U.S. patent application number 09/730556 was filed with the patent office on 2001-07-05 for method for exciting lamb waves in a plate, in particular a container wall, and an apparatus for carrying out the method and for receiving the excited lamb waves.
This patent application is currently assigned to Endress Hauser GmbH Co.. Invention is credited to Getman, Igor, Hardell, Alexander, Lopatin, Sergej.
Application Number | 20010006318 09/730556 |
Document ID | / |
Family ID | 7933699 |
Filed Date | 2001-07-05 |
United States Patent
Application |
20010006318 |
Kind Code |
A1 |
Getman, Igor ; et
al. |
July 5, 2001 |
Method for exciting lamb waves in a plate, in particular a
container wall, and an apparatus for carrying out the method and
for receiving the excited lamb waves
Abstract
In order to excite Lamb waves in a plate (8) or container wall
(8), it is proposed that an IDT transducer (interdigital
transducer), which is known per se and comprises a layer (1)
composed of piezoelectric material, and on one of whose surfaces
two electrodes (2, 3) which engage in one another like fingers are
applied, be acoustically coupled to the plate (8) or container wall
(8) by means of the surface facing away from the electrodes (2, 3).
If a radio-frequency AC voltage is applied to the connections (6,
7) of the electrodes (2, 3), then a thickness vibration is
initiated in the piezoelectric layer (1). If the speed of
propagation or speed of sound of the longitudinal waves which
result in this thickness vibration in the piezoelectric layer (1)
is matched to the phase velocity with which an so mode Lamb wave
propagates in the material of the plate (8) or of the container
wall (8), then only so mode Lamb waves are initiated by the
resonance effect in the plate (8) or container wall (8), by virtue
of the acoustic coupling between the piezoelectric layer (1) and
the plate (8) or container wall (8). The matching process can be
carried out by selecting the distance (D) between the fingers (4,
5) of the electrodes (2, 3) such that this distance (D) is equal to
half the wavelength .lambda./2 of the thickness vibration to be
initiated in the piezoelectric layer (1).
Inventors: |
Getman, Igor; (Lorrach,
DE) ; Lopatin, Sergej; (Lorrach, DE) ;
Hardell, Alexander; (Lorrach, DE) |
Correspondence
Address: |
Felix J. D'Ambrosio
JONES, TULLAR & COOPER, P.C.
P.O. Box 2266 Eads Station
Arlington
VA
22202
US
|
Assignee: |
Endress Hauser GmbH Co.
|
Family ID: |
7933699 |
Appl. No.: |
09/730556 |
Filed: |
December 7, 2000 |
Current U.S.
Class: |
310/313R |
Current CPC
Class: |
G10K 11/36 20130101;
H03H 9/02228 20130101 |
Class at
Publication: |
310/313.00R |
International
Class: |
H01L 041/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 22, 1999 |
DE |
199 61 857.7 |
Claims
1. A method for exciting Lamb waves in a plate, in particular in a
container wall, according to which an IDT transducer (interdigital
transducer), which is known per se and comprises a layer (1)
composed of piezoelectric material and on one of whose surfaces two
electrodes (2, 3), which engage in one another like fingers, are
applied, is coupled acoustically to the plate (8) or container wall
(8) by the surface facing away from the electrodes (2, 3), and a
radio-frequency AC voltage is applied to the connections (6, 7) of
the electrodes (2, 4), which AC voltage causes cyclically occurring
material compressions and rarefactions, a so-called thickness
vibration, in the piezoelectric layer (1) between the electrode
fingers (4, 5), in which the speed of propagation or the speed of
sound of the longitudinal waves resulting from this in the
piezoelectric layer (1) is matched to the phase velocity with which
an s.sub.0 mode Lamb wave propagates in the material of the plate
(8) or of the container wall (8), in that the distance (D) between
the fingers (4, 5) of the electrodes (2, 3) is selected such that
it is equal to half the wavelength .lambda./2 of the longitudinal
wave to be initiated in the piezoelectric layer (1).
