JPS61164116A - Air flow rate measuring instrument - Google Patents

Abstract

PURPOSE:To accurately measure an air flow rate with a simple circuit, by detecting the time lapsed, during which the set temperature of an electric heater in an air flow path reaches the low level from the intermediate level and using the detected time lapsed as the flow rate signal of the air flow. CONSTITUTION:The air flow rate measuring instrument of this invention is composed of a bridge circuit consisting of an electric heater provided in an air flow path, resistance for temperature compensation, and plural fixed resistances, electric current supplying means which supplies two levels of electric currents, large and small, to the bridge circuit, setting means which sets three levels of temperatures, high, intermediate, and low, to the electric heater, inspecting means which inspects that the temperature of the electric heater reaches one of the set temperatures, maintaining means, switching means which switches the electric current supplied from the electric current supplying means to the bridge circuit to the small level, and detecting means which detects the time lapsed during which the electric heater reaches the low set temperature from the intermediate set temperature. The time lapsed is outputted in the form of digital signals as an air flow rate signal. Therefore, accurate air flow rate measurement can be performed with a simple circuit configuration.

Description

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

[Conventional technology]

When controlling an engine electronically, it is necessary to constantly monitor the operating state of the engine. As a means for monitoring this operating state, there is a means for detecting the engine rotation speed, a means for detecting the engine temperature, a means for detecting the exhaust temperature, a means for detecting the opening degree of the throttle valve, and the intake air flow rate is measured. Means are provided. Various methods have been adopted or developed as means for measuring the intake air flow rate, such as a thermal intake air flow rate measuring device. This is a device in which an electric heater made of a platinum resistance wire having temperature-dependent characteristics is installed in the intake conduit of the engine, and the intake air flow rate is measured from the output signal of the heater.

In other words, the air flow measuring device for bulk powder controls the electric heater to a constant temperature using an analog-controlled current, but the amount of heat radiated from the electric heater changes depending on the air flow rate. The resistance value of the electric heater changes, and this change in resistance value is detected and measured. Specifically, the device measures the air flow rate based on the change in the current value flowing through the electric heater.

[Problem that the invention seeks to solve]

However, in the case of such an electric heater configured to control heating to a constant temperature using an analog-controlled current, the measured current value changes while the air flow rate changes, for example, by a factor of 100. It only changes by about twice, and its measurement sensitivity is extremely low. Therefore, in order to use this air flow rate measuring device for engine control, it is necessary to provide an offset processing means for the detection signal amplification circuit, which poses a problem in that the control circuit for this becomes complicated.

Furthermore, when configuring an engine control device using a microcomputer, it is necessary to convert the analog output signal from the air flow rate measuring device into digital data and supply it to the control circuit. Analog-to-digital conversion must be performed with precision.

That is, an expensive high-resolution A/D converter is required.

Furthermore, since ripples caused by flow disturbances are superimposed on the output signal, simply converting the output signal directly into a digital signal has the problem that measurement accuracy is degraded by the ripples.

Therefore, an object of the present invention is to provide an air flow rate measuring device that can accurately measure air flow rate, has a simplified circuit configuration, and can be effectively used in engine control. It is.

[Means for solving problems]

In order to solve the above problems, in the present invention, as shown in FIG. 8, an electric heater provided in the air flow path, a temperature compensation resistor also provided in the air flow path,
An air flow measuring device equipped with a bridge circuit composed of a plurality of fixed resistors, comprising: current supply means for supplying two levels of current, large and small, to the bridge circuit; a setting means for setting a set temperature of three levels, large, medium, and small, whose resistance value changes according to a change in the resistance value of the temperature compensation resistor whose resistance value changes according to the air temperature; a detecting means for detecting that the set temperature set by the means has been reached; a maintaining means for maintaining the temperature of the electric heater at a high level of the set temperature; and in response to a signal sent at a predetermined timing, a switching means for setting the current supplied to the bridge circuit by the current supplying means to a small level; and an elapsed time until the set temperature reaches the small level from the middle level when the temperature of the electric heater changes according to the switching means. and detecting means for detecting the air flow rate.

