Length mode piezoelectric ultrasonic transducer for inspection of solid objects

The transducer is constructed from individual transducer elements arranged in an array and configured to exhibit a predominant, longitudinal mode transversely to the array. The elements are interconnected through thin flexible sheets. Each element is individually damped, and the transducer as a whole is electrically damped through resonance with the clamped capacitance and dissipation. Electrical control permits in-phase operation of all transducer elements or control with preselected phase differences.

BACKGROUND OF THE INVENTION 
The present invention relates to ultrasonic transducers for inspecting 
materials having very low acoustic impedance, for example porous and 
fibrous materials. 
The ultrasonic inspection of low acoustic impedance materials such as 
polyurethane foam or fibrous ceramics and others is a very difficult task 
for a variety of reasons. Ultrasonic inspection is usually carried out by 
means of piezoelectric transducers. The particular piezoelectric materials 
which are suitable for serving as active elements in ultrasonic 
transducers have an acoustic impedance which is much larger than the 
acoustic impedance of foam or of fibrous ceramic material. By way of 
example, lead-zirconate-titanate, a typical piezoelectric material, has an 
acoustic impedance which is almost 700 times the acoustic impedance of 
polyurethane foam, and about 500 times the acoustic impedance of fibrous 
silica ceramic. In other words, there is an inherent, significant mismatch 
in the acoustically active and generating material of the transducer on 
the one hand, and certain materials to be inspected on the other hand. 
As such a transducer interfaces with low impedance material for purposes of 
transmitting thereto acoustic signals, most of the vibrations will be 
reflected back into the transducer, and very little energy will propagate 
into the material to be inspected. While a sufficiently strong inspection 
signal can be generated simply by driving the transducer with sufficient 
power, most of the electric energy applied to the transducer will remain 
therein and will be dissipated in some fashion. Accordingly, the 
transducer will ring so that short range echo signals returning to the 
transducer are readily obscured. Intensive damping of tranducers of 
available construction was found to be inadequate because it desensitizes 
the transducer for receiving echo signals to such an extent that only very 
strong echos can be detected. 
The problem outlined above is compounded by the fact that transducers must 
be sufficiently broad banded for reasons of adequate resolution. Moreover, 
the transducers must have a sufficiently wide aperture to emit a 
relatively large wave front while capturing return echos over a 
sufficiently wide geometric range and area. It was found that conventional 
transducers vibrate in a variety of modes but only one mode, namely the 
mode oscillating in the direction normal to the interface with the object 
to be inspected, is of interest. Limiting the band width and/or providing 
for broad banded strong damping (to impede ringing) for eliminating the 
unwanted modes desensitizes, again, the transducer, and weaker echo 
signals will not be detected. 
The problem is further compounded by the fact that porous and fibrous 
material attentuate high frequency acoustic signals to such an extent that 
the signal fails to penetrate sufficiently deep into the materials 
inspected. Lower frequencies have a better penetration than higher 
frequencies, but ringing is more pronounced at lower frequencies. As was 
mentioned above, such ringing tends to obscure echos at lower frequencies, 
particularly if the echos are weak. These problems and alternative 
attempts to solve them are discussed in a paper by me and another 
"Proceedings 10th Symposium on NDE," San Antonio, Tex., Apr. 23-25, 1975, 
published later in that year. 
Upon considering the foregoing, it must be borne in mind that as long as 
piezoelectric transducers are to be used, the very high acoustic impedance 
mismatch with a porous or fibrous material is an inevitable constraint. 
Different piezoelectric materials may be discovered in the future but, 
broadly speaking, it cannot be expected that one will find always the 
suitable piezoelectric transducer material for each kind of material to be 
inspected. Additionally, the dependency of the penetration depth of 
ultrasonic vibrations on frequency is an inherent property. Thus, the 
detection of deep penetration echo signals makes mandatory the use of as 
low an inspection frequency as possible. 
Considering these conditions as outlined above, it must readily be said 
that the ultrasonic inspection of construction parts made of porous or 
fibrous materials has not yet been adequately solved, and the difficulties 
encountered originate with basic properties of the materials involved. 
DESCRIPTION OF THE INVENTION 
It is an object of the present invention to provide a new and improved type 
of piezoelectric transducer for the ultrasonic inspection of low 
impedance, for example, porous or fibrous materials. 
