U.S. patent application number 11/191848 was filed with the patent office on 2005-11-24 for flowmeter.
Invention is credited to Furuki, Shinya, Koike, Atsushi, Miyajima, Hiromitsu, Takahata, Takayuki, Yamagishi, Kiyoshi.
Application Number | 20050261842 11/191848 |
Document ID | / |
Family ID | 27531009 |
Filed Date | 2005-11-24 |
United States Patent
Application |
20050261842 |
Kind Code |
A1 |
Yamagishi, Kiyoshi ; et
al. |
November 24, 2005 |
Flowmeter
Abstract
A flowmeter for achieving a fluid flow rate value by using
calibration curves on the basis of the output of a detection
circuit using a bridge circuit (73) containing as constituent
resistors respective temperature sensing elements of a flow rate
detector containing a heating element (33) and a fluid temperature
detector in an indirectly heated type flow rate sensor unit. The
bridge circuit (73) varies the circuit characteristic value in
plural steps by a multiplexer (731) for selectively connecting the
output terminal and any one of the connection terminals between the
in-series connected resistors. Plural calibration curves are
provided in association with the steps of the circuit
characteristic value, and any one of the plural calibration curves
is selected in accordance with the step of the circuit
characteristic value selected by the multiplexer (731). The flow
rate range to be measured is set every calibration curve, and the
multiplexer (731) is controlled in accordance with the fluid flow
rate value thus achieved, and the calibration curve corresponding
to the flow rate range to which the flow rate value belongs.
According to this flowmeter, the flow rate can be measured with
excellent precision over a board flow rate range.
Inventors: |
Yamagishi, Kiyoshi;
(Ageo-shi, JP) ; Koike, Atsushi; (Ageo-shi,
JP) ; Takahata, Takayuki; (Ageo-shi, JP) ;
Miyajima, Hiromitsu; (Ageo-shi, JP) ; Furuki,
Shinya; (Ageo-shi, JP) |
Correspondence
Address: |
PITNEY HARDIN LLP
7 TIMES SQUARE
NEW YORK
NY
10036-7311
US
|
Family ID: |
27531009 |
Appl. No.: |
11/191848 |
Filed: |
July 28, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11191848 |
Jul 28, 2005 |
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10111578 |
Apr 26, 2002 |
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10111578 |
Apr 26, 2002 |
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PCT/JP00/07624 |
Oct 30, 2000 |
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Current U.S.
Class: |
702/45 |
Current CPC
Class: |
G01F 1/6986 20130101;
G01F 1/699 20130101; G01F 1/6842 20130101; G01F 1/684 20130101;
G01F 1/6965 20130101 |
Class at
Publication: |
702/045 |
International
Class: |
G06F 019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 29, 1999 |
JP |
11-309007 |
Dec 9, 1999 |
JP |
11-350363 |
Dec 28, 1999 |
JP |
11-373631 |
Feb 16, 2000 |
JP |
2000-037974 |
Mar 16, 2000 |
JP |
2000-074634 |
Claims
1. (canceled)
2. (canceled)
3. (canceled)
4. (canceled)
5. (canceled)
6. (canceled)
7. A thermal type flowmeter having a heating element and a flow
rate detection circuit containing a temperature sensing element for
flow rate detection disposed so as to be affected by the heating of
the heating element and perform heat transfer from/to fluid, the
heating of the heating element being controlled on the basis of a
voltage applied to the heating element, the applied voltage to the
heating element being controlled on the basis of the output of the
flow rate detection circuit and the flow rate of the fluid being
measured on the basis of the applied voltage, characterized in that
the applied voltage to said heating element is equal to the total
voltage of a base voltage the value of which is set every
predetermined time period and invariable within each predetermined
time period, and an addition voltage which has a fixed value and is
variable in voltage applying time, a comparator for comparing the
output of said flow rate detecting circuit with a reference value
is provided to output a binary signal having a first level
indicating that the heating of said temperature sensing element is
insufficient and a second level indicating the other cases, the
binary signal output from said comparator is sampled at a
predetermined period to count the appearance frequency of the first
level every predetermined time period and achieve the count value
thereof every predetermined time period, the base voltage is
adjusted so that when the count value is within a predetermined
range, the value of the base voltage in a subsequent predetermined
time period is not changed, when the count value is larger than the
upper limit of the predetermined range, the value of the base
voltage in the subsequent predetermined time period is increased by
a predetermined step value and when the count value is smaller than
the lower limit of the predetermined range, the value of the base
voltage in the subsequent time period is reduced by the
predetermined step value, and the addition voltage is applied
during only a time period when the binary signal output from the
comparator has the first level.
8. The thermal type flowmeter as claimed in claim 7, wherein the
addition voltage is set to two times to four times of the step
value of the base voltage.
9. The thermal type flowmeter as claimed in claim 7, wherein the
predetermined range of the count value has a lower limit value
which is smaller than a half of the sampling frequency within the
predetermined time period and larger than zero, and has an upper
limit value which is larger than a half of the sampling frequency
within the predetermined time period and smaller than the sampling
frequency.
10. A thermal type flowmeter having a heating element and a flow
rate detection circuit containing a temperature sensing element for
flow rate detection disposed so as to be affected by the heating of
the heating element and perform heat transfer from/to fluid, the
heating of the heating element being controlled on the basis of a
voltage applied to the heating element, the applied voltage to the
heating element being controlled on the basis of the output of the
flow rate detection circuit and the flow rate of the fluid being
measured on the basis of the applied voltage, characterized in that
the applied voltage to said heating element is equal to the total
voltage of a base voltage the value of which is set every
predetermined time period and invariable within each predetermined
time period, and an addition voltage which has a fixed value and is
variable in voltage applying time, a comparator for comparing the
output of said flow rate detecting circuit with a reference value
is provided to output a binary signal having a first level
indicating that the heating of said temperature sensing element is
insufficient and a second level indicating the other cases, the
binary signal output from said comparator is sampled at a
predetermined period to count the appearance frequency of the first
level every predetermined time period and achieve the count value
thereof every predetermined time period, the base voltage is
adjusted so that when the count value is within a predetermined
range, the value of the base voltage in a subsequent predetermined
time period is not changed, when the count value is larger than the
upper limit of the predetermined range, the value of the base
voltage in the subsequent predetermined time period is increased by
a predetermined step value and when the count value is smaller than
the lower limit of the predetermined range, the value of the base
voltage in the subsequent time period is reduced by the
predetermined step value, the addition voltage is applied during
only a time period when the binary signal output from the
comparator has the first level, and a data interpolating
calculation is made by using an instantaneous flow rate converting
table comprising plural individual calibration curves which
indicate the relationship between the applied voltage to said
heating element and the instantaneous flow rate every discrete
temperature value, thereby achieving an instantaneous flow rate
value at an environmental temperature.
11. The thermal type flowmeter as claimed in claim 10, wherein the
individual calibration curves are created for discrete values of
possible values of the voltage to be applied to said heating
element, and when the instantaneous flow rate value is achieved, a
data interpolation calculation is carried out to achieve the
instantaneous flow rate value corresponding to a voltage value
applied to said heating element.
12. The thermal type flowmeter as claimed in claim 11, wherein the
discrete values are set to the minimum values of values having the
same high-order bit values of possible digital values of the
voltage to be applied to said heating element, and when the data
interpolation calculation is carried out, the instantaneous flow
rate values corresponding to first discrete values having the same
high-order bit values as the voltage value applied to said heating
element and second discrete values whose high-order bit values are
larger than that of the voltage value applied to said heating
element by 1 are achieved by the individual calibration curves.
13. The thermal type flowmeter as claimed in claim 10, wherein the
addition voltage is set to two times to four times of the step
value of the base voltage.
14. The thermal type flowmeter as claimed in claim 10, wherein the
predetermined range of the count value has a lower limit value
which is smaller than a half of the sampling frequency within the
predetermined time period and larger than zero, and has an upper
limit value which is larger than a half of the sampling frequency
within the predetermined time period and smaller than the sampling
frequency.
15. A flowmeter having an indirectly heated type flow rate sensor
unit in which a flow rate detector containing a heating element and
a temperature sensing element for flow rate detection is joined to
a heat transfer member for flow rate detection, a fluid flow rate
value being achieved with a calibration curve on the basis of the
output of a detection circuit containing the temperature sensing
element for flow rate detection, characterized in that the
calibration curve comprises three portions corresponding to three
areas of the output value of said detection circuit, and the three
portions are represented by quaternary functions which use the
output of said detection circuit as variables and are different in
coefficient, a fluid flow rate value being achieved by using the
portion of the calibration curve which corresponds to the area to
which the output value of said detection circuit belongs.
16. The flowmeter as claimed in claim 15, wherein the calibration
curve is represented as follows:
f=a.sub.1V+b.sub.1v+c.sub.1v.sup.2+d.sub.1v+e.sub-
.1(0<v<v.sub.1)
f=a.sub.2v.sup.4+b.sub.2v.sup.3+c.sub.2v.sup.2+d.sub- .2v+e.sub.2
(v.sub.1<v<v.sub.2) f=a.sub.3v.sup.4+b.sub.3v.sup.3+c.su-
b.3v.sup.2+d.sub.3v+e.sub.3 (v.sub.2.ltoreq.v) wherein f represents
the fluid flow rate, v represents the output of the detection
circuit, and a.sub.1, b.sub.1, c.sub.1, d.sub.1, e.sub.1; a.sub.2,
b.sub.2, c.sub.2, d.sub.2, e.sub.2; a.sub.3, b.sub.3, c.sub.3,
d.sub.3, e.sub.3 represent coefficients.
17. The flowmeter as claimed in claim 15, wherein said detection
circuit comprises a bridge circuit.
18. The flowmeter as claimed in claim 15, wherein said flow rate
sensor unit has a fluid temperature detector containing a fluid
temperature detecting temperature sensing element and a fluid
temperature detecting heat transfer member joined to said fluid
temperature detector, and said detection circuit contains said
fluid temperature detecting temperature sensing element.
19. A thermal type flowmeter which includes a casing member having
a fluid flow passage extending from a fluid flow-in port to a fluid
flow-out port, a flow rate detecting unit which is secured to the
casing member and varies in electrical characteristic value in
accordance with the flow of the fluid in the fluid flow passage
through the heat exchange between the flow rate detecting unit and
the fluid in the fluid flow passage, and a fluid temperature
detecting unit which is secured to the casing member and varies in
electrical characteristic value in accordance with the temperature
of the fluid through the heat exchange between the fluid
temperature detecting unit and the fluid in the fluid flow passage,
the flow rate of the fluid being also detected on the basis of the
electrical characteristic value of the fluid temperature detecting
unit, characterized in that a fluid residence area is formed at the
upstream side, with respect to the flow of the fluid, of each of
the position at which the heat exchange between the flow rate
detecting unit and the fluid is carried out and the position at
which the heat exchange between the fluid temperature detecting
unit and the fluid is carried out, the fluid residence area having
a flow cross section which is five times or more as large as the
flow cross section at the position where the heat exchange between
said flow rate detecting unit and the fluid is carried out or at
the position where the heat exchange between said fluid temperature
detecting unit and the fluid is carried out.
20. The thermal type flowmeter as claimed in claim 19, wherein the
flow cross section of the fluid residence area is ten times or more
as large as the flow cross section at the position where the heat
exchange between said flow rate detecting unit and the fluid is
carried out or at the position where the heat exchange between said
fluid temperature detecting unit and the fluid is carried out.
21. The thermal type flowmeter as claimed in claim 19, wherein the
volume of said fluid residence area is 50 times or more as large as
the volume per unit length of the fluid flow passage in the fluid
flow direction at the position where the heat exchange between said
flow rate detecting unit and the fluid is carried out or at the
position where said fluid temperature detecting unit and the fluid
is carried out.
22. The thermal type flowmeter as claimed in claim 19, wherein said
fluid flow passage comprises a first flow passage part
intercommunicating with said fluid flow-in port, and a second flow
passage part intercommunicating with said fluid flow-out port, at
which the heat exchange between said flow rate detecting unit and
the fluid is carried out and the heat exchange between said fluid
temperature detecting unit and the fluid is carried out, said fluid
residence area is located between said first flow passage part and
said second flow passage part, and the flow cross section of said
first flow passage part is smaller than the flow cross section of
said fluid residence area.
23. The thermal type flowmeter as claimed in claim 22, wherein said
second flow passage part has a part extending in parallel to said
fluid residence area at the position where the heat exchange
between said flow rate detecting unit and the fluid is carried out
and at the position where the heat exchange between said fluid
temperature detecting unit and the fluid is carried out.
24. The thermal type flowmeter as claimed in claim 22, wherein a
filter is interposed at the intercommunication portion between said
fluid residence area and said second flow passage part.
25. The thermal type flowmeter as claimed in claim 19, wherein said
casing member is formed of metal.
26. The thermal type flowmeter as claimed in claim 19, wherein said
flow rate detecting unit is designed so that a heating element, a
flow rate detecting temperature sensing element and a flow rate
detecting heat transfer member extending into said fluid flow
passage are arranged so as to perform heat transfer thereamong, and
said fluid temperature detecting unit is designed so that a fluid
temperature detecting temperature sensing element and a fluid
temperature detecting heat transfer member extending into said
fluid flow passage are arranged so as to perform heat transfer
therebetween.