〔Example〕

Hereinafter, one embodiment of the present invention will be described based on 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. and,
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.

The electric heater 10 has a structure in which a platinum resistance wire is fixed with a hook attached to the inside of a flow rate measuring tube 9, as shown in FIG. It has a structure in which a platinum thin film resistance element is fixed on a stay attached to the inside of a tube 9.

FIG. 1 shows the entire electronic circuit and sensor control circuit 20 of this air flow measuring device. A constant voltage source 21 (for example, 12) such as a battery is connected to the terminal 291. This terminal 2
91 is connected to the input terminal of a three-terminal regulator 235 via a reverse connection prevention diode 233 and an oscillation prevention capacitor 234, and a constant voltage (
For example, 5■) is output. Capacitor 236 is for ripple suppression. The constant voltage obtained here is connected to the resistor 237.
By attenuating at 238.239, the reference voltage V
r 1 and V r 2 are obtained. A reference voltage is applied to each input terminal i of analog switches 201 and 202.
rI and V r 2 are input, and output terminals O of analog switches 201 and 202 are both input to a manual input terminal of an operational amplifier 203. When the control terminals C of the analog switches 201 and 202 become 1'', the reference voltage Vr is applied to the manual input terminal of the operational amplifier 203.
Either 1 or V r 2 is input.

The transistor 204 amplifies the output voltage of the operational amplifier 203 and supplies it to the bridge input terminal B+.

The bridge circuit 30 includes an electric heater 10 and a temperature compensation resistor 1
1 and resistor 301.302.303.304.305,
It consists of an oscillation prevention capacitor 306, a bridge input terminal B I , bridge output terminals B2, B3,
It has B4 and B5.

Bridge output terminals B3, B4, and BS are connected to the input terminals i of the analog switches 205, 206, and 207, respectively. Also, analog switch 205.20
The output terminals 0 of 6.207 are commonly input to one input terminal of a comparator 208 via a noise limiter capacitor 209. On the other hand, the output terminal B2 of the bridge circuit 30 is input to one input terminal of the operational amplifier 203 and a manual input terminal of the comparator 208. Note that in the operational amplifier 203, the voltage drop across the resistor 301 is equal to the reference voltage v.
The comparator 208 operates to match either r2 or Vrl, and the comparator 208 outputs the reference voltage VrI.

Alternatively, it functions to generate a pulse at the output terminal of one comparator 208 by comparing vr2 with the voltage at the bridge output terminal B3 or BS. The NOT gate 210 inverts the output signal of the comparator 208, and its output is input to one input terminal of the AND gate 212 and the input terminal of the NOT gate 211.
The output of 11 is input to one of the input terminals of AND gate 213, and simultaneously input to one of the input terminals of AND gate 214 and the input terminal of NOT gate 217.

On the other hand, the input terminal 292 of the circuit 20 is 5TAR for flow measurement.
The terminal 292 is an input terminal for the T signal, and the terminal 292 is connected to the resistor 222.
．． 223, it is connected to the base of a transistor 224 for shaping the 5TART signal. transistor 22
The collector of 4 is pulled up with a resistor 225,
Emitter is grounded. The collector of transistor 224 is connected to a NOT gate 228 and an integrating circuit consisting of resistors 229, 231 and capacitor 230. The input terminal of the NAND gate 232 is a NOT gate 2.
The output terminal of 28 is connected to the output of the integrating circuit. The output terminal of NAND gate 232 is connected to the other input terminal of the AND gate 214. Also, N
The output terminal of the OT gate 228 is connected to the NAND gate 23.
In addition to being connected to 2, it is also connected to one input terminal of an AND gate 220.

