Abstrict A thermal type air flow meter that makes it possible to precisely
and easily determine whether a sensor element is a non-defective
or a defective by screening is provided. First and second diaphragm
sections composed of an electrical insulating film are formed in
a semiconductor substrate. A heating resistor and a resistor for
intake air temperature sensor are disposed on them to obtain a sensor
element for thermal type air flow meters. The length of the short
sides W1 of the rectangular first diaphragm section is made substantially
equal to the length of the short sides of the second diaphragm section
rectangular as well. Thus, the pressures applied to each of the
diaphragm sections are substantially identical with each other in
pressurization during screening.
Claims 1. A thermal type air flow meter having a sensor element comprising:
a first diaphragm-section and a second diaphragm-section which are
comprised of respective cavity-portions and insulating diaphragms
being formed at a semiconductor substrate; a thermal sensitive resistor
for air-flow rate measurement being disposed on the first diaphragm-section;
and an intake air temperature-measurement resistor being disposed
on the second diaphragm section, wherein the first diaphragm section
and the second diaphragm section are configured so that a stress
of the first diaphragm section due to a pressure difference of its
both surfaces and that of the second diaphragm section are substantially
identical to each other.
2. A thermal type air flow meter having a sensor element comprising:
a first diaphragm-section and a second diaphragm-section which are
comprised of respective cavity-portions and insulating diaphragms
being formed at a semiconductor substrate; a thermal sensitive resistor
for air-flow rate measurement being disposed on the first diaphragm-section;
and an intake air temperature-measurement resistor being disposed
on the second diaphragm section, wherein both planar shapes of the
first and second diaphragm sections are respectively rectangles
having short sides and long sides, and dimensions of the first and
second diaphragm sections are set so that each size of the short
sides of the second diaphragm section is 0.9 times to 1.1 times
each size of the short sides of the first diaphragm section.
3. A thermal type air flow meter having a sensor element comprising:
a first diaphragm-section and a second diaphragm-section which are
comprised of respective cavity-portions and insulating diaphragms
being formed at a semiconductor substrate; a thermal sensitive resistor
for air-flow rate measurement being disposed on the first diaphragm-section;
and an intake air temperature-measurement resistor being disposed
on the second diaphragm section, wherein a planar shape of the first
diaphragm section is a rectangle having short sides and long sides,
and a planar shape of the second diaphragm is a polygon, and the
dimensions of the polygon are set so that each of opposite sides
having minimum size of the polygon is 0.9 times to 1.1 times each
of the short sides of the rectangle.
4. A thermal type air flow meter having a sensor element comprising:
a first diaphragm-section and a second diaphragm-section which are
comprised of respective cavity-portions and insulating diaphragms
being formed at a semiconductor substrate; a thermal sensitive resistor
for air-flow rate measurement being disposed on the first diaphragm-section;
and an intake air temperature-measurement resistor being disposed
on the second diaphragm section, wherein a planar shape of the first
diaphragm section is a rectangle having short sides and long sides,
and a planar shape of the second section is a polygon comprising
a combination of a plurality of rectangles, and the dimensions of
the polygon are set so that each of opposite sides having minimum
size of the polygon is 0.9 times to 1.1 times each of the short
sides of the rectangle.
5. The thermal type air flow meter according to claim 1 wherein
a distance between a periphery of the first diaphragm section and
the thermal sensitive resistor of the same is equal to a distance
between a periphery of the second diaphragm section and the intake
air temperature-measurement resistor of the same.
6. The thermal type air flow meter according to claim 1 wherein
each protective coat being formed of organic material is provided
on each surface on the peripheries of the first and second diaphragm
sections.
7. The thermal type air flow meter according to claim 1 wherein
the sensor element has a drive unit for driving the sensor element
being provided on the semiconductor substrate.
8. The thermal type air flow meter according to claim 1 comprising:
a computing unit for taking in information of an air flow rate and
an air temperature from the sensor element, computing based on the
information and outputting the result of computation.
9. The thermal type air flow meter according to claim 8 wherein
the computing unit is equipped with an externally writable memory,
and the computing unit carries out computation using information
stored in this memory.
10. The thermal type air flow meter according to claim 8 wherein
the computing unit is equipped with a temperature sensor therein
or in the proximity to thereof, and the computing unit carries out
computation based on temperature information obtained through the
temperature sensor.
11. The thermal type air flow meter according to claim 8 wherein
the drive unit and the computing unit are placed on board of the
semiconductor substrate of the sensor element.
12. The thermal type air flow meter according to claim 11 wherein
the semiconductor substrate has a recessed area between the diaphragm
sections and a group of the drive unit and the computing unit.
Description CLAIM OF PRIORITTY
[0001] The present application claims priority from Japanese application
serial no. 2004-238565 filed on Aug. 18 2004 the content of which
is hereby incorporated by reference into this application.
FIELD OF THE INVENTION
[0002] The present invention relates to a thermal type air flow
meter using a sensor element of semiconductor, and more particularly
to a thermal type air flow meter suitable for measuring an intake
air flow rate of an internal combustion engine.
BACKGROUND OF THE INVENTION
[0003] Recently, a fuel feed rate of an automobile engine (internal
combustion engine) is generally controlled based on an intake air
flow rate. An intake air flow meter is required for this purpose.
Of various types of intake air flow meters, thermal type flow meters
have come into widespread use because of their capability to directly
sense mass air flow rate.
[0004] Of various types of thermal type air flow meters, especially,
those using a sensor element (measuring element) manufactured by
semiconductor micromachining technology are advantageous in terms
of the following: cost reduction, low-power driving, and high-speed
responsibility. Therefore, thermal type air flow meters using a
sensor element based on this semiconductor technology have become
part of mainstream through the years.
[0005] Air flow meters constructed as described below are known
as conventional art: the substrate of their sensor element is provided
with a plurality of diaphragms (thin membrane portions). A heating
resistor, an upstream thermal sensitive resistor to the heating
resistor and a downstream thermal sensitive resistor thereto served
as a flow rate sensor are disposed on these diaphragms. The upstream
and downstream sensitive resistors are disposed adjacently to the
heating resistor. (Refer to Japanese Patent Laid-Open No. 2001-349759
for example.)
[0006] In this thermal type air flow meter according to prior art,
a plurality of diaphragms are provided in line in the direction
orthogonal to the direction of flow of a fluid to be measured. Such
a configuration of diaphragms makes it possible to enhance the strength
of the diaphragms and ensure sensitivity and responsibility of the
sensor.
[0007] The thermal type air flow meter has the following problem:
a measuring error is caused due to a change in intake air temperature.
