JP3064086B2 - Air flow meter and engine control system using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air flow meter and an engine control system, and more particularly, to a starting circuit of an air flow meter using a heating resistor.

[0002]

2. Description of the Related Art Conventionally, a bridge circuit is provided with a heating resistor having a temperature dependency for measuring an air flow rate (a heating control target) and a temperature sensing resistor for compensating the temperature thereof. Connect to the inverting input terminal and non-inverting input terminal of the differential amplifier, and output the output signal of the differential amplifier to a heating current control element (usually composed of a transistor. A so-called heating resistance type air flow meter configured to apply a voltage to an input side of an element to be controlled) is widely known.

In this type of air flow meter, a heating resistor, a temperature-sensitive resistor for temperature compensation, and the like are arranged in a pipe for measuring a flow rate.
Even if the air flow that removes heat from the heating resistor in the bridge circuit changes, the heating current is controlled so that the heating resistor maintains a predetermined resistance value (temperature) to maintain the bridge balance (the midpoint voltage of the bridge). Is controlled to be constant), and there is an advantage that the mass air flow rate can be measured from the heating current, and is used in various fields. Particularly in automobiles, it is used in mass air flow meters in the field of engine control for controlling the mixture ratio of air and fuel, and is evaluated as having high accuracy.

Incidentally, in the heating resistance type air flow meter, if the input offset voltage of the differential amplifier is negative due to variation in characteristics, the air flow meter may not start when the power is turned on. Here, the input offset voltage has a negative characteristic when the heating current control element is an NPN transistor when the power is turned on (when both the inverting and non-inverting inputs of the differential amplifier are zero). This refers to a characteristic in which the output of the differential amplifier goes low, or a characteristic in which the output of the differential amplifier goes high when the heating current control element is a PNP transistor.

Therefore, conventionally, for example, Japanese Patent Publication No. Sho 63-4
As disclosed in Japanese Patent No. 3688, there is proposed a technique in which a predetermined offset voltage is applied to the input side of a differential amplifier when power is turned on, and an initial signal for starting is supplied to a heating current control element.

That is, a voltage dividing resistor is connected in series between power supply lines of a power supply, and when the power is turned on, the divided voltage (about 0.5 V) is applied to one input terminal of a differential amplifier via a diode. By doing so (in this conventional example, as an example, a transistor serving as a heating control element is a PNP transistor and the above-described divided voltage is applied to the inverting input terminal of the differential amplifier), an offset voltage required for startup is generated. . Further, in a normal operating state after start-up (a state in which a signal is output from the bridge), the voltage on the inverting input side of the differential amplifier is much higher than the divided voltage, so that the diode has a cutoff function. Therefore, care is taken that the starting circuit does not affect the heating current control for measuring the air flow rate.

[0007] In addition, Japanese Patent Application Laid-Open No. 58-1585
In the Karman vortex air flow meter disclosed in Japanese Patent Application Publication No. 17-210, an offset voltage required for activation is applied to the input side of the differential amplifier to compensate for the activation of the heating current control element and thus the bridge circuit.

[0008]

Among the above-mentioned conventional start-up compensation techniques, the latter has a start-up circuit on the input side of the differential amplifier and does not have a diode having a cut-off function as in the former, so that the start-up is not possible. The offset voltage affects the equilibrium state of the bridge, and the offset voltage appears as an output error of the differential amplifier in a normal heating resistance type air flow meter other than the Karman vortex type.

On the other hand, even if a diode having a cut-off function is provided in the starting circuit as in the former case, if the diode is deteriorated in insulation due to the influence of heat of the surrounding environment, etc.,
The output current input from the bridge circuit to the differential amplifier leaks through the diode, which lowers the measurement accuracy. For this reason, there is a restriction that the diode itself must be installed in a place where the influence of heat is avoided.
For example, when used for controlling an automobile engine, a diode is mounted on the outer wall of the intake pipe as far away from the engine as the heat source as far as possible from other elements of the air flow meter. Also, it has been difficult to modularize the diode integrally with other elements.