215 is a decimal counter, which plays a central role in the circuit sequence shown in FIG. The output of the AND gate 214 is connected to the clock input terminal φ of this counter 215, and the QO output terminal is connected to one input terminal of the OR gate 216 and the control terminal C of the analog switch 206.
and the first input terminal of the three-man power NOR gate 241. The Q1 output is connected to the other input terminal of the OR gate 216, the control terminal C of the analog switch 205, the second input terminal of the three-man power NOR gate 241, and the flow rate signal output terminal 293 of the sensor control circuit 20. Q2 output is ANDed with clock enable terminal E.
It is connected to the other input terminal of the gate 22o, one input terminal of the OR gate 242, and the third input terminal of the three-way NOR gate 241. This three-man power NOR gate 241
The output of is connected to the other input terminal of OR gate 242. The output of OR gate 242 is analog switch 2
07 control terminal C and AND gate 212.2
The analog switch 207 is connected to the other input terminal of the counter 215, and the analog switch 207 is connected to the other input terminal of the counter 215.
It turns ON when the output of the human-powered NOR gate 241 is "1". In the three-man power NOR gate 241, when the Qo, Q1, and Q2 outputs of the counter 215 are all 0'' at power-on, the analog switches 205, 206, and 207 are all 0.
This is to prevent the analog switch 207 from becoming FF, and in such a case, "1" is output from the output terminal so that the analog switch 207 is always turned on.

218 is a D-type flip-flop, its clock input terminal C is connected to the NOTOR gate 216 output, its reset terminal R is connected to the NOTOR gate 216 output terminal, its data input terminal is pulled up, and its set terminal S is It is pulled down. This flip flop 21
The Q output terminal of B is connected to one input terminal of AND gate 221. The output of the AND gate 220 is connected to the other input terminal of the AND gate 221,
The output terminal of the AND gate 221 is connected to the reset terminal R of the counter 215. Also, AND gate 2
The output of 13 is connected to the control terminal C of the analog switch 202. The output of AND gate 212 is connected to one input terminal of OR gate 240. The output of the OR gate 216 is connected to the other input terminal of the ON gate 240, and the output of the OR gate 240 is
It is connected to the control terminal C of the analog switch 201.

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.

The temperature compensating resistor 11 located at a position within the flow measuring tube 9 that is not affected by the heat generated by the electric heater 10 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 rate measuring device as the embodiment shown in FIG. 1 will be explained using the time chart shown in FIG.

S T A RT in Figure 5, and ■ to ■, and ■
The signals are generated at the points with the same symbols as shown in FIG. Output signal, ■ is the clock input signal of the counter 215, ■ is the Q of the counter 215
, the output signal, ■ is the Q1 output signal of the counter 215 and the output signal of the circuit 20 in FIG. 1, and ■ is the Q2 output signal of the counter 215. Also, ■ is electric heater 1
The waveform of the temperature TH at 0 is the waveform of the voltage VB+ at the input terminal B+ of the bridge circuit 30.

First, the operating state at time to will be described. At this point, if the output of comparator 208 is "1" as shown in FIG.
”, the force of the NOTOR gate 242 becomes “1”. On the other hand,
When a 5TART signal as shown in FIG. 5 is input from the terminal 292, the signal from the collector of the transistor 224 is an inverted version of the 5TART signal, and this inverted signal is sent to the NOTOR gate 242 through the resistor 229, 231 and capacitor 230. Both NAN
By inputting the signal to the D gate 232 and passing it through the NAND gate 232, a signal such as ◯ in FIG. 5 is obtained, which becomes "0" at a fall time of approximately 1 μs in synchronization with the rise of the 5TART signal. This signal is routed to one input terminal of AND gate 214 and the other input terminal to comparator 208.
Therefore, the signal shown in (■) in FIG. 5 is guided to the φ terminal of the counter 215.

At this time, if the count state of the counter 215 is Q or Ql, the signal (■) in FIG.
By counting times, the state of the counter 215 shifts to Q2, but since the Q2 output is connected to the clock enable terminal E, further counting is prohibited and the Q2 output remains at 1''. OR gate 2
Since the output of 42 is 1'', the analog switch 207
is ONL, and the outputs of AND gates 212 and 213 are “0” and ’1, respectively, and the analog switch 202 is turned ON.
The reference voltage Vr2 is applied to the input terminal of the operational amplifier 203.
is applied.

