JP2857491B2 - Hot wire air flow meter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to a flow meter for measuring a flow rate of a fluid represented by air or the like using a line of sight of platinum or the like, and particularly to a flow rate measurement of intake air of an internal combustion engine. It relates to a suitable hot wire air flow meter.

[Conventional technology]

Hot wire air flow meter (hot wire air flow sensor)
Has no moving parts and can directly measure the mass flow rate, and furthermore has the advantage of being able to measure the flow rate over a wide range with an almost constant error rate. It is becoming widely used.

By the way, as a conventional hot-wire air flow meter, for example, as described in Japanese Patent Application Laid-Open No. 61-16026, a resistance change of a hot-wire resistance element for flow rate detection and a temperature-sensitive resistance element for temperature detection is detected. A comparison circuit is provided, and the output of the comparison circuit controls the heating current control NPN transistor of the hot-wire resistance element.

[Problems to be solved by the invention]

In the above prior art, no special consideration is given to the operation when the power supply voltage falls below the set value, and therefore, when applied to the measurement of the intake air flow of an automobile engine, sufficient application of engine startability is provided. There was a problem with the point. That is, at the time of engine start cranking, the terminal voltage of the battery as the power supply is greatly reduced. Therefore, with the conventional hot-wire air flow meter, at the time of engine start, the air flow rate measurement is performed until the terminal voltage of the battery recovers. Since a value cannot be obtained and proper engine control cannot be performed, good startability cannot be provided.

An object of the present invention is to provide a sufficient resistance to a reduction in the power supply voltage to a set value or less, to enable an accurate measurement operation even at the time of engine start cranking, and to sufficiently obtain an improvement in engine startability. To provide a hot wire air flow meter.

[Means for solving the problem]

In order to achieve the above object, the control signal for the base of the NPN transistor for controlling the heating current of the hot-wire resistance element is supplied through a second NPN transistor instead of directly from the comparison circuit, whereby the heating current is controlled. The base current of the control NPN transistor is supplied directly from the power supply.

[Action]

Since the base current of the NPN transistor for controlling the heating current of the hot-wire resistance element is directly supplied from the power supply, a sufficient base current can be supplied to this transistor, and the voltage loss between the collector and the emitter of the transistor is reduced. It can operate sufficiently even with the power supply voltage.

〔Example〕

Hereinafter, a hot-wire air flow meter according to the present invention will be described in detail with reference to the illustrated embodiments.

FIG. 1 shows an embodiment in which the present invention is applied to an intake air flow meter of an internal combustion engine for an automobile. In the drawing, reference numeral 1 denotes a battery for a power supply, 2 denotes a hot wire resistance element (hot wire) for detecting a flow rate, and 3 Is a temperature-sensitive resistance element (cold wire) for temperature detection, 4 is an NPN transistor for heating current control, 5 is a differential amplifier that functions as an amplifier for temperature detection, 6 is a differential amplifier that functions as a comparison circuit, and 7 is a second amplifier. NPN transistor, 8
Is a Zener diode, and 9 to 15 are resistors.

The battery 1 is also used as a power source for starting and controlling the engine.
The terminal voltage greatly decreases and fluctuates.

The hot-wire resistance element 2 is connected in series with the resistor 7 and is connected in parallel to a series circuit of resistors 10 and 11 for dividing the voltage drop of the hot-wire resistance element 2.

On the other hand, the temperature-sensitive resistance element 3 is connected between the output terminal and the inverting input terminal of the differential amplifier 5, and forms a feedback circuit of the differential amplifier 5.

NPN transistor 4 is connected to the positive voltage terminal of battery 1,
The collector-emitter thereof is inserted between the connection point between the hot-wire resistance element 2 and the resistor 10 and serves to control the heating current supplied to the hot-wire resistance element 2.

The non-inverting input terminal of the differential amplifier 5 is connected to the connection point between the hot-wire resistance element 2 and the resistor 9, and the inverting input terminal is connected to the common power supply terminal by the resistor 12, that is, the negative voltage terminal of the battery 1 in this case. , Respectively.

The differential amplifier 6 has its non-inverting input terminal connected to the junction of the voltage-dividing resistors 10 and 11, and its inverting input terminal connected to the output terminal of the differential amplifier 5, respectively. It operates as a comparison circuit for detecting a difference in resistance value change from the resistance element 3, and its output terminal is connected to the base of the second NPN transistor 7 via the resistor 15.

