The flow meter works on the Coriolis principle and has a measuring
tube and at least two energy converters. At least one of the energy
converters is an oscillation exciter for oscillating the tube to
generate a wave traveling along the tube and an oscillation detector.
An evaluating circuit uses the speed of travel of waves in the direction
of flow of fluid through the tube and in the direction opposite
the direction of flow to determine the mass flow of fluid through
1. A mass flow meter working on the Coriolis principle, comprising,
at least one measuring tube for having fluid flow therethrough in
a fluid flow direction, oscillation exciter means for oscillating
the tube to generate waves to travel along the tube in both directions,
and oscillation detector and evaluator means for detecting the speed
of travel of the waves in both said directions to determine the
mass flow of fluid through the tube.
2. A meter according to claim 1 characterized in that two measuring
paths are provided to measure the travelling times of the waves
travelling in and against the direction of flow.
3. A meter according to claim 2 characterized in that said measuring
paths are adjoining with said oscillation excited means being at
opposite ends thereof and said oscillation detector means being
4. A meter according to claim 1 characterized in that a measuring
path is provided which is bounded by two energy converters operable
alternately as said oscillation exciter means and said oscillation
5. A meter according to claim 4 characterized in that said measuring
path is bounded at both ends by said oscillation exciter generator
means and said oscillation detector means which are alternately
operable in pairs.
6. A meter according to claim 1 characterized in that said oscillation
exciter means excites the measuring tube via a short train of oscillations
having a narrow band width.
7. A meter according to claim 6 characterized in that said train
of oscillations consists of a few sinusoidal oscillations in an
enveloping curve corresponding to a Gaussian function.
8. A meter according to claim 6 characterized in that said measuring
tube is clamped at opposite ends thereof, said oscillation exciter
means being operable to excite said tube with a sufficiently high
frequency that measurement of the travelling time occurs before
reflections of the travelling wave from a clamping point of the
measuring tube reach said oscillation detector means.
9. A meter according to claim 1 characterized in that said oscillation
detector means measures the travelling time of said waves based
on a predetermined passage of said waves through zero.
10. A meter according to claim 1 characterized in that an oscillation
damper acting on the measuring tube is associated with said measuring
path on the side of said oscillation detector means remote from
said oscillation exciter means.
11. A meter according to claim 1 characterized in that said oscillation
exciter means excites the measuring tube at such a frequency that
the wave length of the travelling waves is substantially larger
than the diameter of the measuring tube.
12. A meter according to claim 1 characterized by a regulating
circuit for the exciter frequency of said oscillation exciter means
that holds constant the wave length of the wave travelling along
the measuring tube.
13. A meter according to claim 12 characterized in that said regulating
circuit comprises an intergrator which intergrates departures of
the arrival time from a desired value, a voltage-controlled oscillator
which converts the integration result into a cycle frequency, a
store for forming a voltage for exciting said oscillation exciter
means, and a D/A converter which can be read with the aid of a counter
operated at the cycle frequency.
The invention relates to a mass flow meter working on the Coriolis
principle, comprising at least one measuring tube, at least two
energy converters associated therewith in the form of oscillation
exciters or oscillation detectors, and an evaluating circuit using
energy converter signals to determine the mass flow.
Such meters are known in many forms, for example from EP-OS 0 282
217. They are based on the principle that a measuring tube made
to oscillate at its resonance frequency is additionally deformed,
depending on the mass flow, which leads to lagging of the first
measuring tube section and leading of the second measuring tube
section. The phase displacement is a direct measure of the value
of mass flow.
In the known case, the measuring tube consists of a straight tube
section provided with connecting flanges at both ends with resiliant
bellows therebetween. The middle of the tube is associated with
an oscillation exciter which, with the aid of a centrally disposed
oscillation detector, oscillates the measuring tube at its resonance
frequency. In front of and behind it, there is a respective further
oscillation detector serving to determine the positive and negative
phase displacement so that the phase difference can be evaluated.
This measuring device has a phase difference differing from zero
even if there is no flow in the tube. The reason for this may be
clamping forces, different temperature stresses, unevennesses in
the tube material or in the flow medium, phase errors in the detectors
or the associated circuits, and so on. It is therefore necessary
to measure the phase difference that is present without flow after
the meter has been built in and to correct the subsequent flow measurements
by means of corresponding calibration. If a certain accuracy is
required of the measurements, the calibration has to be checked
It is also known (U.S. Pat. No. 4422338) to avoid such manual
error compensation in that use is made of a very special evaluating
circuit with single or multiple integration of the output signal
of the oscillation detectors. The measuring tube is substantially
U-shaped so that the connecting flanges lie in one plane. This reduces
the influence of clamping forces. However, the device has a complicated
Also known are ultrasonic flow meters (DE-PS 34 38 976), in which
two energy converters bounding an obliquely extending measuring
path are provided at opposite sides of the tube wall but axially
displaced from one another. They are utilized alternately as ultrasound
generators and ultrasound receivers. The period of travel of the
sound wave is measured in one direction and then in the other direction.
The amount of flow can be determined from the times of travel and
their difference (reciprocity principle). Zero point departures
are suppressed. However, one disadvantage that remains is the dependence
of the measurement on the flow profile, for which reason calibration
must take place having regard to each medium to be measured.
The object of the invention is to provide a flow meter which requires
no calibration either because of zero point departure or because
of the flow profile.
This problem is solved according to the invention in that in and
against the direction of flow waves can be produced which travel
along the measuring tube, that at least one measuring path is provided
to find the speed of travel in and against the direction of flow,
and that the evaluating circuit determines the mass flow by utilising
the two different speeds of travel.
This construction is based on the recognition that propagation
of the travelling waves of the same frequency is not constant along
a measuring tube loaded because of the flow by Coreolis forces but
depends on the mass flow and the direction of travel. By taking
the speed of travel into account in both directions, one can substantially
suppress the zero point deviation. One thereby achieves the advantages
of the Coreolis principle, (no dependence on the flow profile) and
the advantages of the ultrasonic measuring method (no zero point
departure) without having to accept their disadvantages. The invention
is particularly suitable for very simply designed meters, for example
those with a straight measuring tube, which have a marked departure
from zero during normal measurement but this departure now being
In a particularly simple embodiment, one or two substantially equally
formed measuring paths are provided for measuring the travelling
time of the waves in and against the direction of flow and the evaluating
circuit processes the difference of the traveling times. The speed
measurement can therefore be reduced to a pure measurement of the
It is particularly favourable to provide a measuring path which
is bounded by two energy converters which can be alternately driven
as oscillation excite, and oscillation detectors. With the least
possible constructional expense, one ensures that the same conditions
apply to the measurement in the one direction of travel as they
do in the other direction. Since the principle of reciprocity fully
applies to this case, particularly good results are obtained. Similar
results are achieved by providing a measuring path which is bounded
at both ends by a respective oscillation exciter and an oscillation
detector operable alternately in pairs.
However, there may also be two measuring paths if they are substantially
the same. In this case, it is particularly recommended to provide
two measuring paths which have a common oscillation exciter and
a respective oscillation detector at the opposed ends. The common
oscillation exciter ensures that travelling waves are generated
in both directions at the same frequency without having to take
any additional measures.
It is particularly favourable for the oscillation exciter to excite
the measuring tube by means of a short train of oscillations of
narrow band width. Excitation with a few oscillations will be sufficient
to produce the travelling wave. The narrow band width leads to very
accurate measuring results even with types of excitation in which
the speed of travel depends on the frequency, for example in the
case of bending oscillations. If there is no such dependency, use
can also be made of wide band pulse to excite the oscillations.
In particular, the train of oscillations may consist of a few sinusoidal
oscillations in an enveloping curve corresponding to the Gaussian
function. In contrast with a square pulse, this product of bell
curve and sinusoidal function leads to excitation with a very narrow
It is also advantageous for the oscillation exciter to excite the
measuring tube substantially at such a high frequency that measurement
of the travelling time has taken place before reflections of the
travelling wave at the point of clamping the measuring tube have
reached the oscillation detector. The higher the frequency, the
greater is the possibility that the oscillation detector can detect
a starting zone of the travelling wave sufficient for accurately
determining the time before the detection is impeded by reflections.
A particularly accurate measurement is obtained if the oscillation
detector determines a predetermined passage through zero of the
travelling wave for the purpose of measuring the time of travel.
It may be the first or second passage through zero in a particular
direction, which can be determined with high accuracy. By selecting
a corresponding exciter frequency, one can ensure that this passage
through zero is detected before there is interference from reflections.
In this connection, it is advisable for the measuring path to be
associated on the side of the oscillation detector remote from the
oscillation exciter with an oscillation damper that acts on the
measuring tube. The oscillation damper ensures that reflections
are returned only to a harmless extent to influence the measuring
result. It is also an advantage for the oscillation exciter to excite
the measuring tube at substantially such a frequency that the wave
length of the travelling waves is substantially larger than the
diameter of the measuring tube. This gives well defined and therefore
easily detectable waves.
In a preferred embodiment, there is a regulating circuit for the
exciting frequency of the oscillation exciter, which holds constant
the wave length of the wave travelling along the measuring tube.
It is under this condition that the difference in the travelling
time of the wave in and against the direction of flow will be proportional
to the mass flow.
In a preferred embodiment, provision is made for the regulating
circuit to comprise an integrator which integrates departures in
the arrival time of a desired value, for a voltage-controlled oscillator
to convert the integration result to a cycle frequency, and for
a store which is followed by a D/A converter to be readable with
the aid of a counter operated at the cycle frequency to form a voltage
which excites the oscillation exciter. Every departure of the arrival
time from the desired value leads to a change in the cycle frequency
and thus to a change in the exciting frequency of the oscillation
exciter until the original condition has been re-established.
A preferred example of the invention will now be described in more
detail with reference to the drawing, wherein:
FIG. 1 is a diagrammatic representation of a mass flow meter according
to the invention,
FIG. 2 shows a first modification,
FIG. 3 shows a second modification,
FIG. 4 shows a third modification,
FIG. 5 shows a fourth modification, and
FIG. 6 is a block diagram of one embodiment of an exciter and evaluating
FIG. 1 illustrates a straight measuring tube 1 carrying connecting
flanges 2 and 3 at both ends so that it can be inserted in a tube
circuit. There is a measuring path A1 bounded by a respective energy
converter 4 and 5. These energy converters can each be operated
as an oscillation exciter S and oscillation detector M. Between
the measuring path A1 and the end flanges 2 or 3 there is a respective
damping apparatus 6 or 7.
In the FIG. 2 embodiment, there is a measuring path A2 at the
ends of which there are two energy converters 9 10 11 and 12
namely two oscillation exciters S and two oscillation detectors
M. Of these, the pair 9 12 and the pair 10 11 are operated alternately.
In FIG. 3 there are two measuring paths. The measuring path B1
is bounded by the energy converter 13 in the form of a transmitter
S and the energy converter in the form of an oscillation detector
M, the measuring path C1 is bounded by the energy converter 15 in
the form of an oscillation exciter S and the energy converter 16
in the form of an oscillation detector M.
In FIG. 4 there are again two measuring paths B2 and C2 with energy
converters 17 18 19 and 20. The difference from FIG. 3 is that
the position of the oscillation exciter S and oscillation detector
M has been exchanged.
In FIG. 5 two measuring paths B3 and C3 are provided and they
adjoin each other directly. In the middle, there is an energy converter
21 in the form of an oscillation exciter S. At the same spacing
from it, there are two energy converters 22 and 23 in the form of
oscillation detectors M.
The oscillation exciters can excite the measuring tube 1 in such
a way that bending oscillations, displacement oscillations, torsion
oscillations or other oscillations occur as travelling waves. Use
is made of the fact that the propagation speed of the travelling
waves depends not only on the stiffness and mass of the tube as
well as the density of the flowing medium but also on the mass flow
and its direction. The principle of the invention is therefore suitable
for all measuring tubes which do not impede such propagation of
the waves. If the medium to be measured is compressible, one should
select oscillation excitations at which the compressibility will
not detrimentally influence the measuring result.
The excitation may take place with a wide band voltage pulse. However,
since the speed of travel often depends on the frequency, one obtains
more accurate measuring results if excitation takes place at a definite
frequency or with a short train of oscillations of narrow band width.
Such a train of oscillations is obtained if a few sinusoidal oscillations,
for example three or four, are employed in an enveloping curve which
corresponds to the Gaussian function or bell curve. The frequency
is desirably selected to be so low on the one hand that travelling
waves are produced with a wave length which is much higher than
the diameter of the measuring tube and so high on the other hand
that a passage through zero serving for the certain determination
of the arrival time has occurred at the oscillation detector before
there is superimposition by wave sections that have been reflected
at the clamping flanges 2 and 3. An exciter and evaluating circuit
24 determines the travelling time between commencement of the excitation
of the travelling wave and the arrival at the oscillation detector
M and from this determines the flow of mass. One should wait with
the next oscillation excitation until the measuring tube 1 has come
to rest again.
The oscillation exciters S can be operated electromagnetically,
electrostatically, piezo electrically, hydraulically, magnetically,
magnetostrictively, thermally or in some other known manner. In
general, one part of the oscillation exciter is secured to the measuring
tube 1 and a second part to a second measuring tube or fixed with
respect to the housing.
The oscillation detectors M, which may be similarly constructed
in two parts, respond to the position, speed or acceleration of
the measuring tube. From the signals obtained, one can then find
the starting section of the travelling wave, for example a predetermined
positive passage through zero.
The oscillation detectors M may operate optically, piezo electrically,
electromagnetically, magnetostrictively, or electrostatically, be
in the form of strain gauges or operate in some other known manner.
The oscillation dampers 6 and 7 can, for example, be of corrugated
It will be assumed that in all the examples the direction of flow
In the operation of the FIG. 1 embodiment, the measuring tube 1
is first of all caused to oscillate with the aid of the energy converter
4. The travelling time of the wave thus created in the direction
of flow X is determined with the aid of the energy converter 5.
The latter is then used as an oscillation exciter S and the time
of travel of the wave in the opposite direction is determined by
the energy converter 4. The difference between these two travelling
times, which is also a measure of the difference between the travelling
speeds, is processed in the exciter and evaluating circuit 24.
It can be shown that: ##EQU1## wherein Q.sub.m =mass flow
V.sub.x =speed of the flowing medium
Mu=mass of the flowing medium
.omega.=exciter frequency as circuit frequency
T.sub.t+ =time of travel of wave along measuring path in flow direction
T.sub.t- =time of travel of wave along measuring path against flow
D.sub.t =difference T.sub.t- -T.sub.t+
E=modulus of elasticity
I=moment of inertia
L=length of measuring path.
The travelling time T.sub.t obtained without mass flow is calculated
from (T.sub.t- +T.sub.t+)/2. Since the frequency is readily found
and the other parameters are constant, the mass flow can be calculated
If one keeps the expression (T.sub.t- .times..omega.) or (T.sub.t-
.times.T.sub.t+ .times..omega..sup.2) constant, the mass flow will
only depend on the difference D.sub.t in the travelling times. This
can be effected by a regulating circuit 25 in the exciter and evaluating
circuit 24 which ensures that the wave lengths are kept constant
at the tube. For example, in the case of a change in the travelling
time, one can determine the alteration factor and subsequently divide
the frequency by this factor. This can, for example, be achieved
with the aid of a loop having a closed phase. Attention is drawn
to the example of FIG. 6.
The FIG. 2 embodiment functions similarly because the pairs of
energy converters 9 and 12 or 11 and 10 are alternately made effective.
Here, again, the travelling time of the waves is measured alternately
on the same measuring path A2 in the direction of flow X and against
the direction of flow X.
In the FIG. 3 embodiment, the measuring path B1 serves to measure
the travelling time in the direction of flow X and the measuring
path C1 serves to measure the flow against the direction of flow
X. The measurement can take place alternately or simultaneously.
In the FIG. 4 embodiment, the measuring path B2 serves to measure
the travelling time against the direction of flow X and the measuring
path C2 serves to measure the travelling time in the direction of
flow X. Again, the measurements may take place alternately or simultaneously.
In the FIG. 5 embodiment, the measuring path B3 serves to measure
the travelling time against the direction of flow X and the measuring
path C3 serves to measure the travelling time in the direction of
flow X. Here, too, the measuring paths may be utilized alternately
In all cases, the conditions are similar to those described in
conjunction with FIG. 1. Their measuring accuracy is, however, somewhat
lower because the reciprocity condition is not accurately fulfilled
but only approximated.
The drawings only show straight measuring tubes. However, any other
shapes of measuring tube are possible, especially U-shaped and loop-like
tubes. The only prerequisite is that they should not hinder the
propagation of travelling waves.
FIG. 6 illustrates one embodiment of an exciter and evaluating
circuit 24 with a built-in regulating circuit 25. The two energy
converters 4 and 5 are alternately connected to the exciting transmitter
circuit S1 and the detecting receiver circuit M1 with the aid of
a change-over switch 26. This manner of operation is controlled
by a logic circuit 27. The latter also predetermines a particular
time of arrival (desired value) at the outlet 28. A detector 29
compares this desired value with the existing value determined by
the receiver circuit M1. The departure is integrated in an integrator
I. The integration result is converted in a voltage-controlled oscillator
VCO into a frequency which is proportional to the voltage. This
frequency constitutes the cycle frequency for a counter 30 which,
on the one hand, controls the logic circuit 27 and on the other
hand calls for digital values from a store 31 (PROM) that are converted
into a voltage in a digital/analogue converter DAC. To excite the
oscillation exciter with the aid of the transmitter circuit S1
use is made of a counter run which produces three or four sinusoidal
oscillations in a bell-shaped enveloping curve. If the travelling
time is too high relatively to the desired value, the cycle frequency
of the VCO and thus the exciter frequency will drop and, if the
travelling time is too low, the cycle frequency will rise.
In this way, the product of the travelling time T.sub.t and the
frequency .omega. is kept constant. The mass flow now only depends
on the difference in travelling times.
To determine this, the desired value predetermined by the logic
circuit 27 and the departure determined by the detector 29 are fed
to a time measuring circuit 32 of which the time scale is fixed
by an oscillator 33. The travelling times found each time in the
direction of flow and against the direction of flow are evaluated
by a microcomputer 34. The result can be detected in an outlet apparatus
35 such as a display, printer or the like.