An electromagnetically excited vibrator with arms carrying baffle
members at their ends that are immersed in the flow to be measured
is mounted at one or more vibration nodes to reduce loss of vibratory
energy to the casing of the flow meter. A flat magnetically conducting
cross shape can be mounted at a node in the middle, provided each
cross beam of the cross is vibrated in flexure, and the two cross
beams vibrate in phase opposition (counter stroke). Such a vibrator
is excited by a similarly crossed pair of E-shaped cores with exciter
and secondary windings on the middle legs of the cores, or by a
coaxial re-entrant core with permanent magnet wafers set in the
rim at respective locations separated by an air gap from the arms
of the cross. In each case the vibrator can be firmly affixed to
the central leg or legs of the core. A two-ended rod system can
be excited by magnetostriction, operating differentially on a split
middle portion of the core, while the latter is supported at two
nodes located between the split middle portion and the respective
ends that carry baffle members. Evaluation circuits in all cases
measure the vibration damping caused by fluid flow.
1. Throughout meter for flowing media comprising an electromagnetically
vibratory body having portions thereof located in a flow duct so
as to be damped to an extent dependent on the flow throughput in
the duct and a magnet system for exciting the vibrations of said
body and for compensating for the damping of said vibrations, said
meter further comprising the improvement wherein:
said vibratory body (6) is made of magnetically conducting material
in cross shape with beams (42) carrying baffle members (39) at their
said magnet system is arranged to vibrate said beams in flexure
so that the crossed beams vibrate with the ends of both beams vibrating
in the same sense as the result of excitation alternately attracting
in opposite directions first the ends of one beam and then the ends
of the other, the one beam returning by spring action while the
other is attracted.
2. Throughput flow meter as defined in claim 1 in which a tubular
casing is provided for guiding a flowing medium past said baffle
members of said vibratory body, and in which said vibratory body
is disposed centrally in and transversely of said tubular casing
and is connected at the midpoint of its cross shape to a support
fixed with respect to said casing.
3. Throughput flow meter as defined in claim 2 in which said magnet
system comprises two E-shaped cores (12) arranged with the pole
surfaces at the ends of their respective middle legs facing each
other on opposite sides of said vibrating body and having their
outer poles arrayed with respect to each other at the same angle
at which said beams of said vibratory body cross each other, said
middle legs of said cores each carrying coils (45) and said coils
being held in fixed relation to said casing.
4. Throughput flow meter as defined in claim 3 in which the middle
portion of said vibratory body is held between said pole surfaces
of said middle legs of said cores (12) and in which the pole surfaces
on the outer legs of said cores are separated by an air gap from
the adjacent beam portions of said vibratory body.
5. Throughput flow meter as defined in claim 4 in which permanent
magnets are set in the ends of said middle legs of said cores for
providing a magnetic bias flux.
6. Throughput flow meter as defined in claim 4 in which permanent
magnets are set in the ends of the outer poles of said cores for
providing a magnetic bias flux.
7. Thoughput flow meter as defined in claim 3 in which a driving
and evaluation circut is provided for said magnet system and said
vibratary body comprising:
means for generating two alternating voltages of the same frequency
differing in phase by 90 and for applying said two alternating voltages
respectively to excitation windings (1213) of said respective coils
secondary windings (1415) provided respectively in said coils
(45) and means for summing the outputs of said secondary windings,
a phase discriminator circuit (18) for comparing the phase of the
output of said summing circuit with a wave derived from said means
for generating said alternating voltages and for producing therefrom
a measurement signal.
8. Throughput flow meter as defined in claim 3 in which there is
provided an excitation and evaluation circuit comprising
exciter windings (1213) included in said coils (45) on said middle
secondary windings (1415) included in said coils (45) on said
middle core legs;
means for generating alternating electric voltages of the same
frequency and differing from each other by 180 in phase;
means for superimposing a constant d.c. current on waves produced
respectively by said alternating electric voltages and supplying
the resultant waves and currents respectively to said exciter windings;
means for summing the voltages induced in said secondary windings
a phase discriminator circuit for comparing the phase of the output
of said summing means to a wave derived from said means for providing
said alternating voltages and for thereby producing a measurement
9. Throughput flow meter as defined in claim 7 or claim 8 in which
a bandpass filter (17) is interposed between said summing means
(16) and said phase discriminator circuit (18).