Thermal type air flow meters constructed as described below to cope
with this are also known as conventional art: an intake air temperature
sensor is installed at part of the sensor element of a thermal type
air flow meter, and the intake air temperature is measured with
it to correct an air flow rate measurement. Thus, measuring errors
due to a change in intake air temperature are reduced so that the
air flow rate can be measured with higher accuracy.
[0008] In this case, the intake air temperature sensor must be
separately installed. This increases the number of parts of the
thermal type air flow meter, and further requires mounting structure
and wiring for installing the intake air temperature sensor. This
results in increased production cost.
[0009] A technology to integrate an intake air temperature sensor
with the sensor element of a thermal type air flow meter has solved
this problem. Namely with this technology, the number of parts and
wiring for electrically connecting the intake air temperature sensor
can be reduced.
[0010] Furthermore, the intake air temperature sensor can be integrally
provided on the sensor element of the thermal type air flow meter
by the following means: the sensor element is formed on a semiconductor
substrate of single crystal silicon using micromachining technology.
In addition, a thermistor, a temperature sensitive resistor, and
the like are formed on the sensor element. As a result, the cost
can be further reduced.
[0011] Description will be given to an example of a thermal type
air flow meter according to conventional art in which a sensor element
is provided with an intake air temperature sensor. FIG. 20 is a
plan view illustrating conventional art; FIG. 21 is a sectional
view taken along the line A-A of FIG. 20; and FIG. 22 is a sectional
view taken along the line B-B of the same. In these drawings, numeral
1 denotes a sensor element.
[0012] Numeral 2 denotes a semiconductor substrate; 3 and 4 denote
diaphragm sections; 5 denotes a heating resistor; 6a and 6b denote
thermal sensitive resistors; 7 and 8 denote cavity portions; 9 denotes
a temperature compensation resistor; 10 denotes an electrical insulating
film; 11 denotes a resistor for intake air temperature sensor; and
12a to 12j denote terminal electrode sections.
[0013] Arrow f indicates the direction in which a fluid to be measured
(intake air) flows relative to the sensor element 1. In these drawings,
therefore, the left side is upstream and the right side is downstream.
[0014] As illustrated in FIG. 21 and FIG. 22 the cavity portions
7 and 8 are formed at the semiconductor substrate 2 from one side.
The electrical insulating film 10 is formed on the other side of
the semiconductor substrate so that these cavity portions 7 and
8 are closed therewith. Thus, the diaphragm sections 3 and 4 are
formed. The heating resistor 5 and the thermal sensitive resistors
6a and 6b are disposed on the diaphragm section 3. The resistor
11 for intake air temperature sensor is disposed on the diaphragm
section 4.
[0015] The thermal sensitive resistor 6a is disposed upstream from
the heating resistor 5 adjacently thereto. Downstream from the heating
resistor 5 the thermal sensitive resistor 6b is similarly disposed
adjacently thereto.
[0016] In order to measure the air flow rate with this sensor element
1 the heating resistor 5 is energized to electrically heat itself.
At this time, the temperature of the heating resistor 5 is controlled
so that it is higher by a certain temperature than the temperature
of the temperature compensation resistor 9 that is, the temperature
of the fluid to be measured. (The temperature of the fluid to be
measured the temperature of the semiconductor substrate 2.)
[0017] In this state, a change in temperature difference is measured
between a pair of the thermal sensitive resistors 6a and 6b disposed
upstream and downstream from the heating resistor 5. When assuming
that air is not flowing now, the distribution of temperature on
the diaphragm section 3 is symmetric between the upstream side and
the downstream side with respect to the heating resistor 5. As a
result, the thermal sensitive resistor 6a and the thermal sensitive
resistor 6b become identical with each other in temperature; therefore,
no difference is produced in resistance value.
[0018] On the other hand, when air flows in the direction of arrow
f, the upstream thermal sensitive resistor 6a is exposed to the
air flow. Therefore, it is much cooled, and its temperature falls.
Meanwhile, the downstream thermal sensitive resistor 6b is exposed
to air heated by the heating resistor 5. Therefore, it is not so
much cooled, and its temperature does not fall so much.
[0019] As a result, a temperature difference corresponding to the
air flow rate is produced between the thermal sensitive resistor
6a and the thermal sensitive resistor 6b, and a difference corresponding
to the air flow rate is also produced in resistance value. Consequently,
this difference in resistance value is sensed to determine the air
flow rate.
[0020] The sensor element is configured so that the intake air
temperature is measured with the resistor 11 for intake air temperature
sensor. For this purpose, the resistor 11 is formed of a resistor
material having a large temperature coefficient at zeroth order
of the resistance. Therefore, the intake air temperature can be
determined by variation in the resistance value of the resistor
11.
[0021] As illustrated in FIG. 22 the resistor 11 for intake air
temperature sensor is placed on the diaphragm section 4 formed by
covering the cavity portion 8 with the electrical insulating film
10. Thus, the thermal capacity is reduced, and the responsibility
to the change in intake air temperature is enhanced.
[0022] The pressure in the intake pipe of an engine can be steeply
increased due to the occurrence of back fire or the like. In such
a case, a pressure almost two times greater than the atmospheric
pressure can be applied to the sensor element of the thermal type
air flow meter, and it can be subject to great mechanical stress.
The sensor elements are required to be free from breakage for a
long time even under such circumstances.
[0023] A defective, for example, a sensor element having damage
to its diaphragm section, can exist in a manufacturing process.
If such a defective is mounted on an engine and the engine is started,
the engine prematurely gets out of order. This is because the sensor
element does not have durability enough to endure the harsh pressure
conditions mentioned above.
[0024] Therefore, failure in the diaphragm sections of a thermal
type air flow meter is one of significant causes of such a premature
failure. For this reason, defectives must be screened out in the
process of manufacture, and screening is a common practice for removing
defectives. In case of the sensor element of the thermal type air
flow meter, screening is carried out by exerting a predetermined
stress on the diaphragm by applying pressure to its diaphragm sections
from one side.
[0025] For example, the following measures are taken in the semiconductor
micromachining process: in the stage of wafer prior to dicing, the
cavity portions in positions where the diaphragm sections of the
sensor element are formed are sealed. The sensor element is placed
in high-pressure environmental test equipment. A pressure difference
is produced between one side and the other side of each diaphragm
section, and stress is applied to the diaphragm sections. In case
of a defective having damage, its diaphragm sections are destroyed.
[0026] With this method, a large amount of sensor elements can
be tested and screened at a time, and efficient screening can be
carried out with ease. The method is effective in providing sensor
elements free from defect.
[0027] The above-mentioned conventional art does not give consideration
to problems that may occur when a temperature sensor for measuring
the intake air temperature is formed on the sensor element of a
thermal type flow meter. It has trouble with provision of sensor
elements free from defect.
