH0755522A - Method and instrument for measuring flow rate - Google Patents

Abstract

PURPOSE:To measure the flow rate of a fluid to be measured with high accuracy and high responsiveness without inviting any temperature error nor being affected by the fitting state of a detector. CONSTITUTION:A cavity section 3 is formed in a semiconductor substrate 1 by leaving an insulating film which is made longer in the direction perpendicular to the flowing direction of a fluid to be measured on the section 3 in a micro- bridge state and a single resistor 4 composed of a zigzag metallic film is formed on the micro-bridge 7. The resistor 4 is made to generate heat by making a large current to flow to the resistor 4 and the resistance value of the resistor 4 is detected after making a small current to flow to the resistor 4 and detecting the resistance value of the resistor 4. Then the flow rate of the fluid to be measured is measured from the difference between both resistance values.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flow rate detector, and more particularly, to the fact that the resistance value of a heated resistor disposed in a fluid flow changes depending on the temperature and the flow velocity of the fluid. The present invention relates to a flow rate detector suitable for application to the above-mentioned thermal type flow meter.

[0002]

2. Description of the Related Art A thermal type flow meter, which has no moving parts and is widely used as a small mass flow meter, has particularly excellent responsiveness. Therefore, recently, an air flow meter for optimizing an air-fuel ratio of an internal combustion engine is used. It is attracting attention for controlling flow rate. The temperature of the resistor heated by the constant power is a temperature at which the amount of heat generated by the heating power and the amount of heat released by the fluid flow are balanced, and the amount of heat released is a function of the flow velocity of the fluid, the specific heat of constant pressure, and the density. The thermal type flow meter is made on the basis of this principle, and heats a resistor arranged in a fluid flow with a constant electric power, and the resistance value of the resistor corresponds to the temperature of the fluid. The mass flow rate of the fluid is calculated by subtracting the resistance value and removing the temperature effect of the fluid.

26 (a) and 26 (b) are views showing the structure of a conventional thermal type flow rate detector, FIG. 26 (a) is a plan view, and FIG.
FIG. 26B is a sectional view taken along the line BB ′ of FIG. 26A, in which a silicon nitride insulating film is formed on a silicon single crystal substrate 81 having a crystal plane of (100), and then the microbridges 82 a and 82 are formed.
A cavity 83 is formed by anisotropic etching while leaving b and 82c, and a permalloy or platinum-based zigzag metal film is deposited on the microbridges 82a, 82b and 82c to form resistors 84, 85 and 86. The central resistor 85 is used as a heater, and resistors 84 and 86 of the same shape having the same resistance value are arranged on both sides, and these resistors 84 and 86 are used as a temperature sensor and a heat receiving sensor, respectively.

The heater 85 is heated at a constant power, and is affected by the heat of the heater 85, and the resistance values of the temperature sensor 84 and the heat receiving sensor 86 on both sides increase. When there is no flow of the fluid to be measured, for example, gas, the temperature sensor 84 and the heat receiving sensor 86 have the same amount of increase in resistance value due to the temperature rise, but the gas moves along the substrate 81 in the direction of the white arrow (FLOW direction). When flowing, the temperature sensor 84 is radiatively cooled by the gas flow and its resistance value is reduced, while the heat receiving sensor 86 is heated by the heat of the heater 85 and heated to increase its resistance value. The difference between the resistance values of the temperature sensor 84 and the heat receiving sensor 86 is a function of the mass flow rate of gas.

FIG. 27 is a perspective view of a flow sensor disclosed in Japanese Unexamined Patent Publication No. 4-72523. The flow sensor has openings 92 and 93 which are opened left and right in a central portion of a semiconductor substrate 91 by anisotropic etching. And a thin film microbridge 95 spatially isolated in a bridge shape is formed on the upper part of the hollow portion 94, and a thin film heater 96 and the heater 9 are formed on the microbridge 95.
The temperature measuring resistors 97 and 98 sandwiching 6 from both sides are arranged, and the temperature measuring resistor 99 for measuring the ambient temperature of the thin film is arranged at the upstream corner of the semiconductor substrate 91. The upper surface of each resistor is covered with an insulating film.

The flow sensor shown in FIG. 27 has a cavity 9
4 A heater 96 is attached to the micro bridge 95 formed on the upper surface.
Resistance temperature detectors 97 and 98 are formed so as to sandwich the heater 96, and the resistance value of the ambient temperature resistance detector 99 is used as a reference to measure the temperature when the heater 96 is heated to a constant temperature and gas flows. The flow rate or flow rate of the fluid to be measured is calculated by obtaining the change in resistance value due to heat radiation and heating of the temperature resistors 97 and 98 as a voltage value.

[0007]

As described above, in the conventional thermal mass flowmeter, the conduction of heat generated by the heater (heating element) made of a resistor is arranged at a predetermined distance from the heating element. A method of detecting with a temperature measuring resistor (heat receiving body) is adopted. Therefore, a pair of heat-generating bodies and heat-receiving bodies having the same size, specific heat and heat capacity as those of the heat-generating bodies are always required, which causes the following problems.

[1] Variation in resistance value between heating element and heat receiving element: (1) Variation in temperature of heating element itself, that is, variation in resistance value of the heating element itself, and variation in temperature of heat receiving element itself, that is, variation in resistance value of the heating element itself. It is necessary to adjust with high precision by an electric circuit corresponding to each. (2) It is necessary that the temperature balance between the heating element and the heat receiving element, that is, the combination of the balance of the resistance values, is highly accurate. However, generally, the heating element and the heat receiving element are provided on one substrate. Since they are combined by using the fine processing process of the semiconductor integrated circuit, they are combined with relatively high accuracy. However, if the resistance value is not well balanced, both the heating element and the heat receiving element will be useless. In the conventional coil-type structure, different heating elements and heat receiving elements are combined, so it is possible to select and combine well-balanced ones.
This coil method is less wasteful than the one which is integrally combined in advance as described above.

(3) It is difficult to design the positions and distances between the heat generating element and the heat receiving element, which limits accuracy. (4) Since there is a distance between the heat generating element and the heat receiving element, the resistance difference cannot be accurately detected when detecting a minute flow rate. (5) The temperature distribution accuracy of each of the heating element and the heat receiving element needs to be high. If the temperature distribution has a variation, it affects the detection output signal, so that circuit adjustment or the like is necessary according to each sensor. (6) As the number of heating elements and heat receiving elements increases, the size of the substrate and the number of extraction wirings increase. If this is installed in the flow of the fluid to be measured, the flow itself is obstructed, an accurate measured value cannot be obtained, and the cost is increased. In addition, the accuracy of detecting the minute flow rate becomes low. (7) If the number of heating elements / heat receiving elements is large, the power consumption increases. (8) If the heat generating element and the heat receiving element are arranged vertically, the heat receiving element is in the upper position and the heat generating element is in the lower position, and the gas flow is from bottom to top, even if the flow is stopped, due to the upward flow caused by the heat generating element. Output is generated in the heat receiver, so it is necessary to consider the mounting posture, etc., and the installation location is limited.

