JPH02179419A - Hot-wire airflow sensor and manufacture thereof - Google Patents

Abstract

PURPOSE:To simplify manufacture and to improve reliability by covering a hollow part with a composite of which the outer layer is formed of glass of a softening point 800 deg.C or above and the inner layer of glass and ceramics, and by burying a coiled metal wire in the inner wall of the hollow part. CONSTITUTION:A sensor 1 is made to be a heating resistor by making a current flow through a coiled platinum wire 2. This wire 2 is wound round in the shape of a coil virtually around the center line in the axial direction of a hollow part 6, and lead wires 3 of a platinum-iridium alloy are connected to the opposite ends of said wire by connecting parts 21 respectively. The hollow part 6 is sealed hermetically with a glass member 5 of a softening point 800 deg.C or above as an outer layer and with a composite member 4 of glass and alumina as an inner layer, and the lead wires 3 are supported by the members 4 and 5. By constructing the sensor in this way, the manufacture can be simplified and the reliability can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a hot-wire air flow sensor used for detecting the intake air amount of an automobile internal combustion engine, and a method for manufacturing the same.

[Conventional technology]

A hot wire air flowmeter uses a heating resistor as a sensor, and the heating resistor is placed in an air flow (thereby, the air flow rate is measured. In other words, the amount of heat dissipated by the heating resistor is determined by the air flow velocity). The current is changed to keep the temperature of the heating resistor constant as the air flow rate changes, and the air flow rate is indirectly measured by detecting this current. This kind of air flow meter has no moving parts and can directly measure the air flow rate, so it is widely used for controlling the air-fuel ratio of automobile internal combustion engines.

As the heating resistor in the hot wire type air flow sensor, for example, as described in Japanese Utility Model Application Publication No. 56-96326, an extremely thin metal wire made of platinum or the like with a diameter of several tens of microns is wound around a bobbin made of ceramic or the like. There is one that uses a spinning bobbin method.

In addition, as described in Japanese Patent Application Laid-Open No. 62-83622,
A bobbinless method is also known in which a metal wire serving as a heating resistor is wound around a metal core wire, coated with glass, and then only the metal core wire is etched away using acid.

[Problem to be solved by the invention]

With the bobbin type, the heat that heats the bobbin itself and the amount of heat that is transmitted to the bobbin and then to the support cannot be ignored, and the transient response to changes in air flow is delayed, so surging occurs when the car suddenly accelerates or decelerates. There was a problem that occurred.

On the other hand, the bobbinless type has excellent responsiveness because there is no bobbin. However, the process of removing the metal core wire around which the metal wire is wound by etching with acid after firing the coated glass is complicated. In addition, etching with acid when removing the metal core cotton will roughen the glass surface, and dust and ionic substances in the air will adhere to it under the usage environment, deteriorating its properties.
There was a problem of decreased reliability.

The object of the present invention is to solve these conventional problems and provide a hot wire air flow sensor that has excellent responsiveness, simplifies manufacturing by eliminating the need for etching, and improves reliability. To provide a manufacturing method.

[Means to solve the problem]

In order to achieve the above object, the present invention covers a cavity with an outer layer of glass having a softening point of 800°C or higher, an inner layer of a composite of glass and ceramics, and a coil-like structure with both ends electrically drawn out to the outside. A metal wire is embedded in the inner wall surface of the cavity.

Among the above structural members, ceramics are A1□Q3. Si
O□.

It consists of simple substances such as SiC, MgO, etc. or composite components thereof, and Al5o is given as a representative example, but A1□0
It is also possible to use any ceramic other than ゜. Further, the glass and alumina composite member obtained by penetrating the outer layer glass member into the inner layer alumina or a mixed member of alumina and glass maintains sufficient strength and provides the hot wire type air flow sensor with high dimensional accuracy. Therefore, the alumina contained in the composite member has a volume fraction of 30 to 70.

