Note: Descriptions are shown in the official language in which they were submitted.
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Vibratory Transducer
FIELD OF THE INVENTION
This invention relates to a vibratory transducer which is particularly suited
for
use in a Coriolis mass flowmeter.
BACKGROUND OF THE INVENTION
To determine the mass flow rate of a fluid flowing in a pipe and particularly
of
a liquid, use is frequently made of measuring devices which induce Coriolis
forces in the fluid and derive therefrom a measurement signal representative
of mass flow rate by means of a vibratory transducer and control and
evaluation electronics connected thereto.
Such transducers and particularly their use in Coriolis mass flow meters have
been known and in industrial use for a long time. U.S. Patent 5,549,009, for
example, discloses a Coriolis mass flowmeter incorporating a vibratory
transducer which responds to the mass flow rate of a fluid flowing in a pipe
and comprises:
- a curved flow tube for conducting the fluid which vibrates in operation and
communicates with the pipe via an inlet-side tube section and an outlet-
side tube section;
- an antivibrator which extends essentially parallel to and oscillates in a
phase opposition to the flow tube and is mechanically connected with the
flow tube
-- by means of at least a first coupler on the inlet side and
-- by means of at least a second coupler on the outlet side;
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- an excitation system for driving the flow tube and the antivibrator at an
excitation frequency; and
- a sensor system for sensing inlet-side and outlet-side vibrations of the
flow
tube,
- wherein a torsional rigidity of the inlet-side tube section and a torsional
rigidity of the outlet-side tube section are adapted to one another and to an
internal system supported by the two tube sections and formed by at least
the flow tube, the antivibrator, the excitation system, and the sensor
system such that the internal system is suspended essentially "torsionally
soft", i.e., in a torsionally nonrigid manner.
As is well known, vibrating flow tubes, for example U-, V-, or S2-shaped
tubes, if excited into cantilever vibrations in a first natural mode, can
cause
Coriolis farces in the fluid passing therethrough. In such transducers, the
first
natural vibration mode chosen for the flow tube is usually the mode in which
the flow tube oscillates about a longitudinal axis of the transducer at a
lowest
natural resonance frequency.
The Coriolis forces thus produced in the fluid result in cantilever vibrations
of
an at least second natural mode being superimposed on the excited,
pendulum-like cantilever vibrations of the so-called useful mode, the
vibrations of the second mode being equal in frequency to those of the useful
mode. In transducers of the kind described, these cantilever vibrations forced
by Coriolis forces, the so-called Coriolis mode, commonly correspond to the
natural mode in which the flow tube also performs torsional vibrations about
a vertical axis that is perpendicular to the longitudinal axis. As a result of
the
superposition of the useful and Coriolis modes, the flow tube vibrations
sensed on the inlet and outlet sides of the tube by means of the sensor
system have a measurable phase difference, which is also dependent on
mass flow rate.
Frequently, the flow tubes of such transducers, which are used in Coriolis
mass flowmeters, for example, are excited in operation at an instantaneous
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resonance frequency of the first natural mode, particularly with the vibration
amplitude maintained constant. As this resonance frequency is also
dependent on the instantaneous density of the fluid in particular,
commercially available Coriolis mass flowmeters can also be used to
measure the density of moving fluids.
One advantage of a curved tube shape is that thermally induced expansion,
particularly in flow tubes with a high expansion coefficient, produce
virtually
no or only very slight mechanical stresses in the flow tube itself andlor in
the
connected pipe. Another advantage of curved flow tubes is that the flow tube
can be made relatively long, so that high sensitivity of the transducer to the
mass flow rate to be measured can be achieved with a relatively short
mounting length and relatively low excitation energy. These circumstances
permit the flow tube to be made from materials having a high expansion
coefficient andlor a high modules of elasticity, such as special steel.
In vibratory transducers with a straight flow tube, the latter is commonly
made from a material having at least a lower expansion coefficient and
possibly a lower modules of elasticity than special steel in order to avoid
axial stresses and achieve sufficient measuring sensitivity. Therefore, such
straight flow tubes are preferably made of titanium or zirconium, but because
of the higher material cost and the generally higher machining cost, such
tubes are far more expensive than those made of special steel.
