JPH0347692B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an air flow rate measuring device used, for example, when measuring the intake air flow rate to an engine.

[Conventional technology]

Conventionally, a flow rate measuring tube is installed in the intake conduit of an automobile engine, and an electric heater made of platinum resistance wire and a temperature dependent resistor for detecting the air temperature are installed inside this flow rate measuring tube, and the intake air flow rate is measured based on the output signals of these. A device that does this has been proposed.

This device has the advantage of being able to measure the flow rate with a small and simple structure, but in the past, the configuration was such that a current was supplied to the electric heater so that the electric heater maintained a constant temperature, and this current value was used to measure the flow rate. The output signal was an analog quantity corresponding to . For this reason, when attempting to digitally process the output signal using a microcomputer or the like, it is necessary to convert the output signal into a digital signal using a high-precision A-D converter, resulting in the problem of increased costs.

Furthermore, since ripples caused by flow rate disturbances are superimposed on the output signal, simply converting the output signal directly into a digital signal poses a problem in that the accuracy deteriorates due to the ripples.

[Problem that the invention seeks to solve]

Therefore, the problems to be solved by the present invention are the above-mentioned problems such as increase in cost and decrease in accuracy.
SUMMARY OF THE INVENTION It is an object of the present invention to provide an air flow rate measuring device that can solve the above-mentioned problems and perform good measurements.

[Means to solve the problem]

In order to solve the above-mentioned problems and achieve the above-mentioned objects, the present invention provides an air flow measuring device equipped with a bridge circuit composed of an electric heater, a temperature compensation resistor, and a plurality of fixed resistors. Current supply means supplies two levels of current, large and small, to the bridge circuit, and the temperature of the electric heater changes in accordance with a change in the resistance value of the temperature compensation resistor whose resistance value changes depending on the air temperature. Big,
and a switching means for switching the current level of the current supply means when two set temperatures of 1 and 2 are reached. The elapsed time is used as an air flow rate signal.

[Example] Hereinafter, the present invention will be explained using examples shown in the drawings. In FIG. 2, an engine 1 is a spark ignition engine for driving an automobile, and intakes air for combustion through an air cleaner 2, an intake conduit 3, and a throttle valve 6. Then, fuel is injected and supplied from an electromagnetic fuel injection valve 5 installed in the intake conduit 3.

The suction conduit 3 is provided with a throttle valve 6 that can be operated arbitrarily by the driver, and a rectifying grid 7 that rectifies the air flow is provided at the connection with the air cleaner 2.

In the suction conduit 3, between the rectifier grid 7 and the throttle valve 6, a small flow rate measuring tube 9 is fixedly installed by a support 8 substantially parallel to the axial direction of the conduit 3. An electric heater 10 made of a platinum resistance wire is installed in the flow rate measuring tube 9, and a platinum thin film resistance element is placed at a position slightly away from the electric heater 10 on the upstream side of the electric heater 10 so as not to detect the heat of the electric heater 10. A compensation resistor 11 is provided.

As shown in FIG. 3, the electric heater 10 has a structure in which a platinum resistance wire is fixed with a hook attached to the inside of the flow rate measuring tube 9, and the temperature compensation resistor 11 is
As shown in FIG. 4, it has a structure in which a platinum thin film resistance element is fixed on a stay attached to the inside of a flow rate measuring tube 9.

FIG. 1 shows the entire electronic circuit and sensor control circuit 20 of this air flow rate measuring device as a first embodiment.

A reference voltage Vr ₂ is applied to the input terminal i of the analog switch 201. Further, a reference voltage Vr ₁ is applied to the input terminal i of the analog switch 202. And the output terminal o of the analog switch 201
and the output terminal o of the analog switch 202 are connected in common to the non-inverting input terminal c of the operational amplifier 203. The output terminal of the operational amplifier 203 is connected to the base terminal of the power transistor 204. The bridge circuit 30 includes an electric heater 10, a temperature compensation resistor 11, and resistors 301, 302, 303, and 3.
04, and bridge input terminal B
1. It has bridge output terminals B2, B3, and B4. The emitter terminal of the power transistor 204 is connected to the bridge input terminal B1 of the bridge circuit 30.
The inverting input terminal of the operational amplifier 203 and the inverting input terminal of the comparator 207 are connected to the bridge output terminal B2, the input terminal i of the analog switch 205 is connected to the bridge output terminal B3, and the input terminal i of the analog switch 206 is connected to the bridge output terminal B4. The resistors 301 and 304 are connected to each other, and the resistors 301 and 304 are commonly grounded. The output terminal o of analog switch 205 and the output terminal o of analog switch 206 are commonly connected to a non-inverting input terminal of comparator 207. Control terminal c of analog switch 202, control terminal c of analog switch 205, input terminal and signal output terminal 290 of inverter 208 are commonly connected to output terminal A of comparator 207. Control terminal c of analog switch 201 and control terminal c of analog switch 206 are commonly connected to an output terminal of inverter 208.

The collector terminal of the power transistor 204 is connected to the positive terminal of the battery 21 to supply current, and the negative terminal of the battery 21 is grounded. Although not shown in the figure, analog switches 201, 202, 205, and 206, an operational amplifier 204, a comparator 207, and an inverter 208
The power is also connected to be supplied from the battery 21.

The operation of the above configuration will be explained.

A predetermined amount of air determined by the opening degree of the throttle valve 6 is drawn into the engine 1 from the air cleaner 2 through the intake conduit 3. A certain proportion of this total intake air passes through the flow measuring tube 9 and is sucked into the engine 1.

Then, the electric heater 1 is installed inside the flow rate measuring tube 9.
The temperature compensating resistor 11 located at a position not affected by heat generation is affected only by the temperature of the air. Furthermore, although the electric heater 10 generates heat by being energized, it is cooled by the intake air.

Next, the operation of all the electronic circuits of the air flow measuring device as the first embodiment shown in FIG. 1 will be explained using the time chart shown in FIG.

First, the operating state at time _t0 will be described. At this point, if the logic level of the output terminal A of the comparator 207 is at the "L" level as shown in FIG. Since the analog switch 201 is in the "ON" state, the reference voltage Vr ₂ is applied to the control terminal c of the operational amplifier 20 via the analog switch 201, as shown in FIG.
It is applied to the non-inverting input terminal of No. 3. Note that at this time _t0 , the comparator 207 is activated as shown in FIG.
Since the level of the output terminal A of the switch is at the "L" level and this signal level is applied to the control terminal c of the analog switch 202, the analog switch 202 is in the "OFF" state. operational amplifier 2
03, power transistor 204, and electric heater 1
The electronic circuit consisting of the resistor 301 and the resistor 301 constitutes a constant current circuit.
The voltage across the operational amplifier 203 is made equal to the voltage Vc at the non-inverting input terminal C of the operational amplifier 203. At this time, the current flowing through the resistor 301, that is, the current _IH flowing through the electric heater 10 is expressed by the following equation.

I _H = (Vr ₂ )/(R ₃₀₁ )...(1) However, R ₃₀₁ is the resistance value of the resistor 301.

Here, the value of the current I _H flowing through the electric heater 10 is set to a large enough current value to raise the temperature T _H of the electric heater 10 to overcome the cooling effect of the intake air. Therefore, the temperature T _H of the electric heater 10 increases linearly with a certain slope as time passes, as shown in FIG. 5.

Further, the resistance value R _H of the electric heater 10 has a certain temperature coefficient K _H and changes according to the temperature T _H of the electric heater 10 according to the relationship shown in the following equation.