The second NPN transistor 7 controls the heating current control NPN transistor 4 by the output of the differential amplifier 6, so that its collector is connected via the resistor 13 to the positive voltage terminal of the battery 1, and The emitters are connected to a common power supply terminal via a resistor 14, respectively, and the collector is further connected to the base of the NPN transistor 4 for controlling the heating current.

Zener diode 8 is for protection from surge voltage.
For this reason, a connection point between the base of the heating current control NPN transistor 4 and the collector of the second NPN transistor 7,
It is connected in parallel with the opposite polarity between it and the common power supply terminal.

Next, the operation of the embodiment shown in FIG. 1 will be described.

As described above, the differential amplifier 6 operates as a comparison circuit that detects a difference in resistance change between the hot-wire resistance element 2 and the temperature-sensitive resistance element 3, and drives the NPN transistor 4 with its output.
It functions to control the heating current supplied to the hot-wire resistance element 2.

Therefore, for this purpose, the condition is that the voltage of the power supplied from the battery 1 must be equal to or higher than a predetermined value.

Then, the power supply voltage V required for the operation of such a hot-wire air flow meter is considered as follows.

That is, first, as in the prior art, it is assumed that the output of the differential amplifier 6 is directly supplied to the base of the NPN transistor 4 to control the heating current. If the voltage value is Vx ', this Vx'
Is represented by the following equation (1): Vx '= V _A + V _B + V _C + V _D (1) where V _A : voltage drop at the resistor 7 V _B : at the hot wire resistance element 2 V _C : The voltage drop between the base and the emitter of the NPN transistor 4 V _D : The voltage drop in the differential amplifier 6 Therefore, in the prior art, the terminal voltage V of the battery 1 is at least this formula (1). Requires the voltage Vx ′ given by

Then, if the voltage value of the power that is required in the embodiment of the present invention of FIG. 1 and Vx, the voltage value Vx is a voltage value V _Y required to operate the NPN transistor 4, This NPN
It can be separated into the voltage value V _Z that required for controlling the base current of the transistor 4, they are each of the following equation (2), represented by equation (3).

V _Y = V _A + V _B + V _C + V _E (2) V _Z = V _D + V _G + V _F + V _H ... (3) where V _E : voltage drop at resistor 13 V _F : The voltage drop between the base and the emitter of the NPN transistor 7 V _G : The voltage drop at the resistor 15 V _H : The voltage drop at the resistor 14 As a result, the condition required for this embodiment to operate is the battery 1 must be greater than or at least equal to any of these voltage values V _Y and V _Z , that is, both of the following conditions must be satisfied simultaneously.

V ≧ V _Y V ≧ V _Z By the way, the relations of the expressions (1) and (2) can be expressed by setting the resistance values of the resistors 14 and 15 to a predetermined value to easily _satisfy the condition of V _Y > V _Z. In this case, the conditions for the power supply necessary for the operation of the hot-wire air flow meter according to this embodiment are as described above, namely,
The voltage V of the battery 1 is a voltage value represented by the above equation (2).
V _Y or more is good.

On the other hand, looking at the embodiment of FIG. 1, the resistor 13 has a considerably small resistance value from a practical point of view, and the resulting voltage drop _VE is also a very small voltage. Are almost negligible, and as a result, the above equation (2) can be approximated as the following equation (3).

V _Y ≒ V _Y ′ = V _A + V _B + V _C (3) Then, from this, the voltage value Vx of the power supply required in the above embodiment is as follows: Vx = V _Y ≒ V _Y ′ Therefore, the voltage V of the battery 1 is expressed as follows: V ≧ Vx = V _Y ≒ V _Y ′ That is, V ≧ Vx = V _Y ′ V ≧ Vx = V _A + V _B + V _C (4) As long as it satisfies, in the above embodiment, the amount of air can be reliably measured.

Therefore, the voltage Vx required for operation in the embodiment of FIG.
Is compared with the voltage Vx 'required in the prior art, the above equations (1) and (4), that is, Vx' = V _A + V _B + V _C + V _D (1) Vx = V _A + V _B + V _C (4) As is apparent from (4), the voltage Vx required for operation in the embodiment of the present invention is the voltage V _D , that is, the differential It can be seen that the operation can be performed with a power supply voltage smaller by the internal voltage drop of the amplifier 6.

