Abstrict A mass flow meter provides improved accuracy in measuring the mass
flow rate of a fluid flowing through a straight measuring tube.
The as measured mass flow rate value is changed by parameters such
as the Young's modules, second moment of area, length, axial force,
etc. of the measuring tube. The parameters change with the strain
caused in the axial direction of the measuring tube. A strain sensor
is installed on a measuring tube, and the as measured mass flow
rate is corrected based on the output of the strain sensor. Since
the strain affects in a different way the substantial mass flow
rate and the zero point offset, the correction is conducted separately
on the substantial mass flow rate and on the zero point offset so
that the accuracy of measurement may be improved. Since the parameters
change also with the temperature of the measuring tube, a temperature
sensor is also installed on the measuring tube so that the accuracy
of measurement may be further improved.
Claims What is claimed is:
1. A mass flow meter for measuring a mass flow rate of a fluid
based on a reactive force generated by accelerating the fluid as
it flows through a straight measuring tube, the mass flow meter
comprising:
a sensing means for detecting a state of the measuring tube, wherein
said state comprises at least one of a strain and a temperature
of the measuring tube; and
a correcting means for correcting an as measured value of the mass
flow rate based on the output of the sensing means, wherein the
correcting means comprises:
a mass flow rate correcting means for correcting a part of the
as measured value of the mass flow rate corresponding to a substantial
mass flow rate of the fluid; and
a zero point correcting means for correcting a part of the as measured
value corresponding to a zero point offset remaining when the fluid
is not flowing;
wherein the zero point offset (Qzc) is calculated by using a known
relationship between the zero-point offset and the state of the
measuring tube based on the output of the sensing means;
wherein the substantial mass flow rate value (Qs) is obtained by
subtracting the zero-point offset value (Qzc) from the as measured
value of the mass flow rate (Qm); and
wherein a substantial mass flow rate after correction (Qmc) is
obtained as a result of correcting the substantial mass flow rate
value (Qs) based on the output of the sensing means.
2. The mass flow meter according to claim 1 wherein the sensing
means comprises a strain sensor for detecting a strain in the axial
direction of the measuring tube.
3. The mass flow meter according to claim 1 wherein the sensing
means comprises a temperature sensor for detecting a temperature
of the measuring tube.
4. The mass flow meter according to claim 1 wherein the sensing
means comprises a strain sensor for detecting a strain in the axial
direction of the measuring tube and a temperature sensor for detecting
a temperature of the measuring tube.
5. The mass flow meter according to claim 1 wherein the mass flow
meter further comprises a frame for fixing the measuring tube at
nodes of vibration thereof, and the sensing means comprises a first
temperature sensor for detecting a temperature of the measuring
tube and a second temperature sensor for detecting a temperature
of the frame.
Description FIELD OF THE INVENTION
The present invention relates to a Coriolis-type mass flow meter
that measures a mass flow rate of a fluid in question based on the
reactive force generated by accelerating the fluid as it flows through
a straight measuring tube. More specifically, the invention relates
to a mass flow meter that corrects a measured mass flow rate based
on a measured strain in the axial direction of the measuring tube
and a temperature of the measuring tube to improve the accuracy
of mass flow rate measurement.
BACKGROUND
A conventional Coriolis-type mass flow meter is disclosed in the
Japanese Examined Patent Publication (Kokoku) No. S60-34683. FIG.