Note that when the circuit 20 is activated, the comparator 208 is always “
It is set to output 1”.

This means that either Qo or Q of the counter 215 is "1".
”, the analog switch 201 and the analog switch 205 or 206 are on, the bridge circuit 30 is not in an equilibrium state, and the electric heater 10 is in a cold state, so the voltage at B2 is lower than the voltage at B3 or B4. Therefore, the comparator 208 outputs "1" at this time, and if Q2 is "1", the AND gate 212 or 213 is activated at the moment when the comparator 208 is activated, regardless of whether it is "O" or "1". outputs “1”, so analog switch 201 or 202 and analog switch 2
07 is on, and in this case also the same as above,
The comparator 208 outputs 1", and also QO, QH
, , Q2 are all "0", the analog switch 207 is turned on by the NOR gate 241,
Further, since the analog switch 201 or 202 is on as in the above case, the comparator 208 outputs "1" in the same manner as in the above case.

For this reason, the comparator 208 is always 1'' at startup.

Operational amplifier 203, transistor 204 and electric heater 1
The electronic circuit consisting of 0 and the resistor 301 constitutes a constant current circuit, and this constant current circuit operates so that the voltage across the resistor 301 and the voltage at the 10 input terminal of the operational amplifier 203 become equal. 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+=(Vr2)/(R3ot) −(1 again,
R301 is the resistance value of resistor 301.

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

Further, the resistance value RH of the electric heater 10 has a certain temperature coefficient KH, and changes according to the temperature TH of the electric heater 10 according to the relationship shown in the following equation.

RH=RHoX (1+KHXTH) -+2) However, R1-10 is the resistance value of the electric heater 10 at 0°C. KH>0° Therefore, the voltage VS+ of the bridge input terminal B+ is
Since it is the sum of the both-end voltage of 1 and the both-end mold pressure of the electric heater 10, it can be expressed by the following equation using equations (1) and (2).

VB 1 =V r 2 +V r 2 XRHo'X
'(1+KHXTH)/R301'... (
3) Since the temperature coefficient K H>O in the equation (3), the voltage VB+ of the bridge input terminal B1 increases as shown in (■) in FIG. 5 in response to an increase in the temperature TH of the electric heater 10.

By the way, the current flowing through the temperature compensation resistor 11 is controlled by the resistor 30 so that the temperature TA 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 resistance values 2 to 305 are set to be small, and the temperature TA of the temperature compensation resistor 11 can be regarded as the air temperature. The resistance value RA of the temperature compensation resistor 11 has a certain temperature coefficient KA, and the resistance value RA of the temperature compensation resistor 11, which can be considered to be in the same temperature state as the intake air temperature TA, is given by the following equation.

RA = RAOX (1 + KAX'TA) - (4 However, RA is the resistance value of the temperature compensation resistor 11 at 0°C. K'A > Oa Now, the voltage VBI of the Bl terminal increases as the temperature of the electric heater 10 rises. As it rises, the B3 terminal, B4 terminal, B
The potential of the S terminal also increases. These potentials VB3, V
B4, VBS is resistance 3.02.303.304.30
The resistance values of 5 are R302, R303, and R304, respectively.
・When R305 is used, it is expressed by the following formula.

VB3=VB IX'(R303+R304+R30
5)/ (R302+R303+R304+R30S
+RA) -(51VB4=VB IX
(R304+R30S)/ (R302+R303+
R304 + R305+ RA ) -+61
Vas=Vs lXR305/ (R302 + R303
+R3'04+R305+RA)... (7) By the way, since the voltage VB2 of the B2 terminal is constant, va
3 "Set temperature T2 for electric heater temperature TH which becomes VB 2, electric heater temperature T which becomes VB4=VB'2
The magnitude relationship of the set temperature T1 with respect to H and the set temperature 'ro with respect to the electric heater temperature TH which becomes B5-VE12 is as follows.
T 2 <'r','< T O.

