Abstrict An improved measuring element for a mass air flow meter is provided
with a temperature sensor for detecting a typical temperature indicative
of the impact of heat on a heating resistor. The element is formed
on a heat transfer pass from a configuration member supporting a
substrate on which the heating resistor for measuring an air flow
rate is formed, and an air temperature sensor for detecting air
temperature, formed on the substrate and out of the air flow impinging
the heating resistor.
Claims What is claimed is:
1. A measuring element for a mass air flow meter, comprising:
a substrate;
an air temperature sensor configured for measuring an air temperature
of an air stream and arranged on the substrate;
a heating resistor for measuring an air flow rate of the air flow
arranged on the substrate;
a housing for supporting the substrate, with the air temperature
sensor and the heating resistor being located in the air stream;
and
a temperature sensor configured for detecting a substrate temperature
operatively arranged on the substrate, wherein the temperature sensor
is located inside of the housing and outside of the air stream,
and the substrate temperature is used for correcting the detected
air stream.
2. The measuring element according to claim 1 wherein the temperature
sensor is configured to detect heat emitted from at least one of
an internal combustion engine and an evaluation circuit and transferred
to the heating resistor.
3. The measuring element according to claim 1 wherein the temperature
sensor and the air temperature sensor are comprised of the same
material as the heating resistor.
4. The measuring element according to claim 1 wherein the temperature
sensor and the air temperature sensor comprise semiconductors.
5. The measuring element according to claim 4 wherein the semiconductors
are one of a thermistor, a diode and a transistor.
6. A mass air flow meter, comprising:
a measuring element including a heating resistor on a substrate
for measuring an air stream;
an operating circuit for supplying a current to the measuring element
and for detecting a signal indicative of the air flow;
a control circuit for calculating the air flow based on a representative
signal;
a temperature sensor configured for detecting a substrate temperature
arranged on the substrate; and
a memory configured for storing compensation data for an air flow
rate-substrate temperature characteristic, wherein the control circuit
is operative to compensate the air flow by using the compensation
data and the temperature obtained from the temperature sensor.
7. The mass air flow meter according to claim 6 wherein a housing
covers the temperature sensor to prevent a direct contact with the
air flow.
8. The mass air flow meter according to claim 6 wherein an air
temperature sensor configured for detecting an air temperature,
which is formed at the substrate outside of the air flow, is in
operative contact with the heating resistor.
Description BACKGROUND AND SUMMARY OF THE INVENTION
This application claims the priority of Japanese application 9-005292
the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a measuring element for a mass
air flow meter and a mass air flow meter and, more particularly,
to an apparatus suitable for the measurement of an air flow rate
taken into an internal combustion engine.
In the prior art, a thermal type air flow meter which can directly
detect an amount of a mass air flow is provided in electronically
controlled fuel injection equipment for an internal combustion engine
of an automobile, etc., for measuring the amount of intake air.
Above all, attention is given to a mass air flow meter and a measuring
element used therefor, manufactured by using the semiconductor micro-machining
technique, from view points of the reduction of cost and the desire
for a low electric power consumption. The mass air flow meter in
which the conventional semiconductor substrate is used is disclosed
in Japanese Patent Application Laid-Open No. 60-142268 (1985). The
technique disclosed in the above publication places the accent on
the reduction of manufacturing cost.
Therefore, the above-mentioned prior art fails to consider an air
flow rate-temperature characteristic when the amount of intake air
is measured. Namely, there is a problem that the accuracy of the
measurement of the air flow rate is insufficient.
The problem of the above described conventional technique is explained
with reference to FIG. 16 which is a plan view of the conventional
measuring element for a mass air flow meter. In FIG. 16 reference
numeral 1 designates a measuring element for the conventional mass
air flow meter. The measuring element 1 has two bridges 23a, 23b
formed by insulator film, and bridging air spaces 22a, 22b, 22c
formed by anisotropic-etching a semiconductor substrate such as
a silicon substrate. The bridge 23a is positioned at an upstream
side of the direction of an air flow, and the bridge 23b at a downstream
side.
A heating resistor 20 is arranged so as to sandwich the air space
22c between the two bridges 23a, 23b. Temperature sensing resistors
21a, 21b are arranged the sides of the heating resistor 20. Further,
an air temperature compensating resistor 4a for measuring the air
temperature is arranged at a portion of the upstream side of the
air space 22a. Because the air spaces 22a, 22b, 22c are formed by
anisotropic-etching the semiconductor substrate, through the opening
portion of the electrically insulated film, they are connected continuously
to one another below the bridges 23a, 23b. In such an air flow meter,
the heating resistor 20 is heated to a predetermined temperature
higher than the air temperature determined by the temperature compensating
resistor 4a. The amount of air flow is measured from the difference
of temperature between the upstream temperature sensing resistor
21a and the downstream temperature sensing resistor 21b, using the
cooling effect of the air stream.
In the prior art constructed as described above, the variation
of the temperature of the intake air is compensated only by a bridge
circuit constructed by using the temperature compensating resistor
4a and the heating resistor 20. Namely, in the prior art, the temperature-dependent
variation of the physical properties of air, e.g., the density,
the dynamic viscosity and the thermal conductivity, is not considered.
Therefore, a suitable air flow amount--air temperature characteristic
can not be obtained. Further, in the event that the sensor is used
in harsh environments such as in an automobile, the heat due to
the increase of the temperature of the internal combustion engine
is conducted to the heating resistor 20 the temperature compensating
resistor 4a and the temperature sensing resistors 21a, 21b, and
thus has adverse effects on the measurement accuracy. This problem
also has not been solved in the prior art.
An object of the present invention is to provide to a measuring
element for a mass air flow meter and a mass air flow meter which
can measure the amount of air flow with a high degree of accuracy.
To attain the above-mentioned object, the present invention preferably
has the following configuration.
According to a first aspect of the present invention, in a measuring
element for a mass air flow meter, a temperature sensor for detecting
a typical temperature indicative of the impact of heat on a heating
resistor, is formed on a heat transfer pass from a configuration
member supporting a substrate on which the heating resistor for
measuring an air flow rate is formed. The heat transfer pass is
positioned at the support portion of the substrate supported by
the configuration member, or at a portion between that support portion
and the heating resistor.
According to another aspect of the present invention, a measuring
element for a mass air flow meter is provided with a temperature
sensor for detecting a typical temperature indicative of the impact
of heat on a heating resistor, formed on a heat transfer pass from
a configuration member supporting a substrate on which the heating
resistor for measuring an air flow rate is formed, and an air temperature
sensor for detecting air temperature, formed on the substrate and
out of a line of the air flow touching the heating resistor.
According to a further aspect of the present invention, a mass
air flow meter has a measuring element for a mass air flow meter
including a temperature sensor for detecting a typical temperature
indicative of the impact of heat on a heating resistor, formed on
a heat transfer pass from a configuration member supporting a substrate
on which the heating resistor for measuring an air flow rate is
formed; an operating circuit for supplying a current to the measuring
element and detecting a signal indicative of the amount of the air
flow; a control circuit for calculating the amount of the air flow
based on the signal indicative of the amount of the air flow; and
a memory for storing the compensation data for an air flow rate-substrate
temperature characteristic. The control circuit compensates the
amount of the air flow by using the compensation data, and the typical
temperature obtained from the substrate-temperature sensor.
According to a further aspect of the present invention, a mass
air flow meter has a measuring element for a mass air flow meter
including a temperature sensor for detecting a typical temperature
indicative of the impact of heat on a heating resistor, formed on
a heat transfer pass from a configuration member supporting a substrate
on which the heating resistor for measuring an air flow rate is
formed; an operating circuit for supplying a current to the measuring
element and detecting a signal indicative of the amount of the air
flow; a control circuit for calculating the amount of the air flow
based on the signal indicative of the amount of the air flow; and
a memory for storing the compensation data for an air flow rate-substrate
temperature characteristic and an air flow rate--air temperature
characteristic. The control circuit compensates the amount of the
air flow by using the compensation data, the air temperature obtained
from the air-temperature sensor and the typical temperature obtained
from the substrate-temperature sensor.
These and other features of the invention that will be apparent
to those skilled in the art from the following detailed description
of the invention, taken together with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of a measuring element for the mass air flow
meter according to a first embodiment of the present invention.
FIG. 2 is a sectional view taken along the line A-A' of the mass
air flow meter of FIG. 1.
FIG. 3 is a sectional view of the mass air flow meter according
to a first embodiment of the present invention.
FIG. 4 is an enlarged view of the measuring element part of FIG.
3.
FIG. 5 is a sectional view taken along the line B-B' of the measuring
element part of FIG. 4.
FIG. 6 is a block diagram showing the circuit of the mass air flow
meter according to the present invention.
FIG. 7 is a circuit diagram for measurement of an air flow rate,
used in the mass air flow meter according to the first embodiment
of the present invention.
FIG. 8 is a circuit diagram for measurement of temperature, used
in the mass air flow meter according to the first embodiment of
the present invention.
FIG. 9 is a view showing the relationship between a portion of
the measuring element shown in FIG. 1 and the temperature distribution
on the line B-B' of the measuring element.
FIGS. 10A and 10B are graphs illustrating the correction of substrate
temperature and the correction of air temperature, respectively.
FIG. 11 is a plan view of a measuring element for the mass air
flow meter according to a second embodiment of the present invention.
FIG. 12 is a circuit diagram for measurement of an air flow rate,
used in the mass air flow meter according to the second embodiment
of the present invention.
FIG. 13 is another circuit diagram for measurement of an air flow
rate, used in the mass air flow meter according to the second embodiment
of the present invention.
FIG. 14 is a plan view of a measuring element for the mass air
flow meter according to a third embodiment of the present invention.
FIG. 15 is a circuit diagram for measurement of an air flow rate,
used in the mass air flow meter according to the third embodiment
of the present invention.
FIG. 16 is a plan view of the above-described conventional measuring
element for a mass air flow meter.
Corresponding numerals and symbols in different figures refer to
corresponding parts unless otherwise specified.
DETAILED DESCRIPTION OF THE DRAWINGS
In FIGS. 1 and 2 the measuring element 1 comprises a semiconductor
substrate 2 of silicon and the like having an air space 6 formed
from the bottom surface to a boundary surface of an insulator film
7a by anisotropic etching. A heating resistor 3 is formed on the
insulator film 7a at the air space 6. A temperature compensating
resistor 4a constructs a bridge circuit with the heating resistor
3 for measuring air temperature. An air temperature sensor or a
second temperature sensor 4b formed at the top portion of the substrate
2 for measuring the air temperature or the temperature of intake
air. Terminal electrodes 8a through 8h are provided on the element
1 and a substrate temperature sensor 5 or a first temperature sensor
is formed between the heating resistor 3 and the terminal electrodes
8a-8h for measuring the temperature of a support portion or the
temperature of the substrate at the support portion 2a (FIG. 5)
where the substrate 2 is supported by another configuration member.
An insulator film 7b is provided for protecting each of the resistors.
In such the configuration, a heating current is supplied to the
heating resistor 3 so that its temperature may become higher by
a predetermined temperature than that of the temperature compensating
resistor 4a arranged at the top of a passage of air flow 9. The
air flow rate is measured from the value of the heating current
supplied to the heating resistor 3 based on the cooling effect
of the heating resistor 3 by the air flow 9. The insulator film
7 in the neighborhood of the air space 6 and the heating resistor
3 are formed to improve measurement accuracy by preventing heat
transfer from the heating resistor 3 to the substrate 2.