27. A thermal type flowmeter which includes a casing member having
a fluid flow passage extending from a fluid flow-in port to a fluid
flow-out port, a flow rate detecting unit which is secured to the
casing member and varies in electrical characteristic value in
accordance with the flow of the fluid in the fluid flow passage
through the heat exchange between the flow rate detecting unit and
the fluid in the fluid flow passage, and a fluid temperature
detecting unit which is secured to the casing member and varies in
electrical characteristic value in accordance with the temperature
of the fluid through the heat exchange between the fluid
temperature detecting unit and the fluid in the fluid flow passage,
a fluid-temperature-compensated flow rate of the fluid being
detected by a detection circuit containing the flow rate detecting
unit and the fluid temperature detecting unit, characterized in
that a fluid residence area is formed at the upstream side, with
respect to the flow of the fluid, of each of the position at which
the heat exchange between said flow rate detecting unit and the
fluid is carried out and the position at which the heat exchange
between said fluid temperature detecting unit and the fluid is
carried out, the flow velocity of the fluid at said fluid residence
area being equal to 1/5 or less of the flow velocity of the fluid
at the position where the heat exchange between the flow rate
detecting unit and the fluid is carried out or at the position
where the heat exchange between the fluid temperature detecting
unit and the fluid is carried out.
28. The thermal type flowmeter as claimed in claim 27, wherein said
fluid residence area is formed so that the flow velocity of the
fluid is equal to {fraction (1/10)} or less of the flow velocity of
the fluid at the position where the heat exchange between the flow
rate detecting unit and the fluid is carried out or at the position
where the heat exchange between the fluid temperature detecting
unit and the fluid is carried out.
29. The thermal type flowmeter as claimed in claim 27, wherein the
volume of said fluid residence area is 50 times or more as large as
the volume per unit length of the fluid flow passage in the fluid
flow direction at the position where the heat exchange between said
flow rate detecting unit and the fluid is carried out or at the
position where said fluid temperature detecting unit and the fluid
is carried out.
30. The thermal type flowmeter as claimed in claim 27, wherein said
fluid flow passage comprises a first flow passage part
intercommunicating with said fluid flow-in port, and a second flow
passage part intercommunicating with said fluid flow-out port, at
which the heat exchange between said flow rate detecting unit and
the fluid is carried out and the heat exchange between said fluid
temperature detecting unit and the fluid is carried out, said fluid
residence area is located between said first flow passage part and
said second flow passage part, and the flow cross section of said
first flow passage part is smaller than the flow cross section of
said fluid residence area.
31. The thermal type flowmeter as claimed in claim 30, wherein said
second flow passage part has a part extending in parallel to said
fluid residence area at the position where the heat exchange
between said flow rate detecting unit and the fluid is carried out
and at the position where the heat exchange between said fluid
temperature detecting unit and the fluid is carried out.
32. The thermal type flowmeter as claimed in claim 30, wherein a
filter is interposed at the intercommunication portion between said
fluid residence area and said second flow passage part.
33. The thermal type flowmeter as claimed in claim 27, wherein said
casing member is formed of metal.
34. The thermal type flowmeter as claimed in claim 27, wherein said
flow rate detecting unit is designed so that a heating element, a
flow rate detecting temperature sensing element and a flow rate
detecting heat transfer member extending into said fluid flow
passage are arranged so as to perform heat transfer thereamong, and
said fluid temperature detecting unit is designed so that a fluid
temperature detecting temperature sensing element and a fluid
temperature detecting heat transfer member extending into said
fluid flow passage are arranged so as to perform heat transfer
therebetween.
Description
TECHNICAL FIELD
[0001] The present invention relates to a fluid flow rate detecting
technique, and particularly to a flowmeter for measuring the flow
rate or integrated flow amount of fluid flowing in a pipe.
Furthermore, the present invention relates to a thermal type
flowmeter such as an indirectly heated type flowmeter or the like,
and further to a thermal type flowmeter having a fluid temperature
compensating function.
BACKGROUND TECHNIQUE
[0002] Various types have been known for a flowmeter [flow rate
sensor] (or current meter [flow velocity sensor]) for measuring the
flow rate (or flow velocity) of various kinds of fluid,
particularly liquid. Of these types of flowmeters, a so-called
thermal type (particularly, indirectly heated type) flowmeter has
been used because the price thereof is lower.
[0003] One of indirectly heated type flowmeters is designed and
used so that a sensor chip comprising a thin-film heating element
and a thin-film temperature sensing element which are laminated on
a substrate through an insulating layer by using the thin film
technique is disposed so as to enable the heat transfer between the
sensor chip and fluid flowing in a pipe. The electrical
characteristic of the temperature sensing element, for example, the
value of the electrical resistance is varied by supplying current
to the heating element to heat the temperature sensing element. The
variation of the electrical resistance value (based on increase of
the temperature of the temperature sensing element) is varied in
accordance with the flow rate (flow velocity) of the fluid flowing
in the pipe. This is because a part of the heating value of the
heating element is transferred into the fluid, the heating value
thus diffused into the fluid is varied in accordance with the flow
rate (flow velocity), and the heating value supplied to the
temperature sensing element is varied in accordance with the
variation of the heating value diffused into the fluid, so that the
electrical resistance value of the temperature sensing element is
varied. The variation of the electrical resistance value of the
temperature sensing element is also varied in accordance with the
temperature of the fluid. Therefore, a temperature sensing element
for temperature compensation is installed in an electrical circuit
for measuring the variation of the electrical resistance value of
the temperature sensing element to reduce the variation of the flow
rate measurement value due to the temperature of the fluid as much
as possible.
[0004] With respect to the indirectly heated type flowmeter using
the thin-film element as described above, JP(A)-11-118566 discloses
an example of the indirectly heated type flowmeter. The flowmeter
disclosed in this publication uses an electrical circuit (detection
circuit) containing a bridge circuit to achieve the electrical
output corresponding to a flow rate of fluid.
[0005] It is general that the output of the electrical circuit of
the flowmeter is not in a simply proportional relationship with the
flow rate value, and variation of the output of the electrical
circuit to the flow rate variation is large in an area where the
flow rate value is small while the variation of the output of the
electrical circuit to the flow rate variation is small in an area
where the flow rate value is large. Therefore, there is a problem
that even when little error occurs on measured flow rate values due
to the variation of the output of the electrical circuit in the
small flow rate value area, the error is increased in the large
flow rate value area (that is, the rate of the flow rate difference
to be discriminable when the measurement is carried out is
increased).
[0006] In order to avoid this problem, it has been hitherto general
that a flowmeter is prepared for each relatively narrow flow rate
range and the characteristic value of the electrical circuit is
properly set every flow rate range. Therefore, if attention is paid
to each individual flowmeter, it has a problem that the dynamic
range of the flow rate measurement is small and the application of
the indirectly heated type flowmeter suffers restriction.
[0007] Therefore, an object of the present invention is to provide
an indirectly heated type flowmeter which can perform a flow rate
measurement with excellent precision over a broad flow rate
range.
[0008] In the flowmeter disclosed in JP(A)-11-118566, the voltage
to be applied to the heating element is varied in accordance with
the variation of the flow rate to thereby vary the heating state of
the heating element so that the temperature sensing element is kept
to a predetermined temperature (heating state), and the flow rate
value is achieved on the basis of the voltage applied to the
heating element at this time.
[0009] The environmental temperature at which the flowmeter is used
is broad. For example, when the flowmeter is used in a cold
district, the temperature of the flowmeter may be kept under
5.degree. C. or less. On the other hand, when the flowmeter is used
in a hot district, the temperature of the flowmeter may be kept at
35.degree. C. or more. Even when the flowmeter is used in the same
district, the environmental temperature of the flowmeter is varied
in accordance with day and night. Accordingly, when the flow rate
value is achieved on the basis of the voltage to be applied to the
heating element as described above, there is a problem that the
measurement value is varied in accordance with the environmental
temperature, which is caused by variation of the characteristic of
the electrical circuit of the flowmeter due to the temperature
variation.
[0010] The present invention has another object to improve the
control of an applied voltage to the heating element in the
indirectly heated type flowmeter as described above and achieve
high precision and high control response without complicating the
circuit construction.
[0011] Further, the present invention has another object to prevent
the variation of the measurement value due to the environmental
temperature in the indirectly heated type flowmeter as described
above, and further enhance the precision of the flowmeter.
[0012] When the flow rate detection is carried out by using the
thermal type flowmeter, the following problems occur due to the
variation of the temperature of fluid for which the flow rate is
detected.
[0013] For example, in a case where a kerosene flow passage is
formed by a pipe so as to extend from a kerosene tank disposed
outdoors to kerosene burning equipment disposed indoors and a
flowmeter is disposed in an indoor portion of the pipe, if there is
a large difference between the outdoor temperature and the indoor
temperature (for example, the difference in temperature may be
equal to about 20.degree. C. in the winter season), kerosene
remaining in the indoor portion of the pipe first passes through
the flowmeter at the initial stage where use of the kerosene
burning equipment is started, and after some amount of kerosene
passes through the flowmeter, kerosene existing in the outdoor
portion of the pipe at the initial stage reaches the flowmeter to
be detected in flow rate.
[0014] In most cases, a fluid temperature detecting unit containing
a temperature sensing element for temperature compensation
installed in a fluid flow rate detecting circuit is disposed at a
position different from that of a fluid flow rate detecting unit,
or even when they are disposed to be near to each other, a
heat-exchange portion of the fluid flow rate detecting unit at
which heat exchange is actually carried out to detect the fluid
flow rate is far away from a heat-exchange portion of the fluid
temperature detecting unit at which heat exchange is actually
carried out to detect the fluid temperature. In these cases, if
fluid which quickly varies in temperature flows into the flowmeter
as described above, there appears temporarily such a state that the
fluid temperature when the heat-exchange with the fluid temperature
detecting unit is carried out is different from the fluid
temperature when the heat-exchange with the fluid flow rate
detecting unit is carried out. Therefore, accurate temperature
compensation cannot be performed and thus the precision of the
fluid flow rate detection is reduced.
[0015] Therefore, the present invention has an object to provide a
flowmeter which can accurately make a fluid temperature
compensation and thus perform accurate flow rate detection even
when the temperature of fluid flowing into the flowmeter quickly
varies.
SUMMARY OF THE INVENTION
[0016] In order to attain the above objects, according to the
present invention, there is provided a flowmeter having an
indirectly heated type flow rate sensor unit in which a flow rate
detector containing a heating element and a temperature sensing
element for flow rate detection is joined to a heat transfer member
for flow rate detection, a fluid flow rate value being achieved
with calibration curves on the basis of the output of a detection
circuit using a bridge circuit containing the temperature sensing
element for flow rate detection as a constituent resistor,
characterized in that the bridge circuit includes circuit
characteristic value variation driving means for varying the
circuit characteristic value thereof in plural steps, plural
calibration curves are provided in association with the respective
steps of the circuit characteristic value, any one of the plural
calibration curves is selected in conformity with the step of the
circuit characteristic value selected by the circuit characteristic
value variation driving means, a fluid flow rate range to be
measured is set every calibration curve, the overall measurement
flow rate range is covered by the plural fluid flow rate ranges,
and the circuit characteristic value variation driving means is
controlled in accordance with the fluid flow rate value achieved to
use one of the calibration curves corresponding to the flow rate
range to which the flow rate value belongs.
[0017] In an aspect of the present invention, the respective
neighboring flow rate ranges are partially overlapped with each
other, and the selective switching from one of the two calibration
curves corresponding to the two partially-overlapped flow rate
ranges to the other calibration curve is carried out at the end
portion of the one flow rate range.
[0018] In an aspect of the present invention, the flow rate sensor
unit has a fluid temperature detector containing a fluid
temperature detecting temperature sensing element and a fluid
temperature detecting heat transfer member joined to the fluid
temperature detector, and the bridge circuit contains the fluid
temperature detecting temperature sensing element as a constituent
resistor.
[0019] In an aspect of the present invention, the circuit
characteristic value variation driving means is a multiplexer for
selectively connecting an output terminal of the bridge circuit to
any one of connection terminals between any two neighboring
resistors of plural resistors which are provided to said bridge
circuit so as to be connected to one another in series.
[0020] In an aspect of the present invention, the circuit
characteristic value variation driving means carries out a
switch-ON/OFF operation of switches of a bypass which are connected
in parallel to at least one of plural resistors which are provided
to the bridge circuit so as to be connected in series to one
another. In an aspect of the present invention, each of the
switches comprises a field effect transistor.
[0021] In order to attain the above objects, according to the
present invention, there is provided a thermal type flowmeter
having a heating element and a flow rate detection circuit
containing a temperature sensing element for flow rate detection
disposed so as to be affected by the heating of the heating element
and perform heat transfer from/to fluid, the heating of the heating
element being controlled on the basis of a voltage applied to the
heating element, the applied voltage to the heating element being
controlled on the basis of the output of the flow rate detection
circuit and the flow rate of the fluid being measured on the basis
of the applied voltage, characterized in that the applied voltage
to the heating element is equal to the total voltage of a base
voltage the value of which is set every predetermined time period
and invariable within each predetermined time period, and an
addition voltage which has a fixed value and is variable in voltage
applying time, a comparator for comparing the output of the flow
rate detecting circuit with a reference value is provided to output
a binary signal having a first level indicating that the heating of
the temperature sensing element is insufficient and a second level
indicating the other cases, the binary signal output from the
comparator is sampled at a predetermined period to count the
appearance frequency of the first level every predetermined time
period and achieve the count value thereof every predetermined time
period, the base voltage is adjusted so that when the count value
is within a predetermined range, the value of the base voltage in a
subsequent predetermined time period is not changed, when the count
value is larger than the upper limit of the predetermined range,
the value of the base voltage in the subsequent predetermined time
period is increased by a predetermined step value and when the
count value is smaller than the lower limit of the predetermined
range, the value of the base voltage in the subsequent time period
is reduced by the predetermined step value, and the addition
voltage is applied during only a time period when the binary signal
output from the comparator has the first level.
