U.S. patent number 3,736,532 [Application Number 05/162,583] was granted by the patent office on 1973-05-29 for ultrasonic delay lines.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the Navy. Invention is credited to Anthony E. Armenakas.
United States Patent |
3,736,532 |
Armenakas |
May 29, 1973 |
ULTRASONIC DELAY LINES
Abstract
Delay lines with a variety of delay versus frequency
characteristics are obtained by fabricating the delay line
apparatus from multilayer materials. In one modification, the delay
line is of rod construction and in another, of strip design.
Inventors: |
Armenakas; Anthony E.
(Beachhurst, NY) |
Assignee: |
The United States of America as
represented by the Secretary of the Navy (Washington,
DC)
|
Family
ID: |
22586262 |
Appl.
No.: |
05/162,583 |
Filed: |
July 14, 1971 |
Current U.S.
Class: |
333/145 |
Current CPC
Class: |
H03H
9/0222 (20130101); H03H 9/40 (20130101) |
Current International
Class: |
H03H
9/40 (20060101); H03H 9/00 (20060101); H03h
009/30 () |
Field of
Search: |
;333/3R,3M,71,72,32 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Smith et al., "Dispersive Rayleigh Wave Delay Line Utilizing Gold
on Lith Niobate," MTT17, 11-69, pp. 1043-1044. .
Electronics, "Microwave Acoustics Surfacing," Electronics 12-23-68,
pp. 95-96. .
Matthews et al., "Observation of Lone Wave Propagation at UHF
Frequencies," Applied Physics Letters, Vol. 14, 1969, pp. 171-172.
.
Daniel et al., "Velocity Measurements of Elastic Surface Waves in
the Layered System ZNS on Al O ," Applied Physics Letters, Vol. 16,
5-1-69, pp. 331-332. .
Tournois et al., "Use of Dispersive Delay Lines for Signal
Processing in Underwater Acoustics," Jr. Acoustical Soc. of Amer.,
Vol. 46, 1969, pp. 517-531. .
J. deKlerk, "Ultrasonic Transducers-Surface Wave Transducers"
Ultrasonics, 1-1971, pp. 35-48. .
J. E. May, Jr., "Wire-Type Dispersive Ultrasonic Delay Lines," IRE
Trans. on Ultrasonic Eng., 6-1960, pp. 44-53. .
C. C. Tseng, "Elastic Surface Waves on Free Surface &
Metallized Surface of Cds, ZNO & PZT-4," Jr. of App. Physics,
Vol. 38, 1967, pp. 4281-4284..
|
Primary Examiner: Rolinec; Rudolph V.
Assistant Examiner: Punter; Wm. H.
Claims
What is claimed is:
1. A solid delay line comprising, in combination,
a first cylindrical shell member made of a material that is capable
of propagating elastic waves;
an input interdigital electrode transducer secured to the inner
wall surface of said shell member at a position adjacent one end
thereof;
an output interdigital electrode transducer secured to the inner
wall surface of said shell member at a location adjacent the other
end thereof;
said transducers being in alignment such that an elastic wave
excited by said input transducer is subsequently detected by said
output transducer;
a second cylindrical shell member made of a different material that
is also capable of propagating elastic waves,
the inner surface of said second cylindrical shell member being
bonded to the outer surface of said first inner cylindrical shell
member over their common length,
the thickness of said first cylindrical shell member being such
that said elastic wave also travels through portions of said second
cylindrical shell member.
2. A delay line comprising in combination
a solid rod made of a first material that is capable of propagating
elastic waves;
an input transducer secured to one end of said rod;
an output transducer secured to the other end of said rod;
a first cylindrical shell member having a length less than that of
said rod bonded to the outer surface of said rod,
said shell member being made of a second material that is also
capable of propagating elastic waves;
a second cylindrical shell member having a length less than that of
said first cylindrical shell member,
said second cylindrical shell member being bonded to the outer
surface of said first cylindrical shell member and being made of a
material that is capable of propagating elastic waves,
The lengths of said rod, and said first and second cylindrical
shell members and the density and stiffness ratios of said rod and
said first and second cylindrical shell members being selected to
achieve the signal time delay desired.
