In a mass flow meter having a "U" shaped conduit through
which a fluid to be measured flows and oscillation means therefor
to cause the "U" shaped conduit to produce a torsional
moment due to the Coriolis force, a torque beam on which strain
gages are mounted is fixed between both legs of the "U"
shaped conduit to detect the torsional moment as an electric quantity.
What is claimed is:
1. A mass flow meter comprising:
a "U" shaped conduit having two legs with each leg terminating
in an end and through which a fluid to be measured flows, both ends
of said "U" shaped conduit being fixed to said base;
a supporting beam, one end thereof being provided with an electromagnet
and the other end thereof being fixed to said base, the beam and
the conduit being positioned on the base and with respect to one
another so that they form a tuning fork structure with the "U"
shaped conduit being oscillated at a natural frequency of said tuning
fork structure; and
a torque beam fixed between the two legs of said "U"
shaped conduit, at least one strain gauge mounted on the torque
beam, said at least one strain gauge being electrically responsive
to the torque transmitted to said torque beam from said "U"
shaped conduit to measure the amount of torque moment.
2. A mass flow meter according to claim 1 wherein the torque beam
has a pair of opposing surfaces, four strain gauges being mounted
on both opposing surfaces of said torque beam and being respectively
equidistant from an adjacent leg, said four strain gauges constituting
a bridge circuit arranged so that strain gauges differently distorted
due to said torque transmitted to said torque beam are connected
in series with one another.
3. A mass flow meter according to claim 1 wherein a pair of opposing
strain gauges are each mounted on one of the pair of opposing surfaces
of said torque beam and said two strain gauges being equidistant
from at least one of said legs, and said two strain gauges being
connected in series with two resistors to form a bridge circuit.
4. A mass flow meter according to claim 1 wherein two strain gauges
are mounted on one of the opposing surfaces of said torque beam
and are equidistant from both legs, the two strain gauges being
connected in series with two resistors to form a bridge circuit.
5. A mass flow meter according to claims 1 2 or 3 wherein said
bridge circuit is supplied with DC input voltage.
FIELD OF THE INVENTION
The present invention relates to a mass flow meter and more particularly
a mass flow measuring device in the form of a "U" shaped
conduit through which a fluid material flows and which is subjected
to oscillation for producing a Coriolis force proportional, to the
mass flow and arranged to measure a torsional moment caused in the
"U" shaped conduit due to the Coriolis force.
BACKGROUND OF THE INVENTION
In this kind of device, the application of strain gages is conceived
as a simple means for the detection of torsional moments in a "U"
shaped conduit. One known device as taught by U.S. Pat. No. 4187721
is shown in FIG. 1.
In the figure, strain gages 1 and 2 are mounted adjacent to the
intersection of inlet leg 3 and base leg 4 and outlet leg 5 and
base leg 4 respectively. Strain gages 1 and 2 which may be viewed
as variable resistors dependent upon the distortion of the adjacent
portion of the "U" shaped conduit are connected with resistors
6 and 7 to form a bridge circuit communicating with a voltage source
as indicated, and connected to AC differential amplifier 8. In the
case of simple oscillation of the "U" shaped conduit,
the resistivity of strain gages 1 and 2 varies equally thereby providing
essentially identical inputs to AC differential amplifier 8. However,
in the event of distortion due to Coriolis forces, one of strain
gages 1 and 2 will increase in resistivity while the other decreases
thereby providing different inputs to AC differential amplifier
8 and providing an output in the form of an AC signal proportional
in magnitude and sense to the different strains imposed upon strain
gages 1 and 2.
The output from AC differential amplifier 8 is directed to synchronous
demodulator 9 which, in conjunction with the conduit oscillation
signal, provides a DC output proportional in magnitude and sense
to the distortion of the "U" shaped conduit as a result
of Coriolis forces.
However, it may well be said that no practicability is found in
such an embodiment. The reasons are that the output from such a
device is extremely small because of the much too small Coriolis
force and that the effects of flow vibration and temperature are
great because of the strain gage installations directly on the conduit.
Furthermore, even under simple oscillation, the outputs from both
strain gages 1 and 2 do not get small enough as a result a cancellation
in the AC differential amplifier due to the phase difference between
both strain gages 1 and 2.
SUMMARY OF THE INVENTION
Accordingly, the object of the present invention is to detect any
torsion in a "U" shaped conduit with higher sensitivity
in a wider range by utilizing strain gages in a mass flow meter
of the type discussed above. The purpose of the present invention
can be fulfilled by arranging a torsion-suffering beam, so called
a "TORQUE" beam of negligible rigidity compared with that
of the "U" shaped conduit, in the form of a bridge between
the parallel legs of the "U" conduit and mounting strain
gages on the toruqe beam.
This and other objects, features and advantages of the present
invention, as well as the invention itself, will become more apparent
from the following description of the preferred embodiments of the
present invention taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a mass flow meter of prior art in
the form of a "U" shaped conduit, in which strain gages
are used as means for detecting torsional moment of the "U"
FIG. 2 is a front view of one embodiment of the present invention;
FIG. 3 is a side view of the embodiment;
FIG. 4 is a view of the embodiment to illustrate the operation;
FIG. 5 is an electric connection diagram of the embodiment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 2 is a view of a "U" shaped conduit 11 through which
a fluid to be measured flows, and a permanent magnet 12 is fixed
to the top end thereof. Both ends of the "U" shaped tube
11 are fixed to a base 13. A support beam 15 with an electromagnet
14 fixed at the top end thereof is arranged facing the "U"
shaped tube 11 and is fixed to the base at the bottom end thereof
to form a tuning fork structure.
A "TORQUE" beam 16 is fixed between both legs of the
"U" shaped conduit 11 adjacent permanent magnet 12 and
strain gages 17 18 19 and 20 are installed on both surfaces of
the beam 16. These strain gages 17 18 19 and 20 are positioned
respectively in an equal distance from the legs as shown in FIG.
The following explanation is given of the operation of flow meter
constructed as above. A Coriolis force is generated in the flow
passing through the "U" shaped tube 11 by oscillating
the tuning fork structure at the natural frequency with an electromagnetic
force created between the electromagnet 14 and the permanent magnet
12 of the opposite "U" shaped conduit 11. The magnitude
of the Coriolis force is proportional to the mass of a fluid flowing
through the "U" shaped conduit 11 and the direction of
the force agrees with the direction of vector product of the movement
direction of the flow and angular velocity to oscillate the "U"
shaped conduit. Thus, a torsional moment or torque is generated
in the "U" shaped conduit 11 by the Coriolis force on
the sides of both legs since the flow direction is reversed on the
input and output sides of the "U" shaped conduit 11.
The torque is given as vibrating torque of the same frequency as
the oscillating frequency and the amplitude thereof is proportional
to the mass flow. The vibrating torque is transmitted to the torque
beam 16 and converted into an electric quantity by distorsion of
four strain gages mounted on the surfaces of the beam. The four
strain gages constitute a bridge circuit supplied with DC voltage
at Ein terminal as shown in FIG. 5 to convert only the vibrating
torque into an electric equivalent output proportional to torque
T, as shown in FIG. 4. Namely, in FIG. 4 if torque T is generated
in the counter clockwise direction, the strain gages 17 and 20 are
compressed while the other gages 18 and 19 are elongated, providing
a vibrating unbalance voltage output from Eout terminal in FIG.
5. On the other hand, the forces of vibration in the Y--Y directions
due to the electromagnet drive can scarcely cause the strain gages
17-20 to distort and even if a bit of distortion may occur therein,
they are compensated for and cancelled with each other in the bridge
circuit in FIG. 5. As for the circuit connection for constituting
the bridge, the strain gage 18 can be replaced with the strain gage
19 and the strain gage 17 with the strain gage 20 in FIG. 5 in a
manner so that the strain gages distorted in the different sense
(compression or elongation) due to the torsional moment are connected
in series with one another.
It is apparent that similar effects can be obtained by the two-gage
method using two strain gages, where for instance, strain gages
are employed for 17 and 18 in FIG. 4 while the other gages 19 and
20 in FIG. 5 are replaced by fixed resistors. Also, another arrangement
is considered in which strain gages 18 and 20 are replaced by fixed
resistors. As for the circuit connection, in the former example
the strain gage 18 which is mounted on the opposite surface of the
torque beam 16 to the strain gages 17 and the same distance from
one leg of the "U" shaped conduit can be exchanged with
the replaced fixed resistors 19 in FIG. 5 and in the later example
the strain gage 19 which is mounted on the same surface with the
strain gage 17 and the same distance from the respective legs of
the "U" shaped conduit can be exchanged with the replaced
fixed resistor 18.
Namely, the distortion of the strain gages in the same sense due
to temperature deviation or oscillation component which occurs,
as the case may be, is compensated for by connecting two strain
gages which are distorted in the different senses from each other
responding to the torsional moment in series in the bridge circuit.
However, the two-gage method is inferior to the four-gage method
The one or three gage method can be also employed, but such nonsymmetric
arrangements have difficulty in balancing the noise components having
the same sense such as mentioned above. Furthermore, a series circuit
of two strain gages or one resistor and one strain gage can be used
in accordance with the present invention, but such a series circuit
cannot compensate for the input-voltage component.
In the bridge circuit, AC voltage may be supplied to Ein terminal
in FIG. 5 on condition that a synchronous demodulator is used as
a detector of the vibrating signal. For the oscillation of the "U"
shaped conduit, the permanent magnet is not always necessary. When
the "U" shaped conduit is formed from a magnetic substance
it can be eliminated, and direct coupling of the electromagnet with
the "U" shaped conduit is also possible.
As is clear from the above, by mounting the strain gages on the
torque beam bridged between legs of the "U" shaped conduit,
the torsional moment thereof can be detected as an electric signal
with high sensitivity. Furthermore, temperature and small signal
characteristics are greatly improved, since the strain gages are
not directly influenced by temperature and vibration of the fluid.
Thus, the mass flow meter in accordance with the present invention
allows for excellent measurement of a single flow or pulse flows
of gas, liquid, slurries or the like with a linear relation between
the mass flow and output under a wide rangeability, high sensitivity
and excellent S/N ratio. Especially, when the bridge circuit is
formed by the strain gages, noise components due to temperature
deviation in fluid and atmosphere, oscillation of the "U"
shaped conduit and input voltage are compensated and a high reliability
of the measurement is attained.