Furthermore, the present invention provides a step of winding a metal wire of a heating resistor around a core wire having sublimation property, for example, a molybdenum (Mo) core wire, and a step of winding a metal wire of a heat generating resistor around a core wire having sublimation property, for example, a core wire of molybdenum (Mo), and a step of winding a metal wire of a heating resistor on a core wire having sublimation properties, and a step of winding a metal wire of a heating resistor on a core wire having a sublimation property, and a step of winding a metal wire of a heating resistor on a core wire having a sublimation property, and a step of winding a metal wire of a heating resistor on a core wire having a sublimation property. A porous alumina or a mixture of alumina and glass is attached to the inner layer, and a porous glass member having a softening point of 800°C or higher, which is higher than the sublimation temperature of molybdenum, is attached to the outer layer. The molybdenum core wire is sublimated and removed while the outer glass member does not lose its porosity by heating them.
Thereafter, the outer layer glass member is heated to a temperature higher than that at which it flows, and the outer layer glass member penetrates into the voids of the porous alumina or alumina and glass mixed member of the inner layer.
The method includes at least a step of performing two-step firing to composite glass and alumina in one heat treatment.

[For production]

In the present invention, the metal wire of the heating resistor is wound into a coil around a sublimable molybdenum core wire, a metal lead wire that becomes an electrical lead-out part is welded to the tip, and the inner layer is made of alumina or a mixture of alumina and glass. A glass member with a softening point of 800°C or higher is attached to the outer layer and then baked to integrate them. However, since the molybdenum core wire is conductive and will short-circuit between the lead wires, it is first heated at 800°C. The molybdenum core wire is sublimated and removed by heating to a temperature higher than that, and then the temperature is raised to a temperature higher than that at which the outer glass member flows, and the outer layer glass member permeates into the inner layer alumina or the alumina and glass mixture member, and the inner layer becomes glass. A two-step firing process is performed to create a composite material of aluminum and alumina.

Therefore, in order not to prevent the sublimation removal of the molybdenum core wire, the glass member attached to the outer layer must maintain sufficient porosity during continuous failure of sublimation of molybdenum. It is necessary to have the characteristics.

For example, a platinum wire with high heat resistance and corrosion resistance is used as the metal wire, and a platinum-iridium alloy is used for the lead wire. In addition, molybdenum oxidizes at high temperatures and sublimates at about 795°C, but when sublimating molybdenum core wires, sufficient oxygen must be supplied and the inner layer of alumina material or a mixture of alumina and glass must be used to prevent sublimation and volatilization. It is necessary that the member and the outer layer glass member have sufficient voids. When removing this molybdenum core wire by sublimation, if the outer layer glass material flows at a temperature lower than the sublimation temperature of molybdenum, the surface will be covered before the molybdenum sublimes, so the core cotton will remain inside. Put it away. In addition, even if the outer layer glass does not flow at a temperature lower than the sublimation temperature of molybdenum, if the softening point is lower than the sublimation temperature of molybdenum, the fluidity will decrease due to the influence of sublimating molybdenum oxide, and the alumina Alternatively, the alumina and glass may not penetrate sufficiently into the mixed member, making it impossible to obtain a dense composite layer of glass and alumina.

Therefore, if glass with a softening point of 800°C or higher is attached as an outer layer, the influence of sublimated molybdenum oxide will be reduced, and after sublimation and removal of the molybdenum core wire, the glass will penetrate into the voids of the alumina or alumina and glass mixed member. , a dense composite layer of glass and alumina can be obtained in one heat treatment.

This eliminates the need for an etching step when removing the core wire, thereby simplifying the work. Furthermore, the problem of reduced reliability due to etching roughness can be solved.

Here, if metal lead wires are welded to both ends of a metal wire wound around a sublimable molybdenum core wire and only a glass member is attached to the surface, the hollow part after the molybdenum core wire has been sublimed and removed is melted. Not only will the glass member be buried, but the strong surface of the glass will cause the coiled metal wire to shrink and deform. However, if an alumina material or a mixed material of alumina and glass is attached to the inner layer and a glass material is attached to the outer layer, the alumina becomes a skeleton and the glass penetrates, making it possible to obtain a dense composite material of glass and alumina. .