The present invention has been made in view of the above points, and an object of the present invention is to realize a starting circuit which guarantees stable measurement accuracy with little variation in output as an air flow meter, and which is used in an installation place or the like. An object of the present invention is to provide an air flow meter which is not so limited.

[0011]

In order to achieve the above object, the present invention basically proposes the following means for solving the problems.

That is, the bridge circuit is provided with a heating resistor for measuring the air flow rate of the heating control object having a temperature dependency, and the midpoint of the bridge circuit is connected to the inverting input terminal and the non-inverting input terminal of the differential amplifier, respectively. An output side of the differential amplifier connected to an input side of a heating current control element (an element for controlling a heating current flowing through the heating resistor), wherein the heating current control element is turned on when power is turned on. An activation circuit for applying an activation signal to the differential amplifier is provided between the output side of the differential amplifier and the input side of the heating current control element, and the activation circuit supplies the bridge circuit with a heating current in an air flow rate measurement range. The starting signal value is set so that a smaller minute current flows.

[0013]

According to the present invention having the above configuration, even when the input offset voltage of the differential amplifier has a negative characteristic when the power is turned on, the start signal is applied to the heating current control element via the start circuit. A drive current for starting flows through the bridge circuit via the heating current control element.

If this starting circuit is provided between the output side of the differential amplifier and the input side of the heating current control element, it does not affect the bridge equilibrium condition on the input side of the differential amplifier, and after power-on (bridge After the circuit is driven), a heating current control signal for maintaining the bridge balance is accurately output from the differential amplifier.

Further, the current flowing through the bridge circuit due to the start signal is set to be smaller than the heating current in the air flow rate measurement range, so that this is not mistaken as a measured air flow rate.

[0016]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.

FIG. 1 is a circuit diagram according to a first embodiment of the present invention,
FIG. 2 is a mounting diagram when the air flow meter of the present embodiment is applied to an engine control system of an automobile.

As shown in FIG. 1, the air flow meter according to the present embodiment is roughly divided into a bridge circuit 1 including a heating wire as a heating resistor RH, and a heating current control element 2 (a heating current control element 2) for controlling the current of the bridge circuit 1. In the present example, an NPN transistor T1 is used) 2, a differential amplifier 3, and a start-up circuit 4.

The bridge circuit 1 has a temperature-dependent heating resistor RH and a temperature-independent heating current detecting element R1 on one branch, and the other branch has temperature dependency on the other branch. A resistor RC for temperature compensation and resistors R8 and R7 having no temperature dependency are provided.

One end of the bridge circuit 1 is connected to a transistor T1.
, And the other end is connected to the (−) side of the power supply VB. The collector of the transistor T1 is connected to the (+) side of the power supply VB. One middle point (between the heating resistor RH and the resistor R1) of the bridge circuit 1 is connected to the non-inverting input terminal of the differential amplifier 3, and the other middle point (between the resistors R8 and R7) is inverted. Connected to input terminal.

The output side of the differential amplifier 3 is connected via the resistors R12 and R10 to the base (input side) of the transistor T1.
Connected to Further, the resistors R11 and R12 are connected in series between the output side of the differential amplifier 3 and the (+) side of the power supply VB, and the resistors R10, R11 and R12 constitute the starting circuit 4. That is, the starting circuit 4 includes a voltage dividing resistor R1 connected in series between the (+) side of the power supply VB and the output side of the differential amplifier 3.
1 and R12 and between the voltage dividing resistors and the NPN transistor T
And a resistor R10 connected between the first resistor and the base.

Here, prior to the description of the operation of the circuit having the above configuration, an example of mounting the air flow meter of this embodiment in an automobile will be described with reference to FIG.

