JPH02108919A - Hot wire type air flowmeter - Google Patents

Abstract

PURPOSE:To obtain the high accurate air flowmeter with good response by performing associative control over the balancing variable resistance element of a resistance bridge circuit for temperature detection and the balancing variable resistance element of a resistance bridge circuit for flow rate measurement. CONSTITUTION:A heating resistance body 1 and an air temperature detection resistance 2 are paired and installed in an air flow passage, whose flow rate is to be measured. The resistance 3 functions as the current detecting resistance of the resistance body 1 and the resistance 7 functions as the current detecting resistance of a variable resistance element 5. A comparing circuit 4 detects the balanced state of the constant temperature control bridge circuit for flow rate measurement including the resistance body 1 and a control signal is supplied to a transistor (TR) 10 in response to the detection to control a current supplied to the resistance body 1. Variable resistance elements 5 and 6 are controlled associatively to the same resistance value at all times with the output of the comparing circuit 8. Then the element 6 constitutes a temperature compensation control bridge circuit with the resistance body 2 and constant current circuits 11 and 12 and the comparing circuit 8 generates the output according to the balanced state of this circuit, so the resistance value of the element 6 is controlled according to the resistance value of the resistance body 2, i.e., air temperature.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a resistance bridge type hot wire air flow meter.
In particular, the present invention relates to a hot wire air flowmeter suitable as an intake flow rate sensor for automobile gasoline engines.

[Prior Art] Among various internal combustion engines, gasoline engines for automobiles in particular have extremely wide control ranges for their rotational speed and output, and are also subject to strict exhaust gas regulations. Accurate air-fuel ratio control is required at all times even during operating conditions.

Therefore, in recent years, microcomputer-controlled engines have been developed that use microcomputers to comprehensively judge various operating conditions such as the intake air flow rate of the engine and control the air-fuel ratio and ignition timing. Control devices are becoming widely adopted.

By the way, one type of intake flow sensor for engine control is the so-called hot wire air flow meter, but with this hot wire air flow meter, correction for air temperature is essential, and for this reason, a resistance bridge circuit is used. It is common to use a resistance element for temperature detection.

However, in the method of including a temperature detection resistance element in such a resistance bridge circuit, it is necessary to use a relatively high resistance value as the temperature detection resistance element. When an element is used, there is a problem in that it is difficult to obtain a device that follows temperature changes well.

Therefore, in order to improve the ability to follow such temperature changes, a proposal is known, for example, in Japanese Patent Application Laid-Open No. 55-43447, but the method proposed in this publication uses the amplification effect of an operational amplifier or the like. The voltage drop that occurs in the air temperature detection resistance element due to this is amplified, so that an element with a relatively low resistance value can be used.

[Problems to be Solved by the Invention] Since the above-mentioned conventional technology amplifies the voltage at the temperature detection resistance element, the input offset voltage of the amplifier used for this amplification is also amplified. Due to the temperature characteristics of this amplified offset voltage that is not taken into account, there is a problem with the followability and stability described above.

An object of the present invention is to eliminate the need for an amplifier to amplify the voltage drop that occurs in a resistance element for air temperature detection, eliminate the problem of offset voltage caused by using this amplifier, and provide temperature detection with a sufficiently low resistance value. It is possible to use resistance elements for
The purpose of the present invention is to easily provide a high-precision hot wire air flowmeter with excellent followability and reliability.

[Means for solving the problem] The above object is to independently provide a resistance bridge circuit for flow rate measurement and a resistance bridge circuit for temperature detection, and to use a variable resistance element for balancing in the resistance bridge circuit for temperature detection. This is achieved by interlocking control with a variable resistance element for balance in a resistance bridge circuit for flow rate measurement.

[Function] Since the resistance bridge circuit is independent for air flow rate measurement and temperature detection, the current value flowing through the heating resistance element for air flow rate measurement and the resistance element for air temperature detection can be compared with their resistance value. can be independently set to any value without any particular relationship, and it becomes possible to use a resistance element for detecting air temperature with a relatively low resistance value without using an amplifier or the like.

[Example] Hereinafter, the hot wire air flow meter according to the present invention will be explained in detail with reference to the illustrated example.

First, FIG. 1 shows the basic configuration of an embodiment of the present invention. In the figure, 1 is a heating resistor, 2 is an air temperature detection resistor, 3 and 7 are resistors for the bridge circuit, 4 and 8 are comparison circuits consisting of operational amplifiers, 5.6 is a variable resistance element whose resistance value can be controlled by an electrical control signal, 9 is an adjustment resistor, 10 is a current control transistor, 11.1
2 is a constant current circuit.

The heat generating resistor 1 and the air temperature detecting resistor 2 are bare and installed in an air flow passage whose flow rate is to be measured, for example, in an intake air flow passage of a gasoline engine.

The resistors 3 and 7 are both resistance elements of the bridge circuit, and among these, the resistor 3 functions as a current detection resistor for the heating resistor 1, and the resistor 7 functions as a current detection resistor for the variable resistance element 5.

The comparator circuit 4 detects the equilibrium state of the resistance bridge circuit including the heat generating resistor 1, that is, the constant temperature control bridge circuit for flow rate measurement, and accordingly provides a control signal to the transistor 10, which is supplied to the heat generating resistor 1. Works to control current.

The variable resistance elements 5 and 6 are both controlled in conjunction with the output of the comparison circuit 8, and are always controlled to have the same resistance value.

At this time, the variable resistance element 6 constitutes a second resistance bridge circuit, that is, a temperature compensation control bridge circuit, together with the air temperature detection resistor 2 and the constant current circuit 11.12, and this temperature compensation control bridge circuit Since the comparator circuit 8 generates an output depending on the balanced state, the resistance value of the variable resistance element 6 is eventually controlled in accordance with the resistance value of the air temperature detection resistor 2, that is, the air temperature.

The operation of this embodiment is as follows. That is, now, the resistance value of the resistor 3 connected in series to the heating resistor 1 is R.
1. If the resistance value of the variable resistance element 5 in the constant temperature control bridge circuit is Rv, and the resistance value of the resistor 7 connected in series to this variable resistance element 5 is R7, then the resistance value of the heating resistor 1 is R. , is the following formula (1) % formula % (1) On the other hand, a constant current circuit 12 applies a constant current to the air temperature detection resistor 2 included in the temperature compensation control bridge circuit to the extent that it does not generate heat. ■. However, the variable resistance element 6 is also supplied with the above current IC by the constant current circuit 11.
A constant current I6 having a smaller value than that is supplied to each of them.

Then, the comparison circuit 8 makes the voltage drops of the air temperature detection resistor 2 and the variable resistance element 6 equal.

The resistance value of the variable resistance element 6 is controlled, and the resistance value Rv of the variable resistance element 6 at this time is expressed by the following equation (2).

Rv ” (Rc+R5)Ic/Is ”””(2)
In this equation, R1 is the resistance value of the resistor 9 for adjusting air temperature characteristics.

Here, as mentioned above, since the resistance values of the variable resistance elements 5 and 6 are controlled in conjunction with each other by the output of the comparator circuit 8, their resistance values RV are made equal, and after all, the resistance values of the variable resistance elements 5 and 6 are controlled in conjunction with each other by the output of the comparison circuit 8. The resistance value of the variable resistance element 5 in the temperature control bridge circuit changes depending on the air temperature, making it possible to provide temperature compensation to the measurement of the air flow rate.

According to this embodiment, since the air temperature detection resistor 2 is not incorporated in the constant temperature control bridge circuit for flow measurement, the value of the current I0 flowing through the resistor 2 can be determined arbitrarily. , and Ic/Is of the resistance value RC of the air temperature detection resistor 2 as the resistance value of the variable resistance element 6.
A resistor with twice the resistance value can be used, and in the end, a sufficiently low resistance value can be used as the resistance value of the resistor 2, thereby obtaining a flowmeter that can sufficiently follow the temperature change of the air. be able to.

It goes without saying that the flow rate measurement value can be obtained as a voltage drop appearing across the resistor 3, for example.

Next, regarding a more specific embodiment of the present invention, the second
This will be explained using figures.

The embodiment shown in FIG. 2 uses two FETs (field effect transistors) 50.60
In the figure, reference numerals 13 and 14 are transistors constituting a current mirror circuit, and 15 to 18 are resistors.

The output of the comparison circuit 8 of the temperature compensation control bridge circuit is connected to the resistor 1.
7.18 to a predetermined value, and divide it into a predetermined value by transistor 13.
14 base, thereby FET50,60
Resistor 15.16 connected between the drain and gate of
F is configured so that a constant current I flows from a constant current circuit 11 between the source and the train.
Control is performed so that the voltage of the ET 60 becomes equal to the voltage of the air temperature detection resistor 2 and the resistor 9, which are configured so that a constant current Ic flows from the constant current circuit 12.

Therefore, also in this embodiment, the resistance value of the FET 50 is Ic/
Since it can be multiplied by It, the resistance value of the air temperature detection resistor 2 can be made sufficiently low, and good responsiveness can be easily obtained.

Next, other embodiments of the present invention will be described.

First, FIG. 3 shows an example using elements 51 and 61 in which a plurality of FETs are connected in series as variable resistance elements, and the resistance connected between the drain and gate of each of these FETs is equal to 15.16. Control is performed so that the current flows, and this makes it possible to use a low-resistance resistor 2 as the air temperature detection resistor 2, as well as to reduce the source resistance per FET.
Since the voltage applied between the drains can be reduced, linearity as a variable resistance element can be greatly improved.

Good accuracy can be obtained.

FIG. 4 also shows an embodiment of the present invention, and this embodiment uses elements 52 and 62 in which a plurality of bipolar transistors are connected in series as variable resistance elements.

Therefore, according to the embodiment shown in FIG. 4 as well, it is possible to use a sufficiently low resistance value as the air temperature detection resistor 2, and the linearity of the variable resistance element can be sufficiently improved, resulting in improved accuracy. Good air flow measurements can be obtained.

In the above embodiments, an FET or a bipolar transistor is used as the variable resistance element, but the present invention is not limited to this, and any variable resistance element can be used as long as the resistance value can be controlled by an electric signal. It goes without saying that this may be implemented.

Further, two variable resistance elements were housed in the same pan cage. A so-called bare transistor may also be used. In such a bare transistor,
It is particularly desirable because it is easy to obtain two elements with well-equalized characteristics and good thermal and thermal coupling.

[Effects of the Invention] According to the present invention, a sufficiently low resistance value can be used as the air temperature detection resistor without providing an amplifier circuit in the constant temperature control circuit of the heat generating resistor for flow measurement. It is possible to easily provide a highly accurate hot wire air flow meter that sufficiently suppresses the influence of offset voltage and has good responsiveness.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the principle configuration of a hot wire air flow meter according to the present invention, FIG. 2 is a circuit diagram showing a specific embodiment of the present invention using a field effect transistor, FIG. 3 is a circuit diagram showing an embodiment of the invention using a plurality of field effect transistors, and FIG. 4 is a circuit diagram showing an embodiment of the invention using bipolar transistors. 1... Heat generating resistor, 2... Air temperature detection resistor. 4.8... Comparison circuit, 5, 6... Variable resistance element. Figure 1 σ; °32 Figure

Claims (1)

[Scope of Claims] 1. A resistor bridge circuit including a heat generating resistor element installed in an air passage on one side, and a current supplied to the heat generating resistor element is connected so that the heat generating resistor element maintains a constant temperature. In a hot-wire air flow meter that measures the air flow rate as a function of the current value by controlling, a temperature detection resistance bridge circuit in which the air temperature detection resistance element is a resistance element on one side of the bridge; An electronic circuit that automatically maintains equilibrium conditions by controlling the resistance value of the balancing variable resistance element of this temperature detection resistance bridge circuit, and an electronic circuit that automatically maintains the equilibrium condition, and the resistance value is adjusted in conjunction with the balancing variable resistance element of this temperature detection bridge circuit. A hot wire type air flowmeter characterized in that a second variable resistance element to be controlled is provided, and the second variable resistance element constitutes a resistance bridge circuit including the heating resistance element. 2. The hot wire air flow meter according to claim 1, wherein the balancing variable resistance element of the temperature detection bridge circuit and the second variable resistance element are both transistors. 3. The hot wire air flow meter according to claim 2, wherein the transistors are a composite pair of transistors that are thermally coupled to each other. 4. The hot wire air flowmeter according to claim 2, wherein the transistor is a composite element in which at least two transistors are connected in series. 5. The hot wire air flow meter according to claim 4, wherein the transistor is a field effect transistor. 6. The hot wire air flow meter according to claim 4, wherein the transistor is a bipolar transistor.