Moreover, the set temperature T O, , T I , T 2 is (5),
As can be seen from equations (6), (7), and (4), the higher the intake air temperature TA is, the higher it is, and the lower the intake air temperature TA is, the lower it is.

Now, since the analog switch 207 is ON, the comparator 208 compares V132 and VB5. The temperature TH of the electric heater 10 rises and the set temperature To
When the voltage VB2 of B2 inputted to the tenth input terminal of the comparator 208 is reached, the voltage B5 inputted to the first input terminal
voltage VB5 becomes high, the output of the comparator 208 becomes "O", and accordingly, the output of the NOT gate 210 becomes "1" and the output of the NOT gate 211 becomes 110M, so the analog switch 201 turns ONL and the analog switch 202 turns OFF. , a reference voltage Vrl is applied to the input terminal of the operational amplifier 203. Then, the operational amplifier 203 operates so that V82=Vrl, so the current It-+ flowing through the electric heater 10 becomes IH-VrI/R30+.

The setting is such that the electric heater 10 is sufficiently cooled even with a very small amount of intake air, and the electric heater temperature TH is immediately set.
becomes below the set temperature To. Then comparator 2 again
The output of 08 becomes "1", and the electric heater 10 is heated again. By repeating the above, the output of the comparator 208 repeats 1'' and O'' at a fast cycle, and the temperature TH of the electric heater 10 is maintained at almost the set temperature To.

When the temperature TH of the electric heater 10 reaches the set temperature To, that is, when the output of the comparator 208 becomes 0, the output of the NOT gate 217 rises.At this time, the Q2 output of the counter 215 becomes 1. The reset terminal of the flip-flop 218 is set to 0'', and the data input terminal is blown up and set to "1", so the Q output of the flip-flop 218 is set to 1''.

At this point, when the 5TART signal is input, a signal that is an inversion of the 5TART signal is generated from the collector side of the transistor 224, which is further inverted by the NOT gate 228 and becomes the same signal as the STA and RT signals. AN
After passing through the D gates 220 and 221, a signal of "1" is input to the reset terminal R of the counter 215. Note that this point corresponds to the measurement start point shown in FIG.

When the counter 215 is reset, the QO output power, ie, the signal ``■'' in FIG. 5 becomes "1," and the Q2 output, ie, the signal ``■'' in FIG. 201 turns on. Therefore,
The reference voltage Vrl is input to the + input terminal of the operational amplifier 203, the electric heater 10 is cooled by the intake air, and the moment the electric heater temperature TH reaches the set temperature TI, the output voltage of the comparator 208 becomes 1''.Then, the counter 21
A clock is input to the φ terminal of the counter 215, and the count state of the counter 215 becomes Ql. When Q becomes 1'', the analog switch 205 and the analog switch 201 turn QN, so the temperature TH of the electric heater 10 is compared with the set temperature T2. At this point, since TH>T2, the comparator 208 The output immediately becomes 0'', and as a result, the output signal of the comparator 208 becomes a pulse signal with a short pulse width, as shown in (■) in FIG.

Now, when the temperature TH of the electric heater 10 further decreases and reaches the set temperature T2, the output of the comparator 208 becomes "1" as shown in ■ in FIG. 5, and the φ terminal of the counter 215 becomes "1" as shown in ■ in FIG. , rises, so the counter 215
5, Ql falls as shown in (■) in FIG. 5, and Q2 rises as shown in (■) in FIG. When Q2 rises, counter 21
Since the clock enable terminal E of 5 also rises, the next 5T
counter 215 until reset by the ART signal.
maintains the state of Q2, and the electric heater 10 returns to the set temperature T.
As mentioned above, the electric heater 10 changes the temperature TH to the set temperature 'ro' in preparation for the next measurement.
controlled to maintain

By the way, as can be seen from ■ and ■ in FIG. 5, the time for the temperature TH of the electric heater 10 to cool down from the set temperature T1 to the set temperature T2 corresponds to the pulse width tf of ■ in FIG. This signal is output from the flow rate signal output terminal 293 connected to the Q1 terminal of the counter 25. This pulse width tf 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 flow rate G is large and small when it is small.