[2] About ambient temperature compensation when detecting gas flow rate: (1) One set of three microbridge resistors is used, the temperature of the gas is detected by the leading resistor with respect to the flow, and the center is used. When the flow rate is detected by the heating element and the heat receiving element of the rear part, and the temperature variation of the flow rate detection is corrected by the temperature information of the leading resistor (FIG. 17).
The example) further aggravates the disadvantages (1) to (8) described above regarding the variation in the resistance values of the heat generating element and the heat receiving element in [1]. (2) When the resistor pattern for detecting the ambient temperature is arranged on the semiconductor substrate (example in FIG. 18), even if the ambient temperature, that is, the gas temperature changes suddenly, the heat capacity of the semiconductor substrate is large. Since the resistance value of the detection resistor slowly responds and changes according to the temperature change of the semiconductor substrate, the accurate temperature of the gas to be measured at the time of measurement is not detected, and highly accurate flow rate detection is impossible. is there.

[0011]

In order to solve the above-mentioned problems, the present invention (1) inserts a single resistor into a fluid to be measured and does not heat the resistor to the resistor. A small current is passed to measure the resistance value of the resistor, then a large current is passed to the resistor to heat the resistor, the resistance value at the time of heating is measured, and the resistance value at the time of heating is calculated from The resistance value when not heated is calculated and subtracted to measure the flow rate of the fluid to be measured, or (2) a single resistor is inserted into the fluid to be measured and the current gradually increases in the resistor. And a resistance value at a small current such that the resistor is not heated by the current and a resistance value at a large current at which the resistor is heated are measured. Measuring the flow rate of the measurement fluid by calculating and subtracting the resistance value of
Alternatively, (3) a cavity formed on the substrate, an insulating thin film bridging a partial upper surface of the cavity, a single resistor provided on the insulating thin film, and a resistor A means for detecting a resistance value of the resistor by applying a small current or a small voltage so as not to heat the resistor, and a resistor for supplying a large current or a large voltage to the resistor. A means for heating the body at a high temperature and detecting a resistance value at the time of the high temperature heating; and a means for calculating and subtracting the resistance value at a small current from the resistance value at a large current of the resistor, the subtracted value Detecting the flow rate of the fluid to be measured, further comprising (4) in (3), a flow rate measurement period in which a small current and a large current subsequent to the small current flow in the resistor; The measurement pause period in which no electric current is applied to the body is repeatedly provided, and the measurement pause period is the temperature of the resistor as the temperature of the fluid to be measured. Or more, the voltage or current applied to the resistor during the flow rate measurement period in (5) above (3) or (4) is a low peak value voltage or current. A pulse and a voltage or current pulse having a high peak value output immediately after the voltage or current pulse having a low peak value, and (6) applying to the resistor in (3) or (4) above. The voltage or current to be applied is a sawtooth voltage or current, or (7) two resistors are inserted in the fluid to be measured, and one of the resistors has a small current or a current that does not heat the resistor. A small voltage is simultaneously applied to the other resistor with a large current or large voltage for heating the resistor, the resistance values of both resistors are measured at the same time, the two resistance values are calculated to obtain the difference, The flow rate of the fluid to be measured is measured from this difference, or (8 A cavity formed on the substrate, an insulating thin film bridging a part of the upper surface of the cavity, two resistors provided on the insulating thin film, and one resistor heating the resistor. Means for simultaneously applying to the other resistor a large current or large voltage for heating the resistor, and a means for calculating and subtracting the resistance values of both resistors. And detecting the flow rate of the fluid to be measured based on the subtracted value, or (9) two insulating thin films bridging a cavity formed on the substrate and a partial upper surface of the cavity. And a resistor provided in each of the insulating thin films, and a small current or a small voltage that does not heat the resistor to one resistor and a large amount that heats the resistor to the other resistor. A means for simultaneously applying a current or a large voltage and a means for calculating and subtracting the resistance values of both resistors are provided. Detecting the flow rate of the fluid to be measured based on a value,
Further, (10) in (8) or (9), a clock pulse generating circuit that gives a timing for driving the two resistors at the same time, and a small current flowing in one of the resistors in synchronization with the clock pulse or A small power pulse generating circuit for applying a small voltage, a large power pulse generating circuit for applying a large current or a large voltage to the other resistor, and a resistance value of the resistor to which the small power pulse is applied and a large power pulse Means for comparing and calculating the resistance value of the resistor, and measuring the flow rate of the fluid to be measured based on the comparison result, and (11) in (8) or (9) above. , A small power pulse and a large power pulse are alternately and repeatedly applied to the one resistor in synchronization with the clock pulse, and a large power pulse and a small current pulse of the other resistor are alternately and repeatedly applied. It is characterized in that the.

[0012]

A resistor is arranged in the flow of the fluid to be measured, a small current or a small voltage is applied to the resistor to detect the ambient temperature from the resistance value, and then a large current or a large voltage is applied. By applying and heating, the ambient temperature and the flow rate of the fluid to be measured are detected from the resistance value, and then the flow rate of the fluid to be measured is measured by calculating the difference between both resistance values.

1 (a) and 1 (b) are structural views for explaining an example of a flow rate detector according to the present invention. FIG. 1 (a) is a plan view and FIG. 1 (b) is shown in FIG. In the sectional view taken along the line BB ′ of a), in the figure, 1 is a substrate, 2 and 10 are insulating films, 3 is a cavity, 4 is a resistor, 5 and 6 are terminals, 7 is a microbridge, and 8 and 9 are. Is a support.

In FIGS. 1A and 1B, the substrate 1 is
It is a silicon single crystal plate in the shape of a square plate whose crystal plane is (100),
An insulating film 2 of silicon nitride is formed on the surface.
Anisotropic edging is applied to the substrate 1 on the diagonal line perpendicular to the white arrow Q direction while leaving the microbridge 7 of the insulating film 2 having a predetermined width, and the crystal plane (100) is formed on the diagonal line of the white arrow Q direction. A cavity 3 having a predetermined depth parallel to the above is formed.
Platinum or permalloy (for example, Ni: 80%, Fe: 20%) is formed in a zigzag shape on the upper surface of the microbridge 7, and the thin film resistor 4 is vapor-deposited, and the terminals 5 and 6 are arranged. Further, an insulating film 10 of silicon nitride is formed on the upper surface of the thin film resistor 4.