2. The method as claimed in claim 1, according to which the
matching of the speed of propagation or speed of sound of the
longitudinal waves which are to be initiated in the piezoelectric
layer (1) to the phase velocity of the s.sub.0 mode Lamb wave in
the material of the plate (8) or of the container wall (8) is
carried out by varying the distance (D) between the fingers (4, 5)
of the electrodes (2, 3) of the IDT transducer by means of FEM
simulation (finite element method).
3. An apparatus for excitation for receiving Lamb waves in a plate
(8), in particular a container wall (8), in which an IDT transducer
(interdigital transducer) which is known per se and comprises a
piezoelectric layer (1) on one of whose surfaces two electrodes (2,
3) which engage in one another like fingers are applied, is
acoustically coupled to the plate (8) or container wall (8) by its
surface facing away from the electrodes (2, 3), and the connections
(6, 7) of the electrodes (2, 3) are connected to a radio-frequency
AC voltage source, in which case the distance (D) between the
fingers (4, 5) of the two electrodes (2, 3) is equal to half the
wavelength .lambda./2 of the thickness vibration in the
piezoelectric layer (1), which is caused by the piezoelectric
effect when the radio-frequency AC voltage is applied, and the
speed of propagation or the speed of sound of the longitudinal
waves which result from the thickness vibration of the
piezoelectric layer (1) is matched to the phase velocity of an
s.sub.0 mode Lamb wave in the material of the plate (8) or
container wall (8), and in which a further, identical IDT
transducer is arranged in the same way on the plate (8) or
container wall (8), at a distance from this first IDT transducer,
for receiving the initiated Lamb waves.
4. The apparatus as claimed in claim 3, in which each of the
electrodes (2, 3) has at least two electrode fingers (4, 5), which
engage in one another like fingers while maintaining the mutual
separation (D).
5. The apparatus as claimed in claim 3, in which each of the
electrodes (2, 3) of the IDT transducer has more than two electrode
fingers (4, 5), which engage in one another like fingers while
maintaining the mutual separation (D).
6. The apparatus as claimed in one of claims 3 to 5, in which the
piezoelectric layer (1) is composed of a piezoelectric ceramic
material, or a piezoelectric composite material.
Description
The invention relates to a method for exciting Lamb waves in a
plate, in particular a container wall, and an apparatus for
carrying out the method and for receiving the excited Lamb
waves.
[0001] The use of Lamb waves in level measurement devices is known
per se. The Lamb waves caused in a container wall by a Lamb wave
exciter in the form of an electroacoustic transducer propagate as a
cyclically sinusoidal deformation on both surfaces and can be
picked up by a Lamb wave receiver provided at a distance away on
the same container wall, and can be converted back into an
electrical signal, provided the Lamb waves can propagate without
any impediment. If, on the other hand, the propagation of the Lamb
waves is attenuated, which is the situation when the container
contents reach a specific level in the region of the level
measurement device, then the signal is nonexistent or is at least
so heavily attenuated that it can be used as an indication of the
level reached.
[0002] Based on the fact that Lamb waves are composed of shear
waves or transverse waves oscillating at right angles to the
surface and of compression or longitudinal waves oscillating
tangentially, Lamb waves can be initiated in a plate or container
wall of certain elasticity by means of ultrasound waves striking
this plate or container wall obliquely.
[0003] According to one known method, these ultrasound waves which
strike the plate or container wall obliquely can be produced with
the aid of a prism or a prismatic molding, which is mounted on the
plate and on whose incline a piezoelectric element is firmly
arranged and is connected via electrodes to an AC voltage source.