Therefore, when the intake air flow rate G is large, the temperature T+ of the electric heater 10 decreases quickly, so the pulse width tf is small, whereas when the intake air flow rate G is small, the pulse width tf becomes large. The flow rate characteristics of this tf are shown in FIG. Here, the cooling of the electric heater 10 by the intake air is performed by pulse @t
This continues during period f, and even if there is turbulence in the flow of intake air, the ever-changing flow rate of the air that has passed near the electric heater heater 10 contributes to a decrease in the temperature TH of the electric heater 10, and the pulse @ During the period tf, the instantaneous flow rate is integrated as a decrease in the temperature TH of the electric heater 10. Therefore, the value of the pulse width tf corresponds to a value very close to the true average value of the intake air flow rate G at the pulse width tf. This integral effect makes it possible to remove ripple components caused by airflow turbulence.
When the intake air flow rate G is determined according to the flow rate characteristics, a stable air flow rate without ripple components can be determined.

Since this pulse width tf decreases as the flow rate increases, the relationship between the air flow rate and the output pulse width approximates a hyperbolic function, so that the reading accuracy does not deteriorate when the air flow rate is small, and high accuracy is maintained even when the engine speed is low. A flow signal is obtained.

By the way, when the intake air temperature TA changes, it is necessary to correct the temperature so that the flow rate characteristics shown in FIG. 6 do not change. A temperature compensation resistor 11 is provided to perform droplet temperature compensation, and together with the electric heater 10 constitutes a bridge circuit 30. Therefore, this temperature compensation mechanism will be described next.

The basis of the temperature compensation mechanism is the condition that even if the intake air temperature TA changes, the difference from the set temperature T2 (T2 TA) does not change, that is, T 2 - T A = CON s t -
+81 Difference between two set temperatures T1 and T2 (TI T2)
The constant of each element constituting the bridge circuit 30 is set so that the above two conditions are satisfied, that is, T1-T2=const-(91).

The purpose of keeping (T2 TA) constant is to keep the heat transfer coefficient between the electric heater 10 and the intake air constant,
The purpose of keeping (TI T2) constant is to make the total amount of heat transferred from the electric heater 10 to the intake air constant within the period tf. Even if the temperature TA changes, the period tf does not change, so the temperature characteristics are compensated.

Next, constants of elements constituting the bridge circuit 30 that satisfy equations (8) and (9) will be described.

First, the conditions of equation (8) will be clarified. The conditions that hold true when the temperature TH of the electric heater 10 reaches the set temperature T2 are as follows:
If the resistance value of the electric heater 10 at the set temperature T2 is RH2, then the following equations (10) and (11) are obtained from the above equation (2) and the equilibrium condition of the bridge circuit 30.

R1-12=RI-10X (1+KHXT2) ・=
(10) (RA+R302)XR30+= RH2X (R303+R304+R305) −(1
1) Also, the resistance value RA of the temperature compensation resistor 11 is as described in (4) above.
It is given by Eq.

RA=RAOX (1+KAXTA) -(41(4
), substituting equation (10) into equation (11), RA%R1-
12 is eliminated and rearranged to obtain the following formula.

T2= ((RAOX (1+KAXTA)+R302
)XR30+ RHOX (R303+R304+R
305))/(RHOXK), 1X(R303+R3
04+R305)) -(12) Substituting equation (12) into equation (8), eliminating and rearranging T2, and focusing on the numerator, the following equation is obtained.

