JPH05312617A - Thermal air flowmeter - Google Patents

Abstract

PURPOSE:To obtain a highly accurate air flowmeter for measuring suction air flow of internal-combustion engine in automobile by suppressing characteristic accuracy error due to the shape of heating element. CONSTITUTION:A heating element 1 comprises a bobbin 2, lead wires 3, an adhesive, a resistor 6, and a glass coat 7, wherein the adhesive is composed of a partial crystal of glass 4 and ceramics 5 while the bobbin 2 is composed of porous alumina. This constitution reduces characteristic fluctuation of thermal air flowmeter by a factor of 2/3 or more as compared with a conventional one.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal air flow meter,
In particular, it relates to a thermal air flow meter for detecting the intake air amount of an internal combustion engine.

[0002]

2. Description of the Related Art A conventional bobbin type thermal air flow meter has a structure in which lead wires are inserted into both ends of a bobbin such as ceramic and the bobbin and the lead wires are sintered, as described in Japanese Utility Model Publication No. 56-96326. is there. Next, a platinum wire is wound around a bobbin, the lead wire and the platinum wire are welded, and the surface of the bobbin is coated with glass.

One of the characteristics of this heating resistor is that glass is used for bonding the bobbin and the lead wire, which prevents the heat generated by the bobbin from escaping to the lead wire, thereby improving the responsiveness. Is to improve.

[0004]

In the heating resistor described in the above-mentioned prior art, no consideration is given to the shape stability of the molten glass when the bobbin and the lead wire are bonded to the glass. There was a drawback that the molten glass would flow to the lead portion which is not necessary for adhesion. For example, if glass flows into the lead wire, welding failure will occur when the platinum wire is welded to the lead wire in the next step. When the molten glass flows on the inner wall of the bobbin or the lead wire, the shape of the completed bobbin type heating resistor varies because the amount is not constant, which causes variations in characteristics such as responsiveness and heat dissipation characteristics. It was

When the glass flows on the inner wall of the bobbin or the lead wire, a gap is formed between the bobbin and the lead wire, so that foreign matter may enter the bobbin, resulting in a problem of reliability.

Another disadvantage of the prior art is that if microscopic cracks are present in the glass, the cracks will develop due to repeated stress of cold heat and external force in a practical state, and the characteristics will change.

On the other hand, for example, a crystalline material such as a glass material or a metal paste may be used as an adhesive. However, when crystalline glass is used, the progress of cracks cannot be suppressed even if the shape stability of the glass during firing can be maintained, and when a metal paste is used,
Although there is no concern about cracks, the heat generated by the bobbin easily escapes to the lead wires and the responsiveness deteriorates, so this does not solve the problem.

An object of the present invention is to prevent molten glass from flowing out to the lead wire or flowing to an unnecessary portion of the inner wall of the bobbin when the bobbin and the lead wire are bonded to each other by glass. It is to prevent welding failure at the time and to reduce variations in characteristics.

It is another object of the present invention to suppress the progress of cracks in the glass bonded portion in a practical state and improve reliability.

[0010]

The above object is achieved by preliminarily mixing powder of alumina, zirconia or the like in the glass material powder used for bonding the bobbin and the lead wire, and bonding by using this powder. It

It is also achieved by using porous alumina as the bobbin material.

[0012]

When an adhesive containing ceramic powder such as alumina or zirconia in glass powder is used, a part of the ceramic is melted into the glass during the glass melting process to form a glass-ceramic composite. At this time, the glass which has not yet been melted becomes a nucleus and the glass is crystallized. This can prevent the glass from flowing out to the lead wire or the inner wall of the bobbin.

Further, since a part of the ceramics remains in the glass even after the bonding, this plays a role of suppressing the development of cracks in a practical state. That is, even if minute cracks occur in the glass, the fine ceramic particles serve as a resistance to suppress the development of cracks.

On the other hand, by using porous alumina as the bobbin, even if both ends of the bobbin are completely covered with the glass when the lead wire and the bobbin are bonded and fired by the glass, the inside of the bobbin Since the expanded air goes out through the porous alumina, it does not push the molten glass under the internal pressure. This can prevent the glass from flowing out to the lead wires.

[0015]

EXAMPLE An example of the present invention will be described below.

FIG. 1 is a structural diagram of a heating resistor 1 according to the present invention. The heating resistor 1 has lead wires 3 on both ends of the bobbin 2.
4 is mixed with a paste prepared by mixing 4 powders of glass and 5 powders of ceramics with an appropriate solvent, bonded by firing, and a metal wire 6 is wound around the surface and further coated with glass 7. As shown in the process diagram of FIG. 2, the bobbin 2 of the heating resistor 1 and the lead wire 3 are connected such that glass 4 powder and ceramic 5 powder are dispersed before firing, but when this is fired, the glass 4 melts. In this process, the ceramic 5 powder is dispersed in the glass and changes into a glass having a different composition from the initial glass 4. Further, in this process, the ceramics 5 powder which is not dispersed becomes a nucleus and a part of the glass is crystallized. The mechanism of this partial crystallization increases the viscosity of the glass and prevents the glass from flowing out to the lead wire or the inner wall of the bobbin during firing.

As a combination of the glass 4 powder and the ceramic 5 powder necessary for partially crystallizing the glass 4, lead borosilicate glass, lead glass, or zinc glass is used for the glass 4,
A study was conducted using alumina as the ceramic 5.

