U.S. patent number 3,693,415 [Application Number 05/161,079] was granted by the patent office on 1972-09-26 for scanning ultrasonic inspection method and apparatus.
Invention is credited to Keith Richard Whittington.
United States Patent |
3,693,415 |
Whittington |
September 26, 1972 |
SCANNING ULTRASONIC INSPECTION METHOD AND APPARATUS
Abstract
A method and apparatus for testing for flaws by ultrasonic
energy in which transducer elements are uniformly spaced in a row
relative to a work piece and successive groups thereof are
energized in a progressive manner along the row, each group being
energized in the same manner so that successive foci are on a path
on the outer surface of the work piece. Preferably, each transducer
element emits a pulse of ultrasonic energy throughout a substantial
angle towards the work piece and the pulses arrive substantially
simultaneously and in phase at a point within the work piece.
Inventors: |
Whittington; Keith Richard
(Greatshelford, EN) |
Family
ID: |
10470328 |
Appl.
No.: |
05/161,079 |
Filed: |
July 9, 1971 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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787287 |
Nov 26, 1968 |
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Foreign Application Priority Data
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Nov 29, 1967 [GB] |
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54,224/67 |
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Current U.S.
Class: |
73/619; 73/622;
73/626; 367/105 |
Current CPC
Class: |
A61B
8/4494 (20130101); G10K 11/346 (20130101); A61B
8/4488 (20130101); A61B 8/4483 (20130101); A61B
8/12 (20130101); A61B 8/4281 (20130101); G01N
29/262 (20130101) |
Current International
Class: |
A61B
8/00 (20060101); A61B 8/12 (20060101); G01N
29/26 (20060101); G10K 11/34 (20060101); G10K
11/00 (20060101); G01n 029/04 () |
Field of
Search: |
;73/67.5-67.9 ;340/15
;343/100.6 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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772,083 |
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Apr 1957 |
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GB |
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941,573 |
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Nov 1963 |
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GB |
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1,400,484 |
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Apr 1965 |
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FR |
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Primary Examiner: Queisser; Richard C.
Assistant Examiner: Beauchamp; John P.
Parent Case Text
This is a continuation-in-part of the U.S. Pat. application Ser.
No. 787,287 filed on Nov. 26, 1968, now abandoned, by Keith Richard
Whittington.
Claims
I claim:
1. A method of testing a test-piece for flaws with ultrasonic
energy comprising the steps of positioning the test-piece relative
to a row of individually energizable transducer elements so that
the transducer elements lie opposite a surface of the test-piece
and each emit a pulse of ultrasonic energy towards said surface
when energized, which pulse overlaps with similarly produced pulses
of energy from neighboring transducer elements in a region of the
test-piece which is to be tested; successively energizing different
predetermined groups of transducer elements from the row so that
the transducer elements of each individual group are energized in a
predetermined timing sequence so as to emit pulses which arrive
substantially in phase at a certain point in said region and create
a focus of local maximum intensity at that point, and so that the
successive foci so created are displaced relative to one another in
directions parallel to said surface of the test-piece; and
detecting ultrasonic echo pulses from any flaws in the region of
each focus using transducer elements in which said echo pulses
generate electrical echo signals which are then processed to give
an indication of the presence of a flaw.
2. A method as claimed in claim 1 in which the transducer elements
of each individual group are energized in a predetermined timing
sequence so as to emit pulses which arrive substantially
simultaneously and in phase at a certain point in said region and
create a focus of local maximum intensity at that point.
3. A method as claimed in claim 2 in which the transducer elements
are such that when energized they each emit a pulse of ultrasonic
waves throughout a substantial angle towards the test-piece.
4. A method as claimed in claim 3 in which each of said groups of
transducer elements is selected to include some of the transducer
elements of the group last energized.
5. A method as claimed in claim 4 in which the test-piece is
positioned relative to a row of individually energizable and
uniformly spaced transducer elements, and in which each group of
transducer elements is selected so as to consist of a predetermined
constant number of neighboring transducer elements, successive
groups of transducer elements being selected to have a
predetermined constant number of transducer elements common to one
another.
6. A method as claimed in claim 5 in which each group of transducer
elements is energized in exactly the same predetermined timing
sequence.
7. A method as claimed in claim 1 in which the transducer elements
which are energized to emit ultrasonic pulses and create foci are
also used to receive the ultrasonic echo pulses.
8. A method as claimed in claim 7 in which one or more of the
transducer elements of each group which are energized to form a
focus are used to receive ultrasonic echo pulses from any flaws in
the region of that focus.
9. A method as claimed in claim 8 in which two or more of the
transducer elements of each group which are energized to form a
focus are used to receive ultrasonic echo pulses from any flaws in
the region of that focus, and in which phase delays are introduced
between the corresponding electrical signals generated in these
transducer elements by the echo pulses so that they are in phase
and reinforce one another when added together subsequently.
10. A method as claimed in claim 1 in which two or more transducer
elements are used to receive the echo pulses from each focus, and
in which phase delays are introduced between the corresponding
electrical signals generated in these transducer elements by the
echo pulses so that they are in phase and reinforce one another
when added together subsequently.
11. A method of testing a cylindrical tube for flaws with
ultrasonic energy comprising the steps of positioning the tube
concentrically within a ring of individually energizable and
uniformly spaced transducer elements so that each of the transducer
elements emits a pulse of ultrasonic energy towards the center of
the tube when energized which overlaps with similarly produced
pulses of energy from neighboring transducer elements in a region
of the tube which is to be tested; successively energizing
different predetermined groups of transducer elements from the ring
so that the transducer elements of each individual group are
energized in a predetermined timing sequence so as to emit pulses
which arrive substantially in phase at a certain point in said
region and create a focus of local maximum intensity at that point,
and so that the successive foci so created are displaced relative
to one another circumferentially around the tube in a progressive
manner; and detecting ultrasonic echo pulses from any flaws in the
region of each focus using transducer elements in which said echo
pulses generate electrical signals which are then processed to give
an indication of the presence of a flaw.