3. A solid delay line comprising in combination
a pair of unequal length strips made of different metals that are
capable of propagating elastic waves;
said strips being bonded together over their common length and
having a thickness such that any elastic wave excited in the longer
strip travels also within the shorter strip when it reaches one end
of this strip;
an input transducer secured to one end face of the longer
strip;
an output transducer secured to the other end face of said longer
strip;
a third strip having a length less than the longer strip of said
pair and bonded to said longer strip over its length,
said elastic wave also traveling within said third strip when it
reaches one end thereof;
the lengths of said strips and their density and stiffness ratios
being selected to obtain the desired signal time delay.
Description
The present invention relates generally to electro-acoustic
apparatus and, more particularly, to guided wave delay lines of the
dispersive and nondispersive type wherein the delay versus
frequency characteristic may have a range of possible slopes or
curvatures.
In the conventional acoustic delay line, a piezoelectric transducer
transforms an electrical signal into a mechanical deformation which
then propagates as an elastic wave along a prescribed path through
the delay medium. In the usual case, such as the simple rod-type
delay line, the elastic wave propagates essentially as a plane wave
in an infinite medium, free from any surface interactions.
In the guided wave acoustic delay line, the cross-sectional
dimensions of, for example, a wire or a rectangular strip are so
chosen relative to the wavelength that the elastic wave interacts
strongly with the boundary surfaces and propagates as a guided
elastic wave motion. Thus, there exists many possible modes of wave
propagation and, in most of these modes, the phase velocity varies
as a function of frequency. In this sense, the delay lines
utilizing these modes are termed "dispersive."
There are, however, exceptions to the above in the case of
zeroth-order modes corresponding to thickness shear in a thin,
rectangular strip and torsional in a small diameter rod. These
zeroth-order modes are nondispersive and, below the cut-off
frequency of the lowest dispersive mode, they propagate as isolated
modes of elastic wave motion. These last two modes thus can operate
without an objectionable signal distortion up to this cut-off
frequency. Additionally, the low velocity of propagation of the
torsional mode, as compared, for example, to the first longitudinal
mode, renders this mode most suitable for nondispersive delay lines
where long delay times in the order of milliseconds are
required.
One of the most important applications of a dispersive guided wave
delay line with a linear delay characteristic is in radar systems
for increasing the range without necessitating a corresponding
increase in peak power. In the usual pulse radar system, range is
increased by increasing the average power radiated while range
resolution is increased by decreasing the pulse length. To increase
range without compromising resolution requires an increase in peak
power which is ultimately limited by voltage breakdown in the
system. One solution to this problem involves the pulse compression
system which operates on the basis that when a short pulse is
transmitted to a linear delay network of positive slope the various
components of the Fourier frequency spectrum of the pulse are
linearly dispersed in time, the higher frequencies being delayed
more than the lower frequencies. The output is a linearly frequency
modulated pulse with an amplitude distribution described by the
function sin x/x. Thereafter, this pulse, when it is returned, for
example, from a remote reflecting target, may be transmitted to a
second delay network having an equal but negative slope so that the
components of the frequency spectrum will be delayed in inverted
order, that is, the higher frequencies being delayed less than the
lower frequencies. Alternatively, the pulse may be compressed by
retransmission through the same delay line used for expanding the
original pulse, provided the order of the frequency is inverted by
modulating with a local oscillator frequency of twice the midband
frequency of the input pulse and selecting the lower sideband of
the modulation products. After such processing, it will be
recognized, the frequency components are restored to their initial
phase relationship and the output pulse will have the same shape as
the input pulse.
In designing a dispersive delay line, made, for example, of a rod
of a given material, the thickness of the rod must be chosen so
that the inflection point of the delay versus frequency curve
occurs at a certain frequency. The linearity and the slope of this
curve may be altered by changing the delaying material. But once
the material is selected, the delay characteristic may be altered
only by changing the length of the rod or by subdividing it into a
series of lengths of different thicknesses. Thus, dispersive delay
lines made of a single material have somewhat inflexible delay
versus frequency characteristics.
It is accordingly a primary object of the present invention to
provide a dispersive delay line with a linear delay versus
frequency characteristic whose slope may be readily selected within
a range of possible values.
It is another object of the present invention to provide a
dispersive delay line with a nonlinear delay versus frequency
characteristic of a desired curvature.