2 (a) and 2 (b) are views for explaining the operating principle of the flow rate detector according to the present invention.
1A, when a fluid at 30 ° C. flows in the direction of the white arrow Q, a current is passed through the resistor 4 to heat the resistor 4 and the contact between both ends of the resistor 4 is shown. The voltage characteristics A ₁ and A ₂ are shown. The flow rate characteristic A ₁ is that the flow rate is 0 liter / hour,
The flow rate characteristic A ₂ is for a flow rate of 2 liters / hour. FIG. 2B shows the temperature characteristic of the resistor 4 when there is no flow, that is, when the flow rate is zero, and the temperature characteristic B ₁ is the fluid temperature. Is 20 ° C
In the case of, the temperature characteristics B ₂ and B ₃ are 30 ° C and 4 ° C respectively when the fluid temperature is 30 ° C.
This is for 0 ° C.

As shown in FIG. 2A, when a resistance value of the resistor 4 is measured by passing a small current of, for example, 2 mA or less (to the extent that the resistor 4 is not heated) to the resistor 4, the resistance value of the resistor 4 is measured. Four
Is hardly heated by this measured current, the output voltage across the resistor 4 changes depending on the temperature of the fluid, but does not change even if the flow rate changes. That is, the curves of the flow rate characteristics A ₁ and A ₂ having different flow rates show that they are irrelevant to the flow rate in this low current range and only to the fluid temperature.

However, when the current flowing through the resistor 4 is increased, for example, when a large current of 8 mA is applied, the resistor 4 is heated. Therefore, when the fluid is flowing, the increase in the resistance value is reduced accordingly. The flow rate characteristic curves A ₁ and A ₂ are separated from each other and show a resistance value depending on the flow rate. That is, when heated at a high temperature of 8 mA, the output voltage is about 3 V in the flow characteristic curve A ₂ .
At A ₁ , the output voltage is 4 V, and the larger the flow rate is, the larger the heat radiation amount is and the smaller the increase in the resistance value is.

On the other hand, when the resistor 4 is arranged in a fluid that does not flow, as shown in the temperature characteristic curve shown in FIG.
When the resistance value of the resistor 4 is measured by passing a small current (a small current that does not cause the resistor to self-heat) to the resistor 4, the resistance value of the resistor 4 is the fluid temperature (that is, the resistor 4). The higher the temperature), the higher the resistance value, and the higher the output voltage corresponding to the fluid temperature. That is, the resistor 4 at this time is
The characteristic as a temperature detection element is shown.

As described above, from the characteristic curves shown in FIGS. 2 (a) and 2 (b), one resistor arranged in the shape of a microbridge is heated at a low temperature to detect the ambient temperature, that is, the fluid temperature, Then, the amount of heat depending on the fluid temperature and the flow velocity of the fluid is detected by heating at high temperature, and the resistance value (output voltage) at low temperature heating is subtracted from the resistance value (output voltage) of the resistor at high temperature heating. The flow rate of the fluid can be calculated by

3 (a) and 3 (b) are views for explaining another embodiment of the flow rate detector according to the present invention.
3A is a plan view, FIG. 3B is a sectional view taken along line BB ′ of FIG. 3A, in which 11 is a substrate, 12 and 20 are insulating coatings, 13 is a cavity, and 14 is Resistors, 15 and 16 are terminals,
Reference numeral 18 is a cantilever-shaped insulating coating, and 19 is an open end.

In the flow rate detecting element shown in FIGS. 3A and 3B, the square cavity 13 is anisotropically etched so that the sides of the square substrate 11 are parallel to each other, and the open end 19 is formed. A cantilever beam-shaped insulating coating film 18 having a groove is formed on the upper surface of the cavity 13, and the terminal 1 is formed on the cantilever beam-shaped insulating coating film 18.
A thin film zigzag resistor 14 having 5, 5 is formed and further insulation-treated with an insulating film 20.

In the flow rate detector shown in FIGS. 3 (a) and 3 (b), the resistor 14 is arranged on the upper surface of the cavity 13 in a cantilever shape, and the fluid to be measured is indicated by a white arrow (Q). 1 (a) and 1 (b) except for flowing in parallel with the sides of the rectangular substrate 11 in the direction.
It is the same as the flow rate detector shown in.

FIGS. 4 (a) and 4 (b) are views for explaining the drive circuit of the flow rate detector according to the present invention. FIG. 4 (a) is a constant current drive circuit, and FIG. 4 (b) is a constant voltage drive circuit. Drive circuit,
In the figure, 21 is a constant current power supply, 22 is a constant voltage power supply, and 23 is a detection element (corresponding to the resistor 4 shown in FIG. 1 or the resistor 14 shown in FIG. 3).

In the constant current drive circuit of FIG. 4A, the constant current power source 21 and the detection element 23 are connected via a switch SW having a contact point, b contact point and c contact point, and the detection element 23 is connected.
The output voltage Vout at both ends of is detected. When switched to the a-contact, the detection element 23 is connected in parallel with the resistor R,
When it is driven by low temperature heating and is switched to the b contact, it is driven at high temperature and driven when it is switched to the c contact, the circuit is opened and the heating is stopped.

In the constant voltage drive circuit of FIG. 4B, the constant voltage power source 22, the reference resistor R ₀ and the detecting element 23 are a-contacts, and b.
They are connected in series via a switch SW having a contact and a c contact, and the voltage Vout across the reference resistor R ₀ is output. A resistor R or a Zener diode corresponding to the same potential is connected in series to the a-contact to form a low-temperature heating drive circuit, and when switched to the b-contact, it is directly connected without a resistor and driven to high-temperature heating to form a circuit. When it is switched to the contact, it is opened,
Heating is stopped.

FIGS. 5A and 5B are diagrams for explaining the operation when the flow rate detector according to the present invention is pulse-driven. FIG. 5A is a pulse current drive waveform, and FIG. ) Shows an output voltage waveform when driven in FIG.