12. A method as claimed in claim 11 in which the transducer
elements are such that when energized they each emit a pulse of
ultrasonic waves throughout a substantial angle towards the
test-piece.
13. A method as claimed in claim 12 in which the transducer
elements of each individual group are energized in a predetermined
timing sequence so as to emit pulses which arrive substantially
simultaneously and in phase at a certain point in said region and
create a focus of local maximum intensity at that point.
14. A method as claimed in claim 13 in which each group of
transducer elements is selected so as to consist of a predetermined
constant number of neighboring transducer elements, and in which
each group of transducer elements is energized in exactly the same
predetermined timing sequence.
15. A method as claimed in claim 14 in which successive groups of
transducer elements are selected to have a predetermined constant
number of transducer elements in common with one another.
16. A method as claimed in claim 15 in which the transducer
elements of each group are energized in a progressive manner with
equal intervals of time between the energization of successive
neighboring transducer elements.
17. A method as claimed in claim 12 in which transducer elements
are energized one at a time in a progressive manner around the ring
so that there are equal intervals of time between the energization
of successive transducer elements, and the transducer elements emit
pulses which form a succession of foci.
18. A method as claimed in claim 17 in which all of the transducer
elements are energized in the said progressive manner around the
ring.
19. A method as claimed in claim 11 in which the transducer
elements which are energized to emit ultrasonic pulses and create
foci are also used to receive the ultrasonic echo pulses.
20. A method as claimed in claim 19 in which one or more of the
transducer elements of each group which are energized to form a
focus are used to receive ultrasonic echo pulses from any flaws in
the region of that focus.
21. A method as claimed in claim 20 in which two or more of the
transducer elements of each group which are energized to form a
focus are used to receive ultrasonic echo pulses from any flaws in
the region of that focus, and in which phase delays are introduced
between the corresponding electrical signals generated in these
transducer elements by the echo pulses so that they are in phase
and reinforce one another when added together subsequently.
22. A method as claimed in claim 11 in which two or more transducer
elements are used to receive the echo pulses from each focus, and
in which phase delays are introduced between the corresponding
electrical signals generated in these transducer elements by the
echo pulses so that they are in phase and reinforce one another
when added together subsequently.
23. Apparatus for testing a test-piece for flaws with ultrasonic
energy comprising a row of separate transducer elements arranged
opposite a surface of the test-piece each of which is adapted to
emit a pulse of ultrasonic energy towards said surface when
energized which overlaps with similarly produced pulses of energy
from neighboring transducer elements in a region of the test-piece
which is to be tested; energizing means to energize each of said
transducer elements individually; control means which is adapted to
control said energizing means so that the latter energizes each of
a succession of different predetermined groups of said transducer
elements in a predetermined manner such that the transducer
elements of each individual group emit pulses which arrive in phase
at a certain point in said region and create a focus of local
maximum intensity at that point, and further such that the
successive foci so created are displaced relative to one another in
directions parallel to said surface of the test-piece; transducer
elements which receive ultrasonic echo pulses from any flaws in the
region of each focus and which generate corresponding electrical
echo signals; and processing means which gates said electrical echo
signals and which is adapted to give an indication of the presence
of a flaw.
24. Apparatus as claimed in claim 23 in which the energizing means
comprises an electrical power source and individual switching
devices connected between each transducer element and said source,
each of said switching devices being under the control of said
control means and being operative to cause said source to energize
the associated transducer element.
25. Apparatus as claimed in claim 24 in which said electrical power
source is a d.c. voltage supply and in which each of the switching
devices is a logic gate.
26. Apparatus as claimed in claim 24 in which the control means
comprises a computer.
27. Apparatus as claimed in claim 24 in which each of said
individual switching devices is uniquely coded and in which the
control means comprises a coded pulse generator which produces a
predetermined sequence of coded pulses which are fed to said
switching devices to operate the corresponding ones and thereby
energize the associated transducer elements.
28. Apparatus as claimed in claim 27 in which said coded pulse
generator includes a binary counter with an input from a pulse
oscillator and with output connections which deliver said coded
pulses.
29. Apparatus as claimed in claim 27 in which said transducer
elements which receive ultrasonic echo pulses are the same
transducer elements which are energized to produce said foci, and
in which said processing means comprises individual switching
devices associated with each of the transducer elements and which
are each uniquely coded and are controlled to gate electrical echo
signals by coded pulses from said coded pulse generator.
30. Apparatus as claimed in claim 23 in which the transducer
elements are such as to emit energy throughout a wide angle towards
said surface when energized.
31. Apparatus for testing a cylindrical tube for flaws with
ultrasonic energy comprising a ring of uniformly spaced transducer
elements arranged concentrically about the tube each of which is
adapted to emit a pulse of ultrasonic energy towards the center of
the tube when energized which overlaps with similarly produced
pulses of energy from neighboring transducer elements in a region
of the tube which is to be tested; energizing means to energize
each of said transducer elements individually; control means which
is adapted to control said energizing means so that the latter
energizes each of a succession of different predetermined groups of
said transducer elements in a predetermined manner such that the
transducer elements of each individual group emit pulses which
arrive in phase at a certain point in said region and create a
focus of local maximum intensity at that point, and further such
that the successive foci so created are displaced relative to one
another in directions parallel to said surface of the tube;
transducer elements which receive ultrasonic echo pulses from any
flaws in the region of each focus and which generate corresponding
electrical echo signals; and processing means which gates said
electrical echo signals and which is adapted to give an indication
of the presence of a flaw.
32. Apparatus as claimed in claim 31 in which the transducer
elements are such as to emit energy throughout a wide angle towards
said surface when ener2ized.
33. Apparatus as claimed in claim 32 in which the control means is
adapted to cause energization of successive groups of transducer
elements, each consisting of a predetermined constant number of
neighboring transducer elements, in the same predetermined
manner.
34. Apparatus as claimed in claim 33 in which the control means is
adapted to cause energization of successive transducer elements in
each group at regular intervals.