In addition, when only one layer of glass and alumina mixed material is attached and fired to obtain a composite member, the softened and fluidized glass fills the voids and becomes densified. This is not preferable because it not only causes shrinkage of the member, making dimensional control difficult, but also causes variation in sensor characteristics due to changes in the shape and dimensions of the coiled metal. Therefore, alumina or a mixture of alumina and glass is attached to the inner layer, and a glass member is attached to the outer layer and then fired. However, if the volume percentage of the attached alumina material is high, the permeability of the glass member into the alumina material is poor, and the resulting composite material is The member has low strength. In addition, when the volume fraction of the alumina member is low, although the permeability of the glass member into the alumina member is good, the proportion of the glass member in the resulting composite member increases, and as mentioned above, the shape of the coiled metal wire due to shrinkage and This will cause dimensional changes.

As a result of experimental studies, if the volume percentage of alumina composited with glass is 70 to 30%, the permeability to the alumina attached to the inner layer of the glass member attached to the outer layer or to the alumina and glass mixed member is good. It was confirmed that a highly accurate hot-wire air flow sensor can be obtained with an inner layer made of a composite material of glass and alumina with sufficient strength, and an outer layer coated with glass.

Here, one of the methods for attaching glass, alumina, or a mixture of alumina and glass is by electrophoresis (deposition by electrophoresis).
Electrodeposition), but there is also a suspension using aluminum nitrate or magnesium nitrate as the electrolyte and ethyl alcohol as the organic solvent. Of course, electrodeposition is possible using electrolytes and organic solvents other than those described above, so electrodeposition is not limited to these. When glass powder is suspended in a solution of aluminum nitrate and magnesium nitrate, which is easy to handle, the glass component dissolves, making the electrodeposition solution unstable and making the electrodeposition layer non-uniform.

Therefore, from the viewpoint of electrodeposition solution management, it is necessary to select a highly stable type of glass to be mixed with alumina.

For example, when trying to obtain a hot wire air flow sensor composed of a glass member with a softening point lower than the sublimation temperature of molybdenum in the outer layer and a composite member of glass and alumina with a softening point lower than the sublimation temperature of molybdenum in the inner layer, As mentioned above, glass members whose softening point is lower than the sublimation temperature of molybdenum react with the sublimating molybdenum and lose their original properties, resulting in a significant drop in fluidity and insufficient penetration into the inner alumina voids. . Therefore, the inner layer of alumina or a mixture of alumina and glass is electrodeposited and fired, and after removing the molybdenum core wire, glass is attached to the outer periphery and fired again.
Two heat treatments are required to fully infiltrate the outer layer glass into the alumina voids in the inner layer to form a composite of alumina and the glass member. According to this method, handling strength is required to adhere the glass to the inner layer portion after the first heat treatment. When the inner layer portion is a single layer of alumina, the sintering temperature is lower than the sintering temperature of alumina, so the strength after removing the molybdenum core wire by sublimation is low. Therefore, a mixed glass and alumina material is electrodeposited on the inner layer portion to increase the strength after firing once, but as described above, the mixed electrodeposition solution of glass and alumina is unstable.

According to the present invention, the sublimation removal of the molybdenum core wire and the infiltration of the outer layer glass member into the inner layer alumina member voids to form a composite of alumina and the glass member can be performed in a single heat treatment.
There is no problem even if the inner layer to be electrodeposited is a single phase alumina. Therefore, a highly stable electrodeposition solution containing only alumina powder suspended can be used, and the electrodeposition solution can be easily managed, resulting in high workability.Of course, the inner layer to be electrodeposited is a mixture of glass and alumina. However, a high-strength, high-precision hot wire air flow sensor can be obtained.

In the hot wire type air flow sensor obtained in this way, the heat generated by energizing the metal wire is not transmitted to the bobbin and escaped to the support body as in the conventional bobbin type, but is mostly transmitted to the air. .

