Note: Descriptions are shown in the official language in which they were submitted.
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Transducer and Method for Measuring a Fluid Flowing in a Pipe
This application is based on Provisional Application No. 60/363,850, filed
on March 14, 2002.
FIELD OF THE INVENTION
This invention relates to a transducer and a method for measuring at least
one physical parameter, particularly a mass flow rate and/or a density
and/or a viscosity, of a fluid flowing in a pipe, particularly of a multiphase
fluid, in a non-invasive manner.
BACKGROUND OF THE INVENTION
In process-measurement and automation technology, physical parameters
of a fluid flowing in a pipe, such as mass flow rate, density, and/or
viscosity, are frequently measured by means of meters which, using a
vibratory transducer traversed by the fluid and a measuring and control
circuit connected thereto, induce reaction forces, such as Coriolis forces
corresponding to the mass flow rate, inertial forces corresponding to the
density, or friction forces corresponding to the viscosity, in the fluid in a
non-invasive manner, and derives therefrom a measurement signal
representing the respective mass flow rate, viscosity, and/or density of the
fluid.
Such vibratory transducers are disclosed, for example, in WO-A 01/33174,
WO-A 00/57141, WO-A 98/07009, WO-A 95/16897, WO-A 88/03261,
U.S. Patent 6,006,609, U.S. Patent 5,796,011, U.S. Patent 5,301,557,
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U.S. Patent 4,876,898, U.S. Patent 4,524;610, EP-A 553 939, or EP-A 1
001 254.
To conduct the fluid, each of the transducers comprises at least one flow
tube held in a support frame and having a bent or straight tube egmer~t
which in operation, driven by an electromechanicar excitation assembly, is
caused to vibrate in order to produce the above-mentioned reaction
forces. To sense vibrations of the tube segment, particularly inlet-side and
outlet-side vibrations, the transducers each comprise a sensor
arrangement which responds to motions of the tube segment.
Aside from such vibration transducers, electromagnetic transducers or
transducers evaluating the transit time of ultrasonic waves transmitted in
the direction of fluid flow, particularly transducers based on the Doppler
principle, are frequently used in process-measurement and automation
technology for in-line measurements. Since the basic construction and the
operation of such electromagnetic transducers are sufficiently described in
EP-A 1 039 269, U.S. Patent 6,031,740, U.S. Patent 5,540,103, U.S.
Patent 5,351,554, or U.S. Patent 4,563,904, for example, and the basic
construction and the operation of such ultrasonic transducers are
sufficiently described in U.S. Patent 6,397,683, U.S. Patent 6,330,831,
U.S. Patent 6,293,156, U.S. Patent 6,189,389, U.S. Patent 5,531,124,
U.S. Patent 5,463,905, U.S. Patent 5,131,279, or U.S. Patent 4,787,252,
for example, a detailed explanation of these principles of measurement
can be dispensed with at this point.
For clarification it should be mentioned that within the scope of the
invention, "non-invasive transducers" means those transducers which do
not comprise any flow bodies which are immersed in the fluid and serve to
influence its flow for the purpose of producing measurement effects. By
contrast, within the scope of this invention, those transducers which, in
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order to measure the fluid, produce vortices in the fluid flow or use baffles,
bluff bodies, floats, or orifice plates are regarded as "invasive
transducers". Such invasive transducers are also familiar to those skilled
in the art and are sufficiently described, for example, in WO-A 01/20282,
WO-A 97/22855, U.S. Patent 6,352,000, U.S. Patent 6,003,384, U.S.
Patent 5,939,643, U.S. Patent 5,922,970, U.S. Patent 5,458,005, U.S.
Patent 4,716,770, U.S. Patent 4,476,728, U.S. Patent 4,445,388, U.S.
Patent 4,437,350, U.S. Patent 4,339,957, EP-A 690 292, EP-A 684,458,
DE-A 39 04 224, DE-A 38 10 889, DE-A 17 98 360, or DE-A 100 01 165.