In accordance with the present invention it is suggested to construct a 
transducer from a plurality of bar-like elements having dimensions which 
are relatively small in the plane of interfacing with the object to which 
they are acoustically coupled for transduction, but the elements are 
comparatively long in the direction extending transversely thereto so as 
to have a dominating, single mode for vibration in that length direction 
which mode is at least substantially the same for all transducer elements 
of the plurality. The small end faces of the elements are arranged in an 
array, preferably of regular spacing, whereby at least some of these 
transducer elements are driven electrically to operate mechanically in 
parallel or at least in a definite phase relation. The elements are 
mechanically interconnected by at least one thin, flexible sheet which 
does not couple them together mechanically in the sense that vibrations 
could be transmitted from one element to the others. Each transducer 
element is additionally provided with a damping cover or pad on its side 
or sides other than the end faces. These damping elements vibrate with the 
elements and dissipate mechanical energy. Generally speaking, damping of 
the longitudinal mode in this fashion suffices, the damping elements do 
not have to be effective, e.g., for any transverse mode. The mechanical 
damping is augmented by electrical damping in a manner known, per se, for 
individual transducers in that an inductance and a damping resistor are 
connected electrically across the transducers. The inductor resonates with 
the clamped capacitance of the transducer at the resonant mode frequency 
of the transducer elements so that a significant amount of driving energy 
is dissipated in the damping resistor. 
While a simple square shaped array of a plurality of elements was found to 
readily suffice for regular inspection, one could use circular, hexagonal 
or other types of arrays. Also, the transducer elements may have prism or 
cylindrical configurations. Essential for the invention is that a 
relatively large transducing aperture is more or less covered by spaced 
apart elements whose end faces have small dimensions in the plane of that 
aperture, but the elements are relatively long in the direction 
transversely to that plane so that only the mode as produced by each of 
the elements in that direction dominates by far as far as amplitude is 
concerned, and any other mode is small and quite remote in the frequency 
spectrum. Furthermore, it was found sufficient to drive all the 
transducers in parallel in the strictest sense, but by introducing phase 
shifts and/or different drive signal amplitudes one may provide for 
focusing or shaping or steering of the resulting ultrasonic beam. Also, 
some of the transducer elements may be used for transmission only, others 
may exclusively receive. Still alternatively, not all transducer elements 
may transmit and receive, some may have these dual functions, while others 
have only one such function.

Proceeding now to the detailed description of the drawing, the figures show 
a new transducer 10 being composed of 25 individual bars 11 made of a 
piezoelectric material such as lead-zirconate-titanate, or PZT for short. 
Each bar shaped prism has a square shaped cross-section but is 
considerably longer than wide and thick. By way of example, each bar is 
0.28 cm by 0.28 cm in cross-section and has a length of about 1.42 cm (or 
0.11 inches by 0.11 inches by 0.6 inches). 
Each bar carries at its square shaped end face a layer of silver 12 and 13, 
and each of these layers is less than 1 mil thick. These layers serve as 
electrodes for exciting the bar in the longitudinal mode or for sensing 
voltage differences across the bar in case the bar is caused to vibrate 
from the outside. This way a plurality of, altogether, 25 individual or 
elemental transducer elements is provided. The sides of each bar 11 are 
covered, at least in parts, by thin slabs 17 of rubber, for example, for 
purposes of damping to be described and discussed more fully below. 
These bars 11 each constitute an elemental transducer or transducer 
element; they are arranged in an array so that their respective end faces 
are co-planar. The bars are spaced so that the distance a from center axis 
to center axis along the rows and columns of the array is the same 
throughout. That arrangement is chosen so that the distance a, being also 
the center to center distance of adjacent bar end faces, approximates a 
wave length of the operating transducer signal in the medium to which the 
transducer is coupled for inspection. Presently it is assumed that the 
transducer is to be used for inspecting a porous part made of fibrous 
silica ceramic. Therefore the distance is a little under half a cm (about 
2/5 of a cm) for an inspection and operation frequency of about 100 
kiloherz. The length of each transducer bar is of course equal to half a 
wave length of the longitudinal resonant mode frequency of the bar. 