R _H = R _HO × (1 × K _H × T _H ) … (2) However, R _HO is the resistance value of the electric heater 10 at 0°C. K _H >0.

Therefore, since the voltage V _B1 at the bridge input terminal B1 is the sum of the voltage across the resistor 301 and the voltage across the electric heater 10, it can be expressed by the following equation using equations (1) and (2).

V _B1 = Vr ₂ + Vr ₂ × R _HO × (1 + K _H × T _H )/R ₃₀₁ … (3) And, since the temperature coefficient K _H > 0 in equation (3), the temperature T _H of the electric heater 10 In response to the increase, the voltage V _B1 at the bridge input terminal B1 increases as shown in FIG.

By the way, the current flowing through the temperature compensation resistor 11 is controlled by the resistors 302, 303, and 304 so that the temperature T _A of the temperature compensation resistor 11 does not become higher than the air temperature due to the amount of heat generated by the temperature compensation resistor 11. The value is set to a small value, and the temperature T _A of the temperature compensation resistor 11 can be regarded as the air temperature. The resistance value R _A of the temperature compensation resistor 11 has a certain temperature coefficient K _A , and the resistance value R _A of the temperature compensation resistor 11, which can be considered to be in the same temperature state as the intake air temperature T _A , is calculated by the following formula. is given by

R _A = R _AO × (1 × K _A × T _A ) …(4) However, R _AO is the temperature compensation resistance 1 at 0°C.
1 resistance value. K _A >0.

Here, the bridge output terminal B of the bridge circuit 30 composed of the temperature compensation resistor 11, the electric heater 10, and the resistors 301, 302, 303, and 304 is
The voltage ΔV2 between 4 and B2 is set to be a negative voltage as shown in FIG.

At time _t0 , the level of the output terminal A of the comparator 207 is at the "L" level as shown in FIG. Since it is applied to terminal c,
The analog switch 206 is in the “ON” state,
The voltage at bridge output terminal B4 is applied to the non-inverting input terminal of comparator 207 via analog switch 206. Therefore, at time t ₀ , the input voltage ΔV ₁ of the comparator 207 is as shown in FIG.
-B2 becomes equal to the voltage ΔV ₂ and becomes a negative voltage. As a result, the output terminal A of the comparator 207
As shown in FIG. 5, the level of is maintained at the "L" level at time _t0 .

Note that at this time _t0 , the level of the output terminal A of the comparator 207 is at the "L" level as shown in FIG. 5, and this signal level is applied to the control terminal c of the analog switch 205.
Analog switch 205 is in the "OFF" state.

When the time reaches _t1 , the temperature T _H of the electric heater 10 increases to the first set temperature _T1 as shown in FIG. The resistance value R _H of 10 increases to R _H1 given by the following equation.

R _H1 = R _H0 × (1 + K _H × T ₁ ) (5) Here, the resistor 302 is set so that the voltage ΔV ₂ between the bridge output terminals B4 and B2 becomes 0V at time t ₁ as shown in FIG. 5. , 303, 304 resistance value
R ₃₀₂ , R ₃₀₃ and R ₃₀₄ are set respectively. That is, since the bridge circuit 30 is in a balanced state at time _t1 , the following equation clearly holds true.

(R _A + R ₃₀₂ + R ₃₀₃ ) × R ₃₀₁ = R _H1 × R ₃₀₄ (6) When the voltage ΔV ₂ between the bridge output terminals B4 and B2 exceeds 0V at time t ₁ as shown in FIG. 5,
The input voltage ΔV ₁ of the comparator 207 to which the same voltage as this ΔV ₂ is applied is also 0V as shown in FIG.
exceed. As a result, at time _t1 , the level of the output terminal A of the comparator 207 changes from the "L" level to the "H" level as shown in FIG.
In response to this change, the analog switch 201
An "L" level signal is applied to the control terminal c via the inverter 208, turning it "OFF", and in turn, the analog switch 202 is turned "ON" because a "H" level signal is applied to the control terminal c. Therefore, _{the reference voltage Vr 1 is} applied to the non-inverting input terminal C of the operational amplifier 203 instead of the reference voltage Vr 2 as shown in FIG. At this time, the current flowing through the resistor 301, that is, the current _IH flowing through the electric heater 10, is determined by substituting Vr ₂ in equation (1).
By changing Vr to ₁ , it can be expressed by the following formula.

I _H = (Vr ₁ )/(R ₃₀₁ ) (7) Moreover, the voltage V _B1 of the bridge input terminal B1 can be expressed by the following equation by changing Vr ₂ in equation (3) to Vr ₁ .

V _B1 = Vr ₁ + Vr ₁ + R _HO × (1 + K _H × T _H ) / R ₃₀₁ … (8) By the way, at time t ₁ , the change from “L” level to “H” level shown in FIG. Correspondingly, the analog switch 206 goes to the “OFF” state, and the analog switch 205 goes to the “OZ” state instead.
Therefore, the voltage at the bridge output terminal B3 is applied to the non-inverting input terminal of the comparator 207 via the analog switch 205 instead of the voltage at the bridge output terminal B4. Therefore, after time _t1 , the input voltage ΔV ₁ of the comparator 207 becomes equal to the voltage ΔV ₃ between the bridge output terminals B3 and B2 shown in FIG. 4, as shown in FIG. 5, and becomes a positive voltage. As a result, the level of the output terminal A of the comparator 207 maintains the "H" level after time _t1 , as shown in FIG. 5.

The reference voltage Vr ₁ is set to a value such that the current I _H of the electric heater 10 is sufficiently small, and the amount of heat taken by the intake air due to the cooling effect is greater than the amount of heat generated by the electric heater 10 due to this current I _H. . Therefore, as shown in FIG. 5, the temperature T _H of the electric heater 10 directly decreases with a certain slope as time passes after time _t1 .

At time _t2 , the temperature T _H of the electric heater 10 decreases to the second set temperature _T2 as shown in FIG. The resistance value R _H of 10 is reduced to R _H2 shown by the following equation.

R _H2 = R _HO × (1 + K _H × T ₂ ) … (9) Here, the resistor 302 is connected so that the voltage ΔV ₃ between the bridge output terminals B3 and B2 becomes 0V at time t ₂ as shown in FIG. , 303, 304 resistance value
R ₃₀₂ , R ₃₀₃ , and R ₃₀₄ are set. That is, since the bridge circuit 30 is in a balanced state at time _t2 , the following equation clearly holds true.

(R _A + R ₃₀₂ ) x R ₃₀₁ = R _H2 x (R ₃₀₃ + R ₃₀₄ )...(10) When the voltage ΔV ₃ between the bridge output terminals B3 and B2 falls below 0V at time t ₂ as shown in FIG. ,
The input voltage _ΔV1 of the comparator 207 to which the same voltage as this ΔV3 is applied is also 0V as shown in FIG.
cut. As a result, at time _t2 , the level of the output terminal A of the comparator 207 changes from the "H" level to the "L" level as shown in FIG. Corresponding to this change, the analog switch 202 receives a "L" level signal to the control terminal c, turning it into the "OFF" state, and in turn, the analog switch 201 receives a "H" level signal via the inverter 208. Since it is applied to the control terminal c and becomes in the "ON" state, the reference voltage Vr ₂ is applied to the non-inverting input terminal C of the operational amplifier 203 instead of the reference voltage Vr ₁ as shown in FIG.
At this time, the current flowing through the resistor 301, that is, the current _IH flowing through the electric heater 10, is expressed by equation (1). Further, the voltage V _B1 of the bridge input terminal B1 is expressed by equation (3).

