Abstrict The invention relates to a mass flow meter operable on the Coriolis
principle having two rectilinear juxtaposed parallel arranged measuring
tubes mechanically interconnected at their ends. An oscillator between
the tubes produces opposite oscillatory movement of the tubes with
a harmonic oscillation superimposed on the fundamental oscillation.
Sensors between the tubes on opposite sides of the oscillator sense
relative movement between the tubes and generate signals corresponding
thereto. A circuit responsive to these signals determines resonant
frequencies of the fundamental and harmonic oscillations and derives
therefrom a correcting value which gives effect to axial stresses
in the measuring tubes to determine a corrected mass flow.
Claims We claim:
1. A mass flow meter operable on the Coriolis principle, comprising,
two rectilinear juxtaposed parallel arranged tubes mechanically
interconnected at their ends, wye tube fittings connecting adjacent
ends of said tubes to provide for connections to supply and discharge
pipes and for parallel flow through said tubes, oscillator means
between said tubes for producing opposite oscillatory movement of
said tubes with a harmonic oscillation superimposed on the fundamental
oscillation, first and second sensor means between and at opposite
ends of said tubes and spaced from said oscillator means for sensing
relative movement between said tubes and generating first and second
signals corresponding thereto, means responsive to said first and
second signals for generating a first value of mass flow, frequency
determining circuit means responsive to said first signal for determining
the resonant frequencies of said fundamental and harmonic oscillations
and for deriving therefrom a correcting value which (gives effect
to) is a measure of axial stresses in said measuring tubes and means
for applying said correcting value to said first value to determine
a corrected mass flow.
2. A meter according to claim 1 including a correcting circuit
which forms a quotient from said resonant frequencies of said fundamental
and harmonic oscillations, said correcting value being a predetermined
function of said quotient.
3. A meter according to claim 2 wherein said correcting circuit
includes a store for receiving data of said predetermined function
and generates said correcting value from said determined quotient.
4. A meter according to claim 1 characterized in that said harmonic
oscillation corresponds to the third harmonic wave.
5. A meter according to claim 1 wherein said oscillater means is
disposed substantially midway between the ends of said tubes, and
said first sensor means being positioned approximately onefifth
the length of said tubes from one end thereof.
6. A meter according to claim 1 wherein said circuit means includes
exciter circuit means for driving said oscillator means having an
input connected to said first sensor means, said exciter circuit
means having fundamental and harmonic oscillation branches, said
harmonic oscillation branch having a selection filter arrangement
and amplifier means for selecting said resonant frequency of said
harmonic oscillations, and a summation element connected to said
oscillator means which receives amplified signals of both of said
branches.
7. A meter according to claim 6 characterized in that said summation
element is a summation amplifier with AGC regulation.
8. A meter according to claim 6 characterized in that said fundamental
oscillation branch has phase correcting means to set said exciter
circuit means in phase with said first signal, and said harmonic
oscillation branch has phase reversal means to set said harmonic
oscillation branch in phase with said harmonic oscillations of said
first signal.
9. A meter according to claim 6 including a voltagecurrent transformer
connected between said summation element and said oscillator means.
10. A meter according to claim 6 wherein said selection filter
arrangement includes a band filter with a selection frequency predetermined
by timing pulses, and a pulse generator for generating said timing
pulses having a timing frequency which is a multiple of the frequency
of the harmonic in said harmonic oscillation branch.
11. A meter according to claim 10 including amplifier means for
the output of said harmonic oscillation branch, said pulse generator
including a phase locking circuit having first and second inputs
and an output, and a 1:N divider between said output and second
input.
12. A meter according to claim 11 having a starting circuit including
logic circuit means connected to said phase locking circuit, further
input means for said summation element, said logic circuit means
transmitting a square signal to said further input means when said
phase locking circuit is under voltage and this circuit is not yet
locked.
13. A meter according to claim 6 wherein said frequency determining
circuit has two frequency signal outputs, and means connecting said
outputs to said fundamental oscillation branch and said harmonic
oscillation branch.
Description The invention relates to a mass flow meter on the Coriolis principle,
wherein two juxtaposed measuring tubes are mechanically interconnected
at their ends and connected for parallel flow by means of two tube
connectors which are connected at their nonconfronting ends to
a supply or discharge passage having a connector at its end, wherein
an oscillator is provided, which sets the measuring tubes into opposite
fundamental oscillations, and wherein the measuring tubes are associated
at a spacing from the oscillator with sensors for receiving measuring
signals from which the flow of mass can be determined.
In a known meter of this kind U.S. Pat. No. 4491025 a cylindrical
container provided at its ends with connectors for the supply and
withdrawal of the medium to be measured and at the middle with dividing
walls carries two tubes bent into U shape communicating with the
interior of the container at both sides of the dividing walls. The
container therefore defines the tube connectors and the supply and
withdrawal passages. The adjacent limbs of the U tubes are mechanically
interconnected near the container with straps which define the ends
of the actual measuring tubes which can be oppositely oscillated
by the oscillator. The oscillator is applied at the middle of the
curved web of the U. The sensors are disposed at the transition
between the curves and the straight limbs of the tube. The particular
mass flow can be determined from the difference in the phases of
the oscillatory motion at both ends of the U curve. Since the oscillating
measuring tubes must have a certain length but project laterally
from the container, the meter becomes laterally bulky.
The object of the invention is to provide a mass flow meter of
the aforementioned kind that is laterally more compact.
This problem is solved according to the invention in that the measuring
tubes are straight and parallel, that the oscillator produces a
harmonic superimposed on the fundamental oscillation, and that a
frequency determining circuit is provided which determines from
a measuring signal the values of the resonance frequencies of the
fundamental and harmonic oscillations for the purpose of deriving
therefrom a correcting value which takes axial stresses in the measuring
tubes into account to determine a corrected mass flow.
In this construction, straight measuring tubes are used instead
of bent ones. The lateral extent is therefore small. The measuring
tubes can extend parallel to the conduit in which the meter is connected.
However, since the tube connectors are now widely spaced from one
another, changes in length occur as a result of temperature fluctuations.
If, as is usual, the tube connectors and connections form a solid
unit which is spacially fixed by being applied to the conduit, the
change in length will lead to axial stresses in the measuring tubes,
by which the oscillatory behaviour is altered and there will be
errors in measurement. Axial stresses can also occur through incorrect
clamping of the device and for other reasons. The axial stresses
have different effects on the fundamental and harmonic oscillations.
Consequently, if excitation is not only by means of a fundamental
oscillation but also with a superimposed harmonic, the size of the
axial force can be derived from the two frequencies and hence also
a correcting value for compensating the measuring error. Thus, despite
axial stresses which are inevitable with temperature changes, the
mass flow meter is adapted to give corrected values of mass flow.
Preferably, a correcting circuit is provided which forms a quotient
from the frequencies of the fundamental and harmonic oscillations,
the correcting value being a predetermined function of said quotient.
The ratio of the two frequencies is a particularly simple measure
of the axial stresses and hence also of the correcting value. This
function could even represent a correcting factor which can be particularly
easily linked with the measuring result.
In particular, the correcting circuit may comprise a store for
receiving date of the predetermined function and automatically make
the correcting value available by reason of the determined quotient.
The store therefore assumes the function of a table or computing
rule. Since the correcting value is given automatically, it is constantly
available.
A particularly simple circuit is obtained if an evaluating circuit
for determining the mass flow from measuring signals received by
two spaced sensors is followed by a multiplication element which
is fed with the correcting value determined from the quotient so
as to determine the corrected mass flow.
With particular advantage, the harmonic oscillation corresponds
to the third harmonic wave. This can readily be excited by the same
position as the fundamental oscillation. In addition, compared with
other harmonics it has the largest amplitude, so that it can be
readily detected if the sensor is suitably placed.
In a preferred form of the invention, the oscillator is disposed
substantially in the middle of the straight measuring tubes and
at least one sensor is disposed at a spacing of 15 to 25%, preferably
about 20%, from the end of the measuring tube. By means of the central
arrangement, the fundamental and third harmonic oscillations are
excited under optimum conditions. The special position of the sensor
ensures that the third harmonic will be detected near its greatest
amplitude and the fundamental oscillation will likewise be detectd
with an adequate amplitude.
With particular advantage, the oscillator is fed by an exciter
circuit comprising an input connected to a sensor, a fundamental
oscillation branch provided with an amplifier, a harmonic oscillation
branch provided with a selection filter arrangement and an amplifier,
and a summation element which precedes the output and receives the
amplified signals of both branches. With the aid of the harmonic
oscillation branch, the harmonic can be separately treated and amplified
so that it can be added in a predetermined ratio to the signal of
the fundamental oscillation branch. In this way, one ensures that
sufficient excitation energy is available for the harmonic. Otherwise,
the preferably adjustable admixing can be so selected that evaluation
of the phase displacement of the fundamental frequency for determining
the measured flow quantity is not influenced by the harmonic.
It is favourable if the summation element is a summation amplifier
with AGC (automatic gain control) regulation. The energising power
is therefore so regulated that the measuring signals have a certain
size permitting their evaluation.
In addition, each branch should contain a phase correcting element.
Small correcting values suffice for the fundamental oscillation.
Considerable phase rotations may be necessary for the harmonics,
for example a phase reversal for the third harmonic.
Further, it is advisable for a voltagecurrent transformer to be
connected between the summation element and oscillator. In this
way, one eliminates phase displacements on account of the inductance
of the coils of the oscillator and measurement errors associated
therewith.
With particular advantage, the selection filter arrangement comprises
a band filter with a selection frequency predetermined by timing
pulses and a pulse generator is provided of which the frequency
is a multiple of the frequency of the harmonic in the harmonic oscillation
branch and is made to follow same. In this way, one ensures that,
despite the changes in the harmonic occurring with axial stresses,
the selection filter arrangement will always accurately tune its
mean frequency to the existing harmonic frequency. This avoids the
phase rotations occuring on frequency changes with a solis filter.
In particular, the pulse generator may comprise a phase locking
circuit of which the first input is connected by way of a comparator
to a section of the harmonic oscillation branch following the amplifier
and the second input is connected by way of a 1:N divider to its
output. This gives a particularly simple construction for the pulse
generator which depends on the harmonic frequency.
Further, it is advisable to have a starting circuit in which the
summation element has a further input which receives a square signal
by way of a logic circuit when the first input of the phase locking
circuit is under voltage and this circuit is not yet locked. This
can also initiate excitation of the harmonic so that phase locking
occurs after a short time and the selection filter can operate normally.
It is also advantageous if the frequency determining circuit is
formed by utilising the exciter circuit and comprises two frequency
signal outputs each connected by way of a comparator to a section
of the fundamental oscillation branch or harmonic oscillation branch
that follows the amplifier. Signals of the frequencies to be determined
are simply obtained at the frequency signal outputs.
An example of the invention will now be described in more detail
with reference to the drawing, in which:
FIG. 1 is a diagrammatic representation of a mass flow meter with
associated circuit;
FIG. 2 shows an embodiment of a sensor;
FIG. 3 shows an embodiment of an oscillator;
FIG. 4 shows the oscillating behaviour of a measuring tube; and
FIG. 5 shows an example of an exciter circuit.
The mass flow meter 1 shown in FIG. 1 comprises two measuring tubes
2 and 3 which are straight and parallel. At their ends, they are
mechanically interconnected by crossstruts 4 and 4a. The measuring
tubes are connected for parallel flow with the aid of two tube connectors
5 and 6. The passages 7 and 8 serving for supply and withdrawal
are provided at their nonconfronting ends with an end connector
9 or 10. With its connectors 9 and 10 the meter can therefore be
included in a conduit containing the medium to be measured.
Substantially in the middle of the tubes there is an oscillator
11 comprising a permanent magnet 12 connected to the measuring tube
2 and a drive coil 13 connected to the measuring tube 3. At substantially
equal spacings in front of and behind this oscillator, there are
two sensors 14 and 15 each comprising a permanent magnet 16 or 17
connected to the measuring tube 2 and an induction coil 18 or 19.
These have a spacing of about 20% of the measuring tube length from
the end of the measuring tube. If a periodic exciter current I is
fed to the oscillator, the two measuring tubes 2 and 3 will oscillate
in opposite senses. By reason of the oscillating motion, a measuring
signal U1 and U2 is induced in the induction coils 18 and 19 of
the sensors 14 and 15 that is in the form of a voltage proportional
to the velocity of the movements of the measuring tubes relatively
to each other.
A particularly effective example of a sensor is shown in FIG. 2.
The reference numerals are increased by 100 in relation to FIG.
1. A permanent magnet 116 magnetised as south pole S and north pole
N next to each other transversely is opposite an induction coil
118 with an axis parallel to the measuring tubes.
A particularly effective example of oscillator 111 is shown in
FIG. 3. A permanent magnet 112 likewise magnetised transversely
next to each other as south pole S and north pole N is disposed
within a drive coil 113 consisting of a carrier 120 of nonmagnetisable
material.
An exciter circuit 21 to be explained in more detail in conjunction
with FIG. 5 receives the measuring signal U.sub.1 at its input 22
by way of a conduit 23 and delivers the exciter current I to the
oscillator 11 by way of its output conduit 24. The exciter circuit
21 is such that the exciter current brings the measuring tubes into
resonance in regard to the fundamental oscillation F.sub.1 and their
third harmonic F.sub.3as diagrammatically shown in FIG. 4. The
fundamental oscillation F.sub.1 of each measuring tube occurs between
the full line F.sub.1 and the broken line. The amplitude of the
third harmonic F.sub.3 is considerably less than shown and superimposed
on the fundamental oscillation. The measuring signal U.sub.1 is
fed to the one input 25 and the measuring signal U.sub.2 by way
of a conduit 26 to the other input 27 of a phase detector 28 which,
by reason of the phase displacement of the fundamental oscillation
in both measuring signals delivers an uncorrected flow value Q.sub.1
at its output 29. This is based on the known fact that, by reason
of the Coriolis force, the mass of the medium flowing through the
measuring tubes displaces the phase of the tube oscillations initiated
by the oscillator 11 over the tube length. The phase displacement
is most easily determined in that the time difference between the
occurrence of the zero points is found in both measuring signals
U.sub.1 and U.sub.2. This is proportional to the uncorrected value
Q.sub.1 of the mass flow.
By reason of temperature fluctuations or solely its clamping, the
meter clamped in position by its connectors 9 and 10 undergoes axial
loading. The axial stresses caused thereby likewise lead to a change
in the oscillating behaviour, so that the uncorrected flowQ.sub.1
is in error. For this reason, a part of the exciter circuit 21 forms
a frequency determining circuit 30 which makes available at the
outputs 31 and 32 the determined resonance frequencies f.sub.1 and
f.sub.3 for the fundamental oscillation and third harmonic. The
two frequencies are fed to a correcting circuit 33 which forms a
quotient from these frequencies f.sub.1 and f.sub.3 in a first section
34. By reason of this quotient, a data stor 35 is given a correcting
factor k which is transmitted to a multiplication element 36. Accordingly,
the corrected flow Q.sub.2 =k.times.Q.sub.1 can be indicated in
a display unit 37 or otherwise processed. The upper harmonics are
here designated with an ordinate which is referred to as a fundamental
oscillation with the ordinate 1. By reason of the temperature and
the crosssection of the measuring tubes, the resonance frequencies
of these oscillations are not necessarily in a precise whole number
relationship to each other.
The construction of the exciter circuit is evident from FIG. 5.
Together with the measuring tube system, it forms oscillator means
of which the tube system represents the resonance circuit and the
exciter circuit gives the required loop amplification and feedback.
As a result, the system automatically sets itself to the resonance
frequencies of the tube system. It is therefore possible to resonate
the tube system simultaneously with the resonance frequencies f.sub.1
and f.sub.3 of the fundamental and harmonic oscillations. The measuring
signal U.sub.1 is fed by way of a preamplifier A1 to a fundamental
oscillating branch 38 and a harmonic oscillation branch 39. The
fundamental oscillation branch 38 comprises a phase correcting ciruit
PC1 and an amplifer A2. Since the fundamental oscillation in the
measuring signal U.sub.1 is substantially in phase with the fundamental
oscillation in the exciter current I, Only a slight correction is
necessary in the phase correcting circuit PC1. The harmonic oscillation
branch 39 comprises a high pass filter HPF, a phase correcting circuit
PC2 a selection filter SF and an amplifier A3. The measuring signal
U.sub.1 contains the third harmonic out of phase with the harmonic
in the exciter current I. For this reason, the phase correcting
circuit PC2 effects a phase reversal. The output signal of branch
38 is fed by way of a summation resistor R1 to a summation amplifier
A4 to which there is also fed by way of a summation resistor R2
the output signal of branch 39 which is tapped at a potentiometer
P1 so as to select the ratio of fundamental oscillation and harmonic
in the output signal in such a way, that on the one hand a marked
third harmonic is present in the measuring tube but on the other
hand the evaluation of the phase position of the fundamental oscillation
is not affected in the phase detector 28. The measuring signal U.sub.1
amplified in the preamplifier A1 is also fed to an automatic amplifying
regulator AGC which compares the amplitude of the amplified measuring
signal with a desired value settable at a potentiometer P2 and,
depending thereon, so regulates the amplification of the summation
amplifier A4 that, as is diagrammatically illustrated by a potentiotmeter
P3 in the return circuit, the measuring signal amplitude corresponds
to the desired value. The output value of the summation amplifier
A4 is fed by way of a voltagecurrent transformer U/I and a terminal
stage E to the oscillator 11 as current I.
In order that the harmonic, in this case the third, can be filtered
out cleanly, the high pass filter HPF which blocks for lower frequencies
is supplemented by the selection filter SF of which the mean frequency
determining the filtering function is determined by timing pulses
i.sub.t which are produced by a pulse generator 40 and supplied
by way of a line 41 at a pulse frequency f.sub.t n times the harmonic
frequency f.sub.3. For this purpose, the one input 41 of a phase
locking circuit PLL is connected by way of a comparator K1 which
herein is functionally equivalent to a Schmidt trigger to the output
of amplifier A3 of the harmonic oscillation branch 39 and the second
input 42 is connected by way of a divider T to the output 43 of
the phase locking circuit. The latter conventionally consists of
the series circuit of a phase comparator, a low pass filter and
a voltagecontrolled oscillator. The pulse frequency f.sub.t is
a whole number multiple of the harmonic frequency f.sub.3. N can
for example have the value 64. With the aid of potentiometers P4
and P5 the selection filter SF can additionally be set. It is a
socalled tracking filter, for example of type MF 10 by Messrs.
National. Because the mean frequency of the selection filter SF
follows the resonance frequency f.sub.3 of the harmonic, one ensures
that the filter is very accurately tuned to this frequency f.sub.3
i.e. the third harmonic is amplified whereas all other frequencies
are heavily damped.
A starter circuit A4 comprises a logic circuit 44 with two NAND
elements N1 and N2. The NAND element N2 feeds the summation amplifier
A4 by way of a third summation resistor R3 with randomly occurring
square pulses whenever square pulses are present at the output 45
of comparator K1 and it is simultaneously indicated by the occurrence
of a signal 0 at a further output 46 of the phase locking circuit
PLL that no phase locking has as yet taken place. On the other hand,
if the signal 1 occurs at output 46 on locking, i.e. during normal
operation, the NAND element N2 remains blocked. The irregularly
occuring square pulses produce an oscillation at varying frequencies.
By reason of the construction of the exciter circuit 21 the fundamental
and third harmonic oscillations will soon predominate, so that normal
operating conditions are rapidly attained.
In such an exciter circuit 21 the frequency determining circuit
30 can have a very simple construction. The output 31 need merely
be connected by way of a comparator K2 to the output of amplifier
A2 in the fundamental oscillation branch 38 and the output 32 to
the output 45 of the comparator K1 of the harmonic oscillation branch
39. Square pulses of resonance frequency f.sub.1 of the fundamental
oscillation will then occur at output 31 and square pulses of the
resonance frequency f.sub.3 of the third harmonic at output 32.
The function for determining the correcting factor k is easily
determined experimentally in the following manner. First, in two
attempts one ascertains the resonance frequencies for the fundamental
and harmonic oscillations in dependence on the axial force loading
the measuring tubes, the axial force preferably being standardised
to Euler's bending force. This shows that both frequencies change
but the resonance frequency of the fundamental oscillation much
more so than that of the harmonic. If one interlinks these two frequencies
in any formula, for example by forming a ratio, one obtains a clear
relationship to the instantaneous axial loading condition. If in
a further test series, the axial force is varied at constant mass
flow, one obtainsstarting from the unloaded conditiona correcting
factor k which depends on the axial force. With the aid of both
tests, one can therefore interlink this correcting factor and the
two resonance frequencies in a function. This function may be stored
in the store 35.
Instead of the correcting factor k, one can use an additive correcting
value if the correcting circuit 33 is fed with the value for the
uncorrected flow Q.sub.1.
To determine the axial force and the correcting value dependent
thereon, one can also use the resonance frequencies of oscillations
other than the fundamental or third harmonic. In particular, one
can use the second harmonic for this purpose but this requires excitation
at a position other than the middle and thus a higher excitation
energy. At higher harmonics, one has to make do with smaller oscillating
amplitudes.