35. Apparatus as claimed in claim 34 in which said energizing means
comprises an electrical power source and individual switching
devices connected between each transducer element and said source,
each of said switching devices being under the control of said
control means and being operative to cause said source to energize
the associated transducer element.
36. Apparatus as claimed in claim 35 in which each of said
individual switching devices is uniquely coded and in which the
control means comprises a coded pulse generator which produces a
predetermined sequence of coded pulses which are fed to said
switching devices to operate the corresponding ones and thereby
energize the associated transducer elements.
37. Apparatus as claimed in claim 36 in which said coded pulse
generator includes a binary counter with an input from a pulse
oscillator and with output connections which deliver said coded
pulses.
38. Apparatus as claimed in claim 37 in which said transducer
elements which receive ultrasonic echo pulses are the same
transducer elements which are energized to produce said foci, and
in which said processing means comprises individual switching
devices associated with each of the transducer elements and which
are each uniquely coded and are controlled to gate electrical echo
signals by coded pulses from said coded pulse generator.
39. Apparatus as claimed in claim 38 in which said binary counter
of the coded pulse generator operates cyclically, counting up from
a progressively differing count during successive cycles and
resetting to zero automatically and counting up to the next
starting count during each cycle.
40. Apparatus as claimed in claim 39 in which the state of said
binary counter at the end of each cycle is used to produce a
corresponding coded pulse to gate the electrical echo signal from a
transducer element.
41. Apparatus as claimed in claim 40 in which successive gated
electrical echo signals are used to produce successive horizontally
spaced vertical deflections in a cathode ray tube display.
42. Apparatus as claimed in claim 32 in which the control means is
adapted to cause energization of all of the transducer elements one
at a time in a progressive manner around the ring so that there are
equal intervals of time between the energization of successive
transducer elements.
Description
This invention relates to a method of, and apparatus for, testing a
test-piece with ultrasonic energy to enable it to be tested for
flaws and, in particular, relates to the testing of tubes.
U.S. Pat. Nos. 3,021,706 and 3,052,115 both disclose ultrasonic
testing system comprising ultrasonic transducer elements which each
emit a directional or parallel beam of ultrasonic wave energy
towards the test-piece to test a corresponding part of it when
energized, and which are sequentially energized one at a time so
that a succession of such beams are formed to test a corresponding
succession of different parts of the test-piece. The direction of
the beam emitted by each transducer element and thus its angle of
incidence on the test-piece is fixed and is determined by the
orientation of the transducer element.
U.S. Pat. No. 3,166,731 discloses an ultrasonic testing system
comprising ultrasonic transducer elements which each emit a
directional beam of ultrasonic wave energy towards a test-piece and
which are all arranged to emit their beams parallel to one another
in the same direction to produce a resultant beam with a single
wavefront. The shape of the wavefront and the direction of advance
of the resultant beam are controlled by controlling the energizing
sequence of the transducers. A linear array of transducers is used
to produce a resultant beam with a planar wavefront which advances
in a direction inclined to the component beams by energizing the
transducers at regular intervals in a progressive manner along the
array. The angle of inclination of the resultant beam is determined
by the delay introduced between the energization of successive
transducers, and can be varied thereby so that the beam scans
through an angle relative to the test-piece.
U.S. Pat. No. 3,086,195 discloses an ultrasonic testing system
comprising an ultrasonic transducer element formed from a plurality
of co-planar parts, and means for applying oscillatory electrical
energy to the parts with progressively different time delays so
that ultrasonic wave energy from each forms a convergent beam which
comes to a focus within the body of a test-piece. The illustrated
transducer element comprises a plurality of concentric ring
elements which are all energized to form a focus on the central
axis of the transducer element at a distance from the plane of the
transducer element determined by the time delay between successive
concentric ring elements. This transducer element is placed in
contact with the surface of the test-piece and the depth of the
focus formed by the element within the body of the test-piece is
varied by the said time delay. To scan the focus in a direction
parallel to the surface of the test-piece the transducer element
has to be moved across the surface of the test-piece. It is further
suggested that the transducer element might take the form of a
plurality of parallel strips to form a line focus.
A disadvantage common to all of these known systems is that they
all lack flexibility, in that none will allow a beam of ultrasonic
wave energy to be produced with any required angle of incidence at
any point or region on the surface of the test-piece opposite the
transducer elements. An object of the present invention is to
overcome this disadvantage.
This is achieved according to the invention by arranging that
individual transducer elements in a row can be energized so that
they each emit a pulse of ultrasonic wave energy which overlaps
with similarly produced pulses from neighboring transducer elements
in a region of the test-piece which is to be tested, and
successively energizing different predetermined groups of
transducer elements from the row so that the transducer elements of
each individual group emit pulses which arrive substantially in
phase at a certain point in said region and create a focus of local
maximum intensity at that point, and so that the successive foci so
created are displaced relative to one another in directions
parallel to said surface of the test-piece.
In this way successive focused beams of ultrasonic energy are
formed by successive groups of transducer elements, the focus of
each beam being spaced from the last so that an effective focus is
created which scans parallel with the surface of the
test-piece.
The focus at any point can be formed by pulses from any of a
plurality of different groups of transducer elements, the mean
angle of incidence of the ultrasonic beam incident at the focus for
each group being different. Thus the mean angle of incidence at any
point on the surface of the test-piece can be selected as required.
This is particularly useful when the invention is applied to the
testing of tubes and like articles, as it allows the mean angle of
incidence of the focused beam to be maintained constant as the
focus scans across it, so as to allow the maximum possible transfer
of energy into the wall of the tube.
Preferably, the transducer elements in each individual group are
energized so that they emit pulses which arrive substantially
simultaneously as well as in phase at the focus. Relatively short
pulses can then be employed more effectively to form foci.