[0022] In an aspect of the present invention, the addition voltage
is set to two times to four times of the step value of the base
voltage. In an aspect of the present invention, the predetermined
range of the count value has a lower limit value which is smaller
than a half of the sampling frequency within the predetermined time
period and larger than zero, and has an upper limit value which is
larger than a half of the sampling frequency within the
predetermined time period and smaller than the sampling
frequency.
[0023] In order to attain the above objects, according to the
present invention, there is provided a thermal type flowmeter
having a heating element and a flow rate detection circuit
containing a temperature sensing element for flow rate detection
disposed so as to be affected by the heating of the heating element
and perform heat transfer from/to fluid, the heating of the heating
element being controlled on the basis of a voltage applied to the
heating element, the applied voltage to the heating element being
controlled on the basis of the output of the flow rate detection
circuit and the flow rate of the fluid being measured on the basis
of the applied voltage, characterized in that the applied voltage
to the heating element is equal to the total voltage of a base
voltage the value of which is set every predetermined time period
and invariable within each predetermined time period, and an
addition voltage which has a fixed value and is variable in voltage
applying time, a comparator for comparing the output of the flow
rate detecting circuit with a reference value is provided to output
a binary signal having a first level indicating that the heating of
the temperature sensing element is insufficient and a second level
indicating the other cases, the binary signal output from the
comparator is sampled at a predetermined period to count the
appearance frequency of the first level every predetermined time
period and achieve the count value thereof every predetermined time
period, the base voltage is adjusted so that when the count value
is within a predetermined range, the value of the base voltage in a
subsequent predetermined time period is not changed, when the count
value is larger than the upper limit of the predetermined range,
the value of the base voltage in the subsequent predetermined time
period is increased by a predetermined step value and when the
count value is smaller than the lower limit of the predetermined
range, the value of the base voltage in the subsequent time period
is reduced by the predetermined step value, the addition voltage is
applied during only a time period when the binary signal output
from the comparator has the first level, and a data interpolating
calculation is made by using an instantaneous flow rate converting
table comprising plural individual calibration curves which
indicate the relationship between the applied voltage to the
heating element and the instantaneous flow rate every discrete
temperature value, thereby achieving an instantaneous flow rate
value at an environmental temperature.
[0024] In an aspect of the present invention, the individual
calibration curves are created for discrete values of possible
values of the voltage to be applied to the heating element, and
when the instantaneous flow rate value is achieved, a data
interpolation calculation is carried out to achieve the
instantaneous flow rate value corresponding to a voltage value
applied to the heating element. In an aspect of the present
invention, the discrete values are set to the minimum values of
values having the same high-order bit values of possible digital
values of the voltage to be applied to the heating element, and
when the data interpolation calculation is carried out, the
instantaneous flow rate values corresponding to first discrete
values having the same high-order bit values as the voltage value
applied to the heating element and second discrete values whose
high-order bit values are larger than that of the voltage value
applied to the heating element by 1 are achieved by the individual
calibration curves.
[0025] In an aspect of the present invention, the addition voltage
is set to two times to four times of the step value of the base
voltage.
[0026] In an aspect of the present invention, the predetermined
range of the count value has a lower limit value which is smaller
than a half of the sampling frequency within the predetermined time
period and larger than zero, and has an upper limit value which is
larger than a half of the sampling frequency within the
predetermined time period and smaller than the sampling
frequency.
[0027] In order to attain the above objects, according to the
present invention, there is provided a flowmeter having an
indirectly heated type flow rate sensor unit in which a flow rate
detector containing a heating element and a temperature sensing
element for flow rate detection is joined to a heat transfer member
for flow rate detection, a fluid flow rate value being achieved
with a calibration curve on the basis of the output of a detection
circuit containing the temperature sensing element for flow rate
detection, characterized in that the calibration curve comprises
three portions corresponding to three areas of the output value of
the detection circuit, and the three portions are represented by
quaternary functions which use the output of the detection circuit
as variables and are different in coefficient, a fluid flow rate
value being achieved by using the portion of the calibration curve
which corresponds to the area to which the output value of said
detection circuit belongs.
[0028] In an aspect of the present invention, the calibration curve
is represented as follows:
f=a.sub.1v.sup.4+b.sub.1v.sup.3+c.sub.1v.sup.2+d.sub.1v+e.sub.1
(0.ltoreq.v<v.sub.1)
f=a.sub.2v.sup.4+b.sub.2v.sup.3+c.sub.2v.sup.2+d.sub.2v+e.sub.2
(v.sub.1.ltoreq.v.vertline..sub.2)
f=a.sub.3v.sup.4+b.sub.3v.sup.3+c.sub.3V.sup.2+d.sub.3v+e.sub.3
(v.ltoreq.v)
[0029] wherein f represents the fluid flow rate, v represents the
output of the detection circuit, and a.sub.1, b.sub.1, c.sub.1,
d.sub.1, et; a.sub.2, b.sub.2, c.sub.2, d.sub.2, e.sub.z; a.sub.3,
b.sub.3, c.sub.3, d.sub.3, e.sub.3 represent coefficients.
[0030] In an aspect of the present invention, the detection circuit
comprises a bridge circuit. In an aspect of the present invention,
the flow rate sensor unit has a fluid temperature detector
containing a fluid temperature detecting temperature sensing
element and a fluid temperature detecting heat transfer member
joined to the fluid temperature detector, and the detection circuit
contains the fluid temperature detecting temperature sensing
element.
[0031] In order to attain the above objects, according to the
present invention, there is provided a thermal type flowmeter which
includes a casing member having a fluid flow passage extending from
a fluid flow-in port to a fluid flow-out port, a flow rate
detecting unit which is secured to the casing member and varies in
electrical characteristic value in accordance with the flow of the
fluid in the fluid flow passage through the heat exchange between
the flow rate detecting unit and the fluid in the fluid flow
passage, and a fluid temperature detecting unit which is secured to
the casing member and varies in electrical characteristic value in
accordance with the temperature of the fluid through the heat
exchange between the fluid temperature detecting unit and the fluid
in the fluid flow passage, the flow rate of the fluid being also
detected on the basis of the electrical characteristic value of the
fluid temperature detecting unit, characterized in that a fluid
residence area is formed at the upstream side, with respect to the
flow of the fluid, of each of the position at which the heat
exchange between the flow rate detecting unit and the fluid is
carried out and the position at which the heat exchange between the
fluid temperature detecting unit and the fluid is carried out, the
fluid residence area having a flow cross section which is five
times or more, preferably ten times or more, as large as the flow
cross section at the position where the heat exchange between the
flow rate detecting unit and the fluid is carried out or at the
position where the heat exchange between the fluid temperature
detecting unit and the fluid is carried out.
[0032] In order to attain the above objects, according to the
present invention, there is provided a thermal type flowmeter which
includes a casing member having a fluid flow passage extending from
a fluid flow-in port to a fluid flow-out port, a flow rate
detecting unit which is secured to the casing member and varies in
electrical characteristic value in accordance with the flow of the
fluid in the fluid flow passage through the heat exchange between
the flow rate detecting unit and the fluid in the fluid flow
passage, and a fluid temperature detecting unit which is secured to
the casing member and varies in electrical characteristic value in
accordance with the temperature of the fluid through the heat
exchange between the fluid temperature detecting unit and the fluid
in the fluid flow passage, a fluid-temperature-compensated flow
rate of the fluid being detected by a detection circuit containing
the flow rate detecting unit and the fluid temperature detecting
unit, characterized in that a fluid residence area is formed at the
upstream side, with respect to the flow of the fluid, of each of
the position at which the heat exchange between the flow rate
detecting unit and the fluid is carried out and the position at
which the heat exchange between the fluid temperature detecting
unit and the fluid is carried out, the flow velocity of the fluid
at the fluid residence area being equal to 1/5 or less, preferably
{fraction (1/10)} or less, of the flow velocity of the fluid at the
position where the heat exchange between the flow rate detecting
unit and the fluid is carried out or at the position where the heat
exchange between the fluid temperature detecting unit and the fluid
is carried out.
[0033] In an aspect of the present invention, the volume of the
fluid residence area is 50 times or more as large as the volume per
unit length of the fluid flow passage in the fluid flow direction
at the position where the heat exchange between the flow rate
detecting unit and the fluid is carried out or at the position
where the fluid temperature detecting unit and the fluid is carried
out.
[0034] In an aspect of the present invention, the fluid flow
passage comprises a first flow passage part intercommunicating with
the fluid flow-in port, and a second flow passage part
intercommunicating with the fluid flow-out port, at which the heat
exchange between the flow rate detecting unit and the fluid is
carried out and the heat exchange between the fluid temperature
detecting unit and the fluid is carried out, the fluid residence
area is located between the first flow passage part and the second
flow passage part, and the flow cross section of the first flow
passage part is smaller than the flow cross section of the fluid
residence area.
[0035] In an aspect of the present invention, the second flow
passage part has a part extending in parallel to the fluid
residence area at the position where the heat exchange between the
flow rate detecting unit and the fluid is carried out and at the
position where the heat exchange between the fluid temperature
detecting unit and the fluid is carried out.
[0036] In an aspect of the present invention, a filter is
interposed at the intercommunication portion between the fluid
residence area and the second flow passage part. In an aspect of
the present invention, the casing member is formed of metal. In an
aspect of the present invention, the flow rate detecting unit is
designed so that a heating element, a flow rate detecting
temperature sensing element and a flow rate detecting heat transfer
member extending into the fluid flow passage are arranged so as to
perform heat transfer thereamong, and the fluid temperature
detecting unit is designed so that a fluid temperature detecting
temperature sensing element and a fluid temperature detecting heat
transfer member extending into the fluid flow passage are arranged
so as to perform heat transfer therebetween.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 is a circuit diagram showing an embodiment of a
flowmeter according to the present invention;
[0038] FIG. 2 is a diagram showing a part of the construction of
the embodiment of the flowmeter according to the present
invention;
[0039] FIGS. 3A and 3B are schematic cross-sectional views showing
a flow rate sensor unit of the embodiment of the flowmeter
according to the present invention;
[0040] FIG. 4 is a perspective view showing the flow rate sensor
unit of the embodiment of the flowmeter according to the present
invention;
[0041] FIGS. 5A and 5B are schematic cross-sectional views showing
a modification of the flow rate sensor unit shown in FIGS. 3A and
3B;
[0042] FIG. 6 is an exploded perspective view showing the
construction of a flow rate detector;
[0043] FIG. 7 is an exploded perspective view showing the
construction of a fluid temperature detector;
[0044] FIG. 8 is a diagram showing an example of calibration curves
in the flowmeter of the present invention;
[0045] FIG. 9 is a partial circuit diagram showing another
embodiment of the flowmeter according to the present invention;
[0046] FIG. 10 is a circuit diagram showing an embodiment of the
flowmeter according to the present invention;
[0047] FIG. 11 is a partial detailed diagram of the circuit diagram
of FIG. 10;
[0048] FIG. 12 is a partial detailed diagram of the circuit diagram
of FIG. 10;
[0049] FIG. 13 is a cross-sectional view showing a flow rate
detection portion of the flowmeter;
[0050] FIG. 14 is a cross-sectional view showing a flow rate
detecting unit;
[0051] FIG. 15 is a time chart showing heater voltage control;
[0052] FIG. 16 is a time chart showing variation of a heater
voltage and calculation of a flow rate value;
[0053] FIG. 17 is a circuit diagram showing an embodiment of the
flowmeter according to the present invention;
[0054] FIG. 18 is a partial detailed diagram of the circuit diagram
of FIG. 17;
[0055] FIG. 19 is a diagram showing an instantaneous flow rate
converting table;
[0056] FIG. 20 is a flowchart showing the operation of a flow rate
integrating circuit of the flowmeter;
[0057] FIG. 21 is a diagram showing error measurement data of the
flowmeter;
[0058] FIG. 22 is a schematic diagram showing a flow rate detection
system of the flowmeter according to the present invention;
[0059] FIG. 23 is a cross-sectional view showing the flow rate
detecting unit of the flowmeter according to the present
invention;
[0060] FIG. 24 is a diagram showing an example of a calibration
curve in the flowmeter according to the present invention;
[0061] FIG. 25 is a cross-sectional view showing an embodiment of
the flowmeter according to the present invention;
[0062] FIG. 26 is a partial cross-sectional view showing the
embodiment of the flowmeter according to the present invention;
[0063] FIG. 27 is a front view showing the embodiment of the
flowmeter according to the present invention;
[0064] FIG. 28 is a right side view showing the embodiment of the
flowmeter according to the present invention;
[0065] FIG. 29 is a bottom view of the embodiment of the flowmeter
according to the present invention when a lid member is
removed;
[0066] FIG. 30 is a left side view showing the embodiment of the
flowmeter according to the present invention;
[0067] FIG. 31 is a plan view showing the embodiment of the
flowmeter according to the present invention; and
[0068] FIG. 32 is a schematic diagram showing a flow rate detection
system of the embodiment of the flowmeter according to the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0069] Preferred embodiments according to the present invention
will be described hereunder with reference to the accompanying
drawings.