In FIG. 5A, the switch SW whose base point of the constant current drive circuit of FIG. 4A is d is a contact, b contact, and c.
The relationship between the time when the contacts are switched at a predetermined time interval and the current supplied to the detecting element 23 is shown, and
Is a period in which a small current of 2 mA is supplied in the low temperature heating period, and the following b-d period is 8 in the high temperature heating period.
A period in which a large current of mA is supplied, and the following periods cd are open periods and heating rest periods. FIG. 5B is a diagram showing the output voltage of the detection element 23, and points P and Q show the time when the output voltage becomes stable, that is, the measurement timing.

FIGS. 6A, 6B, 6C and 6D show the operating principle of the flow rate detector according to the present invention when the temperature of the fluid to be measured changes and when the flow rate changes. 6 (a) is a diagram for separately explaining, FIG. 6 (a) is a fluid temperature change on the time axis, FIG. 6 (b) is a flow rate change on the time axis, FIG. 6 (c) is an applied current waveform, and FIG. ) Indicates a detected output voltage waveform corresponding to the change in fluid temperature and the change in flow rate.

The current supplied to the detecting element 23 is as shown in FIG.
As shown in (c), a pulse consisting of a small current of 2 mA and a large current of 8 mA is supplied with a predetermined rest time, and in the case of the illustrated example, this pulse current is the time t.
₀ ~t _1, t ₁ ~t respectively between _2, t ₂ ~t ₃ is output once. On the other hand, as shown in FIG. 6 (a), the fluid temperature is ₂ from the constant temperature of 30 ° C. to the period of time t _{1 to} t 2.
The flow rate changes to 0 ° C or 40 ° C, and the flow rate is as shown in FIG.
As shown in FIG.
A period of ₂ ~t _3, 0 liters / hour, or is intended to change 4 liters / hr of. Therefore, the fluid temperature during a period of time t ₀ ~t _1, does not change the flow rate both changed only fluid temperature during a period of time t ₁ ~t _2, it has changed only flow during the period of time t ₂ ~t ₃ .

As a result, the detected output voltage waveform is shown in FIG.
As shown in (d), the output voltage corresponding to the fluid temperature and the constant flow rate becomes during the period of time t _{0 to} t ₁ , and during the period of time t _{1 to} t ₂ , the fluid temperature change is added to The voltage is, and the subtraction value obtained by subtracting the output voltage at the time of driving the small current from the output voltage at the time of driving the large current is the same and constant, and it can be seen that there is no change in the difference in the detected voltage due to the temperature change. Further, in the period of time t _{2 to} t ₃ ,
The output voltage becomes, according to the flow rate, but the output voltage at the time of small current drive is the same even if the flow rate changes because the fluid temperature does not change, and the output voltage at the time of large current drive is It changes due to the influence of heat dissipation, so if you subtract the output voltage when driving a small current from the output voltage when driving a large current,
It can be seen that the voltage corresponding to the flow rate is detected. actually,
Fluid to be measured, the temperature also flow also changes, t ₁ and the temperature change between ~t _2, it is assumed that the flow rate changes between t ₂ ~t ₃ is combined, the detection voltage (measured at a large current By subtracting the detection voltage (change in temperature of the fluid to be measured) at low current from the change in fluid temperature and the change in flow rate, the change in flow rate of the fluid to be measured can be detected.

FIGS. 7 (a) and 7 (b) are diagrams showing other examples of the drive current waveform and output voltage waveform of the flowmeter according to the present invention. Within a predetermined time width from time t ₁ to t ₂ , Figure 7 (a)
As shown in FIG. 5A, when driven by a triangular wave (sawtooth) current that is proportional to time, an output voltage with a time delay is detected as shown in FIG. 7B. Similar to the measurement timings P and Q, if the resistance value of the resistor is measured during the small current driving and the large current driving at the measurement timings P and Q at which the temperature of the resistor is stable, it is completely different from the above example. The flow rate can be measured in the same manner. Thus, by driving with a triangular constant current source,
Unlike the resistors R and R ₀ of FIG. 1, the driving is performed with only one power source without being affected by the characteristics of the resistors R and R ₀ , and the measurement can be performed with high accuracy. You just need to set Q.

FIGS. 8 (a) and 8 (b) are diagrams showing other current waveforms for driving the detection element of the flowmeter according to the present invention, respectively. FIG. 8 (a) shows that the change rate of the current changes with time. FIG. 8 (b) shows a gradually increasing current waveform having an approximate triangular waveform that is curved in a convex shape that is small, and FIG. 8B is an increasing current waveform that is an approximately triangular waveform that is curved in a concave shape in which the rate of change of the current increases with time. If it is a source, it can be used in exactly the same manner as the case of the triangular wave current drive of FIG.

FIG. 9 is a block diagram showing an example of the drive circuit of the flowmeter according to the present invention. In the figure, 24 is a circuit for supplying a constant current to the sensor (constant current circuit), 25 is a coefficient setting circuit, and 26 is a coefficient setting circuit. Is a hold circuit, 27 is a subtraction circuit, and 28 is an output terminal. The amplifier circuit 24 is loaded with a series resistance of a detection element 23 and a reference resistance R whose one end is grounded, and the voltage of the reference resistance R is an inverting input of a circuit (constant current circuit) 24 that supplies a constant current to the sensor. The switch SW ₁ having a, b and c contacts is connected to the non-inverting input terminal. The reference voltage V is applied to the a contact of the switch SW _1.
V _REF 2, c contact is grounded to _REF 1, b contact, and reference voltages V _REF 1 and V _REF 2 are V _REF 1 = 2R (mV) (1) V _REF 2 = 8R (mV) (2) Is set.

Further, a switch SW ₂ having a and b contacts is connected to the output end of a circuit (constant current circuit) 24 for supplying a constant current to the sensor, and the a contact has low temperature heating (resistor is A coefficient K for converting the ambient temperature when calculating the flow rate when a small current that does not heat is applied) is set, for example, a coefficient setting circuit 25 with variable amplification degree according to K
Is connected to the b-contact, which is connected to one input end of the subtraction circuit 27 and inputs the output voltage V _D when a large current is applied during high temperature heating. A switch SW ₃ having an a contact and a c contact is provided between the coefficient setting circuit 25 and the subtraction circuit 27.
And the hold circuit 26 is connected. Switch S
The a contact of W ₃ is connected to the hold circuit 26.

The switches SW ₁ , SW ₂ and SW ₃ of the drive circuit configured as described above are interlocked, and the a, b and c contacts of each of the switches SW ₁ , SW ₂ and SW ₃ are simultaneously switched by switching. To be When switched to the a-contact, a voltage equal to the reference voltage V _REF 1 is applied to the reference resistor R connected to the inverting input terminal of the amplifier circuit 24, and the detection element 23 receives 2
A constant current of mA flows. Similarly, when switching to b contact,
A voltage equal to the reference voltage V _REF 2 is applied to the reference resistor R, and a constant current of 8 mA flows through the detection element 23.

