Abstrict A thermally-sensitive type flow meter measures a flow rate with
a high accuracy by eliminating a noise generated in a source voltage.
The flow meter includes a sensor chip comprising a substrate carrying
sensing elements. First and second resistors are provided on first
and second bridges formed over a depression formed on the substrate.
The first bridge is positioned on an upstream side and the second
bridge is positioned on a downstream side. The first and second
resistors generate a heat by the same voltage source. First and
second temperature measuring resistors are provided adjacent to
the first and second resistors, respectively, to measure a temperature
of the first and second resistors. Third and fourth temperature
measuring resistors are provided for measuring the temperature of
the fluid. A temperature control unit, including the first, second,
third and fourth temperature measuring resistors, controls a temperature
of each of the first and second resistors to be constant. A flow
rate of the fluid is determined based on a voltage difference between
a first voltage applied to the first resistor and a second voltage
applied to the second resistor.
Claims What is claimed is:
1. A thermally-sensitive type flow meter for measuring a flow rate
of a fluid, said flow meter including a sensor chip comprising a
substrate carrying sensing elements, said flow meter comprising:
first and second bridges formed over a depression formed on said
substrate, said first and second bridges being arranged along the
direction of flow so that said first bridge is positioned on an
upstream side and said second bridge is positioned on a downstream
side;
first and second resistors for generating a heat, said first and
second resistors provided on the respective one of said first and
second bridges, said first and second resistors being heated by
the same voltage source;
first and second temperature measuring resistors provided adjacent
to the respective one of said first and second resistors;
third and fourth temperature measuring resistors for measuring
a temperature of the fluid, said third and fourth temperature measuring
resistors being located at positions not influenced by a temperature
of said first and second resistors;
a temperature control unit, including said first, second, third
and fourth temperature measuring resistors, for controlling a temperature
of each of said first and second resistors to be constant; and
a flow rate determining unit for determining a flow rate of the
fluid flowing around said first and second bridges, a determination
being made based on a voltage difference between a first voltage
and a second voltage, said first voltage being measured across said
first resistor, said second voltage being measured across said second
resistor.
2. The flow meter as claimed in claim 1 wherein said first and
second bridges extend perpendicular to the direction of flow.
3. The flow meter as claimed in claim 1 wherein said first bridge
extends perpendicular to the direction of flow and said second bridge
extends parallel to the direction of flow.
4. The flow meter as claimed in claim 1 wherein said substrate
is comprised of a semiconductor substrate having a (110) plane and
a <100> axis, said depression being formed on a surface corresponding
to the (110) plane, said first bridge extending in a direction 55
degrees inclined with respect to the <100> axis.
5. The flow meter as claimed in claim 1 wherein said substrate
is comprised of a semiconductor substrate having a (110) plane and
a <100> axis, said depression being formed on a surface corresponding
to the (110) plane, said first bridge extending in a direction parallel
to the <100> axis.
6. The flow meter as claimed in claim 1 further comprising a flow
direction determining unit for determining a direction of flow based
on whether said voltage difference is a positive value or a negative
value.
7. The flow meter as claimed in claim 1 wherein said first bridge
and said second bridge are formed over separate depressions.
8. The flow meter as claimed in claim 1 wherein said third and
fourth temperature measuring resistors are provided on a bridge
formed over a depression formed on said substrate.
9. A thermally-sensitive type flow meter for measuring a flow rate
of a fluid, said flow meter including a sensor chip comprising a
substrate carrying sensing elements, said flow meter comprising:
first and second bridges formed over a depression formed on said
substrate, said first and second bridges being arranged along the
direction of flow so that said first bridge is positioned on an
upstream side and said second bridge is positioned on a downstream
side;
first and second resistors for generating a heat, said first and
second resistors provided on the respective one of said first and
second bridges, said first and second resistors being heated by
the same voltage source;
first and second temperature measuring resistors for measuring
a temperature of the fluid, said first and second temperature measuring
resistors being located at positions not influenced by a temperature
of said first and second resistors;
a temperature control unit, including said first and second resistors
and said first and second temperature measuring resistors, for controlling
a temperature of each of said first and second resistors to be constant;
and
a flow rate determining unit for determining a flow rate of the
fluid flowing around said first and second bridges, a determination
being made based on a voltage difference between a first voltage
and a second voltage, said first voltage being measured across said
first resistor, said second voltage being measured across said second
resistor.
10. The flow meter as claimed in claim 9 wherein said first and
second bridges extend perpendicular to the direction of flow.
11. The flow meter as claimed in claim 9 wherein said first bridge
extends perpendicular to the direction of flow and said second bridge
extends parallel to the direction of flow.
12. The flow meter as claimed in claim 9 wherein said substrate
is comprised of a semiconductor substrate having a (110) plane and
a <100> axis, said depression being formed on a surface corresponding
to the (110) plane, said first bridge extending in a direction 55
degrees inclined with respect to the <100> axis.
13. The flow meter as claimed in claim 9 wherein said substrate
is comprised of a semiconductor substrate having a (110) plane and
a <100> axis, said depression being formed on a surface corresponding
to the (110) plane, said first bridge extending in a direction parallel
to the <100> axis.
14. The flow meter as claimed in claim 9 further comprising a
flow direction determining unit for determining a direction of flow
based on whether said voltage difference is a positive value or
a negative value.
15. The flow meter as claimed in claim 9 wherein said first bridge
and said second bridge are formed over separate depressions.
16. The flow meter as claimed in claim 9 wherein said first and
second temperature measuring resistors are provided on a bridge
formed over a depression formed on said substrate.