Therefore, high responsiveness unique to the bobbinless method can be obtained.

It also has a softening point of over 800°C, corrosion resistance,
Because it is coated with glass that has excellent thermal shock resistance,
Since the sensor performance does not deteriorate depending on the usage environment, a highly reliable hot wire type air flow needle can be obtained.

Therefore, embodiments of the present invention will be described with reference to the drawings.

[Embodiment 1] FIG. 1 is a partially cutaway sectional view showing an embodiment of the hot wire type air flow sensor of the present invention.

In FIG. 1, a hot wire type air flow sensor 1 is made into a heating resistor by passing a current through a coiled platinum wire 2. The platinum wire 2 is wound into a coil shape approximately centered on the axial center line of the cavity 6, and a lead wire 3 made of platinum iridium alloy is connected to the connecting portion 21 at each end of the platinum wire 2.
connected with.

The cavity 6 has a sealed structure with a glass member 5 as an outer layer and a composite member 4 of glass and alumina as an inner layer, and the lead wire 3 is supported by the glass member 5 and the composite member 4. Further, the platinum wire 2 is embedded in the inner wall surface of the inner layer of the composite member 4 of glass and alumina, so that the platinum wire 2 is firmly supported by the composite member 4.

Next, a method for manufacturing the hot wire air flow sensor 1 will be described with reference to FIG.

First, as shown in FIG. 2(a), platinum wire 2 is spirally wound around the outer periphery of molybdenum core wire 7 using an automatic winding machine. In this example, a platinum wire 2 with a diameter of 30 μm was wound around a molybdenum core wire 7 with a diameter of 0.5 mm+. Next, as shown in FIG. 2(B), the molybdenum core cotton 7 is cut into a predetermined length (4III11 in this example), and lead wires 3 made of platinum-iridium alloy are attached to both ends of the platinum wire 2. Welding is performed at the connecting portion 21.

In this case, the lead wire 3 used had a diameter of 0.13 mm.

Thereafter, around the molybdenum core wire 7 around which the platinum wire 2 is wound, an alumina member is applied by electrophoresis at a filling rate of about 50.
%, and a glass member was further attached to the top surface by a dipping method. Then, by firing the product thus obtained in an oxidizing atmosphere furnace, the molybdenum core wire 7 was sublimed and removed to form a cavity 6, as shown in FIG. 2(C). Moreover, two layers, the composite member 4 of alumina and glass and the glass member 5, could be formed around the cavity 6.

The glass member 5 used here is made by Alzos Sing.
The glass has a viscosity of 10'' poise at a temperature of 860°C and a viscosity of 104 poise at a temperature of 1180°C. In the firing shown in FIG. 2(C), as the temperature rises, the oxidation of the molybdenum core wire 7 progresses and l'loo
3, and when the temperature reaches 795°C, hoo sublimates, but since the glass member 5, which has a viscosity of 107.6 poise at a temperature of 860°C, that is, a softening point of 860″C, retains voids, The sublimate of l'1oos volatilizes from the voids of the alumina member and the glass member 5.
When the temperature reaches 1200"C after completing sublimation removal of the molybdenum core wire by holding for a time, the viscosity of the glass member 5 becomes 104 poise or less and begins to penetrate into the voids of the alumina member, and is held at 1200°C for 2 hours. However, the glass member 5 completely penetrated into the voids of the alumina member.
A hot wire type air flow sensor 1 was obtained in which a dense composite member 4 of glass and alumina was formed and covered with a glass member 5. As shown in an enlarged view in FIG. 2(d), this hot wire type air flow sensor 1 has a glass member 5 that covers the inside of the platinum wire 2 wound into a coil, thereby making the platinum wire 2 stronger. was able to support.

According to this method, compared to the conventional bobbinless method, the complicated work of etching and removing the core wire is eliminated, and the sublimation removal of the metal core yA7 with sublimation property and the composite formation of the glass member 5 and the alumina member are performed in one step. Since the firing can be done in one go, work efficiency has been greatly improved.