FIG. 10 is a waveform diagram in each part of the drive circuit shown in FIG. 9, and the operation of the drive circuit will be described below with reference to FIGS. 9 and 10. Switch SW ₁ (SW ₂ , SW ₃
Also driven synchronization) is, FIG. 10 (a), the switched at a period of the driving voltage pulse by a time t ₁ ~t ₂ and time t ₂ ~t ₃ voltage waveform (b), the detection element 23 in accordance with the switching Drive currents of 2 mA and 8 mA shown in FIG. When switched to the a-contact, the voltage V shown in FIG. 10 (d) proportional to the ambient temperature is applied to the a-contact of the switch SW ₂ .
s is output. The voltage Vs is input to the coefficient setting circuit 25 and multiplied by a preset coefficient K, and the voltage V's (= KVs) shown in FIG. 10 (e) is output. K is the voltage V output when the switch SW ₂ is switched to the a contact.
s is, because it is not correctly proportional value to the ambient temperature, a factor for correcting so as to correspond to the voltage Vs and the ambient temperature, K = - is given by (V _D flow variation) / Vs (3). As shown in FIG. 10F, the hold circuit 26 outputs V ″ s (= V ′s) equal to the voltage V ′s. The subtraction circuit 27 receives the voltages V _D and V ″ s. A voltage V proportional to the flow rate shown in FIG. 10 (h) is output to the terminal 28. That is, V = V _D −V ″ s (4) is obtained.

FIG. 11 is a drive circuit block diagram for explaining another embodiment of the flowmeter according to the present invention, in which 29 is a voltage detection circuit, 30 is a current detection circuit, and 31 is a division circuit. In addition, the same reference numerals as those in FIG. 9 are attached to the other portions having the same operations as those in FIG. The drive circuit shown in FIG. 11 is a drive circuit for detecting the flow rate from the resistance change of the detection element 23. That is, when the detection element 23 is driven with a small current and when it is driven with a large current, the voltage across the detection element 23 is detected by the voltage detector 29 and input to the division circuit 31 in any period. On the other hand, the voltage across the reference resistor R having a known resistance value is detected by the current detector 30 and input to the division circuit 31, and the division circuit 31 calculates the resistance value of the detection element 23. The circuits below the switch SW ₂ perform flow rate calculation by subtracting the output voltage during the small current pulse driving from the output voltage during the large current pulse driving based on the same principle as the driving circuit shown in FIG.

12 (a), (b), (c), (d),
12E is a timing chart for explaining the driving operation of the flowmeter according to the present invention. FIG. 12A is a driving current pulse corresponding to the driving pulse shown in FIG.
12B is a sawtooth wave drive current waveform corresponding to the drive current shown in FIG. 7A, FIG. 12C is a clock pulse, FIG. 12D is a measurement ON / OFF signal, and FIG. ) Indicates a reset signal.

The measurement timings P and Q in FIGS. 5B and 7B define the voltage value measurement timing when the output voltage stabilizes.
a and the measurement timing tb at the time of high temperature driving are the detection element 2
12 is uniquely determined if the timing t ₀ for heating and driving No. ₃ is determined, and the pulsed drive waveform of FIG.
(B) The time t _a to the point a at which the sawtooth drive waveform and the line AA change and the time t to the point b at which the line BB changes.
_{At b} , the respective measurement timings P and Q are determined.

Further, the driving time t _{0 to} t ₁ and the driving rest time t _{1 to} t ₂ are determined by the measurement ON / OFF signal of FIG. 12D, and the driving completion time t ₁ is shown in FIG. The reset signal of e) is output. However, these times _{t a, t b, t 1} , t 2 may be determined by measuring the number shown in FIG. 12 (c) clock pulse.

FIG. 13 is a block diagram showing an example of a measurement timing generation circuit according to the present invention. In the figure, 32 is a clock generation circuit, 33a, 33b and 33c are counters, and a circuit 34 is a reference voltage (driving current) generation circuit. Is.

From the clock generation circuit 32, FIG.
The clock pulse shown in FIG.
Then, when the preset number of pulses corresponding to the time ta is reached, the measurement timing pulse a is output, and similarly,
The counter circuit 33b outputs a measurement timing pulse b when the number of pulses corresponding to the time t _b is reached.
Further, the counter circuit 33c outputs a pulse corresponding to the drive time t ₁ and the drive pause time t ₂ of the measurement ON / OFF signal, and the counter circuit 3 outputs the pulse at the time t _1.
3a and 33b are reset, a pulse at the time t ₂ opens the counter gates of the counter circuits 33a and 33b, and at the same time, the reference voltage generating circuit 34 is activated to drive the detection element. ) Output E.

FIG. 14 is an example of a circuit for generating the above-mentioned reset signal, and FIG. 15 is a diagram showing a time chart of the reset pulse generating circuit of FIG. When the measurement ON / OFF signal (output signal of the counter 33c) shown in FIG. 15 (b) is input to the input end, the signal shown in FIG. 15 (c) corresponding to the ON / OFF signal is output with a delay of one clock pulse. To be done. Figure 15
Inversion signal of (b) (output of the inversion circuit 36) and F · F35
15D is output as an AND output (output of the AND circuit 37) with the clock pulse width shown in FIG. 15D.

FIG. 16 is a block circuit showing an example of a sawtooth wave current drive circuit for driving the detection element according to the present invention.
A resistor R ₀ for receiving the input of the reference voltage E, and a switch 39 for discharging the feedback capacitor C ₀ by the integration circuit reset signal composed of the resistor R ₀ , the feedback capacitor C ₀ and the OP amplifier 38 are provided. A sawtooth voltage V ₁ proportional to the time t is output within the defined period. V ₁ = (E / _CO R _O ) t (5) The voltage V ₁ has an input resistance R _a that is the same as the reference resistance R _S.
The voltage V ₁ is input to the constant current circuit composed of the P amplifier 40.
A sawtooth-shaped constant current i proportional to the current flows through the detection element. That is, from (5)

[0045]

[Equation 1]

Is obtained. FIG. 17 is a block circuit diagram for explaining an embodiment of a flow rate output circuit by the sawtooth wave current drive of the detection element according to the present invention.
42 is an amplifier circuit, 43 is a coefficient setting circuit, 44 and 45 are hold circuits, and 46 is a subtraction circuit. The sawtooth constant current i shown in FIG. 16 flows through the detection element 23, and the voltage across the detection element 23 is input to the amplifier circuits 41 and 42 connected in parallel. The amplifier circuit 41 measures the output voltage V _s during low current driving, and a coefficient setting circuit 43 and a hold circuit 44 are connected in series. On the other hand, the amplifier circuit 42
Is for measuring the output voltage V _D at the time of driving at a high current, and the hold circuit 45 is connected to the output voltage V _D.