By the way, at time _t2 , in response to the change from the "H" level to the "L" level shown in FIG.
Therefore, the voltage at the bridge output terminal B4 is applied to the non-inverting input terminal of the comparator 207 via the analog switch 206 instead of the voltage at the bridge output terminal B3. Therefore, after time _t2 , the input voltage ΔV ₁ of the comparator 207, as shown in FIG. 52, becomes equal to the voltage ΔV ₂ between the bridge output terminals B4 and B2 shown in FIG. 5, and becomes a negative voltage. As a result, the level of the output terminal A of the comparator 207 maintains the "L" level after time _t2 , as shown in FIG. 5. After time t ₂ , again
The current I _H given by equation (1) flows through the electric heater 10 and the amount of heat generated _increases , and as shown in FIG. will continue to increase. and time t ₀
At time t ₃ , the electric heater 10
The temperature T _H reaches the first set temperature T ₁ .

By repeating the above operations, the temperature T _H of the electric heater 10 generates a triangular waveform between the set temperatures T ₁ and T ₂ as shown in FIG. 5, and correspondingly, as shown in FIG. Flow signal output terminal 290 shown in
outputs a pulse train flow rate output signal that alternately repeats "H" level and "L" level. The "H" level period tf of this pulse train corresponds to the period in which the temperature T _H of the electric heater 10 in FIG. is electric heater 1
It is clear that 0 is the period during which it is heated.

Next, the “H” level period tf of the flow rate output signal
The relationship between the "L" level period tr and the intake air amount G will be described.

As shown in FIG. 5, during the "H" level period tf,
The temperature T _H of the electric heater 10 decreases over time. The speed of this reduction is determined by the rate at which the amount of heat stored in the electric heater 10 is taken away by the cooling effect of the intake air, and this cooling effect is large when the intake air amount G is large, and small when it is small. .

Therefore, when the intake air amount G is large, the temperature T _H of the electric heater 10 decreases quickly, so the "H" level period tf is small, whereas when the intake air amount G is small, the "H" level period is small. tf becomes larger. This tf
Figure 6 shows the flow rate characteristics. Here, the cooling of the electric heater 10 by the intake air is during the "H" level period tf.
Even if there is turbulence in the flow of intake air, the ever-changing flow rate of the air passing near the electric heater 10 will keep the temperature of the electric heater 10 constant.
This contributes to a decrease in T _H , and the momentary flow rate during the "H" level period tf is integrated as a decrease in the temperature T _H of the electric heater 10. Therefore, “H”
The value of the level period tf corresponds to a value extremely close to the true average value of the intake air amount G during the "H" level period tf. This integral effect makes it possible to remove the ripple component caused by airflow turbulence, so when the intake air amount G is determined from the "H" level period tf according to the flow rate characteristics of tf shown in Figure 6, the ripple component is It is possible to obtain a stable air flow rate without any problems.

In addition, when determining the air flow rate from the pulse width of the "H" level period of the flow rate output signal, since this pulse width becomes smaller as the flow rate increases, the relationship between the air flow rate and the output pulse width approximates a hyperbolic function,
The reading accuracy does not deteriorate when the air flow rate is small, and a highly accurate flow rate signal can be obtained even when the engine speed is low.

Next, the “L” level period tf of the flow rate output signal
It will be explained that the intake air amount G can also be determined from .

As shown in FIG. 5, during the "L" level period tr, the temperature T _H of the electric heater 10 increases with time. The speed of this increase is determined by the rate at which the amount of heat generated by the current I _H supplied to the electric heater 10 increases the temperature T _H of the electric heater 10 and the rate at which it is taken away by the cooling effect of the intake air. When G is large, most of the heat generation is taken away by the cooling effect, so the temperature T _H of the electric heater 10 increases slowly.
When the amount of intake air G is small, only a portion of the calorific value is taken away by the cooling effect, so the temperature of the electric heater 10 decreases.
T _H increases rapidly.

Therefore, when the intake air amount G is large, the temperature T _H of the electric heater 10 increases slowly, so the "L" level period tr is large, whereas when the intake air amount G is small, the "L" level period tr is long. becomes smaller. This tr
Figure 6 shows the flow rate characteristics. As is clear from FIG. 6, the flow rate characteristics of tr are opposite in magnitude to the flow characteristics of tf.

When the intake air amount G is calculated from the "L" level period tr according to the flow rate characteristics of tr shown in Fig. 6, the reason is exactly the same as when the intake air amount G is calculated from the "H" level period tf. , it is possible to obtain a stable air flow rate without ripple components.

By the way, if the intake air temperature T _A changes,
It is necessary to perform temperature correction so that the flow rate characteristics shown in FIG. 6 do not change. In order to perform this temperature compensation, a temperature compensation resistor 11 is provided, and an electric heater 10
Together with this, a bridge circuit 30 is constructed. Therefore, this temperature compensation mechanism will be described next.

The basis of the temperature compensation mechanism is the intake air temperature T _A
Conditions in which the difference (T ₂ - _{T A} ) from the set temperature T ₂ does not change even if the temperature changes, that is, T ₂ - _{T A} = const...( ₁₁ ) The _difference (T ₁ - T ₂ ) does not change, that is, T ₁ -T ₂ =const (12) The constants of each element constituting the bridge circuit 30 are set so as to satisfy the above two conditions.

The purpose of keeping (T ₂ − T _A ) constant is to use electric heater 1.
0 and the intake air, and the purpose of keeping (T ₁ - T ₂ ) constant is to keep the heat transfer coefficient constant between the electric heater 10 and the intake air within the period tf or the period tr. The purpose is to keep the total amount of heat constant, and if the heat transfer coefficient and total amount of heat are kept constant, the period tf or period tr will not change even if the intake air temperature T _A changes, and therefore the temperature characteristics will be compensated. Ru.

Next, the constants of the elements constituting the bridge circuit 30 that satisfy the above equations (11) and (12) will be described. First, let us clarify the conditions of equation (11). electric heater 10
The conditions that are satisfied when the temperature T _H reaches the second set temperature T ₂ are the above-mentioned equations (9) and (10).

R _H2 = R _HO × (1 + K _H × T ₂ ) … (9) (R _A + R ₃₀₂ ) × R ₃₀₁ = R _H2 × (R ₃₀₃ + R ₃₀₄ ) … (10) Also, R _A is expressed in the above equation (4). Given.

R ₄ =R _AO ×(1+K _A ×T _A )...(4) Substitute equations (4) and (9) into equation (10), eliminate and rearrange R _A and R _H2 , and obtain the following equation.

T ₂ = [{R _AO × (1 + K _A × T _A ) + R ₃₀₂ } × R ₃₀₁ −
R _HO × (R ₃₀₃ + R ₃₀₄ )] / {R _HO × K _H × (R ₃₀₃ +
R ₃₀₄ )} …(13) Substitute equation (13) into equation (11), eliminate and rearrange T ₂ ,
If we focus on the molecule, we get the following formula.