(RAOXKAXR301R1-10XKHX (R30
3+R304+R305)) XTA+(RAO+R
302) XR301 (R303+R304+R305)
XR+o=const... (13) In equation (13), since the right side is unchanged, the left side must also be unchanged. However, since the intake air temperature TA is a variable, the coefficient of TA needs to be zero. That is, RAOXKAXR301RHOXKHX (R303+R
304+R305)=O-(14) Transforming equation (14), (RAOXKA)/(RHOXKH)=' (R3
03+R304+R305)/R30+...(15) Equation (15) means that the resistance value RHO of the electric heater 10 at 0°C multiplied by the temperature coefficient KH is the combined value RH.
OXKI-1, the resistance value RAO of the temperature compensation resistor 11 at 0°C, the temperature coefficient KA, and the multiplication are the combined values RAOXKA
The ratio between the resistance value R301 of the resistor 301 and the resistance value R301 of the resistor 303 is
, the resistance value of the resistor 304, and the resistance value of the resistor 305 are set to be equal to the sum of the resistance values (R303+R304+R305), the equation (8) can be satisfied regardless of the intake air temperature TA.

Next, the conditions of equation (9) will be clarified.

Conditions that are satisfied when the temperature TH of the electric heater 10 reaches the set temperature T+ are the following equations (16) and (17), where the resistance value of the electric heater 10 at that time is RHI.

RHI=RHOX (+XT+ to 1+) −(16)
(RA+R302+R303)XR3o+=RHIX
(R304+R305) −(17)(4),
Substituting equation (16) into equation (17), eliminating and rearranging RASRl-11, the following equation is obtained.

TI = ((RAOX (1+KAXTA)+R30
2+R303) XR30+- RHOX (R304+R305)) / (RHOXKHX (R304+ R30S)') -(18)(12
), by substituting equation (18) into equation (9), we get T + , T
2 is eliminated and rearranged to obtain the following formula.

R301XR303X ((R302+R303+R3
04+R305+RAO)+RAOXKAXTA)=
cons t - (19) Equation (19) means that even if the intake air temperature TA changes, the RAO
The term XKAXTA is (R302+R303+R304+
If you set it very small compared to R305 + RAO) (
The left side of equation 19) can be considered constant. Therefore (
9) 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 (15) and (19), the flow rate characteristics shown in FIG. 6 can be maintained even if the intake air temperature TA changes. It is clear that the temperature characteristics can be compensated without changing.

The digital flow rate output signal output from the flow rate signal output terminal 293 of this air flow rate measuring device is led to the fuel control circuit 15 as shown in FIG. The intake air flow rate G according to the flow rate characteristics shown
Calculate. The fuel control circuit 15 outputs an injection pulse signal to open the fuel injection valve 5 based on the calculated intake air flow rate 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.

Furthermore, as shown in FIG. 1, by configuring the bridge circuit 30 with elements including the electric heater 10 and the temperature compensation resistor 11, the temperature compensation conditions can be calculated using the above equation (15) and (19).
As shown in the equation, since it is determined only by the elements constituting the bridge circuit 30, the temperature compensation can be easily adjusted.

In addition, in this air flow measuring device, the timing to start measuring the air flow rate is determined by the 5TART signal that arrives from the outside, so if this 5TART signal is synchronized with the engine rotation, the intake air amount G Engine rotation synchronized sampling becomes possible and a stable air flow rate signal can be output.

Furthermore, in this air flow measuring device, the set temperature 'ro of the electric heater 10, which is controlled to a constant temperature, is set higher than the set temperatures T + and T 2 of the other two points involved in flow measurement. , electric heater 1 generated during constant temperature control
A ripple at the temperature TH of 0 does not occur as a ripple in the flow rate measurement result, and highly accurate air flow rate measurement is possible.

In the above first embodiment, platinum resistance wire was used for the electric heater 10, but as shown in FIG. 7, platinum, nichrome, copper,
Even if an electric heater 10' having a structure in which a thin film resistor 101 made of nickel or the like is formed is placed inside the flow rate measuring tube 9 in parallel to the air flow, the flow rate can be measured in the same way as when a platinum resistance wire is used.