Transducers of the kind disclosed in U.S. Patent 5,549,009, i.e., transducers
With a single curved flow tube and with an antivibrator, particularly one
extending parallel to the flow tube, have proved especially effective in
applications where the fluid to be measured has an essentially constant or
only very slightly varying density. For such applications, it is readily
possible
by means of the antivibrator oscillating in operation at the same frequency
as, but in phase opposition to, the flow tube to nearly completely neutralize
those transverse forces which were induced in the transducer as a result of
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alternating lateral motions of the oscillating flow tube, thus virtually
keeping
such transverse forces away from the connected pipe.
If used for fluids with widely varying densities, such a transducer has
practically the same disadvantage as a transducer without antivibrator,
particularly as compared to transducers with two parallel flow tubes.
It turned out that the aforementioned forces produced in the transducer
cannot be completely balanced with such an antivibrator. As a result, the
above-mentioned internal system, oscillating as a whole about the
transducer's longitudinal axis, may also start to vibrate laterally.
Accordingly,
these lateral vibrations of the internal system force an additional elastic
deformation of the inlet-side and outlet-side tube sections and consequently
may cause flexural vibrations in the connected pipe. In addition, such lateral
vibrations may cause cantilever vibrations very similar to, and thus
practically
indistinguishable from, the Coriolis mode to be excited in the empty flow
tube, and this, in turn, would render the measurement signal that ought to
represent the mass flow rate of the fluid unusable.
On the other hand, as is well known, a significant advantage of a single flow
tube transducer over a transducer having two parallel flow tubes is that no
manifolds are necessary to connect the flow tubes with the pipe. Such
manifolds, on the one hand, are expensive to make and, on the other hand,
represent flow bodies with a strong tendency to sedimentation or clogging.
One way of reducing density-dependent transverse forces is described, for
example, in U.S. Patent 5,287,754 or in U.S. Patent 5,705,754. In the
transducers disclosed therein, the transverse forces produced by the
vibrating single flow tube, which oscillate at medium or high frequencies, are
kept away from the pipe by means of an antivibrator that is heavy compared
to the flow tube, and by coupling the flow tube to the pipe relatively
loosely,
i.e., practically by means of a mechanical low-pass filter. Unfortunately,
however, this causes the antivibrator mass required to achieve sufficient
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damping of the transverse forces to increase disproportionately with the
nominal diameter of the flow tube.
This represents a big disadvantage for such transducers, since the use of
5 such massive components always entails both increased assembly costs
during manufacture and increased costs during installation of the measuring
device in the pipe. In addition, it is difficult to ensure that the lowest
natural
frequency of the transducer, which decreases with increasing mass, is still
far from the likewise rather low natural frequencies of the connected pipe.
Thus, use of such a transducer in industrial Coriolis mass flowmeters is
limited to relatively small nominal flow tube diameters up to about 10 mm.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a transducer which is
particularly suited for a Coriolis mass flowmeter and which in operation, even
if it uses only a single straight flow tube, is well balanced dynamically over
a
wide fluid density range and nevertheless has comparatively little mass.
To attain this object, the invention provides a vibratory transducer for a
fluid
flowing in a pipe, said transducer comprising:
- a curved flow tube for conducting the fluid which vibrates in operation and
communicates with the pipe via an inlet-side tube section and an outlet-
side tube section;
- an antivibrator which in operation oscillates in a phase opposition to the
flow tube and is mechanically connected with the flow tube
-- by means of a first coupler on the inlet side and
-- by means of a second coupler on the outlet side;
- an excitation system for vibrating the flow tube and the antivibrator at an
excitation frequency; and
- a sensor system for sensing inlet-side and outlet-side vibrations of the
flow
tube,
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- wherein an internal system formed by at least the flow tube, the
antivibrator, the excitation system, and the sensor system, oscillating about
a longitudinal axis of the transducer which is essentially in alignment with
the inlet-side tube sections, forces
-- a torsion of the first and second couplers about the longitudinal axis and
-- an essentially torsional elastic deformation of at least parts of the inlet-
side and outlet-side tube sections, and
- wherein in order to achieve a torsionally soft oscillation of the internal
system,
-- at least the first coupler, tuned to a torsional rigidity of the inlet-side
tube
section, and
-- at least the second coupler, tuned to a torsional rigidity of the outlet-
side
tube section, are so dimensioned that
--- an inlet-side inherent torsion eigenmode of the first coupler and of the
inlet-side tube section has a natural frequency approximately equal to
the excitation frequency, and
--- an outlet-side inherent torsion eigenmode of the second coupler and of
the outlet-side tube section has a natural frequency essentially equal to
the natural frequency of the inlet-side inherent torsion eigenmode.
