JP2511975B2 - Hot wire air flow meter - Google Patents

Description

The present invention relates to a thermal air flow meter, and more particularly to a thermal air flow meter for detecting the intake air flow rate of an internal combustion engine.

[Conventional technology]

In the conventional thermal air flow meter, as described in Japanese Patent Laid-Open No. 59-31412, the support for holding the heating resistor is made of a plastic base integrated with the housing.

[Problems to be solved by the invention]

As a result, the temperature rises due to the self-heating of the power transistor, which is part of the drive circuit, and the operating environment becomes harsh and the service life of the connection part of the power transistor shortens. It becomes an obstacle to raising the internal temperature.

An object of the present invention is to provide a thermal type flow meter with improved reliability by lowering the temperature of the components of the drive circuit.

[Means for Solving the Problems] The above object is to install a heating resistor for measuring an air flow rate in an air passage, and a support for holding the heating resistor.
A thermal air flow meter including a drive circuit for controlling the current of the heating resistor and taking out the output voltage of the heating resistor as a signal corresponding to the amount of air, and a metal case member to which the driving circuit is adhesively fixed. In one aspect, one surface is in contact with the constituent member of the air passage, and the other surface is provided with a metal base member to which the metal case member is bonded.

[Action]

When the base material plastic is changed to aluminum, its thermal conductivity increases significantly from 0.19Kcal / mh ℃ to 180Kcal / mh ℃. Therefore, the self-heating of the power transistor forming a part of the drive circuit can be released to the outside, and the temperature rise of the power transistor and the like can be prevented.

〔Example〕

Embodiments of the invention will be described below with reference to the drawings. FIG. 1 is a structural diagram of a heating resistor, and FIG. 2 is a circuit diagram showing the operating principle.

The same element is used for the heating resistor 1 for detecting the intake air flow rate and the intake air temperature measuring resistor 2, and the diameter φ0.5mm,
Wind a platinum wire 102 on a bobbin 101 of alumina with a length of 2 mm,
After welding both ends to the lead wire 103, a thin glass coating 104 was applied on the surface. These are as shown in the top view of the thermal air flow meter of FIG. 3 and the sectional structure view of FIG. It is installed in a bypass passage of a body 107 having a main passage 105 through which most of the intake air passes and a bypass passage 106 through which a part of the intake air flows.

The drive circuit 100 includes a heating resistor 1 and an intake air temperature measuring resistor 2, operational amplifiers 3, 4, power transistors 5,
The power transistor 5 is composed of resistors 6 to 10, and the collector terminal 11 of the power transistor 5 is the positive electrode of the battery, the earth terminal 12 of the resistor 6 is the positive electrode of the battery, and the connection point 13 between the resistor 6 and the heating resistor 1 is 13 An input terminal of a microcomputer that controls the engine using the output signal of the thermal air flow meter is connected to the.

The heating resistor 1 is heated by the power transistor 5 and its temperature is kept higher than the intake air temperature by a constant temperature. At this time, the intake air temperature measuring resistor 2 is designed to pass only a small current such that heat generation can be ignored, and the intake air temperature measuring resistor 2 has the same value as the intake air temperature. Detect as resistance value.
Here, when the airflow hits the heating resistor 1, the drive circuit is operated so that the temperature difference between the heating resistor 1 and the intake air temperature measuring resistor 2 is always kept constant. This operation is performed by adding the voltage drop of the resistor 6 generated by the current flowing through the heating resistor 1 to the voltage obtained by dividing the potential difference between both ends of the heating resistor 1 by the resistors 7 and 8 and the voltage drop of the resistor 6 described above. Feedback is applied so that the voltage amplified by the operational amplifier 3 is always equal. Therefore, when the air flow rate changes, the current flowing through the heating resistor 1 changes, and the air flow rate is read by the voltage drop of the resistor 6 according to the current.

By the way, in the present invention, the holders 17 are formed by insert-molding the supports 14 and 15 for holding the heating resistor 1 and the aluminum base 16. Further, the fine bank 18 having the drive circuit portion is adhered to the metal case 19 and then fixed to the base 16, and the plastic housing 20 is also fixed at the same time. In such a configuration, the amount of heat generated by the operational amplifier 3 of the drive circuit unit can be released to the body 107 via the Al base 16 and the temperature rise of the drive circuit unit can be prevented.

Furthermore, if, for example, an Odor pin 21 is press-fitted into a part of the Al base 16 and connected to the case 19 by spot welding for grounding, the resistance to radio interference can be improved, and the structure is significantly simpler than the conventional example. It is possible to reduce the cost of parts and improve productivity. Further, the warp amount can be reduced as compared with the resin base, and the stress generated when the screw is tightened can be reduced, so that the crack of the drive circuit section can be prevented.

[Advantages of the Invention] According to the present invention, the temperature of the components of the drive circuit can be reduced, and a thermal air flow meter with improved reliability can be provided.

[Brief description of drawings]

FIG. 1 is a structural view of a heating resistor, FIG. 2 is a circuit diagram showing the operating principle, FIG. 3 is a top view of a thermal air flow meter of the present invention, and FIG. 4 is a sectional structural view thereof. 1 ... Heating resistor, 2 ... Air temperature measuring resistor, 14,15
…… Support, 16 …… Base, 19 …… Case, 20 …… Housing, 21 …… Pin.

Claims (1)

(57) [Claims] 1. A heating resistor which is installed in an air passage and measures an air flow rate, a support which holds the heating resistor, a current of the heating resistor is controlled, and an output voltage of the heating resistor is controlled. In a thermal type air flow meter provided with a drive circuit for taking out as a signal corresponding to the amount of air, and a metal case member to which the drive circuit is adhered and fixed, one surface is in contact with a constituent member of the air passage and the other surface is A thermal air flow meter, characterized in that a metal base member, to which the metal case member is adhered, is provided in the.