Another object of the present invention is to provide a dispersive
delay line operating in the zeroth torsional mode in rods or in the
zeroth face shear mode, in thin rectangular strips.
Another object of the present invention is to provide a
nondispersive delay line which is capable of delaying high
frequency signals wherein the acoustic signal is propagated as an
interface disturbance.
Other objects, advantages and novel features of the invention will
become apparent from the following detailed description of the
invention when considered in conjunction with the accompanying
drawings wherein:
FIG. 1 is a family of curves showing the variations of the specific
group delay with frequency for rods of different materials;
FIG. 2 is a plot of the same rods but with a different ratio of
core thickness to casing thickness;
FIG. 3 illustrates one modification of the invention wherein the
delay line is composed of concentric, cylindrical members;
FIG. 4 shows an alternative construction utilizing multilayered,
rectangular strips;
FIG. 5 shows the variation of the specific group delay with
frequency of waves traveling in the zeroth face shear mode in a
structure like that of FIG. 4;
FIG. 6 shows an alternative arrangement wherein the mechanical
deformation is in the form of a Stoneley wave; and
FIG. 7 shows a delay line wherein the mechanical deformation is in
the form of a Rayleigh wave.
Briefly and in general terms, the objects of the invention
enumerated above are realized by fabricating the delay line
apparatus from multilayer materials instead of the unitary material
heretofore employed in prior art structures. With multilayer
materials, it will be appreciated, a broader choice of design
parameters are available and a greater selection of delay versus
frequency characteristics may be obtained. Thus, in one
modification, the delay line consists of a plurality of concentric,
cylindrical members made from different materials. The geometrical
and physical properties of the individual members may be selected
to yield the performance curves desired. In an alternative
embodiment, a plurality of rectangular strips are utilized to
achieve the same flexibility. The first construction is utilized
with waves traveling in the zeroth torsional mode or in the first
axisymmetric, nontorsional mode. The second construction is
utilized with waves propagating in the zeroth face shear mode or in
the first longitudinal mode.
Referring now to FIG. 1, the various curves shown illustrate the
variation of the specific group delay with frequency of waves
traveling in the first axisymmetric, nontorsional mode in a
composite rod consisting of a circular core of one material bounded
by and bonded to a circular casing of another material. It will be
seen that all of the curves, which represent different combinations
of materials having various density and stiffness ratios, .rho. and
.mu., possess an inflection point, such as points 1, 2 and 3 in
FIG. 1, and that a linear range of delay versus frequency occurs
over an operating range adjacent to these points. The ratio of core
thickness to casing thickness H is 4.5.
In FIG. 2 this ratio equals one-third, and it will be observed that
the same combinations of materials now yield a different set of
delay versus frequency curves. The inflection points are displaced
from those of FIG. 1, and the linear regions occur at different
frequencies.
A guided delay line making use of the above characteristics is
shown in FIG. 3. The apparatus consists of a solid inner rod or
core member 10 made of a first material. A shorter, circular casing
11 of a second material is bonded thereto, and a still shorter
length of circular casing 12 of a third material is bonded to this
casing. Attached to opposite ends of the central core 10 are the
piezoelectric input and output transducers 13 and 14, respectively.
As is well known, the orientation and construction of these
transducers and the manner in which the input transducer is excited
determine the particular mode excited in the delay line.
Once the variation of delay versus frequency for a composite rod
consisting of a core member of one material, having a circular
casing of another material bonded thereto, such as 10 and 11, has
been established, the addition of a third layer, such as 12, it
will be appreciated, allows an added degree of freedom in the
design of the delay line. Changing the cross section of these
elements, likewise, permits the designing of dispersive delay lines
having a still wider variety of delay versus frequency
characteristics. This may be explained qualitatively by noting that
this arrangement constitutes a series combination of cross sections
and, consequently, the delay at frequency f.sub.i of the rod
assembly is given as
(1) D(f.sub.i) = d.sub.i1 L.sub.1 + d.sub.i2 L.sub.2 + d.sub.i3
L.sub.3 + d.sub.i2 L.sub.4 + d.sub.i1 L.sub.5.
where d.sub.i1 d.sub.i2 and d.sub.i3 are the specific delays of the
various portions of the rod, at the frequency f.sub.i ; L.sub.1,
L.sub.2 045207230 , L.sub.3, L.sub.4, L.sub.5 are the lengths of
the portions. The choice of the materials 1, 2, 3 and the relative
thickness of the layers affects the values of the specific delays
d.sub.i1, d.sub.i2, d.sub.i3. However, the delay versus frequency
characteristic will also depend upon the lengths L.sub.1, L.sub.2,
L.sub.3, L.sub.4, L.sub.5.