The air flow rate Q Figure 2 is to be measured on the horizontal axis, and a characteristic view taken respectively the minimum required supply voltage V _B on the vertical axis, respectively the characteristics of the present invention in solid lines, and the characteristics of the prior art by the dashed line As is apparent from this figure, according to the present invention, it is understood that the air flow rate can be sufficiently measured even with a considerably low power supply voltage as compared with the related art.

Therefore, according to the embodiment shown in FIG. 1, even when the voltage of the battery 1 is considerably reduced at the time of starting cranking of the engine, it is possible to reliably measure the intake flow rate. Engine control such as quantity control can be started from a very early point in time when the engine is started, and good startability can be easily provided.

By the way, in the embodiment of FIG. 1, the Zener diode 8 is provided so that a disturbance surge can be absorbed, and a stable flow meter side can be provided even in an environment such as an automobile engine which is easily affected by a disturbance surge. It is possible to easily obtain highly reliable engine control.

Also, in this embodiment, due to the presence of the resistors 11 and 15,
An RC series circuit including the internal junction capacitance of the Zener diode 8 and the resistor 13 and an RC series circuit including the resistor 15 and the internal junction capacitance of the second PNP transistor 7 are formed.
As a result, the function of suppressing high-frequency peaks is given to the frequency characteristics of the circuit meter, and in addition to being able to operate at a low voltage, the effect that the phase margin is widened is obtained. The reliability can be greatly improved.

FIG. 3 shows the frequency characteristics of one embodiment of the present invention in comparison with those of the prior art. In this figure, the upper side represents the gain characteristic, and the lower side represents the phase characteristic. The solid line is the characteristic of the present invention, and the broken line is the characteristic of the prior art.
Therefore, as is apparent from this characteristic, in the embodiment of the present invention, since the gain is greatly reduced in the high frequency region,
There is little fear of oscillation and the like, and a stable operation can be obtained.

〔The invention's effect〕

According to the present invention, the base current of the transistor for supplying the heating current to the hot-wire resistance element can be sufficiently supplied with a low power supply voltage, so that the lower limit of the operable power supply voltage can be sufficiently reduced, and The intake air flow rate can be sufficiently measured even at the time of starting cranking, and accurate engine control can be started at an early stage, so that good starting performance can be reliably obtained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a hot-wire air flow meter according to the present invention, and FIGS. 2 and 3 are characteristic diagrams for explaining the operation, respectively. DESCRIPTION OF SYMBOLS 1 ... Battery, 2 ... Hot wire resistance element, 3 ... Temperature sensitive resistance element, 4 ... NPN transistor for heating current control, 5, 6
…… Differential amplifier, 7 …… Second NPN transistor, 8…
... Zener diode.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masayoshi Suzuki 2520 Odaikoba, Katsuta-shi, Ibaraki Co., Ltd.Sawa Plant, Hitachi, Ltd. (56) References JP-A-60-237322 (JP, A) JP-A-61-16026 (JP, A) JP-A-63-45508 (JP, A) (58) Int.Cl. ⁶ , DB name) G01F 1/68 G01P 5/12

Claims (3)

(57) [Claims] 1. A detecting circuit for detecting a change in the resistance value of a hot-wire resistance element for detecting a flow rate and a resistance value of a temperature-sensitive resistance element for detecting a temperature,
In a hot wire air flow meter of a type in which an NPN transistor for controlling a heating current of the hot wire resistance element is controlled by an output of the detection circuit, a first resistor connected between a collector and a base of the NPN transistor; A second collector-emitter connected between the base of the transistor and the common power supply terminal;
A hot-wire air flow meter, comprising: an NPN transistor; and an output terminal of the detection circuit connected to a base of the second NPN transistor. 2. A hot-wire air flow meter according to claim 1, wherein a Zener diode is connected between a collector of said second NPN transistor and a common power supply terminal. 3. The hot-wire air flow rate according to claim 1, wherein a second resistor is connected between an output terminal of the detection circuit and a base of the second NPN transistor. Total.