{R _AO ×K _A ×R ₃₀₁ −R _HO ×K _H ×(R ₃₀₃ +R ₃₀₄ )}×
T _A + (R _AO + R ₃₀₂ ) × R ₃₀₁ − (R ₃₀₃ + R ₃₀₄ ) × R _HO
=const...(14) In equation (14), since the right side is unchanged, the left side must also be unchanged. However, the intake air temperature
Since T _A is a variable, the coefficient of T _A must be 0. That is, R _AO × K _A × R ₃₀₁ − R _HO × K _H × (R ₃₀₃ + R ₃₀₄ ) = 0
…(15) Transforming equation (15), ∴(R _AO ×K _A )/(R _HO ×K _A )=(R ₃₀₃ +
R ₃₀₄ )/R ₃₀₁ ...(16) Equation (16) means that the value R _HO ×K _H , which is the product of the resistance value R _HO of the electric heater 10 at 0°C and the temperature coefficient K _H , The value R _AO is the product of the resistance value R _AO of the temperature compensation resistor 11 at 0°C and the temperature coefficient K _A
If the ratio of ×K _A is set to be equal to the ratio of the resistance value R ₃₀₁ of the resistor 301 and the sum of the resistance values of the resistors ₃₀₃ and 304 (R ₃₀₃ + R ₃₀₄ ), This means that equation (11) can be satisfied regardless of the intake air temperature T _A.

Next, we clarify the conditions for equation (12).

The temperature T _H of the electric heater 10 is the first set temperature T ₁
The conditions that hold true when the equations (5) and (6) are satisfied.

R _H1 = R _HO × (1 + K _H × T ₁ ) …(5) (R _A + R ₃₀₂ + R ₃₀₃ ) × R ₃₀₁ = R _H1 × R ₃₀₄ … (6) (4) and (5) are converted into (6) Substitute into the equation, eliminate R _A and R _H1 , and rearrange to obtain the following equation.

T ₁ = [{R _AO × (1 + K _A × T _A ) + R ₃₀₂ + R ₃₀₃ } ×
R ₃₀₁ −R _HO ×R ₃₀₄ 〕/(R _HO ×K _H ×R ₃₀₄ )
…(17) Substitute equations (13) and (17) into equation (12) to eliminate T ₁ and T ₂ ,
By rearranging, we obtain the following formula.

∴〔(R ₃₀₁ ×R ₃₀₃ )／{R _HO ×K _H ×R ₃₀₄ ×(R ₃₀₃
+R ₃₀₄ )}×{(R ₃₀₂ +R ₃₀₃ +R ₃₀₄ +R _AO ) +R _AO
×K _A ×T _A )=const…(18) Equation (18) means that even if the intake air temperature T _A changes, the term (R _AO ×K _A ×T _A ) is (R ₃₀₂ +R ₃₀₃
+R ₃₀₄ +R _AO ), if set very small compared to
The left side of equation (18) can be considered constant. Therefore, equation (12) can be satisfied.

From the above consideration of temperature compensation conditions, if the constants of each element constituting the bridge circuit 30 are set according to equations (16) and (18), even if the intake air temperature T _A changes, the It is clear that the flow characteristics do not change and the temperature characteristics can be compensated.

Flow rate signal output terminal 290 of this air flow rate measuring device
The digital flow rate output signal output from the second
As shown in the figure, the intake air amount G is calculated from the "H" level period tf or "L" level period tr of the flow rate output signal according to the flow rate characteristics shown in FIG. 6, which is guided to the fuel control circuit 15. The fuel control circuit 15 outputs an injection pulse signal to open the fuel injection valve 5 based on the calculated intake air amount G. As a result, air and fuel with an accurate air-fuel ratio A/F are supplied to the engine 1, and the exhaust gas purification performance, engine output, fuel efficiency, etc. of the engine 1 are improved. Further, as shown in FIG. 1, an electric heater 10 and a temperature compensation resistor 11 are also provided.
By configuring the bridge circuit 30 with elements including the above, temperature compensation conditions are determined only by the elements configuring the bridge circuit 30 as shown in equations (16 and 18) above, making it easy to adjust temperature compensation. become.

In the above first embodiment, a platinum resistance wire was used for the electric heater 10, but as shown in FIG.
Thin film resistor 1 made of platinum, nichrome, copper, nickel, etc.
Even if the electrothermal heater 10' having a structure in which 01 is formed is placed inside the flow rate measurement tube 9 in parallel to the air flow, the flow rate can be measured in the same way as in the case where a platinum resistance wire is used.

Next, FIG. 8 shows the entire electronic circuit and sensor control circuit 20A of this air flow rate measuring device as a second embodiment. The elements constituting the electronic circuit of the second embodiment shown in FIG. 8 are given the same reference numerals as the elements constituting the electronic circuit of the first embodiment shown in FIG. The second embodiment shown in FIG.
The difference from the first embodiment shown in the figure is that an AND gate 210 and a set-reset type flip-flop 21
1 (hereinafter referred to as "flip-flop 211"), a flow rate signal output terminal 291, and a start signal input terminal 292. Further, one input terminal of the AND gate 210 is connected to the output terminal of the inverter 208, and the other input terminal of the AND gate 210 is connected to the analog switch 205 by using the output terminal of the flip-flop 211 and the flow rate signal output terminal 291 in common. It is connected to control terminal c of. The R input terminal of flip-flop 211 is connected to the output terminal of AND gate 210, and the S input terminal is connected to start signal input terminal 292. The Q output terminal of flip-flop 211 is connected to control terminal c of analog switch 206. The connection portions described above are different from the first embodiment shown in FIG.

Next, the operation of all the electronic circuits of the present air flow measuring device as the second embodiment shown in FIG. 8 will be explained using the time chart shown in FIG. 9.

First, the operating state at time _t0 will be described. At this point, if the level of the Q output terminal D of the flip-flop 211 is at the "L" level as shown in FIG. It is in the "OFF" state, and the level of the output terminal of the flip-flop 211 is clearly at the "H" level, and this signal level is applied to the control terminal c of the analog switch 206, so the analog switch 206 is in the "ON" state. state, the voltage at the bridge output terminal B4 of the bridge circuit 30 is applied to the non-inverting input terminal of the comparator 207 via the analog switch 206. Therefore, at time _t0 , the input voltage ΔV ₁ of the comparator 207 is equal to the voltage ΔV ₂ between the bridge output terminals B4 and B2 shown in FIG. 9, as shown in FIG. 92, and is approximately 0V.
It is becoming. Here, bridge output terminal B4-
The reason why the voltage between B2 is reduced from ΔV ₂ to approximately 0V is as follows.
At time t ₀ , the temperature T _H of the electric heater 10 is the 9th
As shown in FIG. 1, the first set temperature is _T1 , and at this time, the bridge circuit 30 is set so that it satisfies the balance condition shown in equation (6) above, as in the first embodiment. Because there is.

That is, at time t ₀ , when the temperature T _H of the electric heater 10 becomes slightly lower than the first set temperature T ₁ , the input voltage ΔV ₁ of the comparator 207 becomes negative, so that the output terminal A of the comparator 207 becomes negative.
The level changes from the "H" level to the "L" level, and in response to this change, the analog switch 202
is in the "OFF" state, and the analog switch 201 is in the "ON" state, so the voltage Vc at the non-inverting input terminal C of the operational amplifier 203 changes from the reference voltage Vr ₁ to the reference voltage Vr ₂ as shown in FIG. Changes to Corresponding to this change, the electric current _IH of the electric heater 10 increases according to equation (1) above, and heat generation increases. When a little time passes from time t ₀ , the temperature T _H of the electric heater 10 becomes slightly higher than the first set temperature T ₁ , and the input voltage ΔV ₁ of the comparator 207 becomes positive, so the output terminal A of the comparator 207
The level of the analog switch 201 changes from the "L" level to the "H" level, and in response to this change, the analog switch 201
is in the "OFF" state and the analog switch 202 is in the "ON" state, so the voltage Vc at the non-inverting input terminal C of the operational amplifier 203 changes from the reference voltage Vr ₂ to the reference voltage Vr ₁ as shown in FIG. . In response to this change, the electric heater 10
The current I _H decreases and heat generation decreases. As time passes further, the temperature T _H of the electric heater 10 becomes slightly lower than the first set temperature T ₁ . By repeating the above operations, the level of the output terminal A of the comparator 207 alternates between "H" level and "L" level at high speed as shown in FIG. 96, and as shown in FIG. 93. The voltage ΔV ₂ between the bridge output terminals B4 and B2 is maintained at approximately 0V to maintain the balance condition (equation (6)) of the bridge circuit 30. As a result, the temperature T _H of the electric heater 10 is the ninth
As shown in FIG. 1, the first set temperature _T1 is maintained.