7 is an isometric view of the conventional mass flow meter that
includes a U-shaped measuring tube 2 fixed to a housing 19 with
fixed (cantilever) mounts at both ends. A beam like resonator 120
is fixed to the housing 19 with fixed (cantilever) mount at an end
(bottom) between both ends 31 and 32 of the U-shaped measuring tube
2. The natural frequencies of the U-shaped measuring tube 2 and
the resonator 120 are set to be the same so that the measuring tube
2 and the resonator 120 may resonate with one another. A driver
5 comprised, for example, of a coil and a magnet is disposed between
the bottom of the U-shaped measuring tube 2 and the top (the other
end) of the resonator 120. The U-shaped measuring tube 2 and the
resonator 120 are vibrated at the resonant frequency by the driver
5 and a driver circuit 8. Speed sensors 6a and 6b comprised of a
coil and a magnet are fixed to both ends of the straight bottom
of the U-shaped measuring tube 2. The output signals from the speed
sensors 6a and 6b are input to a signal processing circuit 9 and
converted therein to a flow rate signal. Detecting means for detecting
the vibration of the measuring tube 2 are not limited to speed sensors
such as the sensors 6a and 6b. Any sensors which can detect the
vibration of the measuring tube, such as displacement sensors, acceleration
sensors, etc., may be used for detecting the vibration of the measuring
tube 2.
The fluid in question flows in a direction indicated by an arrow
U from an inlet tube (not shown) into the U-shaped measuring tube
2 and flows out in a direction indicated by an arrow D to an outlet
tube (not shown) from the U-shaped measuring tube 2 after flowing
through the measuring tube 2. In a first case, in which the flow
rate of the fluid is zero, the U-shaped measuring tube 2 and the
resonator 120 are vibrated at the resonant frequency by the driver
5 and the driver circuit 8. Since the positions at which the speed
sensors 6a and 6b are fixed to execute the same movement, the output
signals output from the speed sensors 6a and 6b include no phase
difference between them. In a second case, in which the fluid is
flowing, the fluid flows through the vibrating measuring tube 2
and the Coriolis force is generated perpendicularly to the fluid
velocity. Since the flow directions are opposite to one another
between both arms of the U-shaped measuring tube 2 the directions
of the generated Coriolis force are also opposite to one another.
Therefore, moment is generated with respect to the O axis that is
the central axis of the U-shaped measuring tube. Due to the generated
moment, a torsional vibration with respect to the 0 axis is superimposed
on a bending vibration with respect to the axis W--W connecting
the ends 31 and 32 of the measuring tube 2. Because of this, the
output signals output from the speed sensors 6a and 6b are detected
as the signals having a phase difference (time difference) between
them. Since the Coriolis force is proportional to the mass flow
rate, the phase difference detected from the output signals output
of the speed sensors 6a and 6b is also proportional to the mass
flow rate. Therefore, the mass flow rate of the fluid is measured
by measuring the phase difference (time difference) between the
output signals of the sensors.
The requirement of shaping a measuring tube in the U-shaped as
shown in FIG. 7 or in the other curved shapes causes a number of
drawbacks including: (1) large pressure loss; (2) difficulty in
cleaning the measuring tube; (3) difficulty in draining the fluid
remaining in the measuring tube after the measurement is over; and
(4) the outer dimensions of the mass flow meter are large. While
these problems may be solved by using a straight measuring tube,
a straight measuring tube causes other inherent problems. For example,
stress or strain is caused and changed in the axial direction of
the straight measuring tube by temperature change of the fluid in
question or the environment. The effects of the stress or strain
negatively impacts the measured value of the mass flow rate and
causes a further measurement error.
In view of the foregoing, it is an object of the present invention
to provide a Coriolis-type mass flow meter that measures a mass
flow rate of a fluid in question based on the reactive force generated
by accelerating the fluid as it flows through a straight measuring
tube, and facilitates improving the accuracy of measurement by correcting
the as measured mass flow rate value based on the measured strain
in the axial direction of the measuring tube and the temperature
of the measuring tube.
