Abstrict An exothermic resistor for use in a hot wire air flow meter, having
a wire (2) in the form of a coil made of a metal, a pair of lead
wires (3) connected to connections (21) formed at the opposite ends
of the coil, and a support member (4) which integrally supports
the connection as well as the coil located between the connections.
An example of the support member is in the form of a tubular cylinder
closed at its both ends. The support member is formed from glass
only or formed of a layer of a glass-ceramic-composite material.
The hot wire air flow meter is designed to be easely mass-produced
and to have improved transient response characteristics with respect
to abrupt changes in the air flow rate.
Claims What is claimed is:
1. A method of manufacturing an air flow meter, comprising the
steps of: preparing a blank constituted by a core wire having a
desired length, a wire wound around said core wire, so as to provide
a wound wire, and used as an exothermic resistance wire, and a pair
of lead wires connected to opposite ends of said wound wire; coating
ceramic particles to said wound wire over the entire surface thereof
and thereafter sintering said ceramic particles; removing said core
wire; and forming a layer of composite material by coating the layer
of said sintered ceramic material with molten glass so that said
glass permeates into said layer of sintered ceramic material.
2. A method of manufacturing an air flow meter according to claim
1 wherein said core wire has flat portions formed at its opposite
ends.
3. A method of manufacturing an air flow meter according to claim
1 wherein the ceramic material has a thermal conductivity of 10
W/m.K.
4. A method of manufacturing an air flow meter, comprising the
steps of: preparing a blank constituted by a core wire having a
desired length, a wire wound around said core wire, so as to provide
a wound wire, and used as an exothermic resistance wire, and a pair
of lead wires connected to opposite ends of said wound wire; forming
a layer of composite material by coating ceramic and glass particles
to said wound wire over the entire surface thereof and thereafter
sintering said ceramic and glass particles; and removing said core
wire.
5. A method of manufacturing an air flow meter according to claim
4 wherein said ceramic particles attached to said wound wire includes
particles of a composite material composed of ceramic and glass
materials.
6. A method of manufacturing an air flow meter according to claim
4 wherein the core wire has opposed ends in a direction of the
desired length, and wherein in the step of forming the layers of
composite material the ceramic and glass particles are coated over
the opposed ends and thereafter sintered.
7. A method of manufacturing an air flow meter according to claim
4 wherein material of the ceramic particles has a thermal conductivity
of 10 W/m.K.
8. A method of manufacturing an air flow meter according to claim
7 wherein the glass particle component of the composite material
is 2 to 60 volume percent of the composite material.
9. A method of manufacturing an air flow meter, comprising the
steps of: preparing a blank constituted by a core wire having a
desired length, a wire wound around said core wire, so as to provide
a wound wire, and used as an exothermic resistance wire, and a pair
of lead wires connected to opposite ends of said wound wire, said
core wire being made of a sublimatable material; coating ceramic
particles to said wound wire over the entire surface thereof and
thereafter sintering said ceramic particles; removing said core
wire by heat treatment so as to sublimate the sublimatable material;
and forming a layer of composite material by coating the layer of
said sintered ceramic material with molten glass so that said glass
permeates into said layer of sintered ceramic material.
10. A method of manufacturing an air flow meter according to claim
9 wherein said core wire has flat portions formed at its opposite
ends.
11. A method of manufacturing an air flow meter, comprising the
steps of: preparing a blank constituted by a core wire having a
desired length, a wire wound around said core wire, so as to provide
a wound wire, and used as an exothermic resistance wire, and a pair
of lead wires connected to opposite ends of said wound wire, said
core wire being made of a sublimatable material; forming a layer
of composite material by coating ceramic and glass particles on
said wound wire over the entire surface thereof and thereafter sintering
said ceramic and glass particles; and sublimating the sublimatable
material so as to remove said core wire, the sublimating being performed
by heat treatment.
12. A method of manufacturing an air flow meter according to claim
11 wherein said ceramic particles attached to said wound wire includes
particles of a composite material composed of ceramic and glass
materials.
13. A method of making an exothermic resistor for an air flow meter,
comprising steps of:
(1) providing a blank constituted by a resistor wire wound around
a sublimatable core material, the resistor wire having opposed ends,
the core material having opposite ends adjacent the opposed ends
of the resistor wire;
(2) connecting a respective lead wire to the opposed ends of the
resistor wire;
(3) forming a porous ceramic material coating on the resistor wire,
the porous ceramic material coating having closed ends at the opposed
ends of the resistor wire; and
(4) effecting sublimation of the sublimatable core material to
remove the sublimatable core material.
14. A method of making an exothermic resistor for an air flow meter
according to claim 13 wherein the step of forming the porous ceramic
coating includes forming the porous ceramic coating on exposed surfaces
of the resistor wire and the core material, including on the opposite
ends of the core material.
15. A method of making an exothermic resistor for an air flow meter
according to claim 14 wherein the porous ceramic coating is provided
on all exposed surfaces of the resistor wire and the sublimatable
core material such that the porous ceramic coating is a continuous
coating.
16. A method of making an exothermic resistor for an air flow meter
according to claim 14 including the further step of applying a
molten glass to the porous ceramic coating so as to fill pores of
the porous ceramic coating with the glass.
17. A method of manufacturing an air flow meter according to claim
13 wherein said sublimation of the sublimatable core is effected
by heating.
18. A method of manufacturing an air flow meter, comprising the
steps of: preparing a blank constituted by a core wire having a
desired length, a wire wound around said core wire, providing a
wound wire, and used as an exothermic resistance wire, and a pair
of lead wires connected to opposite ends of said wound wire; forming
a layer of composite material by coating ceramic and glass particles
to said wound wire over the entire surface thereof and thereafter
sintering said ceramic and glass particles; coating said layer of
composite material with molten glass after the composite material
layer has been formed; and removing said core wire.
19. A method of manufacturing an air flow meter, comprising the
steps of: preparing a blank constituted by a core wire having a
desired length, a wire wound around said core wire, providing a
wound wire, and used as an exothermic resistance wire, and a pair
of lead wires connected to opposite ends of said wound wire, said
core wire being made of a sublimatable material; forming a layer
of composite material by coating ceramic and glass particles on
said wound wire over the entire surface thereof and thereafter sintering
said ceramic and glass particles; coating said layer of composite
material with molten glass after the composite material layer has
been formed; and removing said core wire by heat treatment so as
to sublimate the sublimatable material.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a hot wire air flow meter. More particularly,
this invention relates to a hot wire air flow meter suitable for
measurement of the flow rate of air taken into an internal combustion
engine for a motor vehicle, and also relates to a method of manufacturing
such a hot wire air flow meter.
2. Description of the Prior Art
A hot wire air flow meter has a heating coil which is formed as
an exothermic resistor that is disposed in an air flow path the
flow rate of which is to be measured. In order to eliminate any
reduction in the temperature of the heating coil due to the cooling
effect of the air flow, a current which flows through the air flow
path is increased so as to heat up the coil. The air flow rate is
determined from this increase in the current. This type of air flow
meter can be constituted without employing any movable parts and,
at the same time, it enables direct detection of the mass flow.
For this reason, air flow meters of this type are generally adopted
to perform air-fuel ratio control in the internal combustion engines
of motor vehicles.
The exothermic resistor provided in this type of air flow meter
comprises a very thin metal wire, e.g., a platinum wire having a
diameter of several tens microns. For instance, an exothermic resistor
such as the one disclosed in Japanese Utility Model Laid-Open No.
