JPH0143883B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a hot wire flowmeter, and more particularly to a hot wire flowmeter for measuring the intake air amount of an internal combustion engine.

In internal combustion engines that supply fuel to the engine through fuel injection valves, the opening and closing of the injection valves is controlled according to the amount of intake air taken into the engine, and this amount of intake air is measured. An intake air flow meter is provided for this purpose. As such an intake air flowmeter, there is a hot wire flowmeter that measures the intake air flow rate by extending a predetermined metal wire in the intake air path, passing a predetermined current through the wire, and measuring the resistance change of the metal wire in response to fluctuations in air flow velocity.

In such a conventional hot wire flowmeter, a first voltage dividing circuit including a first and second resistor connected in series and a second voltage dividing circuit including a metal wire and a third resistor connected in series are connected in parallel. A connected circuit, or bridge circuit, is formed. A temperature compensation resistor made of a metal wire is connected in series with the first resistor on the side including the first resistor of the bridge circuit, and the temperature compensation resistor is provided in the intake air path together with the metal wire and the third resistor. There is. The metal wire is heated by electricity during engine operation to become a hot wire, and is cooled to a degree according to the intake air flow rate, changing its resistance value.
The resistance value of the metal wire is detected by the voltage across the third resistor, that is, the output voltage of the second voltage dividing circuit, which changes in proportion to the current flowing through the metal wire, and this voltage is output as a value corresponding to the intake air flow rate. .

Further, a current control circuit is connected to the outputs of the first and second voltage divider circuits, and the current control circuit supplies a current to the bridge circuit according to the potential difference between the divided output voltages of the first and second voltage divider circuits. The bridge circuit operates so that the resistance value of the metal wire always becomes a predetermined value due to the equilibrium condition of the bridge circuit.

However, in such a hot wire flowmeter,
Since the first and second resistors of the first voltage divider circuit are formed on the board together with the current control circuit and installed inside the case, temperature changes due to heat generated by the current control circuit and heat from the engine around the case can cause The resistance values of the first and second resistors change due to temperature changes.
Therefore, there was a problem in that although the bridge circuit was controlled to be in a balanced state, the resistance value of the metal wire was not controlled to be a predetermined value.

SUMMARY OF THE INVENTION An object of the present invention is to provide a hot wire flowmeter that controls the resistance value of a metal wire to a predetermined value even if the resistance values of the first and second resistors change depending on the temperature.

The hot wire flowmeter according to the present invention has a temperature coefficient α ₁ of the first resistance (resistance value: R ₁ ) and a second resistance (resistance value: R ₂ ).
The ratio of the temperature coefficient α ₂ to the temperature coefficient α 2 is α ₁ /α ₂ =R ₁ +R _K /R ₁ (R _K is the resistance value of the temperature compensation resistor) or an approximate value thereof.

Embodiments of the present invention will be described in detail below with reference to the drawings.

In the figure, the metal wire R _H is preferably made of platinum, and the resistors R ₁ are the first, second and third resistors.
to _R3 together form a bridge circuit 1.
Temperature compensation resistor R _K is connected to resistor R ₁ on the side that includes resistor R ₁ .
The resistor R _K is provided in the intake air passage 2 together with the metal wire R _H and the resistor R ₃ . The connection point α between the metal wire R _H and the resistor R _K of the bridge circuit 1 is a power supply terminal, and the connection point b between the resistors R ₂ and R ₃ is grounded. Also, metal wire R _H and resistor R ₃
The connection point c between the resistors R ₁ and R ₂ and the connection point d between the resistors R 1 and R 2 are connected to the differential amplifier circuit 3 . Differential amplifier circuit 3
constitutes a current control circuit 4 together with the transistor _Q1 , and generates a voltage at the output terminal proportional to the potential difference between the connection points c and d, and this voltage is applied to the base of the transistor _Q1 . A power supply voltage V _B is applied to the emitter of the transistor _Q1 , and the collector is connected to the connection point a.