SUMMARY OF THE INVENTION
According to an aspect of the invention, a mass flow meter is provided
for measuring a mass flow rate of a fluid in question based on the
reactive force generated by accelerating the fluid flowing through
a straight measuring tube. The mass flow meter includes a sensing
means for detecting the state of the measuring tube, and a correcting
means for correcting the as measured value of the mass flow rate
based on the output of the sensing means. The correcting means preferably
comprises a mass flow rate correcting means for correcting a part
of the as measured value corresponding to the substantial mass flow
rate of the fluid, and a zero point correcting means for correcting
a part of the as measured value corresponding to a zero point offset
remaining when the fluid is not flowing. The sensing means preferably
comprises a strain sensor for detecting the strain caused in the
axial direction of the measuring tube, or a temperature sensor for
detecting the temperature of the measuring tube. The sensing means
also preferably comprises a strain sensor for detecting the strain
caused in the axial direction of the measuring tube, and a temperature
sensor for detecting the temperature of the measuring tube. It is
also preferable to provide the mass flow meter with a frame for
fixing the measuring tube at the nodes of vibration thereof, and
to install a first temperature sensor for detecting the temperature
of the measuring tube, and a second temperature sensor for detecting
the temperature of the frame.
According to another aspect of the invention, a mass flow meter
is provided that includes a detector unit comprising a measuring
tube, a frame for fixing the measuring tube at the nodes of vibration
thereof, and a driving means for vibrating the measuring tube, and
a vibration detecting means for detecting the vibration of the measuring
tube, a housing for fixing the extended sections of the measuring
tube thereto and for suspending the detector unit therein, a temperature
sensor for measuring the temperature of the measuring tube, the
temperature sensor being installed on a mid-point between a first
location, therein the measuring tube is fixed to the frame, and
a second location, therein the measuring tube is fixed to the housing,
and a correcting means for correcting the as measured value of the
mass flow rate based on the output of the temperature sensor.
According to an aspect of the invention, the mass flow rate correcting
means corrects a part of the measured value corresponding to the
substantial mass flow rate based on the output of the sensing means,
and the zero point correcting means corrects a part of the measured
value corresponding to a zero point offset. As a result the as measured
mass flow rate is totally corrected. The correction is conducted
based on the principle described below. The as measured mass flow
rate value is changed by the parameters such as the Young's modules,
second moment of area, length, axial force, etc. of the measuring
tube. The parameters change with the strain caused in the axial
direction of the measuring tube, the temperature of the measuring
tube, etc. Therefore, a temperature sensor and a strain sensor are
provided to the mass flow meter, and the as measured mass flow rate
value is corrected based on the output signals of the sensors. Since
the strain and temperature of the measuring tube affect in the different
way to the substantial mass flow rate and the zero point offset
of the as measured value, the substantial mass flow rate and the
zero point offset are corrected separately so that the measurement
accuracy may be improved.
Further, the strain in the axial direction of the measuring tube
is obtained from the output of the first temperature sensor for
detecting temperature of the measuring tube and from the output
of the second temperature sensor for detecting the temperature of
the frame and the terminal expansion coefficients of the measuring
tube and frame, and the correcting means corrects the as measured
mass flow rate value by the obtained strain and the output of the
first temperature sensor. In accordance with another aspect of the
invention, a temperature sensor is installed on a mid-point between
a first location, therein the measuring tube is fixed to the frame,
and a second location, therein the measuring tube is fixed to the
housing so that the influence of the temperature sensor on the vibration
of the measuring tube may be suppressed.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in greater detail with reference
to the accompanying drawings, wherein:
FIG. 1 is a top plan view of a first embodiment of a mass flow
meter according to the present invention;
FIG. 2 is a cross section of the mass flow meter of FIG. 1;
FIG. 3 is a cross section of a second embodiment of a mass flow
meter according to the present invention;
FIG. 4 is a top plan view of a third embodiment of a mass flow
meter according to the present invention;
FIG. 5 is a top plan view of a fourth embodiment of a mass flow
meter according to the present invention;
FIG. 6 is a cross section of the mass flow meter of FIG. 5; and
FIG. 7 is an isometric view of a conventional mass flow meter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a top plan view of a first embodiment of a mass flow
meter according to the present invention. A detector unit 1 of the
first embodiment has a straight measuring tube 2 that is fixed at
nodes of vibration by fixers 3a and 3b, respectively. The mass flow
meter also has reinforcing bars 4a and 4b connecting the fixers
3a and 3b. The fixers 3a, 3b and the reinforcing bars 4a, 4b are
integrated into a unitary rectangular frame. The fluid in question
flows from side U to the side D, as illustrated in FIG. 1.
FIG. 2 is a cross section of the mass flow meter of FIG. 1. Referring
to FIG. 2 a driver 5 is fixed to the center of the vibrating section
of the measuring tube 2 and on an adapter 7a. Sensors 6a and 6b
which detect the vibration of the measuring tube 2 are installed
symmetrically on the upstream side and the downstream side on the
measuring tube 2 and on respective adapters 7b and 7c. The adapters
7a, 7b and 7c are disposed between the reinforcing bars 4a and 4b.
The measuring tube 2 is vibrated at the resonant frequency by the
driver circuit 8 and the driver 5 with the positions a and b thereof
fixed as the nodes of vibration. Signals detected by the sensors
6a and 6b are converted in a signal processing circuit 9 to a signal
Qm indicative of a mass flow rate of the fluid in question.
In the structure described above, the Coriolis force is generated
perpendicularly to the flow velocity of the fluid flowing through
the measuring tube 2. The direction of the Coriolis force is opposite
on the upstream side to that on the downstream side separated from
one another by the driver 5 positioned on the boundary between the
upstream section and the downstream section. Therefore, a phase
difference (time difference) is generated in proportion to the mass
flow rate between the sensors 6a and 6b disposed on the upstream
side and the downstream side, respectively.
The mass flow meter of FIG. 1 provided with a straight measuring
tube, shows following merits which are the reversal of the drawbacks
of the curved measuring tube: (1) low pressure loss; (2) it is easy
to clean the measuring tube; (3) the fluid is drained easily after
the measurement is over; and,(4) the outer dimensions of the mass
flow meter are small. However, since the measuring tube 2 is fixed
at both ends of its vibrating section by the fixers 3a, 3b and the
reinforcing bars 4a, 4b, stress or strain is caused in the axial
direction of the measuring tube 2 by temperature changes of the
fluid or the environment when the temperature difference is caused
between the measuring tube 2 and the fixers 3a, 3b or the reinforcing
bars 4a, 4b. Stress or strain is also caused in the axial direction
of the measuring tube 2 by the difference of the thermal expansion
coefficients between the measuring tube 2 and the fixers 3a, 3b
or the reinforcing bars 4a, 4b when the temperature of the detector
unit 1 changes uniformly.
The as measured value of the mass flow rate is usually measured
by the mass flow meter of the first embodiment having a straight
measuring tube, is changed by the Young's modules, second moment
of area, length, axial force, etc. of the measuring tube. Since
the stress or strain caused in the axial direction of the measuring
tube 2 further changes the parameters described above, the measured
mass flow rate is changed to further cause a measurement error.
The measurement error may be reduced and the measurement accuracy
may be improved by installing a strain sensor (comprised of a piezoelectric
element or a strain gauge) on the measuring tube 2 for measuring
the strain change in the axial direction of the measuring tube 2
and by correcting the measured mass flow rate based on the strain
detected by the strain sensor.
It is preferable to conduct the correction in the following way.
The as measured mass flow rate value (phase difference or time difference
in this case) may be divided into a part corresponding to the substantial
mass flow rate and a part remaining when the mass flow rate is zero
(zero point offset). Usually, the strain in the axial direction
of the measuring tube affects both of these parts differently. Therefore,
the measurement accuracy may be improved more by correcting the
as measured mass flow rate value by the measured strain in the ways
different from a part to another of the measured mass flow rate
value.
An example of the correction will be explained in greater detail.
In a case in that the part (zero point offset) Qz, remaining even
when the mass flow rate is zero in the as measured mass flow rate
value Qm, may be assumed to change linearly with the strain value
S detected in the axial direction of the measuring tube, Qz is corrected
as follows.
Here, Qzc is a zero point offset after correction, and .alpha.sz
is a correction factor per unit strain for the zero point offset.
The substantial part Qs corresponding to the as measured substantial
mass flow rate is expresses as follows.
When this Qs may be assumed to change linearly with the strain
value S detected in the axial direction of the measuring tube, Qs
is corrected as follows.
Here, Qmc is a substantial mass flow rate after correction, and
.alpha.ss is a correction factor per unit stain for the part corresponding
to the substantial mass flow rate.
Though the linear correction is applied to the as measured mass
flow rate in the above example, higher order correction may be applied
depending on the characteristics of the mass flow meter.
The influential parameters (Young's modules, second moment of area,
length, etc. of the measuring tube) which affect the as measured
mass flow rate changes with the temperature of the measuring tube
2. The influence of the temperature change of the parameters is
combined with the influence of the strain caused in the axial direction
of the measuring tube. For example, the second moment of area of
the measuring tube changes in a combined fashion through the change
by the temperature and the thermal expansion coefficient of the
measuring tube and through the change by the strain in the axial
direction of the measuring tube and the modules of transverse elasticity
of the measuring tube. The length of the measuring tube changes
in a combined fashion through the change by the temperature and
the thermal expansion coefficient of the measuring tube and through
the change by the strain in the axial direction of the measuring
tube and the modules of longitudinal elasticity of the measuring
tube.
FIG. 3 is a cross section of a second embodiment of a mass flow
meter according to the present invention. The second embodiment
is different from the first embodiment in that a temperature sensor
11 is also installed on the measuring tube 2 for measuring the temperature
of the measuring tube 2. Therefore, the measurement accuracy is
further improved by combining the temperature of the measuring tube
2 detected by the temperature sensor 11 and the strain in the axial
direction of the measuring tube detected by the strain sensor 10.
In this occasion, it is preferable to divide the as measured mass
rate value into a part corresponding to the substantial mass flow
rate and a part remaining when the mass flow rate is zero (zero
point offset), and to conduct corrections separately based on the
temperature and the strain in the axial direction of the measuring
tube.
An example of the correction will be explained in further detail.
In a case in that the part (zero point offset) Qz, remaining even
when the mass flow rate is zero in the as measured mass flow rate
value Qm, may be assumed to change linearly with the temperature
and the strain value S detected in the axial direction of the measuring
tube, Qz is corrected as follows.
Here, Ts is a reference temperature, and .alpha.tz is a correction
factor per unit temperature for the zero point offset.
The substantial part Qs corresponding to the as measured substantial
mass flow rate is expressed as follows.
When this Qs may be assumed to change linearly with the strain
value S detected in the axial direction of the measuring tube and
the temperature Tt of the measuring tube, Qs is corrected as follows.
Here, .alpha.ts is a correction factor per unit temperature for
the part corresponding to the substantial mass flow rate.
Though the linear correction is applied to the as measured mass
flow rate in the above example, higher order correction may be applied
depending on the characteristics of the mass flow meter.
FIG. 4 is a top plan view of a third embodiment of a mass flow
meter according to the present invention. Referring now to FIG.
4 the third embodiment is different from the first and second embodiments
in that a temperature sensor 12 is installed on the center of the
reinforcing bar 4b for measuring the temperature of the frame comprised
of the fixers 3a, 3b and the reinforcing bars 4a, 4b and a temperature
sensor 11 is installed on the measuring tube 2 on the right hand
side thereof for measuring the temperature of the measuring tube
2.
In FIG. 4 since all the stress exerted from outside is exerted
to the frame and not exerted directly to the measuring tube 2 the
influence of the external stress or strain caused in the axial direction
of the measuring tube 2 is reduced to a negligible amount. In this
occasion, the strain caused in the axial direction of the measuring
tube 2 may be assumed to be governed by the temperatures, temperature
difference and the thermal expansion coefficients of the measuring
tube 2 and the frame. Since the thermal expansion coefficients are
determined by the materials, it is possible to estimate the strain
in the axial direction of the measuring tube 2 from the temperatures
of the measuring tube 2 and the frame.