17. A thermally-sensitive type flow meter for measuring a flow
rate of a fluid, said flow meter including a sensor chip comprising
a substrate carrying sensing elements, said flow meter comprising:
first and second bridges formed over a depression formed on said
substrate, said first and second bridges being arranged along the
direction of flow so that said first bridge is positioned on an
upstream side and said second bridge is positioned on a downstream
side;
a third bridge provided between said first and second bridges,
said third bridge being parallel to the direction of flow;
first, second and third resistors for generating a heat, said first,
second and third resistors provided on the respective one of said
first, second and third bridges, said first, second and third resistor
being heated by the same voltage source;
first and second temperature measuring resistors for measuring
a temperature of the fluid, said first and second temperature measuring
resistors being located at positions not influenced by a temperature
of said first and second resistors;
a temperature control unit, including said first and second resistors
and said first and second temperature measuring resistors, for controlling
a temperature of each of said first and second resistors to be constant;
and
a flow rate determining unit for determining a flow rate of the
fluid flowing around said first, second and third bridges, a determination
being made based on a voltage difference between a first voltage
and a second voltage, said first voltage being measured across one
of said first resistor and said second resistor, said second voltage
being measured across said third resistor.
18. The flow meter as claimed in claim 17 wherein said substrate
is comprised of a semiconductor substrate having a (110) plane and
a <100> axis, said depression being formed on a surface corresponding
to the (110) plane, said first and second bridges extending in a
direction 55 degrees inclined with respect to the <100> axis.
19. The flow meter as claimed in claim 17 wherein said substrate
is comprised of a semiconductor substrate having a (110) plane and
a <100> axis, said depression being formed on a surface corresponding
to the (110) plane, said first and second bridges extending in a
direction parallel to the <100> axis.
20. The flow meter as claimed in claim 17 further comprising a
flow direction determining unit for determining a direction of flow
based on whether said voltage difference is a positive value or
a negative value.
21. The flow meter as claimed in claim 17 wherein said first bridge
and said second bridge are formed over separate depressions.
22. The flow meter as claimed in claim 17 wherein said first temperature
measuring resistor is provided on a fourth bridge formed over said
depression and said second temperature measuring resistor is provided
on a fifth bridge formed over said depression, said fifth bridge
being located at a position opposite to said fourth bridge with
respect to said first, second and third bridges.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to flow meters and, more
particularly, to a thermally-sensitive type flow meter using a heating
element formed on a semiconductor substrate.
2. Background of the Invention
Recently, a flow rate measuring method has been suggested using
a thermally-sensitive type flow meter in which heating elements
are provided on a semiconductor substrate so as to measure a temperature
drop due to a cooling effect of fluid flowing around the heating
elements. Such a flow meter is suggested, for example, in Japanese
Laid-Open Patent Application No. 55-119381 and Japanese Patent Publications
No. 3-52028 and No. 6-43906.
FIG. 1 is a cross-sectional view of a part of the thermally-sensitive
type flow meter disclosed in Japanese Patent Publication No. 3-52028.
This flow meter uses a sensor comprising a silicon (Si) substrate
having a surface corresponding to a (100) plane. A depression 2
is formed in the surface corresponding to the (100) plane. Two bridges
3 and 4 extending in a 45-degree direction relative to a <110>
axis of the substrate are provided over the depression 2. Resistor
elements 5 and 6 which correspond to heating elements, are provided
on the respective bridges 3 and 4. FIG. 2 is a circuit diagram of
a driving circuit for the resistor elements 5 and 6. The driving
circuit provides voltage to the resistor elements 5 and 6. The resistor
elements are connected to respective amplifiers 7 and 8 so that
a voltage difference between the output voltage of the amplifiers
7 and 8 is obtained as an output voltage V.sub.0.
In the above-mentioned conventional flow meter, if a flow of fluid
is generated in a direction (X-direction) indicated by an arrow
X of FIG. 1 above the depression 2 of the substrate 1 the resistor
element 5 provided on the bridge 3 is cooled, whereas the resistor
element 6 provided on the bridge 4 is warmed up a little since a
heat is supplied from an upper stream side. Thus, the output voltage
V.sub.0 at the amplifier 9 which is a difference between the voltage
between the amplifiers 7 and 8 corresponds to a component of flow
rate in which a fluctuation in source voltage is eliminated. Accordingly,
a flow measurement can be performed with an improved signal-to-noise
ratio (SN ratio). Additionally, the depression 2 can be fabricated
by an anisotropic etching method since a longitudinal direction
of the bridges 3 and 4 is set in the 45-degree direction relative
to the <110> direction of the substrate 1.
When the depression 2 is formed on the (100) plane of the Si substrate
as shown in FIG. 1 the following problem happens. That is, the
depression 2 is normally formed by using an alkaline solution such
as KOH and utilizing the nature of the substrate that the etching
rate differs depending on directions of crystal faces. A relationship
between the etching rates corresponding to the crystal faces (100),
(110) and (111) is represented as follows.
This relationship is a property of anisotropic etching. When the
depression 2 is formed on the (100) plane by using an alkaline solution
as shown in FIG. 3 an etching rate in the Y-direction corresponding
to the etching of a side wall of the depression 2 is almost equal
to an etching rate in the Z-direction (depth) corresponding to the
etching of the (100) plane. Thus, if the depth of the depression
2 is increased, the etched amount of the side walls of the depression
2 is also increased. That is, side walls are formed as slanting
surfaces, and thus large side etched portions 10 are formed in the
side wall area of the depression 2.
If the large side etched portions 10 are formed as shown in FIG.
3 the design width La of the depression 2 is undesirably increased
to a width Lb. This influences the length of the bridges 3 and 4
That is, the actual length Lb of the bridges 3 and 4 must be considerably
longer than the design length La, as shown in FIG. 4. Accordingly,
a strength of the bridges 3 and 4 is decreased. Additionally, the
depth of the side etched portion in the Y-direction is dispersed
due to dispersion of the anisotropic etching effect. This causes
variation in the thermal isolation of the bridges 3 and 4 which
requires a severe control of the manufacturing process. Further,
the resistance of the resistor elements 5 and 6 is varied since
the temperature of the bridges 3 and 4 is varied due to an increase
of the bridge length. Generally, sensitivity of a thermally-sensitive
type flow meter is dependent on the temperature of the heating element.