The bars 11 are bonded to thin, flexible steel sheets having a thickness of 
about 1 mil to insure proper positioning of the bars in the array while 
interconnecting the electrodes of corresponding bar end faces 
electrically. These sheets 14 and 15 can, therefore, be considered to be 
two common electrodes or feed or input-output electrodes for all of the 
elemental transducers. Common electrical driving signals are applied to 
the sheets when the transducers are to be operated as transmitter, and the 
sheets serve as pick-up electrodes for all elemental transducers when 
functioning as receivers. 
The sheets 14 and 15 are specifically bonded to the electrodes by means of 
a silver paste or a conductive epoxy. The entire assembly of bars and 
sheets is potted in rubber 16 whereby, however, the outer surface of one 
of the sheets, for example sheet 14, remains exposed and thereby defines 
the transducing aperture; the boundary 16' delineates that aperture. The 
physical interconnection of the elemental transducers as provided by 
sheets 14 and 15, together with the potting, establishes the transducer 
array as a structural and operational unit in which, however, 25 points or 
small areas are provided in an array for purposes of electromechanical 
transducing. The exposed sheet 14 with 25 transducer bars in its back 
synthesizes a relatively large aperture, which in this case is about 2.2 
by 2.2 cm. 
The transducer array, as described, has in fact only a single dominating 
mode of vibration which is established by the length or longitudinal mode 
of each of the transducer bars. Due to the fact that each bar is 
considerably longer than wide and thick, hardly any other mode exists, and 
the bars each resonate at practically that one frequency only. Moreover, 
the interconnection of the bars does not couple them together acoustically 
so that the system as a whole does not have any transverse or radial mode 
(see FIG. 6). 
In operation, the aperture--window (16') of the transducer 10 is juxtaposed 
to a surface of an object A for interfacing therewith. As outlined above, 
the acoustic impedance of the individual transducer elements and bars is 
much higher than the acoustic impedance of some of the materials to be 
inspected so that little energy is coupled out of the transducer into 
object A if the transducer operates as a transmitter; most of the energy 
remains in the transducer elements and causes them to ring. Ringing is 
suppressed in a two-fold approach and by combining mechanical and 
electrical damping. 
Mechanical damping is obtained by the slabs 17 made, for example, of 
neoprene rubber. These slabs are bonded to each side of the elemental 
transducers. The rubber vibrates with the transducer bar and introduces 
considerable losses of energy. However, the attenuation is not so strong 
that the sensitivity of the transducer is too severely reduced. Since each 
bar has substantially only one mode the damping needs to be effective for 
that one mode only. The rubber slabs have about the same length dimension 
as the bars have themselves so that they are in fact optimized as to the 
specific damping requirements for this case. 
The mechanical damping thus provided does not, however, entirely suppress 
the ringing. For this reason electrical damping is introduced in addition. 
FIG. 3 shows schematically the transducer circuit. Reference 20 denotes an 
electrical signal source and generator which produces, for example, on 
demand a brief pulse with steep leading and trailing edges or it may 
produce a burst of HF signal having a frequency which is about equal to 
the longitudinal mode frequency of the transducer bars. 
A control circuit 21 controls a switch 22, being actually composed of 
electronic gates, which connects the transducer 10 either to the signal 
source 20 or to a receiver circuit 23 which responds to any voltage signal 
developed across each and all of the elemental transducers. Transmitter 
(source 20) and transducer 10 should be isolated from each other during 
receiving because the low impedance of a typical signal source would 
render the electrical damping ineffective. A switch over in the circuit 
from 20 to 23 occurs directly following the trailing edge of a generator 
pulse or burst. Reference numerals 14', 15' refer to the common electrode 
connection by and through the sheets 14, 15 for the electric circuit which 
drives and monitors the transducer. 
In order to provide electrical damping of any transducer ringing following 
the application of a transmitter signal, an inductance 25 is electrically 
connected in parallel to all the transducer elements. FIG. 4 shows the 
equivalent circuit of the transducer elements. They can be represented 
electrically by a series RCL circuit 27 connected in parallel with its 
clamped capacitance C.sub.o. The inductance 25 is chosen to resonate with 
all the clamped capacitances of the transducer at the operating frequency. 
The energy that is drawn from the transducers is readily dissipated in a 
resistor 26 being connected in parallel to inductance or coil 25. It was 
found that this circuit achieves damping of any residual ringing in the 
transducer so that ring-down time becomes very short. This in turn means 
that ringing has sufficiently decayed before any echo arrives at the 
transducer. 