In tube testing systems the row of transducer elements takes the
form of a ring of uniformly spaced transducer elements which are
adapted to receive the tube concentrically within it. Successive
groups of transducer elements, each comprising a fixed number of
neighboring transducer elements, are energized in the the ring and
each group of transducer elements is preferably energizing the same
manner comprising energizing successive neighboring transducer
elements at regular intervals. Such a simple energizing sequence
can be used to produce in phase foci within the tube wall by
allowing for refraction of the ultrasonic energy at the outer tube
surface as it enters the tube wall, such refraction giving an
additional focusing effect.
Further, in such a tube testing system, the same transducer
elements which are used to form the foci are also used to detect
return echo signals from flaws in the region of each focus by
selecting a particular one of the transducer elements from each
group to act as a receiver for return echo pulses from flaws in the
region of the focus formed by that group.
The invention will now be described by way of example with
reference to the accompanying drawings in which:
FIG. 1 shows a schematic diagram of tube testing apparatus
according to one embodiment of the invention,
FIG. 2a-l shows schematic diagrams of transducer elements suitable
for use in the invention,
FIG. 3 shows an alternative form of control means for use with the
apparatus of FIG. 1,
FIG. 4 shows how a ring of transducer elements can be energized to
form a focus within the wall of a tube,
FIG. 5 shows how a transducer element which emits a parallel beam
of energy forms an effective diverging beam within a tube wall,
and
FIG. 6 shows a schematic diagram of apparatus according to an
alternative embodiment of the invention.
The ultrasonic tube testing system of FIG. 1 comprises a circular
array of uniformly spaced ultrasonic transducer elements E each of
which, when energized, emits a pulse of ultrasonic energy which
radiates as from a line source parallel with the central axis of
the array throughout a substantial angle .alpha. towards the center
of the array as shown in dashed outline in FIG. 1. Each transducer
element is energized by being connected to a common power source P
through an individual gate G which is controlled by a computor C.
For simplicity the connections between the gates G, power source P
and computer C are shown for only a few of the transducer elements
in FIG. 1 though they apply equally to all of them. Further, the
number of elements shown is very much smaller than the number which
would usually be used in practice. The tube T to be tested is
disposed within the array of transducer elements so as to be
concentric with the array. The tube is then tested at individual
points in turn around its circumference by operating the gates in a
controlled manner so as to energize the transducer elements and
form an ultrasonic focus of maximum intensity at each individual
point in turn as described more fully below. Flaws at these
individual points produce return echo signals which are detected by
the same transducer elements E under the control of an analyzer
unit A. In this way a region around the circumference of the tube T
can be tested for flaws the whole length of the tube being tested
by moving the tube T axially relative to the array so that
successive portions along the length of the tube are tested.
The focus is formed at any time by pulses emitted from a
predetermined number of transducer elements in an arc such as the
transducer elements E.sub.1 to E.sub.4 in FIG. 1. These transducer
elements are energized in a predetermined sequence which proceeds
from E.sub.1 to E.sub.4 and which leaves appropriate time intervals
between the energization of successive transducer elements so that
some of the ultrasonic wave energy of each of the pulses emitted by
the four transducer elements arrive simultaneously and in phase at
the focal point F.sub.1 on the surface of the tube T to give a
maximum intensity at that point. The path of propagation of the
energy from each transducer element which contributes to the focal
point F.sub.1 is shown in solid lines and it will be appreciated
that the said appropriate time intervals between energization of
successive transducer elements corresponds to the difference
between the paths of propagation for successive transducer
elements. A focus is similarly produced at the point F.sub.2 a
short time later by energizing the transducer elements E.sub.2 to
E.sub.5 in exactly the same sequence as transducer elements E.sub.1
to E.sub.4. Similarly, other points, each displaced relative to the
last, are formed by energizing transducer elements E.sub.3 to
E.sub.6, E.sub.4 to E.sub.7 and so on. In this way an effective
focus is formed which progresses in steps around the circumference
of the tube.
The formation of successive foci in this way is dependent on each
transducer element emitting ultrasonic energy which is incident
over a corresponding extensive region of the outer surface of the
tube so that it can contribute energy to a focus anywhere within
this region, and on the fact that a circumferentially extending
area on the outer surface of the tube is exposed to energy from all
of the transducer elements of each group so that they can all
contribute energy to a focus anywhere within this area. The
particular energizing sequence applied to the transducer elements
in a group determines the position of the focus within said exposed
area i.e., determines the mean angle of incidence of the energy
forming the focus. Clearly, where as in the system of FIG. 1 the
energizing sequence within each group of transducer elements
remains constant the mean angle of incidence of the energy forming
each focus remains constant and is preferably arranged to be
slightly less than 30 so as to allow the maximum possible energy to
pass into the tube wall in the form of shear waves and thereby give
the maximum efficiency in the detection of flaws in the outer
surface layer.
A standard tube testing programme is employed in the computer C
which incorporates information as to the number of transducer
elements in each group of transducer elements, the number of
transducer elements by which each such group is displaced from the
last to produce successive foci, and the energizing sequence
required within each such group to produce a focus with a
particular associated mean angle of incidence on the outer surface
of a tube of a particular diameter. For tubes with different
diameters within a set range of diameters the computer can adjust
the energizing sequence required automatically. The energizing
sequence employed is calculated using information as to the
position of the point or line from which the transducer elements
emit energy throughout an angle .alpha.. As described below the
position of this line may be different from the actual position of
the transducer element. The range of diameters of tubes which can
be tested using a set array of transducers elements increases with
increasing angles .alpha. due to the correspondingly increased
circumferential area on the outer surface of the tube which is
exposed to energy from all of the transducer elements in a
group.
Each transducer element is energized by applying a sudden step
voltage to it, the power source P providing the voltage and a gate
G switching this voltage when operated by the computer C. As a
result the transducer elements oscillate mechanically at their
resonant ultrasonic frequency and thus create ultrasonic waves in
the coupling medium between the transducers and the tube. These
waves are only emitted for a short time, as the oscillations of the
transducer elements are damped, the transducer elements typically
emitting pulses of five cycles, each cycle being of exponentially
decreasing amplitude over the preceding cycle. Ideally the
transducer elements are all exactly the same so that they all have
the same resonant frequency and the pulses of waves emitted by each
group of four transducer elements and arriving simultaneously at
the focal point are all in phase at the focal point. In this way
the maximum possible intensity is obtained at the focal point. In
practice, however, the transducer elements are usually all slightly
different, thus the pulses although initially in phase may
gradually get out of phase but as the pulses are short this will
only have a slight effect on the intensity at the focal point.