In a first preferred embodiment of the invention, the natural frequency of the
inlet-side inherent torsion eigenmode is lower than the excitation frequency.
In a second preferred embodiment of the invention, the antivibrator extends
essentially parallel to the flow tube.
In a third preferred embodiment of the invention, the antivibrator has a mass
distribution at least similar to that of the flow tube.
In a fourth preferred embodiment of the invention, the antivibrator is tubular
in form.
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In a fifth preferred embodiment of the invention, the antivibrator is
essentially
coaxial with the flow tube.
In a sixth preferred embodiment of the invention, the antivibrator is
essentially identical in shape to the flow tube.
In a seventh preferred embodiment of the invention, counterbalance bodies
are attached to the antivibrator for adjusting the mass distribution of the
antivibrator.
In an eighth preferred embodiment of the invention, the antivibrator is
heavier than the flow tube.
In a ninth preferred embodiment of the invention, a first rotating-mass
counterbalance body is rigidly fixed to the inlet-side tube section to adjust
the
inlet-side torsion eigenmode, and a second rotating-mass counterbalance
body is rigidly fixed to the outlet-side tube section to adjust the outlet-
side
torsion eigenmode.
In a tenth preferred embodiment of the invention, the transducer comprises a
first torsion absorber, which is essentially coaxial with the inlet-side tube
section, and a second torsion absorber, which is essentially coaxial with the
outlet-side tube section.
In an eleventh preferred embodiment of the invention, the transducer
comprises a transducer case fixed to the inlet-side tube section and to the
outlet-side tube section and having a lowest natural frequency which is at
least 20% above the excitation frequency.
The invention further provides a Coriolis mass flowmeter incorporating a
transducer as mentioned above.
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A fundamental idea of the invention is to use such torsional vibrations of the
internal system suspended in the aforementioned manner, which are rather
uncritical for the Coriolis mode and, consequently, for the mass flow rate
measurement, and which are caused essentially by in-phase oscillating
motions of the flow tube and the multivibrator, to neutralize lateral
transverse
forces, which are extremely detrimental to the development of the Coriolis
mode and, consequently, to the measurement of the mass flow rate. This
means that these hitherto undesired torsional vibrations are not merely not
damped out but, by being selectively adjusted for their mechanical coupling
to the pipe, produced in such a way as to achieve a reduction of interfering
effects on the transducer and, thus, an improvement in the transducer's
measuring properties.
To accomplish this, according to the invention, an inlet-side torsion
vibrator,
formed by the inlet-side coupler and the inlet-side tube section, and an
outlet-side torsion vibrator, formed by the outlet-side coupler and the outlet-
side tube section, are mechanically tuned so as to oscillate practically at
resonance with the oscillating internal system. This serves to oscillate the
internal system free from external reaction torques if possible, so that the
internal system is practically perfectly isolated from the inlet-side and
outlet-
side tube sections. As a result of this isolation, a total angular momentum of
the internal system is practically zero. To the same degree as the total
angular momentum, a total lateral momentum of the internal system, and
thus transverse forces derived therefrom and transmissible to the outside,
are reduced to zero by this isolation.
One advantage of the invention is that the transducer is very well balanced
with a relatively small additional amount of mechanical complexity,
particularly over a wide fluid density range, regardless of operational
variations of an internal total mass.
The transducer according to the invention is further characterized by the fact
that the inlet-side and outlet-side tube sections can be kept short, so that
the
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mounting length of the transducer can be substantially reduced while the
high quality of the dynamic vibration isolation remains essentially unchanged.