The hold circuit 44 is held by the measuring timing pulse a correction voltage V _'s after multiplied by a coefficient K to the output voltage V _s of the time of small current drive. On the other hand, the hold circuit 45 holds the output voltage V _D at the time of driving a large current by the measurement timing pulse b. The output voltage V _S ″ (= V _S ') held in the hold circuit 44 during the small current drive and the output voltage V _{D held in the} hold circuit 45 during the large current drive are subtracted by the subtraction circuit 46 to obtain the flow rate. A proportional voltage V is output, and a low-temperature small current drive current of 2 m
Set A, and the large current drive current 8mA at high temperature (6) 2 mA when t = T _a according to equation, t = t _b circuit constants C _O such that 8mA when, R _O, the R _S and E Just do it.

FIG. 18 is a block diagram for explaining another embodiment of the flow rate output circuit by the sawtooth wave current drive unit of the detection element according to the present invention. In the figure, 47 is a reference voltage generation circuit and 48 is a reference voltage generation circuit. Is an amplifier circuit, 49 is an A / D converter, and 50 is a CPU (central processing unit). In the figure, the same reference numerals as those in FIG. Flow rate output circuit shown in FIG. 18, a timing pulse, i.e., measuring ON / OFF signals, measurement timing t _a and t _b reset signal utilizes the CPU50 clock. Further, since the CPU 50 has a memory function, a special hold circuit for holding the output voltage at low temperature and high temperature is unnecessary. Therefore, the detection element 23
Of the output voltage of A /
The digital value of the detected voltage that has been digitally converted by the D converter 49 is read and stored at the measurement timings t _a and t _b by the CPU 50, and further coefficient setting and subtraction processing can be performed to easily obtain the flow rate. .

FIG. 19 is a diagram for explaining an embodiment of a flow rate output circuit when the detection element according to the present invention is driven by a voltage pulse. In the figure, 51a is a reference voltage generation circuit 1, 51b.
The reference voltage generating circuits 2 and 52 are constant voltage circuits, and other parts having the same functions as those in FIG. 18 are designated by the same reference numerals as those in FIG. The reference voltage generation circuit 1 indicated by 51a is a constant voltage circuit for applying a low voltage V _REF 1 to the detection element 23, and the reference voltage generation circuit 2 indicated by 51b.
Is a constant voltage circuit for applying a high voltage V _REF 2 to the detection element 23. Each constant voltage circuit is a terminal P of the CPU 50.
Driven by the timing pulse output from ₀₁ a
A switch SW ₁ having a contact, a b contact and a grounded c contact connected to the a contact and the b contact of the switch SW ₁
Is connected to the non-inverting input of the constant voltage circuit 52.

The constant voltage circuit 52 loads the series resistance of the reference resistance Rs and the detection element 23 whose other end is grounded, and the connection point is connected to the inverting input. Constant voltage circuit 52 of this circuit configuration
In, the voltage Vs across the detection element 23 becomes equal to each voltage applied to the contact of the switch SW ₁ . That is, when the switch SW ₁ has the a-contact, Vs = V _REF 1 when the b-contact is Vs = V _REF 2 When the c-contact has a constant voltage of Vs = 0.

As described above, the current is flowing through the reference resistance Rs when the voltage Vs of the detection element 23 having a resistance value Rs = constant is
Since is = Vs / Rs, a current corresponding to the resistance Rs flows through the detection element 23. That is, when the resistance Rs changes due to temperature or flow rate, the current is also changes accordingly, and the current is flowing through the reference resistance Rs is changed to the voltage Vr (= is · R).
s) is detected by the amplifier circuit 48.

FIG. 20 is a time chart for explaining the operation of FIG. 19, for example, V _REF 1 = 0.5V, V
_{If REF} 2 = 4.5V, Vs = 0.5V during the period of time t ₁ when the switch SW ₁ is connected to the contact a, and Vs = 4.5V during the period of time t ₂ when the switch SW ₁ is connected to the contact b. The period of time t ₃ connected to the c contact is the drive stop period,
A drive pulse similar to the drive pulse shown in FIG. 5a is obtained. That is, the high temperature heating in which SW ₁ is connected to the a-contact corresponds to constant current 2 mA driving, and the low temperature heating in which SW ₂ is connected to the b-contact corresponds to constant current 8 mA driving.

FIG. 21 is a plan view for explaining another embodiment of the flow rate detector according to the present invention. In the figure, 61 is a substrate, 62 is an insulating film, 63 is a cavity, and 64 is a heating resistor. Body, 65 is a detection resistor, 66 is a microbridge,
This flow rate detector includes a semiconductor substrate 6 having a cavity 63.
On the upper surface of 1, a zigzag heating resistor 64 is formed on a microbridge 66 made of an insulating film perpendicular to the flow direction of the fluid, and a zigzag temperature is arranged inside the heating resistor 64. The detection resistor 65 is provided, and two resistors, that is, the heating resistor 64 and the temperature detection resistor 65 are used at the same time for measurement.

FIG. 22 is an electric circuit diagram of the flow meter shown in FIG. 21. In this case, since the temperature detecting resistor 65 is provided between B and C, a constant current is passed between the B and C terminals, The BC
The flow rate can also be measured by obtaining the voltage output Vout in between. The flow rate can also be measured by controlling the current flowing through the resistor 65 so that the voltage output Vout between B and C is kept constant according to the change in the flow rate, and converting this control amount into the flow rate. .

FIG. 23 is a perspective view for explaining another embodiment of the flowmeter according to the present invention, in which 70 is a silicon substrate, 71 and 72 are hollow portions, and 73 and 74 are oxide film substrates. , 75 and 76 are resistors, and in this example, two resistors 7
The measurement is performed by using 5,76 at the same time.