The transducer elements are much smaller than those conventionally
used in such ultrasonic testing systems and thus they require much
less power to energize them. Because of this feature the gates G
which have to switch these voltages and currents to energize the
transducer elements may comprise a number of solid-state devices.
The advantage of using such devices is that they are small and
inexpensive, further they can perform the high speed switching
which is involved in the energizing sequence applied to each group
of transducer elements.
Another advantage of using small transducer elements is that a
great number of them can be provided in the transducer array so
that the distance between successive focal points of the beam on
the tube T, such as F.sub.1 and F.sub.2, is small. In this way the
tube can be examined for faults around substantially the whole of
its circumference.
Examples of suitable piezo-electric transducer elements which emit
ultrasonic wave energy throughout a substantial angle are shown in
FIG. 2. FIGS. 2a), 2b), and 2c) show crossections through curved
transducer elements E which oscillate radially when energized so as
to emit energy as if from a line source f. In FIG. 2a) the
transducer is a cylindrical tube of piezo-electric material and in
FIGS. 2b) and 2c) the transducers are sections from such a tube.
Typically the tube has an outer diameter of 13 centimeters, is 0.05
centimeters thick and 0.6 centimeters long, has a natural
oscillating frequency of 3MHz, and emits ultrasonic energy
throughout an angle of 221/2, 30 or 45. FIGS. 2d) and 2e) show
cross-sections through planar transducer elements E which emit
energy from an edge or end face as if from a line source f, the
elements being made to oscillate along a line parallel to the
emitting edge or end face and being of a width substantially equal
to half of the wavelength of the emitted ultrasonic wave energy.
Typically, such planar transducer elements are 1 centimeter long
and 3 millimeters wide. All of these transducer elements typically
draw a current of 8 milliamperes when a voltage of 20 volts is
applied to them.
Other transducer elements may be spherical transducer elements or
transducer elements comprising a section of a sphere which emit
energy as if from a point source. Also, planar transducer elements
may be used which emit ultrasonic wave energy as if from a point or
line source by providing either a divergent lens in front of each
transducer element, or a pinhole which diffracts the wave energy
passing through it. Another arrangement may comprise a planar
transducer element with a tuned mechanical transformer connected to
a flat face thereof which is oscillated by the transducer element
to emit divergent wave energy.
An example of a system as shown in FIG. 1 which employs small
transducer elements as described above comprises ninety such
transducer elements equispaced in a circle at 4.degree. spacings,
the effective line sources f defined by the transducer elements all
lying on a circle of diameter 15.2 centimeters. Groups of ten
transducer elements at a time are energized to form each focus and
successive groups are displaced progressively in one case by just
one transducer element at a time so that ninety consecutive foci
are formed around the circumference of the tube. The time intervals
between the energization of successive transducer elements in a
group are of the order of 0.1 to 1 micro-seconds. Alternatively,
groups of more or less than 10 transducer elements may be energized
and successive groups may be displaced by two or more transducer
elements at a time so that less than 90 foci are formed around the
circumference of the tube.
The return echo from any flaw in the region of a focus is picked up
by one or more of the transducer elements which was energized to
form that focus. Thus, for each successive group of emitting
transducer elements there is a corresponding group of receiving
transducer elements within the emitting group. These groups of
receiving transducer elements are selected under the control of the
analyzer unit A, the latter comprising individual gates provided in
output connections form the transducer elements so as to allow
gating of electrical echo signals from any of the transducer
elements. In FIG. 1 only a few of the connections between the
analyzer unit A and transducer elements E are shown for
clarity.
In the simple case in which just one transducer element at a time
is used to receive echo signals, the analyzer A operates in step
with the computer C to gate the output signal from successive
individual transducer elements around the array. These successive
transducer elements may be, for example, the transducer element
E.sub.2 for the emitting group E.sub.1 to E.sub.4, the transducer
element E.sub.3 for the emitting group E.sub.2 to E.sub.5, the
transducer element E.sub.4 for the emitting group E.sub.3 to
E.sub.6, and so on.
If two or more transducer elements at a time are used to receive
echo signals, the analyzer unit A operates in step with the
computer C to gate the output signals from successive groups of two
or more transducer elements, and also operates to allow for the
fact that the output signals generated in the different transducer
elements of a receiving group by the same return echo will be out
of phase due to the different path lengths between these transducer
elements and the corresponding focus. This last mentioned operation
consists in introducing appropriate phase delays between the
different output signals so that they are then in phase and
reinforce one another to give a larger signal when added together.
Clearly, the appropriate phase delay required between any two
transducer elements in a receiving group will correspond to the
time interval introduced between the energization of these same
transducer elements when they are energized as part of the
corresponding emitting group. In one example, the same four
transducer elements such as E.sub.1 to E.sub.4 which are used to
form a focus can be used as the receiving transducer elements for
return echoes from that focus. It is preferred to employ two or
more receiving transducer elements at a time as the sensitivity of
the system is improved as compared with a system which employs just
one receiving transducer element at a time.
In an alternative embodiment of the invention the transducer
elements E which emit energy to form successive foci are used
exclusively for this purpose, and separate receiving transducer
elements are provided between the transducer elements E, these
receiving transducer elements being controlled by an analyzer unit
in a similar manner to that described above.
In another alternative embodiment of the invention the apparatus
shown schematically in FIG. 1 for sequentially energizing the
transducer elements E is replaced by apparatus shown schematically
in FIG. 3 and comprising a clock generator S which is connected to
a multi-stage shift register R, all of the stages of the shift
register being connected to each of the transducer elements through
solid-state gates H which are under the control of a computer C.
For simplicity the stages of the shift register are only shown
connected to one transducer element E although similar connections
are made to all of them, and further a five stage register is
shown. The computer C calculates the energizing sequence required
in a group of transducer elements and opens appropriate gates H to
connect each transducer element in the group to an appropriate
stage of the register so that when the clock generator is started
the transducer elements in the group are energized in the correct
sequence to form a focus at a required point. The clock is then
stopped, a new focus calculated by the computer, a new pattern of
gates opened and the clock started again. This arrangement can be
used in testing systems as described above, a standard program
being used in the computer for tube testing if required. An
advantage of this arrangement over the illustrated arrangement,
however, is that it is not dependent on the speed of the computer C
for its proper functioning, whereas the illustrated arrangement is,
and thus it can be used where a sufficiently fast computer is not
available.
In the ultrasonic tube testing system described so far transducer
elements within a group are sequentially energized so as to emit
pulses of ultrasonic energy which arrive at a point simultaneously
and in phase and form a focus of maximum intensity at that point.
To produce this result time delays are introduced between the
energization of successive transducer elements within the group
and, as will be appreciated by considering the propagation paths
shown in solid lines in FIG. 1 between the transducer elements
E.sub.1 to E.sub.4 and F.sub.1, the successive time delays
introduced between the transducer elements will necessarily all be
different, decreasing throughout the energizing sequence. It is
possible, however, to form a less intense focus, but one which may
nevertheless still be suitable for testing purposes by introducing
a suitable constant time interval between the energization of
successive transducer elements. The less intense focus is produced
because the pulses at the focus are then slightly out of phase.
A tube testing system which forms a less intense focus of the above
kind is now described. This system comprises a circular array of
uniformly spaced transducer elements which is arranged
concentrically with the tube to be tested, as in the system shown
in FIG. 1. The transducer elements are energized one at a time at
regular intervals in a progressive manner around the array. Thus,
if a transducer array like that shown in FIG. 1 is used, and the
progress of energization is clockwise, then transducer element
E.sub.1 is energized first, transducer element E.sub.2 is energized
a short time later, and then transducer element E.sub.3 is
energized an equal time after that, and so on around the array. As
a result the transducer elements emit pulses of ultrasonic waves
which form a focus which moves in step with the energizing sequence
in a clockwise direction around a circle within the array. The
radius of the circle is determined by the time intervals between
the energization of successive transducer elements and is suitable
adjusted to bring the focus onto the surface of the tube. This
radius, however, is preferably kept small compared with the radius
of the array as the phase condition will then be almost satisfied
and a more intense focus obtained than would be the case
otherwise.
In energizing the transducer elements separate groups of transducer
elements are not specifically selected but it will be appreciated
that the focus is formed at any one time by the pulses emitted from
just a few of the more recently energized transducer elements which
themselves comprise a group, the oscillations of the transducer
elements which were energized earlier having substantially died
away. In this case where the progress of energization is clockwise
the focus is displaced in a clockwise direction from a line joining
the center of the array to the center of the arc on which the
transducer elements forming the focus lie. Thus the ultrasonic beam
forming the focus is not incident normally on the tube and the
condition that the angle of incidence should be that angle for the
maximum transfer of energy into the wall of the tube is
approached.
Return echo signals can be detected by the transducer elements E or
separate individual transducer elements in the manner described
above in connection with the in phase system of FIG. 1. However,
because of the large number of transducer elements being energized
in such a short time (typically, a complete energizing sequence
running through all 90 transducer elements takes 100 micro-seconds
or less) there is a lot of background noise in the reflected echo
which lowers the sensitivity of the system. This background noise
comprises pulses transmitted through the tube from the transducer
elements on the opposite side of the array from the receiving
transducers. This problem can be overcome by energizing only a
proportion of the transducer elements, say 30, at a time with
longer pulses between the energization of successive groups of 30
transducer elements. Successive groups of 30 transducer elements
may include transducer elements from the group last energized.
Obviously, a complete test cycle then takes longer but the
sensitivity of the system is much improved.
The energizing sequence used in this system is simpler than that
used in the in-phase systems, and thus simpler and less expensive
energizing and control means can be used to perform it. The
energizing and control means may, nevertheless, be similar to those
described above but where a computer is used this need be only
large enough to perform the simpler energizing sequence. At the
same time, however, this system is strictly limited to the testing
of tubes and like articles, and because of the reduced intensity at
the focal point is less efficient in detecting flaws than the
in-phase systems.
In the tube testing systems described so far the transducer
elements have been energized to form foci around the outer
circumference of the tube so as to test for flaws in the outer
surface layer. It will be appreciated, however, that in tube
testing it may be necessary to form foci anywhere within the wall
of the tube to test for flaws therein. This can be done quite
readily without making any changes in the systems described above.
However, in determining the energizing sequence which must be
applied to a group of transducer elements to form a particular
focus the effect of refraction of the ultrasonic energy as it
passes into the wall of the tube must be taken into account.
FIG. 4 illustrates how ultrasonic pulses emitted by three
transducer E.sub.1 to E.sub.3 in an array similar to that of FIG. 1
contribute energy to a focal point F within the wall of the tube T,
the dashed lines showing the angular spread .alpha. of the energy
emitted by each transducer element and the solid lines showing the
path of propagation of the energy actually reaching the focus F.
This diagram shows clearly how the energy is refracted at the outer
surface of the tube, and also shows how this refraction causes an
additional focusing effect in that it causes the energy producing
the focus to become more convergent within the tube wall due to the
progressive change in the angle of incidence of the energy
producing the focus and the circular nature of the array of
transducer elements E. This additional focusing effect is
particularly important in that it can be used so that the path
difference between the paths of propagation of energy reaching the
focus from successive transducer elements is constant, thus
allowing successive transducer elements in a group such as E.sub.1
to E.sub.3 to be energized in the simplest manner with equal
intervals of time between the energizing of each without any
substantial decrease in the intensity of the focus.