[0028] As described with respect to the above conventional art,
a problem arises when a resistor 11 for intake air temperature sensor
is formed on a sensor element 1. Two diaphragm sections exist in
the sensor element 1: a diaphragm section 3 in which a heating resistor
5 and thermal sensitive resistors 6a and 6b are formed; and a diaphragm
section 4 in which the resistor 11 is formed.
[0029] Thus, the test pressure must be varied from one diaphragm
section to another diaphragm section during screening because the
two diaphragm sections are different in size from each other in
the conventional art. Since the sensor element is as small as several
millimeters, it is substantially impossible to adjust the test pressure
from one diaphragm section to another diaphragm section. As a result,
satisfactory screening cannot be carried out.
[0030] If a test pressure most suitable for either diaphragm section
is selected for screening, the other diaphragm section is exposed
to excessive pressure, and its life is shortened. Or, the pressure
is insufficient for the other diaphragm section, and satisfactory
screening cannot be carried out. In either case, provision of sensor
elements free from defect cannot be expected.
[0031] An object of the present invention is to provide a thermal
type air flow meter wherein whether a sensor element is non-defective
or defective can be determined with ease and accuracy by screening.
SUMMARY OF THE INVENTION
[0032] The above object is attained by taking the following construction.
A thermal type air flow meter has a sensor element comprising: a
first diaphragm-section and a second diaphragm-section which are
comprised of respective cavity-portions and insulating diaphragms
being formed at a semiconductor substrate; a thermal sensitive resistor
for air-flow rate measurement being disposed on the first diaphragm-section;
and an intake air temperature-measurement resistor being disposed
on the second diaphragm section. Furthermore, the first diaphragm
section and the second diaphragm section are configured so that
a stress of the first diaphragm section due to a pressure difference
of its both surfaces and that of the second diaphragm section are
substantially identical to each other.
[0033] The above object is also attained by taking the following
construction. Likewise, a thermal type air flow meter has a sensor
element comprising: a first diaphragm-section and a second diaphragm-section
which are comprised of respective cavity-portions and insulating
diaphragms being formed at a semiconductor substrate; a thermal
sensitive resistor for air-flow rate measurement being disposed
on the first diaphragm-section; and an intake air temperature-measurement
resistor being disposed on the second diaphragm section. Furthermore,
both planar shapes of the first and second diaphragm sections are
respectively rectangles having short sides and long sides; and dimensions
of the first and second diaphragm sections are set so that each
size of the short sides of the second diaphragm section is 0.9 times
to 1.1 times each size of the short sides of the first diaphragm
section.
[0034] The above object is also attained by taking the following
construction. Likewise, a thermal type air flow meter has a sensor
element comprising: a first diaphragm-section and a second diaphragm-section
which are comprised of respective cavity-portions and insulating
diaphragms being formed at a semiconductor substrate; a thermal
sensitive resistor for air-flow rate measurement being disposed
on the first diaphragm-section; and an intake air temperature-measurement
resistor being disposed on the second diaphragm section. Furthermore,
a planar shape of the first diaphragm section is a rectangle having
short sides and long sides, and a planar shape of the second diaphragm
is a polygon. The dimensions of the polygon are set so that each
of opposite sides having minimum size of the polygon is 0.9 times
to 1.1 times each of the short sides of the rectangle.
[0035] The above object is also attained by taking the following
construction. Likewise, a thermal type air flow meter has a sensor
element comprising: a first diaphragm-section and a second diaphragm-section
which are comprised of respective cavity-portions and insulating
diaphragms being formed at a semiconductor substrate; a thermal
sensitive resistor for air-flow rate measurement being disposed
on the first diaphragm-section; and an intake air temperature-measurement
resistor being disposed on the second diaphragm section. Furthermore,
a planar shape of the first diaphragm section is a rectangle having
short sides and long sides, and a planar shape of the second section
is a polygon comprising a combination of a plurality of rectangles.
The dimensions of the polygon are set so that each of opposite sides
having minimum size of the polygon is 0.9 times to 1.1 times each
of the short sides of the rectangle.
[0036] At this time, the above object is also attained by making
the following distances equal to each other: the distance between
a periphery of the first diaphragm section and the thermal sensitive
resistor of the same is equal to a distance between a periphery
of the second diaphragm section and the intake air temperature-measurement
resistor of the same.
[0037] At this time, the following construction may be adopted:
each protective coat being formed of organic material is provided
on each surface on the peripheries of the first and second diaphragm
sections.
[0038] At this time, the following construction may be adopted:
the sensor element has a drive unit for driving the sensor element
being provided on the semiconductor substrate. Also, the following
construction may be adopted: the sensor element has a computing
unit for taking in information of an air flow rate and an air temperature
from the sensor element, computing based on the information and
outputting the result of computation.
[0039] At this time, the following construction may be adopted:
the computing unit is equipped with an externally writable memory,
and the computing unit carries out computation using information
stored in this memory. Also, the following construction may be adopted:
the computing unit is equipped with a temperature sensor therein
or in the proximity to thereof, and the computing unit carries out
computation based on temperature information obtained through the
temperature sensor.
[0040] At this time, the following construction may be adopted:
the drive unit and the computing unit are placed on board of the
semiconductor substrate of the sensor element. Also, the following
construction may be adopted: the semiconductor substrate has a recessed
area between the diaphragm sections and a group of the drive unit
and the computing unit.
[0041] When pressure is applied to a rectangle diaphragm section,
maximum stress is exerted on, especially, the peripheral portion
of the diaphragm section. In the peripheral portion, the stress
is especially increased in the center of the long sides.
[0042] This stress is more greatly varied when the length of the
short sides of the diaphragm section is changed than when the length
of the long sides is changed. This indicates that the maximum stress
exerted on a diaphragm section largely depends on the length of
its short sides.
[0043] Therefore, the following can be implemented by making the
lengths of the short sides of the first diaphragm section in rectangular
shape and that of the second diaphragm section substantially equal
to each other: the maximum stresses exerted on these diaphragm sections
can be made identical with each other. A large amount of sensor
elements can be screened at a time with optimum pressure on a plurality
of the diaphragm sections formed in the sensor elements.
[0044] In the above-mentioned example, the diaphragm sections are
formed in rectangular shape; however, the present invention is not
limited to this constitution. To attain the object, it is important
to make identical the maximum stresses exerted on diaphragm sections
when the pressure around a sensor element fluctuates. Even in any
other shape, the same effect can be produced if there are lengths
dependant on the maximum stress and such a construction that these
lengths are unified is adopted, as mentioned above.
[0045] If the maximum stresses exerted on the diaphragm sections
when the pressure around the sensor element fluctuates are identical,
it means the following matter. When an ambient pressure is gradually
increased to increase the stress on the diaphragm sections, all
the diaphragm sections are broken with substantially the same timing.