As a result, the average particle size of alumina is in the range of 1 μm to 15 μm, although the degree of partial crystallization varies slightly depending on the firing conditions and the particle size of alumina mixed in, and the proportion of alumina mixed in the glass is 5 wt%. 30 wt%
Was found. FIG. 1 shows a schematic diagram of a heating resistor manufactured by partial crystallization. In the case of this structure, the viscosity of the glass 4 that has been melted during the firing process increases, and it is difficult for the glass 4 to flow into the lead wire 3. Therefore, since the shape of the heating resistor 1 can be stabilized, variations in responsiveness and heat dissipation characteristics can be reduced.

Further, in the conventional product, when minute cracks were present in the glass 4, there was a phenomenon that the cracks developed due to the repeated stress of cold heat and external force in a practical state and the characteristics were changed. However, according to the present example, it was found that even if a repetitive stress of cold heat or an external force is applied, the progress rate of cracks is slow and the progress of cracks is suppressed in the partially crystallized portion. That is, according to this embodiment, it is possible to improve reliability in a practical state.

As another example, as a result of studying partial crystallization by changing the type of ceramics 5 powder, similar results were obtained with zirconia powder. In this case also glass 4
The investigation was carried out using lead borosilicate glass, lead glass, and zinc glass.

As a result, the firing conditions for partial crystallization and the average particle size of zirconia mixed are in the range of 1 μm to 15 μm, and the ratio of alumina mixed in the glass 4 is 5 w.
It was found to be 30% by weight from t%.

On the other hand, when a large number of heating resistors were manufactured in the embodiment by partial crystallization described above, it was found that the glass 4 started flowing out to the lead wire 3 somewhat. When the cause is investigated, the lead wire 3 is inserted into both ends of the bobbin 2. Therefore, if both ends are covered with glass, the inside of the bobbin 2 becomes a sealed structure, and the air inside expands as the temperature rises during firing. However, it was found that the glass 4 was pushed out.

Another embodiment invented to prevent this will be described below.

FIG. 3 is a schematic diagram in which the lead wire 3 is connected to the bobbin 2 according to this embodiment. That is, by lowering the sintering density of the ceramics, which is the material of the bobbin 2, the air permeability is improved and the internal air expansion due to the temperature rise during firing is escaped from the bobbin. As a result of conducting an examination while changing the sintering density, it was found that when alumina was used as the material of the bobbin 2, the effect was obtained when the sintering density was 85% or less. However, when the sintered density is 50% or less, the strength of the bobbin 2 cannot be obtained, which is insufficient. Therefore, the optimum sintered density range of the bobbin 2 is set to a range of 50% to 85%.

According to this embodiment, the adhesive is ceramics 5.
It is possible to reduce the flow-out of the glass 4 to the lead wire 3 even with the ordinary glass 4 containing no powder.

Further, by producing the heating resistor 1 in combination with the embodiment by partial crystallization described above, a larger effect can be expected.

FIG. 4 shows an example in which the heating resistor 1 is manufactured by this combination and the variation in the flow rate characteristics as a thermal air flow meter is measured. The variation in flow rate characteristics is
It was confirmed that it could be reduced to 3 or less. It was also found that the effect is particularly large on the low flow rate side.

[0028]

According to the above structure, it is possible to reduce the variation in flow rate measurement accuracy, and a great effect can be expected especially on the low flow rate side. A comparison with the conventional product is shown in FIG.

[Brief description of drawings]

FIG. 1 is a sectional view of a heating resistor according to the present invention.

FIG. 2 is a cross-sectional view of the manufacturing process of FIG.

FIG. 3 is a sectional view of a bobbin according to the present invention.

FIG. 4 is a diagram showing measurement results of characteristic variations of the thermal air flow meter according to the present invention.

[Explanation of symbols]

1 ... Heating resistor, 2 ... Bobbin, 3 ... Lead wire, 4 ... Glass, 5 ... Ceramics, 6 ... Metal wire, 7 ... Glass coat.

Claims (8)

[Claims] 1. A means for measuring an intake air flow rate of an electronically controlled fuel injection device for an internal combustion engine, which controls a heating resistor installed in an air passage and a current flowing through the heating resistor, and
In a thermal air flow meter comprising a drive circuit for extracting the output voltage of the heating resistor as a signal corresponding to the air flow rate, glass powder and ceramics are used as the material used for bonding the bobbin and the lead wire of the heating resistor. A thermal air flow meter characterized by using a material mixed with powder. 2. The thermal air flow meter according to claim 1, wherein the ceramic powder is alumina and the average particle size is in the range of 1 μm to 15 μm. 3. The thermal air flow meter according to claim 1, wherein the proportion of alumina mixed with the glass is 5 wt% to 30 wt%. 4. The thermal air flow meter according to claim 1, wherein the ceramic powder is zirconia and the average particle size is in the range of 1 μm to 15 μm. 5. The thermal air flow meter according to claim 1, wherein the ratio of zirconia mixed with the glass is 5 wt% to 30 wt%. 6. A heat type air flow meter, characterized in that a bobbin is made of porous alumina as a bobbin material in a heating resistor having a lead wire inserted therein. 7. The sintered density of bobbin according to claim 6,
A thermal air flow meter characterized by being in the range of 0% to 85%. 8. The bobbin made of porous alumina as a bobbin material, wherein a material obtained by mixing glass powder and ceramic powder is used as a material used for bonding the bobbin of the heating resistor and the lead wire. A thermal air flow meter characterized by using.