The individual components of the delay line may be made of any
material which is suitable for acoustic delay media, such as, for
example, aluminum, nickel-iron alloy, iron, fine grained bronze or
any other fine grained material.
It has been determined by mathematical analysis that the zeroth
torsional mode in a multilayer rod arrangement, such as shown in
FIG. 3, is dispersive. This same mode in a unitary rod, it will be
recalled, is nondispersive. Consequently, this zeroth torsional
mode with its advantages of low signal distortion and low velocity
of propagation may be utilized to provide a new class of dispersive
guided wave delay lines.
In FIG. 4 there is disclosed an analogous multilayer guided wave
delay line fashioned from a plurality of relatively thin,
rectangular strips of different metal. Here, the piezoelectric
input and output transducers 20 and 21 are secured to opposite end
faces of an inner rectangular strip 22. Bonded to its opposite
surfaces are a first pair of shorter rectangular strips 23 and 24.
A second pair of still shorter strips 25 and 26 are bonded to these
strips. Each pair of strips is made of the same material so that
the over-all stepped sandwich has a symmetrical configuration and
composition.
It has also been determined mathematically that the zeroth face
shear mode in a multilayer plate assembly, such as shown in FIG. 4,
is dispersive. This, too, is in contradistinction with the same
mode propagating in a unitary plate of a single material and may be
utilized to provide a new class of dispersive guided wave delay
lines.
It should be appreciated that the consecutive casings, where the
delay line is of a rod design or the consecutive strips where the
line is made of such strips, need not be of shorter length such as
depicted in FIGS. 3 and 4. What is important is that the cross
section of the delay medium changes along its length. It will be
appreciated that the length of proportions L.sub.1, L.sub.2,
L.sub.3, L.sub.4, L.sub.5 and the materials from which these
components are made will be selected in order to achieve the
desired performance curve.
FIG. 5 illustrates the variation of the specific group delay with
frequency of waves traveling in the zeroth face-shear mode in a
three-layer plate construction of the type shown in FIG. 4.
The arrangements as shown in FIGS. 3 and 4 are capable of linearly
delaying pulses having a considerably larger bandwidth than has
been possible heretofore. This improvement is due to the more
extensive linearity of their delay versus frequency
characteristics, as exemplified by the curves of FIG. 5.
The frequencies at which nondispersive delay lines operate without
objectionable signal distortion do not exceed a few megacycles. In
FIG. 6 there is disclosed a composite delay line which is capable
of operating at considerably higher frequencies. The apparatus
consists of a two-layer, rectangular plate made by bonding together
two different strips, 30 and 31, of similar dimensions. The
composite plate is driven by an interdigital electrode transducer
32 which excites a Stoneley wave which propagates as an interface
disturbance. The output signal is removed by a second interdigital
electrode transducer 33. It will be appreciated that the individual
comblike strips that form each transducer are provided with
suitable insulating coatings to protect against shorting by the
confronting surfaces of the two strips. One of the materials that
lends itself to this type of delay line is silicon which exhibits
relatively low losses in the microwave signal region.
FIG. 7 shows an arrangement in which the signal is propagated as an
inner surface Rayleigh wave in a hollow cylinder made of two
concentric shells 40 and 41. One of the conditions for operation is
that the shear velocity in the material of the inner cylinder be
smaller than in the material of the outer layer. The Rayleigh waves
are produced by an interdigital electrode transducer 42 affixed to
the inner wall surface of the inner shell of the hollow composite
cylinder. The output signal is extracted by a similar transducer
located at the other end of the cylinder.
Obviously many modifications and variations of the present
invention are possible in the light of the above teachings. It is
therefore to be understood that within the scope of the appended
claims the invention may be practiced otherwise than as
specifically described.
* * * * *