[0070] FIG. 1 is a diagram showing the circuit construction of an
embodiment of a flowmeter according to the present invention, and
FIG. 2 is a diagram showing a partial construction of the flowmeter
of FIG. 1.
[0071] FIGS. 3A and 3B are schematic cross-sectional views showing
a flow rate sensor unit according to the embodiment, wherein FIG.
3A shows a state that the flowmeter is secured to a flow passage
member having a fluid flow passage, and FIG. 3B is a
cross-sectional view taken along X-X of the flow rate sensor unit
of FIG. 3A. FIG. 4 is a perspective view showing the flow rate
sensor unit of the embodiment. FIGS. 5A and 5B are schematic
cross-sectional views showing a modification of the flow rate
sensor unit shown in FIGS. 3A and 3B, wherein FIG. 5B is a
cross-sectional view taken along X-X of FIG. 5A. FIG. 6 is an
exploded perspective view showing the construction of a flow rate
detector, and FIG. 7 is an exploded perspective view showing the
construction of a fluid temperature detector. First, the
construction of a flow rate sensor unit according to this
embodiment will be described with reference to FIGS. 3A to 7. As
shown in FIGS. 3A and 3B, a flow rate detector 5 is joined to the
surface of a fin plate 6 serving as a heat transfer member for flow
rate detection, and a fluid temperature detector 9 is joined to the
surface of a fin plate 10 serving as a heat transfer member for
fluid temperature detection. The flow rate detector 5, the fluid
temperature detector 9 and parts of the fin plates 6, 10 are
accommodated in a housing 2.
[0072] As shown in FIG. 6, the flow rate detector 5 is designed in
the form of a chip by laminating, in the following order, a
thin-film temperature sensing element 31 for flow rate detection,
an interlayer insulating film 32, a thin-film heating element 33
with its electrodes 34, 35 and a protection film 36 on a
rectangular substrate of about 0.4 mm in thickness and about 2 mm
in square formed of silicon, alumina or the like and then forming a
pad layer 37 so that the bonding portion of the thin-film
temperature sensing element 31 for flow rate detection and the
heating element electrodes 34, 35 are covered by the pad layer
37.
[0073] As the thin-film temperature sensing element 31 may be used
a metal resistance film having a high and stable temperature
coefficient such as platinum (Pt), nickel (Ni) or the like which is
patterned to have a thickness of about 0.5 to 1 .mu.m and have a
desired shape, for example, a meandering shape. Alternatively, a
manganese oxide-based NTC thermistor or the like may be used. Each
of the interlayer insulating film 32 and the protection film 36 may
be formed of SiO.sub.2 and have a thickness of about 1 .mu.m. As
the thin-film heating element 33 may be used a resistor patterned
to have a thickness of about 1 .mu.m and have a desired shape,
which is formed of Ni, Ni--Cr, Pt or cermet such as Ta--SiO.sub.2,
Nb--SiO.sub.2 or the like. The heating element electrode 34, 35 may
be formed of a Ni thin film having a thickness of about 1 .mu.m, or
both of a Ni thin film of about 1 .mu.m in thickness and a gold
(Au) thin film of about 0.5 am laminated on the Ni thin film. As
the pad layer 37 may be used an Au thin film or Pt thin film of 0.2
mm.times.0.15 mm in area and about 0.1 .mu.m in thickness.
[0074] As shown in FIG. 7, the fluid temperature detector 9 has
substantially the same construction as the flow rate detector 5
except that the heating element 33, etc. are removed from the
construction of the flow rate detector 5, that is, a thin-film
temperature sensing element 31' for fluid temperature detection
similar to the thin-film temperature sensing element 31 for fluid
temperature detection and a protection film 36' similar to the
protection film 36 are laminated in this order on a substrate 30'
similar to the substrate 30, and also a pad layer 37' is formed so
as to cover the bonding portion of the thin-film temperature
sensing element 31' for fluid temperature detection, thereby
finally achieving the fluid temperature detector 9 as a chip.
[0075] One surface of the fin plate 6 (10) at one end portion
thereof is joined to the surface of the flow rate detector 5 (the
fluid temperature detector 9) at the substrate 30 (30') side
thereof through a joint member having excellent thermal
conductivity. The fin plate 6, 10 may comprise a rectangular plate
of about 0.2 mm in thickness and about 2 mm in width which is
formed of copper, duralumin, copper-tungsten alloy or the like.
Silver paste may be used as the joint member.
[0076] As shown in FIGS. 3A and 3B, the housing 2 of the sensor
unit is accommodated in a sensor unit arrangement portion formed in
the flow passage member 14, and the other end portions of the fin
plates 6, 10 extend into the fluid flow passage 13 formed in the
fluid flow passage member 14. The fin plates 6, 10 extend to pass
through the center of the cross section in the fluid flow passage
13 having a substantially circular cross section. The fin plates 6,
10 are arranged along the flow direction (indicated by an arrow in
FIG. 1) of the fluid in the fluid flow passage 13, so that the heat
exchange between the fluid and each of the flow rate detector 5 and
the fluid temperature detector 9 can be excellently carried out
without greatly disturbing the fluid flow.
[0077] The housing 2 and the fluid flow passage member 14 may be
formed of synthetic resin. The respective electrode terminals
(pads) of the flow rate detector 5 and the fluid temperature
detector 9 are connected to the inner lead portions (the in-housing
portions) of the respective leads 7, 11 by Au wires 8, 12. Each of
the leads 7, 11 extend to the outside of the housing 2 so as to be
partially exposed to the outside, thereby forming an outer lead
portion.
[0078] In FIGS. 3A and 3B, the flow rate detector 5, the fluid
temperature detector 9, parts of the fin plates 6, 10 and the inner
lead portions are sealingly accommodated in the housing 2 by resin
filling. However, a space 23 may be formed in the housing 22 as
shown in a modification of FIGS. 5A and 5B.
[0079] Next, the circuit construction of the flowmeter of this
embodiment having the sensor unit as described above will be
described with reference to FIGS. 1 and 2.
[0080] As shown in FIG. 1, alternating power of 100V is used as a
power supply source, and DC voltages of +15V, -15V and +5V are
output from a DC converting circuit 71 by using the alternating
power of 100V. The DC voltage of +15V output from the DC converting
circuit 71 is input to a voltage stabilizing circuit 72.
[0081] The stabilized DC voltage supplied from the voltage
stabilizing circuit 72 is supplied to a bridge circuit 73. As shown
in FIG. 2, the bridge circuit 73 contains the temperature sensing
element 31 for flow rate detection and resistors 74, 90 which are
connected to one another in series, and the temperature sensing
element 31' for temperature compensation and resistors 75, R1 to R7
which are connected to one another in series. The bridge circuit 73
is equipped with a multiplexer 731 serving as circuit
characteristic value variation driving means, and the multiplexer
731 selectively connects the connection terminal b to one of the
connection terminals c1 to c8 each of which is connected to a point
between the respective two neighboring resistors of the resistors
75, R1 to R7. The characteristic value of the bridge circuit 73 can
be varied in plural steps by the selection of any one of the
connection terminals 1 to c8. The potential Va, Vb at the point a,
b of the bridge circuit 73 is input to a differential amplifying
circuit 76 of variable amplification factor. The output of the
differential amplifying circuit 76 is input to an integrating
circuit 77.
[0082] The output of the voltage stabilizing circuit 72 is supplied
to the thin-film heating element 33 through a field effect
transistor 81 for controlling the current to be supplied to the
thin-film heating element 33. That is, in the flow rate detector 5,
the thin-film temperature sensing element 31 executes the
temperature sensing (detecting) operation on the basis of the
heating of the thin-film heating element 33 with being affected by
the endothermic action of fluid to be detected through the fin
plate 6. As the result of the temperature sensing operation is
achieved the difference between the potential Va at the point of a
and the potential Vb at the point of b in the bridge circuit 73
shown in FIG. 2.
[0083] The value of (Va-Vb) is varied due to variation of the
temperature of the temperature sensing element 31 for flow rate
detection and the thin-film temperature sensing element 31' for
fluid temperature detection in accordance with the flow rate of the
fluid. The value (Va-Vb) may be set to zero at a different fluid
flow rate in accordance with selection of one of the terminals c1
to c8 to be connected to the terminal b by the multiplexer 731. At
these flow rates, the output of the differential amplifying circuit
76 is equal to zero, and thus the output of the integrating circuit
77 is equal to a fixed value.
[0084] The output of the integrating circuit 77 is input to a V/F
converting circuit 78 to form a pulse signal having the frequency
(for example, 5.times.10.sup.-5 at maximum) corresponding to the
voltage signal. The pulse signal has a fixed pulse width (time
width) (for example, a desired value from 1 to 10 microseconds).
For example, when the output of the integrating circuit 77 is equal
to 1V, a pulse signal having a frequency of 0.5 kHz is output. When
the output of the integrating circuit 77 is equal to 4V, a pulse
signal having a frequency of 2 kHz is output. The bridge circuit
73, the differential amplifying circuit 76, the integrating circuit
77 and the V/F converting circuit 78 constitute the detection
circuit.
[0085] The output of the V/F converting circuit 77 is supplied to
the gate of a transistor 81, and current is supplied to the
thin-film heating element 33 through the transistor 81 the gate of
which is supplied with the pulse signal. Accordingly, the thin-film
heating element 33 is supplied with a divided voltage of the output
voltage of the voltage stabilizing circuit 72 through the
transistor 81 in the form of a pulse at the frequency corresponding
to the output value of the integrating circuit 77, so that the
current flows through the thin-film heating element 33
intermittently to thereby heat the thin-film heating element 33.
The frequency of the V/F converting circuit 77 is set on the basis
of high-precision clocks which are set on the basis of oscillation
of a temperature-compensated type quartz oscillator 79.
[0086] The pulse signal output from the V/F converting circuit 77
is counted by a pulse counter 82. A microcomputer 83 converts the
pulse count result (pulse frequency) to the corresponding flow rate
(instantaneous flow rate) on the basis of a reference frequency
generated in a reference frequency generating circuit 80, and
integrates the flow rate thus achieved with respect to the time,
thereby calculating an integrated flow rate.
[0087] The selection of the connection between the connection
terminal b and one of the connection terminals c1 to c8 through the
multiplexer 731 is controlled by the microcomputer 83 as shown in
FIG. 2. The selection of the connection terminals c1 to c8 by the
microcomputer 83 and the conversion to the flow rate are performed
as follows.
[0088] That is, calibration curves for the conversion to the flow
rate are stored in a memory 84. FIG. 8 shows examples of the
calibration curves. These calibration curves contain S.sub.1,
S.sub.2, S.sub.3, . . . , and these calibration curves are used in
association with the circuit characteristic value steps when the
connection terminals c1, c2, c3 . . . are selected by the
multiplexer 731, respectively. The calibration curves S.sub.1,
S.sub.2, S.sub.3, . . . constitute a data table achieved by
selectively connecting the connection terminal b and each of the
connection terminals c1, c2, c3, . . . and, under this state,
measuring the output (pulse frequency) of the pulse counter 82
every actual flow rate of the fluid.
[0089] In FIG. 8, the calibration curve S.sub.1 is mainly used when
the connection terminal c1 of the multiplexer 731 is selected to
measure a flow rate range from 0 to F.sub.12, S.sub.2 is mainly
used when the connection terminal c2 of the multiplexer 731 is
selected to measure a flow rate range from F.sub.11 to F.sub.22,
S.sub.3 is mainly used when the connection terminal c3 of the
multiplexer 731 is selected to measure a flow rate range from
F.sub.21 to F.sub.32, and so forth. Here, as shown in FIG. 8,
F.sub.1, <F.sub.12<F.sub.21<F.sub.22<F.sub.31<F-
.sub.32, and the flow rate value F.sub.11 corresponds to the output
values fill, f.sub.112 of the calibration curves S.sub.1, S.sub.2
respectively, the flow rate value F.sub.12 corresponds to the
output values f.sub.121, f.sub.122 of the calibration curves
S.sub.1, S.sub.2 respectively, the flow rate value F.sub.21
corresponds to the output values f.sub.211, f.sub.212 of the
calibration curves S.sub.2, S.sub.3 respectively, the flow rate
value F.sub.22 corresponds to the output values f.sub.221,
f.sub.222 of the calibration curves S.sub.2, S.sub.3 respectively,
and so forth. That is, the neighboring flow rate ranges are
partially overlapped with each other, and the overall measuring
flow rate range is covered by these flow rate ranges.
[0090] The microcomputer 83 first instructs the multiplexer 731 to
select a connection terminal cn to measure a certain flow rate
range [for example, makes an instruction to select the connection
terminal c2 to measure the flow rate range from F.sub.11 to
F.sub.22] when the flow rate of the fluid to be detected is
measured. Thereafter, the pulse frequency achieved from the counter
82 is converted to the flow rate by using a calibration curve Sn
[for example, the calibration curve S.sub.2]. When the flow rate
value thus achieved is within the flow rate range corresponding to
the selected connection terminal cn [for example, from F.sub.11 to
F.sub.22], the selection of the connection terminal cn [for
example, c2] by the multiplexer 731 is kept.
[0091] On the other hand, when the flow rate value achieved is out
of the flow rate range corresponding to the selected connection
terminal cn [out of the range from F.sub.11 to F.sub.22], the
microcomputer instructs the multiplexer 731 to select the
connection terminal cm [for example, c3] in order to measure the
flow rate range [for example, from F.sub.21 to F.sub.32] to which
the flow rate value achieved belongs. Likewise, the pulse frequency
achieved from the counter 82 is converted to the flow rate by using
a calibration curve S.sub.1 [for example, S.sub.3]. When the flow
rate value thus achieved is within the flow rate range [for
example, from F.sub.21 to F.sub.32] corresponding to the selected
connection terminal cm, the selection of the connection terminal cm
by the multiplexer 731 is kept. On the other hand, when the flow
rate value thus achieved is out of the flow rate range [for
example, from F.sub.21 to F.sub.32] corresponding to the selected
connection terminal cm, the microcomputer instructs the multiplexer
731 to select a connection terminal to measure a flow rate range to
which the flow rate value thus achieved belongs.
[0092] Likewise, on the basis of the measured flow rate value
achieved, the microcomputer 83 controls the multiplexer 731 to
achieve a bridge circuit characteristic required to measure the
flow rate value concerned at all times (specifically, select one of
the connection terminals), and performs the flow rate value
measurement based on a proper calibration curve.
[0093] When some variation occurs in the flow rate while the flow
rate is measured and thus the flow rate is out of the flow rate
range corresponding to the selected calibration curve, the
selection of the current calibration curve is switched to the
calibration curve corresponding to a flow rate range which is
adjacent to and partially overlapped with the current flow rate
range (the selection of the connection terminal is switched under
the control of the multiplexer 731). Accordingly, the selective
switching between the calibration curves for the neighboring flow
rate ranges is carried out with directionality at the end portion
of each flow rate range (for example, only the switching from the
calibration curve S.sub.3 to the calibration curve S.sub.2 is
carried out at the flow rate value F.sub.21, and only the switching
from the calibration curve S.sub.2 to the calibration curve S.sub.3
is carried out at the flow rate value F.sub.22) as shown in FIG. 8.
With this setting, even when variation of the flow rate occurs in
the neighborhood of the switching flow rate value occurs, it is
unnecessary to selectively switch the calibration curve frequently,
and thus stability of the measurement can be kept.
[0094] As shown in FIG. 8, each of the calibration curves has a
moderate slope (the rate of variation of the output pulse frequency
to variation of the flow rate) in the flow rate range in which the
calibration curve is mainly used, and the moderate flow rate
variation with respect to the output variation can be implemented.
Accordingly, in the flowmeter of this embodiment which is
controlled to use the calibration curve S (indicated by a solid
line in FIG. 8) constructed by predetermined respective flow rate
range portions of the calibration curves, the flow rate measurement
can be performed in a broad flow rate range with excellent
precision.
[0095] When the flow rate of the fluid is increased/reduced, the
output of the differential amplifying circuit 76 is varied in
polarity (varied in accordance with the sign (positive or negative)
of the resistance-temperature characteristic of the temperature
sensing element 31 for flow rate detection) and magnitude in
accordance with the value of (Va-Vb), and the output of the
integrating circuit 77 is varied in accordance with this variation
of the output of the differential amplifying circuit 76. The
variation rate of the output of the integrating circuit 77 can be
adjusted by setting the amplification factor of the differential
amplifying circuit 76. The response characteristic of the control
system is set by the integrating circuit 77 and the differential
amplifying circuit 76.
[0096] When the fluid flow rate is increased, the temperature of
the temperature sensing element 31 for flow rate detection is
reduced. Therefore, such an output (higher voltage value) that the
heating value of the thin-film heating element 33 is increased
(that is, the pulse frequency is increased) can be achieved from
the integrating circuit 77, and the bridge circuit 73 is set to the
equilibrium state at the time point when the output of the
integrating circuit is equal to the voltage value corresponding to
the fluid flow rate.
[0097] On the other hand, when the fluid flow rate is reduced, the
temperature of the temperature sensing element 31 for flow rate
detection is increased. Therefore, such an output (lower voltage
value) that the heating value of the thin-film heating element 33
is reduced (that is, the pulse frequency is reduced) can be
achieved from the integrating circuit 77, and the bridge circuit 73
is set to the equilibrium state at the time point when the output
of the integrating circuit 77 is equal to the voltage corresponding
to the fluid flow rate.
[0098] That is, in the control system of this embodiment, the
frequency (corresponding to the heating value) of the pulse current
to be supplied to the thin-film heating element 33 is set so that
the bridge circuit 73 is set to the equilibrium state, and
implementation of the equilibrium state (the response of the
control system) as described above can be performed within 0.1
second, for example.
[0099] Accordingly, according to the flowmeter of this embodiment,
even when the flow rate value to be detected is varied in a broad
range and thus it is deviated from each flow rate range, a proper
bridge circuit characteristic conformed with the flow rate range to
which the flow rate value to be detected belongs can be immediately
set, so that the flow rate measurement can be performed with high
precision on the basis of the bridge circuit characteristic thus
set.
[0100] The instantaneous flow rate and the integrated flow rate
thus achieved are displayed on a display portion 25, and also
transmitted to the outside through a communication line comprising
a telephone line or other networks. Further, the data of the
instantaneous flow rate and the integrated flow rate may be stored
in the memory 84 if necessary.
[0101] In FIG. 1, reference numeral 85 represents a backup battery
(for example, cell).
[0102] According to the above-described embodiment, the pulse
signal generated in the V/F conversion circuit 78 is used to
measure the flow rate, and it is easy to sufficiently reduce the
error of the pulse signal due to the temperature variation.
Therefore, the errors of the flow rate value and the integrated
flow rate value achieved on the basis of the pulse frequency can be
reduced. Further, according to this embodiment, the control of the
current supply to the thin-film heating element 33 is performed by
ON/OFF based on the pulse signal generated in the V/F converting
circuit 78. Therefore, the probability that a control error due to
the temperature variation occurs is extremely small.
[0103] Further, this embodiment uses a minute chip containing the
thin-film heating element and the thin-film temperature sensing
element as the flow rate detector, so that the high-speed response
as described above can be implemented and the precision of the flow
rate measurement is excellent.
[0104] FIG. 9 is a partial circuit diagram showing another
embodiment of the flowmeter according to the present invention. In
FIG. 9, the elements and parts having the same functions as shown
in FIGS. 1 to 8 are represented by the same reference numerals. In
this embodiment, the construction of the bridge circuit 73
containing the circuit characteristic value variation driving means
is different from that of the embodiment shown in FIGS. 1 to 8,
however, the other portions are substantially the same as the
embodiment shown in FIGS. 1 to 8.
[0105] In this embodiment, the bridge circuit 73 contains the
in-series connection between the temperature sensing element 31 for
flow rate detection and the resistor 74 and the in-series
connection between the temperature sensing element 31' for
temperature compensation and resistors 75, r0 to r3. The circuit
characteristic value variation driving means of this embodiment
carries out the switch-on/off operation of a bypass containing
field effect transistors FET1 to FET3 (switching means) which are
respectively connected to the in-series connected resistors r1 to
r3 of the bridge circuit in parallel. That is, switching signals
from switching terminals SW1 to SW3 of a switching circuit 732
controlled by the microcomputer 83 are input to the gates of the
field effect transistors FET1 to FET3, respectively. The field
effect transistors FET1 to FET3 are designed so that the
source-drain resistance values thereof under the switch-ON state
are sufficiently lower than r1 to r3 (for example, several tens
m.OMEGA.) and the source-drain resistance values thereof under the
switch-OFF state are sufficiently higher than r1 to r3 (for
example, several M.OMEGA.). Accordingly, the composite resistance
value of the in-series connected portion of the resistors r0 to r3
with the bypass in the bridge circuit 73 is varied as shown in the
following Table 1 in accordance with the ON (for example, 4V) or
OFF (for example, 0V) state of the switching signals output from
the switching terminals SW1 to SW3 when r0=10 .OMEGA., r1=10
.OMEGA., r2=20 .OMEGA., r3=40 .OMEGA..
1 TABLE 1 Composite resistance value of SW3 SW2 SW1 in-series
connection portion r0 to r3 ON ON ON 10 .OMEGA. ON ON OFF 20
.OMEGA. ON OFF ON 30 .OMEGA. ON OFF OFF 40 .OMEGA. OFF ON ON 50
.OMEGA. OFF ON OFF 60 .OMEGA. OFF OFF ON 70 .OMEGA. OFF OFF OFF 80
.OMEGA.
[0106] The combination of the switching signals output from the
switching terminals SW1 to SW3 of the switching circuit 732 is
controlled by the microcomputer 83 as described above to vary the
characteristic value of the bridge circuit in plural steps as in
the case of the embodiment shown in FIGS. 1 to 8, and the flow rate
can be measured in the same manner.
[0107] FIG. 10 is a circuit diagram showing an embodiment of the
flowmeter according to the present invention, and FIGS. 11 and 12
are partially detailed diagrams of FIG. 10. FIG. 13 is a
cross-sectional view of a flow rate detection portion of the
flowmeter according to the embodiment, and FIG. 14 is a
cross-sectional view of a flow rate detecting unit or flow rate
sensor unit.
[0108] As shown in FIG. 13, a fluid flow passage 20a is formed in a
casing member 20 formed of a material having excellent thermal
conductivity such as aluminum or the like. The fluid flow passage
20 intercommunicates with a fluid flow-in port (not shown) at the
lower side thereof and with a fluid flow-out port (not shown) at
the upper side thereof, so that the fluid flows in from the fluid
flow-in port, passes upwardly along the fluid flow passage 20a and
then flows out from the fluid flow-out port (as indicated by an
arrow).
[0109] A flow rate detecting unit 24 and a fluid temperature
detecting unit 26 are secured to the casing member 20 so as to face
the flow passage 20a. As shown in FIG. 14, in the flow rate
detecting unit 24, a flow rate detector 42 is joined to the surface
of a fin plate 44 serving as a heat transfer member through a joint
member 46 having excellent thermal conductivity, and the electrode
pad of the flow rate detector 42 and an electrode terminal 48 are
connected to each other by a bonding wire 50. The flow rate
detector 42 and the bonding wire 50, a part of the fin plate 44 and
a part of the electrode terminal 48 are accommodated in a housing
52 formed of synthetic resin. The flow rate detector 42 is designed
as a chip by forming a thin-film temperature sensing element and a
thin-film heating element on a rectangular substrate so that the
thin-film temperature sensing element and the thin-film heating
element are insulated from each other, the substrate being formed
of silicon, alumina or the like to be about 0.4 mm in thickness and
about 2 mm in square.
[0110] The fluid temperature detecting unit 26 corresponds to the
construction achieved by using a fluid temperature detector in
place of the flow rate detector 42 in the flow rate detecting unit
24. In the fluid temperature detecting unit 26, "apostrophe (')" is
affixed to the same reference numerals for the members associated
with those of the flow rate detecting unit 24. The fluid
temperature detector has the same construction as the flow rate
detector 42 except that the thin-film heating element is removed
from the flow rate detector 42.
[0111] As shown in FIG. 13, the end portions of the fin plates 44,
44' projecting from the housings 52, 52' of the flow rate detecting
unit 24 and the fluid temperature detecting unit 26 extend into the
flow passage 20a of the casing member 20. The fin plates 44, 44'
extend so as to pass through the center in the cross section of the
flow passage portion 8 having substantially the circular cross
section. The fin plates 44, 44' are arranged along the flowing
direction of the fluid in the flow passage 20a, so that the heat
transfer can be excellently performed between the fluid and each of
the flow rate detector 42 and the fluid temperature detector
.DELTA.' without greatly affecting the flow of the fluid.
[0112] The tip portions of the electrode terminals 48, 48' of the
flow rate detecting unit 24 and the fluid temperature detecting
unit 26 penetrate through a circuit board 60 secured to the casing
member 20, and is connected to a flowmeter electrical circuit
portion formed on the circuit board 60. A protection cover 62 is
secured to the casing member 20 to protect the circuit board
60.
[0113] As shown in FIG. 10, a DC voltage is supplied from a
reference power supply circuit 102 to a sensor circuit (detection
circuit) 104. The sensor circuit 104 comprises a bridge circuit as
shown in FIG. 12. The bridge circuit 104 comprises a flow rate
detecting thin-film temperature sensing element 104-1 of the flow
rate detecting unit 24, a fluid temperature compensating thin-film
temperature sensing element 104-2 of the fluid temperature
detecting unit 26 and resistors 104-3, 104-4. The potential Va, Vb
at the point a, b of the bridge circuit 104 is input to a
differential amplifying circuit (amplifier) 106, and the output of
the differential amplifying circuit 106 is input to a comparator
108. The comparison result of the output voltage signal of the
amplifier 106 and a reference voltage (Vref) is output as a binary
signal from the comparator 108. When the output signal of the
amplifier 106 is lower than the reference voltage (Vref), the
comparator 108 outputs a low (L) level [first level] signal, and
when the output signal of the amplifier 106 is equal to or higher
than the reference voltage (Vref), the comparator 108 outputs a
high (H) level [second level] signal.
[0114] Further, as shown in FIG. 10, the DC voltage from the
reference power supply circuit 102 is supplied to a thin-film
heating element 112 of the flow rate detecting unit 24 through a
transistor 110 for controlling the current to be supplied to the
heating element 112. That is, in the flow rate detector 24, the
thin-film temperature sensing element 104-1 carries out the
temperature sensing operation on the basis of the heating of the
thin-film heating element 112 while being affected by the
endothermic action of the fluid to be detected through the fin
plate 44. As a temperature sensing result, the difference between
the potential Va at the point a of the bridge circuit 104 and the
potential Vb at the point b of the bridge circuit 104 shown in FIG.
12 is achieved.
[0115] The value of (Va-Vb) is varied due to variation of the
temperature of the flow rate detecting temperature sensing element
104-1 in accordance with the flow rate of the fluid. By properly
presetting the characteristic of the bridge circuit 104 and
properly setting the reference voltage (Vref) of the comparator
108, the output voltage signal of the amplifier 106 can be set to
the reference voltage (Vref) of the comparator when the thin-film
temperature sensing element 104-1 is kept to a predetermined
heating state (that is, the temperature of the thin-film
temperature sensing element 104-1 is equal to a predetermined
value). In other words, the reference voltage (Vref) of the
comparator is set to be equal to the value of the output voltage
achieved from the amplifier 106 when the thin-film temperature
sensing element 104-1 is kept under the predetermined heating
state.
[0116] When the fluid flow rate is increased/reduced, the output of
the comparator 108 varies. The heating of the thin-film heating
element (the heater for the sensor) is controlled by using the
output of the comparator 108. CPU 120 is used to control the
heating of the thin-film heating element 112 and further calculate
the flow rate. The output of the comparator 108 is input through
PLD 122 to a heater control circuit 124 of CPU 120 as shown in FIG.
10. The output of the heater control circuit 124 is converted to an
analog signal by a D/A converter 128, the analog signal thus
achieved is input to an amplifier 130 to be amplified, and then the
output voltage signal of the amplifier 130 is input to the base of
the transistor 110. A signal is transmitted from the heater control
circuit 124 to a flow rate integrating calculation circuit 132 in
CPU 120, and a calculation result, etc. are output from the flow
rate integrating calculation circuit 132 to a display portion 134,
whereby necessary displays are made on the display portion 134.
[0117] As shown in FIG. 11, PLD 122 has a synchronizing circuit
122a, an edge detecting circuit 122b and a 125-counter 122c. The
heater control circuit 124 has an "L" level counter 124a, a
comparison circuit 124b and a heater voltage circuit 124c.
[0118] A clock signal is input from a 4 MHz clock circuit 136 to
CPU 120. The clock signal is converted to 1 MHz clock by a
frequency-dividing circuit 138 in CPU 120 and then input to the
125-counter 122c in PLD 122 and the "L" level counter 124a in the
heater control circuit 124.
[0119] The output of the comparator 108 is passed through PLD 122
and then input to the "L" level counter 124a to be sampled every 1
.mu.second (predetermined period). The "L" level counter 124a
counts an appearance frequency of "L" level within 125 .mu.seconds
(predetermined time period) set by the 125-counter 122c. The data
of the count value (count data CD) thus achieved by the counter
124a are input to the comparison circuit 124b to be compared with a
predetermined range. The predetermined range may has a lower limit
value (for example, 43) which is smaller than a half (62.5) of the
sampling frequency (125) within the predetermined time period of
125 .mu.sec and larger than zero, and has an upper limit value (for
example, 82) which is larger than a half of the sampling frequency
within the predetermined time period of 125 .mu.sec and smaller
than the sampling frequency (125).
[0120] In the heater voltage circuit 124c, a control voltage [this
term is sometimes used in the same meaning as the heater applied
voltage in this specification because it corresponds to an applied
voltage to the heater 112] input to the transistor 130 to control
the applied voltage to the heater 112 for sensor is equal to the
sum of a base voltage (Eb) and addition voltage (Ec). The base
voltage is selected from discrete values which are preset at
intervals of predetermined step value. The value of the base
voltage is fixed within each predetermined time period, and the
heating of the heater is roughly controlled by the base voltage.
The addition voltage has a fixed value, however, the applying time
period or applying timing thereof is variable, so that the heating
of the heater is finely controlled by the addition voltage. It is
appropriate that the addition voltage is set to a value which is
from twice to four times as high as the step value of the base
voltage.
[0121] The comparison circuit 124b instructs the heater voltage
circuit 124c to keep the previous base voltage in the next
predetermined time period when the count data CD is not less than
the lower limit value Nd and not more than the upper limit value
Nu. Further, when the count data CD is less than the lower limit
value Nd, the comparison circuit 124b instructs the heater voltage
circuit 124c to reduce the base voltage from the previous base
voltage by 1 step value in the next predetermined time period. When
the count data CD is more than the upper limit value Nu, the
comparison circuit 124b instructs the heater voltage circuit 124c
to increase the base voltage from the previous base voltage by 1
step value in the next predetermined time period.
[0122] The output of the comparator 108 is passed through PLD 122
and then input to the heater voltage circuit 124c. On the basis of
the input signal, the heater voltage circuit 124c allows
application of the addition voltage (Ec) during the period when the
input signal is kept under "L" level, and prohibits application of
the addition voltage (Ec) during the other period.
[0123] The heater voltage control as described above will be
described in detail with reference to the time chart of FIG.
15.
[0124] FIG. 15 shows the time-variation in the relationship between
the output signal (the input signal to the comparator 108) of the
amplifier 106 connected to the bridge circuit 104 and the reference
voltage (Vref) of the comparator 108, and also shows the
time-variation of the output signal of the comparator 108. Further,
FIG. 15 shows variation of the count data CD input to the
comparison circuit 124b achieved in the "L" level counter 124a
every 125 .mu.seconds, and also shows the time-variation of the
heater applied voltage (Eh) in association with the variation of
the count data CD. Further, FIG. 15 schematically shows the
time-variation of an actual flow rate in association with the time
variation of the heater applied voltage (Eh).
[0125] The lower limit and upper limit values Nd and Nu set in the
comparison circuit 124b are set to 43 and 82, respectively. In case
of 43.ltoreq.CD.ltoreq.82, on the basis of the instruction from the
comparison circuit 124b, during the predetermined time period of
125 .mu.seconds subsequent to the predetermined time period at
which the count data CD is achieved, the base voltage Eb is not
changed, but kept to the value at the just-before predetermined
time period in the heater voltage circuit 124c. In case of
CD<43, on the basis of the instruction from the comparison
circuit 124b, during the predetermined time period of 125
.mu.seconds subsequent to the predetermined time period at which
the count data CD is achieved, the base voltage Eb is reduced by
only 1 step voltage value (in this case, 10 mV) from the value at
the just-before predetermined time period in the heater voltage
circuit 124c. In case of CD>82, on the basis of the instruction
from the comparison circuit 124b, during the predetermined time
period of 125 .mu.seconds subsequent to the predetermined time
period at which the count data CD is achieved, the base voltage Eb
is increased by only 1 step voltage value (in this case, 10 mV)
from the value at the just-before predetermined time period in the
heater voltage circuit 124c.
[0126] On the other hand, in the heater voltage circuit 124c, a
prescribed addition voltage (Ec: 30 mV in this case) is applied
during the period when the output signal of the comparator 108 is
under "L" level, and no addition voltage is applied during the
period when the output signal of the comparator 108 is under "H"
level.
[0127] As described above, according to this embodiment, on the
basis of count data CD achieved within a prescribed time period,
the base voltage within the subsequent prescribed time period is
properly set, and the addition voltage applying time period is
properly set in accordance with the output of the comparator. By
combining these two control operations (setting operations), the
response of the control can be enhanced, the precision of the flow
rate measurement can be enhanced and the thermal hysteresis can be
reduced with a simple device construction.
[0128] The predetermined time period, the predetermined period, the
base voltage step value, the addition voltage value and the other
parameters may be properly set in consideration of the predicted
maximum variation of the flow rate so as to support this predicted
maximum variation.
[0129] As described above, the heating of the thin-film heating
element 112 is controlled so that the temperature of the flow rate
detecting temperature sensing element 104-1 is equal to a
predetermined value (that is, the heating state of the flow rate
detecting temperature sensing element 104-1 is set to a
predetermined state) irrespective of the variation of the fluid
flow rate. At this time, the voltage (the heater applied voltage)
applied to the thin-film heating element 112 corresponds to the
fluid flow rate, so that it is taken out as the flow rate output in
the flow rate integration calculating circuit 132 shown in FIGS. 10
and 11. For example, the instantaneous flow rate is output every
0.5 second and the instantaneous flow rate thus output is
integrated to achieve the integrated flow rate.
[0130] That is, as shown in FIG. 11, the integration value
(.SIGMA.Ec) of the addition voltage Ec applied for 0.5 second is
achieved on the basis of the count data CD for each predetermined
time period achieved in the "L" level counter 124a, the integration
value (.SIGMA.Eb) of the base voltage Eb applied for 0.5 second is
achieved on the basis of the base voltage value Eb achieved in the
heater voltage circuit 124c, and the total value
(.SIGMA.Ec+.SIGMA.Eb=.SIGMA.Eh) is achieved (see FIG. 16). This
value is converted to the instantaneous flow rate value by using
the calibration curves (instantaneous flow rate conversion table)
which are measured and stored in advance. This instantaneous flow
rate conversion table is a table indicating the relationship
between the integration value of the heater applied voltage for 0.5
second and the flow rate value. Specifically, the instantaneous
flow rate conversion table intermittently shows the relationship
between the integration value of the heater applied voltage and the
flow rate value, and thus data are complemented to achieve the flow
rate value from the actually-achieved integration value of the
heater applied voltage. The integrated flow rate value is obtained
by integrating the instantaneous flow rate values.
[0131] The output of the flow rate is displayed by the display
portion 134. On the basis of the instruction from CPU 120, the
instantaneous flow rate value and the integrated flow rate value
may be properly stored in a memory, and these data may be
transmitted to the outside through a communication line such as a
telephone line or other networks.
[0132] FIG. 17 is a circuit diagram showing an embodiment of the
flowmeter according to the present invention, and FIG. 18 is a
partially detailed diagram of the flowmeter of FIG. 17.
[0133] This embodiment has the same construction as the embodiment
shown in FIGS. 10 to 16 except that an environmental temperature
measuring circuit 140 and an A/D converter 142 shown in FIGS. 17
and 18 are added. The environmental temperature measuring circuit
140 is provided to the flowmeter electrical circuit portion (not
shown) on the circuit board 60 shown in FIG. 13, and measures the
temperature (environmental temperature) at this position. The
environmental temperature measuring circuit 140 may be constructed
by using a temperature sensing resistor such as a platinum resistor
or the like, and outputs the electrical signal corresponding to the
environmental temperature (which is mainly determined by the effect
of the external temperature and the temperature of the fluid).
[0134] In this embodiment, the same operation as the embodiment
shown in FIGS. 10 to 16 is performed except the measurement of the
environmental temperature and the processing using the measurement
result.
[0135] According to this embodiment, in the same manner as the
embodiment shown in FIGS. 10 to 16, the integration value
(.SIGMA.Ec) of the addition voltage Ec applied for 0.5 second is
achieved on the basis of the value of the count data CD for each
predetermined time period which is achieved by the "L" level
counter 124a, the integration value (.SIGMA.Eb) of the base voltage
Lb applied for 0.5 second is achieved on the basis of the base
voltage Eb achieved in the heater voltage circuit 124c, and then
the total value thereof (.SIGMA.Ec+.SIGMA.Eb=.SIGMA.Eh) (see FIG.
16). This total value is equal to any one of discrete values
(represented by digital values of 29 bits in this embodiment). This
value is converted to the instantaneous flow rate value by using
the calibration curves (instantaneous flow rate conversion table)
which are measured and stored in advance. The instantaneous flow
rate conversion table is a data table indicating the relationship
between the integration value of the heater applied voltage for 0.5
second and the flow rate value.
[0136] In this embodiment, the instantaneous flow rate conversion
table comprises plural individual calibration curves formed every
discrete temperature value. FIG. 19 shows an example of the
instantaneous flow rate conversion table as described above.
Individual calibration curves T.sub.1 to T.sub.4 for four discrete
temperature values t.sub.1 to t.sub.4 (5.degree. C., 15.degree. C.,
25.degree. C. and 35.degree. C.) are shown in FIG. 19. In FIG. 19,
each calibration curve is illustrated as a continuous line,
however, this illustration is used for convenience's sake of
description. Actually, the association relationship between each of
the discrete heater applied voltages (integration values for 0.5
second). . . E.sub.ARm-1, E.sub.ARm, E.sub.ARM+1, E.sub.ARm+2, . .
. shown in FIG. 19 and the instantaneous flow rate is shown. In
this embodiment, the instantaneous flow rate conversion table does
not show the relationship for all the possible heater applied
voltage integration values. That is, the integration values of
these possible heater applied voltage are sectioned into plural
groups in order of the magnitude, and the minimum values of the
respective groups are represented by . . . E.sub.ARm-1, E.sub.ARm,
E.sub.ARm+1, E.sub.ARm+2, . . . . The minimum values of those
values which have the same high-order 8 bits in digital values that
possibly represent the heater applied voltages may be used as the
above discrete representative values. In this case, 256
representative values are provided.
[0137] Further, as shown in FIGS. 17 and 18, an environmental
temperature value (for example, represented by a digital value of
10 bits) t is input from the environmental temperature measuring
circuit 140 through the A/D converter 142 to the flow rate
integration calculating circuit 132.