A specific example of a hot wire air flow meter using this hot wire air flow sensor 1 is shown in FIG. In this specific example, the same one as the hot wire type air flow rate sensor 1 is used as the resistance temperature detector 8 to measure the air temperature. Hot wire air flow sensor 1
As shown in FIG. 3, the body 8 has a main passage 81 and a bypass passage 82 for intake air.
It is fixed to the support body 9 in the bypass passage 82 of No. 3.

Figure 4 shows a specific example of a detection circuit for a hot wire air flow meter, including a hot wire air flow sensor 1, a resistance temperature detector 8, and an operational amplifier 10.
．． 11. It is composed of a power transistor 12, a capacitor 13, and resistors 14-18. In addition, the (+) pole of a battery (not shown) is connected to the power transistor 12 (7? collector terminal 19), the (=) pole of the battery is connected to the earth terminal 20 of the resistor 14, and the resistor 14 and the hot wire type The connection points 31 of the air flow sensor 1 are connected to input terminals of a microcomputer (not shown) that controls the engine using the output signal of the hot wire air flow needle.

In such a configuration, the power transistor 12 supplies current to the hot wire air flow sensor 1 to heat it,
The temperature is controlled so that the temperature is always higher than that of the resistance temperature detector 8 by a certain amount. Only a minute current, which generates negligible heat, is passed through the resistance temperature detector 8, and the temperature of the intake air is detected using this current, which is used for correcting the temperature of the intake air. Here, when the airflow hits the hot wire air flow sensor 1, the detection circuit operates to control the temperature difference between the hot wire air flow sensor 1 and the temperature sensing resistor 8 to be always constant. Therefore, when the air flow rate changes, the current flowing through the hot wire air flow sensor 1 changes, and the air flow rate is measured by the voltage drop that appears across the resistor 14 in accordance with the current.

FIG. 5 shows the response characteristics of the hot wire air flowmeter of this example. Air flow rate ranges from low flow rate of approximately 20 kg/h to high flow rate of approximately 20 kg/h.
The voltage of the hot wire air flowmeter when switched to 0 kg/h was measured, converted to flow rate, and shown on the vertical axis. As is clear from the figure, the time required to reach the final value is significantly improved compared to the conventional bobbin type, and is equivalent to the conventional bobbinless type.

Therefore, even when a car suddenly accelerates or decelerates, the hot-wire air flow meter can output a signal following the true change in air volume, making it possible to determine the appropriate amount of injection from the injector and solving the problem of surging.

The reason for this improved response is that the heat generated in the platinum wire 2 of the hot wire air flow sensor 1 does not heat the bobbin or escape to the support through the bobbin as in the conventional bobbin type. , in order to respond accurately to changes in air volume.

Further, the hot wire type air flow meter of this embodiment has higher reliability than the conventional bobbinless type. This is because the hot wire air flow sensor of this example is coated with glass that has excellent corrosion resistance and thermal shock resistance, so it has good resistance to environments such as air during use, and This is because in the bobbinless method, the glass surface was rough due to etching with acid, but in this example, the surface was smooth.

[Example 2] In the same manner as in Example 1, a diameter of 30 μm was obtained using an automatic winding machine.
The platinum wire 2 was wound around a molybdenum core cotton 7 with a diameter of 0.5 dish, and the lead wire 3 of platinum-iridium alloy with a diameter of 0.13 m+n was welded to both ends of the cut piece, which was cut to a length of 4fflII+ for one sensor. A mixed member of alumina and glass was deposited by electrophoresis so that the filling rate was approximately 50%, and a glass member 5 was further deposited by dipping.