As shown in FIG. 1 the temperature compensating resistor 4a and
the second temperature sensor 4b are positioned at the top portion
of the substrate 2 and at a portion projected into the air flow
passage. As a result, the heating resistor 3 has no influence on
the heated air flow 9 whether forward flow or backward flow. Further,
because the second temperature sensor 4b is formed on the portion
of the substrate out of the region of the air flow touched by the
heating resistor 3 the heat impact from the heating resistor 3
is avoided, and thus the air temperature is correctly detected.
In FIG. 3 the measuring element 1 shown in FIG. 1 is installed
in a mass air flow meter. More specifically, a mass air flow meter
having the measuring element 1 is mounted in an intake pipe of an
automobile internal combustion engine to measure the amount of the
aspirated air flow.
The mass air flow meter according to the present embodiment comprises
the measuring element 1 a housing part 12 and an external circuit
13. The measuring element 1 is arranged in a sub-tube 11 within
an air intake pipe 10. The external circuit 13 is electrically connected
through the housing part 12 to a terminal electrode 8 of the measuring
element 1. While the aspirated air flow is normally flowed into
a direction of the air flow 9 designated by an arrow, it may be
flowed into a direction opposed to the direction of the arrow according
to the condition of the internal combustion engine (a reverse flow).
As shown in FIGS. 4 and 5 the measuring element 1 is supported
by the housing part 12 at the support portion 2a of the substrate
2 so that two sides of each of the temperature compensating resistor
4a and the second temperature sensor 4b can be exposed to the air
flow 9. Further, the external circuit 13 having a terminal electrode
14 and a signal processing circuit (including an operating circuit
and a control circuit described later) is also fixed to the housing
part 12.
The measuring element 1 and the external circuit 13 that is, the
terminal electrodes 8a through 8h, 14 are electrically connected
by wire-bonding through gold lines 15 to each other. Thereafter,
the first temperature sensor 5 the gold lines 15 electrode terminals
8a-8h and 14 and the external circuit 13 are sealed and protected
by the protective casing 12a to avoid direct contact with air. Therefore,
the temperature of the substrate 2 can be measured correctly, independently
of the effect of the air flow 9. Namely, the variation of the measurement
temperature of the first temperature sensor 5 due to the impact
of heat transfer from the air flow can be suppressed.
As to the installed measuring element 1 the top face and the bottom
face of the air space 6 are completely isolated by the insulator
film 7a and the lid plate 12b (which is isolated from or integrated
with the housing part 12), respectively, from the air flow, as seen
from FIGS. 2 and 5. Because the mass air flow meter according to
the present embodiment has an air space 6 not opening to the air
flow different from the prior art, the dust which causes undesired
influences on the measurement of the air flow rate never accumulate
within the air space. As a result, it is possible to measure the
air flow rate with a high degree of accuracy.
FIG. 6 schematically shows the mass air flow meter 24 of the first
embodiment. The mass air flow meter according to second and third
embodiments described below also has the same circuit configuration
as that shown in FIG. 6.
The mass air flow meter 24 includes the measuring element 1 shown
in FIG. 1 and the external circuit 13. The external circuit has
an operating circuit 13a for supplying an electric current to the
measuring element 1 and detecting an signal indicative of the amount
of an air flow. A control circuit 13b has an input circuit for inputting
the air-flow-amount signal from the measuring element 1 through
an A/D converter, a CPU for calculating and processing the amount
of the air flow, and an output circuit for outputting the processed
results. A memory 13c is provided for pre-storing data for compensation
of the air flow rate (signal) measured based on the substrate temperature
(ex. the temperature of the support portion) detected by the first
temperature sensor 5 and/or air temperature (ex. the temperature
of intake air) detected by the second temperature sensor 4b.
FIG. 7 shows a bridge circuit for measurement of an air flow rate
including the heating resistor 3 of FIG. 1 the temperature compensating
resistor 4a and a part of the operating circuit 13a of FIG. 6. The
operating circuit 13a includes a differential amplifier 16 a transistor
17 a power source 18 and resistors 19a, 19b. FIG. 8 shows a measuring
circuit including the first temperature sensor 5 of FIG. 1 the
second temperature sensor 4b and a part of the operating circuit
13a of FIG. 6.
FIG. 9 shows the relationship between a portion of the measuring
element shown in FIG. 1 and the temperature distribution on the
line B-B' of the measuring element. Namely, the top portion of FIG.
9 shows the temperature distributions at the measuring element 1
the external circuit 13 and the housing part 12. FIG. 10(a) shows
an air flow--support portion temperature characteristic of the mass
air flow meter 24 and FIG. 10(b) shows an air flow--aspirated air
temperature characteristic of the mass air flow meter 24.
Next, in operation, the amount of air flow is measured as follows.
In the bridge circuit shown in FIG. 7 the resistance value of each
of the resistors 19a, 19b is set so that the temperature (resistance
value) of the heating resistor 3 positioned at the center of the
measuring element 1 can become higher by a predetermined value (e.g.,
150.degree. C.) than the temperature (resistance value) of the temperature
compensating resistor 4a corresponding to the air temperature. When
the temperature of the heating resistor 3 is lower than the set
value, a potential difference occurs between the electric potentials
A and B at the central point of the bridge circuit. As a result,
the transistor 17 is turned "on" by an output C of the
difference amplifier 16 and the heating current is supplied to
the heating resistor 3. When the temperature of the heating resistor
3 reaches the set value, the transistor 17 is turned "off"
by an output C of the difference amplifier 16 and the heating current
is cut off. Namely, the feedback control is carried out so that
the temperature of the heating resistor 3 may be maintained at the
set value. The electric potential at the point A through which the
heating current is supplied is output to the control circuit 13b
as a signal indicative of the air flow rate.
The temperature of the intake air (the air temperature) and that
of the support portion (the substrate temperature) are measured
as follows. As shown in FIG. 8 a reference voltage (Vref) is applied
to the first temperature sensor 5 formed between the heating resistor
3 and the terminal electrode 8 and the second temperature sensor
4b formed at the top of the measuring element 1. Terminal voltages
D, E of the temperature sensors are output to the control circuit
13b as the air temperature (Ta) and the substrate temperature (Tw),
respectively.
In this embodiment, the same platinum film used as the material
for both the heating resistor 3 and the temperature compensating
resistor 4a is also used for the first temperature sensor 5 and
the second temperature sensor 4b. Because the temperature is proportional
to the electric resistance in such a metal film, the temperature
from the terminal voltages of the first and the second temperature
sensors can be measured. Although the reference voltage Vref is
used in the circuit of FIG. 8 a constant current source can be
used instead of the voltage source. Further, while the same platinum
film used for the heating resistor 3 and the temperature compensating
resistor 4a is used in this embodiment, a semiconductor temperature
sensor such as a thermistor, a diode, a transistor, etc. also can
be used as the first temperature sensor 5 and the second temperature
sensors 4a. In other words, while from a view point of manufacturing,
the same kind of resistor sensors as the heating resistor 3 and
the temperature compensating resistor 4a (the sensor of which the
thin film is formed of the same material and by the same process)
is used in this embodiment, any semiconductor temperature sensors
formed of the same material and by the same process as a thermistor,
a diode, a transistor, etc. also can be used.
Because the temperatures of an air intake pipe 10 and the housing
apart 12 shown in FIG. 3 are increased due to the heat from an automobile
internal combustion engine and the like, there is a concern that
the heat is transferred to the measuring element 1 and thus a measurement
error occurs, namely, the temperature characteristic is deteriorated.
The top portion of FIG. 9 shows the temperature distributions at
the measuring element 1 the external circuit 13 and the housing
part 12 given when the temperature of the housing part 12 for supporting
the measuring element 1 is increased.
Referring again to FIG. 9 which shows the temperature distribution
on the line B-B' of the measuring element, the heat from the internal
combustion engine is transferred from the housing part 12 to the
external circuit 13 and to the terminal electrode 8 of the measuring
element 8 the first temperature sensor 5 the heating resistor
3 the temperature compensating resistor 4a, the second temperature
sensor 4b, etc. Therefore, undesirable heat is transferred to the
configuration members of the mass air flow meter such as the heating
resistor 3 and the temperature compensating resistor 4b, and has
an adverse effect on the measurement accuracy of the air flow rate.
Further, the heat from the external circuit 13 itself may have
an adverse effect on the measurement accuracy of the air flow rate.
To improve the air flow rate-substrate temperature characteristic
relevant to the above-mentioned adverse effect, the correction of
the substrate temperature with respect to the air flow rate is made
in the present invention by using the measuring element 1 in which
the first temperature sensor 5 is formed between the heating resistor
3 and the terminal electrode 8. Namely, the first temperature sensor
5 is formed between the heating resistor 3 and the terminal electrode
8 at the support portion 2a of the substrate 2 or in the neighborhood
of the heating resistor 3 (in a measuring element of different configuration,
the temperature compensating resistor 4a). The air flow rate is
corrected by using the detected temperature from the first temperature
sensor 5 and the air flow rate-substrate temperature characteristic.
In other words, the substrate temperature sensor or the first temperature
sensor for detecting the typical temperature (e.g., the substrate
temperature, the temperature of the support portion, etc.) is formed:
(1) at the support portion 2a (the portion of a measuring element
at which the measuring element is fixed to another configuration
member),
(2) at a portion between the support portion 2a and the heating
resistor 3 or
(3) in the neighborhood of the heating resistor 3
as the portion or position suitable to detect the typical temperature
or the factor indicative of the impact of the heat transfer from
another configuration member to the heating resistor when the measuring
element is fixed to the above member.
The temperature compensating resistor or a temperature sensing
resistor described can be used in lieu of the heating resistor for
measurement of an air flow rate on which undesirable heat transfer
can have an effect. Accordingly, the heating resistor can be replaced
with the temperature compensating resistor or the temperature sensing
resistor.
FIG. 10(a) shows an air flow--support portion temperature characteristic
of the mass air flow meter 24. The abscissa designates the air flow
rate (Q), and the ordinate designates the error of measurement (.DELTA.Q/Q)
in which the reference value is taken when the substrate temperature
is at 25.degree. C. In the prior art meter in which compensation
of the substrate temperature was not made, about 10% error at low
air flow amount occurred when the substrate temperature (Tw) was
at 80.degree. C. In the present invention, the substrate temperature
(Tw) can be compensated by using the measuring element 1 in which
the first temperature sensor 5 is formed between the heating resistor
and the terminal electrode 8. Further, because the air flow rate
(Q) measured by the heating resistor 3 is input to the control circuit
13b, it is possible to compensate the air flow rate with respect
to the substrate temperature (or the support portion) by retrieving
the compensation data made from the air flow rate-substrate temperature
characteristic and pre-stored in the memory 13c, and processing
such data in the CPU of the control circuit 13b.
By applying the substrate-temperature-compensation according to
the present invention, the error of an air flow rate is zero, namely
equally that error at the 25.degree. C. standard substrate temperature.
It is also possible to pre-store an air flow rate-substrate temperature
characteristic inherent in each of the mass air flow meters and
compensate the air flow rate with respect to the substrate temperature.
The measured air flow amount (Q), the substrate temperature (Tw)
and the compensation values may be pre-stored in a map, or as an
approximation-error-function. The compensation of air temperature
(or intake air temperature) will be explained next. In the prior
art described above, the variation of the intake air temperature
is compensated only by a bridge circuit constructed by using the
temperature compensating resistor 4a and the heating resistor 20.
Namely, in the prior art, the temperature-dependent variation of
the physical properties of air, the density, the dynamic viscosity
and the thermal conductivity, is not considered.
Therefore, a suitable air flow amount--air temperature characteristic
can not be obtained.
Now, the solid line in FIG. 10(b) designates a prior art air flow--aspirated
air temperature characteristic in which compensation of air temperature
was not made. As described above, the abscissa designates the air
flow rate (Q), and the ordinate designates the error of measurement
(.DELTA.Q/Q) in which the reference value is taken when the substrate
temperature is at 25.degree. C. While at a low air temperature (Ta)
(e.g., -30.degree.C.), the air flow rate has a minus error in its
low flow region, and a plus error in its high flow region, at a
high temperature (80.degree. C.), the error of the air flow rate
shows the reverse. Because the air temperature (Ta) can be measured
by the second temperature sensor 4b, and the air flow rate (Q) measured
by the heating resistor 3 is input to the control circuit 13b in
the present invention, the air flow rate is compensated with respect
to the substrate temperature (or the support portion) by retrieving
the compensation data made from the air flow rate-substrate temperature
characteristic shown in FIG. 10(a) and pre-stored in the memory
13c, and processing this data in the CPU of the control circuit
13b.
By using the substrate-temperature-compensation according to the
present invention, the error of an air flow rate is reduced to zero,
namely equalling the error at the 25.degree. C. standard substrate
temperature. The air flow rate-substrate temperature characteristic
inherent in each of the mass air flow meters can be pre-stored and
the air flow rate can be compensated with respect to the substrate
temperature. The measured air flow amount (Q), the substrate temperature
(Tw) and the compensation values may be pre-stored in a map, or
as an approximation-error-function. While the dedicated second temperature
sensor 4b is used for the measurement of the air temperature (Ta)
in this embodiment, the air temperature also can be measured by
calculating the variation of a resistance value of the temperature
compensating resistor 4a in the CPU of the control circuit 13b,
without using the second temperature sensor 4b.
FIG. 11 is a plan view of a measuring element for the mass air
flow meter according to a second embodiment of the present invention.
FIG. 12 is a diagram of an operating circuit for measurement of
an air flow rate used in the mass air flow meter according to the
second embodiment of the present invention and having a bridge circuit
with the heating resistor 20 and the temperature compensating resistor
4a. Further, FIG. 13 is a diagram of another output circuit for
measurement of an air flow rate, used in the mass air flow meter
according to the second embodiment of the present invention, including
a bridge circuit with the operating circuit 13a and the temperature
sensing resistors 21a, 21b, respectively, arranged upstream and
downstream of the heating resistor 20.
The difference between the measuring elements of the first and
second embodiments is as follows. In the first embodiment, a directly
heating type measuring element is used, in which a heating current
supplied to the heating resistor 3 is used as a signal indicative
of the air flow rate. However, in the second embodiment, the air
flow rate and the direction of the air flow is determined from the
difference in temperature between the temperature sensing resistors
21a, 21b, respectively, arranged upstream and downstream of the
heating resistor 20. This is the same type as the prior art meter
of FIG. 16.
Next, in operation, the amount of air flow is measured as follows.
In the bridge circuit shown in FIG. 12 the temperature of the heating
resistor 20 is set so as to become higher by a predetermined value
than the temperature of the temperature compensating resistor 4a.
The difference of electric potentials between an F point and a G
point of the bridge circuit including the temperature sensing resistors
21a, 21b, respectively, arranged upstream and downstream of the
heating resistor 20 is amplified by a difference amplifier 16a.
The direction of an air flow (corresponding to the sign of a voltage
at an H point) and the air flow rate signal (corresponding to the
absolute value of the voltage at the H point) is output to the control
circuit 13b.
Because the first temperature sensor 5 and the second temperature
sensor 4b are formed on the measuring element 1 in the same way
as the first embodiment, and the substrate temperature (Tw) and
the air temperature (Ta) are input to the control circuit 13b, the
air flow rate is compensated with respect to the substrate temperature
and the air temperature by retrieving the compensation data made
from the air flow rate-substrate temperature characteristic and
air flow amount--air temperature characteristic, and pre-stored
in the memory 13c, and processing this data in the CPU of the control
circuit 13b.