At time _t1 , the start signal shown in FIG. 9 is input from the fuel control circuit 15 to the start signal input terminal 292. “L” of this start signal
At the rising edge from the level to the "H" level, the flip-flop 211 is set, and the level of the Q output terminal D of the flip-flop 211 changes from the "L" level to the "H" level as shown in FIG. In response to this change, the analog switch 20
6 is in the "OFF" state, and the analog switch 205 is in the "ON" state, and the same voltage as the positive voltage ΔV ₃ between the bridge output terminals B3 and B2 shown in FIG. It is applied to the input voltage ΔV ₁ of the comparator 207 as shown in FIG. As a result, the level of the output terminal A of the comparator 207 is stabilized at the "H" level as shown in FIG.
Corresponding to this level, the analog switch 201 is turned "OFF" and the analog switch 202 is turned "ON", so that the voltage Vc at the non-inverting input terminal C of the operational amplifier 203 becomes the reference voltage Vr as shown in FIG. Stable at level ₁ .

When the reference voltage Vr ₁ is applied to terminal C,
As in the case of the first embodiment shown in FIGS. 1 and 5, the temperature T _H of the electric heater 10 linearly decreases with a certain slope as shown in FIG. 91 as time passes.

When the time reaches _t2 , the temperature T _H of the electric heater 10 decreases to the second set temperature _T2 as shown in FIG. At _t2 , the bridge circuit 30 enters a balanced state, the above equation (10) is established, and the voltage _ΔV3 between the bridge output terminals B3 and B2 falls below 0V as shown in FIG. The input voltage ΔV ₁ of the comparator 207 to which the same voltage as this voltage ΔV ₃ is applied also falls below 0V as shown in FIG. 92. As a result, at time _t2 , the level of the output terminal A of the comparator 207 is as shown in FIG.
The signal changes from "H" level to "L" level as shown in FIG. Corresponding to this change, the output of the inverter 208 changes from the "L" level to the "H" level, and this signal is applied to one input terminal of the AND gate 210, and the other input terminal 12 is applied to the Q output of the flip-flop 211. An "H" level signal is applied to the terminal. Therefore, at time _t2 , as a result of the AND logic processing of the two "H" level signals applied to the input terminal of the AND gate 210, the level of the output terminal of the AND gate 210 changes from the "L" level to the "H" level. Since this signal level is applied to the R input terminal of flip-flop 211, flip-flop 211 is reset.
As shown in FIG. 9, at time _t2 , the level of the Q output terminal D of the flip-flop 211 changes from the "H" level to the "L" level.

By the way, at time _t2 , in response to the change of the Q output terminal D of the flip-flop 211 from the "H" level to the "L" level shown in FIG. is in the "ON" state, so instead of the voltage ΔV ₃ between the bridge output terminals B3 and B2, the same voltage as the voltage ΔV ₂ between the bridge output terminals B4 and B2 is applied to the input voltage ΔV ₁ of the comparator 207.

Also, at time _t2 , the analog switch 20
2 is in the “OFF” state, analog switch 201
is in the "ON" state, the voltage Vc at the non-inverting input terminal C of the operational amplifier 203 changes from the reference voltage Vr ₁ to the reference voltage Vr ₂ as shown in FIG. 9. When the reference voltage Vr ₂ is applied to terminal C, the first
As in the case of _the first embodiment shown in FIGS.
As shown in FIG. 1, it increases linearly with a certain slope.

At time _t3 , the temperature T _H of the electric heater 10 reaches the first set temperature _T1 , as shown in FIG. 91.

After time _t3 , the state is the same as time _t0 , and as shown in FIG. 9, the level of the output terminal A of the comparator 207 alternates between "H" level and "L" level at high speed, The temperature T _H of 10 is maintained at the first set temperature T ₁ as shown in FIG.

By repeating the above operations, the temperature T _H of the electric heater 10 is maintained at the first set temperature T ₁ before the start signal is applied to the start signal input terminal 292, as shown in FIG. It starts decreasing immediately after the start signal is applied, and starts increasing immediately after reaching the second set temperature _T2 ,
Once the first set temperature T ₁ is reached again, the temperature is maintained at the first set temperature T ₁ until a start signal is applied. The temperature T _H of this series of electric heaters 10
In response to the change in the flow rate signal output terminal 291 shown in FIG. 9, a pulse whose "H" level period continues for a certain period of time is output from the flow rate signal output terminal 291 shown in FIG.

It is clear that the "H" level period tf of this pulse corresponds to the period in which the temperature T _H of the electric heater 10 in FIG. 91 decreases, that is, the period in which the electric heater 10 is cooled by the intake air.

The relationship between the "H" level period tf of the flow rate output signal and the intake air amount G is clearly equal to the flow rate characteristic of tf shown in FIG.

Unlike the first embodiment, the present air flow measuring device as the second embodiment described above sets the air flow measurement start time, for example, time t ₁ to the fuel control circuit 15.
It is important that the start signal can be determined based on the start signal emitted from the start signal.

That is, for example, the time t ₁ at which the start signal is issued
If it is synchronized with the rotation angle of the crankshaft of engine 1, the temporal fluctuations in the air flow rate in the intake conduit that occur periodically due to engine 1 will affect this air flow measurement device, resulting in stable measurement. This solves the problem of not outputting a flow rate output signal. Engine rotation synchronous sampling of the so-called intake air amount G becomes possible, and a more accurate and stable air flow rate signal can be output.

Next, FIG. 10 shows a sensor control circuit 20B of the present air flow rate measuring device as a third embodiment. However, the same parts as the sensor control circuit 20A as the second embodiment shown in FIG. 8 are omitted, and only the different parts are shown in FIG. Elements that are the same as the constituent elements are given the same reference numerals.

The third embodiment shown in FIG. 10 is the same as the second embodiment shown in FIG.
The difference from the embodiment is that one input terminal of the AND gate 210 is connected to the output terminal A of the comparator 207, and the other input terminal is connected to the flip-flop 21.
The output terminal of the AND gate 210 is connected to the S input terminal of the flip-flop 211, and the R input terminal of the flip-flop 211 is connected to the start signal input terminal 292.

Next, the operation of all the electronic circuits of the present air flow measuring device as a third embodiment in which the electronic circuit shown in FIG. 8 is modified as shown in FIG. 10 will be explained using the time chart shown in FIG. 11. do.

First, the operating state at time _t0 will be discussed. At this point, the level of the Q output terminal D of the flip-flop 211 is "H" as shown in FIG.
level, this signal level is applied to the control terminal c of the analog switch 205, so the analog switch 205 is in the "ON" state, and the level of the output terminal of the flip-flop 211 is clearly at the "L" level. Since this signal level is applied to the control terminal c of the analog switch 206, the analog switch 206 is in the "OFF" state, and the bridge circuit 3
The voltage at the zero bridge output terminal B3 is applied to the non-inverting input terminal of the comparator 207 via the analog switch 205. Therefore, at time t ₀ , the input voltage ΔV ₁ of the comparator 207 is the 11th
As shown in FIG. 2, it is equal to the voltage ΔV ₃ between the bridge output terminals B3 and B2 shown in FIG.
It is becoming.