During the use of non-invasive in-line transducers it turned out that in the
case of inhomogeneous fluids, particularly of multiphase fluids, the
measurement signals produced, in spite of the viscosity and density being
maintained virtually constant, particularly under laboratory conditions, are
subject to considerable nonreproducible variations and may thus become
practically unusable for the measurement of the respective physical
parameter.
In U.S. Patent 4,524,610, a possible cause of this problem in the
operation of vibratory transducers is indicated, namely the fact that
parasitic inhomogeneities introduced by the fluid into the flow tube, such
as gas bubbles, may be trapped at the inside wall of the tube. To avoid
this problem, it is proposed to install the transducer so that the straight
flow tube is in an essentially vertical position, so that the trapping of such
parasitic, particularly gaseous, inhomogeneities is prevented.
This, however, is a very specific solution which is only conditionally
realizable, particularly in industrial process measurement technology. On
the one hand, the pipe into which the transducer is to be inserted would
have to be adapted to the transducer and not vice versa, which probably
cannot be conveyed to the user. On the other hand, the flow tubes, as
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mentioned, may also have a curved shape, so that the problem cannot be
solved by adapting the mounting position, either. It also turned out that the
above-described distortions of the measurement signal cannot be
appreciably reduced even if a vertically installed straight flow tube is used.
Variations in the measurement signal in the presence of a flowing fluid
cannot be prevented in this manner, either.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a method and a
transducer which even in the case of inhomogeneous, particularly
multiphase, fluids provides accurate, but at least easily reproducible and
highly robust measurement signals substantially regardless of the
instantaneous density distribution within the fluid flowing in the connected
pipe, but particularly substantially regardless of the concentration of any
parasitic inhomogeneities, and substantially regardless of the mounting
position of the flow tube.
To attain this object, the invention provides a method of measuring at
least one physical parameter, particularly a mass flow rate and/or a
density and/or a viscosity, of a fluid flowing in a pipe, the method
comprising the steps of: causing a swirl in a flowing fluid about a swirl axis
aligned with a direction of fluid flow for forcing a density distribution in
the
fluid which is as symmetric with respect to the swirl axis as possible;
causing the fluid rotating about the swirl axis to flow through at least one
flow tube of a non-invasive transducer inserted into the pipe for producing
reactions in the fluid corresponding to the parameter to be measured; and
sensing reactions in the fluid for generating at least one measurement
signal influenced by the parameter to be measured.
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Furthermore, the invention provides a transducer for generating a
measurement signal corresponding to at least one physical parameter of a
fluid flowing in a pipe, the transducer comprising: at least one flow tube of
predetermined lumen for conducting the fluid, which flow tube
5 communicates at its inlet and outlet ends with the pipe; an excitation
assembly for causing reactions in the fluid within the at least one flow
tube, the reactions in the fluid being produced in a non-invasive manner;
and a sensor arrangement for sensing the reactions in the fluid and for
generating the measurement signal, with means provided in an inlet area
of the transducer or at least in the vicinity thereof which cause a swirl in
the entering fluid and, thus, a rotational motion in the fluid flowing within
the flow-tube lumen relative to the flow tube about an axis of rotation lying
in the direction of fluid flow.
In a development, the invention provides a meter for measuring at least
one physical parameter of a fluid flowing in a pipe, particularly a meter
suitable for implementing the method according to the invention, which
comprises the transducer according to the invention.
In a embodiment of the method of the invention, the fluid-conducting pipe
is vibrated for producing reaction forces in the fluid to be measured which
correspond to the parameter to be measured and react on the vibrating
flow tube, and vibrations of the flow tube are sensed to generate the at
least one measurement signal.
In a first embodiment of the transducer of the invention, the at least one
flow tube communicates with the pipe via an inlet tube section and an
outlet tube section, and the means for causing the swirl are at least partly
disposed within the inlet tube section.
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In a second embodiment of the transducer of the invention, the means for
causing the swirl are at least partly disposed within the at least one flow
tube.
In a third embodiment of the transducer of the invention, the means for
causing the swirl are at least partly disposed within the pipe.
In a fourth embodiment of the transducer of the invention, the means for
causing the swirl comprise at least one turbulator extending into the
flowing fluid, particularly a stationary turbulator.
In a fifth embodiment of the transducer of the invention, the turbulator
comprises at least one baffle extending into the flowing fluid.
In a sixth embodiment of the transducer of the invention, the turbulator
has the form of propeller.
In a seventh embodiment of the transducer of the invention, the turbulator
has the form of a helix.
In an eighth embodiment of the transducer of the invention, the turbulator
is helicoidal.
In a ninth embodiment of the transducer of the invention, the means for
causing the swirl are fixed to the inside wall of the inlet tube section
andlor
to the inside wall of the flow tube.
In a tenth embodiment of the transducer of the invention, the means for
causing the swirl are held.against the inside wall of the inlet tube section
and/or against the inside wall of the flow tube.
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In an eleventh embodiment of the transducer of the invention, the means
for causing the swirl are designed as a rifling formed in the inside wall of
the inlet tube section and/or in the inside wall of the flow tube.
In a twelfth embodiment of the transducer of the invention, the means for
causing the swirl have an effective length in the direction of flow which is
at least equal to a nominal diameter of the pipe.
In a thirteenth embodiment of the invention, the transducer is installed in
an essentially horizontal pipe.
In a fourteenth embodiment of the invention, the transducer is installed in
an essentially vertical pipe.
In a fifteenth embodiment of the transducer of the invention, in order to
produce reaction forces acting in the fluid, the at least one flow tube is
vibrated by means of the excitation assembly, and vibrations of the flow
tube are sensed by means of the sensor arrangement.
In a first embodiment of the meter of the invention, the physical parameter
to be measured is a mass flow rate.
In a second embodiment of the meter of the invention, the physical
parameter to be measured is a density.
In a third embodiment of the meter of the invention, the physical
parameter to be measured is a viscosity.
A basic idea of the invention is to induce in the fluid, by means of the
rotational motion about the swirl axis, centrifugal forces such that at least
within the flow-tube lumen, a density distribution is forced which is as
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symmetric in respect of the swirl axis of the rotating "fluid column" as
possible and, thus, largely reproducible. Compared to the above-
described nonreproducible variations, a possible slight measurement
error, particularly a slight error in the measured density value, which is to
be expected mainly in the case of liquids with parasitic gaseous
inhomogeneities because such gas occlusions are concentrated at the
center of the flow-tube lumen as a result of the centrifugal forces can be
considered negligibly small.
The invention is predicated particularly on the recognition that the above-
described variations of the measurement signals are caused not only by
the fact that gas bubbles, for example, are trapped at the inside wall of the
flow tube, but particularly by the fact that this takes place essentially
chaotically and, hence, in a nonreproducible or unpredictable manner. In
other words, if conventional transducers with straight or slightly curved
flow tubes are used for inhomogeneous, particularly multiphase, fluids, the
variations of the measurement signals can be attributed largely to the
essentially chaotic distribution of inhomogeneities in the fluid, such as
entrained gas bubbles, and, thus, to a constantly varying, but practically
undetectable density distribution within the fluid in the flow-tube lumen.
One advantage of the invention, particularly of the method according to the
invention, is that it can be used in practically all known in-line
transducers,
particularly for all flow measurement principles, and, consequently, both in
vibratory transducers and, for instance, in electromagnetic or ultrasonic
transducers.
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BRIEF DESCRIPTION OF THE DRAWINGS
The invention and further advantages will become more apparent from the
following description of embodiments taken in conjunction with the
accompanying drawings. Like parts are designated by like reference
characters throughout the various figures of the drawings; reference
characters that were already assigned have been omitted in subsequent
figures if this contributes to clarity. In the drawings:
Fig. 1 is a perspective view of a meter for measuring at least one
physical parameter of a fluid flowing in a pipe;
Fig. 2 is a first perspective view of an embodiment of a vibratory
transducer suitable for the meter of Fig. 1;
Fig. 3 is a second perspective view of the transducer of Fig. 2;
Fig. 4 shows an embodiment of an electromechanical excitation
assembly suitable for the transducer of Figs. 2 and 3; and
Figs. 5a show embodiments of means according to the invention for
to 5d causing a swirl in the fluid to be measured.
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 shows schematically a meter with a transducer 10, preferably
5 housed in a transducer case 100, and with meter electronics (not shown),
housed in an electronics case 200 and electrically connected to
transducer 10. The meter serves in particular to sense a physical
parameter of a fluid flowing in a pipe (not shown), particularly a mass flow
rate m and/or a density p and/or a viscosity rl, and to map it into a
10 measured value representing this parameter. The meter can also be used
to measure a volumetric flow rate of the fluid, for example.
To that end, in operation, reactions are produced in the fluid in a non-
invasive manner by means of the meter-electronics-driven transducer 10
which are dependent on the parameter to be measured and react on
transducer 10 in a measurable manner, i.e., which can be detected using
sensor technology and converted into useful input signals for subsequent
evaluation electronics. Such reactions may be, for instance, volumetric-
flow-rate-dependent, electromagnetically generated voltages, mass-flow-
rate-dependent Coriolis forces, density-dependent mass intertial forces,
and/or viscosity-dependent friction or damping forces, etc. "Producing
reactions in the fluid in a non-invasive manner", as already indicated at
the beginning, means here that the sensed reactions, which correspond to
the parameter to be measured, are produced without any flow bodies
additionally immersed in and changing the fluid flow.
For the case where the meter is designed to be coupled to a Fieldbus, the,
preferably programmable, meter electronics include a suitable
communication interface for data communication, e.g., for the
transmission of the measurement data to a higher-level stored program
control or a higher-level process control system.
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Figs. 2 and 3 show an embodiment of a transducer 10 in the form of a
physical-to-electrical vibratory transducer assembly. The construction of
such a transducer assembly is described in detail in U.S. Patent
6,006,609, for example. Such transducers are already used in
commercially available Coriolis mass flowmeter-densimeters as are
offered by the applicant with its "PROMASS I" series, for example.
To conduct the fluid to be measured, transducer 10 comprises at least
one flow tube 13 of a predeterminable, elastically deformable lumen 13A
and a predeterminable nominal diameter, which has an inlet end 11 and
an outlet end 12. "Elastic deformation" of lumen 13A as used herein
means that in order to produce reaction forces within the fluid, i.e.,
reaction forces descriptive of the fluid, namely shear or friction forces, but
also Coriolis forces and/or mass inertial forces, during operation of
transducer 10, a three-dimensional shape and/or a spatial position of
lumen 13A are changed in a predeterminable cyclic manner, particularly
periodically, within an elasticity range of flow tube 13; see, for example,
U.S. Patent 4,801,897, U.S. Patent 5,648,616, U.S. Patent 5,796,011,
and/or U.S. Patent 6,006,609.
At this point is should be noted that instead of a transducer according to
the embodiment of Figs. 2 and 3, virtually any of the transducers for
Coriolis flowmeter-densimeters which are known to the person skilled in
the art, particularly a flexural mode transducer with a bent or straight flow
tube vibrating exclusively or at least in part in a flexural mode, can be
used for implementing the invention. Further implementations of
transducer assemblies suitable for use as transducer 10 are described, for
example, in U.S. Patent 5,301,557, 5,357,811, 5,557,973, 5,602,345,
5,648,616, or 5,796,011, which are incorporated herein by reference. It is
also possible to use conventional electromagnetic or ultrasonic
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transducers, for example. Materials suited for flow tube 13, here an
essentially straight tube, are titanium alloys, for example. Instead of
titanium alloys, other materials commonly used for such flow tubes,
particularly for bent tubes, such as stainless steel, tantalum, or zirconium,
may be employed.
Flow tube 13, which communicates with the fluid-conducting pipe via an
inlet tube section 13+ and an outlet tube section 13# in the usual manner,
is clamped in a rigid support frame 14, particularly in a flexurally and
torsionally stiff frame, so as to be capable of vibratory motion, the support
frame preferably being enclosed by a transducer case 100. Flow tube 13
as well as inlet and outlet tube sections 13+, 13# preferably are integrally
formed from a single tubular semifinished product; if necessary, they may,
of course, be of multipart construction.
Support frame 14 is fixed to inlet tube section 13+ by means of an inlet
plate 213 and to outlet tube section 13# by means of an outlet plate 223,
the two plates being penetrated by respective extension pieces of flow
tube 13. Support frame 14 has a first side plate 24 and a second side
plate 34, which are fixed to inlet plate 213 and outlet plate 223 in such a
way as to extend essentially parallel to and in spaced relationship from
flow tube 13; see Figs. 2 and 3. Thus, facing side surfaces of the two side
plates 24, 34 are also parallel to each other.
Advantageously, a longitudinal bar 25 serving as a balancing mass for
absorbing vibrations of flow tube 13 is secured to side plates 24, 34 in
spaced relationship from flow tube 13. As shown in Fig. 3, longitudinal bar
25 extends essentially parallel to the entire oscillable length of flow tube
13. If necessary, longitudinal bar 25 may, of course, be shorter.
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Thus, support frame 14 with the two side plates 24, 34, inlet plate 213,
outlet plate 223, and the optional longitudinal bar 25 has a longitudinal
axis of gravity which is essentially parallel to a central flow tube axis 13B,
which joins inlet end 11 and outlet end 12.
In Figs. 2 and 3, it is indicated by the heads of the screws shown that the
aforementioned fixing of side plates 24, 34 to inlet plate 213, to outlet
plate 223, and to longitudinal bar 25 may be done by screwing; it is also
possible to use other suitable forms of fastening familiar to those skilled in
the art.
If transducer 10 is to be nonpermanently connected with the pipe,
preferably a first flange 19 and a second flange 20 are formed on inlet
tube section 13; and outlet tube section 13#, respectively, see Fig. 1;
instead of flanges 19, 20, so-called Triclamp connections, for example,
may be used to provide the nonpermanent connection with the pipe, as
indicated in Fig. 2 or 3. If necessary, however, flow tube 13 may also be
connected with the pipe directly, e.g., by welding or brazing.
To produce the above-mentioned reaction forces in the fluid, during
operation of transducer 10, flow tube 13, driven by an electromechanical
excitation assembly 16 coupled to the flow tube, is caused to vibrate in the
so-called useful mode at a predeterminable frequency, particularly at a
natural resonance frequency which is also dependent on the density p of
the fluid, whereby the flow tube is elastically deformed in a
predeterminable manner.
In the embodiment shown, the vibrating flow tube 13, as is usual with such
flexural mode transducer assemblies, is spatially, particularly laterally,
deflected from a static rest position; the same applies to transducer
assemblies in which one or more curved flow tubes perform cantilever
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vibrations about a corresponding longitudinal axis joining the respective
inlet and outlet ends, or to those in which one or more straight flow tubes
perform only planar flexural vibrations about their longitudinal axis. For the
other case where transducer 10 is a radial mode transducer assembly and
the vibrating flow tube is symmetrically deformed in the usual manner as
is described, for example, in WO-A 95/16897, the flow tube is essentially
left in its static rest position.
Excitation assembly 16 serves to produce an excitation force acting on
flow tube 13 by converting electric excitation power supplied from the
meter electronics. The excitation power serves virtually only to
compensate the power component lost in the vibrating system because of
mechanical and fluid friction. To achieve as high an efficiency as possible,
the excitation power is preferably adjusted so that essentially the
vibrations of flow tube 13 in the useful mode, e.g., those at a lowest
resonance frequency, are maintained.
For the purpose of transmitting the excitation force to flow tube 13,
excitation assembly 16, as shown in Fig. 4, has a rigid,
electromagnetically and/or electrodynamically driven lever arrangement 15
with a cantilever 154 and a yoke 163, the cantilever 154 being rigidly fixed
to flow tube 13. Yoke 163 is rigidly fixed to an end of cantilever 154
remote from flow tube 13, such that it lies above and extends transversely
of flow tube 13. Cantilever 154 may be a metal plate, for example, which
receives flow tube 13 in a bore. For further suitable implementations of
lever arrangement 15, reference is made to the above-mentioned U.S.
Patent 6,006,609. As is readily apparent from Fig. 2, lever arrangement
15, here a T-shaped arrangement, is preferably arranged to act on flow
tube 13 approximately midway between inlet end 11 and outlet end 12, so
that in operation, flow tube 13 will undergo its maximum lateral deflection
at its midpoint.
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To drive the lever arrangement 15, excitation assembly 16, as shown in
Fig. 4, comprises a first excitation coil 26 and an associated first armature
27 of permanent- magnet material as well as a second excitation coil 36
and an associated second armature 37 of permanent-magnet material.
The two excitation coils 26 and 36, which are preferably electrically
connected in series, are fixed to support frame 14 on both sides of flow
tube 13 below yoke 163, particularly nonpermanently, so as to interact in
operation with their associated armatures 27 and 37, respectively. If
necessary, the two excitation coils 26, 36 may, of course, be connected in
parallel.
As shown in Figs. 2 and 4, the two armatures 27, 37 are fixed to yoke 163
at such a distance from each other that during operation of transducer 10,
armature 27 will be penetrated essentially by a magnetic field of excitation
coil 26, while armature 37 will be penetrated essentially by a magnetic
field of excitation coil 36, so that the two armatures will be moved by the
action of corresponding electrodynamic and/or electromagnetic forces.
The motions of armatures 27, 37 produced by the magnetic fields of
excitation coils 26, 36 are transmitted by yoke 163 and cantilever 154 to
flow tube 13. These motions of armatures 27, 37 are such that yoke 163 is
displaced from its rest position alternately in the direction of side plate 24
and in the direction of side plate 34. A corresponding axis of rotation of
lever arrangement 15, which is parallel to the above-mentioned central
axis 13B of flow tube 13, may pass through cantilever 154, for example.
Particularly in order to hold excitation coils 26, 36 and individual
components of a magnetic brake assembly 217, which is described below,
support frame 14 further comprises a holder 29 for electromechanical
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excitation assembly 16. Holder 29 is connected, preferably
nonpermanently, with side plates 24, 34.
In the transducer 10 of the embodiment, the lateral deflections of the
vibrating flow tube 13, which is firmly clamped at inlet end 11 and outlet
end 12, simultaneously cause an elastic deformation of its lumen 13A; this
elastic deformation extends virtually over the entire length of flow tube 13.
Furthermore, due to a torque acting on flow tube 13 via lever arrangement
15, torsion is induced at least in sections of flow tube 13 about central axis
13B simultaneously with the lateral deflections, so that the flow tube
vibrates in a mixed flexural and torsional mode serving as a useful mode.
The torsion of flow tube 13 may be such that the direction of a lateral
displacement of the end of cantilever 154 remote from flow tube 13 is
either the same as or opposite to that of the lateral deflection of flow tube
13. In other words, flow tube 13 can perform torsional vibrations in a first
flexural and torsional mode, corresponding to the former case, or in a
second flexural and torsional mode, corresponding to the latter case. In
the transducer 10 according to the embodiment, the natural resonance
frequency of the second flexural and torsional mode, e.g., 900 Hz, is
approximately twice as high as that of the first flexural and torsional mode.
For the case where flow tube 13 is to perform vibrations only in the
second flexural and torsional mode, excitation assembly 16
advantageously incorporates a magnetic brake assembly 217 based on
the eddy-current principle, which serves to stabilize the position of the axis
of rotation. By means of magnetic brake assembly 217 it can thus be
ensured that flow tube 13 always vibrates in the second flexural and
torsional mode, so that any external disturbing effects on flow tube 13 will
not result in a spontaneous change to another flexural and torsional
mode, particularly to the first. Details of such a magnetic brake assembly
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are described in U.S. Patent 6,006,609, for example; furthermore, the use
of such magnetic brake assemblies is known from transducers of the
aforementioned "PROMASS I" series.
At this point it should be mentioned that in the flow tube 13 deflected in
this manner according to the second flexural and torsional mode, the
central axis 13B is slightly deformed, so that during the vibrations, this
axis
spreads a slightly curved surface rather than a plane. Furthermore, a path
curve lying in this surface and described by the midpoint of the central
axis of the flow tube has the smallest curvature of all path curves
described by the central tube axis.
To detect the deformations of flow tube 13, transducer 10 comprises a
sensor arrangement with at least a first sensor 17, which provides a first,
particularly analog, sensor signal in response to vibrations of flow tube 13.
As is usual with such transducers, sensor 17 may be formed, for instance,
by an armature of permanent-magnet material which is fixed to flow tube
13 and interacts with a sensor coil held by support frame 14.
Sensor types especially suited for sensor 17 are those which sense the
velocity of the deflections of the flow tube based on the electrodynamic
principle. It is also possible to use acceleration-measuring electrodynamic
or displacement-measuring resistive or optical sensors, or other sensors
familiar to those skilled in the art which are suitable for detecting such
vibrations.
In a embodiment of the invention, sensor arrangement further comprises a
second sensor 18, particularly a sensor identical to the first sensor 17,
which second sensor 18 provides a second sensor signal representing
vibrations of the flow tube. In this embodiment, the two sensors 17, 18 are
positioned at a given distance from each other along flow tube 13,
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particularly at the same distance from the midpoint of flow tube 13, such
that sensor arrangement will detect both inlet-side and outlet-side
vibrations of flow tube 13 and provide the corresponding sensor signals.
The first sensor signal and, if present, the second sensor signal, which
usually each have a frequency corresponding to the instantaneous
vibration frequency of flow tube 13, are fed to the meter electronics (not
shown).
To vibrate the flow tube 13, excitation assembly 16 is supplied from meter
electronics with a likewise oscillating, unipolar or bipolar excitation
current
of adjustable amplitude and adjustable frequency, such that in operation,
excitation coils 26, 36 are traversed by this current to produce the
magnetic field necessary to move armatures 27, 37. Thus, the excitation
force required to vibrate flow tube 13 can be monitored and adjusted in
amplitude, e.g. by means of a current- and/or voltage-regulator circuit
using at least one of the sensor signals, and in frequency, e.g. by means
of a phase-locked loop, in the manner familiar to those skilled in the art.
The excitation current delivered by the meter electronics is preferably a
sinusoidal current, but it may also be a pulsating, triangular, or square-
wave alternating current, for example.
As is usual in vibration meters of the kind described herein, the frequency
of the excitation current is equal to the predetermined vibration frequency
of flow tube 13, and is therefore preferably set at an instantaneous natural
resonance frequency of the fluid-carrying flow tube 13.~
It should be mentioned that, if the transducer is a non-invasive
electromagnetic flow sensor, instead of the excitation assembly shown
above, an excitation assembly in the form of a coil assembly will be used
in the manner familiar to those skilled in the art, which, when traversed by
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an excitation current, produces a magnetic held in the fluid flowing in the
flow tube. The sensor arrangement will then be an electrode arrangement
which picks off a measurement voltage induced in the fluid by means of
the above-mentioned magnetic field.
If the transducer is a non-invasive ultrasonic flow sensor, the excitation
assembly will be in the form of an ultrasonic transducer which, when
traversed by an excitation current, couples ultrasonic waves into the fluid
flowing in the flow tube. Then, an ultrasonic transducer will also be used
for the sensor arrangement, which extracts ultrasonic waves from the fluid
and converts them into a corresponding measurement voltage.
Since the transducer 10 shown in Figs. 1 to 4 is a multivariable transducer
which for detecting, alternately or simultaneously, the mass flow rate, m,
of the fluid by means of the two sensor signals and/or the density, p, by
means of the excitation frequency and/or the viscosity, rl, by means of the
excitation current, for the further explanation of the invention and for the
sake of consistency and clarity, the sensor signals, the excitation current,
or the above-mentioned measurement voltages are henceforth classed
under the term "measurement signal".
As mentioned, investigations have shown that the measurement signal
corresponding to the parameter to be measured, i.e., the first sensor
signal or the excitation current, for example, may be influenced to a
considerable extent by an instantaneous density distribution in the fluid
flowing in flow tube 13, particularly by a concentration and distribution of
possible parasitic inhomogeneities.
To improve the measurement signals, particularly to increase their
robustness to such inhomogeneities, according to the invention, means
are provided in an inlet area of transducer 10 or at least in the vicinity
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thereof which cause a swirl in the entering fluid, and thus a rotational
motion in the fluid within the flow-tube volume about an imaginary axis of
rotation lying in the direction of fluid flow. For the case where the flow
tube
is straight, the imaginary axis of rotation practically coincides with the
central flow-tube axis 13B.
In a embodiment of the invention, the flow-conditioning means for causing
the swirl comprise at least one turbulator 30 extending into the flowing
fluid. The turbulator 30, which preferably rests in the tube lumen, may be,
for instance, a separate propeller-shaped, helical, or helicoidal component
which, as indicated in Figs. 5a to 5d, is disposed at least in part in inlet
tube section 13+ or at least in part directly in flow tube 13. Particularly if
the means for causing the swirl are in the form of a helical or helicoidal
component as shown in Figs. 5a and 5b, they can be inserted into inlet
tube section 13+ and/or flow tube 13 while being subjected to stress, and
thus be held by spring force against the respective inside wall.
At this point it should be noted that the means for causing the swirl,
particularly the turbulator 30 in the form of a separate component, may
also be disposed at least in part within the pipe supplying the fluid to
transducer 10. For instance, the turbulator 30 may be disposed within a
short pipe section which is separately inserted in the pipe upstream of the
transducer 10. Furthermore, the means for causing the swirl may also be
formed by a multiply angled section of the pipe, for example.
In a further embodiment of the invention, turbulator 30 comprises at least
one baffle 30a extending into the flowing fluid at an angle, particularly a
baffle formed in the manner of a turbine swirl blade. Preferably, the
turbulator comprises two or more such baffles 30a, 30b, 30c, 30d, which,
as indicated in Fig. 5d, are so disposed that turbulator 30 takes the form
of a guide wheel, for instance of a turbine stator. The at least one baffle
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30a is preferably fixed to the inside wall of inlet tube section 13+; if
necessary, however, it may also be secured to the inside wall of the fluid-
supplying pipe.
In another embodiment of the invention, the means for causing the swirl
are designed in the manner of a rifled barrel as a rifling formed in the
inside wall of inlet tube section 13+ and/or of flow tube 13.
Investigations have also shown that the means for causing the swirl
advantageously should have an effective length in the direction of flow
which is at least equal to the nominal diameter of the pipe. Particularly
good measurement results were achieved if a reduction in the effective
cross-sectional area of inlet tube section 13+ and/or of the pipe resulting
from the installation of turbulator 30 was either kept very small or
compensated for by an increase in the respective nominal cross-sectional
area.
A particular advantage of the invention lies in the fact that the transducer
using the means for causing the swirl can be operated in virtually any
mounting position, particularly also in an essentially horizontal pipe, with
essentially unchanged measurement accuracy.
A further advantage is that, if the invention is used in vibratory
transducers, the amount of excitation current required to vibrate the flow
tube 13, and hence the required amount of energy, is substantially smaller
than in conventional transducers, particularly if the fluid to be measured
contains a high proportion of gaseous inhomogeneities.
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
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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.
