JPH11262087

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DESCRIPTION JPH11262087
[0001]
The invention relates to an electromechanical transducer, a circular flexural oscillator plate
coupled to the electromechanical transducer, and means for acting on the acoustic radiation of
the flexural oscillator plate. The flexible vibrator plate is configured to be excited by the system
operating frequency to be in a high-order flexural vibration state, and in this vibration state, a
node line is formed on the flexible vibrator plate, and the node A first oscillatory antinode and a
second oscillatory antinode, which alternately oscillate in opposite phase, are located between
the lines and an acoustic wave is emitted by the flexible oscillator plate into the transmission
medium in contact with its one side, Alternatively, the flexible vibrator plate relates to an acoustic
transducer that is excited by a sound wave coming from a transmission medium to be in a
flexural vibration state.
[0002]
Acoustic transducers of this type are used, for example, as acoustic transmitters and / or acoustic
receivers for distance measurement by acoustic methods. In this case, the propagation time for
the sound wave emitted from the acoustic transmitter to reach the reflective object and the
propagation time for reflection from the object to return to the acoustic receiver are measured. If
the speed of sound is known, this propagation time is a measure for the distance to be measured.
The frequency of the sound wave may be in the audible range or in the ultrasonic range. In most
cases, distance measurement is performed according to the pulse transit time scheme, whereby
short acoustic pulses are transmitted and echo pulses reflected by the object are received. In this
case, the same acoustic conversion system can be used alternately as an acoustic transmitter and
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an acoustic receiver.
[0003]
A widespread application of such distance measurements using sound waves is the filling level
measurement. For this purpose, the acoustic transducer emits sound waves downward towards
the filling above the measured filling and above the maximum filling level and receives echoes
reflected upward on the filling surface. Mounted as it is. In this case, the measured sound wave
propagation time represents the distance from the acoustic transducer to the surface of the
filling, and if the built-in height of the acoustic transducer is known, the measured filling level
can be calculated from this distance.
[0004]
In order to achieve a long reach in distance measurement using sound waves, an efficient, highperformance acoustic transducer for the purpose of ensuring that the received echo signal
remains at a sufficient intensity for evaluation is necessary. The efficiency here depends mainly
on two factors: Matching of acoustic transducers to the impedance of the transmission medium.
For directional impedance matching of the acoustic transducer during sound wave transmission /
reception, a flexible vibrator plate used in known acoustic transducers is used. The transmission
medium for sound waves in the filling level measurement is a gas, for example air, the same
being true for many other applications. Conventional electro-mechanical transducers, such as
piezoelectric transducers, magnetostrictive transducers, etc., generally have an acoustic
impedance that differs significantly from that of air or other gaseous transmission media. Thus,
in the case of the known acoustic transducers, they excite large-area flexible oscillator plates,
which form an original acoustic transmitter or receiver and produce a good impedance match to
air or other gaseous transmission media. It is only used to
[0005]
A large-area flexible vibrator plate is also advantageous for obtaining the desired directivity. This
is because, as is well known, as the spread of the radiation surface becomes larger than the
wavelength, the radiation lobe bundle becomes narrower. However, in the case of an acoustic
conversion system having a flexural oscillator plate placed in a high-order flexural vibration
state, the interfering problem is that sound waves of opposite phase are emitted by vibration
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antinodes that vibrate in opposite phase alternately. They interfere with each other.
[0006]
In order to avoid such undesirable radiation patterns, according to the magazine "The Journal of
the Acoustical Society of America", Vol. 51, No. 3 (Part 2), p. 953-p. It is known to form the
oscillating flexure plate regions corresponding to each alternately to different thicknesses. The
difference in thickness is chosen such that the sound waves emitted from the thicker regions are
given a phase rotation of 180 °. In this way, the sound waves emitted from all the vibrational
antinodes are in phase, so that the radiation pattern has a maximum radiation formed in the form
of sharply bundled lobes in the axial direction. However, the manufacture of such a flexible
oscillator plate is complicated and costly. Moreover, an acoustic transducer provided with such a
flexible vibrator plate has a very narrow band. The reason is that the 180 DEG phase rotation
only occurs for a very specific frequency which is determined by the structure of the flexible
oscillator plate.