Thus, the flow velocity detecting element shown in FIG. 1A has one detecting element which is the resistor 4 having the microbridge structure as described above, and one detecting element is connected continuously. For example, low temperature heating and high temperature heating are performed every 50 ms. In reality, the surrounding environment does not suddenly change within such a minute time, but ideally, it is sufficient if the detection element can be heated at low temperature and high temperature at the same time to detect the difference in resistance value. However, in the detector having the structure shown in FIG.
It is impossible to heat at low temperature and high temperature in the same place at the same time.

FIGS. 24 (a) to 24 (d) show FIGS.
6 is a time chart when the flow rate is obtained using the detector shown in FIG. FIG. 24 (a) shows clock pulses a ₁ , a ₂ , a ₃ ... Which are transmitted from a clock circuit (not shown) at equal time intervals. Figure 24
(B) is a pulse b ₁ , b ₂ , b ₃ of a low current value for heating the resistor 65 (76) between the terminals C and B in synchronization with the clock pulses a ₁ , a ₂ , a ₃ ... The resistor 65 (76) is heated to a low temperature by this pulse current, and the resistance value proportional to the temperature is detected.

At the same time, clock pulses a ₁ , a ₂ , a ₃ ...
Synchronously to heat the resistor 64 between the terminals B-A (75) to a pulse c _1, c _2, c ₃ of a high current value, the resistor 64 (75) heated to a high temperature by the pulsed current, temperature And the resistance value proportional to the flow velocity is detected.

FIG. 24D shows clock pulses a ₁ ,
A value obtained by subtracting the resistance value of the resistor 65 (7, 6) heated at a low temperature from the resistance value of the resistor 64 (75) heated at a high temperature by a flow velocity calculation circuit (not shown) in synchronization with a ₂ , a ₃ ... Then, the pulses d ₁ , d ₂ and d ₃ have a peak value proportional to the flow velocity.

FIGS. 25A, 25B, and 25C are time charts when the flow velocity is obtained using the flow velocity sensor shown in FIG. 23. The clock pulse shown in FIG. a ₁ , a ₂ , a ₃ ... Also have the same intervals as the clock pulses a ₁ , a ₂ , a ₃ ... Shown in FIG. 24 (a), and the resistors 75, 76 are supplied with the clock pulses a ₁ , a ₂ ,. Driven in synchronization with a ₃ .

The driving method shown in the time chart of FIG. 25 is a further improvement of the driving method shown in FIG. 24. The low temperature side resistors 75 and 76 are driven alternately at high and low temperatures, for example, one resistor 75 is driven. When driven at high temperature, the other resistor 75
Is driven at low temperature, or vice versa.

That is, the clock pulse (a
_When the resistor 76 between the terminals B and C synchronized with ₁ ) is heated at a low temperature by the current pulse b ₁ , the resistor 75 between the terminals B and A is
Is heated to a high temperature with a current pulse c ₁ . At the timing of the clock pulse a ₁ , from the c ₁ pulse representing the temperature / flow velocity,
The b ₁ pulse difference representing only the temperature is obtained, and at the timing of the next clock pulse a ₂ , the flow velocity is detected based on the difference between the high current pulse driving of the b ₂ pulse and the low current driving of the C ₂ pulse.

[0063]

As is apparent from the above description, the present invention has the following effects. (1) In the past, since the heating element and the heat receiving element were arranged at a distance (separated from each other), it was not possible to detect a flow rate change shorter than the time during which the flow travels with high accuracy. The flow rate detector of the invention can detect a flow rate in response to a rapid flow rate change. (2) Since the thermal response of the detection element is excellent, the time followability of temperature compensation is faster than that of the conventional method. (3) In the conventional example, since temperature detection is performed at two or three places, if the ambient temperature is different, it would be wasteful to correct the temperature. However, according to the present invention, the ambient temperature distribution is In a steep installation environment, all of the measurements are made at one location (one piece), so the temperature is corrected correctly. (4) If there are two or three microbridges, the resistor on the upstream side of the flow disturbs the flow or becomes flow resistance, which makes precise measurement difficult, but if there is only one microbridge Less disruptive, improving accuracy. (5) The number of detection elements is one, and even if there are individual variations such as resistance values and temperature rises, they are not used in combination as in the conventional case, and therefore the yield is good. (6) Since it is a self-detection (temperature compensation) method, there is no element that greatly depends on the positional relationship, such as minute flow rate measurement, and good accuracy can be obtained. (7) Even if there are variations in the temperature distribution of the heating elements themselves, the yield is good without much effect. (8) Since only one detection element is required, the board size is compact and flow cannot be prevented. The number of wires is 2 to 4, which is much smaller than that of the conventional example. Similarly, the size is small and the cost is low, and the size is compact, so that the flow rate distribution in a minute region can be measured. (9) Power consumption is reduced. (10) Installation is free regardless of the mounting posture position.

[Brief description of drawings]

FIG. 1 is a structural diagram for explaining an embodiment of a flow rate detector according to the present invention.

FIG. 2 is a diagram for explaining the operating principle of the flow rate detector according to the present invention.

FIG. 3 is a diagram for explaining another embodiment of the flow rate detector according to the present invention.

FIG. 4 is a diagram for explaining an example of a drive circuit of a flow rate detector according to the present invention.

FIG. 5 is a diagram for explaining an example of a drive current waveform and an output voltage waveform of the flow rate detector according to the present invention.

FIG. 6 is a diagram for explaining the operation of the flow rate detector according to the present invention when the fluid flow rate and the fluid temperature change.

FIG. 7 is a diagram showing another example of a drive current waveform and an output voltage waveform of the flowmeter according to the present invention.

FIG. 8 is a diagram showing another example of the drive current waveform of the flowmeter according to the present invention.

FIG. 9 is a block diagram showing an example of a drive circuit of a flow meter according to the present invention.

FIG. 10 is a waveform diagram in each part of the drive circuit shown in FIG.

FIG. 11 is a drive circuit block diagram for explaining another embodiment of the flowmeter according to the present invention.

FIG. 12 is a timing chart explaining the measurement timing of the flowmeter according to the present invention.

13 is a block diagram showing an example of an electric circuit for executing the measurement timing shown in FIG.

14 is a diagram showing an example of a reset circuit suitable for use in the circuit shown in FIG.

FIG. 15 is a time chart of the reset circuit of FIG.

FIG. 16 is a block diagram showing an example of a sawtooth wave current drive circuit for a detection element according to the present invention.

FIG. 17 is a block circuit diagram for explaining an embodiment of a flow rate output circuit by driving a detection element according to the present invention with sawtooth wave current.

FIG. 18 is a block diagram for explaining another embodiment of the flow rate output circuit by the saw-tooth wave current drive of the detection element according to the present invention.

