Title: Mass flowmeter and method for correcting the measurement signal of a mass flowmeter
Abstract: Disclosed is a method for correcting the measurement signal of a mass flowmeter for flowing media of the type which works on the Coriolis principle and has at least one straight measuring tube conveying the flowing medium, at least one oscillation generator acting on the measuring tube, at least one measurement value sensor detecting Coriolis forces and/or Coriolis oscillations based on Coriolis forces and outputting a measurement signal and a supporting tube accommodating the measuring tube, the oscillation generator and the measurement value sensor, whereby the measuring tube and the supporting tube are connected to one another in a manner excluding relative axial movements and the axial spacing of the fixing points of the supporting tube on the measuring tube represents the oscillation length of the measuring tube. The method includes the steps of detecting the stress state of the measuring tube, detecting the stress state of the supporting tube, and correcting the measurement signal on the basis of the detected stress state of the measuring tube and the detected stress state of the supporting tube. A flowmeter for implementing the method is also disclosed.
Patent Number: 6,889,561 Issued on 05/10/2005 to Hussain, et al.
| Inventors:
|
Hussain; Yousif A. (Weston Fevell, GB);
Jukes; Edward P. (Wellingborough, GB)
|
| Assignee:
|
Krohne A.G. (Basel, CH)
|
| Appl. No.:
|
625084 |
| Filed:
|
July 23, 2003 |
Foreign Application Priority Data
| Dec 16, 2002[DE] | 102 58 962 |
| Current U.S. Class: |
73/861.357 |
| Intern'l Class: |
G01F 001/84 |
| Field of Search: |
73/861,861.354,861.355,861.356,861.357
|
References Cited [Referenced By]
U.S. Patent Documents
| 5381697 | Jan., 1995 | van der Pol.
| |
| 5576500 | Nov., 1996 | Cage et al.
| |
| 5773727 | Jul., 1998 | Kishiro et al.
| |
| 5796012 | Aug., 1998 | Gomi et al.
| |
| 5827979 | Oct., 1998 | Schott et al.
| |
| 5850039 | Dec., 1998 | Van Cleve et al.
| |
| 6164140 | Dec., 2000 | Kalinoski.
| |
| 6327915 | Dec., 2001 | Van Cleve et al.
| |
| 6397685 | Jun., 2002 | Cook et al.
| |
| 6516674 | Feb., 2003 | Poremba.
| |
Primary Examiner: Lefkowitz; Edward
Assistant Examiner: Mack; Corey D.
Attorney, Agent or Firm: Cesari and McKenna, LLP, McKenna; John F.
Claims
1. A mass flowmeter for flowing media which works on the Coriolis principle comprising,
at least one straight measuring tube conveying the flowing medium,
at least one oscillation generator acting on the measuring tube,
at least one measurement value sensor detecting Coriolis forces and/or Coriolis
oscillations based on Coriolis forces and outputting a measurement signal,
a supporting tube accommodating the measuring tube, the oscillation generator
and the at least one measurement value sensor,
at least one first stress sensor for detecting the stress state of the measuring
tube,
wherein the first stress sensor comprises a length-change sensor,
a correction device for correcting the measurement signal, the at least one measuring
tube and the supporting tube being connected to one another at spaced-apart fixing
points in a manner excluding relative axial movements and the axial spacing of
said fixing points representing the oscillation length of the measuring tube, and
the at least one measurement value sensor and the at least one first stress sensor
being connected to the correction device, in order to feed to the correction device
the measurement signal and the stress signal outputted by the at least one first
stress sensor, and
at least one second stress sensor detecting the stress state of the supporting
tube, wherein the second stress sensor comprises a length-change sensor, said at
least one second stress sensor being connected to the correction device in order
to feed to the correction device the stress signal outputted by the at least one
second stress sensor, so that a measurement signal can be outputted from the correction
device that is corrected on the basis of the stress signal outputted by the at
least one first stress sensor and the stress signal outputted by the at least one
second stress sensor.
2. The mass flowmeter according to claim 1, wherein the correction device includes
means for providing an empirically determined correction function for determining
the corrected measurement signal.
3. The mass flowmeter according to 2, wherein said at least one first and second
stress sensors comprise wire strain gages.
4. The mass flowmeter according to any one of 2, wherein said at least one first
stress sensor is orientated in the longitudinal direction of the measuring tube
and/or the at least one second stress sensor is orientated in the longitudinal
direction of the supporting tube.