56-96326/1981 is formed in such a manner that a metal wire provided
as an exothermic resistance wire is wound around a core wire, that
is, a bobbin made of a ceramic material.
Another type of exothermic resistor is disclosed in an already
filed patent application (now U.S. Pat. No. 4790182) which is
a bobbinless exothermic resistor in the form of a coil formed from
a metal wire and overcoated with glass except for opposite end portions
which are welded to a support for the exothermic resistor.
In the case of an exothermic resistor formed of a metal wire wound
around a core wire or bobbin made of a ceramic material employing
one of the above conventional techniques, the quantity of heat heating
up the body of the bobbin and the quantity of heat transmitted through
the bobbin to the support on which the exothermic resistor rests
are not negligible. There is therefore a problem of retardation
of the transient response to any fluctuation in the air flow, in
particular, resulting in the occurrence of surging when the vehicle
is sharply accelerated or decelerated. In addition, it is necessary
during the process of manufacturing exothermic resistors to perform
a coil winding operation for each exothermic resistor, which makes
automatization of the process difficult.
In contrast, the bobbinless type of exothermic resistor has improved
response characteristics and can be manufactured with an improved
degree of automatization because the coil winding operation can
be continuously performed for a plurality of resistors of this type.
However, it is difficult to handle the opposite end portions of
the wire which are not coated with glass, and there is a problem
regarding a reduction in the ease with which the operation of securing
the resistor to the support can be performed in the manufacturing
process. In addition, the layer of coating glass which acts as a
support member for supporting the coiled portion of the wire must
have a substantial thickness so as to ensure the specified strength
of the final products. In consequence, heat transfer between the
wire and the air flow is obstructed due to the glass layer having
inferior heat conductivity, thereby causing deterioration in the
transient response characteristics.
In the structure of the bobbinless type of exothermic resistor,
the inner surface of the cylindrical member which is formed by means
of glass coating (in which the wire extends helically) is brought
into contact with the outside air. If any dust and/or ionic substances
are contained in the air the flow rate of which is to be measured,
the dust and/or ionic substances become attached to the inner surface
of the cylindrical member, or, in the worst case, the inner space
of the cylindrical member becomes filled with accumulated dust.
In this worst case, the heat generating from the wire is transmitted
through the medium of the dust, thereby impairing the advantage
of the bobbinless type. If the inner space of the resistor is filled
with attached and accumulated ionic substances, short circuiting
takes place between adjacent coiled portions of the wire, and characteristics
specific to the exothermic resistor are thereby changed. In a method
of manufacturing the conventional bobbinless exothermic resistor,
a step of removing a bobbin after a coil has been formed by winding
a metal wire around this bobbin is adopted, and chemical etching
is utilized as a means for removing the bobbin, thereby necessitating
an additional process for performing this etching. This makes the
overall manufacturing process more complicated.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a hot wire
air flow meter having an exothermic resistor and a method of manufacturing
the same, the hot wire flow meter being capable of being manufactured
with an increased degree of automatization as well as being easily
handled and having suitable response characteristics.
It is another object of the present invention to provide a hot
wire air flow meter having an exothermic resistor, the exothermic
resistor being capable of maintaining suitable response characteristics
in the face of the influence of dust or ionic substances contained
in the air as well as being protected against any deterioration
in its overall characteristics.
It is still another object of the present invention to provide
a method of manufacturing a hot wire air flow meter in which a complicated
step previously forming part of the manufacturing process is eliminated.
To these ends, the present invention provides a hot wire air flow
meter having an exothermic resistor having a metal wire or an exothermic
resistance wire wound in a coil, metal lead wires connected to opposite
ends of this coil, and a support member made of, for example, a
glass, the support member supporting (integrally fixing) connections
between the wound wire and the lead wires as well as the wound wire
located between these connections.
This type of hot wire air flow meter is manufactured by a method
including: preparing a blank constituted by a metal core wire having
a desired length, a metal wire wound around the core wire and used
as an exothermic resistance wire, and a pair of lead wires welded
to opposite ends of the wound wire; overcoating the wound wire with
a glass material such that this glass material also covers welded
portions; sintering the overcoating; and thereafter removing the
core wire. If a glass core is used in place of the core wire, there
is no need for the step of removing the core.
In accordance with the present invention, a metal wire is continuously
wound around a metal core wire or a glass core line to form a lengthwise
blank having a length corresponding to a plurality of exothermic
resistors successively disposed in line. At this time, the metal
wire is continuously wound at a time by an automatic winding machine
for the plurality of exothermic resistors, thereby remarkably increasing
the degree of automatization of the manufacturing process.
Thereafter, the thus-prepared blank constituted by the core wire
and the resistance wire is cut into pieces, each having a desired
length, and lead wires are welded to opposite ends of each piece.
Welded portions, as well as the resistance wire located between
the welded portions, are overcoated with a glass so that they are
fixed integrally. In consequence, the exothermic resistance wire
is supported (integrally fixed) by the lead wires and the glass
member, and this exothermic resistor is easy to handle since there
is no need for an operation of connecting thin wires to the support
for the exothermic resistor.
The metal core wire short-circuits the coil between the lead wires
since it has electro-conductive properties. It is therefore removed
by means of, for example, etching using an acid. It is possible
to improve the exothermic portion in terms of mechanical strength
and corrosion resistance by the overcoating glass. It is thereby
ensured that the heat caused by energization of the resistance wire
can be almost entirely transmitted to the air without heating the
bobbin or core having a large heat capacity or being transmitted
to the support via the bobbin as in the case of the conventional
bobbin-type exothermic resistor. It is therefore possible for the
hot wire air flow meter to be improved in the response characteristics
with respect to abrupt changes in the air flow rate and to output
signals by suitably following up actual changes in the air flow
rate, thereby optimizing the fuel supply control and solving the
problem of surging, etc.
If the glass core line is used, there is no need for the step of
removing the core since the glass core is nonelectro-conductive.
In this case, it is possible to reduce the thickness of the glass
overcoating so as to avoid any considerable increase in the total
heat capacity because the mechanical strength of the exothermic
resistor can be maintained by the core. As a result, the response
characteristics do not become inferior compared with the case where
the metal core wire is removed.
A method of filling the cavity formed by removing the metal core
wire with a glass is also possible. The resulting resistor has the
same characteristics as the resistor having the glass core.
The resistance wire is ordinarily formed of a platinum wire superior
in terms of thermal resistance and corrosion resistance. Instead,
it can be formed of a tungsten wire. The lead wires may be wires
made of a platinum-iridium.
If the glass is baked at an excessively high temperature, the platinum
wire becomes embrittled and the electrical characteristics thereof
become changed. Heating at a temperature higher than about 1200.degree.
C. for a long time must be avoided. Correspondingly, to perform
baking at a temperature lower than 1200.degree. C., a glass material
having a viscosity of 10.sup.4 to 10.sup.7 poise at temperatures
of 800 to 850.degree. C. is used as the glass material for supporting
the exothermic resistor. It is not always necessary that the thermal
expansion coefficient of the glass is equal to that of the platinum
wire (90.times.10.sup.-7 /.degree. C.). However, this is preferred
in terms of reduction in the stress due to the heat cycle during
operation. If the core wire is removed by etching using an acid,
it is necessary to prevent the glass from becoming greatly eroded.
The acid resistance of the glass, as well as the viscosity properties,
strongly relates to the connective strength of the structure of
the glass. It was confirmed that, in the case of a glass having
this viscosity-strength, erosion was limited to a depth of not greater
than 1 .mu.m. This glass is also suitable in terms of water resistance
and oil resistance in an applied state. A glass the viscosity of
which becomes reduced at a lower temperature can be baked at a lower
temperature but the acid, water and oil resistances of this glass
are insufficient.
A glass having the above-described suitable properties can be selected
from lead-potash glass, lead-soda glass, lead-potash-soda glass,
soda-lime glass, soda-barium glass, potash-lime glass, potash-barium
glass and borosilicate glass.
A molybdenum wire or nickel-iron alloy wire is used as the metal
core wire. With respect to these materials, heating at a temperature
higher than 1200.degree. C. in the atmosphere is not preferable.
They can be used in combination with a glass having the above-described
properties. Specifically, the thermal expansion coefficient of the
nickel-iron alloy wire can be adjusted to that of the platinum wire,
enabling a reduction in the thermal stress at the time of baking
of the glass.
To attain the above objects, the present invention provides a hot
wire air flow meter comprising an exothermic resistor including
a glass member in the form of a cylindrical tube provided as a support
member, and a metal wire in the form of a coil extending helically
along the inner surface of the glass member and coaxially therewith
and having its both ends electrically led to the outside of the
glass member, wherein the glass member is closed at its both ends.
This exothermic resistor is manufactured by a method including
steps of: winding a metal wire as an exothermic resistance wire
around a sublimatable core wire; covering the wound wire and the
core wire with a porous glass material; and performing heat treatment
to sublimate the core wire and bake the glass.
The above cylindrical glass member having the inner surface along
which the coil is formed is closed at its both ends, thereby preventing
dust or ionic substances in the air from attaching the inner surface
of the glass member or filling the cavity thereof. There is no possibility
of any extraneous substance reaching the surface along which the
coil is formed. The properties of the resistor are stabilized in
terms of thermal or electrical effects, thereby attaining the above
object.
In the manufacturing process, a sublimatable material is used for
the core wire, and the core wire is sublimated at the time of baking
of the glass by the heat required for this baking. It is therefore
possible to eliminate the need for the etching step with respect
to the removal of the core wire, thereby simplifying the process.
To attain the above objects, the present invention also provides
an air flow meter comprising an exothermic resistor having an exothermic
resistance wire or metal wire, a pair of lead wires connected to
the wire, and a support member covering and supporting the exothermic
resistance wire, wherein the support member is formed of a layer
of a composite material composed of ceramic and glass materials.
The performance of this type of exothermic resistor can be effectively
improved if this resistor has a cavity and if the glass component
of the composite material forming the support member forms a surface
layer thereof and also forms a continuous phase which reaches the
cavity.
A ceramic material having a thermal conductivity of at least 10
W/m.K may be selected as the ceramic component of the composite
material layer, which is also effective. It is preferable to set
the proportion of the glass component of the composite material
layer to 2 to 60 volume percent. The glass component of the composite
material layer may include a glass having a softening temperature
not higher than 700.degree. C. and another glass having a softening
temperature higher than 700.degree. C., which is effective.
The exothermic resistor may have another type of structure in which
the exothermic resistance wire is formed of a film circuit formed
on a ceramic substrate while the support member is formed of the
ceramic substrate and a layer of a composite material composed of
ceramics and glass.
To attain the above objects, the present invention also provides
a method of manufacturing an air flow meter, including steps of:
preparing a blank constituted by a metal core wire having a desired
length, a metal wire wound around the core wire and used as an exothermic
resistance wire, and a pair of lead wires connected to opposite
ends of the wound wire; depositing ceramic particles to the wound
wire over the entire surface thereof and thereafter sintering the
ceramic particles; removing the core wire; and forming a layer of
composite material by coating the layer of sintered ceramic material
with molten glass so that the glass permeates into the layer of
sintered ceramic material. With respect to the step of preparing
a blank constituted by a metal core wire, a metal wire wound around
the core wire, and lead wires, it is possible to adopt a method
of continuously winding a metal wire used as an exothermic resistance
wire around a long metal core wire, cutting a blank thereby formed
into pieces each having the desired length, and thereafter connecting
the lead wires to the opposite ends of the wound metal wire, or
a method of connecting the lead wires to opposite ends of the metal
core wire (having the desired length), connecting one end of the
metal wire used as an exothermic resistor to one of the lead wires,
winding this resistance wire around the core wire, and thereafter
connecting the other end to the other lead wire.
The layer of composite material composed of ceramics and glass
may be formed by coating a mixed ceramic and glass particles to
the wound metal wire over the entire area thereof and thereafter
sintering this material. A glass coating may be formed over the
composite material layer in a molten glass coating manner. Instead
of this method of mixing ceramic and glass particles, it is possible
to adopt a method of forming particles from a composite material
composed of ceramics and glass, coating the metal wire wound around
the metal core wire with these particles, and thereafter sintering
this material.
The present invention also provides an air flow meter of a motor
vehicle having the above-described exothermic resistor and a driving
circuit which controls the current flowing through the exothermic
resistor and takes out the voltage output from the exothermic resistor
as a signal corresponding to the air flow rate. It also provides
an anemometer having the above-described exothermic resistor and
a means for detecting the temperature of the exothermic resistor.
In accordance with the present invention, the opposite ends of
the resistance wire are connected to the lead wires, and therefore
there is no need for an operation of connecting thin metal wires
to the support, thereby making the resulting resistor easy to handle.
In particular, the resistance wire, as well as the connections between
the wire and the lead wires, is ordinarily covered with the support
member so that the resistance wire and the lead wires are fixed.
In consequence, the exothermic resistance wire is supported by the
lead wires and the support member, thus realizing a structure suitable
for handling of the resistor.
The core wire short-circuits the coil between the lead wires since
it has electro-conductive properties. It is therefore removed by,
for example, etching using an acid, or oxidation and sublimation
at an increased temperature in the atmospheric air. The exothermic
portion can be improved in the mechanical strength and the resistance
to environmental influences by virtue of the composite material
coating. The heat generated by the electrical current through the
resistance wire is transmitted to the air via the composite material
layer. It is possible to set the thermal conductivity of this composite
material layer to at least about ten times as high as that of a
glass, which is about 1 W/m.K, if this composite material layer
is composed of this glass and a ceramic material having a thermal
conductivity of at least 10 W/m.K. It is therefore possible to avoid
a considerable retardation of the change in the heat transfer rate
in response to a change in the air flow rate, thereby improving
the transient response characteristics.
If the coating layer formed on the resistance wire is baked at
an excessively high temperature, the platinum wire becomes embrittled
and the electrical characteristics thereof become changed. Heating
at a temperature higher than about 1200.degree. C. for a long time
must be avoided. For this reason, if the wire is coated with only
a ceramic material having a high thermal conductivity, baking is
not suitably performed and the strength of the exothermic resistor
becomes inadequate. If the composite material layer is formed by
coating the baked ceramic layer with a molten glass so that the
glass permeates into this layer, it has an adequate strength as
well as a high degree of thermal conductivity. If ceramic and glass
materials are simultaneously coated to the resistance wire and are
thereafter sintered, a composite material coating layer having an
improved strength can be obtained by the sintering effect of the
glass even if the composite material is sintered at a temperature
not higher than 1200.degree. C. In the case where there are pores
remaining in the composite material layer, the layer is further
coated with a molten glass to obtain a higher strength.
The uniformity of the components of the composite material layer
can be improved if particles formed from a composite material composed
of ceramic and glass materials are attached as a coating to the
resistance wire and are thereafter baked. It is thereby possible
to improve the accuracy with which the compounding ratio control
is performed.
In terms of ease of coating, it is preferable to use, as the coating
glass, a glass having a softening temperature lower than 700.degree.
C.
If the glass component of the composite material layer forms a
continuous phase, the strength of the support member is increased.
If the proportion of the glass component is excessively small, the
strength of the support member becomes inadequate and, if the proportion
of the glass component is excessively large, the effect of the composition
making use of the ceramic material in order to increase the thermal
conductivity is reduced. A suitable range of the proportion of the
glass component of the composite material layer is 2 to 60 volume
percent.
A molybdenum wire of a nickel-iron alloy used as the metal core
wire can be removed by etching using an acid. The molybdenum wire
can be removed during sintering of the coating layer since it sublimates
by oxidizing in the air.
A type of air flow sensor element, that is, an exothermic resistor
having an exothermic circuit constituted by a film pattern formed
on an alumina substrate, and a support member which covers this
film circuit and the alumina substrate has a reduced thickness.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal cross-sectional view of an exothermic
resistor for use in a hot wire air flow meter of the present invention;
FIGS. 2A to 2E are diagrams of the process of manufacturing the
exothermic resistor shown in FIG. 1;
FIG. 3 is a longitudinal cross-sectional view of an exothermic
resistor for use in another embodiment of the present invention;
FIGS. 4A to 4D are diagrams of the process of manufacturing the
exothermic resistor shown in FIG. 3;
FIG. 4E is an enlarged illustration of a portion of the exothermic
resistor shown in FIG. 4D;
FIG. 5 is an illustration of another example of the method of manufacturing
the exothermic resistor in accordance with the present invention;
FIG. 6 is a longitudinal cross-sectional view of an exothermic
resistor for use in still another embodiment of the present invention;
FIGS. 7A to 7E are diagrams of the process of manufacturing the
exothermic resistor shown in FIG. 6;
FIG. 8 is a cross-sectional view of a state in which ceramic and
glass materials are attached to an exothermic resistance wire wound
around a core wire;
FIG. 9 is a graph of the relationship between the volume percent
of the glass component and the anti-crushing strength;
FIG. 10 is a graph of the relationship between the volume percent
of the glass component and the response time;
FIGS. 11A to 11D are diagrams of another example of the method
of manufacturing the exothermic resistor shown in FIG. 6;
FIGS. 12A to 12D are cross-sectional views of the process of manufacturing
an exothermic resistor with a film circuit;
FIG. 13 is a perspective view of an exothermic resistor with a
film circuit for use in a further embodiment of the present invention;
FIG. 14 is a cross-sectional view of a hot wire air flow meter;
FIG. 15 is a circuit diagram of a driving circuit of the hot wire
air flow meter shown in FIG. 14; and
FIG. 16 is a graph of the response characteristics of the hot wire
air flow meter shown in FIG. 14.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiment 1
FIG. 1 shows the structure of an exothermic resistor 1 for use
in a first embodiment of the present invention. The exothermic resistor
1 for detecting the intake air flow rate has a length of platinum
wire 2 in the form of a coil, a pair of lead wires 3 made of a platinum-iridium
alloy connected to opposite ends of the platinum wire coil 2 and
a glass member 4 which supports the platinum wire 2 and connections
21 between the lead wires 3 and the platinum wire 2.
A method of manufacturing the exothermic resistor 1 will be described
below with reference to FIGS. 2A to 2E.
FIG. 2A shows a core wire or a bobbin 5 which has a diameter of
0.5 mm and is made of a Ni-Fe alloy having a Ni content of 52% and
around which the platinum wire 2 is to be wound. As shown in FIG.
2B, a length of platinum wire 2 having a diameter of 20 .mu.m was
continuously wound around the core wire 5 over a length corresponding
to two or more exothermic resistors by employing an automatic coil
winding machine. As shown in FIG. 2C, a resistor blank thereby formed
was cut into pieces, each having a length of 6 mm, and a pair of
lead wires 3 made of a platinum-iridium alloy and having a diameter
of 0.13 mm were welded to opposite ends of each piece at connections
21. Then, as shown in FIG. 2D, the connections 21 and the platinum
wire 2 were overcoated with a glass material 4 in order to form
a support member, and thereafter underwent sintering. The glass
material 4 was a lead-potash-soda glass having a composition consisting
of, by weight, 56% of SiO.sub.2 30% of PbO, 6% of K.sub.2 O, 6%
of Na.sub.2 O, 1% of CaO, and 1% of Al.sub.2 O.sub.3. The viscosity
of this glass was measured and found to be 10.sup.6.3 poise at 800.degree.
C. and 10.sup.6 poise at 850.degree. C. To perform overcoating,
a solution was prepared which contained denatured alcohol and water
as solvents and magnesium nitrate and aluminum nitrate provided
as electrolytes, and in which the above glass material was dispersed;
and glass powder was attached to the platinum wire by electrophoresis
in such a manner that the platinum wire in the state shown in FIG.
2C was placed in the suspension for electrophoretic deposition as
a cathode while an aluminum plate was placed in the same suspension
as an anode, a voltage of 30V then being applied through the electrodeposition
liquid between these electrodes. Thereafter, the thus-processed
piece was sintered in an electric furnace at 800.degree. C. for
6 minutes. The thickness of the glass was about 100 .mu.m. FIG.
2E shows a state in which the core wire has been removed by immersing
the sintered piece in a mixed acid composed of nitric acid and sulfuric
acid at 80.degree. C. for 1 hour. The glass became eroded by the
mixed acid to a depth of 1 .mu.m or less. The exothermic resistor
1 in the state shown in FIG. 2E has a strength sufficient enough
to resist being broken when it is handled by a pincette, and it
can be treated as a single element. It is therefore easy to handle
in the succeeding assembly process, thus improving the ease with
which operations relating to handling of the exothermic resistor
can be performed.
Embodiment 2
Exothermic resistors of the structure shown in FIG. 1 were manufactured
in the same manner as Embodiment 1 by using various types of glass.
Table 1 shows the compositions of glasses used. Table 2 shows the
viscosity of each glass measured at 800.degree. C. and 850.degree.
C., the temperature at which each piece was sintered, whether or
not the platinum wire was embrittled, and whether or not each exothermic
resistor was broken when it was handled by a pincette after the
core wire had been removed. A state in which the glass became eroded
during etching of the core wire such that the platinum wire was
exposed and was partly unwound is included in the kinds of breakage
to be put in the table.
As can be understood from Table 2 it is necessary for a glass
having a viscosity higher than 10.sup.7 poise at 800.degree. C.
to be sintered at a temperature higher than 1200.degree. C. Sintering
at this temperature embrittles the platinum wire. A glass having
a viscosity lower than 10.sup.4 poise may be sintered at a lower
temperature, but it tends to erode during etching and is inferior
in terms of strength.
Of the exothermic resistors listed in FIG. 2 each of those free
from embrittlement of the platinum wire and breakages (those making
use of glasses b, e, f, h, and i) was used to constitute a hot wire
air flow meter shown in FIG. 14. It was proved that, as shown in
FIG. 16 response characteristics of hot wire air flow meters thereby
made were superior than those of an air flow meter making use of
the conventional bobbin-type exothermic resistor.
TABLE 1 __________________________________________________________________________
Glass Composition (percent by weight) __________________________________________________________________________
a SiO.sub.2 35 PbO 58 K.sub.2 O 7 b SiO.sub.2 50 PbO 35 K.sub.2
O 5 Na.sub.2 O 8 Al.sub.2 O.sub.3 2 c SiO.sub.2 70 PbO 12 K.sub.2
O 6 Na.sub.2 O 7 CaO 5 d SiO.sub.2 60 BaO 12 MgO 5 Na.sub.2
O 12 Al.sub.2 O.sub.3 11 e SiO.sub.2 65 CaO 2 BaO 13 K.sub.2
O 15 Al.sub.2 O.sub.3 5 f SiO.sub.2 72 CaO 4 MgO 3 Na.sub.2
O 20 Al.sub.2 O.sub.3 1 g SiO.sub.2 53 B.sub.2 O.sub.3 9 Al.sub.2
O.sub.3 20 CaO, 15 BaO 3 h SiO.sub.2 65 B.sub.2 O.sub.3 18 Al.sub.2
O.sub.3 7 Na.sub.2 O 6 BaO 4 i SiO.sub.2 70 B.sub.2 O.sub.3 16
Al.sub.2 O.sub.3 4 K.sub.2 O 4 PbO 6 j SiO.sub.2 80 B.sub.2 O.sub.3
13 Al.sub.2 O.sub.3 2 Na.sub.2 O 4 CaO 1 __________________________________________________________________________
TABLE 2 ______________________________________ Embrittle- Viscosity
Sintering ment of Types of (poise) temperature platinum glass 800.degree.
C. 850.degree. C. (.degree.C.) wire Breakage ______________________________________
a 10.sup.4 10.sup.3 690 No Yes b 10.sup.5.2 10.sup.4.7 730 No No
c 10.sup.7.3 10.sup.6.6 1210 Yes No d 10.sup.4.6 10.sup.3.7 560
No Yes e 10.sup.5.5 10.sup.5 760 No No f 10.sup.6 10.sup.5.5 800
No No g 10.sup.9 10.sup.8.4 1250 Yes No h 10.sup.6.5 10.sup.5.8
760 No No i 10.sup.6.9 10.sup.6.1 860 No No j 10.sup.7.8 10.sup.6.9
1220 Yes No ______________________________________
Embodiment 3
A molybdenum wire having a diameter of 0.5 mm was used as the core
wire 5. A platinum wire was wound around this core wire, a resistor
blank thereby formed was cut into pieces and a pair of lead wires
were welded to each piece in the same manner as the manufacturing
process shown in FIGS. 2A to 2C. Glass was attached to the welded
portions and to the platinum wire by a dipping method, the composition
of this glass consisting of, by weight, 74% of SiO.sub.2 9% of
CaO, 8% of K.sub.2 O, 8% of Na.sub.2 O, and 1% of Al.sub.2 O.sub.3.
The viscosity of the glass was 10.sup.6.2 poise at 800.degree. C.
and 10.sup.5.5 at 850.degree. C. The blank piece was thereafter
heated in the electric furnace at 1000.degree. C. for 30 minutes,
thereby sintering the glass. In this case, the ambient atmosphere
inside the electric furnace was atmospheric air, and the molybdenum
core wire was removed during sintering by being oxidized and sublimated.
An exothermic resistor similar to that shown in FIG. 2E was thus
obtained.
The thus-obtained exothermic resistor had a work strength enough
for the succeeding assembly process and a hot wire air flow meter
making use of this exothermic resistor exhibited improved response
characteristics, as shown in FIG. 16.
As can be understood from the foregoing, the present invention
is effective irrespective of the method of coating glass and the
method of removing the core wire.
Embodiment 4
Glass fiber filaments having a diameter of 10 .mu.m were bundled
to form a glass line having a diameter of 0.3 mm. This glass line
was used as a core wire, and a platinum wire was wound around this
core wire. A resistor blank thereby formed was cut into pieces,
each having a length of 10 mm. Lead wires were welded to opposite
ends of each piece. The same glass as that used in Embodiment 1
was attached to each piece by electrophoresis in such a manner that
the core and the platinum wire, including the welded portions, were
covered with the glass over the entire area thereof. The thickness
of the glass was 2/5 of the thickness of the glass of Embodiment
1. Thus-formed piece was heated at 900.degree. C. for ten minutes,
thereby sintering the glass.
An exothermic resistor thereby obtained had a strength greater
than that of the exothermic resistor of Embodiment 1 and was not
broken when dropped from a level of 1 m high to the floor. The rising
rate of response characteristics was slightly higher than that in
the case of Embodiment 3 but it was reduced when the flow rate
was higher than about 100 kg/h and it was substantially equal to
that in the case of the third embodiment with respect to a range
of response time after 30 ms.
Embodiment 5
An exothermic resistor of the type shown in FIG. 2E was manufactured.
In this case, the thickness of glass coated and sintered was set
to a half of that of the first embodiment. Another glass having
a composition consisting of, by weight, 35% of SiO.sub.2 58% of
PbO, and 7% of K.sub.2 O was dispersed in an organic solvent, and
a cavity of the blank piece formed by removing the core wire was
filled with this dissolved glass. The blank piece with the glass
was heated and sintered in the electric furnace at 650.degree. C.
for ten minutes.
The second glass filling the cavity and subjected to baking had
many pores. However, the obtained exothermic resistor exhibited
a strength substantially equal to that of the exothermic resistor
of the first embodiment, and it also exhibited response characteristics
similar to those of the third embodiment.
Embodiment 6
FIG. 3 shows the structure of an exothermic resistor for a hot
wire air flow meter which represents a further embodiment of the
present invention.
A glass member 4 in the form of a cylindrical tube shown in FIG.
3 is provided with a length of platinum wire 2 which extends helically
in the inner surface of the cylindrical glass member such that the
coil thereby formed is coaxial with the glass member 4. Opposite
ends of this platinum wire coil 2 are connected to a pair of lead
wires 3 supported on the glass member 4 so that the coil is electrically
led to the outside of the glass member 4. The glass member 4 is
closed at its both ends by the same material as itself.
Embodiment 7
A method of manufacturing an exothermic resistor of this structure
will be described below with reference to FIGS. 4A to 4D.
As shown in FIG. 4A, a length of platinum wire 2 having a diameter
of 20 .mu.m is continuously wound by an automatic winding machine
around the core wire 5 made of molybdenum and having a diameter
of 0.4 mm. As shown in FIG. 4B, the resistor blank shown in FIG.
4A is cut into pieces, each having a length of about 6 mm for one
exothermic resistor, and a pair of lead wires 3 having a diameter
of 0.13 mm and made of a platinum-iridium alloy are welded to opposite
ends of each piece at connections 21. Then, as shown in FIG. 4C,
a glass material 41 is applied, by electrophoresis, over outer surfaces
of each piece formed by cutting from the molybdenum core wire 5
and the platinum wire 2 wound around the core wire and is baked
in an oxidizing atmosphere. The glass material 41 is, for example,
a SiO.sub.2 --B.sub.2 O.sub.3 --PbO glass having a viscosity of
10.sup.6.5 poise at 800.degree. C. and a viscosity of 10.sup.4.2
at 850.degree. C. As the temperature in the sintering process of
the glass material 41 rises, oxidation of the molybdenum core wire
5 is promoted so that the core material becomes MoO.sub.3. When
heated at 795.degree. C., MoO.sub.3 is sublimated while the glass
material 41 having a viscosity of 10.sup.6.5 at 800.degree. C. maintains
sufficient open pores, so that sublimated MoO.sub.3 is dispersed
through the open pores between particles of the glass material 41
thus removing the molybdenum core wire 5. Thereafter, the temperature
was held at 950.degree. C. for 20 minutes, thereby completing sintering
of the glass material. During this process, the glass material 41
reacts with sublimated MoO.sub.3 and the fluidity of the glass is
thereby reduced so that the glass becomes porous and the smoothness
of the glass surface becomes inadequate. For this reason, a secondary
layer of glass material 42 is formed over the surface of the glass
41 and is baked in an oxidizing atmosphere by the electric furnace.
The glass material 42 is, for example, a ZnO--B.sub.2 O.sub.3 --SiO.sub.2
glass having a viscosity of 10.sup.8 poise at 600.degree. C. and
a viscosity of 10.sup.4 poise at 690.degree. C. During sintering
at 720.degree. C. for 20 minutes, the glass material 42 adequately
fills pores of the porous primary layer of glass 41 and forms a
smooth outer surface, thereby completing the exothermic resistor
1 shown in FIG. 3. In the case of the exothermic resistor obtained
by this method, that is, by being covered with the secondary glass
layer 42 after the molybdenum core wire has been removed and by
being thereafter sintered, the glass layer extends even inside the
coil formed by the platinum wire 2 as shown in FIG. 4E by being
enlarged, so that the platinum wire 2 can be supported more securely.
In the thus-constructed exothermic resistor, the cylindrical glass
member having the inner surface in which the coil is formed is closed
at its both ends. Therefore there is no possibility of dust or ionic
substances becoming attached to the inner surface of the glass member
or filling the inner space of the glass member. In consequence,
there is no possibility of any extraneous substance entering the
area in the vicinity of the coil. It is thereby possible to stabilize
the performance of the exothermic resistor with respect to thermal
or electrical effects.
The above method of manufacturing the exothermic resistor is advantageous
because the sublimation of the core can be effected simultaneously
with sintering of the glass member if the core wire 5 is made of,
for example, molybdenum. It is thus possible to remove the core
without performing any special processing such as etching. This
contributes to simplification of the manufacturing operations.
Embodiment 8
Another example of the method of manufacturing the type of exothermic
resistor shown in FIG. 3 will be described below.
A length of platinum wire 2 having a diameter of 20 .mu.m is continuously
wound by an automatic winding machine around the core wire 5 made
of molybdenum and having a diameter of 0.4 mm. A resistor blank
thereby formed is cut into pieces each having a length of about
6 mm for one exothermic resistor element or body. The lead wires
3 having a diameter of 0.13 mm and made of a platinum-iridium alloy
are welded to opposite ends of each piece at the connections 21.
By electrophoresis, the glass material 4 is attached to outer surfaces
of the molybdenum core wire 5 and the platinum wire 2 except for
opposite ends of the molybdenum core 5. Thus-prepared blank piece
is sintered in an oxidizing atmosphere by the electric furnace.
FIG. 5 shows the state of the blank piece after this sintering.
In this case, the glass material 4 is, for example, a ZnO--B.sub.2
O.sub.3 glass which has a viscosity of 10.sup.4 poise at 680.degree.
C. and which crystallizes at 750.degree. C. and remelts at a temperature
higher than about 1000.degree. C. As the temperature in the sintering
process of the glass material 4 rises, oxidation of the molybdenum
core wire 5 is promoted so that the core material becomes MoO.sub.3.
The softened glass is sealed at 680.degree. C. and crystallize at
750.degree. C. so that the shape of the glass is stabilized. The
sintering temperature is thereafter raised so that MoO.sub.3 is
sublimated, thereby removing the molybdenum core 5. The sintering
is thereafter continued at 950.degree. C. for 20 minutes before
it is finished. After the sintering has been completed, openings
through which sublimated MoO.sub.3 is dispersed are left at opposite
ends of the glass member 4. These openings are closed by melting
the glass by the heat of a flame, thereby obtaining an exothermic
resistor of the type shown in FIG. 3.
Embodiment 9
Still another example of the method of manufacturing the type of
exothermic resistor shown in FIG. 3 will be described below.
A length of platinum wire 2 having a diameter of 20 .mu.m is wound
by an automatic winding machine around the core wire 5 made of molybdenum
and having a diameter of 0.4 mm. A resistor blank thereby formed
is cut into pieces each having a length of about 6 mm for one exothermic
resistor element or body. The lead wires 3 having a diameter of
0.13 mm and made of a platinum-iridium alloy are welded to opposite
ends of each piece at the connections 21. A coating of the glass
material 4 is formed by electrophoresis. Thereafter, the thus-prepared
blank piece is sintered in an oxidizing atmosphere by the electric
furnace. FIG. 3 shows the state of the blank piece after this sintering.
In this case, an Al.sub.2 O.sub.3 --P.sub.2 O.sub.4 glass having
a viscosity of 10.sup.6.7 poise at 820.degree. C. and a viscosity
of 10.sup.4 poise at 910.degree. C. is selected as the glass material
4. As the temperature in the sintering process of this glass material
rises, the molybdenum core wire 5 is oxidized and sublimated at
795.degree. C. so that it is removed. The baking is thereafter continued
at 1080.degree. C. for 1 hour before it is finished, thus obtaining
an exothermic resistor of the type shown in FIG. 3.
As described above, the present invention was exemplified with
respect various compositions of the glass material 4 shown in FIG.
3. If a glass material has a viscosity higher than 10.sup.4 poise
at 800.degree. C. and a viscosity lower than 10.sup.7 poise at 1000.degree.
C., it can be used to form the type of exothermic resistor shown
in FIG. 3. Also, glass materials of various compositions were tried
with respect to the method in which the glass is formed as shown
in FIG. 5. If a glass material is crystalizable at any temperature
lower than 790.degree. C. and capable of maintaining its shape at
a temperature lower than 900.degree. C., it can be used to form
the type of exothermic resistor shown in FIG. 3.
In the above described embodiments, electrophoresis is utilized
to form the glass coating. However, methods other than the method
of using electrophoresis, including a method of applying a glass
material in the form of paste, are applicable to the manufacture
of the exothermic resistor 1 shown in FIG. 3.
Embodiment 10
FIG. 6 shows the structure of an exothermic resistor which represents
a still further embodiment of the present invention. Lead wires
3 made of a platinum-iridium alloy are connected to opposite ends
of a length of exothermic resistance wire 2 in the form of a coil
made of platinum. The exothermic resistance wire 2 including connections
21 is covered with a layer of composite material 4 composed of
ceramic and glass materials which constitute a support member.
A method of manufacturing this type of exothermic resistor will
be described below with reference to FIGS. 7A to 7E.
FIG. 7A shows a molybdenum core wire 5 having a diameter of 0.5
mm and provided as the core around which the platinum wire is to
be wound. The core wire 5 has circular column portions having a
length of 5 mm and flat portions 5A having a length of 2 mm, the
circular column portions and the flat portions 5A being alternately
disposed. As shown in FIG. 7B, a length of platinum wire (exothermic
resistor) 2 having a diameter of 30 .mu.m was wound by an automatic
winding machine around the core wire 5 over a length for two or
more resistor elements. As shown in FIG. 7C, a resistor blank thereby
formed was cut at the centers of the flat portions into pieces,
and a pair of lead wires 3 made of a platinum-iridium alloy and
having a diameter of 0.13 mm were welded to opposite ends of each
piece at the connections 21. The flat portions 5A were provided
with a view to improving the facility with which the lead wires
3 was placed on and attached to the core wire 5. The flat portions
were formed by plastic working based on pressing. It is preferable
in terms of ease of working that the flat portions are made symmetrical
about a horizontal plane. As shown in FIG. 7D, the composite member
4 was formed over the exothermic resistance wire 2 and is thereafter
sintered. To apply a material to form this layer, a solution was
prepared which contained denatured alcohol and water as solvents
and magnesium nitrate and aluminum nitrate provided as electrolytes,
and in which particles of alumina and powder of PbO--SiO.sub.2 glass
mixed at a ratio: 95:5 were dispersed; and particles of the alumina
and the glass were attached to the platinum wire by electrophoresis
in such a manner that the platinum wire in the state shown in FIG.
7C was placed in the suspension for electrophoretic deposition as
a cathode while an aluminum plate was placed in the same suspension
as an anode, a voltage of 40V then being applied through the suspension
for electrophoretic deposition between these electrodes. FIG. 8
schematically illustrates this state of coating in which a glass-alumina
layer 53 having pores 54 and constituting a porous layer was coated
to outer surfaces of the platinum wire 52 wound around the molybdenum
core wire 51. As shown in FIG. 7D, the thus-prepared piece was heated
in the electric furnace at 900.degree. C. for 1 hour so that the
molybdenum core wire was oxidized and sublimated, and this piece
was heated at 1100.degree. C. for 30 minutes, thereby sintering
the electrocoating layer. The thickness of the sintered layer 4
was about 80 .mu.m. The softening temperature of the glass used
to form this layer was 850.degree. C., and the sintered layer 4
became porous but had a strength large enough to prevent itself
from breaking during handling. As shown in FIG. 7E, this layer was
coated with powder of PbO--B.sub.2 O.sub.3 --SiO.sub.2 glass having
a softening temperature of 680.degree. C. and thereafter underwent
baking at 850.degree. C. for 90 minutes so that this glass permeated
through the sintered layer 4 thereby forming a composite material
layer 4A. From observation of a cross-section of the thus-obtained
exothermic resistor, it was found that the coating glass formed
a surface layer and also reached the cavity formed by the removal
of the molybdenum core wire, thereby forming a continuous phase.
The volume percent of the glass contained in the composite material
layer was 32%, and the force required to crush the thus-obtained
exothermic resistor is 2.1 kg while the anti-crushing strength of
a resistor manufactured by the conventional technique is about 0.5
kg at most.
Embodiment 11
A wire having a diameter of 0.5 mm and made of a Ni-Fe alloy having
a 53% nickel content was used as the core wire. In the same process
as that shown in FIGS. 7A to 7E, a platinum wire was wound around
this core wire, a resistor blank thereby formed around the core
wire was cut into pieces, and lead wires were welded to each cut
piece. Alumina particles were coated to the welded portions and
to the platinum wire by a dipping method. This method resides in
a process in which a solution is prepared by dispersing alumina
particles in an organic solvent (terpineol); and the exothermic
resistor blank constituted by the core, the platinum wire wound
around the core and the lead wires connected to the platinum wire
is dipped in this solution and is taken out therefrom, thereby coating
alumina particles to the exothermic resistor. In this case, one
end of the core was not coated with alumina particles. Each blank
piece was heated at 1500.degree. C. for 2 minutes, thereby sintering
the alumina. The blank piece was thereafter immersed in a mixed
acid composed of nitric acid and sulfuric acid at 80.degree. C.
for 3 hours, thereby removing the core by the etching manner. The
resistor piece was thereafter coated with powder of PbO--SiO.sub.2
glass having a softening temperature of 600.degree. C. and thereafter
underwent baking at 820.degree. C. for 90 minutes so that this glass
permeated through the baked alumina layer, thereby forming a composite
material layer. The anti-crushing strength of the thus-obtained
exothermic resistor was 1.8 kg, and the volume percent of the glass
in the composite material layer was 41%.
Embodiment 12
Exothermic resistors similar to those of Embodiments 10 and 11
and having glass components of different volume percents in the
composite material layers were manufactured. Of these exothermic
resistors, one having a glass component of a smaller volume percent
was manufactured in such a manner that ceramic particles and glass
particles were simultaneously coated, at a desired mixing ratio,
to the platinum wire by electrophoresis in the same manner as in
the case of Embodiment 10 and were sintered under conditions for
enabling the glass to suitably melt. The exothermic resistor was
completed without performing the succeeding glass coating. To form
each of exothermic resistors of this type, composite material particles
were preliminarily formed by mixing ceramic particles and glass
particles at a desired ratio, heating this mixture so that the glass
was molten, cooling to solidify the same, and pulverizing the composite
material thereby obtained; and the composite material particles
thereby formed were coated to the platinum wire, thereby improving
the uniformity of the materials constituting the composite material
layer. Since, in the case of attachment of particles based on the
electrophoresis method, action of electric charges on the surfaces
of particles is utilized, the manner of attachment varies depending
upon the type of particle. For this reason, the ceramic and the
glass are not always attached while being maintained at the same
compounding ratio as that at which they are originally mixed, and
there is therefore a possibility of occurrence of non-uniformity
of the compounding ratio with respect to the area over which the
mixed particles are attached. However, it is possible to avoid this
problem by preparing composite material particles each of which
contains the ceramic and glass materials mixed at a predetermined
compounding ratio. Silicon carbide, silicon nitride and aluminum
nitride were also used as ceramic components other than the alumina.
Thermal conductivities of alumina, silicon carbide, silicon nitride,
and aluminum nitride are 21 W/m.K, 40 W/m.K, 12 W/m.K, and 21 W/m.K,
respectively. When these materials other than the alumina were used,
sintering after the coating of the composite material was performed
in an inert gas.
The anti-crushing strength and the response time were examined
with respect to exothermic resistors thereby manufactured. FIG.
9 shows the relationship between the ratio of the volume of glass
component to the volume of the composite material layer (volume
percent) and the anti-crushing strength, the abscissa representing
the volume percent R and the ordinate representing the anti-crushing
strength F (kg). The broken line F.sub.0 indicates a level of anti-crushing
strength required for an ordinary exothermic resistor, and the anti-crushing
strengths of the exothermic resistors manufactured in accordance
with the present invention fall into a region between the solid
lines C and D. The anti-crushing strength varies over a certain
range depending upon factors including the type of ceramic component
of the composite material. FIG. 10 shows the relationship between
the volume percent of the glass component and the response time,
the abscissa representing the volume percent R and the ordinate
representing the response time T (ms). The response time also varies
depending upon factors including the type of ceramic component of
the composite material.
When the volume percent of the glass component of the composite
material layer was less than 2%, the strength of the layer was so
small that it was impossible to handle the resistor piece by employing
a pincette or the like. After the composite material had been sintered
at a high temperature for a long time in order to increase the strength,
the characteristics of the platinum wire changed, which impaired
the desired characteristics of the exothermic resistor.