10. Throughput flow meter as defined in claim 1 in which said magnet
system includes an electromagnet having a coaxially reentrant cup
core (26) on the reentrant portion of which are provided an exciter
winding (27) and a secondary winding (28) and into the rim (44)
of which permanent magnets (29) are set opposite the respective
portions of said beams of said vibratory body passing adjacent thereto
across an airgap therefrom.
11. Throughput flow meter as defined in claim 10 in which a protective
body shaped convexly in a surface of revolution on the upstream
side thereof is provided upstream of said electromagnet.
12. Throughput flow meter as defined in claim 11 in which said
protective body (31) is connected to said rim portion (44) of said
electromagnet and is provided with slots (45) for passage of said
beams (42) of said vibratory body so that said baffle members may
be exposed to fluid flow.
13. Throughput flow meter as defined in claim 10 in which said
electromagnet is connected for excitation and evaluation purposes
in a circuit comprising: alternating voltage wave generating means
(7) connected to a controlled current source (32) for providing
an alternating current in said exciter winding (27);
a phase discriminator circuit (18) for comparing the phase of the
output voltage of said secondary winding (28) with the phase of
a wave derived from said alternating voltage wave generating means
and producing a measurement signal.
14. Throughput flow meter as defined in claim 13 in which a bandpass
filter (17) is interposed between said secondary winding (28) and
phase discriminator circuit (18).
15. Throughput flow meter as defined in claim 7 8 13 or 14 in
which for calibration purposes a phase shifter (19) is interposed
in a reference voltage supply to said phase discriminator circuit
This invention concerns a mass throughput meter for measuring the
flow of a fluid by means of a vibrating device that is damped in
a manner that varies with the rate of flow.
Flow rate meters utilizing a vibrating baffle member are described
in U.S. Pat. No. 4024759. Excitation of the flow meters there
described require a large amount of energy, however. For reliable
operation of the flow meter an external current source is required
from which much energy may be drawn. This requires great energy
consumption which is particularly not to be neglected if a multiplicity
of flow meters are to be operated. Also, when installed in vehicles,
the known devices are disadvantageous because they unduly load the
The Invention. It is an object of the present invention to provide
a flow meter having low energy consumption that is nevertheless
reliable and accurate.
Briefly, in a first embodiment, an electromagnetically vibratory
body made of magnetically conducting material in cross shape and
carrying baffle members at the ends of the cross is excited by a
magnet system into vibration of the beams of the cross in flexure,
so that both ends of both beams vibrate in unison. This system of
vibration porvides a single node in the center where the vibrating
body can be mounted.
In another embodiment, a magnetostrictive rod structure with baffle
members at its two ends is magnetically excited to vibrate in flexure,
in this case being mounted at two nodal points and having a split
middle portion composed of two parallel legs each wound with a coil
and provided with remanent magnetization in such a way that passing
alternating current through the coils will vibrate the rod structure
in flexure in a plane passing through the axis of the duct through
which the flow medium passes.
The flow meters of the invention have the advantage that only a
very small amount of vibration energy, and therefore only a small
supply current, is necessary for operation. Furthermore, the measuring
head located in the fluid flow can be made relatively sturdy and
resistant to shock. The coil windings acting as sources for the
measurement signals, moreover, have small internal resistance, so
that the signals have little vulnerability to electrical disturbances
in the neighborhood. Finally, the flow meter of the invention can
be made very small in size, and no temperature or heat dissipation
problems are presented.
In the case of the above-mentioned first embodiment, the cross-shaped
vibrator can conveniently be connected to a fixed structure in the
casing at the mid-point of the cross. This assures that little vibration
energy leaks away to the casing of the flow meter. For excitation
of the vibrator it is advantageous to use E-shaped cores, on the
middle legs of which the windings are located, the middle legs being
aligned on opposite sides of the vibrator with mutually facing pole
surfaces, while the outer poles of the cores are angularly displaced
by an angle corresponding to the angle between the beams of the
vibrator cross. The cores, like the middle of the cross, can be
fixably mounted in the casing. With this arrangement, the vibrator
is particularly easy to excite, and the construction is sturdy and
little subject to disturbance. In this construction, it is advantageous
to hold the middle of the vibrator cross between the pole surfaces
of the middle legs of the core, while an air gap is provided between
the pole surfaces of the outer core legs and the vibrating surfaces
of the beam strips of the core. Such an arrangement provides a simple
mounting of the vibrator, with respect to the housing, and a reliable
In the above-described embodiment, it is further advantageous,
in a first modification thereof, to provide permanent magnets connecting
each central core leg tip with the middle of the vibrator cross.