FIG. 19 is a diagram for explaining an embodiment of a flow rate output circuit when the detection element according to the present invention is driven by a voltage pulse.

20 is a diagram showing an example of a time chart for explaining the operation of FIG.

FIG. 21 is a plan view for explaining another embodiment of the flow rate detector according to the present invention.

22 is an electric circuit diagram for explaining an example of a flow rate measuring method of the flow rate detector shown in FIG.

FIG. 23 is a perspective view for explaining another embodiment of the flow meter according to the present invention.

FIG. 24 is a time chart when a flow rate is measured using the flowmeter shown in FIGS. 21 and 23.

FIG. 25 is another time chart in the case of measuring the flow rate using the flowmeter shown in FIG.

FIG. 26 is a partial cross-sectional view showing the structure of a conventional flowmeter.

FIG. 27 is a view showing another structure of the conventional flowmeter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Substrate, 2, 10 ... Insulation film, 3 ... Cavity, 4 ... Resistor, 5, 6 ... Terminal, 7 ... Microbridge, 8, 9 ... Supporting part, 11 ... Substrate, 12 ... Insulating coating, 13 … Cavity, 1
4 ... Resistor, 15, 16 ... Terminal, 18, 20 ... Insulating film, 21 ... Constant current source, 22 ... Constant voltage source, 23 ... Detecting element, 24 ... Amplifying circuit, 25 ... Coefficient setting circuit, 26 ... Hold circuit , 27 ... Subtraction circuit, 28 ... Output terminal, 29 ... Voltage detector, 30 ... Current detector, 31 ... Division circuit, 32 ...
Clock generation circuit, 33a, 33b, 33c ... Counter, 34 ... Reference voltage generation circuit, 35 ... Flip-flop, 36 ... Inversion circuit, 37 ... AND circuit, 38 ... OP amplifier, 39 ... Switch, 40 ... OP amplifier, 41, 42
... Amplification circuit, 43 ... Constant setting circuit, 44, 45 ... Hold circuit, 46 ... Subtraction circuit, 47 ... Reference voltage generation circuit, 4
8 ... Amplifier circuit, 49 ... A / D, 50 ... CPU, 61 ... Substrate, 62 ... Insulating coating, 63 ... Cavity, 64 ... Heating resistor, 65 ... Detection resistor, 66 ... Microbridge, 7
0 ... Silicon substrate, 71, 72 ... Cavity, 73, 74 ...
Oxide film substrate, 75, 76 ... Resistor.

Claims (11)

[Claims] 1. A single resistor is inserted in a fluid to be measured,
A small current that does not heat the resistor is applied to the resistor to measure the resistance value of the resistor, and then a large current is applied to the resistor to heat the resistor, and the resistance during heating is measured. A flow rate measuring method comprising: measuring a value, calculating the resistance value when not heating and subtracting the resistance value from the resistance value when heating to measure the flow rate of the fluid to be measured. 2. A single resistor is inserted in the fluid to be measured,
A gradually increasing current is passed through the resistor, and the resistance value at a small current such that the resistor is not heated by the current and the resistance value at a large current at which the resistor is heated are measured, The flow rate measuring method is characterized in that the flow rate of the measurement fluid is measured by calculating and subtracting the resistance value at the time of small current from the resistance value of 1. 3. A cavity formed on a substrate, an insulating thin film bridging a part of the upper surface of the cavity, a single resistor provided on the insulating thin film, and a resistor. A means for detecting a resistance value of the resistor by applying a small current or a small voltage so as not to heat the resistor, and a resistor for supplying a large current or a large voltage to the resistor. A means for heating the body at a high temperature and detecting a resistance value at the time of the high temperature heating; and a means for calculating and subtracting the resistance value at a small current from the resistance value at a large current of the resistor, the subtracted value A flow rate measuring device for detecting the flow rate of the fluid to be measured based on the above. 4. A flow measurement period during which a small current flows through the resistor and a large current follows the small current, and a measurement pause period during which no current flows through the resistor are repeatedly provided, and the measurement pause period includes The flow rate measuring device according to claim 3, wherein the temperature of the resistor is equal to or longer than the time required to return to the temperature of the fluid to be measured. 5. The voltage or current applied to the resistor during the flow rate measurement period is a voltage or current pulse having a low peak value,
The flow rate measuring device according to claim 3 or 4, wherein the voltage or current pulse of high peak value is output immediately after the voltage or current pulse of low peak value. 6. The flow rate measuring device according to claim 3, wherein the voltage or current applied to the resistor is a sawtooth voltage or current. 7. Two resistors are inserted in the fluid to be measured,
A small current or a small voltage that does not heat the resistor is applied to one resistor, and a large current or a large voltage that heats the resistor is applied to the other resistor at the same time. A flow rate measuring method characterized in that the values are measured at the same time, both resistance values are calculated to obtain a difference, and the flow rate of the fluid to be measured is measured from the difference. 8. A cavity formed on a substrate, an insulating thin film bridging a partial upper surface of the cavity, two resistors provided on the insulating thin film, and one of the resistors. A means for simultaneously applying a small current or a small voltage that does not heat the resistor and a large current or a large voltage that heats the resistor to the other resistor, and calculates the resistance values of both resistors. A means for subtracting, and detecting the flow rate of the fluid to be measured based on the subtracted value. 9. A cavity formed on a substrate, two insulating thin films bridging a part of the upper surface of the cavity, resistors provided in each of the insulating thin films, and one resistor. A means for simultaneously applying a small current or a small voltage that does not heat the resistor and a large current or a large voltage that heats the resistor to the other resistor, and the resistance values of both resistors are calculated. And a means for performing subtraction, and detects the flow rate of the fluid to be measured based on the subtracted value. 10. A clock pulse generation circuit that gives a timing to drive the two resistors at the same time, and a small power pulse generation circuit that applies a small current or a small voltage to one of the resistors in synchronization with the clock pulse. , A high power pulse generation circuit for applying a large current or a large voltage to the other resistor,
And a means for comparing and calculating the resistance value of the resistor applied with the small power pulse and the resistance value of the resistor applied with the high power pulse, and measuring the flow rate of the fluid to be measured based on the comparison result. The flow rate measuring device according to claim 8 or 9, wherein: 11. A small power pulse and a large power pulse are alternately and repeatedly applied to the one resistor in synchronization with the clock pulse, and a large power pulse and a small current pulse of the other resistor are alternately repeated. The flow rate measuring device according to claim 8 or 9, wherein the flow rate measuring device is applied.