The temperature of intake air (the air temperature) and that of
the support portion (the substrate temperature) are output to the
control circuit 13b as the air temperature (Ta) and the substrate
temperature (Tw), respectively.
FIG. 14 is a plan view of a measuring element for the mass air
flow meter according to the third embodiment of the present invention.
FIG. 15 is a diagram of an operating circuit for measurement of
an air flow rate, used in the mass air flow meter according to the
third embodiment of the present invention and includes a heating
resistor 20 the temperature compensating resistor 4a and a temperature
sensing resistor 21a.
While the first embodiment is of a directly heating type, an indirectly
heated type is used in the third embodiment, in which the temperature
of the heating resistor 20 is detected by the temperature sensing
resistor 21a arranged in the neighborhood of the heating resistor
20.
Because the first temperature sensor 5 and the second temperature
sensor 4b are formed on the measuring element 1 in the same way
as the first embodiment, and the substrate temperature (Tw) and
the air temperature (Ta) are input to the control circuit 13b, the
air flow rate is compensated with respect to the substrate temperature
and the air temperature by retrieving the compensation data made
from the air flow rate-substrate temperature characteristic and
the air flow rate--air temperature characteristic, and pre-stored
in the memory 13c, and processing this data in the CPU of the control
circuit 13b, as the previous embodiment.
Three embodiments have been explained hereinabove. Also in the
event that another type air flow amount detecting system is used,
in which the first temperature sensor 5 and the second temperature
sensor 4b are formed on the measuring element 1 the air flow rate
can be compensated in the following way.
The substrate temperature (Tw) from the first temperature sensor
5 formed between the heating resistor 3 and the terminal electrodes
8a-8h and the air temperature (Ta) from the second temperature sensor
4b (or the temperature sensing resistor 4a) are input to the control
circuit 13b. The compensation data made from the air flow rate-substrate
temperature characteristic and the air flow rate--air temperature
characteristic is pre-stored in the memory 13c. The data is retrieved
and processed in the CPU of the control circuit 13b, as the previous
embodiment.
A concrete example of the measuring element for a mass air flow
meter will be explained next with reference to FIGS. 1 and 2. As
the insulator film 7a, silicon oxide (SiO.sub.2), silicon nitride
(SiN), etc. are formed at a thickness of 5 micron on the substrate
2 of silicon semiconductor by a CVD method. Further, the group of
resistors comprised of the heating resistor 3 the temperature sensing
resistor 4a, the second temperature sensor 4b and the first temperature
sensor 5 are formed from platinum deposited at a thickness of 0.2
micron on the same substrate 2 by a sputtering method. Next, the
resist is formed in a predetermined shape by the conventional photolithographic
etching technology. Thereafter, the platinum is patterned by an
ion-milling method.
After the terminal electrodes 8a-8h are formed by a gold plating
method, the insulator film 7b is formed at a thickness of 0.5 micron
as a protective film on the portions beside the terminal electrodes.
Finally, the air space 6 is formed from the back face of the silicon
substrate 2 through a mask of silicon oxide (SiO.sub.2) by anisotropic
etching, and then a chip-like measuring element 1 is obtained by
cutting.
While this invention has been described with reference to illustrative
embodiments, this description is not intended to be construed in
a limiting sense. Various modifications and combinations of the
illustrative embodiments as well as other embodiments of the invention,
will be apparent to those of skill in the art upon reference to
the description. It is, therefore, intended that the appended claims
encompass any such modifications or embodiments.
Although the invention has been described and illustrated in detail,
it is to be clearly understood that the same is by way of illustration
and example, and is not to be taken by way of limitation. The spirit
and scope of the present invention are to be limited only by the
terms of the appended claims. |