Here, the reason why the voltage ΔV ₃ between the bridge output terminals B3 and B2 is approximately 0V is that at time t ₀ , the temperature T _H of the electric heater 10 is the second set temperature T as shown in FIG. 11. ₂ , and at this time the first
As in the embodiment, the bridge circuit 30 is adjusted so that the bridge circuit 30 satisfies the balance condition shown in equation (10).
This is because it is set to 0. That is, time t ₀
In this case, when the temperature T _H of the electric heater 10 slightly changes from the second set temperature T ₂ , the sensor control circuit 20B corrects this change as in the case of time t ₀ in the second embodiment. is activated, the level of the output terminal A of the comparator 207 alternately repeats between the "H" level and the "L" level at high speed as shown in FIG. 11, and the level of the bridge output terminal B3- as shown in FIG. Voltage between B2 ΔV ₃
is maintained at approximately 0V to maintain the balance condition (formula (10)) of the bridge circuit 30. As a result, the temperature T _H of the electric heater 10 is maintained at the second set temperature T ₂ as shown in FIG. 11.

At time _t1 , the start signal shown in FIG. 11 is input from the fuel control circuit 15 to the start signal input terminal 292. At the rising edge of this start signal from the "L" level to the "H" level, the flip-flop 211 is reset and the first
1 As shown in FIG. 7, the level of the Q output terminal D of the flip-flop 211 changes from the "H" level to the "L" level. In response to this change, the analog switch 205 goes to the "OFF" state, and the analog switch 206 goes to the "ON" state in its place.
Bridge output terminal B4 as shown in Fig. 11 2 and 3
The same voltage as the negative voltage ΔV ₂ between -B2 is applied to the input voltage ΔV ₁ of the comparator 207.
As a result, the level of the output terminal A of the comparator 207 is stabilized at the "L" level as shown in FIG.
2 becomes “OFF” and analog switch 2
Since 01 is in the “ON” state, the operational amplifier 203
The voltage Vc at the non-inverting input terminal C of is stabilized at the level of the reference voltage _Vr2 as shown in FIG.

When the reference voltage Vr ₂ is applied to terminal C,
As in the case of the first embodiment shown in FIGS. 1 and 5, the temperature T _H of the electric heater 10 increases linearly with a certain slope as shown in FIG. 11 as time passes.

When the time reaches _t2 , the temperature T _H of the electric heater 10 increases to the first set temperature _T1 as shown in FIG. At t ₂ , the bridge circuit 30 becomes in a balanced state, the above equation (6) is established, and the voltage ΔV ₂ between the bridge output terminals B4 and B2 becomes as shown in FIG.
Cut 0V. The input voltage ΔV ₁ of the comparator 207 to which the same voltage as this voltage ΔV ₂ is applied is also the 11th
Cut off 0V as shown in Figure 2. As a result, at time _t2 , the level of the output terminal A of the comparator 207 changes from the "L" level to the "H" level as shown in FIG.
Change in level. This signal level is applied to one input terminal of AND gate 210, and the "H" level signal from the output terminal of flip-flop 211 is applied to the other input terminal. Therefore, at time _t2 , the two "H" level signals applied to the input terminal of the AND gate 210
As a result of the AND logic processing, the level of the output terminal of the AND gate 210 changes from "L" level to "H" level, and this signal level changes to the level of the output terminal of the flip-flop 210.
Since the signal is applied to the S input terminal of the flip-flop 211, the flip-flop 211 is set, and the level of the Q output terminal D of the flip-flop 211 changes from the "L" level to the "H" level at time _t2 , as shown in FIG.

By the way, at time _t2 , the analog switch 205 goes into the "ON" state and the analog switch 206 goes into the "ON" state in response to the change of the Q output terminal D of the flip-flop 211 shown in FIG. 11 from the "L" level to the "H" level. Since it is in the "OFF" state, the same voltage as the voltage ΔV ₃ between the bridge output terminals B3 and B2 is applied to the input terminal ΔV _{1 of the comparator 207 instead of the voltage ΔV 2 between} the bridge output terminals B4 and B2.

Also, at time _t2 , the analog switch 20
2 is in the "ON" state and the analog switch 201 is in the "OFF" state, so the voltage Vc at the non-inverting input terminal C of the operational amplifier 203 changes from the reference voltage Vr ₂ to the reference voltage Vr ₁ as shown in FIG. do. When the reference voltage Vr ₁ is applied to terminal C, the first
As in the case of _the first embodiment shown in FIGS.
1. As shown in FIG. 1, it decreases linearly with a certain slope.

At time _t3 , the temperature T _H of the electric heater 10 reaches the second set temperature _T2 , as shown in FIG. 11.

After time t ₃ , the state is the same as time t ₀ , and the 11th
As shown in FIG. 6, the output terminal A of the comparator 207
The level of is alternately repeated between the "H" level and the "L" level at high speed, and the temperature T _H of the electric heater 10 is
is maintained at the second set temperature _T2 as shown in FIG. 11.

By repeating the above operations, the 11th
As shown in FIG. 1, the temperature T _H of the electric heater 10 is maintained at the second set temperature T ₂ before the start signal is applied to the start signal input terminal 292, and starts increasing immediately after the start signal is applied. death,
Immediately after reaching the first set temperature T ₁ , it begins to decrease,
Once the second set temperature is reached again, the temperature is maintained at the second set temperature _T2 until the start signal is applied. In response to this series of changes in the temperature T _H of the electric heater 10, a pulse whose "L" level period continues for a certain time is output from the flow rate signal output terminal 291 shown in FIG. 11.

The “L” level period tr of this pulse is shown in FIG.
A period during which the temperature T _H of the electric heater 10 increases as shown in
That is, it is clear that this period corresponds to a period in which the electric heater 10 heats up by overcoming the cooling effect of the intake air.

The relationship between the "L" level period tr of the flow rate output signal and the intake air amount G is clearly equal to the flow rate characteristic of tr shown in FIG.

The present air flow measuring device as the third embodiment described above is different from the first embodiment in that the air flow measurement start time t ₁ can be determined by the start signal issued from the fuel control circuit 15 in the second embodiment. This is important as in the example, and it becomes possible to sample the intake air amount G in synchronization with the engine rotation, and it is possible to output a stable air flow rate signal.

Next, FIG. 12 shows a sensor control circuit 20c of the present air flow rate measuring device as a fourth embodiment. However, the same parts as the sensor control circuit 20 as the first embodiment shown in FIG. 4 are omitted, and only the different parts are shown in FIG. Elements that are the same as the constituent elements are given the same reference numerals.

The fourth embodiment shown in FIG. 12 is similar to the first embodiment shown in FIG.
The difference from the embodiment is that one terminal of a resistor 305 is connected to the bridge output terminal B2, and this resistor 305
The other terminal of the resistor 306 is connected to one terminal of the resistor 306, the other terminal of this resistor 306 is connected to the bridge output terminal B4, and the resistor 305 and the resistor 30
6 is connected to the input terminal i of the analog switch 206.

Next, the operation of all the electronic circuits of the present air flow rate measuring device as a fourth embodiment in which the electronic circuit shown in FIG. 1 is changed as shown in FIG. 12 is shown in FIGS.
This will be explained using the time chart shown in the figure. 1st
FIG. 3 is a time chart showing the operation of the air flow measuring device as the first embodiment shown in FIG. 1, and FIG. 4 is a time chart showing the operation of the present air flow measuring device as a fourth embodiment.