Another important feature which is noticeable in FIG. 4 is that
when producing a focus within the tube wall two transducer elements
such as E.sub.1 and E.sub.3 which do not emit energy which is
incident over a common area at the surface of the tube can
nevertheless contribute energy to the same focus F within the tube
wall due to refraction at the outer surface of the tube. Thus, the
angular spread of the emitted energy is not so important as it is
when considering a focus at the surface of the tube. In fact, it is
even possible to employ transducer elements such as shown in FIG. 5
which emit a parallel beam of ultrasonic energy. After refraction
at the outer surface of the tube such a beam becomes divergent,
effectively coming from a point or lines source E.
As has already been mentioned above the mean angle of incidence at
the outer surface of the tube of that energy forming the focus is
preferably kept constant at slightly less than 30.degree. so as to
allow the maximum possible energy to pass into the tube wall. The
importance of this requirement can be more readily seen when a
focus is formed within the tube wall proper.
The focus F formed by a group of transducer elements such as
E.sub.1 to E.sub.3 can be formed anywhere within the wall of the
tube by appropriate choice of the energizing sequence applied to
the transducer elements. Further, the focus can be formed either
directly after the focused energy has been refracted or after it
has been reflected internally once or more times from the surfaces
of the tube. A succession of such foci can be formed one after the
other around the tube by energizing successive overlapping groups
of transducer elements such as E.sub.1 to E.sub.3 to E.sub.4 and so
on, the energizing sequence within each group remaining constant
and comprising energizing successive transducer elements at regular
intervals of time. Alternatively, a focus can be formed which scans
continuously around the tube by energizing the transducer elements
E.sub.1, E.sub.2, E.sub.3, E.sub.4 and so on in a progressive
manner around the tube with equal intervals of time between the
energization of successive transducer elements and without any
regard to selecting individual groups of transducer elements. The
means employed to energize the transducer elements and to detect
return echo signals from flaws in the region of the focus may take
the various forms already described above.
The apparatus illustrated schematically in FIG. 6 is generally
suitable for controlling energization of a row of transducer
elements so that successive similar groups of transducer elements
are energized in a progressive manner along the row and successive
transducer elements in each group are energized at regular
intervals which remain unchanged from group to group. The apparatus
further serves to control the detection of return echo signals by
selecting a particular one of the transducer elements from each
group to act as a receiver for return echo pulses from flaws in the
region of the focus formed by that group. Basically, the apparatus
operates by uniquely coding each transducer element and supplying a
suitable sequence of coded pulses to energize them and to select
them for receiving purposes. The illustrated system is specifically
designed for use with a ring of ninety uniformly spaced transducer
elements and serves to energize successive groups of 10 neighboring
transducer elements which are each displaced by one transducer
element from the last. The transducer elements are coded 0 to 89
progressively around the ring and a series of coded pulses 0 to 9,
1 to 10, 2 to 11, and so on are used to energize the groups of
correspondingly coded transducer elements. The coded pulses are
produced by two binary counters 1 and 2 which are fed pulses from a
variable frequency pulse oscillator 3 the frequency setting of
which determines the length of the intervals between energization
of successive transducer elements in each group, and the rate at
which successive groups of transducer elements are energized is
determined by the repetition frequency of a pulse generator 4.
Stop and start buttons 5 and 6 and a clear button 7 are used to
control the apparatus. The clear button 7 is operated initially to
reset both of the binary counters 1 and 2 to the zero state, it
setting a bistable 8 and thereby causing the latter to supply a
reset signal to both counters. The bistable 8 is then automatically
reset itself by a reset signal which is generated by the counter 2
when in its zero state and which is supplied to the bistable 8
through a connection 9. After the clear button 7 has been operated
the start button 5 can be operated to allow the supply of pulses
from the oscillator 3 to the counters 1 and 2. The stop and start
buttons control setting and re-setting, respectively, of a bistable
circuit 10 which has an output connection to the input of a gate
11. The gate 11 also has input connections from the oscillator 3
and generator 4. Operation of the start button 3 produces an output
signal from the bistable 10 which allows the gate 11 to pass a
coincidence pulse every time that the oscillator 3 and generator 4
produce output pulses simultaneously. This pulse sets two bistable
circuits 12 and 13 causing the first to open a gate 14 so that it
passes oscillator pulses from the oscillator 3 to the two binary
counters 1 and 2, and causing the second to open transmitting gates
15 so that coded pulses corresponding to these input oscillator
pulses are gated from the counter 1. Successive coincidence pulses
are produced in this way and cause the counter 1 to generate
successive sets of coded pulses to energize groups of transducer
elements until such time as the stop button is operated to reset
the bistable 10 and close gate 11.
The required coding pattern of successive output pulses from the
counter 1 is provided by arranging that the counter 1 is a binary
counter with a maximum count capacity of 89, and that the counter 2
is a binary counter with a maximum count capacity of ninety and
controls the gate 14. After the first coincidence pulse is passed
by the gate 11 both counters count up from zero together and in
each of the states 0 to 9 the counter 1 passes a correspondingly
binary coded output pulse through a binary/decimal converter unit
16 and the open transmitting gates 15 to a decoder unit 17. On the
count of 10 the counter 2 generates a reset signal which is
supplied through a connection 18 to the bistable 13 resetting the
latter and thus closing the transmitting gates 15. Thus, as both
counts count on past nine none of the coded output signals, 10
onwards, from the counter 1 pass to the decoder 17. Eventually, the
nintieth oscillator pulse causes the counter 1 to reset itself to
zero automatically and puts the counter 2 into its ninetieth state.
The next oscillator pulse thereafter puts the counter 1 into its
one state and causes the counter 2 to reset itself to zero
automatically and to pass a reset signal through a connection 19 to
bistable 12 which responds by closing the gate 14 to stop any
further oscillator pulses passing to the counters.
The two counters are now out of step with one another by one count
and when the second coincidence pulse opens the gate 14, counter 1
counts up from 1 to 10 while counter 2 counts up from 0 to 9.
Counter 2 again closes the transmitting gates 15 on the count of 10
which stops the coded pulses 11 onwards from passing between the
counter 1 and the decoder 17. Further, both counters are eventually
reset to zero but because they are out of step by one count in
counting up from zero, counter 1 has reset and reached the count of
two by the time counter 2 has reset to zero and closed gate 14.
Thus, on receipt of the third coincidence pulse the two counters
count up from zero out of step with one another by two counts and
the transmitting gates 15 passes the coded pulses 2 to 11 before
they are again closed by counter 2 on the count of ten and the
counter 1 resets and finally reaches the count of three before
counter 2 resets to zero. Similarly, the fourth coincidence pulse
causes the transmitting gates 15 to pass the coded pulses 3 to 12,
the fifth coincidence pulse causes the transmitting gates 15 to
pass the coded pulses 4 to 13, and so on. The 81st, 82nd, and 83rd
coincidence pulses will cause the transmitting gates 15 to pass the
coded pulses 80 to 89, 81 to 0, and 82 to 1 respectively, and
eventually the 91st coincidence pulse will cause the transmitting
gates 15 to pass the coded pulses 0 to 9 again.
Successive trains of ten coded pulses are therefore fed to the
decoder unit 17 which is adapted to respond to each coded pulse in
turn by opening a correspondingly coded gate to energize an
associated transducer element in the ring so that successive
corresponding groups of transducer elements are energized The time
intervals between successive pulses in each train is determined by
the repetition frequency of the oscillator 3 and is preset so that
the group of transducer elements energized by each train of coded
pulses emits ultrasonic energy to form a focus at a particular
point. The time interval between each train of pulses is determined
by the repetition frequency of the generator 4 and is preset to
define a receiving interval after each energizing interval during
which a particular one of the transducer elements of the group last
energized is used to receive any return echo signals from
flaws.
The receiving interval after each energizing interval is triggered
automatically by the same coincidence pulse passed by the gate 11
to start that energizing interval. This pulse operates a first
monostable circuit 20 which, after a set delay ending after counter
2 resets to zero, operates a second monostable circuit 21. The
operated monostable 21 remains in its astable state for a set time
ending before the gate 11 passes the next coincidence pulse to
start the next energizing interval. While in the astable state it
supplies an output signal to receiving gates 22 and thereby holds
them open to allow a coded pulse derived from the output of the
counter 1 to pass from a binary/decimal converter unit 23 to a
decoder unit 24. The receiving gates 22, converter 23 and decoder
24 are all similar to the transmitting gates 15, converter 16 and
decoder 17 and they operate in a similar manner except that they
handle only the one coded pulse derived from the output of the
counter 1 and that the decoder unit 24 operates a correspondingly
coded gate to gate electrical echo pulses from an associated
transducer element rather than to energize that element. The coded
pulse may correspond directly to the state of the counter 1 at the
end of the energizing interval this pulse being fed to the
converter 23 through an addition unit 25 which is preset so as to
have no effect on the pulse. Alternatively, however, the addition
unit 25 can be preset so as to add any number from one to eight to
the coding number of the pulse from the counter 1. By these means
it is possible to arrange that any one of the last nine of the
transducer elements which are energized in an energizing interval
can be used as a receiver in the following receiving interval.
If the addition unit is set to zero the coding of the pulse fed to
the decoder 24 will be 1 after the first coincidence pulse, 2 after
the second coincidence pulse, and so on. Clearly, if the addition
unit is set to add one or any other number up to eight the coding
of all of these pulses will be increased by one or the appropriate
number.
The electrical echo pulses gated from the transducer element in
each receiving interval are used to produce a visual display on a
cathode ray tube 26, the pulses being fed to an integrating and
amplifier unit 27 which feeds the resulting output to the Y-plates
of the cathode ray tube 26. The Y-deflection corresponding to each
of the ninety different receiving intervals is displayed at a
different X-coordinate on the screen of the cathode ray tube by
using the coincidence pulses from the gate 11 to drive a staircase
generator 28 with an output to the X-plates of the cathode ray
tube. Each coincidence pulse causes the generator 28 to deflect the
beam one step in the X-direction and the generator 28 allows 89
such deflections progressively across the screen before returning
to the start again. Also, the cathode of the cathode ray tube 26 is
fed a bright up pulse for each separate Y-deflection which is
derived from the output of the monostable 21 and which is delayed
by a monostable 29 so as to coincide with the corresponding
Y-deflection. The display on the screen as shown at 30, therefore,
comprises a succession of 90 horizontally spaced vertical lines
each proportional to the energy of the return echo signals from a
different focus.
In the system illustrated in FIG. 6, the oscillator 3, generator 4,
two binary counters 1 and 2, the decoder 17 and intermediate gates
and bistable circuits all correspond to the computer C in the
system illustrated in FIG. 1; and the other monostable circuits,
decoder 24 and cathode ray tube 26 correspond to the analyzer unit
A in the system of FIG. 1, where the analyzer unit A is coupled
with the computer C. It will be appreciated, however, that the
system of FIG. 6 is only suitable for testing tubes or like
cylindrical test-pieces whereas the system of FIG. 1 can be used
for testing test-pieces other than tubes if the computer C is
programmed appropriately. Further, it will be appreciated that the
transducer array shown in FIG. 1 need not necessarily be circular
and may be linear in an alternative embodiment of the
invention.
Further, in other alternative embodiments of the testing systems
according to the invention, the single ring of transducer elements
as shown in the illustrated embodiments may be supplemented by
further rings which form a cylindrical array of transducer
elements. The effective focus can then be scanned along the tube as
well as around it thus enabling transverse flaws (i.e., flaws
perpendicular to the axis of the tube) to be detected.
In yet other alternative embodiments of the invention instead of
arranging the transducer elements in an array which is spaced from
the surface of the test-piece, the transducer elements may be
arranged in contact with the surface of the test-piece so as to
transmit ultrasonic energy directly into the latter. In such
embodiments there will be no refraction of the transmitted energy
at the surface and foci will be formed directly as shown in FIG.
1.
* * * * *