[0007]
According to the acoustic conversion device known from European patent application EP-PS 0
039 986, the flexible oscillator plate regions corresponding to alternating antinodes are again
180 ° in phase with the sound waves originating from every other oscillating antinode. The
rotation is configured to be applied so that the sound waves emitted from all the vibrational
antinodes are substantially in phase. For this purpose, in the corresponding area in the radiation
plane of the flexible oscillator plate, a small loss of acoustically propagating material is attached,
the thickness of which is chosen such that the desired phase rotation is achieved. Closed cellular
foam plastics or non-foam plastics have been proposed as low-loss, acoustically propagating
materials used for this purpose. Such material must be cut according to the shape of the
vibrating antinodes and glued onto the flexure oscillator plate. This leads to problems when the
transducing device is in operation, in particular when it is subjected to mechanical stresses or
chemical effects, as is the case for fill level measurements. Bonded plastic parts can be easily
damaged and have only a slight resistance to many chemically aggressive media. In addition, the
risk of deposit formation due to dusty, powdery or sticky fillings which impair the functionality is
increased.
[0008]
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In the case of an acoustic transducer known from German patent application DE-PS 36 02 351,
an acoustic radiation shaper is provided in order to control the acoustic radiation, which has an
acoustically impervious acoustic blocking member. There is. This member is spaced from the
flexure oscillator plate and further in front of the oscillating antinodes that are acoustically
separated from it and vibrate in phase with each other while oscillating in antiphase with those
antinodes In front of the remaining vibration antinodes, there is a region through which the
sound waves pass. By this acoustic radiation shaper, only the in-phase sound wave is emitted
from the curved transducer plate, while the sound wave of the opposite phase is suppressed by
the sound wave blocking member.
[0009]
SUMMARY OF THE INVENTION The object of the present invention is therefore to provide an
acoustic transducer of the type mentioned at the outset which has good directivity and the action
of disturbing noise, dirt, deposit formation and aggressive media. To be extremely strong.
[0010]
SUMMARY OF THE INVENTION According to the present invention, this object is achieved by
vibration on the back side of the flexible vibrator plate opposite to the transmission medium, in
phase with each other and in antiphase with the first vibration antinode. The solution is achieved
by the fact that one mass ring is mounted concentrically with respect to the center point of the
flexible oscillator plate at the second oscillating antinode.
[0011]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the case of an acoustic
transducer according to the invention, the vibration antinode is made to vibrate with a reduced
amplitude due to the action of a mass ring attached to a second vibration antinode oscillating in
phase. On the other hand, the oscillation amplitude of the first oscillation antinode oscillating in
the opposite phase to this second oscillation antinode becomes large.
Thus, sound waves emitted from alternating vibrational antinodes that are in antiphase and
interfere with each other will have significantly different amplitudes as a result, and as a result,
the weakened sound waves are suppressed and the intensity is significantly higher. Only the inphase sound waves of i.e. propagate perpendicularly to the flexural oscillator plate in the main
radiation direction.
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And by this, a radiation pattern with directivity will be generated in the main radiation direction.
The end face of the acoustic transducer which is exposed to the influence of the surroundings is
then formed solely by the smooth and flat front part in the flexure oscillator plate, while all
means for controlling the acoustic radiation are in the surroundings of the flexure oscillator
plate. It is placed on the back side protected from the influence. This makes the acoustic
transducer very resistant to the action of dirt, deposits and even aggressive media. Therefore, the
acoustic transducer is particularly suitable for use under severe ambient conditions, as can be
found in industrial applications.
[0012]
Advantageous embodiments of the invention are indicated in the dependent claims.
[0013]
The invention will now be described in detail on the basis of examples with reference to the
drawings.
[0014]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The acoustic transducer 10 shown
in FIG. 1 has a casing 112 with a tubular portion 12.
Its one end is sealed by the bottom 13 and is connected at its opposite open end to the widening
14, which is in the form of a shallow dish with a rim 15.
A cable through hole 16 is provided at the opening of the bottom portion 13. The entire casing
11 is rotationally symmetrical about its axis A-A, so that the rim 15 of the flared portion 14 is
circular.
[0015]
Disposed within the tubular portion 12 is an electromechanical transducer 20, which in the
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illustrated embodiment is a piezoelectric transducer. This converter consists of two piezo
elements 21 and 22. In this case, the intermediate electrode 23 is disposed between the two
outer electrodes 24 and 25 in a sandwich-like manner. A sandwich block consisting of the
piezoelectric elements 21 and 22 and the electrodes 23, 24 and 25 is clamped between the
support member 26 and the coupling member 27. Both outer electrodes 24, 25 are electrically
connected to one common connection lead 28. The intermediate electrode 23 is connected to the
second connection lead 29. Therefore, while both piezo elements 21 and 22 are mechanically
connected in series, they are electrically connected in parallel.
[0016]
A thin circular flexure oscillator plate 30 is arranged in the flat spreader 14, which is
mechanically connected to the electromechanical transducer 20 via a rod 31. The rod 31 projects
through an axial hole in a bushing 32 mounted in the center of the flexible oscillator plate 30,
wherein the rod 31 is screwed, for example screwed, pressed, welded or soldered with the
bushing 32. It is fixed in position and connected in an appropriate manner. The flexible vibrator
plate 30 is disposed at a distance from the bottom of the expanding portion 14 of the casing. Its
diameter is somewhat larger than the inner diameter of the rim 15 and somewhat smaller than
the inner diameter of the opening 33 formed in the end face of the rim 15. At the opening 33,
the peripheral portion of the flexible vibrator plate 30 is clamped between the two O-rings 35
and 36 by the holding ring 34. It should be noted that the retaining ring 34 can be attached to
the rim 15 in any suitable manner, for example by screwing, melting, soldering or gluing. O-rings
35, 36 are used to separate the sound propagating in the solid between the flexure oscillator
plate 30 and the casing 11, and by them at the same time the undesirable unknown material
surrounding the periphery of the flexure oscillator plate 30 Is prevented from entering the
interior of the casing 11.
[0017]
The front face 30a of the flexible transducer plate 30 which is in contact with the transmission
medium (e.g. air, to which sound waves are emitted or received therefrom) is completely smooth
and flat. On the other hand, a concentric circular mass ring 40 is attached to the back surface
30b of the flexural oscillator plate 30 opposite to the transmission medium and within the casing
expansion 14 as shown in FIG. It is shown as a plan view in FIG. 2 as a figure. The mass ring 40
can be connected to the flexure oscillator plate 30 in any suitable manner. This can be made
integral with the flexure oscillator plate 30 as well as the central bushing 32 as depicted in the
embodiment of FIG. 1, for example by cutting from a solid metal plate it can. However, it can also
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be manufactured as a separate part, in which case it can be attached, for example by welding,
soldering or bonding on the flexible vibrator plate 30. Again, the mass ring 40 may be made of
metal. The portion of the back surface 30 b of the flexible vibrator plate 30 where the bushing
32 and the mass ring 40 are not provided is covered by the foamed plastic material 41 and the
thickness thereof is smaller than the height of the mass ring 40. The entire other internal space
in the casing 11 is filled with a potting material 42 of high damping plastic, in which part of the
mass ring 40 of foamed plastic material 41 is also embedded. The foamed plastic material 41
prevents the potting material 42 from coming into contact with the flexible vibrator plate 40. The
foamed plastic material 41 can be constructed, for example, of polyethylene or polybutadiene.
For the potting material 42, it is also possible to use a polyurethane-based two-component
casting resin known under the trademark "Nafturan" or a silicone rubber known under the
trademark "Eccosil". .
[0018]
The acoustic conversion device 10 depicted in FIG. 1 is for the purpose of converting the
electrical vibration into a sound wave emitted in the direction of the axis A-A, or alternatively the
sound wave coming in that direction into the electrical vibration. Used for conversion purposes.
In the case of FIG. 1, the transmitting / receiving device is vertically located below the acoustic
transducer, which corresponds to the usual installation if the acoustic transducer is used in
accordance with the sound sounding method for filling level measurement. It is a thing. In this
application case, the acoustic transducer is mounted above the highest filling level, where the
sound wave travels down the air and travels down to the surface of the filling, where it is
reflected and so on It returns to the acoustic converter as an echo signal. The propagation time of
the sound wave gives the distance between the surface of the filling and the acoustic transducer,
and the filling level can be calculated from this distance. The sound waves are usually delivered
as short pulses for transit time measurement, and the time period until the echo pulse arrives is
measured. In this case, the illustrated acoustic conversion device can be used alternately as an
acoustic transmitter and an acoustic receiver.
[0019]
It will be appreciated that the acoustic transducer can be actuated in any direction, for example
for other purposes such as distance measurement.
[0020]
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In all cases, in order to realize a large reach with the best possible efficiency, that is to say that an
echo signal strong enough to be received can be obtained with as little transmission power as
possible. Must meet the requirements.
すなわち、１． The acoustic transducer is well matched to the acoustic impedance of the
transmission medium, eg air.
[0021]
２． The sound beam (sound beam) is focused as sharply as possible in the desired transmission
direction, ie in the direction of the axis A-A.
[0022]
The flexible vibrator plate 30 is used as an acoustic radiator to meet the first requirement. When
an alternating voltage is applied to the electrodes 23, 24, 25 via the connecting leads 28, 29, the
piezo elements 21, 22 carry out a thickness oscillation, which causes a coupled resonance tuned
to the members 26, 27. The device is excited into a longitudinal resonant vibration which is
transmitted to the rod 31 so that it vibrates longitudinally in the direction of the axis A-A. The
system operating frequency, ie the AC voltage frequency, ie the frequency of the mechanical
vibration generated by the piezoelectric transducer, is significantly higher than the flexural
vibration natural frequency of the flexural oscillator plate 30, so that the flexural oscillator plate
30 is excited by the rod 31, It becomes a high-order flexural vibration state. The large-area
flexural oscillator plate 30 in a high-order flexural vibration state provides good impedance
matching to the air or other gaseous transmission medium that is the transmission medium.
[0023]
In order to meet the second requirement, a mass ring 40 mounted on the back surface 30b of the
flexible vibrator plate 30 is used. The function of the mass ring 40 and the effects achieved
thereby will now be described with reference to FIGS. 3 and 4.
[0024]
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FIG. 3 depicts the vibration characteristics of a portion of a conventional type of flexible
oscillator plate excited to a higher order flexural vibration state. This diaphragm according to the
prior art is constituted by a thin metal plate which is smooth and flat on both sides and has a
uniform thickness. The straight line M represents the midplane of the flexural oscillator plate in
the rest position. In the excited state, a concentric nodal line K is formed on the flexural oscillator
plate, which remains in the resting position on the central plane M during oscillation. The
spacing of the nodal line K is determined by the system operating frequency. That is, all nodal
lines have an equal spacing λ / 2, which corresponds to the half wavelength of the generated
bending wave formed on the bending plate 30 at the system operating frequency. Ring-shaped
diaphragm portions are located between the nodal lines K, which alternately form a first
vibrational antinode B1 and a second vibrational antinode B2. All the first oscillation antinodes
B1 oscillate in phase with one another. All the second vibrational antinodes B2 likewise vibrate in
phase with one another but in antiphase with the first vibrational antinode B1. According to FIG.
3, the oscillation states of the oscillation antinodes B1 and B2 at the time corresponding to the
maximum deviation in one direction are drawn in solid lines, while the maximums shifted in the
opposite direction, ie 180 ° phase The oscillating state at the point in time corresponding to the
shift is depicted in dashed lines. In this case, the amplitudes of the shifts are of the same
magnitude for the oscillating antinodes B1 and B2, but are drawn here to be exaggerated for the
sake of clarity.
[0025]
Each oscillating antinode produces an acoustic wave that propagates in the adjacent transmission
medium. However, when it comes to the desired directivity, sound waves formed from adjacent
vibration antinodes have the problem that they are in opposite phase to each other. In this case,
with the prior art acoustic transducing device shown in FIG. 3, those sound waves which are
alternately of opposite phase have the same amplitude, so that with respect to the plane M of the
flexural oscillator plate The sound waves cancel each other in the desired direction of
propagation, which is vertical. With such a distribution of sound waves, directivity formed in an
axial direction perpendicular to the flexural oscillator plate can not be obtained. Rather, the
directional pattern will have strong radial side lobes located concentric to the above axial
direction and other sub-blips that are weaker. As a result of such poor directivity, a large
proportion of the delivered acoustic energy is lost without return to the acoustic transducer,
especially if the measuring distance is very long. The acoustic conversion device also has the
same directivity pattern as that for transmission when receiving.
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[0026]
FIG. 4 shows the vibrational characteristics of the flexural oscillator plate 30 according to FIG. 1
provided with a mass ring 40. The mass rings 40 are arranged as follows. That is, when vibrating
according to the system operating frequency, one mass ring 40 is positioned at the center of
each other vibrational antinode B2, while the first vibration antinode B1 is provided with the
mass rings 40. There is no. As an additional mass is provided, the second vibrational antinode B2
vibrates with a suppressed amplitude about the central plane M of the flexural oscillator plate 30.
The distance between the two nodal lines K at which the second oscillation antinode B2 provided
with the mass ring 40 is located is reduced to λ / 2-Δ, and the first oscillation antinode B1 is
located between The spacing between the two nodal lines K increases correspondingly to λ / 2 +
Δ. As a result, the first vibration antinode B1 vibrates at an amplitude significantly larger than
that of the second vibration antinode, and the sound wave emitted by the first vibration antinode
B1 is correspondingly the second vibration. It will have a significantly greater amplitude than the
sound waves emitted by the antinode B2. Thus, sound waves in antiphase parallel to one another
are no longer completely cancel each other out, ie the sound waves from the first vibrational
antinode B1 are only slightly weakened, while from the second vibrational antinode B2 Sound
waves are completely suppressed. This allows acoustic radiation to be obtained for the acoustic
device of FIG. 1 with directivity formed in the direction of the axis A-A, i.e. perpendicular to the
plane of the flexible vibrator plate 30.
[0027]
The mass rings 40 must be equally spaced, in order to cause the ring-shaped diaphragm parts of
the first vibration antinode B1 located therebetween to vibrate in phase at the same resonance
frequency. The resonant frequency can be varied by the ring spacing and the thickness of the
pattern. Furthermore, it should be noted that the center spacing of the oscillating antinodes is
smaller than the wavelength of the sound waves in air. This is because otherwise structural
interference of the sound waves from the individual vibrational antinodes causes additional
secondary maxima in the directivity characteristic.
[0028]
By slightly offsetting the adjustment of the individual ring diaphragm parts, the radial amplitude
distribution or directivity characteristic can be matched to the predetermined requirements. The
distribution can, for example, be fitted to a Gaussian or Kaiser-Bessel distribution in order to
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reduce the secondary maxima in the directivity characteristic.
[0029]
In the case of distance measurement by pulse acoustics, as already mentioned, the acoustic
transducer is used alternately as a transmitter and receiver. Due to the post-oscillations that
occur each time an acoustic pulse is delivered, the acoustic transducer can not immediately act as
a receiver, so that there is a dead time when echo pulses from nearby targets become
unreceivable become. The shortest measurable distance is referred to as block distance or section
distance. In order to reduce this distance the post-vibration must be kept as short as possible,
which can be achieved by a corresponding damping. In the case of the acoustic transducer shown
in FIG. 1, such damping is preferably achieved in the partially high-damping potting material 42
of the mass ring 40 mounted on the back surface 30b of the flexible vibrator plate 30. Achieved
by embedding. This improves the pulse characteristics of the acoustic transducer and
significantly reduces post-vibration.
[0030]
FIG. 5 depicts a variant embodiment of the acoustic conversion device shown in FIG. In this case,
the difference with the acoustic transducer according to FIG. 1 is firstly that the
electromechanical transducer 20 is not coupled to the flexible oscillator plate 30 via a bushing
attached at its center, but rather to the innermost It is connected via a mass ring 40. For this
purpose, a coupling member 48 is attached to the end of the rod 31 and this member is
connected to the end face of the innermost mass ring 40 opposite to the flexible vibrator plate
30. For this reason, the excitation of the flexible vibrator plate 30 is performed at the second
vibration antinode B2, and is not performed at the first vibration antinode B1 as in the
embodiment of FIG. Since the second vibration antinode B2 vibrates with an amplitude smaller
than that of the first vibration antinode B1, such type of excitation naturally causes the amplitude
conversion of the acoustic transducer, that is, the remarkableness of the acoustic transducer High
efficiency is obtained. Since all mass rings 40 oscillate in phase and with the same amplitude, it is
also possible to connect the electromechanical transducer 20 with a plurality of mass rings 40
via coupling members 48.
[0031]
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Yet another difference between the embodiment according to FIG. 5 and the embodiment
according to FIG. 1 is that a mass ring 50 is also attached to the first vibration antinode on the
back surface 30b of the flexural oscillator plate 30. This is a mass disc 51 which is contracted at
the central vibration antinode. The mass of these mass rings 50 and mass disc 51 is significantly
smaller than the mass of each of mass rings 40. These additional small mass members 50, 51
make it possible to adjust the resonance frequency of the ring-shaped diaphragm part forming
the first oscillation antinode.
[0032]
Both of the above configurations of the embodiment of FIG. 5 that make a difference to the
embodiment of FIG. 1 are independent of one another. That is, the excitation of the flexible
oscillator plate 30 through the mass ring 40 can be applied even when the mass members 50
and 51 are not provided, while the mass ring in the form of the mass ring 50 is used. It is also
possible to attach in the embodiment of FIG.
[0033]
A notable point of this acoustic transducer in all cases is that while the acoustic transducer end
face exposed to ambient influences is formed exclusively by the smooth and flat front portion in
the flexural oscillator plate 30, The means of control are all located on the back side of the
flexure oscillator plate 30 which is protected from the effects of the surroundings. Therefore, the
acoustic transducer according to the invention is very resistant to the action of dirt, deposit
formation and aggressive media.
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