Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IN-LINE MEASURING DEVICE
Field of Invention
The invention relates to an inline measuring device having a
vibratory-type measurement pickup, especially a Coriolis mass-
flow/density measuring device for a medium, especially a two, or
more, phase medium flowing in a pipeline, as well as a method for
producing by means of such a measurement pickup a measured value
representing a physical, measured quantity of the medium, for
example a mass flow rate, a density and/or a viscosity.
Background
In the tecnnology of process measurements and automation, the
measurement of physical parameters of a medium flowing in a
pipeline, parameters such as e.g. the mass flow rate, density
and/or viscosity, such inline measuring devices, especially
Coriolis mass flow measuring devices, are used, which bring about
reaction forces in the medium, such as e.g. Coriolis forces
corresponding to the mass flow rate, inertial forces corresponding
to the density, or frictional forces corresponding to the
viscosity, etc., by means of a vibratory measurement pickup
inserted into the course of the pipeline carrying the medium and
traversed during operation by the medium, and by means of a
measurement and operating circuit connected therewith. Derived
from these reaction forces, the =measuring devices then produce a
measurement signal representing the particular mass flow rate, the
particular viscosity and/or the particular density of the medium.
Inline measuring devices of this type, utilizing a vibratory
measurement pickup, as well as their manner of operation, are
known per se to those skilled in the art and are described in
detail in e.g. WO-A 03/095950, WO-A 03/095949, WO-A 03/076880, WO-
A 02/37063, WO-A 01/33174, WO-A 00/57141, WO-A 99/39164, WO-A
98/07009, WO-A 95/16897, WO-A 88/03261, US 2003/0208325, US-B
= 6,691,583, US-B 66 51 51 13, US-B 6,513,393, US-B 6,505,519, US-A
6,006,609, US-A 5,869,770, US-A 5,796,011, US-A 5,616,868, US-A
5.602,346, US-A 5,602,345, US-A 5,531,126, US-A 5,301,557, US-A
5,253,533, US-A 5,218,873, US-A 5,069,074, US-A 4,876,898, US-A
4,733,569, US-A 4,660,421, US-A 4,524,610, US-A 4,491,025, US-A
4,187,721, EP-A 1 291 639, EP-A 1 281 938, EP-A 1 001 254 or EP-A
553 939.
For guiding the medium, the measurement pickups include at least
one measuring tube with a straight tube segment held in a, for
example, tubular or box-shaped, support frame. For producing the
above-mentioned reaction forces during operation, the tube segment
is caused to vibrate, driven by an electromechanical' exciter
arrangement.
For registering vibrations of the tube segment,
particularly at its inlet and outlet ends, the measurement pickups
additionally include an electrophysical sensor arrangement
reacting to movements of the tube segment.
In the case of Coriolis mass flow measuring =devices, the
measurement of the mass flow rate of a medium flowing in a
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pipeline rests, for example, on having the medium flow through the
measuring tube inserted into the pipeline and oscillating during
operation laterally to a measuring tube axis, whereby Coriolis
forces are induced in the medium. These, in turn, effect that the
inlet and outlet end regions of the measuring tube oscillate
shifted in phase relative to one another. The magnitude of this
phase shift serves as a measure of the mass flow rate.
The
oscillations of the measuring tube are, to this end, registered by
means of two oscillation sensors of the above-mentioned sensor
arrangement separated from one another along the length of the
measuring tube and are transformed into oscillation measurement
signals, from whose phase shift relative to one another the mass
flow rate is derived.
Already the above-mentioned US-A 4,187,721 mentions, further, that
the instantaneous density of the flowing medium can also be
measured by means of such inline measuring devices, and, indeed,
on the basis of a frequency of at least one of the oscillation
measurement signals delivered from the sensor arrangement.
Moreover, usually also a temperature of the medium is directly
measured in suitable manner, for example by means of a temperature
sensor arranged on the measuring tube.
Additionally, straight
measuring tubes can, as is known, upon being excited to torsional
oscillations about a torsional oscillation axis extending
essentially parallel to, or coinciding with, the longitudinal axis
of the measuring tube, effect that radial shearing forces are
produced in the medium as it flows through the tube, whereby
significant oscillation energy is withdrawn from the torsional
oscillations and dissipated in the medium. As a result of this, a
considerable damping of the torsional oscillations of the
oscillating measuring tube occurs, so that, additionally,
electrical exciting power must be added, in order to maintain the
torsional oscillations.
On the basis of the electrical exciting
power required to maintain the torsional oscillations of the
measuring tube, the measurement pickup can also be used to
determine, at least approximately, a viscosity of the medium;
compare, in this connection also US-A 4,524,610, US-A 5,253,533,
US-A 6,006,609 or US-B 6,651,513.
It can, consequently, assumed,
without more in the following, that, even when not expressly
stated, modern inline measuring devices using a vibratory
measurement pickup, especially Coriolis mass flow measuring
devices, have the ability to measure, in any case, also density,
viscosity and/or temperature of the medium, especially since these
are always needed anyway in the measurement of mass flow rate for
the compensation of measurement errors arising from fluctuating
density and/or viscosity of the medium; compare, in this
connection, especially the already mentioned US-B 6,513,393, US-A
6,006,609, US-A 5,602,346, WO-A 02/37063, WO-A 99/39164 or also
WO-A 00/36379.
In the application of inline measuring devices using a vibratory
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measurement-pickup, it has, however, become evident, as also
discussed, for example, in JP-A 10-281846, WO-A 03/076880, EP-A 1
291 639, US-B 6,505,519 or US-A 4,524,610, that, in the case of
inhomogeneous media, especially two, or more, phase media, the
oscillation measurement signals derived from the oscillations of
the measuring tube, especially also the mentioned phase shift, can
be subject to fluctuations to a considerable degree and, thus, in
some cases, can be completely unusable for the measurement of the
desired physical parameters, without the use of auxiliary
measures, this in spite of keeping the viscosity and density in
the individual phases of the medium, as well as also the mass flow
rate, practically constant and/or appropriately taking them into
consideration.
Such inhomogeneous media can, for example, be
liquids, into which, as is e.g. practically unavoidable in dosing
or bottling processes, a gas, especially air, present in the
pipeline is entrained or out of which a dissolved medium, e.g.
carbon dioxide, outgasses and leads to foam formation. As other
examples of such inhomogeneous media, emulsions and wet, or
saturated, steam can be named.
As causes for the fluctuations
arising in the measurement of inhomogeneous media by means of
vibratory measurement pickups, the following can be noted by way
of example: the unilateral clinging or deposit of gas bubbles or
solid particles, entrained in liquids, internally on the measuring
tube wall, and the so-called "bubble-effect", where gas bubbles
entrained in the liquid act as flow bodies for liquid volumes
accelerated transversely to the longitudinal axis of the measuring
tube.
While, for decreasing the measurement errors associated with two,
or more, phase media, a flow, respectively medium, conditioning
preceding the actual flow rate measurement is proposed in WO-A
03/076880, both JP-A 10-281846 and US-B 6,505,519, for example,
describe a correction of the flow rate measurement, especially the
mass flow rate measurement, based on the oscillation measurement
signals, which correction rests especially on the evaluation of
deficits between a highly accurately measured, actual medium
density and an apparent medium density determined by means of
Coriolis mass flow measuring devices during operation.
In particular, pre-trained, in some cases even adaptive,
classifiers of the oscillation measurement signals are proposed
for this.
The classifiers can, for example, be designed as a
Kohonen map or neural network, and the correction is made either
on the basis of some few parameters, especially the mass flow rate
and the density measured during operation, as well as other
features derived therefrom, or also using an interval of the
oscillation measurement signals encompassing one or more
oscillation periods.
The use of such a classifier brings, for
example, the advantage that, in comparison to conventional
Coriolis mass flow/density meters, no, or only very slight,
changes have to be made at the measurement pickup, in terms of
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mechanical construction, the exciter arrangement or the operating
circuit driving such, which are specially adapted for the
particular application.
However, a considerable disadvantage of
such classifiers includes, among others, that, in comparison to
conventional Coriolis mass flow measuring devices, considerable
changes are required in the area of the measured value production,
above all with regards to the analog-to-digital transducer being
used and the microprocessors. As, namely, also described in US-B
6,505,519, required for such a signal evaluation, for example, in
the digitizing of the oscillation measurement signals, which can
exhibit an oscillation frequency of about 80 Hz, is a sampling
rate of about 55 kHz or more, in order to obtain a sufficient
accuracy. Stated differently, the oscillation measurement signals
have to be samples with a sampling ratio of far above 600:1.
Beyond this, also the firmware stored and executed in the digital
measurement circuit is correspondingly complex.
A further
disadvantage of such classifiers is that they must be trained and
correspondingly validated for the conditions of measurement
actually existing during operation of the measurement pickup, be
it regarding the particulars of the installation, the medium to be
measured and its usually variable properties, or other factors
influencing the accuracy of measurement.
Because of the high
complexity of the interplay of all these factors, the training and
its validation can occur ultimately only on site and individually
for each measurement pickup, this in turn meaning a considerable
effort for the startup of the measurement pickup. Finally, it has
been found, that such classifier algorithms, on the one hand
because of the high complexity, on the other because of the fact
that usually a corresponding physical-mathematical model with
technically relevant or comprehensible parameters is not
explicitly present, exhibit a very low transparency and are,
consequently, often difficult to explain.
Accompanying this
situation, it is clear that considerable reservations can occur on
the part of the customer, with such acceptance problems especially
arising when the classifier, additionally, is self-adapting, for
example a neural network.
As a further possibility for getting around the problem of
inhomogeneous media, it is proposed, for instance, already in US-A
4,524,610 to install the measurement pickup such that the straight
measuring tube extends essentially vertically, in order to
prevent, as much as possible, a deposition of such disturbing,
especially gaseous, inhomogeneities.
Here, however, one is
dealing with a very special solution which cannot always be
implemented, without more, in the technology of industrial process
measurement. On the one hand, in this case, it can happen,
namely, that the pipeline, into which the measurement pickup is to
be inserted, might have to be adapted to the measurement pickup,
rather than the reverse, which can mean an increased expense for
implementing the measurement location.
On the other hand, as
already mentioned, the measuring tubes might have a curved shape,
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in which case the problem cannot always be solved satisfactorily by an
adapting of
the installation orientation anyway. It has, moreover, been found in this case
that the
aforementioned corruptions of the measurement signal are not necessarily
prevented
with certainty by the use of a vertically installed, straight measuring tube
anyway.