5. A mass flowmeter for flowing media, which works on the Coriolis principle comprising,
at least one straight measuring tube conveying the flowing medium,
at least one oscillation generator acting on the measuring tube,
at least one measurement value sensor detecting Coriolis forces and/or Coriolis
oscillations based on Coriolis forces and outputting a measurement signal,
a supporting tube accommodating the measuring tube, the oscillation generator
and the at least one measurement value sensor,
at least one first stress sensor for detecting the stress state of the measuring
tube, the first stress sensor being a wire strain gauge which is oriented in the
longitudinal direction of the measuring tube,
a correction device for correcting the measurement signal, the at least one measuring
tube, and the supporting tube being connected to one another at spaced-apart fixing
points in a manner excluding relative axial movements and the axial spacing of
said fixing points representing the oscillation length of the measuring tube, and
the at least one measurement value sensor and the at least one first stress sensor
being connected to the corrective device, in order to feed the correction device
the measurement signal and the stress signal outputted by the at least one first
stress sensor, and
at least one second stress sensor detecting the stress state of the supporting
tube, the second stress sensor being a wire strain gauge which is oriented in the
longitudinal direction of the supporting tube, said at least one second stress
sensor being connected to the correction device in order to feed to the correction
device the stress signal outputted by the at least one second stress sensor, so
that a measurement signal can be outputted from the correction device that is corrected
on the basis of the stress signal outputted by the at least one first stress sensor
and the stress signal outputted by the at least one second stress sensor.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a mass flowmeter for flowing media which works on the
Coriolis principle, with at least one straight measuring tube conveying the flowing
medium, with at least one oscillation generator acting on the measuring tube, with
at least one measurement value sensor detecting Coriolis forces and/or Coriolis
oscillations based on Coriolis forces and outputting a measurement signal, with
a supporting tube accommodating the measuring tube, the oscillation generator and
the measurement value sensor, with a stress sensor for detecting the stress state
of the measuring tube and with a correction device for correcting the measurement
signal, whereby the measuring tube and the supporting tube are connected to one
another in a manner excluding relative axial movements and the axial spacing of
the fixing points of the supporting tube to the measuring tube represents the oscillation
length of the measuring tube and whereby the measurement value sensor and the stress
sensor detecting the stress state of the measuring tube are connected to the correction
device, in order to feed to the correction device the measurement signal and the
stress signal outputted by the stress sensor detecting the stress state of the
measuring tube.
The invention further relates to a method for correcting the measurement signal
of a mass flowmeter for flowing media, which works on the Coriolis principle and
has at least one straight measuring tube conveying the flowing medium, at least
one oscillation generator acting on the measuring tube, at least one measurement
value sensor detecting Coriolis forces and/or Coriolis oscillations based on Coriolis
forces and outputting a measurement signal and a supporting tube accommodating
the measuring tube, the oscillation generator and the measurement value sensor,
whereby the measuring tube and the supporting tube are connected to one another
in a manner excluding relative axial movements and the axial spacing of the fixing
points of the supporting tube to the measuring tube represents the oscillation
length of the measuring tube and whereby the stress state of the measuring tube
is detected.
With mass flowmeters for flowing media which work on the Coriolis principle,
so-called Coriolis mass flowmeters, a distinction is basically made between, on
the one hand, devices whose measuring tube is designed curved, e.g. loop-shaped,
and on the other hand, devices whose measuring tube is essentially straight. Furthermore,
a distinction is made with the Coriolis mass flowmeters in question between, on
the one hand, those that have only one measuring tube, and on the other hand, those
that have two measuring tubes. In the case of the forms of embodiment of the Coriolis
mass flowmeters with two measuring tubes, these can lie in a row or parallel to
one another from the flow technology standpoint.
Forms of embodiment of Coriolis mass flowmeters with which the measuring tube
is designed straight, or with which the measuring tubes are designed straight,
can, in view of the mechanical structure, be produced simply and consequently at
relatively low cost. The Coriolis mass flowmeters obtainable in this way are compact
and lead to only low pressure loss.
The drawback with such Coriolis mass flowmeters with which the measuring tube
is designed straight, or with which the measuring tubes are designed straight,
is that both length changes of thermal origin and stresses of thermal origin as
well forces and moments acting from outside lead to measurement errors and to mechanical
damage, i.e. to stress cracks.
2. Description of the Prior Art
A mass flowmeter and a method for correcting the measurement signal of a mass
flowmeter,
as described at the outset, are known for example from DE 42 24 397 C1. With the
Coriolis mass flowmeter described there, there is provided, as a stress sensor
for detecting the stress state of the measuring tube, a length-change sensor which
detects changes in the oscillation length of the measuring tube in order to correct
the measurement signal in dependence on the oscillation length and the stress.
Due to the fact that a length-change sensor detecting changes in the oscillation
length of the measuring tube is provided in the case of this Coriolis mass flowmeter
known from the prior art, a change in the oscillation length and in the axial stress
state of the measuring tube influencing the oscillation frequency of the measuring
tube can be taken into account, as a result of which measurement errors can be
reduced or eliminated. With the additional provision of a temperature sensor, it
is possible to reduce or eliminate measurement errors based on temperature changes
of the measuring tube on the one hand and those based on forces acting on the measuring
tube from the outside on the other hand. The length-change signals originating
from the length-change sensor are a direct measure of changes in the oscillation
length of the measuring line, irrespective of their origin, and an indirect measure
of changes in the axial stress state of the measuring tube, also irrespective of
their origin. The length-change sensor for detecting the changes in the oscillation
length of the measuring tube thus makes it possible to detect changes in the oscillation
length of the measuring tube and changes in the axial stress state of the measuring
tube and to reduce or eliminate errors based thereon in the measurement signal
when determining the value of the mass flow rate.
As far as the measurement errors arising due to temperature changes are concerned,
the following further applies: the temperature dependence of the modulus of elasticity
influences the oscillation frequency and the flexibility of the measuring tube
and thus the measurement signal outputted by the measurement value sensor. As a
result of this knowledge, a temperature sensor detecting the temperature of the
measuring tube is provided for the measurement-signal correction dependent on the
measuring-tube temperature. In this regard, reference is also made to the article
"Direct mass flow rate measurement, in particular with the Coriolis method" by
W. Steffen and Dr. W. Stumm in "measurement, testing and automation", 1987, pp. 301-305.
Furthermore, a Coriolis mass flowmeter is known from the prior art,
with which the ongoing temperature dependence of the measurement signal is taken
into account by the fact that a temperature sensor detecting the temperature of
the supporting tube is provided for the measurement-signal correction dependent
on the supporting-tube temperature. This is described in DE 36 32 800 A1 and in
EP 0 261 435 B1. The correction measures described there make provision such that
the temperature-sensor signals generated by the two temperature sensors are inputted
into a correction device which is intended to remove the temperature influence
on the measurement signal.
All the previously described devices and measures that have been taken with Coriolis
mass flowmeters to obtain a measurement-signal correction dependent on stress and
temperature have led to an improvement in the ascertainment of the mass flow rate
signal. The known measures are not, however, fully satisfactory, since it emerges
that the ascertained mass flow rate signals continue to be bound up with errors,
even though small ones.
SUMMARY OF THE INVENTION
Proceeding from this, the problem of the invention is to make available
such a Coriolis mass flowmeter and such a method for correcting the measurement
signal of a Coriolis mass flowmeter, with which an ongoing correction of the measurement
signal and thus of the ascertained value for the mass flow rate can be achieved.
Proceeding from the Coriolis mass flowmeter described at the outset, the
previously derived and expounded problem is solved by the fact that a stress sensor
detecting the stress state of the supporting tube is provided, and the stress sensor
detecting the stress state of the supporting tube is connected to the correction
device in order to feed to the correction device the stress signal outputted by
the stress sensor detecting the stress state of the supporting tube, so that a
measurement signal can be outputted by the correction device that is corrected
on the basis of the stress signal outputted by the stress sensor detecting the
stress state of the measuring tube and the stress signal outputted by the stress
sensor detecting the stress state of the supporting tube.
According to the invention, therefore, in addition to the stress sensor
for measuring the stress state of the measuring tube, there is provided a further
stress sensor, i.e. one which detects the stress state of the supporting tube.
The oscillation length and the axial stress state of the measuring tube, on the
one hand, and the length and the axial stress state of the supporting tube, on
the other hand, can thus be ascertained, whereby length changes and the stress
state of the measuring tube and the supporting tube are influenced to a differing
degree by different influencing factors.
If the information concerning the stress state originating from the measuring
tube and the supporting tube, respectively, is used jointly for the measurement-signal
correction dependent on length and stress, a more effective correction of the measurement
signal can be achieved compared with the measures known hitherto. In particular,
it is the case that the stress sensor on the supporting tube is influenced only
slightly by the temperature of the medium flowing through the measuring tube. The
stress sensor on the supporting tube is, however, influenced much more markedly
by the stresses acting from outside on the Coriolis mass flowmeter, such as traction,
compression or torsion, and the ambient temperature. The ambient temperature for
its part can influence the stress sensor on the measuring tube only slightly, since
the temperature of the measuring tube is essentially determined by the temperature
of the medium flowing through the measuring tube. As a result, it is essentially
information concerning external influences that is provided by the stress sensor
provided on the supporting tube, so that, in combination with the information through
the stress sensor for the stress state of the measuring tube, an improved measurement-signal
correction dependent on length and stress is enabled overall.
There are various options for ascertaining the corrected measurement signal
on the basis of the stress signal outputted by the stress sensor detecting the
stress state of the measuring tube and the stress signal outputted by the stress
sensor detecting the stress state of the supporting tube. Within the scope of a
theoretical model, for example, the interrelationships and dependencies of the
factors influencing the measurement signal can thus be ascertained from the stress
states of the supporting tube and the measuring tube in order to obtain a correction
function. As a rule, however, mathematical correction conventions for the measurement
signal cannot be given in a closed form, so that recourse must be taken to iteration
and approximation methods.
According to a preferred embodiment of the invention, provision is made
such that an empirically determined correction function is provided in the correction
device in order to ascertain the corrected measurement signal. To ascertain such
a correction function, a medium with a known mass flow rate is for example passed
through the measuring tube, and the stress states of the measuring tube and the
supporting tube are determined with this known mass flow rate. A series of value
triplets thus results, from which an empirical correction function can be determined
by means of a matching procedure, a so-called fit procedure.
Various stress sensors can be used as the stress sensors for the stress state
of the measuring tube and the supporting tube. In particular, it is not necessary
for the stress sensors to be fixed directly on the measuring tube and on the supporting
tube. Stress sensors for contactless stress measurement are in fact also known.
According to a preferred development of the invention, however, provision is made
such that length-change sensors, i.e. wire strain gauges in particular, are provided
as stress sensors, with which wire strain gauges stress and length changes in the
measuring tube and the supporting tube can be detected. Such wire strain gauges
fixable on the measuring tube and on the supporting tube are well known from the
prior art as measuring sensors for mechanical magnitudes, such as small expansions,
compressions, bends and torsions as well as the respective elastic stresses. The
effective measuring element of such a wire strain gauge consists, for example,
of a thin resistance wire, which is applied in a looped or zigzag form on an expandable
strip of a substrate material, such as plastic. If the wire strain gauge is applied
on a deformable body, such as the measuring tube and the supporting tube of the
Coriolis mass flowmeter, the wire strain gauge experiences, in the presence of
loading, the same expansions or compressions as the measuring tube and the supporting
tube itself, which leads to an elongation and transverse contraction of the resistance
wire in the case of an expansion or to a shortening and transverse bulging of the
resistance wire in the case of a compression and thus always to a change in the
electrical resistance of the resistance wire. This change in the electrical resistance
is proportional to the expansion or compression and thus, according to Hooke's
law, to the elastic stress.
In principle, different orientations of the stress sensors on the supporting
tube
and the measuring tube come into consideration. According to a preferred development
of the invention, however, provision is made such that the stress sensor detecting
the stress state of the measuring tube is orientated in the longitudinal direction
of the measuring tube and/or the stress sensor detecting the stress state of the
supporting tube is orientated in the longitudinal direction of the supporting tube.
In this way, it ensures that, in fact, the length changes detected by the stress
sensors are changes in the actual oscillation length of the measuring tube and
the actual length of the supporting tube.
Proceeding from the initially described method for correcting the measurement
signal of a Coriolis mass flowmeter, the problem derived and expounded above is
solved by the fact that the stress state of the supporting tube is detected and
the measurement signal is corrected on the basis of the detected stress state of
the measuring tube and the detected stress state of the supporting tube.
Preferred embodiments of the method according to the invention emerge,
moreover, by analogy with the preferred embodiments of the Coriolis mass flowmeter
according to the invention.
In detail, there are a large number of possibilities for configuring and developing
the mass flowmeter according to the invention and the method according to the invention
for correcting the measurement signal of a Coriolis mass flowmeter. In this regard,
reference is made to the dependent claims and to the following detailed description
of a preferred invention embodiment, reference being made to the drawing.
BRIEF DESCRIPTION OF THE DRAWING
In the drawing, the single FIGURE shows in cross-section a Coriolis mass flow-meter
according to a preferred example of embodiment of the invention.
DESCRIPTION OF THE PREFERED EMBODIMENT
The Coriolis mass flowmeter shown in cross-section in the FIGURE has a single
straight measuring tube
1, an oscillation generator
2 acting on measuring
tube
1 and two measurement value sensors
3 detecting Coriolis forces
and/or Coriolis oscillations based on Coriolis forces. Furthermore, a supporting
tube
4 for accommodating measuring tube
1, oscillation generator
2 and measurement value sensors
3 is provided. In order to detect
the stress state of measuring tube
1, there is arranged thereon a stress
sensor
5, i.e. in the form of a wire strain gauge. The latter is connected
to a correction device
6 for correcting the measurement signal, in order
to feed to said correction device the stress signal outputted by the stress sensor
5 detecting the stress state of measuring tube
1. Correction device
6 is further connected to measurement value sensors
3, in order also
to feed the measurement signals coming from measurement value sensors
3
to correction device
6. As is well known, a signal proportional to the mass
flow rate through measuring tube
1 emerges directly through the phase shift
of the oscillation signals picked up by the two measurement value sensors
3.
Apart from stress sensor
5 for detecting the stress state of measuring
tube
1, there is also provided a stress sensor
7 for detecting the
stress state of supporting tube
4, also in the form of a wire strain gauge.
The stress signal detected by stress sensor
7 on supporting tube
4
is also fed to correction device
6. A measurement signal corrected on the
basis of the stress signal outputted by stress sensor
5 detecting the stress
state of measuring tube I and the stress signal outputted by measurement value
sensor
7 detecting the stress state of supporting tube
4 is then
outputted by correction device
6, i.e. specifically a corrected signal for
the mass flow rate through measuring tube
1.
Measuring tube
1 and supporting tube
4 are connected to one
another in a manner excluding relative axial movements, whereby the axial spacing
of the fixing points of supporting tube
4 on measuring tube
1 represents
the oscillation length of measuring tube
1. Two connection rings
8
joined to the supporting tube at the ends are provided for fixing supporting tube
4 to measuring tube
1. Finally, an external accommodation cylinder
9 is provided as a housing for the Coriolis mass flowmeter, said accommodation
cylinder accommodating the assembly unit consisting of measuring tube
1,
oscillation generator
2, measurement value sensors
3, supporting
tube
6 and connection rings
8. Accommodation cylinder
9 has
two connection rings
10 joined at the ends, to which a joining flange
11
projecting outwards is joined in each case. Joining pipes
12 connected to
measuring tube
1 project through connection rings
10 up to joining
flange
11. According to the present preferred embodiment of the invention,
measuring tube
1 and joining pipes
12 are designed in one piece,
so that overall it involves a continuous pipe. In order to protect joining pipes
12, the latter are surrounded by a reinforcing cylinder
13.
With the Coriolis mass flowmeter according to the currently described preferred
embodiment of the invention, therefore, a total of two wire strain gauges are provided,
i.e. one for detecting the stress state of measuring tube
1 and one for
detecting the stress state of supporting tube
4. As already explained in
detail above, the oscillation length and the axial stress state of measuring tube
1, on the one hand, and the length and the axial stress state of supporting
tube
4, on the other hand, can thus be ascertained, whereby length changes
and the stress state of measuring tube
1 and supporting tube
4 are
influenced to a differing degree by different influencing factors.
In particular, it is the case that stress sensor
7 on supporting tube
4
is influenced only slightly by the temperature of the medium flowing through measuring
tube
1, but much more markedly by the stresses acting from outside on the
Coriolis mass flowmeter, such as traction, compression or torsion, and the ambient
temperature. The ambient temperature can, in turn, influence stress sensor
5
on measuring tube
1 only slightly, since the temperature of measuring tube
1 is essentially determined by the temperature of the medium flowing through
measuring tube
1.
With the currently described Coriolis mass flowmeter according to the preferred
embodiment of the invention, therefore, the information concerning the respective
stress state originating respectively from measuring tube
1 and supporting
tube
4 can be used jointly to correct the measurement signal, i.e. in respect
of the measurement-signal influences dependent on length and stress, by the application
of an empirically determined correction function in correction device
6.
As a result, an effective error correction of the measurement signal can thus be obtained.
*