Accordingly, at higher flow rate, an upstream heating element and
a downstream heating element shows almost the same temperature and
there is no linearity between the flow rate and difference value
between the output voltages of the resistor elements 5 and 6. Thus,
it is difficult to achieve an accurate measurement by the above-mentioned
conventional method.
Additionally, Japanese Patent Publication No. 7-3351 and Japanese
Laid-Open Patent Application No. 3-248018 disclose thermally-sensitive
type flow meters in which a difference in two output voltages is
obtained so as to improve an SN ratio.
The flow meter disclosed in Japanese Patent Publication No. 7-3351
has heating elements on opposite surfaces of a supporting member.
The heating elements are connected in series, and the front face,
for example, of the supporting member is positioned perpendicular
to a flow stream line and toward the upstream side. In this state,
the heating elements are heated so that an average temperature of
the heating elements of the front face and back face (upstream and
downstream) is always maintained to be a predetermined temperature
higher than the temperature of the fluid. A flow rate is measured
by a voltage difference between the two heating elements. In this
method, variation in the resistance of the heating element positioned
on the upstream side is large and is linear to the flow rate and
that of the heating element positioned on the downstream side is
small and is non-linear to the flow rate because of the turbulent
flow occurred at the edge of the supporting member. Thus, an output
(voltage difference of the heating elements) versus flow rate characteristic
curve becomes non-linear, and thus it is difficult to take a measurement
with a high accuracy.
In the flow meter disclosed in Japanese Laid-Open Patent Application
No. 3-248018 heaters are rolled up around the bypath of a fluid
passage at the upstream position and the downstream position. A
flow rate of the fluid is measured by a voltage difference applied
to the heaters by controlling the voltages provided to the heaters
so that the resistances or the temperatures of the heaters are equalized
to each other. In this method, a large power is necessary for heating
the tube at the bypath. Thus, this method is not appropriate for
an application in which a measurement should be taken in a small
area with a low power consumption.
SUMMARY OF THE INVENTION
It is a general object of the present invention to provide an improved
and useful thermally-sensitive type flow meter in which the above
mentioned problems are eliminated.
A more specific object of the present invention is to provide a
thermally-sensitive type flow meter which can measure a flow rate
with a high accuracy by eliminating a noise generated in a source
voltage.
Another object of the present invention is to provide a thermally-sensitive
type flow meter which can measure a flow rate based on a linear
relationship between a flow rate of a fluid and a voltage difference
between two heating elements.
Another object of the present invention is to provide a thermally-sensitive
type flow meter in which a temperature of a heating element is measured
without a time delay.
In order to achieve the above-mentioned objects, there is provided
according to one aspect of the present invention a thermally-sensitive
type flow meter for measuring a flow rate of a fluid, the flow meter
including a sensor chip comprising a substrate carrying sensing
elements, the flow meter comprising:
first and second bridges formed over a depression formed on the
substrate, the first and second bridges being arranged along the
direction of flow so that the first bridge is positioned on an upstream
side and the second bridge is positioned on a downstream side;
first and second resistors for generating a heat, the first and
second resistors provided on the respective one of the first and
second bridges, the first and second resistors being heated by the
same voltage source;
first and second temperature measuring resistors provided adjacent
to the respective one of the first and second resistors;
third and fourth temperature measuring resistors for measuring
a temperature of the fluid, the third and fourth temperature measuring
resistors being located at positions not influenced by a temperature
of the first and second resistors;
a temperature control unit, including the first, second, third
and fourth temperature measuring resistors, for controlling a temperature
of each of the first and second resistors to be constant; and
a flow rate determining unit for determining a flow rate of the
fluid flowing around the first and second bridges, a determination
being made based on a voltage difference between a first voltage
and a second voltage, the first voltage being measured across the
first resistor, the second voltage being measured across the second
resistor.
According to this invention, when the fluid flows around the first
and second bridges, the first resistor provided on the first bridge
and the second resistor provided on the second bridge are subjected
to the flow of the fluid. Thus the first resistor is cooled. The
cooling effect on the first resistor is proportional to the flow
rate. On the other hand, the temperature of the second resistor,
which is provided on the downstream side of the first resistor,
is not changed since a heat generated in the first resistor is transmitted
to the second resistor via the fluid flow or on the second bridge
a heat from the upstream part of the resistor is transferred to
the downstream part of it via the fluid flow. Thus, a higher voltage
is applied to the first resistor to maintain a constant temperature,
whereas a voltage applied to the second resistor is maintained to
be constant. This means that the first voltage measured across the
first resistor is increased as the flow rate is increased, and the
second voltage measured across the second resistor is constant irrespective
of the flow rate. Since the same source voltage is used for applying
the first voltage and the second voltage, the voltage difference
between the first voltage and the second voltage is not effected
by fluctuation or noise generated in the source voltage. Additionally,
a linear relationship is obtained between the voltage difference
and the flow rate, resulting in a flow measurement with a high accuracy.
Additionally, there is provided according to another aspect of
the present invention a thermally-sensitive type flow meter for
measuring a flow rate of a fluid, the flow meter including a sensor
chip comprising a substrate carrying sensing elements, the flow
meter comprising:
first and second bridges formed over a depression formed on the
substrate, the first and second bridges being arranged along the
direction of flow so that the first bridge is positioned on an upstream
side and the second bridge is positioned on a downstream side;
first and second resistors for generating a heat, the first and
second resistors provided on the respective one of the first and
second bridges, the first and second resistors being heated by the
same voltage source;
first and second temperature measuring resistors for measuring
a temperature of the fluid, the first and second temperature measuring
resistors being located at positions not influenced by a temperature
of the first and second resistors;
a temperature control unit, including the first and second resistors
and the first and second temperature measuring resistors, for controlling
a temperature of each of the first and second resistors to be constant;
and
a flow rate determining unit for determining a flow rate of the
fluid flowing around the first and second bridges, a determination
being made based on a voltage difference between a first voltage
and a second voltage, the first voltage being measured across the
first resistor, the second voltage being measured across the second
resistor.