〔Effect of the invention〕

As described above, according to the present invention, a bridge circuit consisting of an electric heater provided in the air flow path, a temperature compensation resistor also provided in the air flow path, and a plurality of fixed resistors is provided. In the air flow measuring device, the current supply means supplies two levels of current, large and small, to the bridge circuit, and the electric heater has a resistance value that changes depending on the air temperature. a setting means for setting a set temperature of three levels, large, medium, and small, which changes according to a change in the resistance value of the compensation resistor; and a temperature of the electric heater reaching the set temperature set by the setting means. a detecting means for detecting the current, a maintaining means for maintaining the temperature of the electric heater at a high level of the set temperature, and a current supply means supplying the current to the bridge circuit in response to a signal sent at a predetermined timing. comprising a switching means for setting the current to a small level, and a detecting means for detecting the elapsed time from the medium level to the small level of the set temperature when the temperature of the electric heater changes according to the switching means, and the elapsed time Since the air flow rate measuring device is characterized in that it uses the air flow rate signal as the air flow rate signal, the measurement result of the flow rate signal is output as a digital signal called pulse width.
When executing this measurement result with a device that performs digital processing such as a microcomputer, the signal format is extremely matched, so a high-precision A-D converter is required to digitize the analog signal as in the past. In addition, since it is equipped with a bridge circuit that includes an electric heater and a temperature compensation resistor, temperature compensation can be adjusted only with the elements that make up the bridge circuit, making it simple and easy to use. Since ripples caused by flow turbulence can be removed, highly accurate and stable air flow measurement with little fluctuation is possible.In addition, the switching means is executed in response to a signal sent at a predetermined timing. Since the detection means is executed in accordance with the switching means, it is possible to synchronize the signal sent at this predetermined timing with the engine rotation,
By synchronizing with the engine rotation, conditions regarding changes in the state of the intake air that occur synchronously with the engine rotation, such as pulsation, can be set to be approximately the same at each measurement timing, making it possible to perform more stable flow measurements. The electric heater is controlled at a constant temperature at a high level until the signal sent at the timing is input, but this temperature is set higher than the other two temperature settings that affect the air flow rate. Therefore, ripples in the temperature of the electric heater that occur during constant temperature control do not affect the measurement results of the air flow rate, which has the excellent effect of enabling more stable flow measurement.

[Brief explanation of the drawing]

FIG. 1 is an overall circuit diagram of an air flow measuring device as an embodiment of the present invention, FIG. 2 is a time chart showing the operation of the fluid flowing according to the present invention, and FIG. A flow rate characteristic diagram showing the relationship between the flow rate output signal output from the overall circuit and the intake air flow rate, Figure 7 is a configuration diagram showing another embodiment of the electric heater shown in Figure 2, and Figure 8 is a flow rate characteristic diagram showing the relationship between the flow rate output signal output from the overall circuit and the intake air flow rate. - It is a block diagram showing the schematic configuration of the light. 10.10'... Electric heater, 11... Temperature compensation resistor, 20... Sensor control circuit, 30... Bridge circuit, 293... Flow rate signal output terminal, To, Tl+
72...Set temperature, Vrl, Vr2...Reference voltage.

Claims (1)

[Claims] In 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 a resistance value change of the temperature compensation resistor whose resistance value changes according to the air temperature with respect to the temperature of the electric heater. a setting means for setting a set temperature of three levels, large, medium, and small, which changes according to the temperature; a detecting means for detecting that the temperature of the electric heater has reached the set temperature set by the setting means; maintaining means for maintaining the temperature of the electric heater at a high level of the set temperature; and controlling the current supplied to the bridge circuit by the current supply means to a small level in response to a signal sent at a predetermined timing. a switching means; and a detection means for detecting the elapsed time from the medium level to the small level of the set temperature in the temperature change of the electric heater according to the switching means, and detects the elapsed time as an air flow rate signal. An air flow measuring device characterized by: