Abstrict A flow meter is provided which employs a thermal flow sensor and
a correcting flow meter, for example, a Karman vortex flow meter
such that the output from the thermal flow sensor is corrected by
the Karman vortex flow meter, thereby making it possible to accurately
and stably measure the flow rate of fluids in a wide range even
if the composition of fluids changes.
Claims What is claimed is:
1. A method of correcting a flow rate using a flow meter comprising
a thermal flow sensor and a correcting flow meter means for correcting
a flow rate value measured by said thermal flow sensor in a predetermined
range by a flow rate value measured by said correcting flow meter
means, said method comprising the steps of:
(a) starting said method of correcting when an output from said
thermal flow sensor remains within a predetermined flow rate range
during a first predetermined time;
(b) integrating outputs from said respective thermal flow sensor
and correcting flow meter means during a second predetermined time
following said first predetermined time;
(c) determining whether an integrated value measured by said correcting
flow meter means is within said predetermined flow rate range;
(d) correcting a value measured by said thermal flow sensor based
on said integrated value measured by said correcting flow meter
means when said integrated value measured by said correcting flow
meter means is determined to be within said predetermined flow rate
range at said step (c); and
(e) returning to said step (a) without performing a correction
if said integrated value measured by said correcting flow meter
means is determined to be out of said predetermined flow rate range
at said step (c).
2. A method of correcting a flow rate as in claim 1 wherein said
correcting step includes determining a correction factor based on
the ratio of the integrated flow rate as measured by the correcting
flow meter to be integrated value as measured by the thermal flow
meter.
3. A measure of correcting a flow rate as in claim 2 wherein said
correcting step further includes applying said correction factor
to a flow rate value as measured by said thermal flow meter.
4. A method of correcting a flow rate as in claim 3 wherein said
correcting flow meter is a Karman vortex flow meter.
5. A method of correcting a flow rate as in claim 2 wherein said
correcting flow meter is a Karman vortex flow meter.
6. A method of correcting a flow rate as in claim 1 wherein said
correcting flow meter is a Karman vortex flow meter.
7. A method of correcting a flow rate as in claim 1 including
the further step of starting said correcting flow meter when an
output from said thermal flow sensor remains within a predetermined
flow rate range during a first predetermined time.
8. A method of correcting a flow rate as in claim 1 including
the further step of inhibiting for a predetermined interval further
correction of said flow rate value measured by said thermal flow
sensor after said correcting a value measured by said thermal flow
sensor in step (d).
9. A method of correcting a flow rate as in claim 8 wherein said
predetermined interval is relatively short if the correction in
step (d) is large and is relatively long if the correction in step
(d) is short.
10. A method of correcting a flow rate as in claim 1 including
the further step of storing without execution of step (d) said integrated
flow sensor output and said correcting flow meter output if the
difference in their values exceeds a predetermined amount.
11. A method of correcting a flow rate as in claim 1 wherein said
integrated thermal flow sensor output value and said integrated
correcting flow meter output value are determined sequentially.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a measurement of fluid flows, and more
particularly to a composite-type flow meter and a measuring method
therefor which is adapted to correct a flow rate sensed by a thermal
flow sensor by means of a correcting flow meter such as a Karman
vortex flow meter.
2. Description of the Prior Art
There have been employed a variety of sensors for measuring flow
rates of gaseous fluids. One of these sensors is a thermal flow
sensor which is typically adapted to heat a resistive sensor element
by feeding a current thereto and measure a flow of a gas, making
use of the fact that the sensor element is cooled by the flow of
the gas to cause changes in the resistance of the sensor element.
Recently, flow sensors of a type manufactured by the semiconductor
technology, or so-called micro flow sensors have been known. The
micro flow sensor is advantageous over the conventional thermal
flow sensor in a remarkably fast response, a high sensitivity, a
low power consumption, the adaptability to a mass production and
so on.
The thermal flow sensor is generally disposed in a circular or
rectangular conduit constituting a flow meter to measure a gas flowing
through the conduit.
The conventional thermal flow meter, however, generates an output
related to a mass flow, so that for a measurement of a volume flow
the composition of gases under measurement must have been previously
determined. Therefore, if the composition of gases changes in course
of measurement, a correct value cannot be measured. In addition,
the conventional thermal flow meter implies a problem that its output
drifts due to aging changes during a long term of service.
Meanwhile, there has been known a Karman vortex flow meter as a
highly accurate and stable meter for measuring volume flow. The
Karman vortex flow meter, however, is not capable of performing
measurement in a low flow range.
OBJECTS AND SUMMARY OF THE INVENTION
In view of the problems mentioned above, it is an object of the
present invention to provide a flow meter which is capable of highly
accurately and stably measuring the flow rate of fluids from a low
flow range to a high flow range.
It is another object of the present invention to provide a flow
meter which is capable of accurately measuring the flow rate of
fluids in a wide range even if the composition of gases changes.
To achieve the above objects, the present invention proposes a
flow meter which employs an accurate and stable correcting flow
meter, for example, a Karman vortex flow meter for correcting outputs
derived from a thermal flow sensor having a wide measuring range.
According to a first aspect of the present invention, there is
provided a flow meter comprising:
a conduit for introducing a fluid flow under measurement;
a thermal flow sensor placed at a location on the inner wall of
the conduit; and
a correcting flow meter means for correcting a flow rate value
measured by the thermal flow sensor in a predetermined range by
a flow rate value measured by the correcting flow meter means.
According to a second aspect of the present invention, there is
provided a method of correcting a flow rate using a flow meter comprising
a thermal flow sensor and a correcting flow meter means for correcting
a flow rate value measured by the thermal flow sensor in a predetermined
range by a flow rate value measured by the correcting flow meter
means, the method comprising the steps of:
(a) starting the correcting flow meter means when an output from
the thermal flow sensor remains within a predetermined flow rate
range during a first predetermined time;
(b) integrating outputs from the respective thermal flow sensor
and correcting flow meter means after a second predetermined time;
(c) determining whether a value measured by the correcting flow
meter means is within the predetermined flow rate range;
(d) correcting a value measured by the thermal flow sensor based
on the value measured by the correcting flow meter means when the
value measured by the correcting flow meter means is determined
to be within the predetermined flow rate range at the step (c);
and
(e) returning to the step (a) without performing a correction if
the value measured by the correcting flow meter means is determined
to be out of the predetermined flow rate range at the step (c).
The above and other objects, features and advantages of the present
invention will become apparent from the following detailed description
of the preferred embodiments with reference to the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a functional block diagram illustrating a basic structure
of a flow meter according to the present invention;
FIG. 2 is a graph used for explaining the principle of the present
invention;
FIG. 3 is a flowchart illustrating a basic algorithm of a correcting
method according to the present invention;
FIG. 4 is a longitudinal sectional view illustrating a flow meter
of a first embodiment of the present invention;
FIG. 5 is a cross-sectional view taken along a line A--A in FIG.
4;
FIG. 6 is a longitudinal sectional view illustrating a second embodiment
of the present invention;
FIG. 7 is a schematic diagram illustrating a main structure of
a Karman vortex flow meter employed in the present invention;
FIG. 8 is a sectional view illustrating a pressure difference detecting
unit of the Karman vortex flow meter shown in FIG. 7;
FIGS. 9a, 9b are respectively a longitudinal sectional view and
a cross-sectional view taken along a line B--B illustrating a modified
vortex generating member of the Karman vortex flow meter employed
in the present invention; and
FIGS. 10a-10d are cross-sectional views illustrating other possible
shapes of the vortex generating member.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a functional block diagram illustrating a basic configuration
of a flow meter according to the present invention. A thermal flow
sensor 1 preferably, a semiconductor-type micro flow sensor as
mentioned above is disposed at a location on the inner wall of a
conduit 11 and driven by a first driving circuit 3. There is an
additional flow meter means, for example, a Karman vortex flow meter
means 2 also disposed in the conduit 11 as a correcting flow meter
and driven by a second driving circuit 4. A controller 5 is composed
of a microprocessor, not shown, a first integrating circuit 6 for
integrating measured values from the thermal flow sensor 1 through
the first driving circuit 3 a second integrating circuit 7 for
integrating measured values from the Karman vortex flow meter means
2 through the second driving circuit 4 and a correcting circuit
for executing a predetermined correction based on outputs from the
integrating circuits 6 7 controlling the driving circuits 3 4
and generating an output signal indicative of a corrected flow value,
whereby a flow rate measured by the thermal flow sensor 1 is corrected
by a flow rate measured by the Karman vortex flow meter means 2
in a predetermined measuring range.
FIG. 2 is a graph for explaining the principle of a correcting
method implemented in the present invention, where the abscissa
represents the flow rate and the ordinate outputs of the thermal
flow sensor 1 and the Karman vortex flow meter means 2. More specifically,
FIG. 2 illustrates the linealized output characteristic I of the
thermal flow sensor (MF) and the output characteristic II of the
Karman vortex flow meter (KV). The sensor 1 and the meter means
2 have previously been adjusted such that they respectively output
an identical value for an identical flow with a given fluid and
given ambient conditions.
The characteristic equation of the thermal flow sensor 1 after
linearization is expressed by the following equation (1)
where Q represents a flow, V.sub.MF an output value, and C a constant.
In the present embodiment, when an output value from the thermal
flow sensor 1 represents a flow rate between a lower limit value
F.sub.1 and an upper limit value F.sub.2 for example, at a point
F.sub.A, during more than a predetermined period, the Karman vortex
flow meter means 2 is started. The range between F.sub.1 and F.sub.2
of the Karman vortex flow meter is a range within which a measured
value by the Karman vortex flow meter is used for correcting the
thermal flow sensor. There are two mode of operations of the flow
meter. One mode is that the Karman vortex flow meter is operated
for correction when a value detected by the thermal flow sensor
remains within this range for a predetermined time. The other mode
is that the thermal flow sensor and the Karman vortex flow meter
are parallelly operated and when it is found that a measured value
from the Karman vortex flow meter is within the measurable range
between F.sub.1 and F.sub.2 a value indicated by the thermal flow
sensor is corrected by a value from the Karman vortex flow meter.
When the Karman vortex flow meter means 2 is started, outputs of
the respective thermal sensor 1 and Karman vortex flow meter means
2 are integrated for a predetermined period and mean values thereof
V.sub.KV1 V.sub.MF1 are calculated. A correction of the output
value from the thermal sensor 1 is performed by multiplying the
aforementioned equation (1) with a ratio (V.sub.KV1 V.sub.MF1) A
corrected flow Q' is therefore given by the following equation (2):
where C' represents a constant.
FIG. 3 is a flowchart illustrating a basic algorithm of a correcting
method of the present embodiment. The operation of the flow meter
shown in FIG. 1 will be described with reference to FIG. 3. First,
the flow rate of a fluid is measured by the thermal flow sensor
at step 101. An output Q.sub.MF derived at step 102 is checked whether
it exists within a measurable range of the Karman vortex flow meter
means 2; that is within a range between F.sub.1 and F.sub.2 (F.sub.1
.ltoreq.Q.sub.MF .ltoreq.F.sub.2). If the output Q.sub.MF is not
within this range, the execution returns to step 101. otherwise,
a second measurement is performed by the thermal flow sensor 1 at
step 104 and the same operation is repeated predetermined times
at steps 104-109. If the output Q.sub.MF is detected to be still
within the range between F.sub.1 and F.sub.2 or if the output remains
within the range during the operations performed at steps 101-109
the Karman vortex flow meter is started at step 110. Next, at step
111 a measurement is performed again by the thermal flow sensor
1. An output Q.sub.MF derived at step 112 is checked again whether
it exists within the range between F.sub.1 and F.sub.2 at step 113.
If the output Q.sub.MF is not within the range, the execution returns
to the initial state of step 101. On the contrary, if it is within
the range, the execution proceeds to step 114 where it is determined
whether a predetermined period T.sub.1 has elapsed or not. The period
T.sub.1 starts at step 111. If the answer to the question at step
114 is negative (N), the execution returns to step 111 and the above-mentioned
operations at steps 111-114 are repeated until the predetermined
period T1 has elapsed. When it is determined at step 114 that the
predetermined period T1 has elapsed, the execution proceeds to steps
115 199 in parallel.
At step 115 a series of further measurements is performed by the
thermal flow sensor 1 outputs derived at step 116 are integrated
at step 117 and it is determined at step 118 whether or not a predetermined
period T2 has elapsed from the start of step 115. If the predetermined
period T2 has not elapsed, the operation performed at the steps
115 to 118 are repeated until the period T2 has elapsed. When the
period T2 has elapsed, the execution proceeds to step 124.
On the other hand, at step 119 a measurement is performed by the
Karman vortex flow meter means 2 in parallel with step 115 outputs
derived at step 120 are integrated at step 121 and it is determined
at step 122 whether or not a predetermined period T2 has elapsed
from the start of step 15. If the predetermined period T2 has not
elapsed, the operation performed at the steps 119 to 122 are repeated
until the period T2 has elapsed. When the period T2 has elapsed,
the Karman vortex flow meter means 2 is stopped at step 123 followed
by the execution proceeding to step 124.
After thus integrating the outputs of the respective thermal flow
sensor 1 and Karman vortex flow meter means 2 it is determined
at step 124 whether or not a correction should be made in accordance
with the integrated flow rate value or a mean flow rate value calculated
from the integrated value. If it is determined that the correction
should not be made (N), the execution returns to step 101 whereas
if it is determined that the correction should be made (Y), the
correction is executed at step 125. The correction may be made based
on the ratio of an integrated value of the flow rate measured by
the thermal flow sensor 1 to an integrated value of the flow rate
measured by the Karman vortex flow meter means 2 or the ratio of
a mean flow rate value of the thermal flow sensor 1 to that of the
Karman vortex flow meter means 2. More specifically, when a measured
flow rate value lies within the measurable range of the Karman vortex
flow meter means 2 or between F1 and F2 the value is corrected
in accordance with the foregoing equation (2) based on a flow rate
value measured by the Karman vortex flow meter means 2. Subsequently,
these operations are continually or intermittently performed.
It will be appreciated that the employment of the Karman vortex
flow meter means 2 for correction allows the thermal flow sensor
1 to be used as a highly accurate and widely measurable mass flow
meter even if the composition of gases changes. Also, a drift of
output from the thermal flow sensor 1 due to a long term service,
is corrected by the above-mentioned correction. The Karman vortex
flow meter means 2 is operated only when it is needed, so that the
power consumption can be reduced.
In the operations shown in the flowchart of FIG. 3 after the correction
at step 125 has been completed and the execution has returned to
the initial state of step 101 the next correction may be prevented
for a predetermined period even if the conditions for executing
the correction are satisfied. This predetermined preventing period
is chosen to be short if a correction amount is large while long
if a correction amount is small, whereby the correction is executed
more times as fluctuation in the composition of gases is larger,
which results in removing useless operations and accordingly reducing
the power consumption.
At step 124 if the difference between the outputs from the thermal
flow sensor 1 and the Karman vortex flow meter means 2 or the ratio
of the output from the thermal flow sensor 1 to the output from
the Karman vortex flow meter means presents a value above a predetermined
value, the difference may be stored without executing the correction.
If the difference or ratio is not changed in the next execution
loop, that is, if the difference in ratio between the two measurements
is below a predetermined value, the correction may be executed.
This additional function is useful in preventing malfunctions.
Further, at steps 117 and 121 if sampling values of both flow
meters largely scatter during the integration, that is, if a flow
is not stable, the integration time may be prolonged so as to perform
a precise correction even if the flow is largely fluctuating.
At steps 117 and 121 if the results of integrations of the outputs
from the thermal flow sensor 1 and the Karman vortex flow meter
means 2 show that the output from the Karman vortex flow meter 2
only is below its measurable range, the correction may be cancelled
and the output value be stored. The correction may not be resumed
unless the integrated output value of the karman vortex flow meter
means 2 becomes larger than the stored value by a predetermined
value. If the output from the Karman vortex flow meter means 2 comes
off the measurable range thereof, the operation may be immediately
stopped and return to the initial state at step 101.
Also, the thermal flow sensor 1 and the Karman vortex flow meter
means 2 may be alternately operated, such that when the output of
the Karman vortex flow meter lies in the measurable range for a
predetermined period, the outputs of the thermal flow sensor 1 and
the Karman vortex flow meter means 2 are integrated for a predetermined
period. Then, if the output of the Karman vortex flow meter still
remains in the measurable range, this value is used to correct a
measured value of the thermal flow sensor 1 as described above.
Since the determination of whether to perform the correction is
made directly based on the output of the Karman vortex flow meter
means 2 a precise correction operation is ensured. The Karman vortex
flow meter means 2 may occasionally output a value different from
that of the thermal flow sensor 1 for an identical flow rate. Also
in this case, after the correction at step 125 has been completed
and the execution has returned to the initial state of step 101
the next correction may be prevented for a predetermined period
even if the conditions for executing the correction are satisfied.
This predetermined preventing period is chosen to be short if a
correction amount is large while long if a correction amount is
small, whereby the correction is executed more times as fluctuation
in the composition of gases is larger, which results in removing
useless operations and accordingly reducing the power consumption.
If the difference between the outputs from the thermal flow sensor
1 and the Karman vortex flow meter means 2 or the ratio of the output
from the thermal flow sensor 1 to the output from the Karman vortex
flow meter means 2 presents a value above a predetermined value,
the value or difference may be stored without executing the correction.
If the difference or ratio is not changed in the next execution
loop, that is, if the difference in the ratio between the two consecutive
measurements is below a predetermined value, the correction may
be executed, whereby malfunctions can be avoided as described above.
Further, if sampling values of both flow meters largely scatter
during the integration, that is, if a flow is not stable, the integration
time may be prolonged so as to perform a precise correction even
if the flow is largely fluctuating.
FIG. 4 shows the structure of the flow meter of the present embodiment
in cross-section, and FIG. 5 is a sectional view taken along a line
A--A in FIG. 4. An arrow F indicates the direction of the flow.
A conduit 11 for introducing a gas to be measured to the flow sensors
is made up of an entrance path 12 a throat 14 made narrower than
the entrance path 12 through a restriction 13 and an exit path
16 made wider than the throat 14 through an enlarger 15. In the
entrance path 12 there are disposed a plurality of rectifying screens
17.sub.1 -17.sub.4 having a diameter substantially equal to the
inner diameter of the entrance path 12. The respective screens 17.sub.2
-17.sub.4 have their peripheral portions interposed between respective
adjacent two fixing spacers 18.sub.1 -18.sub.4. The outermost screen
17.sub.1 is interposed between the spacer 181 and a threaded ring
19.sub.1 which is engaged with thread formed on the inner wall of
an end portion of the entrance path 11 to thereby tightly fasten
the screens 17.sub.1 -17.sub.4. The screens 17.sub.1 -17.sub.4 are
thus disposed with predetermined intervals by the spacers 18.sub.1
-18.sub.4. The inner diameter of the respective spacers 18.sub.1
-18.sub.4 and the threaded ring 19.sub.1 is coincident with the
inner diameter of the beginning of the restriction 13 so as to avoid
unevenness on the inner wall of the entrance path 12. The exit path
16 is also provided with a screen 17.sub.5 at a location immediately
behind the enlarger 15 which is supported by a spacer 18.sub.5
and pressed by a threaded ring 19.sub.2 which in turn is engaged
with thread formed on the inner wall of an end portion of the exit
path 16.
The throat 14 is formed with a recess 20 for mounting a sensor
in a longitudinal central portion of the outer wall. In this recess
20 there is disposed a thermal flow sensor 21 constituting a thermal
flow sensors 1 such that its sensor elements are located on the
inner surface of the throat 14 through a sensor fixing hole formed
through the wall of the throat 14 at a location immediately behind
the restriction 13. The thermal flow sensor 21 after mounted in
the recess 20 is covered with a lid 31 and fixed by a screw 32.
The location where the sensor 21 is mounted, where turbulence in
flow is remarkably reduced by virtue of the screens 17.sub.1 -17.sub.4
and the restriction 13 provides a stable measuring environment
from a low flow rate range to a high flow rate range.
At a location downstream of the thermal flow sensor 21 in the throat
14 a Karman vortex flow meter means 2 is disposed which is composed
of a cylindrical vortex generating member 22 a pressure measuring
hole 23 formed on the inner wall of the conduit at a location downstream
of the vortex generating member 22 and a pressure detecting element
24 for detecting the pressure prevailing in the pressure measuring
hole 23. As is well know in the art, Karman vortices generated downstream
of the vortex generating member 22 in accordance with the rate of
a gas flowing in the conduit causes a change in pressure in the
pressure measuring hole 23 which is detected by the pressure detecting
element 24. Then, the number of vortices generated in a unit time
or a vortex frequency f (Hz) is counted to thereby calculate the
flow rate of a gas under measurement by the following equation (3):
where S.sub.t represents a constant.
The Karman vortex flow meter means 2 thus constructed is advantageous
in that it is not influenced by the composition, density, temperature,
pressure and so on of an ordinary fluid to thereby ensure accurate
and stable measurement of a mass flow as well as its measurable
range is extended toward a low flow region because of the vortex
generating member 22 located in the throat 14 where the fluid flow
is faster than other locations in the conduit.
FIG. 6 shows a second embodiment of the present invention, where
parts corresponding to those in FIG. 4 are designated the same reference
numerals. In FIG. 6 a Karman vortex flow meter means 2 composed
of a vortex generating member 22 a pressure measuring hole 23 and
a pressure detecting element 24 in a manner similar to the first
embodiment shown in FIG. 4 is disposed in an exit path 16 and
two rectifying screens 17.sub.6 17.sub.7 are provided in front
of the vortex generating member 22. This structure, since the Karman
vortex flow meter means 2 is disposed in a wide conduit portion,
is advantageous in facilitating the manufacturing of the flow meter
as well as reducing output errors due to an error in the size of
the vortex generating member 22. The rectifying screens 17.sub.6
17.sub.7 disposed behind the enlarger 15 effectively prevents turbulence
of flow occurring in the enlarger 15 from influencing the output
of the Karman vortex flow meter means 2.
FIG. 7 shows another example of a Karman vortex flow meter which
may be employed in the present invention. This Karman vortex flow
meter is composed of a vortex generating member 22 disposed in the
exit path 16 pressure measuring holes 23 formed at a location downstream
of the vortex generating member 22 through the walls diametrically
spaced with each other, a tube 25 coupled to communicate the pressure
measuring holes 23 and a pressure difference fluctuation detecting
unit 26 for detecting a pressure difference between two pressure
measuring holes by measuring the rate of a fluid flow in the tube
25 caused by such pressure difference. FIG. 8 is a cross.sectional
view of the pressure difference fluctuation detecting unit 26. A
thermal flow sensor 29 capable of detecting flow rates in two directions,
is mounted on a sensor mounting board 32 arranged in a flow channel
28 formed inside a flow housing. Pressure inlet and outlet ports
30 are communicated with each other through a tube 25 whereby a
fluctuation in a pressure difference between the pressure measuring
holes 23 is measured by the thermal flow sensor 29 disposed in the
flow channel 28. A pressure change caused by vortices is generally
quite minute so that a pressure detecting element having a relatively
large diaphragm should be employed for obtaining a sufficient sensitivity,
thereby resulting in providing a large size flow meter. However,
the structure of the present embodiment, where the tube for communicating
the pressure difference detecting unit 25 with the two pressure
measuring holes 23 is integrated to the conduit 11 provides a high
sensitivity and a compact size.
It should be noted that the Karman vortex flow meter is not limited
to the type employed in the above-mentioned embodiment, and one
which has a pressure measuring hole integrated with a vortex generating
member 22 as shown in FIGS. 9a, 9b, may be employed. The shape
of the vortex generating member 22 may be alternatively selected
from those shown in FIGS. 10a-d. Incidentally, an arrow 40 in FIG.
10 indicates the direction of a fluid flow.
Since many changes could be made in the above construction and
many apparently widely differing embodiments of the present invention
could be made without departing from the scope thereof, it is intended
that all matters contained in the above description or shown in
the accompanying drawings shall be interpreted as illustrative and
not in a limiting sense. |