Here, the mixing ratio of the mixed member was 20% glass and 80% alumina, and this glass member was
It is an n-type glass and has a viscosity of 10'·6 poise at a temperature of 860"C and a viscosity of 104 poise at 1180°C, and was made the same member as glass member 5. These were placed in an oxidizing atmosphere furnace. When the molybdenum core wire 7 is fired at
ob, is sublimed and removed. After holding at 850°C for 5 hours to complete the sublimation removal of the molybdenum core wire, it was heated to 1200°C.
The firing was completed by holding the alumina and glass mixture for 2 hours. However, in order to facilitate the penetration of the glass member 5, which is the same material, the alumina and glass mixture were made of alumina that was denser than that obtained in Example I. While forming the glass composite member 4,
A hot wire air flow sensor 1 whose surface was covered with a smooth glass member was obtained.

When the characteristics of a hot wire air flow meter using this hot wire air flow sensor were measured, results similar to those described in Example 1 were obtained.

[Example 3] The following experiments were conducted using glass members and ceramic members of various compositions and various types of ceramic members shown in Fig. 1. If the ceramic material has the above characteristics and the ceramic volume percentage of the composite material made by infiltrating glass into a ceramic material or a mixed material of ceramic and glass is 30 to 70%, the hot wire air flow rate shown in Fig. 1 is as follows. I was able to obtain sensor 1.

In this embodiment, the ceramic member or the mixed member of ceramics and glass was attached by electrophoresis, or the glass member 5 was attached by dipping. However, other methods other than electrophoresis or dipping may also be used, for example, in the form of a paste. Even if the method
A hot wire air flow sensor 1 shown in FIG. 1 can be obtained.

Therefore, the present invention can be applied to glass members in general having the characteristics described in the present invention, and ceramic members or composite members of ceramics and glass in general having the characteristics described in the present invention, even if other than those described in the examples. It can be applied to all methods of obtaining heating resistors by coating with methods other than electrophoretic.

〔Effect of the invention〕

As is clear from the above explanation, according to the hot wire type air flow sensor of the present invention, the surface is coated with glass having excellent corrosion resistance and impact resistance, so that dust, ionic substances, etc. in the air Characteristics do not deteriorate and high reliability can be maintained. Furthermore, since the inside is hollow, it is possible to obtain high responsiveness equivalent to that of a conventional bobbinless system.

Furthermore, according to the manufacturing method of the present invention, not only does the complicated etching work using acid become unnecessary, but also the sublimation removal of the sublimable metal core wire and the composite formation of glass and ceramics can be performed in one firing. As a result, work efficiency can be greatly improved.

[Brief explanation of the drawing]

Fig. 1 is a partially cutaway sectional view showing an embodiment of the hot wire air flow sensor of the present invention, Fig. 2 (a), (b), (C)
. (d) is a process diagram showing an example of the method for manufacturing a hot wire air flow sensor according to the present invention; FIG. 3 is a configuration diagram showing a specific example of a hot wire air flow meter using the hot wire air flow sensor;
FIG. 4 is a circuit diagram showing a specific example of the detection circuit of the air flow meter, and FIG. 5 is a characteristic diagram showing a comparison of response characteristics between the conventional method and the present invention. 1... Hot wire air flow sensor, 2... Platinum wire,
3... Lead wire, 4... Composite member, 5... Glass member, 6... Cavity part, 7... Molybdenum core wire.

Claims (3)

[Claims] (1) The outer layer is glass with a softening point of 800°C or higher, the inner layer is a composite of glass and ceramics, and the cavity is covered with a coiled metal wire, both ends of which are electrically drawn out to the outside. A hot wire type air flow sensor characterized by being disposed embedded in the inner wall surface of. (2) The ceramics contained in the composite of the inner layer are:
The hot wire air flow sensor according to claim 1, characterized in that it has a volume fraction of 30 to 70%. (3) The process of winding a metal wire around a sublimable metal core wire, attaching a ceramic or a mixed member of ceramics and glass to the metal wire wound around the metal core wire, and then attaching a glass member to the surface of the metal wire. and a covering step, by firing the product obtained in this way, the metal core wire is sublimated and removed, and the glass member is infiltrated into the gap between the ceramic or a mixed member of ceramics and glass to form a composite. 1. A method for manufacturing a hot wire air flow sensor, comprising the steps of: