U.S. patent number 3,891,869 [Application Number 05/394,407] was granted by the patent office on 1975-06-24 for piezoelectrically driven ultrasonic generator.
This patent grant is currently assigned to Scarpa Laboratories, Inc.. Invention is credited to Thomas J. Scarpa.
United States Patent |
3,891,869 |
Scarpa |
June 24, 1975 |
Piezoelectrically driven ultrasonic generator
Abstract
Ultrasonic system for measuring fluid flow by comparing the
responses from pulsed beams transmitted obliquely in opposite
directions through fluid flowing in a pipe section. The transducer
assembly includes a driving element comprising a piezoelectric
generator in the form of a disk with a high mass backing element
bonded to one face and an acoustic wave transformer bonded to the
other face. The wave transformer element varies in cross-section in
an axial direction, comprising disks of maximum dimension at the
generator face and at the radiating face, which are separated by a
connecting portion of minimum dimension. In preferred form, the
wave transformer is shaped like a barbell. An advantage of this
configuration is that the beam pattern is a function of frequency,
having a dominant central lobe at about 45 kilocycles per second
which becomes relatively depressed with respect to increasing side
lobes as the frequency is increased. Although the invention has
been described in general terms, the primary uses of the system and
components as disclosed herein are for the measurement of the flow
of gases.
Inventors: |
Scarpa; Thomas J. (Edison,
NJ) |
Assignee: |
Scarpa Laboratories, Inc.
(Metuchen, NJ)
|
Family
ID: |
23558829 |
Appl.
No.: |
05/394,407 |
Filed: |
September 4, 1973 |
Current U.S.
Class: |
310/325;
73/861.27; 116/DIG.19; 116/137A; 310/26; 310/334; 367/152 |
Current CPC
Class: |
G01F
1/66 (20130101); Y10S 116/19 (20130101) |
Current International
Class: |
G01F
1/66 (20060101); H04r 017/00 () |
Field of
Search: |
;310/26,8.2,8.3,8.7,9.1,9.4 ;73/67.5R,67.6,67.8,69,71.5 (U.S./
only)/ ;73/194A,194B,194E ;340/8MM ;259/1 ;116/137A,DIG.19
;181/.5J,.5AG ;51/59SS |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Budd; Mark O.
Attorney, Agent or Firm: Pugh, Esq.; Martha G.
Claims
What is claimed is:
1. A transducer assembly comprising in combination:
an ultrasonic generator which includes a pair of piezoelectric
ceramic wafers having substantially parallel opposite major
surfaces comprising electrode means interposed between the internal
major surfaces of said wafers and on the two external surfaces
thereof, said generator designed to vibrate in the direction of the
longitudinal axis of said transducer assembly in a principal
resonant-thickness mode of vibration.
a cylindrical backing element having a pair of substantially flat
end surfaces,
means interposed between one said end surface of said cylindrical
backing element and one of the opposite surfaces of said generator
for bonding said backing element to said one face,
a solid transforming member formed to include at its opposite ends
a pair of substantially circular parallel disks coaxial with said
ultrasonic generator each having a diameter slightly exceeding the
diameter of said generator, said disks spaced apart along the axis
thereof by a connecting portion symmetrical about the said axis and
declining therebetween to a minimum diameter not exceeding about
one-third the diameter of said disks,
one of said disks being bonded in acoustical transfer relation to
the other face of said generator, and the other said disk being
free to function as an ultrasonic radiator.
2. The combination in accordance with claim 1 wherein the flat
major surfaces of said cylindrical backing element and of said
solid transforming member each have engraved thereon a series of
concentric grooves,
and said bonding means includes an epoxy cement interposed between
said grooves in the one face of said backing element and one of the
outer surface of said piezoelectric generator,
and means including an epoxy cement interposed between said grooves
in the major face of said transforming member and the outer surface
of said piezoelectric generator.
3. The combination in accordance with claim 2 wherein said
electrode means interposed between the major surfaces of said
generator wafers and on the external surfaces thereof comprises
electrically conducting epoxy.
4. The combination in accordance with claim 2 wherein the axial
length of said transducer assembly from a plane bisecting the
thickness of said radiator plate to the other end surface of said
backing element approximates an integral number of
quarter-wavelengths in the principal vibrating frequency of said
transducer, and wherein the diameter of said disks approximates a
half-wavelength in said vibrating frequency.
5. The combination in accordance with claim 2 wherein the ratio
between the mass of said backing element and the mass of said
transforming member including said radiator plate is at least about
1:1.
6. The combination in accordance with claim 1 wherein said
transducer assembly is enclosed in a tubular inner housing, the end
of the cavity of said housing containing the backing element of
said asembly being partially closed by a plug having a central bore
for accommodating a pair of energizing leads connected across
terminals on the electrode means of said ultrasonic generator,
said end plug terminating in an annular flange,
an externally screw-threaded hollow cylindrical outer housing
having an enlarged cylindrical inner chamber constructed to
accommodate the flanged end of said tubular inner housing,
means comprising a series of o-rings of elastomer material
constructed to fit between said tubular inner housing and the inner
chamber of said outer housing for acoustically isolating said
innner housing from said outer housing,
said outer housing having a coaxially disposed extension,
an electrical coaxial connector containing a solid insulating shell
between a pair of conductors comprising a central conductor and an
outer coaxial shell accommodated in the extension of said housing,
said outer shell being in electrical contact with the said external
housing,
means for connecting one of the electrode terminals of said
ultrasonic generator to the said central conductor,
and means for providing an electrical conducting path between the
other electrode terminal of said ultrasonic generator and said
external housing.
7. the combination in accordance with claim 6 wherein said means
for providing an electrical conducting path between the other
electrode terminal of said ultrasonic generator and said external
housing comprises a conducting strap interposed therebetween.
8. The combination in accordance with claim 6 wherein said means
for providing an electrical conducting path between the other
electrode terminal of said ultrasonic generator and said external
housing includes conducting material interposed into said o-rings.
Description
BACKGROUND OF THE INVENTION
This relates in general to ultrasonic flowmeter systems and, more
specifically, to the transducer assemblies employed in such
systems, particularly for measurement of the flow of gas.
In ultrasonic flowmeter systems of a type disclosed in the prior
art, the velocity of fluid flowing in a test pipe section is
measured by transmitting a pulsed ultrasonic beam obliquely across
the pipe section from a transmitting transducer interposed at one
point in the pipe to a receiving transducer interposed at a second
position in the pipe, spaced up or downstream from the first. When
an ultrasonic wave is transmitted through the test fluid, the time
required for such transmission is dependent on the velocity of the
ultrasound in the fluid, the length of the path over which the beam
is transmitted and the velocity at which the fluid is flowing. The
velocity of fluid flow either adds to or subtracts from the
transmission time, depending on whether the beam is transmitted
against or in the direction of the stream of flow. Thus, a feedback
circuit produces a signal having a frequency of repetition which
varies in accordance with the time required for the ultrasound
waves to travel over a predetermined path in the fluid.
Such ultrasonic systems are well-known in the art, as disclosed,
for example in U.S. Pat. No. 2,669,121 to R. L. Garman et al.,
issued Feb. 16, 1954, and by others.
In accordance with prior art practice, the transducers used for
transmitting and/or receiving the ultrasonic beam in such a system
were so constructed that the maximum lobe of the emitted beam was
directed in substantially a normal direction from the face of the
transducer. Thus, in order to provide a beam capable of crossing
the stream of fluid flowing in the pipe at an oblique angle, it was
necessary to build a shoulder or supporting nipple onto the surface
of the pipe. This obviously requires precise modifications in the
pipes to which such a system is applied. Moreover, it creates a
series of recesses in the inner pipe wall, associated with the
obliquely positioned transducers which tend to cause interruptions
or turbulence in the fluid flow being measured.
In addition, particularly in measurement of the flow rates of gases
or low density liquids, problems arise in impedance matching the
radiating member of the transducer assembly to the fluid in such a
way as to avoid unwanted reflections and thereby energy loss.
SUMMARY OF THE INVENTION
Accordingly, it is a principal object of the present invention to
provide an ultrasonic system for measuring fluid flow in pipe
systems which is substantially more sensitive, more efficient and
more economically constructed than the systems of the prior
art.
A particular object of the present invention is to provide an
ultrasonic system for measuring fluid flow in pipes transporting
gases and low density fluids in which the ultrasonic transducing
assemblies are impedance matched to the measured fluid.
Another object of the invention is to provide a system in which
alterations in the test pipe surface for installing transducers are
minimized.
A further object of the invention is to provide a system in which
turbulence and other distortions in the flow of the test fluid are
minimized.
Another object of the invention is to provide an ultrasonic system
including transducer assemblies which are constructed to radiate
narrow, high velocity beams.
Another object of the present invention is to provide an ultrasonic
system including a transducer assembly in which the principal beam
direction with reference to the radiating face may be controlled as
a function of frequency.
These and other objects, features and advantages will be apparent
to those skilled in the art as set forth in the invention
hereinafter.
The present invention relates to ultrasonic systems for measuring
fluid flow in a pipe in which one or more pulsed beams is
transmitted obliquely through the fluid from a transmitting
transducer interposed through the pipe wall in one position to a
receiving transducer interposed through the pipe wall in another
position, the effect of the fluid flow velocity on the pulse
transmission time being computed electronically.
In order to facilitate the operation of such a system, the present
invention contemplates a novel type of transducer assembly which
includes a driving element comprising a longitudinal thickness-mode
piezoelectric generator in the form of a disk having a backing
element of relatively high mass bonded to one face and an acoustic
wave transformer bonded to the other face. The whole assembly is
preferably an integral number of quarter-wavelengths along the
principal axis in the vibrating frequency of the system. The
transformer diminishes in cross-section from a maximum at the bond
with the driving element to a relatively small cross-sectional
dimension along its axis at a parallel plane remote from the bond.
The small cross-sectional dimension is broadened out at the
radiating end to form a center driven radiating disk of relatively
small mass. The diameter of the radiating disk is preferably a
half-wavelength in the operating frequency; and, it is disposed
parallel to the face of the piezoelectric generator and normal to
the axis of the transformer. The mass ratio between the backing
element, which may comprise a metal cylinder, and the transforming
element lie anywhere within the range 2:1 to 30:1, thereby
producing a particle velocity transformation of from 2:1 to 100:1
between the face of the piezoelectric generator and the radiating
disk. The transformer, including the disk, functions to step down
the impedance by increasing the area of contact at the radiating
surface, which moves in small arc vibrations at high velocity.
Thus, the low impedance at the face of the radiating disk is
substantially equal to that of air, or any other low density fluid.
This has the advantage of greatly increasing the efficiency of the
transducer assembly by reducing the energy losses from distortions
and reflections at the interface. A tight beam of small angular
divergence is radiated by transducer assemblies designed in
accordance with the present invention. This permits the ultrasonic
beams so radiated to be transmitted and received over greater
distances than with the same expenditure of energy using prior art
structures.
In accordance with a preferred form of the invention disclosed
herein, the solid energy transforming member may assume a shape
somewhat like a barbell with disks of equal diameter on the two
ends connected by a cylindrical axial portion having a diameter
about one-third that of the end disks. In the disclosed embodiment,
the left-hand disk which is bonded to the face of the piezoelectric
generator is substantially thicker than the radiating disk.
In accordance with a variation, the cross-sectional dimension of
the transforming member decreases exponentially until it reaches a
minimum cross section of, say, one-tenth the diameter bonded to the
piezoelectric element. The minimum cross-sectional plane is then
bonded or otherwise integrally connected in driving relation to the
center of a thin disk about the same diameter as the piezoelectric
generating element. There are numerous other suitable shapes for
the transforming element of the present invention, the only
requirement being that the cross-sectional dimension decrease in a
manner symmetrical about the longitudinal axis from a maximum at
the area of contact with the piezoelectric generator surface to a
minimum along the axis at a plane removed from the generator
surface; and, that the minimum cross-sectional area function as the
central driving means for an expanded radiating surface comprising
a relatively thin disk normal to the axis. Such a transformer
configuration bonded to one face of a piezoelectric generator, to
the other face of which is bonded a backing element of relatively
higher mass, constitutes a transducer assembly with numerous
advantages. It produces a highly efficient narrow ultrasonic beam
impedance matched to gas and low density liquids.
As a further feature of the present invention, the transducer
assembly described in the foregoing paragraphs is supported in a
coaxial acoustically isolating housing for providing electrical
connections between the energizing circuit and the
piezoelectrically active transducer, which may comprise a sandwich
transducer of the bimorph type.
A particular advantage of the structure of the present invention is
that the beam pattern has been found to be a function of frequency.
Whereas at a frequency of about 45 kilocycles per second, the beam
has a dominant central lobe and relatively suppressed side lobes, I
have found that by increasing the operating frequency to about 90
kilocycles per second, the central lobe is depressed and the side
lobes become dominant.
When transducers of the type disclosed herein are incorporated in
an ultrasonic flowmeter of the general circuit arrangement
disclosed by Garman et al., supra, substantial advantages ensue in
that such a flowmeter is especially adapted to measure the flow
rates of gas and other low density fluids with high efficiency.
Moreover, assuming the transducer assemblies are operated in a
frequency range producing dominant side lobes, it becomes
unnecessary to build shoulders or nipples into the pipe wall for
respectively supporting the individual transducer assemblies in an
oblique direction with reference to the axis of the pipe. Instead,
the transmitting and/or receiving transducer assemblies may be
interposed into the test pipe section in directions respectively
normal to the pipe surface. In this type of operation the pulsed
side lobe beams are transmitted and/or received obliquely in the
pipe from two or more identically operated transducers positioned
in obliquely spaced-apart positions so that while one transducer
assembly transmits an obliquely directed beam, another transmitter
assembly is positioned to receive the obliquely transmitted beam,
and vice versa.
Other objects, features and advantages will be understood from a
detailed study of the specification hereinafter with reference to
the drawings of which the following is a short description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an overall perspective showing of the narrow beam
transducer assembly of the present invention;
FIG. 2A is a longitudinal section through the plane indicated by
the arrows 2--2 of FIG. 1;
FIG. 2B is a modification of a section of the left of the plane
x--x in FIG. 2A, including a feedback crystal;
FIG. 3A is an enlarged sectional showing of the backing element 3
of FIGS. 1 and 2;
FIG. 3B is an end elevation showing the face 3b;
FIG. 4 is an enlarged side elevational showing of the backing
element 3 of FIG. 1, showing the recessed area for the screw hole
3d;
FIG. 5A is an enlarged side elevational showing of the acoustical
transformer 2 of FIG. 1;
FIG. 5B is an end elevational showing of the left-hand end face 2a
of the acoustic transformer 2 of FIG. 5A;
FIG. 6 is a showing in perspective of the transducer assembly in
one form of housing for incorporation into a test pipe system;
FIG. 7 is a longitudinal section of the assemblage of FIG. 6;
FIG. 8 is a schematic cross-sectional showing of the transducer
assembly disclosed in FIGS. 1 and 2, indicating the general outline
of the ultrasonic beam radiated at an operating frequency of about
45 kilocycles per second, comprising an enlarged central lobe and
minimal side lobes;
FIG. 9 is a schematic cross-sectional showing of the transducer
assembly disclosed in FIGS. 1 and 2, indicating the general outline
of the ultrasonic beam radiated at an operating frequency of about
90 kilocycles per second, comprising enlarged side lobes, the
central lobe being substantially suppressed;
FIG. 10 is a schematic circuit showing of an ultrasonic flowmeter
of the pulsed beam type employing a rectangular array of four
transducer assemblies of a type shown in FIG. 8, driven at about 45
kilocycles per second, and each characterized by a beam having a
prominent central lobe;
FIG. 11 is a schematic circuit showing of an ultrasonic flowmeter
of the pulsed beam type employing a rectangular array of four
transducer assemblies of a type shown in FIG. 9, driven at about 90
kilocycles per second, and each characterized by a beam having a
pair of enlarged side lobes and a suppressed central lobe; and
FIG. 12 is a schematic circuit showing of an ultrasonic flowmeter
of an alternative from to that indicated in FIG. 11 employing a
triangular array of three transducer assemblies of a type shown in
FIG. 9, driven at about 90 kilocycles per second, and each
characterized by a beam having a pair of enlarged side lobes and a
suppressed central lobe. Detailed Description
Reference will now be made to FIGS. 1 and 2A of the drawings which
show a preferred from of the transducer assembly of the present
invention.
The radiating face 2b of the transducer assembly may be considered
to be a center driven thin plate, vibrating in a flexural mode. The
driving force is a longitudinal or compressional wave that is
converted by the geometry into a flexural, radial wave in the disk.
In such a transition, the phenomenon of velocity dispersion occurs
which makes the resonant frequency of the disk difficult to
mathematically predict and is usually arrived at by empirical
experimentation.
The velocity constant c.sub.f for the flexural wave in the disk 2b
may coarsely be computed from an equation that is derived from a
fourth order differential equation given by P. M. Morse, Vibration
and Sound, second edition, McGraw Hill, New York, 1948, pp. 154 and
209.
For a plate whose width > a: c.sub.f = [Y.sub.o K.sup.2 /.rho.(1
- 6.sup.2)] 1/4 (2 .pi. f).sup.1/2
where:
c.sub.f = Flexural velocity
Yo = Youngs Modulus
a = Plate thickness, of 2b as indicated in FIG. 2
k = radius of gyration (for a plate = a/.sqroot.12
.rho. = Density of the plate
6 = Poisson's Ratio
f = Frequency of vibrations.
The combination 1, disclosed in FIG. 1, in perspective, has two
visible elements, namely, the acoustic wave radiator 2 and the
backing element 3. As shown in the sectional view of FIG. 2A, a
composite piezoelectric driving unit of the bimorph type comprising
elements 4a,4b is mounted contiguously with the circular inner
faces of the backing element 3 and the acoustic wave radiator 2, or
transforming element.
The active elements 4a,4b may be any piezoelectric crystalline body
known in the art, formed or polarized to vibrate principally in a
resonant-thickness mode of vibration. For the purposes of the
present invention, elements 4a,4b are preferably disk-shaped wafers
of lead-zirconate-titanate dimensioned in accordance with the
desired frequency. In the present illustrative embodiment, they are
0.500 inch in diameter and, say, 0.078 inch thick; and are
constructed to vibrate in a longitudinal-thickness mode of a
resonant frequency of about one megahertz. It will be understood
that the wafers 4a,4b are thicker or thinner, depending on the
desired resonant frequency of the principal mode. Disks of
lead-zirconate, titanate, of a type and dimension suitable for the
uses of the present invention, may be obtained commercially in
various diameters and in thicknesses of 0.5 inch or less.
As a step in their preparation, the piezoelectric wafers 4a,4b are
polarized in accordance with well-known prior art practice by the
application of a polarizing voltage in a thickness direction
substantially normal to the major faces of the wafer, while the
temperature is lowered through the Curie temperature. Moreover, the
elements 4a,4b are aged to give them stable electroacoustic
characteristics, poling and aging having been carried out by the
suppliers in a manner taught, for example, in a bulletin entitled
"Aging of Ferroelectrics," New Jersey Ceramic Research Station,
Rutgers University, New Brunswick, Technical Report No.1, Rt. 1,
TR-59-128 (July 1, 1959) (AD-227-737-1004).
An important step in the process of preparing these piezoelectric
elements is the careful cleansing and preparation of the contacting
surfaces prior to assemblage into any of the combinations disclosed
hereinafter. For example, the wafers 4a,4b when obtained
commercially, or after prepolarizing treatment, may have permanent
electrode coatings on their major parallel surfaces. For some
embodiments of the present invention these are carefully removed by
lapping to a depth of 0.005 inch with any of the lapping compounds
known in the art, such as, for example, aluminum oxide and
water.
After the foregoing treatment the surfaces of the ceramic wafers
4a,4b are further cleaned by exposing the elements to ultrasonic
cleaning action in a container of, for example, an isoprophyl
alcohol or acetone, or any similar solvent which is characterized
by rapid evaporation, and which is not readily absorbed by the
ceramic. The ultrasonic vibrations are generated in the cleansing
liquid by means of, for example, a 100 to 200 watt generator
operation at a frequency of, say, 40 kilocycles per second, for
about 10 minutes.
In addition to the use of lead-zirconate-titanate, which is the
piezoelectric disclosed in the present illustrative example, any
other ceramic or piezoelectric crystalline materials may be
employed for the purposes of the present invention which are
capable of producing an element which vibrates in the
longitudinal-thickness mode, such as, for example, barium titanate,
x-cut quartz, etc.
As pointed out hereinbefore, the crystal elements 4a,4b are
sandwiched together with an electrode coating, preferably an
acoustic conducting epoxy characterized by a conductivity of at
least about 0.01 ohm-centimeter at 25.degree. C., providing a
coating which forms the inner electrode 5. A suitable conductive
cement for this purpose can be obtained from the Hysol company as
No. 4238, which is cured with Hysol hardener No. 3469, or a similar
combination. The external electrodes 6a,6b comprises similar
coating applied to the external surfaces of elements 4a,4b. As
previously stated, the piezoelectric sandwich 4a,4b is interposed
between the cylindrical backing element 3 and the acoustical
transforming element 2, which will now be described.
Although the piezoelectric element 4a,4b is disclosed herewith as a
bimorph, it will be understood that a single crystal wafer can be
used. Moreover, although elements 4a,4b are disclosed as being
cylindrical in the present illustrative example, it will be
understood that they can be other shapes, such as rectangular.
Referring to the FIGS. 3A, 3B and 4, it will be seen that in the
present example the backing element 3 comprises a substantially
cylindrical metal element of stainless steel having an outer
diameter of 0.625 inch and an axial length of 0.410 inch. The
right- and left-hand faces 3a and 3b each have circular recesses
symmetrical about the axis which are 0.02 inch deep and have a
diameter of 0.500 inch. The inside faces of each of these recesses
are respectively grooved with a series of annular grooves 0.010
inch deep and spaced apart 0.050 inch. The recess of right-hand
face 3b serves to accommodate the face of the piezoelectric element
4a, together with the cementing epoxy mixed with conducting coating
which is forced into the indentations to provide a better bond.
Adjacent the left-hand face is a lengthwise indentation 3c about
0.250 inch wide and 0.360 inch deep which is rounded at the inner
end to accommodate a screw hole 3d which is 4/40 inches in diameter
with a tap one-fourth inch deep. This serves for mounting the
backing element 3 when the transducer has been assembled. It will
be understood that in addition to stainless steel, as disclosed for
the present illustrative example, backing element 3 may
alternatively be formed of any high density element, such as high
carbon steel, monel metal, or even ceramic material having
sufficient density to serve as a loading element. Although the
backing element 3 is disclosed herein as being cylindrical, it will
be understood it is not limited to that shape.
In addition to the piezoelectric elements 4a,4b, a modificaton of
the invention shown in FIG. 2B includes an auxiliary piezoelectric
element 7 for feedback purposes, which may be of any thickness
between, for example, 0.15 inch and 0.78 inch. This is bonded into
the recess of the left-handed surface of the backing element 3 in
the same manner that the composite element 4a,4b is bonded to the
right-hand recess. Similarly, electrode coatings 8 and 9 are
included which may also comprise conducting epoxy of the type
previously described.
The unique element of the combination disclosed is the energy
transforming and acoustic wave radiating element 2. This
combination is preferably formed of a metal such as aluminum,
titanium, monel or stainless steel.
In accordance with the present invention, the element 2 has
substantially smaller mass than the element 3. However, the ratio
of the mass of the backing element 3 to the acoustic wave
tranforming-radiating element 2 may lie anywhere within the range
1:1 up to 30:1 or 40:1 or higher.
The function of the impedance member 2 is two-fold. One function is
to provide a substantial increase of the particle velocity from its
driving end 2a to its driven and 2b. Another function is to provide
a driven end having an impedance substantially matched to air or
gas impedance, which is center-driven and has a large radiating
surface constructed to withstand high temperatures and pressures,
and substantially resistant to corrosion, This is accomplished by
providing an element shaped, for example, as shown in FIG. 5A, in
which the left-hand, or driven, portion comprises a disk-like end
2a which has a diameter 0.750 inch and is cylindrical having a
thickness in an axial direction of 0.135 inch. As explained with
reference to the element 3, the left-hand face of the disk-like end
2a has a recess 10 matching the recess in the right-hand end of the
cylindrical element 3 for mounting the face 4b of the piezoelectric
sandwich element 4a,4b. As described with reference to the backing
element 3, the recessed portion of the left-hand face 2a is
provided with a series of annular grooves 11 which are 0.010 inch
deep and spaced apart 0.050 inch for receiving the conducting epoxy
coating on the contacting face of the piezoelectric sandwich
element 4b. In the present embodiment, the transforming member 2 is
0.625.+-.0.005 inch long, in an axial direction, tapering conically
to a diameter at mid-section of about 0.250 inch. The right-hand
end is expanded in cross section to a diameter of 0.750 inch,
forming a plate-like radiating member 2b 0.090 inch thick, the
radiating face being made parallel within .+-.0.002 inch to the
face of the driven end 2a which is in contact with the electrode
coating on the face of piezoelectric active element 4b. The
diameter of the radiating element 2b preferably approximates a
half-wavelength in the vibrating frequency of the transducer;
altough it will be apparent that this is only approximate, as the
system is designed to operate over a range of frequencies. In order
to have a pronounced main lobe the diameter of the disk should be
limited to under one wavelength. However, the smaller the diameer,
the less is the impedance match. If the diameter exceeds one
wavelength, the vibration of the disk breaks up into multiple modes
and produces a suppressed main lobe and pronounced side lobes,
generally two main lobes with a multiplicity of minor lobes. The
axial length of the combination, as shown in FIG. 2A, from the
center of the radiating end 2b to the left-hand face of the backing
element 3, preferably approximate an integral number of
quarter-wavelengths in the vibrating frequency.
It will be understood that although a specific embodiment of the
transforming element 2 is disclosed herein by way of illustative
example, the transforming element can take other forms within the
teaching of the present invention, in which the driving means
applied at the center of the right-hand radiating element 2b serves
to step-up the particle velocity anywhere within the range 2:1 to
100:1; and, the disk element 2b, which is disposed at the high
velocity end of the transformer, serves to substantially step-down
the impedance between the transforming element and the contacting
medium by substantially increasing the area of contact.
It will be apparent that the manner in which the cross section of
the transforming means 2 is reduced between the elements 2a,2b may
assume the form of any one of a number of cross-sectional curves
well-known in the art, such as cone, parabola, catenary, etc., as
long as the cross-sectional dimension is tapered from a large
diameter at the driving end to a substantially small diameter at
the plane at which the radiating member 2b is applied, so as to
provide a velocity step-up.
Referring now to FIGS. 6 and 7 of the drawings, there is shown a
preferred type of housing including a pipe fitting for a transducer
assembly of the type described with reference to FIGS. 1 through 5B
of the drawings.
FIG. 6 is an external perspective showing of the transforming
assembly in the housing; whereas, FIG. 7 shows the complete
assembly in longitudinal section.
The transducer assembly comprising the active piezoelectric
sandwich element 4a,4b, backed by cylinder 3 and disposed in
driving relation to a solid acoustic transformer 2, is enclosed in
the stainless steel tubular housings 12 and 13. The latter are
fitted together in longitudinal relation with the right-hand end of
tube 12 bonded into a slight recess in the left-hand end of tube
13. The latter is 0.895 inches in outer diameter and 0.748 inches
in inner diametr, being slightly recessed at the left-hand end, as
indicated. Tube 13 is fitted coaxially around the solid transformer
element 2 so that the right-hand face 2b of the latter protrudes
about 0.015 inches from the end of the tube in an axial direction.
Tubular housing 12 is 0.850 inch in outer diameters, 0.748 inches
in inner diameter and 1.187 inches in axial length. Screwed onto
the left-hand end of the tubular housing 12 is an end plug 12b
having an annular flange 12c which is one-fourth inch thick, 1.020
inch in outer diameter. At its right-hand end, plug 12b is reduced
in diameter to form a screw-threaded nipple about 0.733 inch in
outer diameter which protrudes one-fourth inch coaxially for
internal engagement with a matching screw-threaded recess in the
inner end of the tube 12. An axial bore 12a which is 0.078 inch in
diameter, for accommodating lead wire 16, extends through the
flange 12b and the nipple.
Flange 12c provides an annular shoulder extending outwardly about
0.075 inches in a radial direction from the screw-threaded surface
of the nipple. The flange 12c includes an annular notch 12d which
is 0.068 inches wide and 0.035 inch deep, symmetrically disposed
between the opposite faces of the flange. The left-hand face of the
plug 12b also includes an annular o-ring groove having a means
diameter 0.95 inch, which is 0.04 inch wide and 0.015 inch deep. An
annular collar 21 which is 0.065 inch in outer diameter and 0.0884
inch in inner diameter one-fourth inch in axial length, and
externally screw-threaded, is fitted near the left-hand end of tube
12, providing an annular clearance 0.05 inch wide with the shoulder
of flange 12c. The aforesaid three annular notches are constructed
to retain three o-rings 22, 23 and 24, which serve to acoustically
isolated the inner transducer assembly and its housing from tubular
brass enclosure 14, in a manner which will now be described.
The tubular enclosure 14 comprises an enlarged open right-hand end
14a, externally screw-threaded at 111/2 turns per inch. It is
13/8inch in maximum outer diameter, 0.680 inch in overall length
and slightly tapered from left to right in accordance with the
National Plumbing code taper for one-inch pipe. The open end of
enclosure 14 forms a cavity 1.041 inch in diameter and 0.567 inch
along the axis, terminating at its left-hand end in an annular
inner shoulder 14c which is about one-fourth wide in a radial
direction. The cavity is internally screw-threaded at its open
right-hand end at a pitch of 32 turns per inch.
At its left-hand end, tubular enclosure 14 has a square tubular
extension 14b, symmetrical about the axis, the outer dimension of
which is 0.70 inch on a side, and which extends externally
three-eighth inch to the left of the flanged screw-threaded portion
14a. The diameter of the cylindrical inner cavity of enclosure 14
drops off from the inner shoulder 14c for an axial length of about
0.113 inch, to form a smaller cylindrical inner cavity 14f which is
one-half inch in diameter and 0.4 inch along the axis. This is
partly closed at the left-hand end, except for an axial opening 14g
which is 0.209 inch in diameter to accommodate an "Amphenol"
BNC-type connector, as will be described. In the present example,
the axial depth of opening 14g is 0.055 inch; and, it leads into an
annular recess 14h which is 0.351 inch in diameter and 0.045 inch
deep at the center of the rectangular left-hand face of the
extension 14b. The latter is constructed to mate with an
Amphenol-type plug assembly 18, which will now be described.
The "Amphenol" coaxial-type connector is of particularly the
general form indicated in J. V. Malek et al., U.S. Pat. No.
3,054,981, issued Sept. 18, 1962, or C. W. Concelman U.S. Pat. No.
3,103,548, issued Sept. 10, 1963. It comprises a tubular outer
conductor 18a of silver, or other conducting material, three-eighth
inch in outer diameter, one-third inch thick, which extends
five-eighth inch in axial length, terminating at the center of a
rectangular plate 18b, eleven-sixteenth inch by eleven-sixteenth
inch and three-sixteenth inch thick. This has screw holes to
accommodate the screws 19 at its four corners, so that it mates
adjacent the left-hand face of the rectangular extension 14b of the
outer housing 14, in contact with gasket 20, about one thirty
seconds inch thick, comprising rubber, "Neoprene" or another
similar elastomer. A pair of diametrically opposite protuberances
18c provide means for locking shell 18a in electrical connection
with an external coaxial lead. It will be apparent to those skilled
in the art that the specific dimensions of pipe 14b and pipe plug
18b will vary so that each Amphenol type connector 18 is fitted to
the pipe plug securely. Before the connector is put in place, a
quantity of silicone rubber is internally into the opening 14g to
secure the connector. After the connector is in place, the silicone
is allowed to cure 24 hours, after which a test is made to
determine that there is no air leakage around the connector.
Amphenol connector 18 has a cental conductor 18d, about
one-sixteenth inch in diameter, embedded in an annular insulating
member 18e, three-sixteenth inch in diameter. This combination is
embedded internally in a solid conducting sheath 14g which makes
solid integral contact with the inside of shell 18a, which extends
from a position about five-eighth inch inside the left-hand end and
continuing through to the right-hand face of the rectangular
connecting plate 18b, where it protrudes about one-sixteenth inch
beyond the surface of the plate, surrounding the insulating portion
18e, which protrudes an additional one-sixteenth inch, surrounding
the axial conductor 16 which is connected to the high potential
electrode coating 5 of piezoelectric elements 4a,4b to which it is
soldered or otherwise electrically connected.
The flanged end 12c of tubular housing 12 fits into the cavity in
the screw-threaded outer connector 14a so the left-hand face rests
against the o-ring 22, one inch in outer diameter and one-sixteenth
inch in thickness, resting against internal shoulder 14c. The
o-ring 22 is preferably rubber or a rubber substitute, such as
Neoprene or any other insulating material having a similar
coefficient of elasticity. The second similar o-ring 23 is
interposed in the annular notch 12d; and, the third similar o-ring
24 rests against the shoulder formed by the right-hand face of the
flange 12c. As previously mentioned, the function of these three
gaskets is to acoustically isolate the external housing from the
transducer assembly. The combination, including o-ring 22, 23 and
24, is held in place by the annular collar 21, the rings being
screw-fitted into the internal threads of the outer tubular housing
14a. The left-hand end of collar 21 contains two or three
peripheral notches which are one-eighth inch wide and 0.09 inch
deep. These are symmetrically spaced around the circumference of
the collar.
Assuming the acoustically isolating o-ring 22, 23 and 24 to be
nonconducting or insulating material, a conducting tab 9, about
one-fourth inch wide and one-half inch long, is force-fit between
the external surface of the cylindrical shell 12 and the internal
surface of the screw-threaded collar 21, so as to make electrical
contact between the two, providing a conducting path for the
electrode coatings 6a,6b which are connected together to make
electrical contact with the transformer face 2a, and hence,
internal surface of the shell 12.
As an alternative to using the conducting tab 9, the elastomer
o-rings 20, 22, 23 and 24 are formed of a mixture of elastomeric
material with metallic powder, having a conductivity of at least
0.01 ohm-centimeter at 25.degree. c. For example, a suitable
conductive epoxy cement can be obtained from the Hysol Company as
No. 4238, which is cured with Hysol hardener No. 3469 or a similar
combination.
Referring to FIG. 8 of the drawings, let us assume that a
transducer assembly of the type previously described is connected
to a conventional high frequency oscillator operating at a
frequency of the order of 45 kilocycles per second. The ultrasonic
radiation pattern of the plate 2b, indicated in FIGS. 1 and 2A,
includes a centrally directed ultrasonic beam having a major
central lobe which is substantially symmetrical about the x axis.
Laboratory measurements show that at a distance of one foot from
the center of the plate 2a, along the x axis, the beam intensity
varies in a relative manner as illustrated.
If a receiving meter is moved in a plane perpendicular to the face
of the radiator through a series of points of identical ultrasonic
intensity, a plotting of these points indicates a pronounced
central lobe a, flanked by a pair of substantially symmetrically
disposed side lobes b and c, considerably weaker intensity. These
patterns assume measurements made in air or another gas.
If, however, the frequency of the oscillator, operating at the same
voltage and power, is increased to about 95 kilocycles per second,
the radiation pattern changes to the general form indicated in FIG.
9. Measurements at the same level of ultrasonic intensity in a
plane perpendicular to the face of the radiator produce a series of
readings which, when plotted, indicate a radiation pattern of the
form shown in FIG. 9 of the drawings. As indicated, the central
lobe d is now suppressed, and a relatively larger amount of energy
is contained in the side lobes. It is apparent that the operation
of the transducer assembly at the 95 kilocycles per second
frequency, in such a manner that it includes the prominent side
lobes as shown, has many applications not hitherto shown in the
art. Two of these will be described with reference to FIGS. 11 and
12 of the drawings.
First, however, a gas flowmeter circuit of FIG. 10 will be
described wherein are employed four transducer assemblies of the
type indicated in FIGS. 1 and 2A, which are operated at a pulse
modulated frequency of the order of 45 kilocycles per second to
produce in each case a beam pattern having a single prominent lobe
of the type indicated in FIG. 8 of the drawings. The operation of
the circuit shown in FIG. 10 is substantially similar to that
described in U.S. Pat. No. 2,669,121 issued to R. L. Garman et al.
on Feb. 16, 1954, entitled Supersonic Flowmeter. The ultrasonic
flowmeter shown schematically in FIG. 10 is designed so that the
velocity of fluid in a pipe section is determined directly, the
indication obtained being independent of the velocity of sound in
the fluid. Different fluids transmit sound at different velocities,
the velocity of travel through any particular fluid varying with
temperature. By providing a device whose measured output is
independent of such a variable, the necessity for recalibration and
the allowance for temperature errors and other variables are
avoided.
When an ultrasonic wave is transmitted through a fluid, the time
required for such transmission is dependent on the velocity of
sound in the fluid, the length of the path over which the
ultrasonic wave is transmitted and the velocity of the fluid
itself, the latter either adding to or subtracting from the
transmission time depending on whether the ultrasonic wave is
transmitted against or with the stream of flow.
In the flowmeter shown in FIG. 10, the time required for
transmission of the ultrasonic waves over a predetermined path in
the measured fluid is determined by the use of a feedback circuit
which produces a pulsed signal whose repetition frequency is a
function of the transmission time. This leaves two variables:
namely, the velocity of the fluid to be determined and the velocity
of sound through the fluid. In order to eliminate the velocity of
the sound by cancelling it out from the final indicating
instrumentality, two feedback circuits, driven from a common source
of power, are utilized. One circuit includes amplifier 42 and pulse
modulated, high frequency alternating current oscillator 44 which
energizes transmitting transducer assembly 34 to transmit an
ultrasonic beam to receiving transducer assembly 36, so that the
beam obliquely intersects the fluid path. The other, which includes
amplifier 39 and pulse modulated, high frequency alternating
current oscillator 31, energizes transmitting transducer assembly
33 to transmit an ultrasonic beam to receiving transducer assembly
37, so that the beam obliquely intersects the fluid path in the
opposite direction. Each of the transducer assemblies 34, 35, 36
and 37 is so operated that it directs a principal ultrasonic beam
substantially normally to the radiating surface. Hence, the axes of
transducer assemblies 34 and 35 are respectively mounted in nipples
32a and 33a which are disposed obliquely in the wall of pipe
section 30. Transducer assemblies 36 and 37 are similarly mounted
in obliquely disposed pipe nipples 33b and 32b. In such an
arrangement, each separate path will generate a signal of a
different repetition frequency. The frequency of the signal in one
path, namely from transducer assemblies 34 to 36, will be greater
than the means by an amount which is proportional to the velocity
of the fuid; and, the frequency of the signal in the other path,
from transducer assemblies 35 to 37, will be less than the means by
a like amount. Signals are derived from both paths and are
impressed in opposing relation on the mixer circuit 46. The
difference signal from mixer circuit 46, varying only as the
velocity of the fluid, is used to provide an indication in meter
47, which is a function of the velocity of fluid flow in the
pipe.
The foregoing will be more clearly understood by reference to the
description in the aforementioned U.S. Pat. No. 2,669,121 to Garman
et al., supra, which describes in detail the manner of operation of
the circuit of FIG. 10, with the exception of the transducer
assemblies 34, 35, 36 and 37, which are specifically of the form
described in FIGS. 1, 2A, et seq. of this application, operated to
radiate a beam having a dominant central lobe, in the manner of
FIG. 8.
Assume for further example that transducer assemblies 34, 35, 36
and 37 are replaced with transducer assemblies 34', 35', 36' and
37' which are operated in the manner indicated in FIG. 9 of the
drawings, at a frequency of the order of 95 kilocycles per second;
and further, assume that the circuit to which they are connected is
substantially of the configuration and operation shown and
described hereinbefore with reference to FIG. 10.
Because of suppressed central lobe and the angular spread of
approximately thirty degrees between the side lobes e and f (FIG.
9), it is no longer necessary to modify the surface of the pipe
section to provide special nipples 32a, 32b, 33a and 33b in which
to mount the transducer assemblies as indicated in FIG. 10.
Instead, pipe section 30', as shown in FIG. 11, is substantially
unmodified, except for the drilling of an opening in a direction
normal to the pipe wall for the positioning of each of transducer
assemblies 34', 35', 36' and 37' in their respective housings. The
latter are screwed, or otherwise closely fitted, to protrude in a
normal direction through the wall of pipe section 30', so that the
radiating face 2b of each of the transducer assemblies is
substantially flush with the inner surface of the pipe. Referring
to FIG. 9, it will be seen that each of the aforementioned
transducer assemblies, when driven at a frequency of about 95
kilocycles per second, generates a pair of dominant ultrasonic side
lobes for transmitting and/or receiving, which are directed
obliquely, each at an angle .theta./2 relative to the direction of
fluid flow in the pipe, where .theta. is the angle between the
dominant side lobes. At the radiating face of each transducer
assembly, one lobe is directed upstream, and the other downstream.
Thus, with this modification, the system shown in FIG. 11 has its
transducer assemblies 34', 35', 36' and 37' connected in
substantially the same manner as the corresponding transducer
assemblies of FIG. 10, in a pulse transmission and receiving system
constructed and operated in a substantially similar manner
thereto.
It will be appreciated that alternative arrangements of the
flowmeter systems of FIGS. 10 and 11 can be made using, for
example, two transducer assemblies positioned as indicated by 34'
and 36', instead for four, with the circuit modified so that the
direction of wave transmission is reversed at alternate intervals
by alternately switching each transducer between the transmitting
and receiving circuit.
Another alternative flowmeter system, making use of side beam
transducer assemblies operated as indicated in FIG. 9, is shown in
FIG. 12 of the drawings. This includes a conventional pipe section
50 in which there are drilled three openings in a triangular
arrangement into which are interposed three transducer assemblies
51, 52 and 53, of the form shown and described in FIGS. 1 and 2 of
the drawings, and operated as shown in FIG. 9. These are interposed
in a direction normal to the pipe wall so that the radiating
surface 2b of each of the transducer assemblies is substantially
flush with the inner surface of pipe 50. Transducer assembly 51,
which is located at the apex of the triangle, is connected to the
output of the pulse transmitting circuit 49. The two transducer
assemblies 52 and 53, which are located at the base angles of the
triangle, are respectively connected to the input terminals of
receivers 54 and 55, the latter being respectively tuned to two
different high frequency channels designated as No. 1 and No. 2.
The outputs of these two receiver channels are modulated with
gating pulses derived from the gate control circuit 56 which, in
turn, is synchronized with the operation of the transmitter circuit
49. The two output channels from receivers 54 and 55 are imposed on
the divider circuit 57 to provide a difference signal which
actuates the flowmeter indicator 58. Details of the operation of
this circuit may be better understood from a study of U.S. Pat. No.
3,204,456, issued Sept. 7, 1965, to W. Welkowitz, entitled
Ultrasonic Flowmeter.
It will be seen that the side beam transducer assembly 51 which
simultaneously directs two beam with an angle of .theta. between
them from the radiating surface thereof to the receiving surfaces
of transducer assemblies 52 and 53, provides a novel and
advantageous system of operation.
Although the patterns disclosed in FIGS. 8 through 12 have been
described in general terms, it will be understood that these
specific patterns, as shown, are based on measurements made with
air as the fluid medium.
It will be understood that the invention is not limited to the
specific forms of transducer assemblies shown and described herein
by way of example; or to the specific flowmeter circuit
arrangements herein shown and described, but is limited only by the
scope of the claims hereinafter set forth.
* * * * *