According to this invention, similar to the previously mentioned
invention, since the same source voltage is used for applying the
first voltage and the second voltage, the voltage difference between
the first voltage and the second voltage is not effected by fluctuation
or noise generated in the source voltage. A linear relationship
is obtained between the voltage difference and the flow rate, resulting
in a flow measurement with a high accuracy. Additionally, the change
in the temperature of the first and second resistor is measured
by the change in the resistances of the first and second resistors.
Thus, there is no time delay in measuring the change in the temperature
of the first and second resistors which time delay may occur where
the change in temperature is measured by a separate element.
In one embodiment of the present invention, the first and second
bridges may extend perpendicular to the direction of flow. Alternatively,
the first bridge may extend perpendicular to the direction of flow
and the second bridge may extend parallel to the direction of flow.
Additionally, the substrate may be comprised of a semiconductor
substrate having a (110) plane and a <100> axis, the depression
being formed on a surface corresponding to the (110) plane, the
first bridge extending in a direction 55 degrees inclined with respect
to the <100> axis. Alternatively, the substrate may be comprised
of a semiconductor substrate having a (110) plane and a <100>
axis, the depression being formed on a surface corresponding to
the (110) plane, the first bridge extending in a direction parallel
to the <100> axis. This eliminates an under-etching of the
depression formed on the substrate.
Additionally, a flow direction determining unit may be provided
for determining a direction of flow based on whether the voltage
difference is a positive value or a negative value.
Further, the first bridge and the second bridge may be formed over
separate depressions.
Additionally, the temperature measuring resistors for measuring
the fluid temperature may be provided on a bridge formed over a
depression formed on the substrate.
There is provided according to another aspect of the present invention
a thermally-sensitive type flow meter for measuring a flow rate
of a fluid, the flow meter including a sensor chip comprising a
substrate carrying sensing elements, the flow meter comprising:
first and second bridges formed over a depression formed on the
substrate, the first and second bridges being arranged along the
direction of flow so that the first bridge is positioned on an upstream
side and the second bridge is positioned on a downstream side;
a third bridge provided between the first and second bridges, the
third bridge being parallel to the direction of flow;
first, second and third resistors for generating a heat, the first,
second and third resistors provided on the respective one of the
first, second and third bridges, the first, second and third resistors
being heated by the same voltage source;
first and second temperature measuring resistors for measuring
a temperature of the fluid, the first and second temperature measuring
resistors being located at positions not influenced by a temperature
of the first and second resistors;
a temperature control unit, including the first and second resistors
and the first and second temperature measuring resistors, for controlling
a temperature of each of the first and second resistors to be constant;
and
a flow rate determining unit for determining a flow rate of the
fluid flowing around the first, second and third bridges, a determination
being made based on a voltage difference between a first voltage
and a second voltage, the first voltage being measured across one
of the first resistor and the second resistor, the second voltage
being measured across the third resistor.
According to this invention, when the first resistor is on the
upstream side with respect to the third resistor, the voltage difference
is obtained by using the voltage applied to the first resistor and
the voltage applied to the third resistor. When the second resistor
is on the upstream side with respect to the third resistor, the
voltage difference is obtained by using the voltage applied to the
second resistor and the voltage applied to the third resistor. Thus,
an accurate flow measurement is obtained for either the normal direction
or the reverse direction of the flow.
In one embodiment, the substrate may be comprised of a semiconductor
substrate having a (110) plane and a <100> axis, the depression
being formed on a surface corresponding to the (110) plane, the
first and second bridge extending in a direction 55 degrees inclined
with respect the <100> axis. Alternatively, the substrate
may be comprised of a semiconductor substrate having a (110) plane
and a <100> axis, the depression being formed on a surface
corresponding to the (110) plane, the first and second bridge extending
in a direction parallel to the <100> axis.
A flow direction determining unit may further be provided for determining
a direction of flow based on whether the voltage difference is a
positive value or a negative value.
Additionally, the first bridge and the second bridge may be formed
over separate depressions.
Further, the first temperature measuring resistors may be provided
on a fourth bridge formed over the depression and the second temperature
measuring resistors may be provided on a fifth bridge formed over
the depression, the fifth bridge being located at a position opposite
to the fourth bridge with respect to the first, second and third
bridges.
Other objects, features and advantages of the present invention
will become more apparent from the following detailed descriptions
when read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a part of a conventional thermally-sensitive
type flow meter;
FIG. 2 is a circuit diagram of a driving circuit for resistor elements
shown in FIG. 1;
FIG. 3 is a plan view of a depression shown in FIG. 1;
FIG. 4 is a cross-sectional view taken along a line IV--IV of FIG.
3;
FIG. 5 is a plan view of a sensor chip used in a thermally-sensitive
type flow meter according to a first embodiment of the present invention;
FIG. 6 is a circuit diagram of a driving circuit for the sensor
chip shown in FIG. 5;
FIG. 7 is a graph showing a relationship between voltages output
from differential amplifiers and fluid velocity;
FIG. 8 is a graph showing a relationship between a voltage difference
output from a differential amplifier and the fluid velocity;
FIG. 9 is a cross-sectional view showing a cross section of an
example of a depression formed in the sensor chip shown in FIG.
5;
FIG. 10 is a cross-sectional view showing a cross section of another
example of the depression formed in the sensor chip sown in FIG.
5;
FIG. 11 is a plan view of a sensor chip according to a second embodiment
of the present invention;
FIG. 12 is a circuit diagram of a driving circuit for driving the
sensor chip shown in FIG. 11;
FIG. 13 is a plan view of a sensor chip used in a flow meter according
to a third embodiment of the present invention.