[0138] In the flow rate integration calculating circuit 132, the
data interpolation calculation is carried out according to the
procedure shown in FIG. 20 on the basis of the count data value CD,
the base voltage Eb and the environmental temperature value t by
using the instantaneous flow rate conversion table as described
above to achieve the instantaneous flow rate value.
[0139] That is, the heater applied voltage value .SIGMA.Eh
(=.SIGMA.Ec+.SIGMA.Eb) corresponding to the total value of the
integration value of the addition voltage Ec and the integration
value of the base voltage Eb for 0.5 second is first calculated
(S1).
[0140] Next, the individual calibration curves T.sub.n, T.sub.n+1
(in the example of FIG. 19, n=2; that is,
T.sub.2[t.sub.2=15.degree. C.], T.sub.3[t.sub.3=25.degree. C.])
satisfying t.sub.n.ltoreq.t<t.sub.n+1 for the environmental
temperature value t measured (22.degree. C. in the example of FIG.
19) are selected (S2).
[0141] Next, the voltage values E.sub.ARm and E.sub.ARm+1
satisfying E.sub.ARm.ltoreq..SIGMA.Eh<E.sub.ARm+1 are achieved.
That is, the representative value E.sub.ARm of the group having the
value ([10110100] in the example of FIG. 19) represented by the
high-order 8 bits of .SIGMA.Eh and the representative value
E.sub.ARm+1 of the group having the value ([10110101] in the
example of FIG. 19) achieved by adding the value of the high-order
8 bits of .SIGMA.Eh with "1" are achieved. The voltage values
E.sub.ARm and E.sub.ARm+1 are converted to instantaneous flow rate
values F.sub.b, F.sub.a; F.sub.B, F.sub.A on the individual
calibration curves T.sub.n, T.sub.n+1 (S3).
[0142] Next, the instantaneous flow rate values F.sub.ab, F.sub.AB
on T.sub.n, T.sub.n+1 corresponding to .SIGMA.Eh are achieved from
values F.sub.b, F.sub.a; F.sub.B, F.sub.A by the data interpolating
calculation. At this time, the following equations (1), (2) are
used (S4).
F.sub.ab=(F.sub.a-F.sub.b)
(.SIGMA.Eh-E.sub.ARm)/(E.sub.ARm+1-E.sub.ARm)+F- .sub.b (1)
F.sub.AB=(F.sub.A-F.sub.B)
(.SIGMA.Eh-E.sub.ARm)/(E.sub.ARm+1-E.sub.ARm)+F- .sub.B (2)
[0143] Next, the instantaneous flow rate value Ft corresponding to
.SIGMA.Eh for the environmental temperature value t is achieved
from F.sub.ab, F.sub.AB by the data interpolating calculation. At
this time, the following equation (3) is used (S5).
Ft=(F.sub.ab-F.sub.AB) (t-T.sub.3)/(T.sub.2-T.sub.3)+F.sub.AB
(3)
[0144] By achieving the instantaneous flow rate value Ft at the
environmental temperature with the data interpolating calculation
as described above, volume of the data of the instantaneous flow
rate conversion table can be reduced. In addition, the
instantaneous flow rate measurement can be performed with extremely
little measurement error due to the environmental temperature. FIG.
21 shows an example of the measurement result of the measurement
error (display error) every flow rate value, which was achieved by
using the flowmeter of this embodiment. It is apparent from FIG. 21
that high precision within .+-.1% in error was achieved.
[0145] In the flow rate integration calculating circuit 132, the
instantaneous flow rate value achieved is also integrated to
achieve the integrated flow rate value.
[0146] The flow rate outputs such as the instantaneous flow rate
value and the integrated flow rate value thus achieved are
displayed on the display portion 134. On the basis of the
instruction from CPU 120, the instantaneous flow rate value and the
integrated flow rate value may be properly stored in a memory, and
further may be transmitted to the outside through a communication
line such as a telephone line or other networks.
[0147] FIG. 22 is a schematic diagram showing the overall
construction of the flowmeter, particularly, the flow rate
detection system of the flowmeter according to the present
invention, and FIG. 23 is a cross-sectional view showing the flow
rate detecting unit.
[0148] First, the construction of a flow rate sensor unit of this
embodiment will be described with reference to FIG. 23. As shown in
FIG. 23, in a flow rate detecting unit 204, a flow rate detector
205 is joined to the surface of a fin plate 224 serving as a heat
transfer member by joint material 226 having excellent thermal
conductivity, and an electrode pad of the flow rate detector 205
and an electrode terminal 228 are connected to each other through a
bonding wire 220. Further, the flow rate detector 205 and the
bonding wire 220, and a part of the fin plate 224 and a part of the
electrode terminal 228 are accommodated in a synthetic esin housing
212. The flow rate detector 205 has the construction as shown in
FIG. 6.
[0149] A fluid temperature detecting unit 206 has the same
construction as the flow rate detecting unit 204 except that a
fluid temperature detector is used in place of the flow rate
detector 205 in the flow rate detecting unit 204. In the fluid
temperature detecting unit 206, the parts corresponding to those of
the flow rate detecting unit 204 are represented by the same
reference numerals affixed with apostrophe "'". The fluid
temperature detector has the same construction as shown in FIG.
7.
[0150] As shown in FIG. 22, the end portions of fin plates 224,
224' projecting from the housings 212, 212" of the flow rate
detecting unit 204 and the fluid temperature detecting unit 206
extend into the fluid flow passage 203 of the fluid flow passage
member 202. The fin plates 224, 224" extend to pass through the
center in the cross section of the fluid flow passage 203 having a
substantially circular cross section. The fin plates 224, 224' are
arranged along the fluid flowing direction in the fluid flow
passage 203, so that excellent heat transfer between each of the
flow rate detector 205 and the fluid temperature detector and the
fluid can be performed without greatly affecting the fluid
flow.
[0151] A DC voltage V1 is applied from a power supply circuit (not
shown) to a bridge circuit 240. The bridge circuit 240 comprises a
thin-film temperature sensing element 231 for flow rate detection
of the flow rate detecting unit 204, a thin-film temperature
sensing element 231' for temperature compensation of the fluid
temperature detecting unit 206 and resistors 243, 244. The
potential Va, Vb at the point a, b of the bridge circuit 240 is
input to a differential amplifying/integrating circuit 246.
[0152] A DC voltage V2 from the power supply circuit is supplied to
a thin-film heating element 233 of the flow rate detecting unit 204
through a transistor 250 for controlling current to be supplied to
the thin-film heating element 233. That is, in the flow rate
detector 205, the thin-film temperature sensing element 231 carries
out the temperature sensing operation on the basis of the heating
of the thin-film heating element 233 with being affected by the
endothermic action of the fluid to be detected through the fin
plate 224. As a result of the temperature sensing is achieved the
difference between the potential Va at the point a of the bridge
circuit 240 and the potential Vb at the point b of the bridge
circuit 240 shown in FIG. 22.
[0153] The value of (Va-Vb) is varied due to variation of the
temperature of the temperature sensing element 231 for flow rate
detection in accordance with the flow rate of the fluid. By
properly setting the resistance values of the resistors 243, 244 of
the bridge circuit 240 in advance, the value of (Va-Vb) can be set
to zero when the fluid flow rate is equal to a desired value
(reference value). At the reference flow rate, the output of the
differential amplifying/integrating circuit 246 is equal to a fixed
value (the value corresponding to the reference flow rate), and the
resistance value of the transistor 250 is equal to a fixed value.
In this case, a divided voltage to be applied to the thin-film
heating element 233 is also equal to a fixed value, and thus the
voltage at the point P at this time indicates the reference flow
rate.
[0154] When the fluid flow rate is increased/reduced, the output of
the differential amplifying/integrating circuit 246 varies in
polarity (varied in accordance with positive/negative sign of the
resistance-temperature characteristic of the temperature sensing
element 231 for flow rate detection) and magnitude, and the output
of the differential amplifying/integrating circuit 246 is varied in
accordance with the variation in polarity and magnitude.
[0155] When the fluid flow rate is increased, the temperature of
the temperature sensing element 231 for flow rate detection is
reduced, and thus the differential amplifying/integrating circuit
246 supplies the base of the transistor 250 with such a control
input that the resistance value of the transistor 250 is reduced to
thereby increase the heating value of the thin-film heating element
233 (that is, to increase the power).
[0156] On the other hand, when the fluid flow rate is reduced, the
temperature of the temperature sensing element 231 for flow rate
detection increases, and thus the differential
amplifying/integrating circuit 246 supplies the base of the
transistor 250 with such a control input that the resistance value
of the transistor 250 is increased to thereby reduce the heating
value of the thin-film heating element 233 (that is, to reduce the
power).
[0157] As described above, the heating of the thin-film heating
element 233 is feed-back controlled so that the temperature
detected by the temperature sensing element 231 for flow rate
detection is equal to a target value irrespective of the variation
of the fluid flow rate. Further, the voltage (the voltage at the
point P) applied to the thin-film heating element 233 corresponds
to the fluid flow rate, and thus it is taken out as an output of
the flow rate.
[0158] The output of the flow rate of this detection circuit is
A/D-converted by an A/D converter 252, and then converted to the
corresponding flow rate (instantaneous flow rate) by CPU 254. The
flow rate thus achieved is integrated with respect to the time to
calculate the integrated flow rate (integrated flow amount). The
values of the instantaneous flow rate and the integrated flow rate
can be displayed by an integrated flow rate display portion 256,
and stored in a memory 284. Further, these values may be
transmitted to the outside through a communication line such as a
telephone line or other networks.
[0159] The conversion from the output of the detection circuit to
the flow rate in CPU 254 is carried out as follows. The calibration
curve for conversion to the flow rate is stored in the memory 284
in advance. An example of the calibration curve is shown in FIG.
24. The calibration curve is represented as follows:
f=a.sub.1v.sup.4+b.sub.1v.sup.3+c.sub.1v.sup.2+d.sub.1v+e.sub.1(0.ltoreq.v-
<v.sub.1)
f=a.sub.2v.sup.4+b.sub.2v.sup.3+c.sub.2v.sup.2+d.sub.2v+e.sub.2(v.sub.1.lt-
oreq.v<v.sub.2)
f=a.sub.3v.sup.4+b.sub.3v.sup.3+c.sub.3v.sup.2+d.sub.3v+e.sub.3(v.sub.2.lt-
oreq.v)
[0160] wherein f [liter/h] represents the fluid flow rate, v[V]
represents the output of the detection circuit, and a.sub.1,
b.sub.1, c.sub.1, d.sub.1, e.sub.1; a.sub.2, b.sub.2, c.sub.2,
d.sub.2, e.sub.2; a.sub.3, b.sub.3, c.sub.3, d.sub.3, e.sub.3
represents coefficients.
[0161] In the case shown in FIG. 24,
v.sub.1=7.0 [V]
v.sub.2=8.0 [V]
a.sub.1=+1.99933E-1
b.sub.1=-4.84409E+0
c.sub.1=+4.44365E+1
d.sub.1=-1.82380E+2
e.sub.1=+2.81911E+2
a.sub.2=+3.45600E-1
b.sub.2=-8.77327E+0
c.sub.2=+8.40224E+1
d.sub.2=-3.58917E+2
e.sub.2=+5.75936E+2
a.sub.3=+6.55492E+0
b.sub.3=-2.13636E+2
c.sub.3=+2.61702E+3
d.sub.3=-1.42694E+4
e.sub.3=+2.92043E+4
[0162] Accordingly, it is sufficient to store only the function
style of f=av.sup.4+bv.sup.3+cv.sup.2+dv+e, two threshold values
v.sub.1, v.sub.2 serving as boundaries of three areas of the output
values of the detection circuit (the first area of
0.ltoreq.v<v.sub.1, the second area of
v.sub.1.ltoreq.v<v.sub.2, and the third area of
v.sub.2.ltoreq.v), and the values of a to e (a, to e.sub.1, a.sub.2
to e.sub.2, a.sub.3 to e.sub.3) every area as the content of the
calibration curve to be stored in the memory 284, and thus the
capacity of the memory 284 may be small.
[0163] FIG. 24 is a graph in which actual measurement values of the
flow rate are plotted. These measurement values show the
relationship between the output value of the detection circuit of
the flowmeter when fluid is made to flow into the flowmeter of this
embodiment and the flow rate value achieved by actually measuring
the volume of the fluid flown in the flowmeter, and it is apparent
from FIG. 24 that these measurement values excellently meet the
calibration curve.
[0164] The technical background to explain that the measurement
values excellently meet the calibration curve will be described.
The flow rate measurement can be also performed with excellent
precision by using a calibration curve represented by one function
over all the area of the output values of the detection circuit.
However, in this case, six-order or higher-order function styles
are needed and the numerical calculation is extremely complicated.
Therefore, according to the present invention, the area of the
output values of the detection circuit is divided into three parts,
and the calibration curves which have different coefficients, but
are represented by the same function style are used for all these
three parts, whereby a four-order function style can be used as the
function style. In addition, the flow rate measurement can be
performed with excellent precision without increasing the memory
capacity so much.
[0165] Accordingly, the memory area used when the calibration curve
is individually set for each of plural environmental temperature
values can be prevented from being greatly increased, and this
makes it easy to measure the environmental temperature and use a
proper calibration curve (if necessary, extrapolation is carried
out with use of two calibration curves) based on the environmental
temperature thus measured, thereby performing the higher-precision
flow rate measurement.
[0166] The calibration curve as described above can be created by
determining five coefficients of the above function style for each
area by using the least squares method on the basis of the actual
measurement as shown in FIG. 24. The threshold value v.sub.1 may be
set so that the flow rate value is within the range from 0.5 to
2.0, and the threshold value v.sub.2 may be set so that the flow
rate value is within the range from 4.0 to 12.0. With this setting,
the calibration curve which excellently meets the actual
measurement values as shown in FIG. 24 can be achieved.
[0167] In CPU 254, one of the three areas to which the output v of
the detection circuit belongs is specified, and then the
calculation using the function having the coefficients
corresponding to the area thus specified is made to achieve the
flow rate value f.
[0168] As described above, in the flowmeter of this embodiment, the
flow rate measurement can be performed with high precision even
when the flow rate value to be detected varies in a broad
range.
[0169] Further, according to this embodiment, the minute chip
containing the thin-film heating element and the thin-film
temperature sensing element is used as the flow rate detector.
Therefore, the high response as described above can be implemented,
and the precision of the flow rate measurement can be enhanced.
[0170] FIG. 25 is a cross-sectional view showing an embodiment of
the flowmeter according to the present invention, and FIG. 26 is a
partial cross-sectional view of the flowmeter of this embodiment.
FIG. 27 is a front view of the flowmeter of this embodiment, FIG.
28 is a right side view of the flowmeter of this embodiment, FIG.
29 is a bottom view of the flowmeter of this embodiment when some
parts are removed, FIG. 30 is a left side view of the flowmeter of
this embodiment, and FIG. 31 is a plan view of the flowmeter of
this embodiment. FIG. 25 is an A-A' cross-sectional view of FIG.
30, and FIG. 26 is a B-B' partial cross-sectional view of FIG.
27.
[0171] In these figures, three portions 304, 306, 308 constituting
a fluid flow passage are formed in a casing member 302 formed of
material having excellent thermal conductivity such as aluminum or
the like. The flow passage portion 304 intercommunicates with a
fluid flow-in port 314, and the flow passage portion 308
intercommunicates with a fluid flow-out port 316. The fluid flowing
from the fluid flow-in port 314 into the flow passage portion 304
passes through the flow passage portion 306 and the flow passage
portion 308 and then flows out from the fluid flow-out port 316 (in
the flowing direction as indicated by an arrow). The flow passage
portion 306 constitutes a fluid residence area. A lid member 303 is
detachably mounted at the lower portion of the casing member 302,
and the lid member 303 constitutes a part of the casing member
302.
[0172] The flow passage portion 304 extends in the horizontal
direction, the flow passage portion 306 extends in the vertical
direction, and the flow passage portion 308 comprises a vertical
portion 308a extending in the vertical direction and a horizontal
portion 308b extending in the horizontal direction. A fluid supply
source side pipe is connected to the fluid flow-in port 314, and a
fluid demand side pipe is connected to the fluid flow-out port
316.
[0173] A male screw 310 is detachably screwed in the casing member
302 so as to close the port intercommunicating with the upper
portion of the flow passage portion 306. A filter 312 including
non-woven fabric comprising glass fiber, plastic fiber or the like
which is held by a proper holder may be interposed in the
intercommunication portion between the flow passage portion 306 and
the flow passage portion 308.
[0174] A flow rate detecting unit 324 and a fluid temperature
detecting unit 326 are secured to the casing member 302 so as to
face the vertical portion 308a of the flow passage portion 308. The
flow rate detecting unit 324 has the construction as shown in FIG.
23. The fluid temperature detecting unit 326 has the same
construction as the flow rate detecting unit 324 except that a
fluid temperature detector is used in place of the flow rate
detector. The fluid temperature detector has the same construction
except that the thin-film heating element is removed from the flow
rate detector.
[0175] The end portions of fin plates 344, 344' projecting from the
housings 352, 352' of the flow rate detecting unit 324 and the
fluid temperature detecting unit 326 extend into the vertical
portion 308a of the flow passage portion 308 of the casing member
302. The fin plates 344, 344' extend to pass through the center in
the cross section of the flow passage portion 308 having the
substantially circular cross section. The fin plates 344, 344' are
arranged along the flow direction of the fluid in the flow passage
portion 308, so that heat transfer can be excellently performed
between each of the flow rate detector 342 and the fluid
temperature detector 342' and the fluid without greatly affecting
the flow of the fluid.
[0176] FIG. 29 is a bottom diagram when the lid member 303
described above is removed, and shows the flow passage portion 306
and the vertical portion 308a of the flow passage portion 308. In
this embodiment, as shown in FIGS. 29 and 25, the flow passage
portion (fluid residence area) 306 is formed so that the cross
section thereof is sufficiently larger than the cross section of
the flow passage portion 308. The cross section of the flow passage
portion 306 is set to be five times or more, preferably ten times
or more as large as that of the flow passage portion 308, The
volume of the flow passage portion 306 is larger than the volume
per unit length of the vertical portion 308a of the flow passage
portion 308 at which the flow rate detecting unit 324 and the fluid
temperature detecting unit 326 are located. The volume of the flow
passage portion 306 is preferably 50 times or more, more preferably
100 times or more, as large as the volume per unit length of the
vertical portion 308a.
[0177] As described above, the flow passage portion 306 is located
at the upstream side, with respect to the fluid flowing direction,
of the vertical portion 308a of the flow passage portion 308 at
which the heat exchange for the flow rate detection is carried out,
and it constitutes an area where the fluid flowing into the flow
passage portion 308 stays temporarily. The fluid flow velocity at
the flow passage portion 306 is smaller than that at the vertical
portion 308a of the flow passage portion 308, and is preferably
equal to 1/5 or less, more preferably {fraction (1/10)} or less, of
the fluid flow velocity at the vertical portion 308a.
[0178] Further, the vertical portion 308a of the flow passage
portion 308 extends in parallel to and proximately to the flow
passage portion 306, and thus it is disposed to be liable to suffer
thermal influence of the fluid in the flow passage portion 306.
[0179] The cross section of the flow passage portion 304 is smaller
than the cross section of the flow passage portion 306, and it is
set to the same level as the cross section of the flow passage
portion 308. Accordingly, the fluid flow velocity in the flow
passage portion 304 is the same level as the fluid flow velocity in
the flow passage portion 308.
[0180] FIG. 32 is a schematic diagram showing a flow rate detection
system of a thermal flowmeter according to this embodiment. A DC
voltage is applied from a constant voltage circuit 402 to a bridge
circuit (detection circuit) 404. The bridge circuit 404 comprises a
flow rate detecting thin-film temperature sensing element 404-1 of
a flow rate detecting unit 324, a temperature compensating
thin-film temperature sensing element 404-2 of a fluid temperature
detecting unit 326 and variable resistors 404-3, 404-4. The
potential Va, Vb at the point a, b of the bridge circuit 404 is
input to a differential amplifying circuit 406, and the output of
the differential amplifying circuit 406 is input to an integrating
circuit 408.
[0181] The DC voltage from a power supply source is supplied to a
thin-film heating element 412 of the flow rate detecting unit 324
through a transistor 410 for controlling current to be supplied to
the thin-film heating element 412. That is, in the flow rate
detector of the flow rate detecting unit 324, the thin-film
temperature sensing element 404-1 carries out the temperature
sensing operation on the basis of the heating of the thin-film
heating element 412 with being affected by the endothermic action
of the fluid to be detected through the fin plate 344. As a result
of the temperature sensing operation, the differential between the
potential Va at the point a of the bridge circuit 404 and the
potential Vb at the point b of the bridge circuit 404 shown in FIG.
32 is achieved.
[0182] The value of (Va-Vb) is varied due to variation of the
temperature of the flow rate detecting temperature sensing element
404-1 in accordance with the flow rate of the fluid. By properly
setting the resistance values of the resistors 404-3, 404-4 of the
bridge circuit 404 in advance, the value of (Va-Vb) can be set to
zero when the fluid flow rate is equal to a desired value
(reference value). At the reference flow rate value, the output of
the integrating circuit 408 is equal to a fixed value (the value
corresponding to the reference flow rate value), and the resistance
value of the transistor 410 is equal to a fixed value. In this
case, a divided voltage to be applied to the thin-film heating
element 412 is also equal to a fixed value, and thus the voltage at
the point P at this time indicates the reference flow rate
value.
[0183] When the fluid flow rate is increased/reduced, the output of
the differential amplifying circuit 406 varies in polarity (varied
in accordance with positive/negative sign of the
resistance-temperature characteristic of the flow detecting
temperature sensing element 404-1) and magnitude, and the output of
the integrating circuit 408 is varied in accordance with the
variation in polarity and magnitude of the output of the
differential amplifying circuit 406.
[0184] When the fluid flow rate is increased, the temperature of
the flow rate detecting temperature sensing element 404-1 is
reduced, and thus the integrating circuit 408 supplies the base of
the transistor 410 with such a control input that the resistance
value of the transistor 410 is reduced to thereby increase the
heating value of the thin-film heating element 412 (that is, to
increase the power).
[0185] On the other hand, when the fluid flow rate is reduced, the
temperature of the flow rate detecting temperature sensing element
404-1 increases, and thus the integrating circuit 408 supplies the
base of the transistor 410 with such a control input that the
resistance value of the transistor 410 is increased to thereby
reduce the heating value of the thin-film heating element 412 (that
is, to reduce the power).
[0186] As described above, the heating of the thin-film heating
element 412 is feed-back controlled so that the temperature
detected by the flow rate detecting temperature sensing element
404-1 is equal to a target value irrespective of the variation of
the fluid flow rate. Further, the voltage (the voltage at the point
P) applied to the thin-film heating element 412 corresponds to the
fluid flow rate, and thus it is taken out as an output of the flow
rate.
[0187] As in the case of the above embodiment, the flow rate output
is properly A/D-converted by the A/D converter, and subjected to
operation processing such as integration, etc. by CPU, and then the
flow rate is displayed by the display portion. On the basis of the
instruction from CPU, the instantaneous flow rate and the
integrated flow rate can be properly stored in the memory. Further,
these data may be transmitted to the outside through a
communication line such as a telephone line or other networks.
[0188] In this embodiment, since the flow passage portion 306
constitutes the fluid residence area, the fluid flow velocity at
the flow passage portion 306 is low, and even when the temperature
of the fluid flowing from the flow passage portion 304 into the
flow passage portion 306 varies sharply, the fluid newly supplied
into the flow passage portion 306 is mixed with the fluid which has
already existed in the flow passage portion 306 before the
temperature variation occurs, and thus there exists a time margin
for averaging of the fluid temperature, so that the temperature
variation of the fluid supplied to the flow passage portion 308 is
moderated. In addition, the casing member 302' is formed of metal
having excellent thermal conductivity, so that even when the
temperature of the fluid flowing into the fluid flow passage of the
casing member 302 varies sharply, the averaging of the temperature
of the fluid in the flow passage is promoted by thermal conduction
of the casing member 302, so that the effect of the sharp variation
of the temperature of the flow-in fluid is moderated.
[0189] As described above, according to this embodiment, the
temperature variation of the fluid in the vertical portion 308a of
the flow passage portion 308 at which the flow rate detecting unit
324 and the fluid temperature detecting unit 326 are located is
moderated. Therefore, even when the temperature of the flow-in
fluid varies sharply, the fluid temperature detected by the fluid
temperature detecting unit 326 is substantially equal to the
temperature of the fluid for which the flow rate is detected in the
flow rate detecting unit 324, so that the fluid temperature can be
surely compensated and the precision of the flow rate detection can
be enhanced. The fluid temperature variation in the vertical
portion 308a of the flow passage portion 308 at which the flow rate
detecting unit 324 and the fluid temperature detecting unit 326 are
located is moderated, so that the operation of the control system
can be stabilized, and from this viewpoint, the flow rate detection
precision can be enhanced.
INDUSTRIAL APPLICABILITY
[0190] As described above, according to the flowmeter of the
present invention, the flow rate can be measured with excellent
precision over a broad flow rate range.
[0191] According to the present invention, by combining the two
types of control operation, one of which is to properly set, on the
basis of the count value achieved within a predetermined time
period, the base voltage within the subsequent predetermined time
period, and the other of which is to properly set the applying
period of the addition voltage in accordance with the output of the
comparator, the response of the heater control can be enhanced, the
precision of the flow rate measurement can be enhanced and the
thermal hysteresis can be reduced without complicating the circuit
construction.
[0192] Further, according to the present invention, the data
interpolation calculation is carried out to achieve the
instantaneous flow rate value at the environmental temperature,
thereby reducing the data volume of the instantaneous flow rate
conversion table, and the variation of the measurement value due to
the environmental temperature is prevented to perform the extremely
high-precision flow rate measurement.
[0193] Still further, according to the flowmeter of the present
invention, the flow rate can be measured with excellent precision
over a board flow rate range without increasing the capacity of the
memory for storing the calibration curves so much.
[0194] Still further, according to the present invention, the fluid
residence area is formed at the upstream of the flow rate detecting
unit and the fluid temperature detecting unit in the fluid flow
passage, whereby the fluid temperature compensation can be surely
performed even when the temperature of flow-in fluid varies
sharply, thereby enhancing the precision of the flow rate
detection.
* * * * *