First, the operation of the first embodiment shown in FIG. 13 will be explained.

The solid line waveform shown in FIG. 13 is exactly the same as the waveform shown in FIG. However, this was an ideal case in which the input offset voltage of comparator 207 was 0V. However, in reality, this input offset voltage deviates from 0V, and the operating waveform of the air flow rate measuring device according to the first embodiment differs from the solid line waveform shown in FIG.

For example, when the input offset voltage of the comparator 207 is a negative voltage Vos, the operating waveform of the air flow rate measuring device changes to the one-dot chain line waveform shown in FIG. Therefore, this one-dot chain line waveform will be explained.

When the one-dot chain line waveform shown in FIG. 13 is compared with the solid line waveform, the following three points become clear.

First, as shown in FIG. 13, at time _t1 , the voltage ΔV ₂ between the bridge output terminals B4 and B2 reaches the voltage -Vos (voltage Vos is negative, so the voltage -Vos becomes positive). Further, as shown in FIG. 13, at time _t2 ', the bridge output terminals B3-B2
The voltage ΔV ₃ between them has also reached the voltage −Vos. Therefore, the input voltage ΔV ₁ of the comparator 207 also changes at time t ₁
The voltage is -Vos at time t2', and the voltage is -Vos at time _t2 '. The reason for this is that the comparator 20
It is clear that since there is an input offset voltage Vos at 7, the comparison reference of comparator 207 is due to a change in voltage -Vos.

Second, the first set temperature of the temperature T _H of the heat _transfer heater 10 _shown in FIG.
The set temperature of has increased from T ₂ to T ₂ ′, and the difference between the two set temperatures (T ₁ − T ₂ ) has become (T ₁ ′ −
T ₂ ′), and (T _{1 ′} −T _{2 ′} ) is larger than (T ₁ −T ₂ ). That is, (T ₁ −T ₂ )<(T ₁ ′−T ₂ ′) …(19) Thirdly, the flow rate signal output terminal 2 shown in FIG.
The "H" level period of the flow rate output signal outputted from 90 increases from tf to tf', and the "L" level period also increases from tr to tr'. In other words, tf＜tf′ …(20) tr＜tr′ …(21) The reason why the relationships in equations (20) and (21) hold is the 13th
The waveform of the temperature T _H of the electric heater 10 shown in Fig. 1 and (19)
It is clear from the equation, for example, the time t ₁ of the solid line waveform
The part from to t ₂ and the dash-dotted line waveform from time t ₁
Comparing the part from time t ₁ to t 2 ′, the slope is the same, but since the difference between the two set temperatures is according to equation (19), the period tf ′ from time t ₁ to t ₂ ′ is longer. from t ₁
The period up to _t2 is greater than tf. Equation (21) also holds true for the same reason.

From the above results, even if the intake air amount G is constant, if the input offset voltage of the comparator 207 is not 0V, the "H" level period tf and the "L" level period tr of the flow rate output signal output from the terminal 290.
As a result, the flow rate characteristics shown in FIG. 6 change, causing an error.

Next, the operation of the fourth embodiment shown in FIG. 14 will be explained. The solid line waveform shown in FIG. 14 is exactly the same as the solid line waveform shown in FIG. 13, and is for the case where the input offset voltage of the comparator 207 is 0V.
The one-dot chain line waveform shown in FIG.
The input offset voltage of 7 is a negative voltage.
This is the waveform when it is Vos. When this waveform of the dashed-dotted line is compared with the waveform of the dashed-dotted line shown in FIG. 13, the following two points become clear.

First of all, the electric heater 10 shown in FIG.
The difference between the two set temperatures T _H (T ₁ − T ₂ ) and (T ₁ ′ − T ₂ ′) are equal. That is, (T ₁ −T ₂ )=(T ₁ ′−T ₂ ′) …(22) Second, the “H” level period tf of the flow rate output signal output from the terminal 290 shown in FIG. L”
In the level period tr, the solid line waveform and the one-dot chain line waveform match. That is, if the intake air amount G is constant, the periods tf and tr do not change even if the input offset voltage of the comparator 207 is 0V, and the flow rate characteristics shown in FIG. 6 do not change and no error occurs. That is, tf=tf'...(23) tr=tr'...(24) Next, these are the conditions for establishing equations (23) and (24). We clarify the conditions for formula (22) to hold.

Voltage ΔV ₄ between terminals B5 and B2 shown in FIG. 14 3
reaches the voltage −Vos at time _t1 . At this time, the bridge circuit 30 slightly deviates from the balanced state shown in equation (6) above. Electric heater 10 at this time
If the resistance value of is R _H1 ', then the following equation holds true.

V _B1 = {(R ₃₀₁ + R _H1 ′) / R ₃₀₁ } × Vr ₂ …(25) Vr ₂ −Vos′ = {R ₃₀₄ / (R ₃₀₂ + R ₃₀₃ + R ₃₀₄ +
R _A )}×V _B1 …(26) However, V _B1 is the voltage of bridge input terminal B1,
Vos' is the bridge output terminal B4-B at time _t1
The voltage ΔV ₂ between the two.

If V _B1 is eliminated and rearranged using equations (25) and (26), the following equation is obtained.

R _H1 ′={R ₃₀₁ −(R ₃₀₂ +R ₃₀₂ +R _A )/R ₃₀₄ }−
{Vos′ × R ₃₀₁ × (R ₃₀₂ + R ₃₀₃ + R ₃₀₄ + R _A )/
(Vr ₂ ×R ₃₀₄ )} …(27) Also, by transforming equation (6), the following equation is obtained.

R _H1 = {R ₃₀₁ × (R ₃₀₂ + R ₃₀₃ + R _A )}/R ₃₀₄ …(28) If the change in resistance of the electric heater 10 at time t ₁ is ΔR _H1 , ΔR _H1 = R _H1 ′−R _H1 ...(29) Substitute equations (27) and (28) into equation (29) and eliminate R _H1 ′ and R _H1 to obtain the following equation.

ΔR _H1 = − (Vos′/Sr ₂ ) × {R ₃₀₁ × (R ₃₀₂ + R ₃₀₃ +
R ₃₀₄ +R _A )}/R ₃₀₄ (30) Here, the voltage Vos' is given by the following equation from the sensor control circuit 20C in FIG.

Vos={R ₃₀₅ /(R ₃₀₅ +R ₃₀₆ )}×Vos'...(31) Next, bridge output terminal B3 shown in FIG.
-B2 voltage ΔV ₃ is the voltage -Vos at time _t2
reach. At this time, the bridge circuit 30 slightly deviates from the balanced state shown in equation (10) above. If the resistance value of the electric heater 10 at this time is R _H2 ', the following equation holds true.

V _B1 = {(R ₃₀₁ +R _H2 ′)/R ₃₀₁ )×Vr ₁ …(32) Vr ₁ −Vos={(R ₃₀₃ +R ₃₀₄ )/(R ₃₀₂ +R ₃₀₃ +
R ₃₀₄ +R _A )}×V _B1 ...(33) By eliminating V _B1 from equations (32) and (33), the following equation is obtained.

R _H2 ′={R ₃₀₁ × {R ₃₀₂ + R _A )/(R ₃₀₃ + R ₃₀₄ )}−
{Vos × R ₃₀₁ × (R ₃₀₂ + R ₃₀₃ + R ₃₀₄ + R _A )} /
{Vr ₁ × (R ₃₀₃ + R ₃₀₄ )} (34) Also, by transforming the equation (10), the following equation is obtained.