FIG. 14 is a plan view of a variation of the sensor chip shown
in FIG. 13; and
FIG. 15 is a plane view Of a sensor chip used in a flow meter according
to a fourth embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A description will now be given, with reference to FIGS. 5 to 10
of a first embodiment of the present invention. FIG. 5 is a plan
view of a sensor chip used in a thermally-sensitive type flow meter
according to the first embodiment of the present invention.
The sensor chip shown in FIG. 5 is formed by a single crystalline
silicon semiconductor substrate 11 having the (110) plane. In FIG.
5 a direction Z (Z-direction) corresponds to a depth direction
which is perpendicular to the (110) plane. A depression 12 is formed
on a surface which corresponds to the (110) plane. A pair of bridges
13 and 14 are formed over the depression 12 so that a longitudinal
direction of the bridges 13 and 14 is perpendicular to a direction
(X-direction) of a fluid flow. The bridges 13 and 14 extend parallel
to each other. The bridge 13 is positioned on the upstream side
of the fluid flow with respect to the bridge 14. The longitudinal
direction (Y-direction) of the bridges 13 and 14 is set to a direction
inclined about 55 degrees with respect to the <100> axis of
the substrate. The longitudinal direction of the bridges 13 and
14 may be parallel to the <100> axis.
A heating element Rh.sub.1 and a temperature measuring element
Rs.sub.1 each of which comprises a resistor, are provided on the
upstream bridge 13. The temperature measuring element Rs.sub.1 is
provided for measuring a temperature of the heating element Rh.sub.1.
A heating element Rh.sub.2 and a temperature measuring element Rs.sub.2
each of which comprises a resistor, are provided on the downstream
bridge 14. The temperature measuring element Rs.sub.2 is provided
for measuring a temperature of the heating element Rh.sub.2. Temperature
measuring elements Rf.sub.1 and Rf.sub.2 are provided on the surface
of the substrate 11 to measure the temperature of the fluid. Thus,
the temperature measuring elements Rf.sub.1 and Rf.sub.2 are located
in positions not influenced by the heating elements Rh.sub.1 and
Rh.sub.2.
FIG. 6 is a circuit diagram of a driving circuit for the sensor
chip shown in FIG. 5. As shown in FIG. 6 the temperature measuring
element Rs.sub.1 the temperature measuring element Rf.sub.1 fixed
resistors R.sub.11 and R.sub.21 and a temperature setting resistor
Rt.sub.1 together form a bridge circuit 15. The bridge circuit 15
is connected to a differential amplifier 16. The differential amplifier
16 is connected to a base of a transistor 17. An emitter of the
transistor 17 is connected to the heating element Rh.sub.1. Similarly,
the temperature measuring element Rs.sub.2 the temperature measuring
element Rf.sub.2 fixed resistors R.sub.12 and R.sub.22 and a temperature
setting resistor Rt.sub.2 together form a bridge circuit 18. The
bridge circuit 18 is connected to a differential amplifier 19. The
differential amplifier 19 is connected to a base of a transistor
20. An emitter of the transistor 20 is connected to the heating
element Rh.sub.2. The bridge circuits 15 and 18 the differential
amplifiers 16 and 19 and transistors 17 and 20 constitute a temperature
control unit 21 for maintaining each of the heating elements Rh.sub.1
and Rh.sub.2 at a constant temperature. Additionally, the transistor
17 and the transistor 20 are provided with a source voltage provided
from the same voltage source +V.
The emitters of the transistors 17 and 20 to which the heating
elements Rh.sub.1 and Rh.sub.2 are respectively connected, are also
connected to a differential amplifier 22 serving as a flow rate
detecting unit. The differential amplifier 22 outputs an amplified
voltage difference V.sub.12 between a voltage V.sub.1 applied to
the heating element Rh.sub.1 (corresponding to a voltage measured
across the heating element Rh.sub.1) and a voltage V.sub.2 applied
to the heating element Rh.sub.2 (corresponding to a voltage measured
across the heating element Rh.sub.2). The voltage difference V.sub.12
is supplied to a flow rate and direction determining unit. The flow
rate and direction determining unit determines a flow rate of the
fluid by referring to a relationship between the output of the differential
amplifier 22 and a flow rate. Additionally, the flow rate and direction
determining unit determines a direction of the flow by checking
whether the voltage difference V.sub.12 is a positive value or a
negative value.
A description will now be given of an operation of the flow meter
according to the first embodiment of the present invention. It is
assumed that the heating elements Rh.sub.1 and Rh.sub.2 are controlled
to maintain a constant temperature higher than a temperature of
the fluid. If a fluid flow is generated in the X-direction in this
condition, a resistance of the temperature measuring element Rs.sub.1
for the heating element is decreased since the upstream bridge 13
is cooled by the fluid flow. Thus, the balance of the bridge circuit
15 is lost, and the voltage difference is detected by the differential
amplifier 16. The difference voltage is amplified by the differential
amplifier 16 and supplied to the transistor 17. The transistor 17
is driven by the output of the differential amplifier 16 to supply
a current to the heating element Rh.sub.1. The heating element Rh.sub.1
is heated until the temperature reaches the above-mentioned constant
temperature. At this time, the voltage V.sub.1 applied to the heating
element Rh.sub.1 is decreased in proportion to an increase in resistance
of the heating element Rs.sub.1 which returns to an initial value
corresponding to a state in which no fluid flow is generated. On
the other hand, the temperature of the downstream bridge 14 hardly
changes when the fluid velocity is low since the fluid heated by
the heating element Rh.sub.1 on the upstream bridge 13 is lead around
the bridge 14. The temperature fall of the bridge 14 is extremely
less than that of the upstream bridge 13 even when the fluid velocity
is increased.
FIG. 7 is a graph showing a relationship between the voltages V.sub.1
and V.sub.2 and fluid velocity when the driving circuit shown in
FIG. 6 is driven by a 3-Volt battery. FIG. 8 is a graph showing
a relationship between the voltage V.sub.12 and the fluid velocity.
As shown in the graph of FIG. 7 the voltage V.sub.1 applied to
the upstream heating element Rh.sub.1 is increased as the fluid
velocity is increased, whereas the voltage V.sub.2 applied to the
heating element Rh.sub.2 is hardly increased as the fluid velocity
is increased. Especially, the voltage V.sub.2 does not exhibit flow
dependency in a low flow rate range.
On the other hand, as shown in FIG. 8 the voltage difference V.sub.12
output from the differential amplifier 22 exhibits a good linearity
from a low flow range to a high flow range with respect to the fluid
velocity.
By taking the difference between the voltages V.sub.1 and V.sub.2
noise or drift in phase of the voltages V.sub.1 and V.sub.2 due
to aging of the elements, offsets of the differential amplifiers
or fluctuation in the source voltage can be canceled. Thus, a signal
corresponding to only a flow component can be obtained. This results
in the good linearity of the voltage difference V.sub.12. Accordingly,
the SN ratio can be remarkably improved as compared to the conventional
apparatus. Especially, an accuracy of flow measurement in a low
flow range is increased.
It is known that an etching rate of a semiconductor substrate is
dependent on each crystal face. When a KOH solution is used as an
etchant, an etching rate of the (111) plane is extremely low as
is appreciated from the previously mentioned relationship (1). In
the present embodiment, the silicon substrate 11 having the (110)
plane is used. The depression 12 is formed in the Z-direction which
corresponds to a depth direction of the (110) plane. The longitudinal
direction of the bridges 13 and 14 corresponds to the direction
about 55 degrees slanting from the <100> axis. Thus, the side
walls 12a and 12b of the depression 12 can be constituted by the
(111) plane.
As discussed above, when an anisotropic etching is applied by using
the KOH solution, an etching rate (in the Y-direction) with respect
to the side walls 12a and 12b (corresponding to the (111) plane)
of the depression 12 can be set to an extremely low speed relative
to an etching rate (in the Z-direction) with respect to a bottom
surface (corresponding to the (110) plane) of the depression 12.
Thus, as shown in FIG. 9 the side walls 12a and 12b can be formed
almost perpendicular to the bottom surface of the depression 12
even when the depth of the depression is increased. Accordingly,
the length of the bridges 13 and 14 can be accurately set to the
design length La. Additionally, the depression 12 can be formed
deeper than the depression of the conventional substrate as shown
in FIG. 4 since the large side etched portion 10 is not formed.
This construction provides a good heat isolation of the bridges
13 and 14 and reduces variation in the temperature of the bridges
13 and 14.
When the longitudinal direction (Y-direction) of the bridges 13
and 14 is set parallel to the <100> axis of the substrate,
the bottom surface of the depression 12 corresponds to the (111)
plane. In this condition, the (111) plane crosses the (110) surface
at an angle of 35 degrees and the depression 12 can be formed in
a V-shape with an anisotropic etching. Thus, the depression 12 can
be formed in a V-shape. In this case also the under-etching of the
depression 12 can be prevented. Thus, the length of the bridges
13 and 14 in the Y-direction can be set to the design length La.
Additionally, as apparent from the graph of FIG. 7 the voltage
V.sub.2 of the heating element Rh.sub.2 of the downstream bridge
14 is small even when the flow rate is in a high flow velocity range
since the downstream bridge 14 is not influenced by an increase
of the flow rate, whereas the voltage of the heating element Rh.sub.1
of the upstream bridge 13 is increased in proportion to the flow
rate since the upstream bridge 13 is cooled by the fluid flow. Thus,
if the voltage difference obtained by subtracting the voltage V.sub.2
from the voltage V.sub.1 is a positive value, it can be determined
that the direction of flow is from the bridge 13 to the bridge 14.
On the other hand, if the voltage difference obtained by subtracting
the voltage V.sub.2 from the voltage V.sub.1 is a negative value,
it can be determined that the direction of flow is from the bridge
14 to the bridge 13. Accordingly, the direction of flow can be determined
by checking whether the voltage difference V.sub.12 output from
the differential amplifier 22 is a positive value or a negative
value. This determination of the direction of flow can eliminate
a measurement error due to a reverse flow when the flow is obtained
by an integral of the flow rate.
A description will now be given, with reference to FIGS. 11 and
12 of a second embodiment according to the present invention. FIG.
11 is a plan view of a sensor chip used in a flow meter according
to the second embodiment of the present invention. FIG. 12 is a
circuit diagram of a driving circuit for driving the sensor chip
shown in FIG. 11. In FIGS. 11 and 12 parts that are the same as
the parts shown in FIGS. 5 and 6 are given the same reference numerals,
and descriptions thereof will be omitted.
In the sensor chip shown in FIG. 11 the heating elements Rh.sub.1
and Rh.sub.2 are provided on the respective bridges 13 and 14. However,
the temperature measuring elements Rs.sub.1 and Rs.sub.2 are not
provided. Other construction of the sensor chip according to the
second embodiment is the same as that of the sensor chip according
to the above-mentioned first embodiment.
As shown in FIG. 12 the heating element Rh.sub.1 the temperature
measuring element Rf.sub.1 the fixed resistors R.sub.11 and R.sub.21
and the temperature setting resistor Rt.sub.1 together form a bridge
circuit 23. The bridge circuit 23 is connected to the differential
amplifier 16. The differential amplifier 16 is connected to the
base of the transistor 17. The transistor 17 is connected between
the voltage source +V and the bridge circuit 23. Similarly, the
heating element Rh.sub.2 the temperature measuring element Rf.sub.2
the fixed resistors R.sub.12 and R.sub.22 and the temperature setting
resistor Rt.sub.2 together form a bridge circuit 24. The bridge
circuit 24 is connected to the differential amplifier 19. The transistor
20 is connected between the voltage source +V and the bridge circuit
24. In this case, the bridge circuits 23 and 24 the differential
amplifiers 16 and 19 and the transistors 17 and 20 constitute a
temperature control unit 25 for maintaining each of the heating
elements Rh.sub.1 and Rh.sub.2 at a constant temperature.
Additionally, the heating element Rh.sub.1 of the bridge circuit
23 and the heating element Rh.sub.2 of the bridge circuit 24 are
connected to the differential amplifier 22. The differential amplifier
22 outputs an amplified voltage difference V.sub.12 between the
voltage V.sub.1 applied to the heating element Rh.sub.1 (corresponding
to a voltage measured across the heating element Rh.sub.1) and the
voltage V.sub.2 applied to the heating element Rh.sub.2 (corresponding
to a voltage measured across the heating element Rh.sub.2). The
voltage difference V.sub.12 is supplied to the flow rate and direction
determining unit.
A description will now be given of an operation of the flow meter
according to the second embodiment of the present invention. When
a fluid flow is generated in the X-direction, the upstream bridge
13 is cooled by the fluid flow. Thus, the resistance of the heating
element Rh.sub.1 is varied and the voltage V.sub.1 applied to the
heating element Rh.sub.1 is also varied. On the other hand, the
resistance of the heating element Rh.sub.2 provided on the downstream
bridge 14 is maintained almost the same, and thus the voltage V.sub.2
applied to the heating element Rh.sub.2 is hardly changed. Accordingly,
the voltage difference V.sub.12 output from the differential amplifier
22 has a good linearity with respect to the flow velocity similar
to the graph shown in FIG. 8.
As mentioned above, since a change in the temperature of the heating
elements Rh.sub.1 and Rh.sub.2 is directly measured by a change
in the resistance of the heating elements, there is no time delay
in measuring the temperature of the heating elements. That is, there
is no time delay as is in the first embodiment in which a temperature
of the heating element is measured by the temperature measuring
element which is adjacent to but physically separated from the heating
element. Thus, the heating elements Rh.sub.1 and Rh.sub.2 of the
present embodiment can be driven by a pulsed voltage, and a width
of the pulse can be decreased. Thus, the flow meter according to
the present embodiment can be operated with a reduced power consumption.
A description will now be given, with reference to FIG. 13 of
a third embodiment of the present invention. FIG. 13 is a plan view
of a sensor chip used in a flow meter according to the third embodiment
of the present invention. In FIG. 13 parts that are the same as
the parts shown in FIG. 5 are given the same reference numerals,
and descriptions thereof will be omitted.
The sensor chip shown in FIG. 13 is formed of a silicon substrate
having the (110) plane similar to the sensor chips according to
the first and second embodiments. In this sensor chip, two depressions
12 are formed on a surface. The bridge 13 is formed over the depression
12 positioned on the upstream side. The bridge 13 extends generally
perpendicular to the direction (X-direction) of fluid flow. The
longitudinal direction of the upstream bridge 13 is slanted about
55 degrees with respect to the <100> axis of the substrate
11. The longitudinal direction of the bridge 13 may be parallel
to the <100> axis. The bridge 14 is formed over the depression
12 positioned on the downstream side of the depression 12 over which
the upstream bridge 13 is formed. The longitudinal direction of
the downstream bridge 14 is perpendicular to the longitudinal direction
of the upstream bridge 13. That is, the downstream bridge 14 is
formed along the direction (X-direction) of fluid flow. The heating
element Rh.sub.1 is provided on the bridge 13 and the heating element
Rh.sub.2 is provided on the bridge 14.
The temperature measuring element Rf.sub.1 is provided on the substrate
11 at a position not influenced by the temperature of the heating
elements Rh.sub.1 and Rh.sub.2. The temperature measuring element
Rf.sub.2 is provided on the substrate 11 at a position not influenced
by the temperature of the heating elements Rh.sub.1 and Rh.sub.2.
The sensor chip according to the present embodiment is driven by
a driving circuit having the same structure as the driving circuit
shown in FIG. 12.
In the present embodiment, the heating element Rh.sub.2 can be
insensitive with respect to the fluid flow by forming the bridge
14 perpendicular to the bridge 13. This is because the bridge 14
is aligned with the direction (X-direction) of fluid flow. That
is, a heat transmitted to the fluid on the upstream portion of the
heating element Rh.sub.2 is transported along the bridge 14 and
thus the downstream portion of the heating element Rh.sub.2 is not
cooled by the fluid flow. Thus, the heating element Rh.sub.2 of
the downstream bridge 14 has an extremely low sensitivity (flow
dependency) with respect to the flow rate. In this case, the voltage
V.sub.2 applied to the heating element Rh.sub.2 can be almost zero
not only in a low flow range but also in a high flow range. Additionally,
an offset voltage included in the voltage V.sub.1 applied to the
heating element Rh.sub.1 is equal to an offset voltage included
in the voltage V.sub.2 applied to the heating element Rh.sub.2.
Thus, a good linearity of the output voltage V.sub.12 which is
a difference between the voltage V.sub.1 and the voltage V.sub.2
can be obtained from a low flow range to a high flow range.
On the other hand, the heating element Rh.sub.1 of the upstream
bridge 13 which is perpendicular to the direction of fluid flow,
has an extremely high sensitivity with respect to the flow rate.
The longitudinal direction of the bridge 13 is set to about 55 degrees
with respect to the <100> axis. Thus, the side walls of the
depression 12 for the bridge 13 are constituted by the (111) plane
as shown in FIG. 9. This structure can eliminate under-etching of
the depression with respect to the longitudinal direction (Y-direction),
and permits an increase in a depth of the depression in the Z-direction.