Such permanent magnets make it unnecessary to provide magnetic bias
with direct current.
In another modified embodiment, it is useful to constitute the
electromagnetic core in the form of a cup core with a re-entrant
central leg coaxially disposed, on which the exciter winding and
a secondary winding are provided, while permanent magnet wafers
are set in the rim portion of the core, opposite the arms of the
cross-shaped vibrator. A device of this coaxial configuration is
easy to manufacture and has the advantage that the coils are in
a protected location. For further protection, it is desirable, if
the open side of the cup core faces upstream, to provide a more
or less hemispherical dome to cover the magnet on the upstream side.
Such a dome is advantageously connected to the rim of the cup core
and slotted for passage of the beam strips of the cross-shaped vibrator.
This provides a particularly good degree of protection for the vibrator
The excitation of the flow meter in the various embodiments utilizing
E-shaped cores may be produced with a generator operating at audio
or supersonic frequency that furnishes two alternating voltages
having a relative phase shift of 90.degree., which respectively
energize the excitation windings of the two E-shaped cores. The
voltages produced in the secondary windings are then summed and
supplied to a phase discriminator to which a reference voltage from
the generator is provided. The output signal of the phase discriminator
is then a measure for the mass flow of fluid.
Another possibility for excitation of such an electromagnet arrangement
is to generate alternating voltages having a relative phase shift
of 180.degree. and to superimpose a constant direct current on the
alternating currents provided by these alternating voltages and
respectively supplied to the exciter coils of the cores. In this
case again, the voltages induced in the secondary windings are summed
and supplied to a phase discriminator. With the use of permanent
magnets set in the magnetic circuit, the superimposition of a direct
current becomes unnecessary. Furthermore, in the embodiments using
a single coaxial core with magnets set in the rim, a single exciter
winding is sufficient to excite the cross-shaped vibrator in a counterstroke
mode when each diammetral pair is made up of magnets poled the same
way and the pairs are oppositely poled.
For suppression of disturbance, it is desirable to provide a bandpass
filter between the summing circuit and the phase discriminator.
For zero setting of the flow meter scale (hereinafter referred to
as calibration, although this is not a complete calibration operation),
it is desirable to interpose a phase shifter between the phase discriminator
and the reference voltage output of the oscillator that generates
the alternating voltage for the system. Another useful circuit for
excitation of the flow meter is to utilize an oscillator or a similar
generator for controlling a controllable current source that supplies
current to the exciter winding. The induced voltage in a single
secondary winding of the coaxial core is, again, supplied to a phase
discriminator to supply a suitable output signal for the flow meter.
In the magnetostrictive embodiment of the invention mentioned above,
it is practical to provide two bores through the rod structure at
nodal points, into which mounting bearing pins or wires can penetrate.
This configuration also makes it possible to prevent any appreciable
amount of vibratory energy going off to the casing. It is preferred
to pass wires of hard spring metal through the bores and to anchor
the wires in bearing posts mounted on a support structure, which
may conveniently be a cylindrical plate or block. A simple support
bearing structure for the magnetostrictive rod structure is thereby
provided. If, as above mentioned, the magnetostrictive vibrator
is split in the middle into two cores, it is advantageous for excitation
to provide a coil on each branch. It is then possible with the same
coils to provide, before putting the device in service, a remanent
bias magnetization of the core. In service, the alternating currents
are supplied to these coils to excite flexure vibrations magnetostrictively.
The cylindrical block preferred for mounting the vibrator can be
reliably held in position in a flow duct casing by means of spokes.
A more or less hemispherical shell can then cover the exposed side
of the vibrator, and the side so covered can conveniently be made
the upstream side. The vibrator is then protected from damage by
any solid particles contained in the flowing medium .
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is further described by way of illustrative example
with reference to the annexed drawings, in which:
FIG. 1 is a basic diagram, more or less in perspective, with a
tubular casing broken away and supporting structures omitted, for
a first embodiment of the invention;
FIG. 2 is a block diagram of a first embodiment of a circuit for
driving the vibrator of FIG. 1;
FIG. 3 is a block diagram of another circuit for driving the vibrator
of FIG. 1;
FIG. 4 is a diagrammatic side elevation view of a modification
of the kind of vibrator and driving mechanism shown in FIG. 1.
FIG. 5 is a cross-sectional view, and
FIG. 6 a top view, partly in section, of another embodiment of
a throughput flow meter vibrator according to the invention;
FIG. 7 is a circuit block diagram of a circuit for use with the
apparatus of FIGS. 5 and 6;
FIG. 8 is a section along the axis of the stream of flow, and
FIG. 9 is a section perpendicular thereto, of another embodiment
of flow meter according to the invention, in this case utilizing
magnetostriction for vibration, and
FIG. 10 is a side view of the magnetostrictive vibrator of the
device of FIGS. 8 and 9 and
FIG. 11 is a modification of FIG. 4 showing an embodiment similar
in behavior to that of FIG. 4.
DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
The flow of mass of a medium can be directly determined by means
of resistance bodies. For this purpose, a baffle body is put into
vibration parallel to the axis of a tube through which the fluid
flows and in which the baffle body is located, the vibrations being
produced at constant amplitude, and the supplementary damping occurring
as the result of the flow of the medium is observed. Further details
regarding the manner of operation of throughput meters utilizing
vibrating baffle discs or plates are to be found, for example, in
U.S. Pat. No. 4024759. The excitation of the vibrating baffle
disc still presents difficulties. An electromagnetic excitation
of the vibrating baffle disc has the advantage that relatively sturdy
and impact resistant construction elements can be used. Furthermore,
the measurement signal sources then coming into consideration have
small internal resistance, so that the signals are therefore only
slightly vulnerable to disturbances. The devices described below
are so constructed that only very little vibration energy leaks
off to the casing of the measuring instrument.
In FIG. 1 a cross 6 of flat spring steel of high magnetic permeability
serves as the vibrator. It has strip cross beams 42 on the ends
of which are baffle discs or plates 39 and the cross strips are
so excited that they move in a counter-stroke fashion. In consequence,
the midpoint of the cross 6 is a vibration node and can therefore
be fixed firmly to the casing 3 without energy loss. The casing
3 is tubular in shape and serves for guiding the fluid of which
the rate of flow is to be measured. Only the internal mechanical
losses of the spring material and the surrounding fluid contribute
to the damping of the vibrator 6. The baffle discs 39 vibrate in
the annular gap between the tubular casing walls of the casing 3
and a cylindrical covering of the central part and of the electromagnetic
system. It is to be considered in this connection that a pressure,
which is always in the same sense or direction, is exerted by the
flowing medium on the baffle discs, which the cross beam strips
42 must withstand without any great amount of bending. In spite
of the stiffness necessary for the strip beams 42 for this purpose,
small excitation forces are sufficient, because the vibrating cross
6 is driven in resonance.
For excitation of the vibrating cross 6 there are two E-shaped
laminated cores 1 and 2 of a highly permeable material that are
disposed in axial symmetry with their pole surfaces facing eath
other, but shifted with respect to each other by 90.degree.. These
cores are affixed in the casing 3 through which the fluid, of which
the flow is to be measured, passes. The middle legs of the cores
are provided with coils 4 and 5 and the pole surfaces of the central
legs confine between them the mid-portion of the cross-shaped vibrator
6 while the pole surfaces of the outer core legs in each case stand
at a small spacing from a beam strip of the cross. For excitation,
an alternating voltage is applied to the windings of the middle
core legs. The resulting currents produce alternating flux and,
resulting therefrom, variable pulling forces between the pole surfaces
and the beams 42 that vary periodically about a finite mean value.
Since the forces have effect comparable to the square of the exciting
current, the pulsation frequency of the forces is twice as great
as the frequency of the exciting current.
If now the cores 1 and 2 are excited with a phase difference for
one relative to the other which is 90.degree., the forces in the
two cores 1 and 2 operate in counter stroke, i.e., the ends of one
of the crossed beam strips are attracted while the ends of the other,
being released, spring back in the same direction. On the reverse
stroke it is the ends of the last-mentioned beam which are attracted,
while the first-mentioned ends spring back. For indication of the
mass flow of the fluid, a circuit can be used as shown in FIG. 2.
On each of the middle core legs, a supplementary winding must be
provided, in which a voltage is induced which is proportional to
the time rate of change of the magnetic flux. By superimposition
of excitation with the fundamental wave and an air gap variation
by the vibrations of the beams 42 in the second harmonic, there
is produced in the flux a third harmonic component that also appears
in the induced signal, with a phase that depends from the damping
of the vibrator. For phase measurement, a reference signal of the
third multiple of the exciting voltage frequency is required.
The vibration frequency is, as already mentioned, a resonant frequency
of the strips having a baffle plate at their ends, and this is conveniently
in the audio or supersonic range.
As shown in FIG. 2 an alternating current source 7 operating at
six times the fundamental frequency is supplied simultaneously to
two frequency dividers 8 and 9. The first frequency divider 8 divides
the frequency of the generator 7 by 3 while the second frequency
divider 9 divides the frequency by 2 . The output signal of the
frequency divider 8 then goes through an inverter 10 to an additional
frequency divider 11 with the division ratio 2:1 and also directly
to the divider 11a with the same division ratio. In this manner,
there are produced two alternating voltage waves of the fundmental
frequency, one shifted 90.degree. in phase from the other. These
serve for energization of the exciter windings 12 and 13 respectively
corresponding to the coils 4 and 5 of FIG. 1. The voltages induced
in the secondary windings 14 and 15 that are respectively separate
winding parts of the coils 4 and 5 are supplied to a summing circuit
16 and added there, after which the resultant signal goes through
a band pass filter 17 the phase discriminator 18. The alternating
voltage at the output of the frequency divider 9 which has three
times the fundamental frequency, serves as the reference signal
for the phase discriminator 18. This reference frequency wave is
passed through a phase shifter 19 to the phase discriminator 18.
The output signal of the system is available at the output of the
phase discriminator 18. The phase shifter 19 is so set that in the
quiescent state of the measuring fluid, the output signal is zero.
If the vibrator 6 is additionally damped by the flow of the fluid,
a phase shift occurs in the third harmonic of the induced voltage,
and this appears at the output as a voltage well-suited to represent
a measure of the damping and therefore a voltage that can be used
for indication of the mass throughput of the fluid.
A second possibility for exciting the vibrating cross 6 in a counter
stroke motion consists in supplying a constant supplementary direct
current in each of the coils 4 and 5 on which the exciting alternating
currents will be superimposed. Such a system is shown in FIG. 3.
In this case, each of the cores 1 and 2 receives a pulsating flux
without any reversal of sign (i.e., of direction of flux). In consequence,
the attraction force now pulsates at the fundamental frequency (instead
of at twice that frequency). There must now be a difference of 180.degree.
between the phases of the alternating component in the fluxes of
the two magnetic circuits, so that the attraction of one core is
at a minimum while the attraction of the other is at a maximum.
Through the combined effect of excitation and air gap size change,
the second harmonic of the base frequency now appears in the flux.
Since the beam strips 42 vibrate mechanically in unison, with their
drives in phase opposition, this overtone wave is compensated. The
combined signal of the secondary coils contains essentially the
fundamental wave. Its phase depends on the amount of damping.
As in the previous example, here also a phase discriminator is
used to evaluate the effect of damping. FIG. 3 shows the circuit
in a block diagram. The output voltage at the fundamental frequency
is produced by the generator 20 and proceeds through an inverter
21 through a first controlled current source 22 while also being
furnished directly to a second current source 23. The second input
of each of the current sources 22 and 23 is connected to the d.c.
voltage source 24. The d.c. voltage of the source 24 is higher than
the negative peak value of the alternating voltage of the generator
20. The pulsating output currents flow through the exciting windings
12 and 13 of the coils 4 and 5 respectively. They induce secondary
voltages that are made available by the windings 14 and 15 and
these voltages are added together in the summing circuit 16 so
that the sum signal can be supplied through the band pass filter
to the phase discriminator 18. The output voltage of the fundamental
frequency generator 20 provides the reference signal for the phase
discriminator 18 after passing through a phase shifter 19 before
application to the phase discriminator. The output signal of the
system is, again, provided at the output of the phase discriminator
18. The phase shifter 19 is so set when the measuring fluid is quiescent
that the output signal of the phase discriminator 18 is then zero.
Then, if the vibrator is additionally damped by flow of the fluid,
a phase shift occurs in the fundamental wave of the induced voltage
and at the output of the circuit there appears a voltage suitable
for representing the amount of damping that is, accordingly, used
as an indication of mass throughput of fluid.
In a further modified embodiment of the invention, permanent magnets
25 are provided which produce a constant magnetic bias in each of
the magnetic circuits. The flux generated by the windings 12 and
13 is then superimposed on the magnetic bias flux, so that with
suitable poling of the magnets, pulsating fluxes acting in counter-stroke
without the occurrence of any flux reversals. This embodiment has
the advantage that the d.c. voltage source 23 of FIG. 3 can be omitted.
The evaluation of the effect of fluid flow can be made in the same
manner as in the preceding example.
FIG. 4 shows, in section, a flow meter head of the kind shown in
FIG. 1. The laminated cores 1 and 2 in E shape, which are offset
from each other by 90.degree., are here recognizable. The coils
4 and 5 on the middle legs of the cores are also evident. In this
case, however, magnetic wafers 25 are set between the pole surfaces
of the middle legs of the E-shaped cores 1 and 2 and the vibrator
6 in order to provide the magnetic bias above described.
FIG. 11 shows a modification of FIG. 4 in which the magnets 25
are set between the pole surfaces of the outer legs of the cores
1 and 2 in order to provide magnetic bias.
A further embodiment that operates in the same way as the embodiment
illustrated in FIG. 4 is shown in FIG. 5. In this configuration
of FIG. 5 there are no separate E-shaped cores, but instead there
is a coaxial electromagnet with a coaxial iron body 26 which may
be described as a re-entrant cup core, the exciter winding 27 and
a secondary winding 28 surrounding the re-entrant core leg in the
center. The windings are mounted on a spool 43. The cross-shaped
vibrator 6 is mounted on the end surface of the re-entrant central
pole of the core 26. Opposite the strips of the cross-shaped vibrator
6 magnet wafers 29 are set in the rim portion 44 of the core 26.
The electromagnet is held in place centrally in the tubular casing
3 of the flow meter by means of spokes 30 (FIG. 6). For protection
against damage, the vibrator is protected on the upstream side by
a hemispherical dome 31. In this dome, slots are milled for free
passage of the strip cross beams of the vibrator. Among the magnets
set opposite the beam strips, both members of a pair at opposite
ends of the same strip are magnetized in the same direction, but
the polarity of one pair is opposed to the polarity of the other
FIG. 6 shows the device of FIG. 5 as seen from upstream (from
the top with reference to FIG. 5). The spokes extending to the casing
3 for holding the vibrator are plainly visible. The dome 31 intended
to protect the exciter system from damage is shown in the middle.
The baffle members 39 project out of the dome 31 and, of course,
are affixed to or integral with the vibrator 6 the rest of which
is covered by the dome 31.
When an alternating current flows in the exciter winding 27 the
beam strips of the vibrator 6 are excited into counter-stroke vibrations.
Their reaction back on the flux in the core is, however, unidirectional
with reference to the fundamental wave, while the even harmonics
are suppressed. The fundamental wave, therefore, predominates in
the total flux. The induced voltage has a phase shift with respect
to the exciting current, and this phase shift is influenced by the
damping of the vibrator. FIG. 7 shows an example of an evaluation
circuit. The alternating frequency generator 7 acting through a
controlled current source 32 excites the winding 27 of the electromagnet.
No d.c. voltage source is necessary because of the permanent magnets
29. The voltage induced in the secondary winding 28 is filtered
by the band pass filter 17 and then supplied to the phase discriminator
18. The latter has its reference voltage provided through the phase
shifter 19 from the output of the alternating frequency generator
7. At the output of the phase discriminator 18 the measurement
signal is again available. Calibration of the circuit is performed
as already described.
In a manner analogous to piezoelectric mass throughput meters,
it is also possible to excite the vibration by pulses, and in this
case, instead of evaluating the damping by the phase shift produced
by it, the pulse response of the system can be observed and the
width or peak value of the exciting pulse can be so determined that
a vibration of some particular amplitude is maintained. In this
case the parameters of excitation are setting magnitudes for amplitude
control and at the same time a measure of the damping. Further details
of this type of operation are obtainable from the previously mentioned
U.S. Pat. No. 4024759. This method is practical, however, only
for systems with magnetic bias, because only such systems inherently
deliver signal voltages with free vibration of the flexure vibrator.
Magnetostrictively excited vibrators can also be usefully substituted
for the corresponding electromagnetic systems. Magnetostrictive
vibrators have the advantage that they tolerate relatively high
temperatures and mechanical stresses without damage.
As the measuring sensor for flow in such a case, a flexure vibrator
is preferably used that is oriented along a diameter in the casing
of the flow meter, with its ends equipped with baffle members vibrating
in the same sense and in a plane passing through the casing tube
axis. Such a device is shown in FIG. 8.
In a casing 38 a block 36 is held in place by spokes not shown
in the drawing. The block 36 carries bearing supports 35 on which
the vibrator 33 is mounted. In order to reduce the loss of vibration
energy to the casing, the vibrator 33 is, as in the case of the
other examples, mounted on node points. The mounting is constituted
by having holes provided through the vibrator 33 in its node points
for the passage in each case of a spring-hard wire which is taken
up tight at both ends in the bearing supports 35. FIG. 9 which
shows a view from above with the dome 31 removed, allows further
details to be recognized. The magnetostrictive vibrator 33 has the
already familiar baffle members 35 at its ends and is mounted on
the bearing supports 35 by means of a hard spring wire 34 pulled
through the holes in its node points and held at both ends in the
supports 35. The supports 35 are connected with the cylindrical
block 36 of which the outer rim is shown in FIG. 9. The block 36
is held in the casing 38 by means of the spokes 37.
In order to excite a flexure vibration about the intended node
points by means of magnetostrictive forces, magnetic alternating
fields must be produced in the mid-portion of the vibrator, which
preferably consists of an alloy having high magnetostriction and
also a specific resistance that is as great as possible. The alternating
fields produce a periodic prolongation or shortening of the material
through which these fields penetrate. For this purpose, the vibrator
33 is advantageously constituted in the manner shown in FIG. 10.
In its middle portion, the vibrator 33 splits into two parallel
cores individually wound with the windings 40 and 41 respectively.
These two core portions form a closed magnetic circuit. This circuit
is preliminarily magnetized so as to produce a remanent magnetic
field, this being done by means of a sudden current (current shock)
through the windings connected in series. The direction of the magnetization
vectors are shown in FIG. 10 by arrows. According to the sign of
the magnetostriction, there occurs in both core legs simultaneously
a shortening or prolongation of equal magnitude, so that no bending
of the vibrator is produced. After magnetization, the coils are
connected either in series or in parallel, in each case in such
a way that henceforth an applied voltage produces parallel running
supplementary flux permeations in the legs, in directions shown
by the broken line arrows for a momentary value of the voltage.
As a result thereof, the magnetizations in the respective core legs
change oppositely, so that there are produced opposite changes in
length that cause bending of the vibrator. If an alternating voltage
is applied to the coils 40 and 41 a flexure vibration takes place.
The amplitude of the vibration, in addition to depending on the
voltage, also depends upon the ratio of the length of the core legs
to their spacing and is accordingly subject to variation by the
shape of the branching. At the ends of the vibrator 33 the baffle
members 39 which are damped by the flow of fluid, are visible in
the drawing. The value of the inductance represented by the two
coils is now varied in a detectable manner. For example, this change
can be measured by means of a bridge circuit, in order to produce
an indication of the mass flow of fluid.
If the coil system composed of the windings 40 and 41 is excited
by a pulse, free damped vibrations appear as the pulse response
of the vibrator, and the decrement of these damped vibrations provides
a measure of the damping. For throughput measurement, this decrement
can, for example, be ascertained, or else the parameters of excitation
for constantly controlled amplitude of a continuous vibration can
be utilized as a measure of the damping.
Although the invention has been described with reference to particular
illustrative embodiments, it will be understood that modifications
and variations are possible within the inventive concept.