Summary
According to one aspect of the present invention, there is provided inline
measuring
device for the measurement of at least one physical, measured quantity of a
medium
guided in a pipeline, which inline measuring device comprises a vibratory-type
measurement pickup and measuring device electronics electrically coupled with
the
measurement pickup, wherein the measurement pickup includes: at least one
essentially straight measuring tube, inserted into the course of the pipeline,
serving
for guiding the medium to be measured, and communicating with the connected
pipeline, an exciter arrangement acting on the measuring tube for causing the
at least
one measuring tube to vibrate, which, during operation, causes the measuring
tube to
vibrate with bending oscillations, and which, during operation, causes the
measuring
tube to vibrate with torsional oscillations about an imaginary measuring tube
longitudinal axis essentially aligned with the measuring tube, and a sensor
arrangement for registering vibrations of the at least one measuring tube and
for
delivering at least one oscillation measurement signal representing
oscillations of the
measuring tube, wherein the measuring device electronics delivers an exciter
current
driving the exciter arrangement, wherein the measuring device electronics
further
determines a first intermediate value and a second intermediate value, the
first
intermediate value corresponding to at least one of a lateral current
component of the
exciter current serving to maintain the lateral oscillations of the measuring
tube and a
damping of the lateral oscillations of the measuring tube, and the second
intermediate value corresponding to at least one of a torsional current
component of
the exciter current serving to maintain the torsional oscillations of the
measuring tube
and a damping of the torsional oscillations of the measuring tube, and wherein
the
measuring device electronics generates, by means of at least one of (i) the at
least
one oscillation measurement signal and (ii) the exciter current, and by means
of the
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first and the second intermediate values , at least one measured value, which
represents said at least one physical quantity to be measured.
According to another aspect of the present invention, there is provided method
for
measuring a physical, measured quantity of a medium flowing in a pipeline by
means
of an inline measuring device including a vibration-type measurement pickup
and a
measuring device electronics electrically coupled with the measurement pickup,
which method comprising: - allowing the medium to be measured to flow through
at
least one straight measuring tube of the measurement pickup, with the
measuring
tube being in communication with the pipeline, and feeding an exciter current
into an
exciter arrangement mechanically coupled with the measuring tube guiding the
medium, for causing the measuring tube to execute mechanical oscillations; -
causing
the measuring tube to execute bending oscillations, and causing the measuring
tube
to execute torsional oscillations, especially torsional oscillations
superimposed on the
lateral oscillations; - registering vibrations of the measuring tube and
producing at
least one oscillation measurement signal representing oscillations of the
measuring
tube; - determining a first intermediate value derived from the exciter
current, which
corresponds to at least one of (i) a lateral current component of the exciter
current
serving to maintain the bending oscillations of the measuring tube and (ii) a
damping
of the lateral oscillations of the measuring tube; - determining, derived from
the
exciter current, a second intermediate value, which corresponds to at least
one of a
torsional current component of the exciter current serving to maintain the
torsional
oscillations of the measuring tube and a damping of the torsional oscillations
of the
measuring tube; and - using, together with the first and second intermediate
values,
at least one of (i) the at least one oscillation measurement signal and (ii)
the exciter
current for producing a measured value representing the physical, measured
quantity
to be measured.
An object of some embodiments of the invention is to provide a corresponding
inline
measuring device, especially a Coriolis mass flow measuring device, that is
suited for
measuring a physical, measured quantity, especially mass flow rate, density
and/or
viscosity, very accurately, even in the case of inhomogeneous, especially two,
or
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more, phase media, and, indeed, especially desirably with a measurement error
of
less than 10% referenced to the actual value of the measured quantity. A
further
object of some embodiments of the invention is to provide a corresponding
method
for producing a corresponding measured value.
For achieving this object, some embodiments of the invention reside in an
inline
measuring device, especially a Coriolis mass flow rate/density measuring
device
and/or a viscosity measuring device, for measuring at least one physical,
measured
quantity, especially a mass flow rate, a density and/or a viscosity, of a
medium,
especially a two, or more, phase medium, conducted in a pipeline. The inline
measuring device includes for this purpose a vibratory measurement pickup and
a
measuring device electronics electrically coupled with the measurement pickup.
The
measurement pickup includes, inserted into the course of the pipeline, a
measuring
tube, especially an essentially straight measuring tube, thus communicating
with the
pipeline and serving to conduct the medium to be measured, an exciter
arrangement
acting on the measuring tube for causing the at least one measuring tube to
vibrate,
and a sensor arrangement for registering vibrations of the at least one
measuring
tube and delivering at least one oscillation measurement signal representing
oscillations of the measuring tube. The exciter arrangement causes the
measuring
tube, during operation, at least at times and/or at least partially, to
vibrate with lateral
oscillations, especially bending oscillations. Additionally, the exciter
arrangement
causes the measuring tube, during operation, at least at times and/or at least
partially, to vibrate with torsional oscillations about an imaginary,
measuring tube
longitudinal axis essentially aligned with the measuring tube, especially an
axis
developed as a principal axis of inertia of the measuring tube. The torsional
oscillations are especially ones which alternate with the lateral oscillations
or are at
times superimposed thereon. The measuring device electronics delivers, at
least at
times, an exciting current driving the exciter arrangement. Further, the
measuring
device electronics determines a first intermediate value, which corresponds to
a
lateral current component of the exciter current serving for maintaining the
lateral
oscillations of the measuring tube and/or to a damping of
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the lateral oscillations of the exciter current. Additionally, the
measuring device electronics determines a second intermediate
value, which corresponds to a torsional current component of the
exciter current serving for maintaining the torsional oscillations
of the measuring tube and/or to a damping of the torsional
oscillations of the measuring tube. By means of the at least one
oscillation measurement signal and/or by means of the exciter
current, as well as with application of the first and second
intermediate values, the measuring device electronics generates,
at least at times, at least one measured value, which represents
the at least one physical quantity being measured, especially the
mass flow rate, the density or the viscosity of the medium.
Additionally, some embodiments of the invention reside in a method
for measuring a physical, measured quantity, especially mass flow
rate, a density and/or a viscosity, of a medium flowing in a
pipeline, especially a two, or more, phase medium, by means of an
inline measuring device having a vibratory measurement pickup,
especially a Coriolis mass flow measuring device, and a measuring
device electronics electrically coupled with the pickup, which
method comprises the following steps:
- allowing a medium to be measured to flow through at least one
measuring tube of the measurement pickup communicating with the
pipeline and feeding an exciter current into an exciter
arrangement mechanically coupled with the measuring tube guiding
the medium, in order to cause the measuring tube to execute
mechanical oscillations,
- effecting lateral oscillations, especially bending oscillations,
of the measuring tube and effecting torsional oscillations of
the measuring tube, especially torsional oscillations
superimposed on the lateral oscillations,
- registering vibrations of the measuring tube and producing at
least one oscillation measurement signal representing
oscillations of the measuring tube,
- determining a first intermediate value derived from the exciter
current and corresponding to a lateral current component of the
exciter current serving for maintaining the lateral oscillations
of the measuring tube and/or to a damping of the lateral
oscillations of the measuring tube,
- determining a second intermediate value derived from the exciter
current and corresponding to a torsional current component of
the exciter current serving for maintaining the torsional
oscillations of the measuring tube and/or to a damping of the
torsional oscillations of the measuring tube, and
- using the at least one oscillation measurement signal and/or the
exciter current, as well as the first and second intermediate
values, for producing a measured value representing the physical
quantity to be measured.
According to a first embodiment of the inline measuring device of
the invention, the measuring electronics determines, derived from
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the at least one oscillation measurement signal and/or from the
exciter current, an initial measured value, which corresponds, at
least approximately, to the at least one quantity to be measured,
and, on the basis of the first and second intermediate values, a
correction value for the initial measured value, and the measuring
device electronics generates the measured value by means of the
initial measured value and the correction value.
In a second embodiment of the inline measuring device of the
invention, the measuring tube, driven by the exciter arrangement,
executes torsional oscillations having a measuring tube torsional
oscillation frequency which is set to be different from a
measuring tube bending oscillation frequency with which the
measuring tube, driven by the exciter arrangement, executes
lateral oscillations.
According to a third embodiment of the inline measuring device of
the invention, the measuring tube communicates with the connected
pipeline via an inlet tube piece opening into an inlet end and via
an outlet tube piece opening into an outlet end, and the
measurement pickup includes a counteroscillator fixed to the inlet
end and to the outlet end of the measuring tube, especially also
mechanically coupled with the exciter arrangement, and vibrating,
at least at times, during operation, especially with phase
opposite to that of the measuring tube.
In a fourth embodiment of the inline measuring device of the
invention, the measuring device electronics determines the
correction value on the basis of a comparison of the first
intermediate value with the second intermediate value and/or on
the basis of a difference existing between the first intermediate
value and the second intermediate value.
According to a fifth embodiment of the inline measuring device of
the invention, the measuring device electronics produces the first
and/or the second intermediate value also on the basis of the
least one oscillation measurement signal.
In a sixth embodiment of the inline measuring device of the
invention, the at least one measured value represents a viscosity
of the medium flowing in the measuring tube, and the measuring
device electronics determines also the initial measured value on
the basis of the exciter current, and/or a component of the
exciter current, driving the exciter arrangement.
According to a seventh embodiment of the inline measuring device
of the invention, the at least one measured value represents a
density of the medium flowing in the measuring tube, and the
measuring tube electronics determines the initial measured value
using the at least one oscillation measurement signal and/or the
exciter current by recognizing that this corresponds to the
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density to be measured and/or to an oscillation frequency of the
at least one oscillation measurement signal.
In an eighth embodiment of the inline measurement device of the
invention, the measuring device electronics determines at least at
times, on the basis of the first and second intermediate values, a
concentration measured value, which represents an, especially
relative, volume and/or mass fraction of a phase of the medium, in
the case of a two, or more, phase medium in the measuring tube.
According to a ninth embodiment of the inline measuring device of
the invention, the sensor arrangement delivers at least one
oscillation measurement signal representing, at least in part,
inlet end lateral oscillations, especially bending oscillations,
of the measuring tube, and at least one second oscillation
measurement signal representing, at least in part, outlet end
lateral oscillations, especially bending oscillations, of the
measuring tube.
In a tenth embodiment of the inline measuring device of the
invention, the at least one measured value represents a mass flow
rate of the medium flowing in the measuring tube, and the
measuring device electronics determines the initial measured value
using the two oscillation measurement signals by recognizing that
this corresponds to the mass flow rate to be measured and/or to a
phase difference between the two oscillation measurement signals.
According to a first embodiment of the method of the invention,
the step of producing the measured value includes the steps of:
- developing, using the at least one oscillation measurement
signal and/or the exciter current, an initial measured value
corresponding at least approximately to the physical quantity to
be measured,
- producing a correction value for the initial value by means of
the first and second intermediate values, and
- correcting the initial measured value by means of the correction
value, for producing the measured value.
In a second embodiment of the method of the invention, the step of
producing the correction value for the initial measurement value
comprises the steps of:
- Comparing the first intermediate value with the second
intermediate value for determining a difference existing between
the two intermediate values and
- determining, taking into consideration the difference existing
between the two intermediate values, a concentration measured
value, which represents, in the case of a two, or more, phase
medium in the measuring tube, an, especially relative, volume
and/or mass fraction of a medium phase.
A basic idea of the invention resides in operating the measurement
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pickup in a dual-mode for the purpose of correcting or
compensating possible measurement errors caused especially by
inhomogeneities in the medium to be measured. In the dual mode,
the measuring tube is caused to vibrate alternately in at least
two oscillation modes which are essentially independent of one
another, namely a lateral oscillation mode and a torsional
oscillation mode.
On the basis of operating parameters of the
measurement pickup determined during the dual-mode operation,
especially the exciter current, the frequencies and/or amplitudes
of the oscillations of the measuring tube, etc., required for
maintaining the lateral and torsional oscillations of the
measuring tube, very exact and amazingly robust correction values
for the actual measured values can be determined in a very simple
manner.
The invention rests, in this connection, especially on the
discovery that the exciter power fed into the measurement pickup
for maintaining the lateral oscillations of the measuring tube can
be affected to a high degree by inhomogeneities in the medium
being measured, inhomogeneities such as e.g. entrained gas bubbles
or solid particles, etc..
In comparison therewith, the exciter
power fed into the measurement pickup for maintaining torsional
oscillations of the measuring tube depend to a considerably lesser
extent on such inhomogeneities, so that, during operation, based
on this exciter power, especially based on the exciter current
component actually fed for maintaining the torsional oscillations,
up-to-the-moment reference values can be determined, with whose
help a comparison of the correspondingly determined measured
values for the lateral oscillations, for example the exciter
current component actually fed for maintaining the lateral
oscillations, can be made.
On the basis of this, for example,
normalized or subtractively executed comparison, an instantaneous
degree of inhomogeneity in the medium can be estimated and,
derived from this, a sufficiently accurate conclusion made as to
the measurement error which has entered the measurement.
The
inline measuring device of the invention is, therefore, especially
suited for the measurement of a physical, measured quantity,
especially a mass flow rate, a density and/or a viscosity, even of
a two, or more, phase medium flowing in a pipeline, especially a
liquid-gas mixture.
An advantage of some embodiments of the invention is that the correction
values to be determined are well reproducible over a large range of
application and, also, the forming rules for determining the correction
values during measurement operation can be formulated relatively simply.
Moreover, these forming rules can be calculated initially with a
relatively small effort. A further advantage of some embodiments of the
invention is, additionally, to be seen in the fact that, in the case of
the inline measuring device of the invention, as compared to a
conventional type, especially such as described in WO-A 03/095950, WO-A
03/095949 or US-A 4,524,610, only in the case of the usually
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digital, measured value production do slight changes have to be made,
these being essentially limited to the firmware, while, both in the case
of the measurement pickup and in the production and preprocessing of the
oscillation measurement signals, no, or only slight, changes are
required. Thus, for example, even in the case of two, or more, media,
the oscillation measurement signals can be sampled, as before, with a
usual sampling ratio of far under 100:1, especially of about 10:1.
Brief Description of Drawings
The invention and further advantageous embodiments will now be explained
in detail on the basis of examples of embodiments presented in the
figures of the drawing. Equal parts are provided in all figures with
equal reference characters; when required in the interest of clarity,
already mentioned reference characters are omitted in subsequent
figures.
Fig. 1
shows an inline measuring device which can be inserted
into a pipeline for measuring a mass flow rate of a fluid
guided in the pipeline,
Fig. 2
shows, in a perspective, side view, an example of an
embodiment for a measurement pickup suited for the
measuring device of Fig. 1,
Fig. 3
shows, sectioned in a side view, the measurement pickup of
Fig. 2,
Fig. 4
shows the measurement pickup of Fig. 2 in a first cross
section,
Fig. 5
shows the measurement pickup of Fig. 2 in a second cross
section,
Fig. 6
= shows, sectioned in a side view, a further example of an
embodiment of a vibratory measurement-pickup suited for
the inline measuring device of Fig. 1,
Fig. 7
shows schematically in the form of a block diagram a
preferred embodiment of a measuring device electronics
suited for the inline measuring device of Fig. 1, and
Figs. 8, 9 show, graphically, measurement data
experimentally
determined using an inline measuring device of the Figs. 1
to 7.
Detailed Description
Fig. 1 shows, perspectively, an inline measuring device 1 suited for
registering a physical, measured quantity, e.g. a mass flow rate m, a
density p and/or a viscosity n, of a medium flowing in a pipeline (not
shown) and for imaging this measured quantity in an instantaneously
representing, measured value Xx. The medium
in
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this instance can be practically any flowable substance, for
example a liquid, a gas, a vapor, or the like.
The inline measuring device 1, for example provided in the form of
a Coriolis mass flow, density and/or viscosity meter, includes
therefor a vibratory measurement pickup flowed-through by the
medium to be measured, an example of an embodiment and
developments being shown in Figs. 2 to 6, together with a
measuring device electronics 50, as illustrated schematically in
Figs. 2 and 7.
Preferably, the measuring device electronics 50
is, additionally, so designed that it can, during operation of the
inline measuring device 1, exchange measurement and/or operational
data with a measured value processing unit superordinated, i.e.
located at a higher level, with respect thereto, for example a
programmable logic controller (PLC), a personal computer and/or a
workstation, via a data transmission system, for example a field
bus system.
Furthermore, the measuring device electronics is
designed such that it can be supplied from an external energy
supply, for example also over the aforementioned field bus system.
For the case in which the vibratory measuring device is provided
for coupling to a field bus or some other communication system,
the, especially programmable, measuring device electronics 50 is
equipped with a corresponding communications interface for a
communication of data, e.g. for the transmission of the
measurement data to the already mentioned, programmable logic
controller or to a superordinated process control system.
For
accommodation of the measuring device electronics 50, an
electronics housing 200 is additionally provided, especially one
mounted externally directly onto the measurement pickup, but
even one possibly set apart from such.
As already mentioned, the inline measuring device includes a
vibratory measurement pickup, which is flowed-through by the
medium to be measured, and which serves for producing, in a
through-flowing medium, mechanical reaction forces, especially
Coriolis forces, dependent on the mass flow rate, inertial forces
dependent on the density of the medium and/or frictional forces
dependent on the viscosity of the medium, forces which react
measurably, i.e. capable of being detected by sensor, on the
measurement pickup.
Derived from these reaction forces
characterizing the medium, e.g. the mass flow rate, the density
and/or the viscosity of the medium can be measured in manner known
to those skilled in the art. In Figs. 3 and 4, an example of an
embodiment of an electrophysical transducer arrangement, serving
as a vibratory measurement pickup 10, is schematically
illustrated.
The mechanical construction and manner of
functioning of such a transducer arrangement is known per se to
those skilled in the art and is also described in detail in US-B
6,691,583, WO-A 03/095949 or WO-A 03/095950.
For guiding the medium and for producing said reaction forces, the
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measurement pickup includes at least one, essentially straight,
measuring tube 10 of predeterminable measuring tube diameter.
Measuring tube 10 is caused, during operation, to vibrate, at
least at times, and is repeatedly elastically deformed thereby.
Elastic deformation of the measuring tube lumen means here, that a
spatial form and/or a spatial position of the measuring tube lumen
is changed within an elastic range of the measuring tube 10 in
predeterminable manner cyclically, especially periodically;
compare, in this connection, also US-A 4,801,897, US-A 5,648,616,
US-A 5,796,011, US-A 6,006,609, US-B 6,691,583, WO-A 03/095949
and/or WO-A 03/095950. It should be mentioned here that, instead
of the measurement pickup shown in the example of an embodiment
having a single, straight measuring tube, the measurement pickup
serving for implementation of the invention can, as well, be
selected from a multiplicity of vibratory measurement pickups
known in the state of the art.
In particular, suited, for
example, are vibratory measurement pickups having two parallel,
straight measuring tubes flowed-through by the medium to be
measured, such as are described in detail also in US-A 5,602,345.
As shown in Fig. 1, the measurement pickup additionally has a
measurement pickup housing 100 surrounding the measuring tube 10,
as well as surrounding possible other components of the
measurement pickup (see also further below). Housing 100 acts to
protect tube 10 and other components from damaging environmental
influences and/or to damp possible outwardly-directed sound
emissions of the measurement pickup. Beyond this, the measurement
pickup housing 100 also serves as a mounting platform for an
electronics housing 200 housing the measuring device electronics
50. To this end, the measurement pickup housing 100 is provided
with a neck-like transition piece, on which the electronics
housing 200 is appropriately fixed; compare Fig. 1.
Instead of
the tube-shaped transducer housing 100 shown here extending
coaxially with the measuring tube, other suitable housing forms
can, of course, be used, such as e.g. box-shaped structures.
The measuring tube 10, which communicates in the usual manner at
inlet and outlet ends with the pipeline introducing, respectively
extracting, the medium to be measured, is oscillatably suspended
in the preferably rigid, especially bending- and twisting-stiff,
transducer housing 100.
For permitting the medium to flow
through, the measuring tube is connected to the pipeline via an
inlet tube piece 11 opening into the inlet end 11# and an outlet
tube piece 12 opening into the outlet end 12#. Measuring tube 10,
inlet tube piece 11 and outlet tube piece 12 are aligned with one
another and with the above-mentioned measuring tube longitudinal
axis L as exactly as possible and are, advantageously, provided as
one piece, so that e.g. a single, tubular stock can serve for
their manufacture; in case required, measuring tube 10 and tube
pieces 11, 12 can, however, also be manufactured by means of
separate, subsequently joined, e.g. welded, stock. For
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manufacture of the measuring tube 10, as well as the inlet and
outlet, tubular pieces 11, 12, practically every usual material
for such measurement pickups can be used, such as e.g. alloys of
iron, titanium, zirconium and/or tantalum, synthetic materials, or
ceramics.
For the case where the measurement pickup is to be
releasably assembled with the pipeline, first and second flanges
13, 14 are preferably formed on the inlet tube piece 11 and the
outlet tube piece 12, respectively; if required, the inlet and
outlet tube pieces can, however, also be connected directly to the
pipeline, e.g. by means of welding or brazing. Additionally, as
shown schematically in Fig. 1, the transducer housing 100 is
provided, fixed to the inlet and outlet tube pieces 11, 12, for
accommodating the measuring tube 10; compare, in this connection,
Figs. 1 and 2.
At least for measuring the mass flow rate m, the measuring tube 10
is excited in a first useful mode of oscillation developed as a
lateral oscillation mode, in which it executes, at least in part,
oscillations, especially bending oscillations, laterally to an
imaginary measuring tube longitudinal axis L, especially such that
it bends laterally outwards, essentially oscillating at a natural
bending eigenfrequency, according to a natural, first form of
eigenoscillation. For the case where the medium is flowing in the
connected pipeline and, consequently, the mass flow rate m is
different from zero, the measuring tube 10, oscillating in the
first useful mode of oscillation, induces Coriolis forces in the
medium as it flows through.
These, in turn, interact with the
measuring tube 10 and result, in the manner known to those skilled
in the art, in an additional, sensor-detectable deformation of the
measuring tube 10 essentially according to a natural, second form
of eigenoscillation coplanarly superimposed on the first form of
eigenoscillation.
The instantaneous shape of the deformation of
the measuring tube 10 is, in such case, especially as regards its
amplitudes, also dependent on the instantaneous mass flow rate m.
As usual in the case of such measurement pickups, anti-symmetric
forms of bending oscillation of two, or four, antinodes can e.g.
serve as the second form of eigenoscillation, the so-called
Coriolis mode.
Since natural eigenfrequencies of such modes of
lateral oscillation of measuring tubes are known to depend, in
special measure, also on the density p of the medium, also the
density p can be measured, without more, by means of the inline
measuring device, in addition to the mass flow rate m.
In
addition to the lateral oscillations, the at least one measuring
is also driven, at least at times, in a torsional mode of
oscillation, for producing viscosity-dependent, shear forces in
the flowing medium.
In this torsional mode of oscillation, the
measuring tube is excited to torsional oscillations about an axis
of torsional oscillation extending essentially parallel to, or
coinciding with, the longitudinal axis L of the measuring tube.
Essentially, this excitement is such that the measuring tube 10 is
twisted about its longitudinal axis L in a form of natural,
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torsional oscillation; compare, in this connection, e.g. also US-A
4,524,610, US-A 5,253,533, US-A 6,006,609 or EP-A 1 158 289. The
exciting of the torsional oscillations can, in such case, occur
either in alternation with the first useful mode of oscillation
and separated therefrom, in a second useful mode of oscillation,
or, at least in the case of mutually distinguishable oscillation
frequencies, also simultaneously with the lateral oscillations in
the first useful mode of oscillation.
Stated differently, the
measurement pickup works, at least at times, in a dual-mode of
operation, in which the at least one measuring tube 10 is caused
to vibrate alternatingly in at least two oscillation modes
essentially independent of one another, namely in the lateral
oscillation mode and in the torsional oscillation mode.
According to one embodiment of the invention, for producing the
mass flow rate-dependent Coriolis forces in the flowing medium,
the measuring tube 10 is excited, at least at times, with a
lateral oscillation frequency, which corresponds as exactly as
possible to a lowest natural bending eigenfrequency of the
measuring tube 10, so that, thus, the laterally oscillating
measuring tube 10, without fluid flowing through it, is
essentially symmetrically bowed outwards with respect to a middle
axis perpendicular to the longitudinal axis L of the measuring
tube and, in doing so, exhibits a single oscillation antinode.
This lowest bending eigenfrequency can be., for example, in the
case of a stainless steel tube serving as the measuring tube 10,
of nominal diameter 20 mm, wall thickness about 1_2 mm and length
about 350 mm, with the usual appendages, about 850 Hz to 900 Hz.
In a further embodiment of the invention, the measuring tube 10 is
excited, especially simultaneously to the lateral oscillations in
the useful mode, with a torsional oscillation frequency fexcTe
which corresponds as exactly as possible to a natural torsional
eigenfrequency of the measuring tube.
A lowest torsional
eigenfrequency can, for example, lie in the case of a straight
measuring tube about in the range of twice the lowest bending
eigenfrequency.
As already mentioned, the oscillations of the measuring tube
are damped, on the one hand, by transfer of oscillation energy,
especially to the medium. On the other hand, however, oscillation
energy can also be withdrawn from the vibrating measuring tube to
a considerable degree by the excitation of components mechanically
coupled therewith into oscillations, components such as e.g. the
transducer housing 100 or the connected pipeline. For the purpose
of suppressing or preventing a possible loss of oscillation energy
to the environment, a counteroscillator 20 is, therefore, provided
in the measurement pickup fixed to the inlet and outlet endc of
the measuring tube 10.
The counteroscillator 20 is, as shown
schematically in Fig. 2, preferably embodied as one piece.
If
required, the counteroscillator 20 can be composed of multiple
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parts, as shown e.g. also in US-A 5,969,265, EP-A 317 340 or WO-A
00/14485, or it can be implemented by means of two separate
counteroscillator portions fixed to the inlet and outlet ends of
the measuring tube 10; compare Fig. 6. The counteroscillator 20
serves, among other things, to balance the measurement pickup
dynamically for at least one, predetermined density value of the
medium, for example a density value most frequently to be
expected, or also a critical density value, to such an extent that
transverse forces and/or bending moments possibly produced in the
vibrating measuring tube 10 are largely compensated; compare, in
this connection, also US-B 6,691,583. Moreover, the
counteroscillator 20 serves for the above-described case, where
the measuring tube 10 is also excited during operation to
torsional oscillations, additionally to produce counter torsional
moments largely compensating such torsional moments as are
produced by the single measuring tube 10 preferably twisting about
its longitudinal axis L, thus holding the environment of the
measurement pickup, especially, however, the connected pipeline,
largely free of dynamic torsional moments. The counteroscillator
20 can, as shown schematically in Figs. 2 and 3, be embodied in
tube shape and can be connected, for example, to the inlet end 11#
and the outlet end 12# of the measuring tube 10 in such a manner
that it is, as shown in Fig. 3, arranged essentially coaxially
with the measuring tube 10. The counteroscillator 20 can be made
of practically any of the materials also used for the measuring
tube 10, thus, for example, stainless steel, titanium alloys, etc.
The counteroscillator 20, which is, especially in comparison to
the measuring tube 10, somewhat less torsionally and/or bendingly
elastic, is likewise caused to oscillate during operation and,
indeed, with essentially the same frequency as the measuring tube
10, but out of phase therewith, especially with opposite phase.
To this end, the counteroscillator 20 is caused to oscillate with
at least one of its torsional eigenfrequencies tuned as accurately
as possible to those torsional oscillation frequencies, with which
the measuring tube is predominantly caused to oscillate during
operation. Moreover, the counteroscillator 20 is adjusted also in
at least one of its bending eigenfrequencies to at least one
bending oscillation frequency with which the measuring tube 10,
especially in the useful mode, is caused to oscillate, and the
counteroscillator 20 is excited during operation of the
measurement pickup also to lateral oscillations, especially
bending oscillations, which are developed essentially coplanarly
with lateral oscillations of the measuring tube 10, especially the
bending oscillations of the useful mode.
In an embodiment of the invention shown schematically in Fig. 3,
the counteroscillator 20 has, for this purpose, grooves 201, 202,
which make possible an exact adjustment of its torsional
eigenfrequencies, especially a sinking of the torsional
eigenfrequencies through a sinking of a torsional stiffness of the
CA 02559564 2011-05-03
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counteroscillator 20. Although the grooves 201, 202 are shown in
Fig. 2 or Fig. 3 essentially uniformly distributed in the
direction of the longitudinal axis L, they can, if required, also
be arranged, without more, distributed non-uniformly in the
direction of the longitudinal axis L.
Moreover, the mass
distribution of the counteroscillator can, as likewise shown
schematically in Fig. 3, also be corrected by means of
corresponding mass balancing bodies 101, 102 fixed to the
measuring tube 10. These mass balancing bodies 101, 102 can be
e.g. metal rings pushed onto the measuring tube 10, or small metal
plates fixed thereto.
For producing mechanical oscillations of the measuring tube 10,
the measurement pickup additionally includes an exciter
arrangement 40, especially an electrodynamic one, coupled to the
measuring tube. The exciter arrangement 40 serves for converting
an electrical exciter power P
- exc fed from the measuring device
electronics, e.g. having a regulated exciter currenti
-exc and/or a
regulated voltage, into an e.g. pulse-shaped, or harmonic, exciter
moment m
-exc and/or an exciter force Fe.c acting on, and elastically
deforming, the measuring tube 10.
For achieving a highest
possible efficiency and a highest possible signal/noise ratio, the
exciter power Pexc is tuned as exactly as possible such that
predominantly the oscillations of the measuring tube 10 in the
useful mode are maintained, and, indeed, as accurately as possible
to an instantaneous eigenfrequency of the measuring tube
containing the medium flowing therethrough.
The exciter force
Fexc as well as also the exciter moment Mexc, can, in this case, as
is shown schematically in Fig. 4 or Fig. 6, each be developed
bidirectionally or, however, also unidirectionally, and can be
adjusted in the manner known to those skilled in the art, e.g. by
means of a current and/or voltage regulating circuit as regards
their amplitude and e.g. by means of a phase locked loop as
regards their frequency. The exciter arrangement 40 can include,
as usual in the case of such vibratory measurement-pickups, for
instance a plunger coil arrangement having a cylindrical exciter
coil attached to the counteroscillator 20 or to the inside of the
transducer housing 100.
In operation, the exciter coil has a
corresponding exciter current iexc flowing through it.
Additionally included in the exciter arrangement 40 is a
permanently magnetic armature extending at least partially into
the exciter coil and fixed to the measuring tube 10. Furthermore,
the exciter arrangement 40 can also be realized by means of a
plurality of plunger coils, or also by means of electromagnets,
such as e.g. shown in US-A 4,524,610 or WO-A 03/095950.
For detecting the oscillations of the measuring tube 10, the
measurement pickup additionally includes a sensor arrangement 51, 52,
which produces, as a representation of vibrations of the measuring
tube 10, a first, especially analog, oscillation measurement
signal si by means of a first oscillation sensor 51 reacting to
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such vibrations. The oscillation sensor 51 can be formed by means
of a permanently magnetic armature, which is fixed to the
measuring tube 10 and interacts with a sensor coil mounted on the
counteroscillator 20 or the transducer housing. To serve as the
oscillation sensor 51, especially such sensors are suited, which
detect a velocity of the deflections of the measuring tube 10,
based on the electrodynamic principle. However, also acceleration
measuring, electrodynamic or even travel-distance measuring,
resistive or optical sensors can be used.
Of course, other
sensors known to those skilled in the art as suitable for
detection of such vibrations can be used. The sensor arrangement
includes, additionally, a second oscillation sensor 52,
especially one identical to the first oscillation sensor 51. The
second sensor 52 provides a second oscillation measurement signal
s2 likewise representing vibrations of the measuring tube 10. The
two oscillation sensors 51, 52 are in this embodiment so arranged
in the measurement pickup, separated from one another along the
length of the measuring tube 10, especially at equal distances
from the halfway point of the measuring tube 10, that the sensor
arrangement locally registers both inlet-end and outlet-end
vibrations of the measuring tube 10 and converts them into the
corresponding oscillation measurement signals 31, s2.
The two
oscillation measurement signals sl, s2, which usually each exhibit
a signal frequency corresponding to an instantaneous oscillation
=
frequency of the measuring tube 10, are, as shown in Fig. 2, fed
to the measuring device electronics 50, where they are
preprocessed, especially digitized, and then suitably evaluated by
means of corresponding components.
According to an embodiment of the invention, the exciter
arrangement 40 is, as, in fact, shown in Figs. 2 and 3, so
constructed and arranged in the measurement pickup, that it acts,
during operation, simultaneously, especially differentially, on
the measuring tube 10 and on the counteroscillator 20.
In the
case of this further development of the invention, the exciter
arrangement 40 is, as, in fact, shown in Fig. 2, advantageously so
constructed and so arranged in the measurement pickup, that it
acts, during operation, simultaneously, especially differentially,
on the measuring tube 10 and on the counteroscillator 20. In the
example of an embodiment shown in Fig. 4, the exciter arrangement
40 has, for such purpose, at least one first exciter coil 41a,
through which the exciter current, or an exciter current
component, flows at least at times during operation. The exciter
coil 41a is fixed to a lever 41c connected to the measuring tube
and acts differentially on the measuring tube 10 and the
counteroscillator 20 via this lever and an armature 41b fixed
externally to the counteroscillator 20.
This arrangement has,
among others, the advantage that, on the one hand, the
counteroscillator 20, and thus also the transducer housing 20, is
kept small in cross section and, in spite of this, the exciter
coil 41a is easily accessible, especially also during assembly.
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Moreover, a further advantage of this embodiment of the exciter
arrangement 40 is that possible used coil cups 41d, which
especially at nominal diameters of over 80 mm, have weights which
can no longer be ignored, are fixable on the counteroscillator 20
and, consequently, have practically no influence on the
eigenfrequencies of the measuring tube 10.
It is to be noted
here, however, that, in case required, the exciter coil 41a can
also be held by the counteroscillator 20 and the armature 41b,
then, by the measuring tube 10.
In corresponding manner, the oscillation sensors 51, 52 can be so
designed and arranged in the measurement pickup that the
vibrations of the measuring tube 10 and the counteroscillator 20
are registered differentially by them.
In the example of an
embodiment shown in Fig. 5, the sensor arrangement 50 includes a
sensor coil 51a fixed to the measuring tube 10, here outside of
all principal axes of inertia of the sensor arrangement 50. The
sensor coil 51a is arranged as close as possible to an armature
51b fixed to the counteroscillator 20 and magnetically so coupled
with such, that a changing measurement voltage is induced in the
sensor coil, influenced by rotary and/or lateral, relative
movements between measuring tube 10 and counteroscillator 20 in
changing their relative position and/or their relative separation.
On the basis of such an arrangement of the sensor coil 51a, both
the above-mentioned torsional oscillations and the excited bending
oscillations can, advantageously, be registered simultaneously.
If necessary, the sensor coil 51a therefor can, however, also be
fixed to the counteroscillator 20 and the armature 51b coupled
therewith can, correspondingly, then be fixed to the measuring
tube 10.
In another embodiment of the invention, measuring tube 10,
counteroscillator 20 and the sensor and exciter arrangements 40,
50 secured thereto are so matched to one another with respect to
their mass distribution, that the resulting inner part of the
measurement pickup, suspended by means of the inlet and outlet
tube pieces 11, 12, has a center of mass MS lying at least inside
of the measuring tube 10, and preferably as close as possible to
the longitudinal axis L of the measuring tube. Additionally, the
inner part is advantageously so constructed that it has a first
principal axis of inertia Tl aligned with the inlet tube piece 11
and the outlet tube piece 12 and lying at least sectionally within
the measuring tube 10. Due to the displacement of the center of
mass MS of the inner part, especially, however, also due to the
above-described position of the first principal axis of inertia
T1, the two oscillation forms assumed in operation by the
measuring tube 10 and largely compensated by the counteroscillator
20, namely the torsional oscillations and the bending oscillations
of the measuring tube 10, are highly mechanically decoupled from
one another; compare, in this connection, also WO-A 03/095950. In
this way, the two forms of oscillation, thus lateral oscillations
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and/or torsional oscillations, are advantageously, without more,
excited separately from one another. Both the displacement of the
center of mass MS and also the first principal axis of inertia TI
toward the longitudinal axis of the measuring tube can, for
example, be considerably simplified by having the inner part, thus
measuring tube 10, counteroscillator 20 and the sensor and exciter
arrangements 50, 40 secured thereto, so constructed and arranged
with respect to one another, that a mass distribution of the inner
part along the length of the measuring tube longitudinal axis L is
essentially symmetrical, at least, however, invariant relative to
an imaginary rotation about the longitudinal axis L of the
measuring tube by 180 (c2-symmetry).
Additionally, the
counteroscillator 20 - here tubularly, especially also largely
axially symmetrically, embodied - is arranged essentially
coaxially with the measuring tube 10, whereby the reaching of a
symmetrical distribution of mass in the inner part is
significantly simplified, and, consequently, also the center of
mass MS is displaced in simple manner close to the longitudinal
axis L of the measuring tube. Moreover, the sensor and exciter
arrangements 50, 40 in the example of an embodiment presented here
are so constructed and arranged relative to one another on the
measuring tube 10 and, where appropriate, on the counteroscillator
20, that a mass moment of inertia produced by them is developed as
concentrically as possible to the longitudinal axis L of the
measuring tube or at least is kept as small as possible. This can
e.g. be achieved by having a common center of mass of sensor and
exciter arrangements 50, 40 lie as close as possible to the
longitudinal axis L of the measuring tube and/or by keeping the
total mass of sensor and exciter arrangements 50, 40 as small as
possible.
In a further embodiment of the invention, the exciter arrangement
40 is, for the purpose of the separated exciting of torsional
and/or bending oscillations of the measuring tube 10, so
constructed and so fixed to the measuring tube 10 and to the
counteroscillator 20, that a force producing the bending
oscillations acts on the measuring tube 10 in the direction of an
imaginary line of force extending outside of a second principal
axis of inertia 12 perpendicular to the first principal axis of
inertia T1, or intersecting the second principal axis of inertia
in, at most, one point. Preferably, the inner part is so embodied
that the second principal axis of inertia T2 is essentially the
above-mentioned middle axis.
In the example of an embodiment
shown in Fig. 4, the exciter arrangement 40 has, for this purpose,
at least one first exciter coil 41a, through which the exciter
current or an exciter current component flows at least at times
during operation.
Exciter coil 41a is fixed to a lever 41c
connected with the measuring tube 10 and via this lever and an
armature 41b fixed externally to the counteroscillator 20, acts
differentially on the measuring tube 10 and the counteroscillator
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20. This arrangement has, among other things, also the advantage
that, on the one hand, the counteroscillator 20 and, consequently,
also the transducer housing 100 are kept small in cross section
and, in spite of this, the exciter coil 41a is easily accessible,
especially also during assembly. Moreover, a further advantage of
this embodiment of the exciter arrangement 40 is that possibly
used coil cups 41d, which especially at nominal diameters of over
80 mm have weights that no longer can be neglected, can likewise
be fixed to the counteroscillator 20 and, consequently, have
practically no effect on the resonance frequencies of the
measuring tube. It should be noted here that, when required, the
exciter coil 41a can also be mounted to the counteroscillator 20
and then the armature 41b is held by the measuring tube 10.
According to a further embodiment of the invention, the exciter
arrangement 40 has at least one, second exciter coil arranged
along a diameter of the measuring tube 10 and coupled with the
measuring tube 10 and the counteroscillator 20 in the same way as
the exciter coil 41a. According to another, preferred embodiment
of the invention, the exciter arrangement has two further exciter
coils, thus a total of four, at least arranged
symmetrically with respect to the second principal axis of inertia
T2. All coils are mounted in the measurement pickup in the above-
described manner.
The force acting on the measuring tube 10
outside of the second principal axis of inertia T2 can be produced
by means of such two, or four, coil arrangements in simple manner
e.g. by having one of the exciter coils, e.g. the exciter coil
41a, exhibit another inductance than the respective others, or by
causing to flow through one of the exciter coils, e.g. the exciter
coil 41a, during operation, an exciter current component that is
different from a respective exciter current component of the
respectively other exciter coils.
According to another embodiment of the invention, the sensor
arrangement 50 includes, as shown schematically in Fig. 5, a
sensor coil 51a arranged outside of the second principal axis of
inertia T2 and fixed to measuring tube 10. The sensor coil 51a is
arranged as near as possible to an armature 51b fixed to the
counteroscillator 20 and is magnetically coupled therewith such
that a changing measurement voltage is induced in the sensor coil,
influenced by rotary and/or lateral relative movements between
measuring tube 10 and counteroscillator 20 as they change their
relative positions and/or their relative separations. Due to the
arrangement of the sensor coil 51a according to the invention,
both the above-described torsional oscillations and the bending
oscillations, excited where appropriate, can be registered in
advantageous manner simultaneously. If required, the sensor coil
51a therefor can, instead, be fixed to the counteroscillator 20
and, in corresponding manner, the armature 51b coupled therewith
can be fixed to the measuring tube 10.
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It is noted here, additionally, that the exciter arrangement 40
and the sensor arrangement 50 can also have, in the manner known
to those skilled in the art, essentially the same mechanical
structure; consequently, the above-described embodiments of the
mechanical structure of the exciter arrangement 40 can essentially
also be transferred to the mechanical structure of the sensor
arrangement 50, and vice versa.
For vibrating the measuring tube 10, the exciter arrangement 40
is, as already mentioned, fed with a likewise oscillating exciter
current
-exc r especially a multifrequency current, of adjustable
amplitude and adjustable exciter frequency fexc such that this
current flows through the exciter coils during operation
and the magnetic fields required for moving the armatures
are produced in corresponding manner. The exciter current iexc can
be e.g. harmonically multifrequent or even rectangular.
The
lateral oscillation exciter frequency fexcL of a lateral current
component i excl. of the exciter current i
-exc required for maintaining
the lateral oscillations of the measuring tube 10 can
advantageously be so chosen and adjusted in the case of the
measurement pickup shown in the example of an embodiment that the
laterally oscillating measuring tube 10 oscillates essentially in
a bending oscillation base mode having a single oscillation
antinode.
Analogously thereto, also a torsional oscillation
frequency f excT of a torsional current component iexcT of the
exciter current i
-exc required for maintaining the torsional
oscillations of the measuring tube 10 can advantageously be so
chosen and adjusted in the case of the measurement pickup shown in
the example of an embodiment that the torsionally oscillating
measuring tube 10 oscillates essentially in a torsional
oscillation base mode having a single oscillation antinode. The
two mentioned current components i
-excL andi
- excT can, depending on
the type of operation selected, be fed into the exciter
arrangement 40 intermittently, thus instantaneously each acting as
the exciter current iexcr or also simultaneously, thus
supplementing one another to form the effective exciter current
iexc =
For the above-described case wherein the lateral oscillation
frequency fexcL
and the torsional oscillation frequency f
-excT with
which the measuring the measuring tube 10 is caused to oscillate
during operation, are adjusted differently from one another, a
separation of the individual oscillation modes can occur both in
the exciter signals and also in the sensor signals, by means of
the measurement pickup in simple and advantageous manner, even in
the case of simultaneously excited torsional and bending
oscillations, e.g. based on a signal filtering or a frequency
analysis. Otherwise, an alternating exciting of the lateral and
torsional oscillations recommends itself.
For producing and adjusting the exciter current jexcr or the
21
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current components lexcL, 1
sexcT r the measuring device electronics
includes a corresponding driver circuit 53, which is controlled by
a lateral oscillation frequency adjustment signal yFNITõ representing
the desired lateral oscillation exciter frequency¨fexcL and by a
lateral oscillation amplitude adjustment signal ypm, representing
the desired lateral oscillation amplitude of the exciter current
iõ, and/or the lateral current component i
excL I as well as, at
least at times, by a torsional oscillation frequency adjustment
signal YRJE representing the torsional oscillation exciter
frequency fexcr and by a torsional oscillation amplitude adjustment
signal yAlr representing the desired torsional oscillation
amplitude of the exciter current i,c and/or the torsional current
component1
¨excT =
The driver circuit 53 can be realized e.g. by
means of a voltage-controlled oscillator or a downstream voltage-
to-current converter; instead of an analog oscillator, however,
also a numerically controlled, digital oscillator can be used to
set the instantaneous exciter current iexc or the components iexcL,
lexcT of the exciter current.
An amplitude control circuit 51 integrated into the measuring
device electronics 50 can serve for producing the lateral
amplitude adjustment signal ymi, and/or the torsional oscillation
amplitude adjustment signal yANIT. The amplitude control circuit 51
actualizes the amplitude adjustment signals yArm, YAET on the basis
of instantaneous amplitudes of at least one of the two oscillation
measurement signals s1, 2 measured at the instantaneous lateral
oscillation frequency and/or the instantaneous torsional
oscillation frequency, as well as on the basis of corresponding,
constant or variable amplitude reference values for the lateral
and torsional oscillations, respectively W
¨Br WT; as appropriate,
also instantaneous amplitudes of the exciter current iõc can be
referenced for generating the lateral oscillation amplitude
adjustment signal yANIL and/or the torsional oscillation amplitude
adjustment signal yANIT; compare Fig. 7. Construction and manner of
operation of such amplitude control circuits are likewise known to
those skilled in the art. As an example for such an amplitude
control circuit, reference is made, moreover, to the measurement
transmitters of the series "PROMASS 80", such as are available
from the assignee, for example in connection with measurement
pickups of the series "PROMASS I".
Their amplitude control
circuit is preferably so constructed that the lateral oscillations
of the measuring tube 10 are controlled to a constant amplitude,
thus an amplitude also independent of the density p.
The frequency control circuit 52 and the driver circuit 53 can be
constructed e.g. as phase-locked loops, which are used in the
manner known to those skilled in the art for adjusting the lateral
oscillation frequency adjusting signal yETE, and/or the torsional
oscillation frequency adjusting signal YEN'''. continuously for the
instantaneous eigenfrequencies of the measuring tube 10 on the
basis of a phase difference measured between at least one of the
22
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oscillation measurement signals sl, s2 and the exciter current iexc
to be adjusted, respectively the instantaneously measured exciter
currenti
-exc. The construction and use of such phase-locked loops
for the driving of measuring tubes at one of their mechanical
eigenfrequencies is described in detail in e.g. US-A 4,801,897.
Of course, other frequency control circuits known to those skilled
in the art can be used, such as are proposed in US-A 4,524,610 or
US-A 4,801,897.
Furthermore, reference is made to the already
mentioned measurement transmitters of the series "PROMASS 80"
respecting a use of such frequency control circuits for vibratory
measurement pickups. Other circuits suitable for use as driver
circuits can be learned from, for example, US-A 5,869,770 or US-A
6,505,519.
According to a further embodiment of the invention, the amplitude
control circuit 51 and the frequency control circuit 52 are, as
shown schematically in Fig. 7, realized by means of a digital
signal processor DSP provided in the measuring device electronics
50 and by means of program code correspondingly implemented in
such and running therein.
The program codes can be stored
persistently or even permanently e.g. in a non-volatile memory
EEPROM of a microcomputer 55 controlling and/or monitoring the
signal processor and loaded upon startup of the signal processor
DSP into a volatile data memory RAM of the measuring device
electronics 50, e.g. RAM integrated= in the signal processor DSP.
Signal processors suited for such applications are e.g. those of
type TMS320VC33 available from the firm Texas Instruments Inc..
It is clear, in this regard, that the oscillation measurement
signals si, s2 need to be converted by means of corresponding
analog-to-digital converters A/D into corresponding digital
signals for a processing in the signal processor DSP; compare, in
this connection, especially EP-A 866,319.
In case required,
adjustment signals output from the signal processor, such as e.g.
the amplitude adjusting signals yAmL, ymTr, or the frequency
adjusting signals ym, ywr, can be, in corresponding manner,
converted from digital to analog.
As shown in Fig. 7, the, if appropriate, first suitably
conditioned, oscillation measurement signals su s2 are
additionally sent to a measurement circuit of the measuring device
electronics 50 for producing the at least one measured value Xx on
the basis of at least one of the oscillation measurement signals
su s2 and/or on the basis of the exciter currenti
¨exc =
According to an embodiment of the invention, the measurement
circuit is constructed, at least in part, as a flow rate
calculator and the measurement circuit serves for determining, in
the manner known per se to those skilled in the art, from a phase
difference detected between the oscillation measurement signals
si, s2 generated in the case of a measuring tube 10 oscillating
laterally at least in part, a measured value Xx serving here as a
23
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mass flow rate measured value and representing, as accurately as
possible, the mass flow rate to be measured.
The measurement
circuit can be any, especially digital, measuring circuit already
used in conventional Coriolis mass flow measuring devices for
determining the mass flow rate on the basis of the oscillation
measurement signals s1, s2; compare, in this connection,
especially the initially mentioned WO-A 02/37063, WO-A 99/39164,
US-A 5,648,616, US-A 5,069,074.
Of course, other measuring
circuits known to those skilled in the art to be suitable for
Coriolis mass flow measuring devices can be used, i.e. measuring
circuits which measure, and correspondingly evaluate, phase and/or
time differences between oscillation measurement signals of the
described kind.
Additionally, the measurement circuit can also serve to utilize an
oscillation frequency of the at least one measuring tube, as
measured, for example, on the basis of at least one of the
oscillation measurement signals s1, s2, for generating a measured
value Xx usable as a density measured value instantaneously
representing a density p to be measured for the medium or a phase
of the medium.
Because the straight measuring tube 10 is, as above described,
caused to execute, during operation, lateral and torsional
oscillations simultaneously or alternatingly, the measurement
circuit can also be used to determine (derived from the exciter
currenti
¨ exc which, it is known, can serve also as a measure for
an apparent viscosity or also a viscosity-density product) a
measured value Xx usable as a viscosity measured value and
instantaneously representing a viscosity of the medium; compare,
in this connection, also US-A 4,524,610 or WO-A 95 16 897.
It is clear in this connection, without more, for those skilled in
the art, that the inline measuring device can determine the
separate measured values Xx for the various measured quantities x
both in a common measuring cycle, thus with equal updating rates,
as well as with different updating rates.
For example, a very
accurate measurement of the usually significantly varying mass
flow rate requires usually a very high updating rate, while the
comparatively less variable viscosity of the medium can, where
appropriate, be updated at larger separations in time.
Additionally, it can, without more, be assumed that currently
determined, measured values Xx can be stored temporarily in the
measuring device electronics and, therefore, be available for
subsequent uses.
Advantageously, the measurement circuit can,
furthermore, also be implemented by means of the signal processor
DS P.
As already mentioned at the start, inhomogeneities and/or the
formation of first and second phases in the flowing medium, for
example gas bubbles and/or solid particles entrained in liquids,
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can lead to the result that a measured value determined in
conventional manner assuming a single-phase and/or homogeneous
medium will not match with sufficient accuracy the actual value of
the quantity x whose measurement is desired, for example the mass
flow rate m, i.e. the measured value must be appropriately
corrected.
This preliminarily determined, provisionally
representing, or at least corresponding, value of the physical
quantity x whose measurement is desired, which value, as already
explained, can, for example, be a phase difference AT measured
between the oscillation measurement signals si, s2, or a measured
oscillation frequency, of the measuring tube 10, is, consequently,
referenced in the following as an initial measured value, or also
a beginning measured value, X'.. From this initial measured value
X'., the measurement electronics 21, in turn, finally derives the
measured value X. representing the physical, measured quantity x
sufficiently accurately, whether the physical, measured quantity x
is the mass flow rate, the density, or the viscosity. Considering
the very comprehensive and very well documented and detailed state
of the art, it can be assumed that the determination of the
initial measured value X'., which, for practical purposes,
corresponds to the measured value generated in conventional
manner, presents no difficulties for those skilled in the art, so
that the initial measured value X'x can be taken as a given for
the further explanation of the invention.
There is already discussion in the state of the art with reference
to the mentioned inhomogeneities in the medium that these can
immediately show up both in the phase difference measured between
the two oscillation signals sl, s2 and in the oscillation
amplitude or the oscillation frequency of each of the two
oscillation measurement signals, respectively exciter current,
thus in practically all of the usually measured, directly or
indirectly, operational parameters of measuring devices of the
described kind.
This is true, especially in the case of the
operational parameters determined with a laterally oscillating
measuring tube, as is treated in WO-A 03/076880 or US-B 6,505,519;
it can, however, also not always be excluded for operational
parameters measured with a torsionally oscillating measuring tube
- compare, in this connection, especially US-A 4,524,610.
Further investigations by the inventors have, however, led to the
surprising discovery that, while it is true, the instantaneous
exciter current i
¨exc and, going along therewith, a damping of the
oscillations of the measuring tube 10 usually likewise measured in
the operation of the measuring device, do depend to a significant
amount on the degree of the inhomogeneity of the two, or more,
phase medium and/or on a concentration of a second medium phase
thereof, for example, thus on a characteristic, a distribution
and/or an amount of gas bubbles and/or solid particles entrained
in a liquid being measured, nevertheless, both for lateral and for
CA 02559564 2011-05-03
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torsional oscillations - at least in the two base modes mentioned
above - a largely reproducible and, consequently, at least
experimentally determinable relationship can be postulated between
the particular current componenti
-excL I
jexcT instantaneously
required for maintaining the lateral, respectively torsional,
oscillation and the instantaneous degree of inhomogeneity of the
two, or more, phase medium, or even the instantaneous
concentration of a second phase of the medium, especially a second
phase acting as a disturbance.
Surprisingly, it has additionally been found that, in spite of the
fact that both an instantaneous damping of the lateral
oscillations and, as is discussed especially in US-A 4,524,610 or
EP-A 1 291 639, an instantaneous damping of the torsional
oscillations are dependent significantly on the degree of the
inhomogeneity or on the concentrations of individual phases of the
medium, simultaneous, or at least contemporaneous, determination
of the instantaneous dampings of both of the oscillation modes
permits an amazingly robust and very reproducible correction of
the intermediate value X'x and, therefore, the generating of a
very accurate measured value Xx.
Further investigations have
shown, namely, that the damping both of the lateral oscillations
and the torsional oscillations is, indeed, very strongly dependent
on the viscosity of the medium to be measured. At the same time,
the damping of the lateral oscillations shows a very strong
dependence on the degree of inhomogeneities of the medium
instantaneously present in the measuring tube 10, while, in
contrast, the dependence of the damping of the torsional
oscillations on inhomogeneities in the medium is far weaker.
According to the invention, for the purpose of improving the
accuracy with which the physical, measured quantity x, for example
the mass flow rate m or the density p, is determined, the
measurement pickup is operated, at least at times, in the
previously mentioned dual-mode, in which the at least one
measuring tube 10 - in turns and/or alternatingly - is caused to
vibrate in the lateral oscillation mode and/or in the torsional
oscillation mode.
For the correction of the first determined,
initial, measured value X'x sought accordingly, the measuring
device electronics 50 determines, during operation, an, especially
digital, first intermediate value X1, which essentially
corresponds to the damping of the lateral oscillation mode, and
an, especially digital, second intermediate value X2, which
essentially corresponds to the medium-dependent damping of the
torsional oscillation mode.
The determining of the first
intermediate value proceeds here essentially based on the lateral
current component i
-excl. of the exciter current- i
exc, especially the
regulated component, required for maintaining the lateral
oscillations, while, for determining the second intermediate value
X2, especially the torsional current component- i
excT I especially
the regulated component, required for maintaining the torsional
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oscillations is taken into consideration.
Using the two intermediate values X1, X2, the measurement circuit
21 additionally determines an, especially likewise digital,
correction value XK for the intermediate value X'x.
The
correction of the intermediate value X'x on the basis of the
correction value XK, as well as the generating of the measured
value Xx, can occur in the measuring device electronics, for
example, based on the mathematical relationship
Xx=1(õ41+Xl()=X'x . (1)
In an embodiment of the invention, the correction value XK is
determined by means of the measuring device electronics based on
the mathematical relationship
XK = KK =(,(1 -X2) (2)
so that this essentially represents a measure of the deviation AD
of damping measured during operation for the principally excited,
lateral and torsional oscillations.
Alternatively, or for
supplementing such, the correction value XK can also be determined
based on the mathematical relationship
=KK''(1 X2) ( 3 )
XI
While, thus, in Eq. (2), the correction value XK is determined on
the basis of a difference AD existing between the intermediate
value X1 and the intermediate value X2, in the case of Eq. (3),
the correction value XK is determined on the basis of a comparison
of the second intermediate value X2 with the first intermediate
value Xl. In this respect, the correction value X0 represents, at
least for a two-phase medium, also a measure for an instantaneous,
relative or absolute, concentration of a first and a second phase
of a medium, especially for gas bubbles in a liquid. Besides the
generating of the actual measured value Xx, the correction value
XK can, therefore, also be converted into a concentration measured
value X0, which represents, in the case of a two, or more, phase
medium in a measuring tube, an, especially relative, volume and or
mass fraction of a phase of a medium. Furthermore, the correction
value XK can also be used to signalize the degree of inhomogeneity
of the medium, or measured values derived therefrom, such as e.g.
a percentage air content in the medium or a volume, quantity or
mass fraction of solid particles entrained in the medium, e.g. on
site or visually perceivable in a remote control room.
Alternatively or additionally, the correction value XK can also
serve for signalizing for the user, for example out of a
comparison with an initially defined limit value, that, for the
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instantaneous flow conditions in the measuring tube 10, the
measured quantity x is being measured with considerable
uncertainty and/or with a large amount of error. Additionally,
the correction value XK can, in this case, also be used to switch
off a signal output which issues the measured value X. for the
measured quantity x of interest during operation.
Further experimental investigations have shown that, for a
measurement pickup of the illustrated example of an embodiment,
the consideration of the instantaneous lateral oscillation
frequency of the vibrating measuring tube can lead to a further
improvement of the accuracy of the measured value X.. Moreover,
by a normalizing of the correction value XK determined on the
basis of Eq. (2) or Eq. (3) on the square root of the
instantaneous lateral oscillation frequency, one finds that the
correction value XI( is essentially proportional to the gas
fraction, at least for the case wherein a liquid, for example
glycerin, contains entrained gas bubbles, for example air;
compare, in this connection, also Fig. 9. Therefore, in a further
development of the invention, Eq. (2) is modified using a lateral
oscillation frequency measured value XfexcL representing the
instantaneous lateral oscillation frequency, as follows:
(XI -X2)
XK = K ______________________________________________________________ ( 4 )
IXfexcL
The determining of the lateral oscillation frequency measured
value can transpire simply e.g. on the basis of the above-
mentioned lateral oscillation frequency adjusting signal yDm.
In the determining of the two intermediate values XI, X2, it is
additionally to be kept in mind that the damping of the
oscillations of the measuring tube 10 is determined both by the
damping component attributable to viscous frictions within the
medium and by a damping component which is practically independent
of the medium.
This latter damping component is caused by
mechanical friction forces, which act e.g. in the exciter
arrangement 40 and in the material of the measuring tube 10.
Stated differently, the instantaneously measured exciter current
exc represents the totality of the frictional forces and/or
frictional moments in the measurement pickup, including the
mechanical frictions in the measurement pickup and the viscous
friction in the medium.
In determining the intermediate values
X1, X2, which, as mentioned, mainly are related to the damping
components of the oscillations of the measuring tube resulting
from viscous frictions in the medium, the mechanical damping
components, which are independent of the medium, must be
appropriately considered, for example they should be separated out
or eliminated.
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For determining the intermediate value XI, therefore, an
embodiment of the invention provides that, from an, especially
digital, lateral current measured value XiexcL instantaneously
representing the lateral current component
¨excL r a correspondingly
associated, lateral, empty-state, electrical current, measured
value KiexcL is subtracted, which represents the mechanical
friction forces in each case arising in the instantaneously
excited, lateral oscillation mode in the measurement pickup in the
case of empty measuring tube 10.
In the same manner, for
determining the intermediate value X2f from an, especially
digital, torsional current measured value XiexcT instantaneously
representing the torsional current component 1õ,T, a torsional,
empty-state, electrical current, measured value KiexcT
i 5
subtracted, which represents the mechanical frictional forces in
each case arising in the instantaneously excited, torsional
oscillation mode in the measurement pickup in the case of empty
measuring tube 10.
According to a further embodiment of the invention, the
determining of the intermediate value X1 occurs, as also shown in
Fig. 8 by way of example on electrical current, measured values
XiexcL XiexcT and empty-state, electrical current, measured values
KiexcL KiexcT experimentally determined for the correction of the
mass flow rate on the basis of the lateral current component¨ i
excL
driving the lateral oscillations and on the basis of the
associated lateral, empty-state, electrical current, measured
value Kieõ,L, especially based on the mathematical relationship
X1 = K1 = (XiexcL K ( 5 )
and/or based on the mathematical relationship
X1 = 1
K ' 1 K iexcL = (
6 )
X.
iexcL
In case required, especially in the case of significantly varying,
vibratory measuring tube, oscillation amplitudes and/or vibratory
measuring tube, oscillation amplitudes deviating from the
calibrated reference values, the lateral current component iexci,
can initially likewise be normalized on the instantaneous
oscillation amplitude of the lateral oscillations of the measuring
tube, for example using the oscillation measurement signals
S2 =
Analogously thereto, also the intermediate value X2 can be
determined based on the mathematical relationship
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CA 02559564 2011-05-03
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=
X2 = K2 = (XtexcT K iexcT
(7)
and/or based on the mathematical relationship
."2 '2 I
X. (
8 )
excf
Each of the empty-state, electrical current, measured values
KiexcLr KiexcT t as also the device-specific coefficients kk, Kki, Klr
K2, K1 or K2' is likewise to be determined during a calibration of
the inline measuring device, e.g. using an evacuated or air-
carrying measuring tube, and stored, or set, appropriately in the
measuring device electronics 50, especially normalized on the
oscillation amplitude measured at such time. It is clear, without
more, for those skilled in the art, that, if necessary, other
physical parameters influencing the empty-state, electrical
current, measured values KiexcL KiexcT such as e.g. an
instantaneous temperature of the measuring tube and/or of the
medium, are to be taken into consideration during the calibration.
For calibrating the measured values pickup, usually two, or
more, different, two, or more, phase, media with varying, but
known, flow parameters, such as e.g. known concentrations of the
individual medium phases of the calibrating medium, whose density
p, mass flow rate m, viscosity n and/or temperature are known, are
caused to flow in turn through the measurement pickup, and the
corresponding reactions of the measured values pickup, such as
e.g. the instantaneous exciter current- i
exc r the instantaneous
lateral oscillation exciter frequency fexcL and/or the
instantaneous torsional oscillation exciter frequency f
- excT are
measured. The adjusted flow parameters and the respectively
measured reactions of the measured operational parameters of the
measurement pickup are related to one another in appropriate
manner and, thus, mapped onto the corresponding calibration
constants. For example, for determining the constants during the
calibration measurements for two calibration media of known
viscosity held as constant as possible and of different
inhomogeneity, which, however, in each case, is formed in a manner
which remains constant, a ratio X.1/x and/or X./x of the
intermediate value X.' determined in each case, respectively, of
the measured value X. determined in each case, to the then, in
each case, current, actual value of the quantity being measured is
formed for known air fraction. For example, the first calibration
medium can be flowing water, or even oil, with entrained air
bubbles, and the second calibration medium can be water which is
as homogeneous as possible. The calibration constants determined
here can then be stored e.g. in the form of digital data in a
table memory of the measuring device electronics; they can,
however, also serve as analog adjustment values for corresponding
computing circuits. It is to be noted here that the calibration
=
CA 02559564 2011-05-03
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of measurement pickups of the described type is a subject known
per se, or at least executable from the above explanations, for
those skilled in the art, and, consequently does not require any
further explanation.
Advantageously, the already mentioned
lateral oscillation amplitude adjustment signal ymc and/or the
torsional oscillation amplitude adjustment signal ymff can be used
for determining the lateral current, measured value XiexcL and/or
the torsional current, measured value XiexcTe since these represent
the exciter current jexcr or its components iexcLr jexcT sufficiently
accurately for the correction.
According to a further embodiment of the invention, for the
already multiply-mentioned case where the measured quantity x to
be registered corresponds to a viscosity, or even a fluidity, and
so the measured value Xx serves as a viscosity measured-value,
also the initial measured-value Xx' is determined on the basis of
the exciter currenti
¨exc driving the exciter arrangement 40 in the
case of a measuring tube at least partially torsionally
oscillating, especially on the basis of the torsional current
component lexcT serving for maintaining the torsional oscillations
of the measuring tube 10.
Taking into consideration the
relationship already described in US-A 4,524,610:
-- lexcT (9)
according to which the torsional current component
reduced
r reduced
by the above-mentioned, torsional, empty-state, electrical
current, measured value KiexcTr correlates very well with the
square root of the actual viscosity, n, at least in the case of
constant density, p, and largely homogeneous medium, in
corresponding manner first a squared value XiliexcT2 of the torsional
current, measured value XiexcT is formed inside the measuring
device electronics, reduced by the torsional, empty-state,
electrical current, measured value KiexcT and derived from the
exciter current iexcr for the determining of the initial measured
value Xx'. Considering that, as, in fact, also explained in US-A
4,524,610, the square of the current does, in fact, provide the
information on the product of density and viscosity, the actual
density, which, for example, can be determined initially, likewise
by means of the inline measuring device, is, moreover, to be taken
into consideration when determining the initial measured value Xx'
in the aforedescribed manner.
In a further embodiment of the invention, for forming the initial
measured value Xn, the square XiexcT2 of the torsional current,
measured value XiexcT is, moreover, by means of a simple, numerical
division, normalized on an amplitude measured value XsT, which
represents instantaneously, in the case of a torsionally
oscillating measuring tube, an operationally determined, in
certain cases varying, signal amplitude of at least one of the
31
CA 02559564 2006-09-13
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oscillation measurement signals sl, s2. It has, namely, also been
found, that, for such viscosity measuring devices having such a
vibratory measurement pickup, especially also in the case of
constantly controlled oscillation amplitude and/or in the case of
simultaneous exciting of lateral and torsional oscillations, a
ratio ;
¨exc/e of the exciter current iexc to a practically not
directly measurable velocity 9 of a movement causing the internal
frictions and, thus, also the frictional forces in the medium, is
a more accurate estimate of the already mentioned damping acting
against the excursions of the measuring tube 10.
Consequently,
for further increasing the accuracy of the measured value X,
especially, however, also for decreasing its sensitivity to
fluctuating oscillation amplitudes of the vibrating measuring tube
possibly arising during operation, it is further provided that,
for the determining of the initial measured value Xx', the
torsional current measured value XiexcT is first normalized on the
amplitude measured value XsT, which represents the above-mentioned
velocity 9 sufficiently accurately.
Stated differently, a
normalized torsional current measured value Xiiõcr is formed
according to the following formula:
)CexcT ¨ XieuT
(10)
l
Xa
The amplitude measured value Xsi is, based on the recognition that
the movement causing the viscous friction in the medium matches
very strongly the movement of the vibrating measuring tube 10
registered locally by means of the sensor 51 or also by means of
the sensor 52, preferably derived using the measuring device
electronics 50, e.g. by the internal amplitude measurement
circuit, from at least one, possibly already digitized, sensor
signal si.
It is noted here, again, that the sensor signal si is
preferably proportional to a velocity of an, especially lateral,
excursion of the vibrating measuring tube 10; the sensor signal sl
can, however, also be proportional to an acceleration acting on
the vibrating measuring tube or to a distance covered by the
vibrating measuring tube 10.
For the case, where the sensor
signal si is designed to be velocity-proportional in the above
sense, this is, of course, to be considered in the determining of
the initial measured value.
The aforementioned functions serving for the production of the
measured value Xx, symbolized by the Eqs. (1) to (10), can be
implemented, at least in part, by means of the signal processor
DSP or e.g. also by means of the above-mentioned microcomputer 55.
The creation and implementation of corresponding algorithms
matching such equations or mimicking the functioning of the
amplitude control circuit 51, respectively the frequency control
circuit 52, and their transformation into program code executable
in such signal processors, is, per se, within the skill of the
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art, and, consequently, does not require any detailed explanation,
particularly once the present disclosure has been reviewed.
Of
course, these equations can also, without more, be represented by
means of corresponding, discretely assembled, analog and/or
digital, simulating circuits in the measuring device electronics
50.
In a further development of the invention, the correction value XK
instantaneously appropriate during operation is determined,
starting from the intermediate values x1, X2, practically directly
by representing in the measuring device electronics, especially in
a program, a unique relationship between a present combination of
the two intermediate values x1, X2 and the correction value XK
belonging therewith.
To this end, the measuring device
electronics 50 additionally has a table memory, in which a set of
predetermined, digital correction values Xici is stored, for
example values determined during the calibration of the Coriolis
mass flow measuring device.
These correction values X1(,, are
accessed directly by the measurement circuit via a memory address
determined by means of the instantaneously valid intermediate
values XI, X2. The correction value XK can be =determined e.g. in
simple manner by comparing a combination of the instantaneously
determined intermediate values XI, X2, for example the above-
mentioned damping difference, with corresponding prestored values
stored for this combination in the table memory and, on the basis
of this comparison, that correction value Xfc, is read out, thus
used by the evaluation electronics
for the further calculations,
which corresponds to the prestored value having the closest match
with the instantaneous combination.
The table memory can be a
programmable, fixed-value memory, thus a FPGA (field programmable
gate array), an EPROM or an EEPROM.
The use of such a table
memory has, among others, the advantage that the correction value
XK is available during runtime very quickly following calculation
of the intermediate values XI, X2-
Moreover, the correction
values Xici entered in the table memory can be predetermined very
accurately, e.g. based on the Eqs. (2), (3) and/or (4) and making
use of the method of least squares.
As can be appreciated, without more, from the above presentation,
a correction of the initial measured value X'x can be carried out,
on the one hand, using few correction factors which are very easy
to determine. Also, the correction can be performed using the two
intermediate values XI, X2 with a computing effort, which is quite
small in comparison to the more complexly developed computing
methods known from the state of the art. An additional advantage
of the invention is to be seen in the fact that at least some of
the described correction factors can be generated, without more,
from the flow parameters determined, for example, by means of
conventional Coriolis mass flow measuring devices, especially the
measured density and/or the - here provisionally - measured mass
flow rate, and/or from the operational parameters usually directly
33
CA 02559564 2006-09-13
34 5L0239-US
measured in the operation of Coriolis mass flow measuring devices,
especially the measured oscillation amplitudes, oscillation
frequencies and/or derived from the exciter current, and,
consequently without noticeable increase of the circuit and
measurement complexity.
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