The square shaped transducer array of 5 by 5 individual transducers 
represents a particular assembly for establishing a particular large, 
effective aperture using transducer elements, each of which having a 
comparatively small effective surface oscillating in a direction normal 
thereto. This arrangement was found to be convenient and practical and 
solves the problems outlined above. FIG. 6 shows the equivalent electrical 
impedance of the transducers plotted against frequency for a large range 
of frequencies. The longitudinal mode of each element has a frequency of 
about 100 kiloherz and the plotted characteristic exhibits no other modes. 
A single transducer having width dimensions similar to the width dimension 
of the array as a whole, has many other modes in that range. FIG. 5 shows 
by way of example such a characteristic of a cylindrical disc covering the 
same aperture area. The figure shows many modes of which the longitudinal 
is but one, and not even the strongest one. For further details on such a 
transducer see the paper referred to above in the chapter on the 
background of the invention. 
The significance of FIG. 6 when compared with FIG. 5 is to be seen further 
in the fact that both of them were generated by devices which did not have 
electrical damping. Thus, the unwanted mode suppression is solely the 
result of the array configuration wherein the individual transducer 
elements are however, mechanically damped by the side slabs 17. 
In summary, it can readily be seen that each transducer element and the 
transducer as a whole is sufficiently damped so that ringing decays within 
a few cycles following a sharp and definite pulse when applied to the 
transducer so that even a short range echo from a rather small flaw or the 
like becomes readily detectible. Since there are no noticeable parasitic 
modes, electrical damping does not have to be excessive so that broad 
banded echos are still readily detected, which, in turn, means that the 
penetration depth of the transducer is as satisfactory as can be expected 
for porous material. 
As shown somewhat schematically in FIG. 7, the electrodes or the transducer 
elements facing the transducing aperture could be connected to separate 
steel strips 14a, 14b, 14c, etc. For example, the end faces of the 
transducer elements pertaining to the same row are connected to such a 
common steel strip. The several strips 14a, 14b, 14c, etc., receive 
electrically driven signals separately and with a predetermined phase 
difference .phi., 2.phi., etc. This way, the emitted wave front is tilted 
and steered in a direction which is not normal to the transducer object 
interface. The phase shift .phi. between the several signals determines 
the steering or tilt angle. For practicing the invention in this manner, 
the transmitter circuit may provide a predetermined signal S, and the 
phase shifted signals S + .phi., S + 2.phi., etc., are produced through 
suitable delays. 
The 5 by 5 array of square shaped transducer prisms is only one mode of 
practicing the inventions though presently deemed the preferred mode. 
However, each transducer element could have still smaller cross sections 
and a larger number of elements may be needed for covering the same 
aperture. It was found that there is no need for such an increase, 
particularly, then, there is no need for increasing the length to cross 
section ratio. The chosen dimensions are sufficient to avoid any 
interfering parasitic modes. On the other hand, a different number of bars 
in the array should be used if a different aperture width is desired. 
The rectangular and square shaped kind of array was found well suited for a 
transducer when used for inspecting material by the standard pulse echo 
method. In cases, however, it may be desirable to use a circular array as 
shown in FIG. 8. Moreover, each bar is of cylindrical construciton, but 
these bars 31 are of similar length. Each round bar has its cylindrical 
surface covered with a rubber hose or several of them being shrunk onto 
the piezoelectric material and serve as damping medium. 
The transducers may be grouped in that the central group is connected to 
one electrode 33 and connecting plate, and the outer ring of transducers 
is connected to a corresponding annulus 34. The opposite ends of the 
transducer elements may be connected by a common sheet. Also, the assembly 
may be potted as described above. Such an arrangement permits separate 
control of the central and of the outer transducers as regards to 
excitation as indicated by the separate blocks in FIG. 7, and labelled 
source 1 and source 2 respectively. 
Particularly, upon choosing different amplitudes and/or phase for the 
energizing signals for inner and outer frequencies the resulting wave 
front being launched into the interior of the object under investigation, 
is shaped and/or focused therewith. 
The invention is not limited to the embodiments described above but all 
changes and modifications thereof not constituting departures from the 
spirit and scope of the invention are intended to be included.