Therefore, the same effect is also produced if such a shape that
diaphragm sections are broken under the same ambient pressure is
adopted.
[0046] According to the present invention, a large amount of sensor
elements can be screened at a time under optimum pressure. Thus,
a thermal type air flow meter that is excellent in mass productivity,
reliability, and measurement accuracy can be provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] FIG. 1 is a plan view of a sensor element in a first embodiment
of a thermal type air flow meter according to the present invention.
[0048] FIG. 2 is a sectional view of the sensor element in the
first embodiment, which is taken along the line C-C of FIG. 1.
[0049] FIG. 3 is a D-D line sectional view of the sensor element
in the f embodiment, which is taken along the line D-D of FIG. 1.
[0050] FIGS. 4A, 4B, and 4C are circuit diagrams of the first embodiment.
[0051] FIG. 5 is an explanatory drawing illustrating the mounting
structure of a sensor element in the first embodiment.
[0052] FIG. 6 is an explanatory drawing of equipment for conducting
pressure tests on the diaphragm sections of the sensor elements
of thermal type air flow meters.
[0053] FIG. 7 is an explanatory drawing illustrating the distribution
of one-dimensional stress exerted on the diaphragm sections in a
sensor element for thermal type air flow meters.
[0054] FIG. 8 is an explanatory drawing illustrating the distribution
of two-dimensional stress exerted on the diaphragm sections in a
sensor element for thermal type air flow meters.
[0055] FIG. 9 is an explanatory drawing illustrating the way stress
exerted on the diaphragm sections in a sensor element for thermal
type air flow meters depends on dimensions.
[0056] FIG. 10 is a plan view of a sensor element in a second embodiment
of a thermal type air flow meter according to the present invention.
[0057] FIG. 11 is a sectional view of the sensor element in the
second embodiment, which is taken along the line E-E of FIG.
[0058] FIG. 12 is a F-F line sectional view of the sensor element
in the second embodiment, which is taken along the line F-F of FIG.
11.
[0059] FIG. 13 is a plan view of a sensor element in a third embodiment
of a thermal type air flow meter according to the present invention.
[0060] FIG. 14 is a plan view of a sensor element in a fourth embodiment
of a thermal type air flow meter according to the present invention.
[0061] FIG. 15 is a block diagram illustrating an embodiment of
the computing unit for temperature correction of a thermal type
air flow meter according to the present invention.
[0062] FIG. 16 is a plan view of a sensor element in a fifth embodiment
of a thermal type air flow meter according to the present invention.
[0063] FIG. 17 is a sectional view of the sensor element in the
fifth embodiment.
[0064] FIG. 18 is a characteristic diagram illustrating an example
of the flow rate measuring error characteristic of a thermal type
air flow meter in relation to a change in intake air temperature.
[0065] FIGS. 19A, 19B, and 19C are characteristic diagrams explaining
transient temperature change in the sensor element of a thermal
type air flow meter.
[0066] FIG. 20 is a plan view illustrating an example of the sensor
element of a thermal type air flow meter according to conventional
art.
[0067] FIG. 21 is a sectional view illustrating an example of the
sensor element of the thermal type air flow meter according to conventional
art, which is taken along the line A-A of FIG. 20.
[0068] FIG. 22 is a sectional view illustrating an example of the
sensor element of the thermal type air flow meter according to conventional
art, which is taken along the line B-B of FIG. 20.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0069] Hereafter, more detailed description will be given to thermal
type air flow meters according to the present invention based on
the embodiments illustrated in the drawings.
[0070] FIG. 1 is a plan view of a sensor element in a first embodiment
of a thermal type air flow meter according to the present invention.
FIG. 2 is a sectional view taken along the line C-C of FIG. 1 and
FIG. 3 is a sectional view taken along the line D-D of the same.
In FIG. 1 numeral 19 denotes the sensor element of the thermal
type air flow meter.
[0071] In the example illustrated in these drawings, the whole
of the sensor element 19 is formed based on a semiconductor substrate
20 comprising a single crystal silicon (Si) plate. A cavity portion
21 and a cavity portion 22 are formed in this semiconductor substrate
20. Both of the cavity portion 21 and the cavity portion 22 are
formed as holes whose planar shape is a rectangle.
[0072] In FIGS. 2 and 3 arrow f indicates the direction in which
a fluid to be measure (intake air) flows relative to the sensor
element 19. In these drawings, therefore, the left side is upstream
and the right side is downstream.
[0073] A diaphragm section 23 as a first diaphragm is formed on
one side of the cavity portion 21 and a diaphragm section 24 as
a second diaphragm is formed on one side of the cavity portion 22.
The diaphragm section 23 is formed by a part of an electrical insulating
film 25 provided on either side (the upper face in FIG. 2) of the
semiconductor substrate 20 which covers the cavity portion 21.
[0074] The electrical insulating film 25 comprises a thin film
of silicon dioxide (SiO.sub.2). Therefore, the diaphragm section
23 also comprises the thin film of silicon dioxide (SiO.sub.2),
and a heating resistor 26 and thermal sensitive resistors 27a and
27b are formed on the surface thereof. One thermal sensitive resistor
27a is disposed upstream from the heating resistor 26 adjacently
thereto. Downstream from the heating resistor 26 the other thermal
sensitive resistor 27b is similarly disposed adjacently thereto.
[0075] The diaphragm section 24 is also formed by a part of the
electrical insulating film 25 provided on either side (the upper
face in FIG. 3) of the semiconductor substrate 20 which covers
the cavity portion 22. A resistor (intake air temperature-measurement
resistor) 28 that serves as an intake air temperature sensor for
measuring the temperature of the fluid to be measured is formed
on the surface of the diaphragm section 24.
[0076] The heating resistor 26 is formed of a thin film of polycrystalline
or single crystal silicon doped with impurity. Thereby, it has a
predetermined resistance value and a predetermined temperature coefficient
at zeroth order of the resistance.
[0077] The thermal sensitive resistors 27a and 27b are also formed
of a thin film of polycrystalline or single crystal silicon doped
with impurity. They are thereby so formed that they have a predetermined
resistance value and a predetermined temperature coefficient at
zeroth order of the resistance.
[0078] Further, the resistor 28 that serves as an intake air temperature
sensor is also formed of a thin film of polycrystalline or single
crystal silicon doped with impurity. Thereby, it has a predetermined
resistance value and a predetermined temperature coefficient at
zeroth order of the resistance.
[0079] In FIG. 1 a temperature compensation resistor 30 is used
for compensating the temperature of the heating resistor 26. This
temperature compensation resistor 30 is also formed of a thin film
of polycrystalline or single crystal silicon doped with impurity.