When the volume percent of the ceramic component of the composite
material layer was less than 40%, that is, the volume percent of
the glass component was higher than 60%, the response characteristics
of the resulting air flow meter became deteriorated as in the case
where the platinum wire was covered with glass alone. As a result,
effects specific to the composition of the ceramic and glass materials
were not obtained.
Embodiment 13
In the case of Embodiments 10 to 12 the wire to be used as an
exothermic resistance element was continuously wound around the
core wire, the resistor blank thereby formed was cut into pieces
each having a desired length, and the lead wires were connected
to each piece. In this embodiment, however, the core wire is cut
into pieces each having a desired length; a pair of lead wires are
connected to opposite ends of each cut core wire; one end of a wire
to be used as an exothermic resistance element was connected to
one of the pair of lead wires; and the other end of the resistance
wire is connected to the other lead wire after the resistance wire
has been wound around the core wire. FIGS. 11A to 11D show procedures
of manufacturing an exothermic resistor in this manner. As shown
in FIG. 11A, a pair of lead wires having a diameter of 0.13 mm and
made of a platinum-iridium alloy are connected to flat portions
of a molybdenum core wire 5 having a diameter of 0.5 mm, the flat
portions being formed at opposite ends thereof. As shown in FIG.
11B, one end a platinum wire (exothermic resistor) 2 having a diameter
of 30 .mu.m is welded to one of the pair of lead wires 3 at a connection
21 and the other end of the platinum wire 2 is welded to the other
lead wire 3 at another connection 21 after it has been wound around
the core wire 5. As shown in FIG. 11C, a composite material is applied
over the exothermic resistance wire 2 and is baked. As shown in
FIG. 11D, the resistor piece is coated with glass powder and thereafter
undergoes sintering so that the glass permeates through the sintered
layer 4 thereby forming a composite material layer 4A.
Embodiment 14
A type of exothermic resistor formed on an alumina substrate will
be described below with reference to FIGS. 12A to 12D and FIG. 13.
A circuit 94 was formed by a lift-off method on an alumina substrate
91 having a width of 4 mm, a length of 10 mm, and a thickness of
0.3 mm. FIG. 12A to 12D show main procedures based on the lift-off
method by illustrating cross-sectional views of the exothermic resistor;
FIG. 12A shows a step of forming a mask 92 on the substrate 91 from
a photoresist; FIG. 12B, a step of forming a film over the substrate
and the mask by applying a platinum paste 93 thereto; FIG. 12C,
a step of cutting the film by dilating the resist by a developer;
and FIG. 12D, a step of removing the mask 92 by an etching manner
and thereafter completing the platinum circuit 94 by baking. The
pattern of the platinum film circuit was formed while the width
of circuit lines was set to 400 .mu.m and the distance between the
lines was set to 100 .mu.m. The resistance of this circuit was 12
.OMEGA.. The pattern had at its opposite ends portions of large
areas to which a pair of lead wires 95 made of a platinum-iridium
alloy were connected by brazing. A paste containing a PbO--SiO.sub.2
glass having a softening temperature of 600.degree. C. and alumina,
the ratio of the proportions of the glass and the alumina being
2:3 was applied over the platinum thick film circuit and was sintered
at 800.degree. C. for 15 minutes, thereby forming a composite material
layer 96. The response speed of an air flow meter constituted by
employing the thus-obtained exothermic resistor 100 was twice as
high as that in the case there the circuit was coated with glass
alone.
FIG. 14 shows a hot wire air flow meter which makes use of the
exothermic resistor 1 of the present invention. In this air flow
meter, a resistor 6 for measuring the temperature of air which is
the same as the exothermic resistor 1 is used in combination with
the exothermic resistor 1. As shown in FIG. 14 the exothermic resistor
1 and the resistor 6 for measuring the temperature of air are secured
to a support 8 disposed in a bypass passage 72 which is formed in
a body 73 and which bypasses part of intake air the majority of
which flows through a main passage 71 also formed in the body 73.
FIG. 15 shows a circuit for driving the hot wire air flow meter,
constituted by the exothermic resistor 1 the air temperature measurement
resistor 6 operational amplifiers 9 and 10 a power transistor
11 capacitor 12 and resistors 13 to 17. The plus terminal of a
battery (not shown) is connected to the collector terminal 18 of
the power transistor 11 while the minus terminal of the battery
is connected to a grounding terminal 19 of the resistor 13. An input
terminal of a microcomputer (not shown) for controlling an engine
by using signals output from the hot wire air flow meter is connected
to a connection 20 between the resistor 13 and the exothermic resistor
1.
In the thus-constructed circuit, an electric current is supplied
to the exothermic resistor 1 by the power transistor 11 in order
to heat up the exothermic resistor, and the temperature of the exothermic
resistor is controlled in such a manner that it is kept higher than
that of the air temperature measurement resistor 6 by constant degrees.
During this control, the air temperature measurement resistor 6
is used to correct the temperature of the intake air by detecting
this temperature while allowing only a very weak current to flow
through the air temperature measurement resistor 6 such that the
heat generated by this current is negligible. As the air flows while
colliding with the exothermic resistor 1 the driving circuit performs
the control operation to constantly maintain the difference between
the temperatures of the exothermic resistor 1 and the air temperature
measurement resistor 6 as described above. This operation is performed
in a feedback manner such that a voltage obtained by dividing the
voltage across the exothermic resistor 1 by means of the resistors
14 and 15 is constantly kept equal to a voltage amplified by the
operational amplifier 9 from a voltage drop across the resistor
13 proportional to the current flowing through the exothermic resistor
1. In consequence, as the air flow rate changes, the current flowing
through the exothermic resistor 1 changes, and the air flow rate
is measured from the voltage drop that appears across the resistor
13 in response to the current.
FIG. 16 shows a graph of response characteristics of this hot wire
air flow meter in accordance with the present invention. The abscissa
represents the time (ms) and the ordinate represents the flow rate
(kg/h). The voltage output from the hot wire air flow meter was
measured when the air flow rate was changed from a low flow rate
of about 20 kg/h to a high flow rate of about 200 kg/h. This voltage
was converted into the flow rate to be plotted along the ordinate.
The curve B indicates a characteristic of the air flow meter making
use of the conventional bobbin-type exothermic resistor, and the
curve A indicates a characteristic of the air flow meter in accordance
with the present invention in comparison with the former. As can
be understood from this graph, the present invention ensures that
the time taken for the flow meter to output the final value can
be remarkably reduced.
It is therefore possible for the hot wire air flow meter to output
signals correctly in response to actual changes in the air flow
rate even at the time of rapid acceleration or deceleration of the
vehicle, thereby optimizing the determination of the injection rate
of the injector and solving the problem of surging.
This remarkable improvement in the response performance is attained
for the reason that the exothermic resistor 1 can rapidly react
to a change in the air flow rate since the heat generating in the
platinum wire 2 of the exothermic resistor 1 is almost entirely
transmitted to the air without heating the bobbin or core or being
transmitted to the support via the bobbin as in the case of the
conventional bobbin-type exothermic resistor.
A type of anemometer was also manufactured which was designed to
utilize a combination of the exothermic resistor of the present
invention and a circuit for detecting the temperature from a change
in the resistance value of this resistor and converting it into
the wind velocity. As a result, this anemometer also exhibited improved
response characteristics. |