JPH0235315A - Hot wire type air flowmeter - Google Patents

Abstract

PURPOSE:To improve response by returning a deformed balanced bridge circuit into an ordinary bridge circuit, and changing the connection to a differential amplifier. CONSTITUTION:A balanced type bridge is composed of resistors 1-3 and a heat resistor element 4 for detecting the flow rate of air. The balanced state of the bridge circuit is detected with an operation amplifier 5. For an operation amplifier 10, resistors 7 and 8 and a heat resistor element 9 for compensating temperature are provided. A voltage output V2 as a function of the flow rate of air becomes as follows: V2=R3R7V1/(R7+R8+Rc)(R2+R3)-R7V01/(R7+R8+ Rc)-V02. The effect term VTOFF of the offset voltage in this expression is as follows: VTOFF=R7V01/(R7+R8+Rc)+V02. When R1=10OMEGA, R2=9kOMEGA, R3=400OMEGA, R7=232OMEGA, R8=11OMEGA and Rc=20OMEGA, VTOFF=0.88V01+V02 is obtained. The effect of the offset can be made sufficiently small. Therefore, a hot wire type air flowmeter having a high response speed and excellent characteristics is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a hot-wire type air flow meter for measuring the intake air flow rate of an internal combustion engine such as a gasoline engine. Regarding air flow meters.

[Conventional technology] In order to fully comply with high-performance exhaust gas regulations for gasoline engines for automobiles, systems that detect intake air flow rate and control the engine have been widely put into practical use. Various types of air flowmeters are used, and one type of air flowmeter is the hot wire type air flowmeter.
- There is a wire airflow sensor.

By the way, as a conventional technology for this hot wire type air flowmeter, for example, there is Japanese Patent Application Laid-Open No. 3F"77, which is said to have low power consumption, sufficient temperature compensation, and excellent characteristics. Are known.

[Problems to be Solved by the Invention] The above-mentioned conventional technology does not give sufficient consideration to the offset voltage of the differential amplifier, so-called operational amplifier, used therein, and has a problem in terms of responsiveness.

That is, as shown in FIG. 3, this conventional technology includes a modified balanced bridge circuit consisting of resistors 1 to 3 and a thermal resistance element for detecting air flow rate, that is, a hot wire 4, and a balanced state of this modified balanced bridge circuit. An operational amplifier 5 for detection, a 1-ranjistalro that is controlled by the output of this operational amplifier 5 and serves to control the current flowing into the modified balanced bridge circuit including the hot wire 4, and resistors 7 and 8 form a feedback circuit for temperature compensation. It consists of an operational amplifier equipped with a thermal resistance element for temperature compensation, that is, a cold wire 9, and a DC power supply 11, and is designed to obtain a voltage output ■2 as a function of air flow rate. The operational amplifiers 5 and 10 are each equipped with Offcella 1.
~Voltage V. Even though there is 1.1 VO2, no consideration has been given to these, and as mentioned above, there is a problem with responsiveness.

To explain this point in more detail, first, in FIG. 3, the output voltage v2 is the input voltage v1 of the bridge circuit and the offset voltage V of the operational amplifiers 5 and 10.

11VO2, and resistance values R1 to R3, R7, R11, and Rc of each resistor 1 to 3.7.8 and cold wire 9.
When expressed as (1), it becomes as shown in equation (1).

Here, R = 10Ω, R2 = 9Ω, R3 = 400
Ω.

If R7=232Ω, R,=11Ω, and Rc-20Ω, then ■ in equation (1). , and■. The coefficients of the terms of 2 are as follows.

Therefore, the offset voltage ■ that operational amplifiers 5 and 10 have independently. □1 VO2 will be amplified approximately six times. Note that how many times it is multiplied depends on the value of the constant in equation (1).

Next, the relationship between the offset voltage and the sensor output will be explained. Similarly, the following equation can be derived from FIG. 3. Note that RR is the resistance value of the wire 4 to the wire 1.

V1=K (vpl-vN1+vo,) --(
4) V2=I. R1--(5) ■9, = ■2+■1・R3...(6) VP,,=A
(v2+v.,)...47)I h
= (vl V2) / RPI
-”'(9) Knee-(v1V2) / (R2+R3
)...(10) When the electric current 8 flowing through the hot wire 4 is derived from the above equation, the following equation is obtained.

The only term that changes on the right side of this equation is R8, and the other terms can be considered constants.

Therefore, when formula (11) is transformed, (A, B, and C are constants), furthermore, for the two operational amplifiers 5 and 10 in FIG.
The same thing is often used, so vo, = vo
2, and the result is:

Here, if we calculate ■,,′, becomes.

On the other hand, the relationship between the heat generation amount of the hot wire 4 and the air flow rate is as follows:
As known from the famous King's law, it is expressed by the following equation.

Ih”R)l= (x+y, rth) (RI4Rt
) ・ ・(15) Here, R2 is the resistance value of the hot wire 4 at room temperature, U is the flow rate, and X and Y are the wire 4 to the hot wire 1.
and a fluid-dependent constant.

Transforming equation (15), we get becomes.

Next, the voltage and current of the circuit in Figure 3 are calculated using equation (14) and (]B
It exists stably at a point that satisfies Equation 6 at the same time. This is shown in FIG.

In this figure, R8 is plotted on the horizontal axis and becomes large, R,=ω
becomes zero. As is clear from equation (14), the offset voltage V. If 1 is large, the curve will shift to the right side, and as shown in the figure, the offset voltage of curves C□-B and -A1 will be ■.

□-1, the voltage to the offset cell l larger than this value■
. , -2, the curve becomes C2-B2-A, and for larger (V,, 3), B3-B3
-A3.

On the other hand, the curve determined from equation (16) is a curve that extends from RH''Rt toward the upper right.

Therefore, A1. which is the intersection of both. A2, A3g B
) gB2. B3. C1, C? ,, B3 becomes the stable existence point.

First, if the flow rate is U=O, the circuit will be A1, A2, or A3 depending on the magnitude of the offset voltage.
It is stable.

Next, if the flow rate changes to U, the stable point is B1.
B2. Move on to B3. At this time, the length of B1A1 is B2
There is a relationship between the length of A2 and the length of B5 and A3, and in a circuit with a large offset voltage, the distance from one stable point to the next stable point will always be long, and the response time will be lengthened by this amount.
This has been revealed both experimentally and analytically.

From the above, it can be seen that reducing the influence of the offset voltage of the circuit is important from the viewpoint of responsiveness.

The above shows the relationship between <Ih'' and RH based on static analysis, but if we look at the relationship between ■2 and vl, it becomes as shown in equation (1), and V□ changes with the change in R. v2 changes, and the result is amplified by the operational amplifier and fed back, causing ■1 to change again.Therefore, dynamically, as shown in equation (1), to be influenced.

[Means for Solving the Problems] The above object is achieved by returning the modified balanced bridge circuit to a normal balanced bridge circuit and changing the connection with the differential amplifier.

[Operation] By using a normal balanced bridge circuit and changing the connection of the differential amplifier for balance detection and the differential amplifier for temperature compensation to this circuit, the sum of the offset voltage components due to these two differential amplifiers is It is smaller than the total value of offset values 1 to 2 that each of these differential amplifiers has individually, and the response is improved.

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

FIG. 1 shows an embodiment of the present invention. This embodiment differs from the conventional example shown in FIG. 3 in that the lower end of the resistor 3 is not connected to the non-inverting input of the operational amplifier 10, ), and the bridge circuit consisting of the resistors 1 to 3 including the hot wire 4 is not a modified balanced type, but is returned to a balanced type.

From this figure, the following formula can be derived.

Vl-VN,,=I2R2...(17)VN1=I
2R3 (18) VP, -VN2= +4 (R8+RC) -...
(+9)VN2=I, R7=--(20)Vp,
VN-+ Vol-○ --(21)■
2-vN2+vo,=O...(22) From the above,...(24) The influence term of the offset voltage in equation (24) is as follows.

If we assume FF, then if we insert the same constants as in equations (2) and (3), we get
VTOFF=0.88Vo, +V,
--(26) This is sufficiently smaller than the influence of the offset in the conventional example shown in FIG. That is, VTOFF=0.88Vo, +Vo2<6V
, 1+6V.

Therefore, according to this embodiment, it is possible to obtain a hot wire type air flowmeter with high response speed and excellent characteristics.

Also, according to equations (25) and (26), it is easy to understand that, and therefore, according to this example,
It can be seen that the overall offset voltage VTOFF is smaller than the sum of the offset voltages that the two operational amplifiers 5 and 10 have individually.

Next, another embodiment of the present invention is shown in FIG.

The difference between the embodiment shown in FIG. 2 and the embodiment shown in FIG. 1 is as follows.
The operational amplifier 10 for temperature compensation is removed from the feedback loop of the operational amplifier 5 for balance detection] 1 and is configured as a path for taking out the output voltage V, and as a result, the output voltage V is temporarily increased without temperature compensation. 2 is generated, temperature compensation is performed by an operational amplifier 10 having a cold wire 9 as a feedback circuit.

The operating conditions for the embodiment of FIG. 2 are derived as follows.

Vl-VN□=I 2R2--(29)V, 1=I,
R,...(30) I 4 (R7+R11+ RC) =V3
・・・・・・(31)VP□−VN,+V,,=O−
-(32)V3=A(V2-VN□-V,,)
-...(33)V, 2=I, -R7--(34) From the above equation, in equation (34), the total offset voltage VTOFF is calculated by combining equations (2) and (3). Entering the same numbers as when calculating, vTorF-0,567 (vo1+■.2) <vo1+
v. Therefore, in this embodiment as well, the magnitude of the offset voltage that is affected by the circuit as a whole can be made smaller than the sum of the offset voltage values that the two operational amplifiers 5 and 10 have individually. It can be seen that the response can be sufficiently improved.

[Effects of the Invention] According to the present invention, the influence of the offset voltage of the differential amplifier can be sufficiently reduced, so that the features of a high-performance hot-wire air flowmeter using multiple differential amplifiers can be fully realized. It can be easily applied to an intake flow meter with excellent responsiveness.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing one embodiment of a hot wire air flowmeter according to the present invention, and FIG. 2 is a circuit diagram showing another embodiment.
Figure 3 is a circuit diagram showing a conventional example of a hot wire air flowmeter;
The figure is a characteristic diagram for explaining the operation. 1 to 3...Resistance, 4 - Wire to Hola 1 (thermal resistance element for detecting air flow rate), 5.10... Operational amplifier, 6
Transistor, 7, 8... Resistor, 9 Cold wire (thermal resistance element for temperature compensation), 11 DC power supply. Agent Patent Attorney Junji Takeshi Department (1 other person) Figures 1 to 3... Resistor 4... °Hot Wire 5.10... Oyaa' Noah 9 6... - Transistor 7.8...・Resistance 9... Cold wire 1... DC power 5 and

Claims (1)

[Claims] 1. A resistance bridge circuit including a thermal resistance element for detecting air flow rate on one side and connected to a DC voltage input terminal and a common potential;
A differential amplifier that detects a voltage balance state of the resistor bridge circuit, and a current control element connected between the DC voltage input terminal and the DC power supply, and the differential amplifier is connected to a control input of the current control element. In the hot wire air flowmeter of the type that controls the heating current of the thermal resistance element for detecting air flow rate by supplying an output of a second circuit whose input and output are connected between the point and the non-inverting input of the differential amplifier;
and a thermal resistance element for temperature compensation connected between the inverting input and the output of the second differential amplifier, and temperature compensation for the detected value of the air flow rate controls the heating current. A hot wire air flow meter characterized in that the hot wire air flow meter is configured to be operated within a feedback loop for 2. A resistance bridge circuit including a thermal resistance element for detecting air flow rate on one side and connected between a DC voltage input terminal and a common potential, a differential amplifier for detecting a voltage equilibrium state of the resistance bridge circuit, and the above-mentioned DC A current control element connected between a voltage input terminal and a DC power supply is provided, and the output of the differential amplifier is supplied to the control input of the current control element to control the heating current of the air flow rate detection thermal resistance element. A second differential amplifier having a non-inverting input connected to a voltage balance detection point on one side of the resistance bridge circuit including the air flow rate detection thermal resistance element; A temperature-compensating thermal resistance element connected between the inverting input and the output of the second differential amplifier is provided so that a temperature-compensated air flow rate detection value can be obtained at the output of the second differential amplifier. A hot wire air flowmeter characterized by having the following configuration.