Thus, the heat isolation for the bridge 13 can be increased. Accordingly,
an SN ratio of the sensor chip according to the present invention
is improved, and an accurate flow measurement can be achieved.
The sensor chip according to the present embodiment has bridges
13 and 14 having only the heating elements Rh.sub.1 and Rh.sub.2.
However, the temperature measuring elements Rs.sub.1 and Rs.sub.2
may be provided adjacent to the respective heating elements Rh.sub.1
and Rh.sub.2 as is in the sensor chip according to the first embodiment.
Additionally, as shown in FIG. 14 the bridges 13 and 14 may be
provided over the same depression 12. In this case, the longitudinal
direction (Y-direction) of the upstream bridge 13 is off set about
55 degrees or is parallel to the axis <100>.
A description will now be given, with reference to a fourth embodiment
of the present invention. FIG. 15 is a plane view of a sensor chip
used in a flow meter according to the fourth embodiment of the present
invention. In FIG. 15 parts that are the same as the parts shown
in FIG. 5 are given the same reference numerals, and descriptions
thereof will be omitted.
In the sensor chip shown in FIG. 15 the depression 12 is formed
in the center of the substrate 11 along the direction (X-direction)
of the fluid flow. In the center of the depression are formed a
first bridge 26 and a second bridge 27. The first and second bridges
are perpendicular to the direction (X-direction) of fluid flow.
A third bridge 28 is connected to the first and second bridges 26
and 27. The heating elements Rh.sub.1 and Rh.sub.2 are provided
on the first and second bridges, respectively. A heating element
Rh.sub.3 is provided on the third bridge.
Additionally, bridges 29 and 30 are formed over the depression
12 perpendicular to the direction (X-direction) of the fluid flow.
The bridge 29 is located at a position upstream of the bridge 26
it is not influenced by the temperature of the heating elements
Rh.sub.1 Rh.sub.2 and Rh.sub.3. The bridge 30 is located at a position
downstream of the bridge 27 where it is not influenced by the temperature
of the heating elements Rh.sub.1 Rh.sub.2 and Rh.sub.3. The temperature
measuring element Rf.sub.1 is provided on the bridge 29 and the
temperature measuring element Rf.sub.2 is provided on the bridge
30.
The sensor chip according to the present embodiment can be driven
by a driving circuit having a basic structure similar to the driving
circuit shown in FIG. 12. The heating elements Rh.sub.1 and the
Rh.sub.2 together correspond to the heating element Rh.sub.1 which
has a high flow dependency, of the driving circuit shown in FIG.
12. The heating element Rh.sub.3 corresponds to the heating element
Rh.sub.2 which has a low flow dependency, of the driving circuit
shown in FIG. 12. That is, a bridge circuit is formed by the heating
elements Rh.sub.1 and Rh.sub.2 and the temperature measuring element
Rf.sub.1. Another bridge circuit is formed by the heating element
Rh.sub.3 and the temperature measuring element Rf.sub.2. These bridge
circuits constitute a temperature controlling unit for maintaining
the temperature of the heating elements to be constant similar to
that of the circuit shown in FIG. 12. Additionally, the temperature
controlling unit is connected to a flow rate and direction determining
unit. The flow rate and direction determining unit obtains the voltage
difference V.sub.13 or V.sub.23 between the voltage applied to the
heating element Rh.sub.1 or Rh.sub.2 and the voltage V.sub.3 applied
to the heating element Rh.sub.3 so as to determine the flow rate.
The direction of fluid flow is also determined by checking whether
the voltage difference V.sub.13 is a positive value or a negative
value.
The substrate may be formed of a single crystalline silicon substrate
having the (110) plane. In this case the longitudinal direction
of the bridges 26 and 27 may be offset about 55 degrees or parallel
to the <100> axis of the substrate 11.
In the above-mentioned construction, the third heating element
Rh.sub.3 of the third bridge 28 is insensitive with respect to the
flow rate since the third bridge 28 extends along the direction
(X-direction) of fluid flow. When the direction of fluid flow is
the X-direction, the voltage V.sub.1 is obtained from the heating
element Rh.sub.1 of the first bridge 26 so as to obtain the voltage
difference V.sub.13. On the other hand, when the direction of fluid
flow is opposite to the X-direction, that is, the flow is reversed,
the voltage V.sub.2 is obtained from the heating element Rh.sub.2
of the second bridge 27 so as to obtain the voltage difference V.sub.23.
Thus, in the present embodiment, an accurate flow measurement can
be achieved for either the normal flow or the reverse flow.
Additionally, in the present embodiment, since the temperature
measuring elements Rf.sub.1 and Rf.sub.2 for measuring the temperature
of the fluid are provided on the respective bridges 29 and 30 a
change in the temperature of the fluid can be immediately detected.
This is because the temperature measuring elements Rf.sub.1 and
Rf.sub.2 are provided on the bridges 29 and 30 having a small heat
capacity. Such a quick response of the measurement of fluid temperature
is required, for example, when flow of a gas such as LPG is measured.
If a large amount of LPG is used in a short time, the temperature
of the LPG may be lower than the ambient temperature because of
the LPG in the liquid phase being cooled due to the vaporization
to the gas phase. In such a case, if the temperature measuring element
is provided on the surface of the substrate 11 having a large heat
capacity, an actual temperature cannot be reflected to the measurement.
However, in the sensor chip according to the present embodiment,
since the temperature measuring elements Rf.sub.1 and Rf.sub.2 are
formed on the bridges 29 and 30 the temperature of the fluid is
always immediately reflected to the resistance of the temperature
measuring elements Rf.sub.1 and Rf.sub.2. This provides an accuracy
to the flow measurement performed by the flow meter according to
the present embodiment.
It should be noted that any combination of the above-mentioned
embodiments may be in the scope of the present invention.
The present invention is not limited to the specifically disclosed
embodiments, and variations and modifications may be made without
departing from the scope of the present invention. |