Electrodynamically producing ultrasonic waves

An electrodynamic transducing device includes two staggerdly coils with a distance of staggering equal to a quarter of a wavelength of ultrasonic waves; unidirectional transmission characteristics of the transducer is obtained by feeding the two coils separately with HF current at a time difference of feeding of the respective two signals to be equal to a quarter of the wavelength such that a reinforcing ultrasonic signal is produced in the direction of the coil receiving said signal earlier in time and complete cancelation of acoustic waves obtains in the opposite direction, whereby in case of equal pitch of said coils the HF signals are of equal contour but of opposite polarity while in case of opposite winding pitch the signals not only have equal signal contour but also the same polarity.

BACKGROUND OF THE INVENTION 
The present invention relates to an electro-dynamic transducing device for 
producing ultrasonic waves with a unidirectional characteristics, and 
including two coils arranged in a lanbda/4 staggered (interdigitized) 
relationship and in the direction of beaming; these two coils are fed with 
high frequency currents so that acoustic signals result which interfer in 
a reinforcing fashion in the direction of beaming but cancel each other 
(or are supposed to cancel) in the opposite direction. 
A transducer of the type to which the invention pertains is, for example, 
disclosed and described in German Pat. No. 2,655,804, particularly column 
8, line 44 et seq. The transducer described a Lamb wave transducer but the 
present invention refers more broadly also to transducer which may use 
surface and plate mode osciallations, also tubular wave kind conductor 
transducers ought to be included. 
Transducers generally of the type in which windings receive simultaneously 
a phase-shifted high frequency signal offers the advantage that, indeed, a 
large ultra-sonic amplitude is produced in the direction of beaming. 
However, this kind of device is disadvantaged by the fact that the feeding 
of a hgh frequency signal will produce initially a first acoustic wave by 
means of the rear coil without concurrent production of an acoustic wave 
by the front coil in that location of the rear coil so that the cancelling 
effect in the undersided direction does not obtain. As a consequence a 
certain parasitic wave is produced which, in turn, may result subsequently 
in undesired parasitic echo effects. 
DESCRIPTION OF THE INVENTION 
It is an object of the present invention to provide a new and improved 
electro-acoustic transducer wherein the cancellation of undesired waves is 
complete and observable up initio. 
In accordance with the preferred embodiment of the present invention it is 
suggested to feed an hf-current to that one of the coils which is situated 
first as seen in the direction of the desired beaming, and the other more 
rearwardly oriented lambda/4 staggered coil receives an hf-powering signal 
delayed by T/4 whereby, however, the signal shape is the same for both 
coils, particularly if they are wound with opposite pitch, while an 
hf-feed signal of similar shape but with opposite polarity is used as the 
two lambda denotes the wavelength (of the acoustic wave) and T is the 
oscillation period. 
Such a transducer, therefore, will produce an acoustic signal in but one 
direction, namely in the direction towards that coil which receives the 
hf-signal earlier than the other one; in other words, the direction of 
acoustic wave propagation is determined by the timing sequence of feeding 
current to the two coils. The first in time operating winding produces an 
acoustic signal which to the extent it propagates opposite to the desired 
direction will be acoustically cancelled by operation of the somewhat 
delayedly produced signal in the other (rear) coil. While on the other 
hand in the direction of desired ultra-sonic wave propagation a maximum 
amplitude is produced just as described with reference to the device in 
German Pat. No. 2,655,804, column 8, line 44 et seq. It is, however, 
important that the feature of the invention is applicable independent of 
the particular mechanical mode by means of which acoustic waves are 
produced.