In FIG. 2, an air flow meter has a body (main air passage) 20 through which intake air (detected air) passes.
A sub-air passage 20a for introducing a part of the intake air is formed therein, and a heating resistor RH serving as an air flow measuring element and a temperature-compensating temperature sensing resistor RC for detecting the temperature of air are formed in the sub-air passage 20a. A case 23 in which the electronic circuit (excluding the resistors RH and RC) shown in FIG.
Is attached.

This type of mounting structure has an advantage in that the heating resistor RH and the temperature compensation resistor RC, which are the measuring elements, are housed in the bent auxiliary air passage 20a to protect against dust. The air flow meter configured as described above is mounted between the air cleaner 21 with the air filter 22 and the throttle body 26 with the throttle valve 27 via ducts 24 and 25.

The air taken into the engine reaches the intake manifold through an air cleaner 21, a duct 24, an air flow meter, a duct 25, and a throttle valve 27. The engine control unit 28 calculates an optimal fuel supply amount (fuel injection pulse) from various parameters relating to the engine state such as the output of the air flow meter and the engine speed, and supplies fuel by driving an injector (not shown). This makes it possible to realize an engine control system that excels in exhaust gas purification and fuel efficiency.

The circuit operation of this embodiment will be described with reference to FIG.

With the differential amplifier 3, the bridge circuit 1
Equilibrium so that Equation 1 is obtained.

[0028]

RH · R7 = R1 · (R8 + RC) RH; resistance value of heat generating resistor RH RC; resistance value of temperature compensation resistor RC

[0029]

RH = RH ₀ (1 + αTh) RC = RC ₀ (1 + αTa) where RH ₀ ; heating resistor R at reference temperature (0 ° C.)
Resistance value of H RC ₀ ; resistance value of temperature compensation resistor RC at reference temperature (0 ° C.) α; temperature coefficient Th; temperature of heating resistor Ta; temperature of air, the heating temperature of heating resistor (Th− Ta) is the number 3
It is shown by the formula.

[0030]

(Equation 3)

On the other hand, the relationship between the heating current Ih of the heating resistor RH and the amount of heat transferred from the heating resistor RH to the air is represented by the KING equation shown in Equation 4.

[0032]

(Equation 4)

Accordingly, the heating current is represented by the following equation (5) and represents the air signal.

[0034]

(Equation 5)

This Ih is detected by the resistor R1 and the output signal V
h is taken out via a detection terminal (not shown) and sent to the engine control unit 28. That is, the bridge circuit 1
The heating circuit Ih controls the heating current Ih so that the relationship between the heating resistor RH and the temperature compensation resistor RC maintains a predetermined temperature difference even if the air flow rate that takes away the heat of the heating resistor RH changes.
(Balance the bridge midpoint voltage)
In this way, the mass air flow rate can be measured from the signal value related to the heating current value Ih.

When the power supply VB is turned on, the differential amplifier 3
When the output has a low level (0 V) due to the negative characteristic of the input offset voltage of the
Since the voltage divided by 12 is applied to the base of the transistor T1 as a start signal via the resistor R10, a minute current flows through the collector of the transistor T1 and the transistor T1
Eventually, the bridge circuit 1 can be activated. At this time, the bridge circuit 1 flowing through the transistor T1
Is set to be smaller than Ih at the minimum value (lower limit value of the air flow rate range) of the air flow rate Q in the equation (5). Will not give.

That is, the transistor T1 can be started sufficiently when the base voltage is 1.5 V (the current Ih flowing through the heating resistor of the bridge circuit 1 at this time is 0.3 mA), while the base voltage at the minimum air flow rate Q is 3.78V
(Ih = 70 mA). Therefore, the resistors R11 and R1
If the ratio of 2 is appropriately set, the influence of the change (insulation deterioration) can be greatly reduced.