In the hot wire flowmeter with the above configuration, the metal wire R _H
The potential difference between the connection points c and d due to the change in the resistance value changes the base voltage of the transistor _Q1 , and the current supplied from the transistor _Q1 to the bridge circuit 1 maintains the resistance value of the metal wire _RH at a predetermined value. increases or decreases. When such control is performed, the equilibrium condition of the bridge circuit 1 is satisfied, R _H = R ₁ + R _K /R ₂ × R ₃ ...(1) (Here, R _H , R ₁ , R ₂ , R ₃ and R _K is metal wire R _H ,
These are the resistance values of resistors R ₁ , R ₂ , R ₃ and R _H. ) becomes. However, in reality, the resistance values of the resistors R ₁ and R ₂ change depending on the ambient temperature t°C caused by the heat of the current control circuit 4, etc., so from equation (1), when the metal wire
The resistance value of R _H is R _H = R ₁ (1 + α ₁ t) + R _K /R ₂ (1 + α ₂ t) × R ₃ ...(2
) α ₁ : Temperature coefficient of resistance R ₁ α ₂ : Temperature coefficient of resistance R ₂ . Therefore, in the hot wire flowmeter according to the present invention, the ratio of temperature coefficients α ₁ and α ₂ is as shown in the following equation.

α ₁ /α ₂ =R ₁ +R _K /R ₁ (3) The reason for this will be explained next. Controlling the resistance value of the metal wire R _H to always be a predetermined value without being affected by the ambient temperature t°C of the resistors R ₁ and R ₂ means that the resistance value of the metal wire R
The resistance value of the metal wire R _H when the ambient temperature is t°C must be equal to the resistance value of the metal wire R _H when the ambient temperature is t°C. In equation (2), when t=0, equation (1) holds, so equation (1)=equation (2) holds true. Therefore, t{R ₁ α ₁ −α ₂ (R ₁ +R _K )}=0 (4). Since it is not affected by the ambient temperature t°C, from equation (4), R ₁ α ₁ −α ₂ (R ₁ +R _K )=0 (5). In other words, if the ratio of _the temperature coefficient α ₂ of the resistance R ₂ to the temperature coefficient α ₁ of the resistance R 1 is determined to be α ₁ /α ₂ = R ₁ + R _K /R ₁ or an approximate value thereof, the resistance
The resistance value of the metal wire R _H can be controlled to always be a predetermined value without being affected by the ambient temperature of R ₁ and R ₂ .

As described above, according to the hot wire flowmeter of the present invention, the ratio of the temperature coefficient α ₂ of the second resistor R ₂ to the temperature coefficient α _{1 of the first resistor R 1 of} the bridge circuit is R ₁ +R _K /R ₁ or this ratio. Since the values are approximate, even if the resistance values of the first and second resistors R ₁ and R ₂ change depending on the ambient temperature, the resistance value of the metal wire is always controlled to a predetermined value. .

[Brief explanation of drawings]

The figure is a circuit diagram showing an embodiment of the hot wire flowmeter of the present invention. Explanation of symbols of main parts, 1...Bridge circuit,
2...Intake air path, 3...Differential amplifier circuit, 4...
Current control circuit.

Claims (1)

[Claims] 1. First resistor connected in series (resistance value: R ₁ )
a first voltage dividing circuit including a metal wire and a third resistor provided in the gas passage and connected in series; and a second voltage dividing circuit including a metal wire and a third resistor provided in the gas passage and connected in series; a temperature-compensating resistor (resistance value: R _K ) of the metal wire connected in series with the voltage difference detecting means for detecting a potential difference between the divided voltages of the first and second voltage dividing circuits; An active element connected in series to one end of the second voltage divider circuit and activated according to the output signal of the potential difference detection means, and a voltage between both ends of the series circuit of the first and second voltage divider circuits and the active element. and a power source that supplies the first
A hot wire flowmeter characterized in that the ratio of the temperature coefficient α ₂ of the second resistance to the temperature coefficient α ₁ of the resistance is R ₁ +R _K /R ₁ or an approximate value thereof.