Therefore, by detecting the temperatures of the measuring tube
2 and the frame with the sensors 11 and 12 the temperature of the
measuring tube 2 as well as the strain caused in the axial direction
of the measuring tube 2 are determined. By correcting the as measured
mass flow rate with these detected values, the measurement accuracy
may be improved. In this occasion, it is preferable to divide the
as measured mass rate value into a part corresponding to the substantial
mass flow rate and a part remaining when the mass flow rate is zero
(zero point offset), and to conduct corrections separately.
Since the strain sensor 10 is not installed on the measuring tube
2 of the third embodiment, the influence of the strain sensor 10
to the vibrating measuring tube 2 can be suppressed more as compared
with the first and the second embodiment where the strain sensor
10 is installed on the measuring tube 2.
An example of the correction will be explained more in detail.
The temperature difference Td between the frame and the measuring
tube 2 is expressed by the temperature Tb of the frame as follows.
When the thermal expansion coefficients of the frame (exactly describing,
the reinforcing bars 4a, 4b in FIG. 4) and the measuring tube 2
are same, the detected value of the strain S caused in the axial
direction of the measuring tube 2 may be assumed to be proportional
to Td. By considering in the same way as before, a following equation
is obtained.
Here, .alpha.dz is a correction factor per unit temperature difference
for the zero point offset, and a ds is a correction factor per unit
temperature difference for the part corresponding to the substantial
mass flow rate.
When the thermal expansion coefficients of the frame (exactly describing,
the reinforcing bars 4a, 4b in FIG. 4) and the measuring tube 2
are different from one another, the detected value of the strain
S caused in the axial direction of the measuring tube 2 is expressed
by the thermal expansion coefficient .alpha.tb of the frame and
the thermal expansion coefficient .alpha.tt of measuring tube as
follows.
Then, by using this S and by considering in the same way as before,
a following equation is obtained.
Though the linear correction is applied to the as measured mass
flow rate in the above example, higher order correction may be applied
depending on the characteristics of the mass flow meter.
FIG. 5 is a top plan view of a fourth embodiment of a mass flow
meter according to the present invention. FIG. 6 is a cross section
of the mass flow meter of FIG. 5. In the fourth embodiment, a detector
unit 20 is suspended in a housing 15 at predetermined point of extended
sections 14a, 14b of the measuring tube 2. By installing a temperature
sensor 11 on one of the extended sections 14a for detecting the
temperature of the measuring tube 2 the influence of the temperature
sensor 11 affecting to the vibrating measuring tube 2 may be suppressed
as compared with the measuring tube 2 with a temperature sensor
installed on its vibrating section. In the fourth embodiment, it
is not always necessary to execute corrections separately on the
part corresponding to the substantial mass flow rate and on the
zero point offset. In some cases, the correction on the zero point
offset may be omitted.
According to an aspect of the invention, correction is executed
separately on the part corresponding to the substantial mass flow
rate and on the part corresponding to the zero point offset based
on the output signals from the sensor for detecting the state parameters
of the measuring tube, e.g. a strain sensor for detecting the strain
caused in the axial direction of the measuring tube, a temperature
sensor for detecting the temperature of the measuring tube, etc.
As a result, the measurement accuracy is improved.
Correction is executed separately on the part corresponding to
the substantial mass flow rate and on the part corresponding to
the zero point offset based on the output signals from two temperature
sensors. As a result, the measurement accuracy is improved to the
same extent as by the stain and temperature sensors but more easily
and cheaply.
According to another aspect of the invention, influence of the
installed temperature sensor to the vibration of the measuring tube
is avoided and the measurement accuracy is further improved.
As a result, the improvement of the measurement accuracy is facilitated
while maintaining the inherent merits of the straight tube mass
flow meters including (1) low pressure loss, (2) ease in cleaning
the measuring tube, (3) ease in draining the remaining fluid out
of the measuring tube, and (4) small outer dimensions.
The invention has been described with reference to certain preferred
embodiments thereof. It will be understood, however, that variations
and modifications are possible within the scope of the appended
claims. |