Proceeding now the the detailed description of the drawings, FIG. 1 
illustrates a tube 1 to be inspected and tested with regard to flaws and 
defects, and for this purpose tube 1 passes progressively through a 
tubular transducer in the direction of arrow 9. The transducer itself is 
comprised of a tube or sleeve 2 with external annular grooves denoted by 
reference numerals 3, 4, 5, and 6. These grooves are spaced apart by a 
distance equal to one forth of the wavelength of the principal acoustic 
waves as there will be produced by and in the inventive transducer. The 
grooves 3, 4, 5, and 6 receive two coils or windings 7 and 8. Herein the 
winding 8 is first in direction of desired wave propagation, the arrow 9 
denotes also the direction of desired wave propagation. The particular 
coil 8, therefore, has a front winding in groove 4 and is connected to the 
second coil circular 8 by means of a connection 10, the connection then 
runs back to groove 6. The second coil 7 being more to the rear vis-a-vis 
the direction of desired wave propagation, has a front winding which runs 
in groove 5 and through a connection 11 to groove 3 and back to groove 5. 
In the particular example, therefore, the two interdigitized coils are 
wound in hf-generators, not shown in FIG. 1, but explained with reference 
to FIG. 3, and these hf-generators feed hf-current to the coils. 
Bevore describing the hf-current feed mechanism, the operation of the 
device in accordance with the invention will be explained with reference 
to FIG. 2. In accordance with the invention coil 8 receives a feeder 
hf-current first in time and the time is denoted by t=0, top diagram of 
FIG. 2. This way two ultra-sonic waves are produced in the tube. These 
waves are denoted by reference numeral 12 and 13. They are, in fact, in 
the instant of production one and the same wave, but propagate in opposite 
directions, i.e. in the desired direction 9 as well as in the opposite 
direction. 
At the time t=T/4 wherein T is the oscillation period of the desired 
ultra-sonic wave, these waves have a contour respectively indicated by 
reference numerals 14 and 15 in the middle diagram of FIG. 2. One can see 
that they have propagated by a distance of lambda/4 from the initial point 
of production to the right and for an equal amount to the left so that, in 
fact, they are in that instant out of phase by lambda/2. In that instance, 
i.e. at the instance of t=T/4 coil 7 is fed with an hf-current being 
delayed in relation to the feeding of hf-current to the coil 8 by T/4. As 
a consequence two "new" ultra-sonic waves 16 and 17 are produced which 
also propagate in opposite directions. In fact, of course, these waves are 
not produced side by side but superimposed so that, in fact, then, a 
complete wave anihilation occurs in the direction opposite to direction 9. 
The newly produced wave is superimposed upon 14 so that in fact a wave 
12-14 propagates in direction 9 and in a phase reinforcing fashion. The 
oscillation and ultra-sonic wave production in the opposite direction 
cancel so that, for example, at the end 19 of the tube 1, no reflection of 
ultra-sonic waves occur. Also, if there are any defects in the tube to the 
left of the transducer, the defect will likewise not produce any echos. On 
the other hand, the ultra-sonic wave propagating in the direction 9 has a 
contour 18 at thetime t=T/2 is shown in the lower diagram of FIG. 2. 
It can readily be seen that the principle of the invention can be expanded 
to include more than two coils and a transducing material in that a 
plurality of such coils are arranged in the axial direction of the system 
and the feeding is timed to obtain a progressive build-up of acoustic 
waves in the desired direction and pairwise cancelation in the opposite 
direction. 
Turning now to the block diagram of FIG. 3, the tube 1 and the two 
transducer coil 7 and 8 are shown again together with schematic indication 
of current feeding. The coil 8 receives a wave signal 22 from a 
transmitter amplifier 21 and the coil 7 receives a wave signal 24 from the 
transmitter amplifier 23. As is schematically shown in FIG. 3, these two 
waves are out of phase by lambda/4. The two amplifiers 21 and 23 are 
operated from the same signal source such as a pulse source 25, producing 
directly a sinusoidal or any other suitable wave contour in the 
transmitter amplifier 21 while the same pulse is fed to the transmitter 
amplifier 23 but through a delay device 26. 26 is a true delay device so 
that the pulse shape as it is effective at the input sides 21 and 23 is 
the same and the two devices 22 and 23 should be of similar construction 
so that the two waves 22 and 24 have the same contour, amplitude and 
frequency differing only by lambda/4 in phase. 
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.