R _H2 = {R ₃₀₁ × (R ₃₀₂ + R _A )} / (R ₃₀₃ + R ₃₀₄ )
…(35) Letting the resistance change valve of the electric heater 10 at time t ₂ be ΔR _H2 , ΔR _H2 = R _H2 ′−R _H2 …(36) Substitute equations (34) and (35) into (36). and eliminate R _H2 ′ and R _H2 to obtain the following equation.

ΔR _H2 = - (Vos/Vr ₁ ) × {R ₃₀₁ × (R ₃₀₂ + R ₃₀₃ +
R ₃₀₄ + R _A )}/(R ₃₀₃ + R ₃₀₄ ) …(37) By the way, in order to make equation (22) hold, ΔR _H1 and ΔR _H2 shown in equations (30) and (37) should be made equal. .

In other words, ΔR _H1 = ΔR _H2 (38) The reason is that the resistance R _H of the electric heater 10 is
It has the temperature characteristic shown in equation (2 ₎ , and if the change in resistance R _H is the same, the change in temperature T _H is also the same, and thus satisfies (22 ₎ .

Substitute equations (30), (31), and (37) into equation (38) and rearrange to obtain the following equation.

∴R ₃₀₅ / (R ₃₀₅ + R ₃₀₆ ) = (Vr ₁ / Vr ₂ ) × {(R ₃₀₃
+R ₃₀₄ )/R ₃₀₄ } (39) In the actual sensor control circuit 20c, the resistance value R 303 of the resistor ₃₀₃ is much smaller than the resistance value R ₃₀₄ of the resistor 304. That is, R ₃₀₃ << R ₃₀₄ (40) Therefore, the following equation holds true.

(R ₃₀₃ + R ₃₀₄ ) / R ₃₀₄ ≒ 1 ... (41) Substituting equation (41) into equation (39), ∴R ₃₀₅ / (R ₃₀₅ + R ₃₀₆ ) ≒ Vr ₁ / Vr ₂ ... (42) (42 ) means that the resistances 305 and 30
6 can be regarded as an attenuator that converts the voltage ΔV ₂ between the bridge output terminals B4 and B2 into the voltage ΔV ₄ between the terminals B5 and B2, and the attenuation rate α of this attenuator is [=R ₃₀₅ / (R ₃₀₅ + R ₃₀₆ ) ] to the reference voltage Vr ₁
If the ratio of Vr ₂ and Vr 2 is made equal to Vr ₁ /Vr ₂ , then Equation (22) will always hold true even if the input offset voltage Vos of the comparator 207 changes, and Equations (23) and (24) will also hold. It is clear that the flow rate characteristics shown in Figure 6 do not change and no errors occur.

Therefore, by installing the above-mentioned attenuator between the bridge circuit 30 and the comparator 207 that detects the bridge output signal, it is possible to prevent the output signal from being affected even if the input offset voltage Vos of the comparator 207 fluctuates. , high accuracy is not required for the comparator 207,
A major advantage is that inexpensive comparators can be used.

〔Effect of the invention〕

As described above, in the present invention, in an air flow measuring device equipped with a bridge circuit composed of an electric heater, a temperature compensation resistor, and a plurality of fixed resistors, the bridge circuit has two levels, large and small. A current supply means for supplying current and an electric heater whose temperature changes in response to a change in the resistance value of the temperature compensation resistor whose resistance value changes depending on the air temperature when two set temperatures, large and small, are reached. and switching means for switching the current level of the current supply means at different times, detects the elapsed time until the temperature of the electric heater reaches one set temperature and the other set temperature, and uses this elapsed time as an air flow rate signal. Therefore, conventionally, when trying to process air flow rate signals into digital signals using a microcomputer, etc., it was necessary to use a high-precision A-D converter, but with the air flow rate measurement device of the present invention, it is possible to directly process digital signals. Since this device outputs It is simple and can be performed using only the element, and ripples caused by turbulence in the air flow rate can be removed, so it has the excellent effect of enabling highly accurate and stable air flow measurement with little fluctuation.

[Brief explanation of drawings]

FIG. 1 is an overall circuit diagram of an air flow rate measuring device as a first embodiment of the present invention, and FIG. 2 is an overall configuration diagram showing an internal combustion engine employing an embodiment of the fluid flow rate measuring device of the present invention. Fig. 3 is a structural diagram of the electric heater shown in Fig. 2, Fig. 4 is a structural diagram of the temperature compensation resistor shown in Fig. 2, and Fig. 5 shows its operation in the overall circuit shown in Fig. 1. Time chart shown, No. 6
The figure is a flow rate characteristic diagram showing the relationship between the flow rate output signal output from the overall circuit shown in Figure 1 and the intake air flow rate, and Figure 7 is a structural diagram showing another embodiment of the electric heater shown in Figure 2. , FIG. 8 is an overall circuit diagram of an air flow measuring device as a second embodiment of the present invention, and FIG.
The figure is a time chart showing the operation in the previous circuit shown in FIG. 8, and FIG. 10 is a part of the air flow measuring device as a third embodiment, which is a partially modified second embodiment shown in FIG. The circuit diagram, FIG. 11 is a time chart showing the operation of the third embodiment shown in FIG. FIG. 13 is a partial circuit diagram of an air flow measuring device as a fourth embodiment in which parts have been changed. FIG. 14 is a time chart showing the operation of the fourth embodiment shown in FIG. 12. 3... Suction conduit, 9... Flow rate measurement tube, 10, 10'
...Electric heater, 11...Temperature compensation resistor, 20,2
0A, 20B, 20C...Sensor control circuit, 21...
Battery, 201, 202, 205, 206... Analog switch, 203... Operational amplifier, 204...
Power transistor, 207...Comparator, 2
08...Inverter, 210...And gate, 21
1...Flip-flop, 30...Bridge circuit, 3
01, 302, 303, 304, 305, 306
...Resistance, 290,291...Flow rate signal output terminal, 2
92...Starter signal input terminal.

Claims (1)

[Scope of Claims] 1. An air flow measuring device equipped with a bridge circuit consisting of an electric heater provided in an air flow path, a temperature compensation resistor also provided in the air flow path, and a plurality of fixed resistors. Current supply means for supplying two levels of current, large and small, to the bridge circuit; and the temperature of the electric heater is controlled by a change in the resistance value of the temperature compensation resistor whose resistance value changes depending on the air temperature. and a switching means for switching the current level of the current supply means when two set temperatures, large and small, which change accordingly, are reached, and the temperature of the electric heater reaches from one set temperature to the other set temperature. An air flow measuring device characterized by detecting the elapsed time until the end of the air flow and using this elapsed time as an air flow rate signal. 2. The air flow measuring device according to claim 1, wherein the switching means switches the current level immediately when the temperature of the electric heater reaches two set temperatures, high and low. 3. The switching means maintains the temperature of the electric heater at one set temperature from the time it reaches this set temperature, and switches the current level according to a signal sent at a predetermined timing, so that the other set temperature is reached. 2. The air flow measuring device according to claim 1, wherein the current level is immediately switched at a certain point in time. 4. The air flow rate measuring device according to claim 1, wherein the air flow rate signal is a digital signal having two levels of "L" and "H". 5. Claim 5, wherein the two set temperatures, large and small, are such that even if the air temperature changes, the difference between the two set temperatures and the difference between the set temperature and the air temperature are maintained at predetermined values. The air flow measuring device according to item 1. 6. The air flow measuring device has a comparator that detects the output signal of the bridge circuit,
2. The air flow measuring device according to claim 1, further comprising an attenuator having a predetermined attenuation rate installed between the bridge circuit and the comparator.