Thereby, it has a predetermined resistance value and a predetermined
temperature coefficient at zeroth order of the resistance.
[0080] At one end (the lower end in FIG. 1) of the sensor element
19 terminal electrode portions 29a to 29j of these resistors are
formed on the surface of the electrical insulating film 25. These
terminal electrode portions 29a to 29j comprise thin film pads of
aluminum (Al), gold (Au), or the like.
[0081] In order to measure the air flow rate with this sensor element
19 the heating resistor 26 is energized to electrically heat itself
and increase its temperature. At this time, the temperature of the
heating resistor 26 is controlled so that it is higher by a certain
temperature than the temperature of the temperature compensation
resistor 30 that is, the temperature of the fluid to be measured
indicated by arrow f. In this state, variation in temperature difference
is measured between a pair of the thermal sensitive resistors 27a
and 27b disposed upstream and downstream from the heating resistor
26. The air flow rate is thereby determined.
[0082] More specific description will be given. When air as the
fluid to be measured is not flowing, the distribution of temperature
on the diaphragm section 23 is symmetric between the upstream side
and the downstream side with respect to the heating resistor 26.
When the temperature distribution is symmetric as mentioned above,
the thermal sensitive resistor 27a and the thermal sensitive resistor
27b are brought to the identical temperature; therefore, no temperature
difference is produced.
[0083] If air flows in the direction of arrow f, the upstream thermal
sensitive resistor 27a is directly exposed to the air flow, and
thus it is much cooled. Meanwhile, the downstream thermal sensitive
resistor 27b is exposed to air heated by the heating resistor 26
and thus it is not so much cooled.
[0084] Therefore, the temperature of the thermal sensitive resistor
27a becomes lower than that of the thermal sensitive resistor 27b,
and a temperature different is produced in correspondence with the
air flow rate.
[0085] When the fluid to be measured flows in the direction opposite
to that of arrow f, the temperature of the thermal sensitive resistor
27b becomes lower than that of the thermal sensitive resistor 27a.
Therefore, the direction of the flow can be determined by comparing
the temperature of the thermal sensitive resistor 27a with that
of the thermal sensitive resistor 27b.
[0086] Description will be given to circuitry for driving the sensor
element 19 with reference to FIGS. 4A to 4C. FIG. 4A illustrates
a circuit for controlling the temperature of the heating resistor
26. As illustrated in the figure, this circuit comprises resistors
with fixed resistance 31 and 32 a differential amplifier 33 a
transistor 34 and a power source 35 in addition to the heating
resistor 26 and the temperature compensation resistor 30.
[0087] A bridge circuit is composed of the heating resistor 26
temperature compensation resistor 30 resistors 31 and resistor
32. The differential voltage of the bridge is sensed with the differential
amplifier 33 and the current supplied from the power source 35
to the bridge is feedback controlled. The current supplied from
the transistor 32 to the heating resistor 26 is controlled by this
feedback control.
[0088] At this time, the heating resistor 26 is caused to produce
heat by the current supplied from the transistor 34 because it has
a predetermined temperature coefficient at zeroth order of the resistance.
As a result, its temperature is increased, and its resistance value
is varied. Consequently, the bridge circuit is balanced at a predetermined
temperature, and Expression (1) below holds. In this expression,
suffix R represents the resistance value of each resistor in FIG.
4A. R26/R31=R30/R32 (1)
[0089] Expression (1) indicates the following: when the temperature
of the temperature compensation resistor 30 that is, the temperature
of a fluid to be measured is increased and the value R30 increases,
the resistance value R26 of the heating resistor 26 also increases.
Therefore, the temperature of the heating resistor 26 is controlled
so that it is constantly higher by a certain temperature .DELTA.Th
than the temperature of the fluid to be measured. The temperature
.DELTA.Th at this time can be arbitrarily set by selecting the fixed
resistance values of the resistors 31 and 32.
[0090] FIG. 4B illustrates a circuit that senses the difference
in resistance value due to a difference in temperature between the
thermal sensitive resistor 27a and the thermal sensitive resistor
27b, and outputs signals related to the air flow rate. As illustrated
in the figure, this circuit comprises the thermal sensitive resistors
27a and 27b and a power source 36. In this circuit, the power source
36 is connected with the series circuit of the thermal sensitive
resistor 27a and the thermal sensitive resistor 27b.
[0091] The thermal sensitive resistors 27a and 27b are disposed
upstream and downstream from the heating resistor 26 in proximity
thereto. Therefore, when air flows on the sensor element 19 in the
direction indicated by arrow f, the thermal sensitive resistor 27a
positioned upstream from the heating resistor 26 is cooled, and
its resistance value is lowered.
[0092] Meanwhile, the thermal sensitive resistor 27b positioned
downstream from the heating resistor 26 is exposed to air heated
by the heating resistor 26. Therefore, it is less prone to be cooled,
and its resistance value is hardly varied. Therefore, when these
thermal sensitive resistors 27a and 27b are connected in series
and a difference in resistance value is taken out as a voltage difference,
a signal corresponding to the air flow rate is obtained.
[0093] FIG. 4C illustrates a circuit that outputs information about
the temperature of a fluid to be measured from variation in the
resistance value of the resistor 28. This circuit comprises the
resistor 28 a resistor with fixed resistance 38 and a power source
37.
[0094] The resistance value of the resistor (intake air temperature-measurement
resistor) 28 that serves as an intake air temperature sensor is
varied with a change in the temperature of the fluid to be measured.
This is because the resistor 28 has a predetermined temperature
coefficient at zeroth order of the resistance. Therefore, when the
resistor 28 in series with the resistor 38 is connected with the
power source 37 and the voltage at the junction point between the
resistor 28 and the resistor 38 is taken out, a signal about the
temperature of the fluid to be measured can be outputted.
[0095] With reference to FIG. 5 description will be given to a
case wherein this sensor element 19 is installed in the intake pipe
to an engine and the intake air flow rate is measured with the thermal
type air flow meter. FIG. 5 is a sectional view illustrating the
way the sensor element 19 is installed in the intake pipe.
[0096] The example illustrated in FIG. 5 is designed so that intake
air is let through the main passage 39 of the intake pipe, as indicated
by arrow f. The sensor element 19 is installed in the sub-passage
40 protruded into the main passage 39. The member 41 constituting
the sub-passage 40 is supported by a casing 42.
[0097] The casing 42 is installed on the wall surface 46 of the
intake pipe by an attaching support 45. A circuit board 43 is installed
in the casing 42 and a drive circuit for driving the sensor element
19 and taking out flow rate signals is mounted on the circuit board
43.
[0098] The sensor element 19 is mechanically supported in the casing
42 and is electrically connected with the drive circuit on the
circuit board 43 through bonding wires 44 or the like.
[0099] An air flow rate signal obtained through measurement with
the thermal type air flow meter provided with this sensor element
19 is taken out through a connector 47. The signal is then supplied
to an engine control computer (not shown), and used in fuel feed
rate control and the like.
[0100] Description will be given to screening that is used in the
process of manufacture of the sensor element of such a thermal type
air flow meter. In this screening, a test is conducted to determine
whether its diaphragm sections have sufficient strength, and defectives
are screened. FIG. 6 is a conceptual drawing illustrating this screening.
In the example illustrated in this figure, a plurality of sensor
elements 19 are placed in pressure test equipment 48 and a large
amount of elements can be screened at a time.
[0101] First, the back sides of the sensor elements 19 are brought
into tight contact with adhesive sheet 49 or the like. Thus, the
cavity portions 21 of the diaphragm sections 23 and the cavity portions
22 of the diaphragm sections 24 are sealed out the surrounding environment.
As illustrated in the figure, the sensor elements are placed on
the table in the pressure test equipment 48.
[0102] Next, a pressurizer, such as a pump, is actuated to increase
the pressure P in the pressure test equipment 48. Thus, warpage
occurs in the diaphragm sections 23 and 24 because the interior
of the cavity portions 21 and 22 is sealed off from the outside.
[0103] FIG. 7 illustrates the distribution of stress exerted on
a diaphragm section due to pressurization during this screening.
At this time, stress .sigma. is exerted on the diaphragm section
24. The stress .sigma. is the sum of potential stress .sigma..sub.0
and the stress .sigma..sub.p produced due to pressurization (.sigma.=.sigma..sub.0+.sigm-
a..sub.p). It can be seen that the stress exerted on the diaphragm
section at this time becomes the maximum stress .sigma..sub.m, especially,
at the edges of the diaphragm section.
[0104] FIG. 8 is a plan view that the distribution of one-dimensional
stress exerted on the diaphragm section, illustrated in FIG. 7
is represented by two-dimensionally. The following is apparent from
this two-dimensional plan view: the stress exerted on the diaphragm
section takes the maximum value .sigma..sub.m at point d and point
e in proximity to the center of the long sides L of the rectangular
diaphragm section 24. Therefore, it can be seen that the vicinities
of point d and point e are areas exposed to large stress and are
especially prone to be broken.
[0105] FIG. 9 illustrates stresses exerted on point d and point
e when the size L of the long sides is varied with the size W of
the short sides fixed in the rectangular diaphragm section (FIG.
8). The horizontal axis represents the ratio of long side size L
to short side size W. Therefore, when the diaphragm section is square-shaped,
L/W=1. If L/W>1 the diaphragm section is rectangular, and W
represents the size of its short sides and is constant. If L/W<1
L represents the size of its short sides, and the length of the
short sides is varied.
[0106] The relation of stress will be considered with attention
focused on the length of the short sides. It is apparent from FIG.
9 that: when L/W<1 (the length of the short sides is varied),
the stress .sigma..sub.m at point d and point e is largely varied;
when L/W>1 (the length of the short sides is fixed) with L/W=1
on the border, it is hardly varied.
[0107] The present invention pays attention to stress variation
relative to L/W. For this reason, the sensor element 19 illustrated
in FIG. 1 is configured so that the following is implemented: letting
the size of the diaphragm section 23 on which the heating resistor
26 is disposed be W1 the size W2 of the diaphragm section 24 on
which the resistor 28 for intake air temperature sensor is made
substantially equal to W1 (W1.apprxeq.W2).
[0108] This is equivalent to that the length of the short sides
of the diaphragm section 23 and that of the diaphragm section 24
are unified. Thus, the maximum stresses respectively exerted on
the diaphragm sections 23 and 24 become identical with each other
in pressurization during screening.
[0109] Further, the following distances d1 and d2 are made identical
with each other: the distance d1 is one between each edge of the
electrical insulating film 25 (diaphragm 23) in place where the
cavity portion 21 is covered and the resistance pattern on the diaphragm
23 illustrated in FIG. 2; and the distance d2 is one between each
edge of the electrical insulating film 25 (diaphragm 24) in place
where the cavity portion 22 is covered and the resistance pattern
on the diaphragm 23 illustrated in FIG. 3. Thus, the stresses exerted
on the electrical insulating film 25 of the individual diaphragm
sections 23 and 24 can be made identical with each other.
[0110] According to this embodiment, therefore, the pressure need
not be adjusted from one diaphragm section to another diaphragm
section in pressurization during screening. A plurality of diaphragm
sections can be simultaneously checked for defect in one cycle of
screening.
[0111] The stress exerted on each diaphragm section 23 or 24 is
largely varied also according to the thickness of the electrical
insulating film 25. Therefore, it is preferable that the thickness
of the electrical insulating film 25 and the structure of the resistance
patterns should be identical as much as possible.
[0112] To make identical the stresses exerted on the diaphragm
sections 23 and 24 formed in the sensor element 19 the diaphragm
sections can be made completely identical with each other in shape
and dimensions. The object can also be sufficiently attained by
this method. In this case, however, there is no degree of freedom
in the shape of the diaphragm section.
[0113] In this embodiment, on the other hand, the lengths of the
short sides of rectangles only have to be unified. Therefore, a
high degree of freedom can be ensured in the shape of the diaphragm
section. Further, a plurality of diaphragm sections can be checked
for defect in one cycle of screening, as mentioned above.
[0114] In this embodiment, as mentioned above, a plurality of diaphragm
sections (the first diaphragm section and the second diaphragm section)
are configured so that their shape is rectangular and the lengths
of their short sides are substantially identical (W1.apprxeq.W2).
Even if the lengths of the short sides differ from each other to
some extent at this time, that is acceptable as long as the difference
is within an allowable range. For example, when the difference in
the length of short sides is within .+-.10%, that is, the ratio
of the lengths of short sides is between 0.9 times and 1.1 times
inclusive, the object can be sufficiently attained.
[0115] FIG. 10 is also a schematic plan view of the sensor element
19 of a thermal type air flow meter, illustrating a second embodiment
of the present invention. FIG. 11 is a sectional view taken along
the line E-E of FIG. 10 and FIG. 12 is a sectional view taken along
the line F-F of the same.
[0116] In the drawing of the second embodiment, the same members
as described with respect to the first embodiment will be marked
with the same numerals, and the description thereof will be omitted.
Description will be given only to constructions, features, and the
like different from those in the first embodiment.
[0117] As is apparent from FIG. 10 FIG. 11 and FIG. 12 the sensor
element 19 in the second embodiment is constructed as follows: a
protective coat 50 formed of organic material is additionally provided
on the surface of the electrical insulating film 25 formed so that
the cavity portion 21 and the cavity portion 22 in the semiconductor
substrate 20 are covered therewith. The other respects are the same
as in the first embodiment.
[0118] Particles of sand and the like can be trapped as dust in
intake air, and collide with the diaphragm sections 23 and 24. The
protective coat 50 functions to absorb the resulting collision energy
and prevent the electrical insulating film 25 from being destroyed.
[0119] In the second embodiment, therefore, the possibility of
the sensor element 19 being damaged can be reduced even if dust
is trapped in intake air. Thus, a more reliable thermal type air
flow meter can be provided.
[0120] At this time, of the surface of the electrical insulating
film 25 the surface of the diaphragm section 23 has the heating
resistor 26 and the thermal sensitive resistors 27a and 27b formed
thereon. Measures must be taken there so as to obtain sufficient
thermal conduction from intake air. For this purpose, the protective
coat 50 is not provided on the surface of the diaphragm section
23 as illustrated in FIG. 11.
[0121] For the organic material for forming the protective coat
50 for example, polyimide is used. With the heat resistance of
polyimide taken into account, it is preferable that the continuous
use temperature should be controlled to 250.degree. C. or below.
Also, in this respect, therefore, the following measures should
be taken when the protective coat 50 is formed of polyimide: the
protective coat 50 should be formed so that the surface of the heating
resistor 26 is not covered therewith, as mentioned above.
[0122] As illustrated in FIG. 11 the protective coat 50 is formed
so that it slightly extends from the peripheral portion of the cavity
portion 21 (diaphragm section 23) to the inside of that at areas
where the electrical insulating film 25 forms the diaphragm section
23. The reason for this is as follows: the peripheral portion of
the diaphragm section 23 is supported on the semiconductor substrate
20; therefore, the peripheral portion is inferior in capability
to absorb the collision energy of particles, and is prone to be
destroyed. The peripheral portion of the diaphragm section 23 where
the electrical insulating film 25 is especially prone to be destroyed
is protected with the protective coat 50.
[0123] As illustrated in FIG. 12 in the area on the surface of
the electrical insulating film 25 where the resistor 28 for intake
air temperature sensor is formed, that is, on the surface of the
diaphragm section 24 the protective coat 50 is formed as follows:
it is formed so that the entire surface of the diaphragm section
including the resistor 28 is covered therewith. The reason for this
is as follows: the resistor 28 for intake air temperature sensor
produces almost no heat, and it is unnecessary to take into account
the thermal insulation for the diaphragm section or the heat resistance
of the organic material constituting the protective coat 50.
[0124] Even if the protective coat 50 is formed in there, as mentioned
above, the stresses exerted on the diaphragm sections can be made
identical with each other by taking the following measures: the
lengths W1 and W2 of the short sides of the two rectangular diaphragm
sections 23 and 24 are made substantially equal. Therefore, a plurality
of diaphragm sections can be checked for defect in one cycle of
screening.
[0125] In the above-mentioned embodiments, diaphragm sections are
configured so that their shape is rectangular, and the lengths of
their short sides are substantially identical. The object can be
similarly attained by any other shape. Also, these cases, there
are representative dimensions with which stress exerted on the electrical
insulating film is most variable in some diaphragm section shape,
like the short sides of a rectangle. Consequently, hereafter, description
will be given to embodiments in other shapes.
[0126] Description will be given to a third embodiment with reference
to FIG. 13. FIG. 13 is also a schematic plan view of the sensor
element of a thermal type air flow meter. Also, in this drawing,
the same members as described with respect to the first embodiment
will be marked with the same numerals, and the description thereof
will be omitted. Description will be given only to constructions,
features, and the like different from those in the first embodiment.
[0127] In the sensor element 19 in the third embodiment illustrated
in FIG. 13 the shape of the diaphragm section 24 on which the resistor
28 for intake air temperature sensor is disposed is polygonal. The
other respects are the same as in the first embodiment described
with reference to FIG. 1 to FIG. 3.
[0128] Also, when the diaphragm section 24 is polygonal as in the
third embodiment, the maximum stress exerted on the diaphragm sections
of the electrical insulating film is varied according to the size
W2 in which the distance between the two opposite sides is minimized.
[0129] Consequently, the stresses exerted on the diaphragm sections
23 and 24 can be made substantially identical with each other by
taking the following measures: the size W2 in which the distance
between the two opposite sides is minimized is made substantially
equal to the size W1 of the short sides of the rectangular diaphragm
section 23.
[0130] Also, according to the third embodiment, therefore, a plurality
of diaphragm sections 23 and 24 can be simultaneously checked for
defect in one cycle of screening. In this case, further, the mechanical
strength of the diaphragm section 24 can be enhanced by making the
diaphragm section 24 polygonal.
[0131] Also, in the third embodiment, the protective coat 50 formed
of organic material may be formed on the surface of the electrical
insulating film 25 as in the second embodiment. With this construction,
the collision energy of particles that collide with the diaphragm
sections 23 and 24 can be absorbed by the protective coat to prevent
the electrical insulating film 25 from being destroyed. As a result,
the enhancement of reliability can be accomplished.
[0132] Description will be given to a fourth embodiment with reference
to FIG. 14. FIG. 14 is also a schematic plan view of the sensor
element of a thermal type air flow meter. Also, in this drawing,
the same members as described with respect to the first embodiment
will be marked with the same numerals, and the description thereof
will be omitted. Description will be given only to constructions,
features, and the like different from those in the first embodiment.
[0133] In the sensor element 19 in the fourth embodiment illustrated
in FIG. 14 the shape of the diaphragm section 24 on which the resistor
28 for intake air temperature sensor is disposed is a cross polygon
comprising a combination of a plurality of rectangles. The other
respects are the same as in the first embodiment described with
reference to FIG. 1 to FIG. 3.
[0134] Also, when the diaphragm section 24 is cross-polygonal as
in the fourth embodiment, the maximum stress exerted on the electrical
insulating film is varied according to the size W2 in which the
distance between the two opposite vertexes is minimized, as illustrated
in the figure.
[0135] Consequently, the stresses exerted on the diaphragm sections
23 and 24 can be made substantially identical with each other by
taking the following measures: the size W2 in which the distance
between the two opposite vertexes is minimized is made substantially
equal to the size W1 of the short sides of the rectangular diaphragm
section 23.
[0136] Also, according to the fourth embodiment, therefore, a plurality
of diaphragm sections 23 and 24 can be simultaneously checked for
defect in one cycle of screening. Also, in this case, further, the
mechanical strength of the diaphragm section 24 can be enhanced
by making the diaphragm section 24 cross-polygonal.
[0137] Also, in the fourth embodiment, the protective coat 50 formed
of organic material may be formed on the surface of the electrical
insulating film 25 as in the second embodiment. With this configuration,
the collision energy of particles that collide with the diaphragm
sections 23 and 24 can be absorbed by the protective coat to prevent
the electrical insulating film 25 from being destroyed. As a result,
the enhancement of reliability can be accomplished.
[0138] FIG. 15 illustrates a computing unit 51 as an embodiment
for enhancing the measurement accuracy in a thermal type air flow
meter using any of the first to fourth sensor elements 19. This
computing unit 51 comprises: an input portion 51A for inputting
air flow rate signals and intake air temperature signals from the
circuits illustrated in FIG. 4B and FIG. 4C; an arithmetic portion
51B that computes air flow rates and intake air temperatures from
these signals with accuracy; an output portion 51C that outputs
the result of computation; a temperature sensor 51D that measures
the temperatures inside and in proximity to the computing unit 51;
and memory 51E in which constants and the like required for computation
are stored, and data can be externally written.
[0139] The computing unit 51 is installed in the thermal type air
flow meter, for example, on the circuit board 43 of the thermal
type air flow meter illustrated in FIG. 5. At this time, one-chip
structure may be adopted. In this case, as illustrated in FIG. 16
the sensor element 19 and the computing unit 51 are formed on the
same semiconductor substrate, or the semiconductor substrate 20
of the sensor element 19 through semiconductor process technologies.
Further, the drive circuit illustrated in FIG. 4A may also be formed
on the same semiconductor substrate.
[0140] FIG. 17 is a sectional view taken along the line G-G of
FIG. 16. As illustrated in FIG. 16 as well, numeral 53 denotes a
recess (hollow) formed on the other side (the lower face in FIG.
16) of the semiconductor substrate 20. The recesses are disposed
side by side between the sensor element 19 and the computing unit
51 on the semiconductor substrate 20. These recesses function to
reduce the sectional area of the semiconductor substrate 20 and
suppress thermal conduction. Therefore, this makes it possible to
minimize heat transmitted from the computing unit 51 to the sensor
element 19.
[0141] By adopting a construction including such a computing unit
51 the following parts can be formed on the same semiconductor
substrate: resistors for measuring air flow rate and air temperature,
a drive circuit for driving these resistors, a circuit that carries
out computation for correction, and the like. As a result, a sensor
element excellent in mass productivity and measurement accuracy
is obtained.
[0142] When such a thermal type air flow meter is applied to engine
control in automobile, it is exposed to harsh conditions under which
the temperature of intake air as a fluid to be measured changes
over a wide range from -30.degree. C. to 80.degree. C. or so. Even
under such harsh conditions, measurement accuracy must be ensured.
FIG. 18 illustrates measuring errors produced in the thermal type
air flow meter when the intake air temperature changes. The horizontal
axis represents intake air temperatures, and the vertical axis represents
the measuring error at each temperature relative to an intake air
temperature of 20.degree. C.
[0143] As apparent from the figure, the measuring error produced
when the intake air temperature changes depends on the air flow
rate, and it is increased especially in the low flow rate range.
Therefore, the measuring error depends both on the intake air temperature
and on the flow rate. At this time, the computing unit 51 corrects
measuring errors produced according to the flow rate of the fluid
to be measured and its temperature, and outputs highly accurate
values.
[0144] Measuring errors that occur when the intake air temperature
changes can be measured in advance. Consequently, air flow rate
signals can be corrected and computed according to conditions by
taking the following measures: the relation between air flow rate
signals measured in advance and measuring errors due to intake air
temperature signals is stored in the memory in the computing unit
51.
[0145] At this time, a problem arises in the case where the sensor
element 19 and the computing unit 51 are formed on the same semiconductor
substrate 20 as described with respect to the embodiment illustrated
in FIG. 16. Heat generated by the computing unit 51 is transmitted
to the resistor 28 for intake air temperature sensor, and the measurement
of intake air temperature is thereby influenced. To improve this,
in the embodiment illustrated in FIG. 15 the temperature sensor
51D is installed inside or in proximity to the computing unit 51.
Thus, to what extent the computing unit 59 generates heat can be
measured, and correction can be carried out.
[0146] In this case, therefore, the following advantage is brought:
even if heat is transmitted to the resistor 28 for intake air temperature
sensor and an error is produced, it can be corrected at the arithmetic
portion 51B based on information from the temperature sensor 51D.
Therefore, intake air temperature signals can be constantly outputted
with accuracy.
[0147] When the embodiment provided with the recesses 53 described
with reference to FIG. 16 and FIG. 17 is adopted, heat transmitted
from the computing unit 51 to the sensor element 19 can be minimized.
Therefore, it is advantageous.
[0148] The relation between intake air temperature and measuring
error with air flow rate taken as a parameter, illustrated in FIG.
18 is a characteristic in steady state in which the flow rate and
the temperature are stable. Therefore, if the intake air temperature
suddenly changes, a transient state thereafter lasts for a while.
[0149] In this case, a problem can arise in correction at the computing
unit 51. This is especially conspicuous when there is variation
in thermal capacity in a flow rate measuring section where the resistor
28 for intake air temperature sensor, the heating resistor 26 and
the like are formed.
[0150] FIGS. 19A to 19C illustrate the states of temperature change
observed when the intake air temperature suddenly changes with respect
to the above embodiment. This sudden intake air temperature change
takes place in a sensor element in which the resistor 28 for intake
air temperature sensor and the flow rate measuring section are different
in thermal capacity. FIG. 19A indicates the intake air temperature;
FIG. 19B indicates the temperature of the resistor 28; and FIG.
19C indicates the temperature of the heating resistor 26.
[0151] As already described, the temperature of the heating resistor
26 is controlled so that it is higher by a certain temperature than
the intake air temperature. In this case, as illustrated in FIG.
19C, its controlled so that it Is higher by 150.degree. C. than
the intake air temperature.
[0152] Thus, when a certain time t1 has passed after the intake
air temperature changes from 0.degree. C. to 50.degree. C., the
various temperatures are as illustrated in the figures: the intake
air temperature is 50.degree. C.; the intake air temperature sensor
temperature is 30.degree. C.; and the temperature of the heating
resistor is 165.degree. C. This state of temperature does not exist
in the relation between the air flow rate and the measuring error
due to intake air temperature, illustrated in FIG. 18. In this case,
therefore, there is the possibility that the computing unit 51 cannot
properly carry out correction or computation.
[0153] To prevent such a problem from arising, the above embodiment
is configured so that the resistor 28 for intake air temperature
sensor is of diaphragm section structure, similarly with the potion
where the heating resistor 26 is formed. By adopting the diaphragm
section structure, the thermal capacity is minimized to quickly
respond to and follow change in intake air temperature. |