The longitudinal wave (ultrasound wave) initiated in the
piezoelectric element by an AC voltage pulse is transmitted through
the prism to the plate, and strikes this plate at an angle which
depends on the included angle 2 of the prism. The proportionality
of the ratio of longitudinal waves and transverse waves to the
resultant Lamb waves, and thus their amplitudes, is then also
dependent on the included angle 2 of the prism. Waves of different
orders can be excited depending on the thickness D of the plate or
of the container wall and on the excitation frequency .omega., of
which, however, only the zero order (zeroth mode) waves reach the
surface and result in a deformation of the surfaces which
propagates cyclically and sinusoidally and may be symmetrical or
antisymmetrical with respect to the center plane. Zero order
symmetrical Lamb waves are of particular importance for practical
use, and these are referred to as the s.sub.0 mode (zeroth
symmetrical mode). (ULTRASONIC TECHNOLOGY, A Series of Monographs,
General Editor Lewis Balamuth, Cavitron Corporation, New York,
N.Y., RAYLEIGH AND LAMB WAVES, Physical Theory and Applications, I.
A. Viktorov, pages 69, 70 and page 82 paragraph 3).
[0004] Special prisms are thus used in order to determine and/or to
adjust the phase velocity of the relevant Lamb wave and the
wavelength on the surface. Furthermore, in this method for Lamb
wave excitation, the size or the dimension of the so-called
excitation zone on the boundary surface between the prism and plate
or container wall is important, and is governed by the projection
of the area covered by the piezoelectric element on the incline of
the prism onto the boundary surface between the prism and plate or
container wall. The magnitude of the maxima of the excited Lamb
waves is directly proportional to this excitation zone (page 84,
paragraph 2, pages 87 to 88 loc. cit.).
[0005] An apparatus for level monitoring in a container, which
makes use of this method, is described in DE 689 03 015 T2.
[0006] Another method uses the cyclic structures on the effective
surface of a Lamb wave exciter resulting from piston vibration. In
this case, compressions are produced at specific points on the
plate or on the container wall, and their cyclic separation is
governed by the phase velocity of the corresponding Lamb wave.
[0007] One problem in the excitation of Lamb waves in a plate or
container wall by means of prisms is the impedance matching at the
contact surfaces between the prism and the container wall, in which
context it must be remembered that the containers whose level is
intended to be monitored may be composed of widely differing
materials, such as steel, brass, aluminum or else glass etc. When
sudden impedance changes occur between the prisms of the Lamb wave
exciter or Lamb wave receiver and the container wall, interference
effects can occur in the form of parasitic waves or harmonics. The
characteristics of the prisms, which are normally manufactured from
plastic, preferably a polymer, may vary within wide limits,
depending on the density, the concentration of the glass phase, the
flow during pressing etc., so that the prisms must be specially
matched to each application. Furthermore, certain characteristics
of plastic prisms are dependent on temperature to an extent which
cannot be ignored. Air holes which occur in the material during
production of plastic prisms can undesirably influence the
characteristics of the prism. From all this, it follows that
production of plastic prisms is highly complex and costly.
[0008] One object of the invention is to provide a method for
producing Lamb waves in a plate or container wall and an apparatus
for carrying out this method, in which method there is no need for
a prism and in which the phase velocity of the Lamb waves which are
produced and the wavelength can be adjusted and matched easily, for
example in an appropriate manner for different plate or container
wall materials in which the Lamb waves are intended to
propagate.
[0009] According to the invention, this is achieved in that an IDT
transducer (interdigital transducer), which is known per se and
comprises a layer composed of piezoelectric material and on one of
whose surfaces two electrodes, which engage in one another like
fingers, are applied, is coupled acoustically to the plate or
container wall by the surface facing away from the electrodes, and
a radio-frequency AC voltage is applied to the connections of the
electrodes, which AC voltage causes cyclically occurring material
compressions and rarefactions, so-called thickness vibration, in
the piezoelectric layer between the electrode fingers, in which the
speed of propagation or the speed of sound of the longitudinal
waves resulting from this in the piezoelectric layer is matched to
the phase velocity with which an s.sub.0 mode Lamb wave propagates
in the material of the plate or of the container wall, in that the
distance D between the fingers of the electrodes is selected such
that it is equal to half the wavelength .lambda./2 of the
longitudinal wave to be initiated in the piezoelectric layer.
[0010] According to the novel method, no prism is required to
excite Lamb waves in a plate or container wall, so that all the
disadvantages associated with such prisms are also avoided, such as
the material-related temperature dependency or negative influences
caused by air holes that occur in the plastic during production of
the prisms.
[0011] Owing to the relatively good match between the speed of
propagation or speed of sound of the longitudinal waves in the
piezoelectric layer and the phase velocity of the s.sub.0 mode Lamb
wave in the plate or container wall, the acoustic coupling between
the piezoelectric layer and the plate or container wall produces
only an s.sub.0 mode Lamb wave in this plate or container wall
after activation of the IDT transducer. Parasitic interference
waves or harmonics are thus prevented.
[0012] The operating frequency of the IDT transducer as a Lamb wave
exciter must be matched to the distance D between the electrode
fingers and may thus also be varied, that is to say matched to
different plate or container wall materials, without any need for
changes to be made to the dimensions of the piezoelectric layer
itself. Matching by varying the distance between the electrode
fingers of an IDT transducer is considerably easier and more
economic than by varying the included angle of the prism in the
known prismatic Lamb wave exciters. In this case, the measured
effectiveness of the IDT transducer as a Lamb wave exciter is
comparable to that of the conventional Lamb wave exciters, in which
a piezoelectric layer is used to excite the Lamb waves on a polymer
prism having a specific included angle.
[0013] The matching of the speed of propagation or speed of sound
of the thickness vibration in the piezoelectric layer and of the
longitudinal waves resulting from this to the phase velocity of the
so mode Lamb wave in the material of the plate or container wall by
varying the distance D between the fingers of the electrodes of the
IDT transducer can be carried out by FEM simulation (finite element
method).
[0014] An apparatus for carrying out the invention comprises an IDT
transducer (interdigital transducer) which is known per se and for
its part comprises a piezoelectric layer on one of whose surfaces
two electrodes, which engage in one another like fingers, are
applied; the surface of this IDT transducer facing away from the
electrodes is acoustically coupled to the plate or the container
wall, and the connections of the electrodes are connected to a
radio-frequency AC voltage source. The distance between the fingers
of the two electrodes is in this case equal to half the wavelength
.lambda./2 of the thickness vibration or longitudinal wave in the
piezoelectric layer caused by the piezoelectric effect when the
radio-frequency AC voltage is applied; the speed of propagation or
speed of sound of the thickness vibration in the piezoelectric
layer, and of the longitudinal waves resulting from this, is thus
matched to the phase velocity of the s.sub.0 mode Lamb wave in the
material of the plate or of the container wall. A physically
identical IDT transducer which is likewise acoustically coupled to
the container wall and is at a distance from the IDT transducer
which acts as the Lamb wave exciter receives the initiated Lamb
waves and converts them back into an electrical signal.
[0015] According to a first embodiment of the invention, the two
electrodes of the IDT transducer have at least two electrode
fingers, which engage in one another like fingers while maintaining
the mutual separation D.
[0016] According to another embodiment of the invention, the two
electrodes of the IDT transducer have more than two electrode
fingers, which engage in one another like fingers while maintaining
the mutual separation D.
[0017] The piezoelectric layer of the IDT transducer is preferably
composed of a piezoelectric ceramic material or of a piezoelectric
composite material.
[0018] The construction, production and matching of the Lamb wave
exciter according to the invention are considerably simpler and
more cost-effective than in the case of the known prismatic Lamb
wave exciters.
[0019] The Lamb wave exciter according to the invention can be used
in conjunction with a Lamb wave receiver constructed in the same
way as a sensor for level measurement devices.
[0020] The invention will be described in more detail in the
following text with reference to the attached drawings, by way of
example, in which:
[0021] FIG. 1 is a perspective illustration of a first embodiment
of a Lamb wave exciter according to the invention,
[0022] FIG. 2 likewise shows a perspective illustration of a second
embodiment of a Lamb wave exciter according to the invention,
[0023] FIG. 3 shows, schematically, the excitation of Lamb waves by
means of a Lamb wave exciter according to the invention, and their
propagation in a plate or container wall,
[0024] FIG. 4 shows an example of the use of a Lamb wave exciter as
shown in FIG. 1 in conjunction with the Lamb wave receiver
constructed in the same way, as a sensor for level measurement on a
container wall, and
[0025] FIG. 5 uses a graph to show the basic profile of the group
velocity c of s.sub.0 mode Lamb waves as a function of the
operating frequency f times the wall thickness d of a plate or
container wall.
[0026] According to the invention, the IDT structure, that is to
say the structure of an interdigital transducer, is used to excite
Lamb waves in a plate or container wall. As already mentioned, the
s.sub.0 mode of the Lamb waves is of primary importance for
practical use. Specifically, the s.sub.0 mode Lamb wave also has
the best sensitivity with respect to a medium, for example a
liquid, which comes into contact with one side of the plate or the
inner surface of a container and attenuates the propagation of the
Lamb waves depending on the filling level reached. Zero order Lamb
waves (s.sub.0 mode) pass through the entire plate or container
wall and result in both surfaces of the plate or container wall
being deformed with the deformation propagating cyclically and
sinusoidally. In the presence of water, for example, a s.sub.0 mode
Lamb wave propagating in the plate or container wall is subject to
an attenuation which is approximately 6 dB per 1 mm of plate or
wall thickness over a constant frequency range of 2.5-5 MHz*mm and
with a distance of 1 cm (between the Lamb wave exciter and the Lamb
wave receiver). The attenuation of the s.sub.0 mode Lamb wave is
thus particularly pronounced in comparison with that of other Lamb
wave modes. The graph in FIG. 5 clearly shows the sensitivity on
the snout-like profile of the group velocity c of the s.sub.0 mode
Lamb wave as a function of the operating frequency f times the wall
thickness d of the container or of the plate in the desired
operating range a and with the speed of sound c.sub.s; the change
is most strongly pronounced in this region. Furthermore, FIG. 5
shows that the operating frequency f of the excited s.sub.0 mode
Lamb waves and the wall thickness d of the container or of the
plate depend on one another, and influence one another, that is to
say a greater wall thickness d requires a lower operating frequency
f for the s.sub.0 mode Lamb wave.
[0027] The invention is based on the relatively good match between
the phase velocity of the s.sub.0 mode Lamb wave in a material
plate or container wall and the speed of propagation or the speed
of sound of a longitudinal wave resulting from a thickness
vibration in a layer composed of piezoelectric material, preferably
a piezoelectric ceramic material.
[0028] The phase velocity of the s.sub.0 mode Lamb wave is, for
example 3000-4000 m/s in a steel plate; the speed of propagation or
the speed of sound of a longitudinal wave in a layer composed of
piezoelectric ceramic material is 3600-4000 m/s.
[0029] A longitudinal wave or thickness vibration which is
initiated in a piezoelectric layer and has this speed of
propagation or speed of sound will thus cause only an s.sub.0 mode
Lamb wave, due to the resonance effect, in this plate or container
wall, provided there is good acoustic coupling between this
piezoelectric layer and a steel plate or a steel container wall;
interference effects due to parasitic waves or harmonics are thus
precluded from the start.
[0030] FIG. 1 shows a first, simple form of a Lamb wave exciter
according to the invention based on the IDT structure of an
interdigital transducer. Two electrodes 2 and 3, which engage in
one another like fingers, are applied on a layer 1 composed of
piezoelectric material, with a constant distance D being maintained
between the fingers 4 of the one electrode 2 and the fingers 5 of
the other electrode 3. A ceramic or composite material is
preferably used as the piezoelectric material for the layer 1.
These materials have, inter alia, the advantage that the speed of
sound is only slightly dependent on the temperature.
[0031] FIG. 2 shows an expanded embodiment of a Lamb wave exciter
constructed on the basis of the same IDT structure, but which
differs from that shown in FIG. 1 by the number n of fingers 4 and
5 of the two electrodes 2 and 3.
[0032] If an appropriate radio-frequency AC voltage is applied to
the connections 6 and 7 of the electrodes 2 and 3, then a thickness
vibration is initiated in the piezoelectric layer 1, and propagates
as symmetrical surface waves along both surfaces of the layer 1, as
is indicated in FIG. 3. As can likewise be seen from FIG. 3, the
distance D between the fingers 4 and 5 of the electrodes 2 and 3
corresponds to half the wavelength .lambda./2 of the wave-like
surface deformations which result from the thickness vibration of
the piezoelectric layer 1, thus resulting in an n*.lambda./2--times
thickness vibration of the layer 1, which lies in the region of the
excitation resonance for the s.sub.0 mode Lamb wave to be excited
in a plate 8 or container wall 8, where n is the number of fingers
4 and 5 of the electrodes 2 and 3.
[0033] Thus, in order to produce an s.sub.0 mode Lamb wave in a
plate 8 or container wall 8, a Lamb wave exciter as shown in FIG. 1
or 2 is fitted parallel on this plate 8 or container wall 8 with
good acoustic coupling. The AC voltage which is applied to the
electrodes 2 and 3 via their connections 6 and 7 causes a thickness
vibration in the piezoelectric layer 1 which, due to the acoustic
coupling between the piezoelectric layer 1 and the plate 8 or
container wall 8 is transmitted to the latter and thus, owing to
the resonance conditions described above between the speed of
propagation of the longitudinal waves in the piezoelectric layer
and the phase velocity of the s.sub.0 mode Lamb wave in a plate or
container wall 8, produces an s.sub.0 mode Lamb wave in this plate
8 or container wall 8, which then propagates in this plate 8 or
container wall 8 as indicated by the arrows 9 in FIG. 3. A Lamb
wave receiver 11, which is arranged on the container wall 8 at a
distance from the Lamb wave exciter 10 and which, as shown in FIG.
4, is constructed as an IDT transducer in the same way as the Lamb
wave exciter 10, can receive this s.sub.0 mode Lamb wave, provided
this Lamb wave can propagate without being attenuated, and can
convert it back into an electrical signal. The s.sub.0 mode Lamb
wave is attenuated during its propagation by a medium which comes
into contact with the rear face of the plate 8 or the inner surface
of the container wall 8, so that the absence of the electrical
signal at the Lamb wave receiver 11 can be used as an indication
that a specific level has been reached in the region of the
measurement arrangement.
[0034] Since, as mentioned, the wavelength .lambda. of the
longitudinal waves in the piezoelectric layer 1 is determined
directly, in the described method for producing an s.sub.0 mode
Lamb wave, by the distance D between the fingers 4 and 5 of the
electrodes 2 and 3 of the IDT transducer, it is possible to match
the operating frequency of the IDT transducer to the phase
frequency of the S.sub.0 mode Lamb wave in a specific material just
by varying this distance D, and in this way to achieve the
respectively best effectiveness for excitation of the Lamb wave,
that is to say the method and the apparatus for carrying out the
method can be used with containers of widely differing material,
for example composed of steel, bronze, aluminum or else glass,
without having to make any changes whatsoever to the dimensions of
the piezoelectric layer 1 itself. The matching of the distance D
between the fingers 4 and 5 of the electrodes 2 and 3 to the
container material can be determined numerically using the FEM
simulation (finite element method). The measured effectiveness of
the IDT Lamb wave exciter is comparable to that of the conventional
Lamb wave exciters, which use a piezoelectric element in
conjunction with a (polymer) plastic prism having a specific
included angle. Since, according to the invention, there is no need
for a prism, this also avoids the disadvantages associated with
such a prism, which were described initially.
* * * * *