Despite its short mounting length, the transducer can be made very light.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention and further advantages will become more apparent by
reference to the following description of an embodiment taken in conjunction
with the accompanying drawings. Like reference characters have been used
to designate like parts throughout the various figures; reference characters
already allotted have been omitted in subsequent figures if this contributes
to
clarity. In the drawings:
Fig. 1a is a perspective side view of a vibratory transducer;
Fig. 1 b is a axial front side view of the transducer of Fig. 1 a;
Fig. 2 is a first graphical plot for the transducer of Fig. 1a1, b;
Fig. 3 is a second graphical plot for the transducer of Fig. 1a, 1b; and
Fig. 4 shows a torsion absorber for the transducer.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
While the invention is susceptible to various modifications and alternative
forms, exemplary embodiments thereof have been shown by way of example
in the drawings and will herein be described in detail. It should be
understood, however, that there is no intent to limit the invention to the the
particular forms diclosed, but on the contrary, the intention is to cover all
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modifications, equivalents, and alternatives falling within the spirit and
scope
of the invention as defined by the intended claims.
Fig. 1 a and 1 b show schematically a meter for moving fluids with a vibratory
5 transducer. The transducer serves to produce in a fluid passing therethrough
mechanical reaction forces, such as mass-flow-rate dependent Coriolis
forces, density-dependent inertial forces, andlor viscosity-dependent friction
forces, which react on the transducer and are measurable, particularly with
sensor technology. Derived from these reaction forces, a mass flow rate m, a
10 density p, andlor a viscosity r~ of the fluid, for example, can thus be
measured in the manner familiar to those skilled in the art.
To conduct the fluid to be measured, the transducer comprises a curved flow
tube 10, particularly a single tube, which is connected via an inlet-side tube
section 11 and an outlet-side tube section 12 to a pipe (not shown) that
supplies the fluid and carries it away. Flow tube 10, inlet-side tube section
11, and outlet-side tube section 12 are in alignment with each other and with
a longitudinal axis A~ and are preferably of one-piece construction, so that
they can be made from a single tubular semifinished product, for example; if
necessary, flow tube 10, inlet-side tube section 11, and outlet-side tube
section 12 may also be made from separate semifinished products that are
subsequently joined together, for instance welded together. For flow tube 10,
virtually any of the materials commonly used for such transducers, such as
steel, Hastelloy, titanium, zirconium, tantalum, etc, may be employed.
For the preferred case where the transducer is to be detachable from the
pipe, a first flange 13 is formed on inlet-side section 11 at an inlet end and
a
second flange 14 is formed on outlet-side tube section 12 at an outlet end; if
necessary, inlet-side and outlet-side tube sections 11, 12 may also be
connected with the pipe directly, for instance by welding or brazing.
Furthermore, as shown in Fig. 1 a, a transducer case 100, for instance a
boxlike case or a case in the form of a hollow cylinder, is fixed, preferably
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rigidly, to the inlet end of inlet-side tube section 11 and to the outlet end
of
outlet-side tube section 12. Transducer case 100 may also serve to mount
an electronics case 200 of the meter.
As shown in Fig. 1 a an 1 b, the transducer further comprises an antivibrator
20 for flow tube 10, which antivibrator is fixed to an inlet end of flow tube
10
by means of an inlet-side first coupler 31 and to an outlet end of flow tube
10
by means of an outlet-side second coupler 32 so as to be capable of
vibratory motion, the second coupler 32 being preferably identical in shape to
the first coupler 31. Coupler 31 may be implemented, for example, with one
or, as shown in Fig. 1 a, two node plates which are fixed to flow tube 10 and
antivibrator 20 at the inlet end; analogously, coupler 32 may be implemented
with node plates fixed to flow tube 10 and antivibrator 20 at the outlet end.
The preferably likewise tubular antivibrator 20 is spaced from and extends
essentially parallel to flow tube 10. Flow tube 10 and antivibrator 20 are
preferably designed to have equal or at least similar mass distributions,
particularly mass distributions proportional to each other, while having
identical surface shapes if possible. It may also be advantageous, however,
to form antivibrator 20 nonidentically to flow tube 10; for instance,
antivibrator
20 may be coaxial with flow tube 10 if necessary.
Preferably, antivibrator 20 is made heavier than flow tube 10.
To permit easy adaptation of antivibrator 20 to a mass distribution effective
at the flow tube, in a further preferred embodiment of the invention,
counterbalance bodies 21 serving as discrete additional masses are
mounted, preferably detachably, on antivibrator 20. Counterbalance bodies
21 may be, for example, disks screwed on to staybolts provided on flow tube
10, or short tube sections slipped over the flow tube. Furthermore, a
corresponding mass distribution over antivibrator 20 may be implemented by
forming longitudinal or annular grooves, for example. A mass distribution
suitable for the respective application can be easily determined by the
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person skilled in the art using the finite element method andlor suitable
calibration measurements, for example.
In operation, flow tube 10, as is usual with such vibratory transducers, is
excited into cantilever vibrations at an excitation frequency feX~ such that
the
flow tube, oscillating in this so-called useful mode about the transducer's
longitudinal axis A~, deflects essentially according to a first natural
vibration
mode shape. At the same time, antivibrator 20 is so excited into cantilever
vibrations as to oscillate in essentially the same mode but in phase
opposition to flow tube 10, which is oscillating in the useful mode. In other
words, flow tube 10 and antivibrator 20 then move in the manner of vibrating
tuning fork tines.
In another preferred embodiment of the invention, the excitation or useful-
mode frequency feX~ is selected to correspond as exactly as possible to a
preferably lowest natural frequency of flow tube 10. If use is made of a flow
tube of special steel with a nominal diameter of
29 mm, a wall thickness of about 1.5 mm, a straight length of about 420 mm,
and a cord length of 305 mm measured from inlet end to outlet end, the
lowest resonance frequency of the tube at zero density is about 490 Hz.
When fluid flows in the pipe, so that the mass flow rate m is nonzero,
Coriolis
forces are induced by the vibrating flow tube 10 in the fluid passing
therethrough. The Coriolis forces react on flow tube 10, thus causing an
additional deformation of the flow tube essentially according to a second
natural vibration mode shape, this deformation being detectable using
sensor technology. An instantaneous form of this so-called Coriolis mode,
which is superimposed on and has the same frequency as the excited useful
mode, is also dependent on the instantaneous mass flow rate m, particularly
with respect to its amplitudes. As is usual with such transducers, the second
natural vibration mode may be the antisymmetric twist mode, for example,
i.e., the natural mode in which flow tube 10, as mentioned above, also
performs torsional vibrations about a vertical axis A2 which is perpendicular
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to longitudinal axis A~ and lies in a single plane of symmetry of the
transducer shown.
To generate mechanical vibrations of flow tube 10, the transducer further
comprises an excitation system 40, particularly an electrodynamic system.
This excitation system serves to convert electric excitation energy EeXo
supplied from control electronics (not shown) housed in electronics case
200, for instance with a regulated current andlor a regulated voltage, into an
excitation force FeXC that acts on flow tube 10, for example in a pulsed
manner or harmonically, and deflects the tube in the manner described
above. Control electronics suitable for adjusting the excitation energy Eex~
are disclosed, for example, in U.S. Patent 4,777,833, 4,801,897, 4,879,911,
or 5,009,109.
As is usual with such transducers, the excitation force Fexc may be
bidirectional or unidirectional and be adjusted in amplitude, for instance by
means of a current- andlor voltage-regulator circuit, and in frequency, for
instance by means of a phase-locked loop, in the manner familiar to those
skilled in the art. The excitation system may be, for example, a simple
solenoid assembly with a cylindrical excitation coil that is mounted on
antivibrator 40 and traversed in operation by a suitable excitation current,
and with a permanent magnetic armature that is fixed to the outside of flow
tube 10, particularly at the midpoint thereof, and rides at least in part in
the
excitation coil. Excitation system 40 may also be implemented with an
electromagnet, for example.
To detect vibrations of flow tube 10, the transducer comprises a sensor
system 50. For sensor system 50, virtually any of the sensor systems
commonly used for such transducers, which senses motions of flow tube 10,
particularly on the inlet and outlet sides, and converts them into
corresponding sensor signals, may be employed. Sensor system 50 may be
formed, for example, by a first sensor, mounted on flow tube 10 on the inlet
side, and a second sensor, mounted on flow tube 10 on the outlet side, in
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the manner familiar to those skilled in art. The sensors may be
electrodynamic velocity sensors, which perform relative vibration
measurements, or electrodynamic displacement sensors or acceleration
sensors, for example. In place of electrodynamic sensor systems, sensor
systems using resistive or piezoelectric strain gages or optoelectronic sensor
systems may be used.
If necessary, sensors adapted for the measurement andlor the operation of
the transducer may be provided in the manner familar to those skilled in the
art, such as additional vibration sensors mounted on antivibrator 20 andlor
transducer case 100, see also U.S. Patent 5,736,653, or temperatur sensors
mounted on flow tube 10, on antivibrator 20 andlor transducer case 100, see
also U.S. Patent 4,68,384 or WO-A OOI102816.
As is readily apparent from the foregoing explanations, antivibrator 20 serves
as a support system for excitation system 40 and sensor system 50.
However, antivibrator 20 also serves to dynamically balance the transducer
for a predetermined fluid density value, for example a value most frequently
expected during operation of the transducer or a particularly critical value,
to
the point that transverse forces produced in the vibrating flow tube 10 and
acting essentially perpendicular to longitudinal and vertical axes A~, AZ are
completely offset by counterforces produced by antivibrator 20, cf. U.S.
Patent 5,549,009. For a flow tube 10 of special steel and at a vibration
amplitude of about 0.03 mm, for example, such transverse forces in flow
tube 10, which are produced as a result of mass accelerations when the tube
is excited into cantilever vibrations in its first natural vibration mode,
would lie
in the range of 45 N.
If, however, the aforementioned transverse forces of flow tube 10 are not
counterbalanced, as is quite possible in a transducer as disclosed in U.S.
Patent 5,549,009, for example, an internal system formed by flow tube 10,
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antivibrator 20 with any counterbalance bodies 21 attached thereto,
excitation system 40, and sensor system 50 and suspended from inlet-side
tube section 11 and outlet-side tube section 12, and the couplers 31, 32
fixed to the internal system, will be deflected laterally from an assigned
static
mounting position. In this manner, the transverse forces may act at least in
part via inlet-side and outlet-side tube sections 11, 12 on the connected
pipe,
thus causing the latter to vibrate as well. Furthermore, such transverse
forces, as a result of an unbalanced suspension of the internal system or
entire transducer, for instance because of virtually unavoidable
manufacturing tolerances, may result in flow tube 10 being additionally
excited into cantilever vibrations in a second natural mode, which are then no
longer distinguishable from the Coriolis mode proper by means of sensors.
As repeatedly mentioned, flow tube 10 can be dynamically balanced solely
by means of antivibrator 20, but only for a single fluid density value, and
only
for a very narrow fluid density range at best.
If the mass of antivibrator 20, which is preferably identical in shape to flow
tube 10, is less than the mass of the fluid-carrying flow tube 10, the
vibrating
flow tube 10 and antivibrator 20 may additionally perform common oscillating
motions about longitudinal axis A~ which, as shown in Fig. 1 b, at least with
the fluid at rest, are essentially in phase with each other and with the
cantilever vibrations of antivibrator 20; if the mass of the fluid-carrying
flow
tube 10 is less than the mass of antivibrator 20, these common or nonlocal
oscillating motions may be in phase with the cantilever vibrations of flow
tube
10. In other words, as a result of unbalances, particularly of density-
dependent unbalances, between flow tube and antivibrator 20, the entire
internal system may perform torsional vibrations about longitudinal axis A~
which are in phase with the cantilever vibrations of flow tube 10 or with
those
of antivibrator 20.
As a result of these torsional vibrations of the internal system, the two
couplers 31, 32, which are virtually rigidly connected with the internal
system,
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are subjected to a corresponding torsion about longitudinal axis A~, i.e.,
they,
too, vibrate, namely in phase with the internal system and with each other.
To the same degree, an essentially torsional, elastic deformation of inlet-
side
and outlet-side sections 11, 12, which are fixed to transducer case 100 and
flow tube 10 so as to be capable of vibratory motion, is forced at least in
parts thereof.
To the inventors' surprise it turned out that merely by suitably tuning the
aforementioned torsion vibrators, namely inlet-side tube section 11 together
with coupler 31 and outlet-side tube section 12 together with coupler 32, the
transducer can be dynamically balanced virtually independently of the
density p of the fluid, so that its sensitivity to internally produced
transverse
forces can be substantially reduced.
To accomplish this, according to the invention, a inlet-side moment of inertia
about longitudinal axis Ai, here adjusted by means of at least coupler 31,
and the torsional rigidity of inlet-side tube section 11 are so adapted to
each
other that an inlet-side inherent torsion eigenmode, i.e., an eigenmode
merely to be computed, of coupler 31 and inlet-side tube section 11 about
longitudinal axis A1 has a natural frequency f~ essentially equal to or less
than the excitation frequency fex~. Furthermore, a outlet-side moment of
inertia about longitudinal axis A~, here adjusted by means of at least coupler
32, and the torsional rigidity of outlet-side tube section 12 are so adapted
to
each other that an outlet-side inherent torsional eigenmode of coupler 32
and outlet-side tube section 12 about longitudinal axis A, has a natural
frequency f2 essentially equal to the natural frequency f~. In the transducer
shown in Fig. 1 a, the tube segments between the two node plates of coupler
31, which practically do not vibrate, must also be taken into account in the
selection of the moment of inertia for adjusting the inlet-side torsion
eigenmode; analagously, the tube segments between the two node plates of
coupler 32 must be added on to the moment of inertia of the coupler 32 in
adjusting the outlet-side torsion eigenmode.
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By adjusting the useful mode and the torsion eigenmode in the manner
described, the internal system, which in operation oscillates at the same
frequency as flow tube 10, which vibrates at the excitation frequency feX~, is
caused to excite practically exactly the inlet-side and outlet-side torsion
eigenmodes. In that case, the torsional vibrations of the internal system are
opposed by no or only very small reaction torques of the two torsion vibrators
vibrating at their natural frequencies f~ and f2, respectively, and in phase
with
the internal system. Thus, in operation, the internal system is mounted so
"torsionally soft" that it can be regarded as being practically perfectly
isolated
from inlet-side and outlet-side tube sections 11, 12.
Because of the fact that despite a practically perfect isolation, the internal
system oscillates about longitudinal axis A~ and does not rotate, no total
annular momentum of the internal system can exist. As a result, however, a
total lateral momentum nearly directly dependent on the total annular
momentum, particularly with similar mass distributions in flow tube 10 and
antivibrator 20, and, consequently, lateral transverse forces derived from
this
total lateral momentum, which may be transmitted from the internal system
to the outside, are also zero. In other words, in the transducer according to
the invention, density-dependent unbalances will result nearly exclusively in
a change in the instantaneous amplitude of the torsional vibrations of the
internal system, but will cause no or only negligibly small displacements of
the internal system from its assigned mounting position.
Investigations on transducers incorporating the above-described flow tube of
special steel have shown that despite a variation of the excitation frequency
fexc over a range of about 100 Hz, which is usual with such transducers and
corresponds approximately to a fluid density range between 0 and 2000 kg
m-3, a maximum transverse force Q* acting on the internal system,
normalized to a maximum interior force produced in couplers 31, 32 by the
antiphase motions of flow tube 10 and antivibrator 20, can be kept well below
5%, i.e., at about 2 N, see Figs. 2 and 3.
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For this aforementioned case and at a length L~~ of inlet-side tube section 11
of about 170 mm, for example, that said inlet-side moment of inertia would
have to be approximately 1.5 ~ 10-3 kg ~ m2 to set the associated torsion
eigenmode at the excitation frequency feXC in the aforementioned manner, cf.
Fig. 2. The parameters then to be set on the concrete transducer to optimally
adjust the inlet-side and outlet-side torsion eigenmodes to the useful mode,
i.e., suitable mass distributions, moments of inertia, torsional rigidities,
and
geometrical dimensions of flow tube 10, antivibrator 20, inlet-side and outlet-
side tube sections 11, 12, and couplers 31, 32, which are derived therefrom,
can be determined in the manner familiar to those skilled in the art using the
finite element method or other computer-aided simulation computations in
conjunction with suitable calibration measurements.
To permit the inlet-side torsion eigenmode to be adjusted as accurately as
possible, particularly if the transducer's mounting length is specified, in a
further preferred embodiment of the invention, at least a first rotating-mass
counterbalance body 33 is rigidly fixed to inlet-side tube section 11,
preferably in proximity to coupler 31, and to correspondingly adjust the
outlet-side torsion eigenmode, at least a second rotating-mass
counterbalance body 43 is rigidly fixed to outlet-side tube section 12,
preferably in proximity to coupler 32. Rotating-mass counterbalance bodies
33 and 34 are preferably disks of identical shape and may be mounted on
inlet-side tube section 11 and outlet-side tube section 12, respectively,
either
concentrically as shown in Fig. 1 a, 1 b, i.e., with their respective
centroids
located on longitudinal axis A~, or eccentrically. For the transducer shown,
the above-mentioned moment of inertia of 1.5 ~ 10-3 kg ~ m2 can thus be
implemented in a very simple manner using two rotating-mass
counterbalance bodies 33, 34 in the form of annular disks of special steel
with a diameter of about 100 mm and a thickness of about 15 mm.
It also turned out that in order to reliably avoid antiphase torsional
vibrations
of the inlet-side and outlet-side torsion vibrators, an additional,
torsionally
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stiff connection from the two couplers 31, 32 or the two rotating-mass
counterbalance bodies 33, 34 to an inner support frame may be
advantageous. Furthermore, the excitation frequency fex~ should preferably
be set at a value not higher than 85% of a lowest natural frequency of
transducer case 100, which acts as an external support frame in the above
sense.
In a further preferred embodiment of the invention, the transducer comprises
an inlet-side first torsion absorber 60 and an outlet-side second torsion
absorber 70, which is essentially identical in shape to the first torsion
absorber 60. The latter serves in particular to suppress torsional moments
caused by the oscillating internal system and transmitted via inlet-side tube
section 11 to the connected pipe andlor the transducer case 100 fixed at the
inlet end.
Torsion absorber 60 is fixed at the inlet end of inlet-side tube section 11 or
at
least in the vicinity of that end, for instance directly to transducer case
100;
analogously, the second torsion absorber, which also serves to avoid
torsional moments on the connected pipe andlor the transducer case 100, is
attached at the outlet end of outlet-side tube section 12.
As shown in Fig. 4, torsion absorber 60 comprises a torsion spring 61 which
is preferably tubular and essentially coaxial with inlet-side tube section 11,
and which is fixed to the inlet end so as to be capable of torsional
vibration,
i.e., of being twisted at least in sections with respect to inlet-side tube
section
11. Furthermore, torsion absorber 60 comprises a preferably disk-shaped
rotating-mass body 62 attached to the torsion spring on the side remote from
the inlet end of inlet-side tube section 11. Torsion spring 61 and rotating-
mass body 62 are so adapted to each other that torsion absorber 60, excited
in operation by the twisting inlet-side tube section 11 andlor by the likewise
slightly deforming transducer case 100 into torsional vibrations about
longitudinal axis A~, vibrates out of phase with respect to, and particularly
in
phase opposition to, the above-mentioned inlet-side torsion vibrator,
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consisting of coupler 31 and inlet-side tube section 11. To support the
excitation of torsion absorber 60, torsion spring 61 may advantageously be
extended up to rotating-mass counterbalance body 33, if present, or up to
coupler 31, and fixed to one of the two. This also serves to reduce lateral
flexural vibrations of torsion absorber 60.
Because of its good dynamic balance even at varying densities p of the fluid
passing through it, the transducer according to the invention is particularly
suited for use in a Coriolis flowmeter, a Coriolis mass flowmeter-densimeter,
or a Coriolis mass flowmeter-densimeter-viscometer.
While the invention has been illustrated and described in detail in the
drawings and forgoing description, such illustration and description is to be
considered as exemplary not restrictive in character, it being understood that
only exemplary embodiments have been shown and described and that all
changes and modifications that come within the spirit and scope of the
invention as described herein are desired to protected.