Since the starting circuit 4 is provided between the output side of the differential amplifier 3 and the input side of the heating current control element 2 (transistor T1), the bridge balance condition of the input side of the differential amplifier 3 is set. After the power is turned on (after driving the bridge circuit), the heating current control signal Vo for maintaining the bridge balance is output from the differential amplifier 3 with high accuracy.
That is, the base current supplied from the power supply VB to the transistor T1 and the collector current are controlled by the heating current control signal Vo, and the heating circuit Ih flows through the bridge circuit 1 so as to maintain the balance of the bridge.

FIG. 3 shows a second embodiment of the present invention (the same reference numerals as those in the first embodiment denote the same or common elements). This embodiment has basically the same circuit configuration as the first embodiment,
The difference is that a Zener diode Dz for obtaining a reference voltage is connected between the voltage dividing resistors R11 and R12 of the starting circuit 4 and the (−) side of the power supply VB. This is for protecting the circuit from an overvoltage such as a surge voltage, and the zener voltage is set to a value slightly higher than the power supply VB.

FIG. 4 shows a third embodiment of the present invention. The differences from the first embodiment will be mainly described.

In this embodiment, the heating current control element 2 has a PNP
The middle point of the bridge circuit 1 where the heating resistor RH is provided is connected to the inverting input terminal of the differential amplifier 3 and the other middle point (the resistor R8.
R7) is connected to the non-inverting input terminal. The starting circuit 4 connects voltage dividing resistors R12 and R13 in series between the (−) side of the power supply VB and the output side of the differential amplifier 3, and connects the voltage dividing resistors R12 and R13 to the base of the transistor T2. Connected.

In the case of this embodiment, since the transistor T2 is of the PNP type, if the output of the differential amplifier 3 is fixed at a high level (the same level as the power supply VB) when the power is turned on, there is no starting means. In this case, the heating control element 2 and thus the bridge circuit 1 are not activated.

Therefore, in the above case, the output of the differential amplifier 3 is stepped down through the voltage dividing resistors R12 and R13 to generate a start signal, and a small collector current required for driving the transistor T2 is generated. Thus, the activation of the bridge circuit 1 is compensated.

In such a configuration, the resistance R12,
By setting R13 appropriately, the same effect as in the first and second embodiments can be obtained.

Further, according to each of the above embodiments, it is not necessary to provide a diode having a cut-off function in the starting circuit unlike the prior art, so that there is no need to consider this thermal deterioration, and the air flow rate including the starting circuit is eliminated. An electronic circuit for measurement can be integrally formed with the module.

FIG. 5 is an open plan view showing an example of mounting the air flow meter, and FIG. 6 is a longitudinal sectional view of a part thereof.

5 and 6, the heating resistor RH and the temperature compensating resistor RC are connected to and supported by pins 37 by welding, respectively. The pin 37 is connected to each lead frame 36 by welding, and the frame 36 includes wires W1 to W4.
Are connected to terminals on the circuit board 31 via the corresponding ones.

On the circuit board 31, the above-described transistor T1 or T2 as the heating current control element 2 and FIGS.
In addition to the IC 33 in which the resistor R12, the differential amplifier 3, and the Zener diode Dz shown in FIG. 4 are integrated, the elements R1, R8, and R3 of all circuits (excluding the heating resistor RH and the temperature compensation resistor RC) are provided.
R7, R10, R11, a noise removing capacitor, and the like are formed by a hybrid IC. The terminals of the power supply VB and the air flow signal Vh are connected to external terminals 3 through wires W5 to W7.
4a to 34c. External terminals 34a to 34c
Is insert-molded on the side of the plastic housing 30.

The pin 37 and the lead frame 36 are integrally formed with the molded body 35,
5 is inserted into the base 32 immediately below the circuit board 31.

The circuit board 31 is made of an aluminum base 3
2 and stored in the housing 30 and the housing 30 is covered with the cover 30A. The cover 30A, the housing 30, and the base 32 are fixed to each other by adhesion.

In the air flow meter described in each of the above embodiments, a diode having a cut-off function is not required in the starting circuit 4, so that the mounting structure as shown in FIGS. And a highly reliable air flow meter can be provided.

In the above-described embodiment, the heating resistor RH is used as an example, and the heating resistor is used as an example. However, the present invention is not limited to this.
In addition, a film type thin film resistor or a semiconductor type may be used.

[0053]

As described above, according to the present invention, it is possible to provide an air flow meter which can be reliably started when the power is turned on, and which has little deterioration in the detection accuracy of the air flow rate even if the starting means is provided. Can be.

[Brief description of the drawings]

FIG. 1 is a circuit diagram according to a first embodiment of the present invention.

FIG. 2 is a system configuration diagram when the above-described embodiment is applied to an air flow meter of an automobile engine control system.

FIG. 3 is a circuit diagram according to a second embodiment of the present invention.

FIG. 4 is a circuit diagram according to a third embodiment of the present invention.

FIG. 5 is a plan view showing a mounting example of the air flow meter of each of the above embodiments.

FIG. 6 is a longitudinal sectional view showing an example of mounting the air flow meter of each of the above embodiments.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Bridge circuit, 2 (T1, T2) ... Heating current control element, 3 ... Differential amplifier, 4 ... Start-up circuit, 20a ... Air flow measurement pipeline, 28 ... Engine control unit, 30 ... Housing, 31 ... Circuit Substrate, 32 base, 36 lead frame, 37 pin, RH heating resistor, RC temperature compensation resistor (temperature-sensitive resistor), R10, R11, R12, R1
3 ... Resistance for starting circuit, Dz ... Zener diode.

Claims (6)

(57) [Claims] A bridge circuit is provided with a heating resistor having a temperature dependency for measuring an air flow rate to be heated and controlled, and a middle point of the bridge circuit is connected to an inverting input terminal and a non-inverting input terminal of a differential amplifier, respectively. An air flow meter connected to an input side of a heating current control element (an element for controlling a heating current flowing through the heating resistor), wherein an output side of the differential amplifier is connected to an input side of the heating current control element. An activation circuit for applying an activation signal to the differential amplifier is provided between the output side of the differential amplifier and the input side of the heating current control element, and the activation circuit supplies the bridge circuit with a heating current in an air flow rate measurement range. An air flow meter characterized by setting a start signal value so that a smaller minute current flows. 2. The differential amplifier according to claim 1, wherein the heating current control element is formed of an NPN transistor, and a middle point of the bridge circuit on which the heating resistor to be heated is provided is the differential amplifier. And the other middle point is connected to the inverting input terminal, and the starting circuit is connected in series between the (+) side of the power supply and the output side of the differential amplifier. An air flow meter comprising a piezoresistor and a resistor connected between the divided resistors and the base of the NPN transistor. 3. The differential amplifier according to claim 1, wherein the heating current control element is constituted by a PNP transistor, and a middle point of the bridge circuit on which the heating resistor to be heated is provided is the differential amplifier. And the other middle point is connected to the non-inverting input terminal, and the starting circuit is connected in series between the (−) side of the power supply and the output side of the differential amplifier. An air flow meter having a pressure resistance, wherein the voltage division resistance is connected to the base of the PNP transistor. 4. The air flow meter according to claim 2, wherein a zener diode for protecting the circuit from an overvoltage such as a surge voltage is added to the starting circuit. 5. The bridge circuit according to claim 1, further comprising a temperature-sensitive resistor for temperature compensation, in addition to the heat-generating resistor, in the bridge circuit. A temperature resistor is arranged in a pipe for measuring the air flow rate, and all circuits of the air flow meter except the heating resistor and the temperature sensitive resistor are formed on a single substrate and housed in a housing with external input / output terminals. On the other hand, the heating resistor and the temperature-sensitive resistor are connected and supported by pins connected to a lead frame, and the air flow meter is configured such that the lead frame and the circuit on the single substrate are connected by wires. . 6. An engine control system which is set so as to control a fuel amount to an engine by an output of an air flow meter provided with the starting circuit according to any one of claims 1 to 5.