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Description  |
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This invention relates to variable acoustic wave energy
transfer-characteristic control devices and, more particularly, to such
devices using multiple piezoelectric layers.
It is known that a single-layered piezoelectric plate with its opposite
faces metallized forms a three-port device. The three ports comprise one
electrical port between the respective electrodes formed by the two
metallized surfaces, across which a voltage can exist, the two mechanical
ports consisting of each of the respective faces of the plate, over which
a force can exist. The impedance of the electrical port depends upon the
boundary conditions at the two electrodes. By closing and opening a switch
connected across the electrodes, the electrical port can be shorted or
opened. Either of the two mechanical ports is in the open-circuit
condition when the corresponding piezoelectric plate face is placed
against a "rigid" wall (i.e. a wall having a very high acoustic impedance)
and is in the short-circuit condition when it is in a vacuum or when it is
in contact with a very low acoustic impedance, such as air.
In the past, such single-layered piezoelectric plates, with respective
electrodes on each of its opposite faces, have been used acoustically
mainly as electro-acoustic transducers for transmitting and/or receiving
sonic or ultrasonic wave energy. More recently, it has been proposed that
one or more of such single-layered piezoelectric plates, along with
appropriate electrical control circuitry, be placed in the path of a
propagating acoustic wave and be used as a variable acoustic wave energy
transfer-characteristic control device exhibiting variable acoustic
reflection and transmission coefficients. Such variable acoustic wave
energy transfer-characteristic control devices are described in the
following articles:
(1) E. K. Grishchenko and L. I. Kholod, "Acoustic Impedance and
Transmissivity of and Electrically Loaded Piezoelectric Element," Sov.
Phys. Acoust. 21, p. 252, 1975.
(2) E. K. Grishchenko, "Acoustic Analog of the Electro-Optic Shutter," Sov.
Phys. Acoust. 21, p. 511, 1975.
(3) E. B. Krivin, "Reflection of a Plane Sound Wave from an Electrically
Loaded Piezoelectric Layer," Sov. Phys. Acoust. 21, p. 144, 1975.
Article (1) discusses a device with a series of two contiguous
piezoelectric plates, each plate having an acoustic thickness equal to
one-half of a given acoustic wavelength. The first plate acts as a
conventional transducer for generating or receiving acoustic wave energy.
The second plate, which has a variable Q circuit electrically connected
across the electrodes thereof, is situated between the first plate and an
acoustic propagating medium and operates as a control element for
controlling the acoustic transmission of the wave energy to or from the
propagating medium as a function of the Q of the variable Q circuit.
Specifically, the transmission approaches unity as the Q approaches zero
and the transmission approaches zero as the Q approaches infinity.
However, in practice, the range between the maximum and minimum Q (and,
hence, the range between the maximum and minimum acoustic transmissivity)
is limited.
Article (2) suggests the use of a control device, of the type described in
Article (1), as an acoustic shutter for selectively transmitting or
reflecting incident acoustic wave energy. Article (3) analyzes more
complex electrically-controlled acoustic networks which may be composed of
a plurality of half-wave piezoelectric plates. Each individual one of the
piezoelectric plates in Article (3) has associated therewith its own
variable electrical load connected across the electrodes thereof, so that
each piezoelectric plate together with its associated electrical load
operates as a separate variable acoustic impedance of the overall acoustic
network. Article (3) suggests that such a controllable acoustic network
may be employed as an acoustic gate by appropriately switching the state
of the respective electrical load of each individual piezoelectric plate
of the acoustic network. Thus, in article (3), the acoustic network is
comprised of a plurality of interconnected substantially independent
single-layered piezoelectric plates, each exhibiting an effective
impedance determined by its own electrical load.
The present invention makes use of a variable acoustic wave energy transfer
characteristic control device including a piezoelectric unit which
comprises a plurality of contiguous layers of piezoelectric material which
defines two outer faces and define an interface between each pair of
adjacent ones of the contiguous layers. Each of the layers has a
respective specified thickness. The piezoelectric unit also comprises
first and second electrodes respectively at the outer faces and at least
one additional electrode at each interface. The variable acoustic wave
energy transfercharacteristic control device also includes an electrical
circuit connected across the first and second electrodes and switch means
for selectively connecting at least one additional electrode to another
electrode which selectively short circuits at least a portion of at least
one of the layers of the piezoelectric unit. Thus, the multi-layered
piezoelectric unit together with the given electrical circuit and the
switch means operate as a unitary acoustic wave energy control device
having at least two states. Also disclosed herein are various acoustic
wave propagating systems incorporating such a unitary acoustic wave energy
control device.
In the Drawings:
FIG. 1 is a schematic diagram of a generalized embodiment of a variable
acoustic wave energy transfercharacteristic control device incorporating
the principles of the present invention;
FIG. 2 is a block diagram of the equivalent mechanical circuit of the
control device of FIG. 1;
FIG. 3 illustrates the change in the transferfunction characteristic of the
control device of FIG. 1 in accordance with the switch condition of the
switches thereof;
FIG. 4 is a block diagram of a first acoustic wave propagating system
utilizing a specific embodiment of the acoustic wave energy control device
of the present invention;
FIG. 4a shows an electrical equivalent circuit of the piezoelectric unit of
FIG. 4 together with the electrical circuit of a matching network
therefor;
FIG. 5 illustrates a second acoustic wave propagating system incorporating
a specific embodiment of the present invention;
FIG. 6 illustrates the third acoustic wave propagating system incorporating
a specific embodiment of the present invention;
FIG. 7 illustrates a fourth acoustic wave propagating system incorporating
a specific embodiment of the present invention; and
FIG. 8 illustrates a fifth acoustic wave propagating system incorporating a
specific embodiment of the present invention.
Referring now to FIG. 1, multi-layered piezoelectric unit 10 may comprise
two or more contiguous layers of piezoelectric material (such as the three
layers, 12, 14 and 16 shown in FIG. 1). Layer 12 has a first specified
thickness a; layer 14 has a second specified thickness b, and layer 16 has
a third specified thickness c.
Unit 10 further includes first electrode 18 at the upper outer face
thereof, second electrode 20 at the lower outer face thereof, additional
electrode 22 at the interface between layers 12 and 14, and additional
electrode 24 at the interface between layers 14 and 16. Electrical circuit
26 is connected across first and second electrodes 18 and 20. Electrical
circuit 26 may include one or more reactive and/or resistive impedance
elements and, in any given case, may or may not include a voltage source
and/or a detector. Switch means comprising first switch 28, second switch
30 and third switch 32 connected to electrodes 18, 20, 22 and 24, may be
utilized to selectively short circuit one or more of piezoelectric layers
12, 14 and 16. In practice, switches 28, 30 and 32 may comprise electronic
gates. The upper outer face of control device 10 is in effective contact
with acoustic propagating medium I and the lower outer face of unit 10 is
in effective contact with acoustic propagating medium II. In any given
case, mediums I and II may be the same or may be different from each
other.
The operation of the generalized embodiment of the present invention shown
in FIG. 1 makes use of one or more of the following electro-acoustic
principles known in the art: An electro-acoustic transducer transforms
electrical parameters into mechanical parameters and mechanical parameters
into electrical parameters. In this transformation the electrical
parameters of voltage, current, inductance, capacitance and electrical
resistance correspond respectively with the mechanical parameters of
force, velocity, mass, compliance and mechanical resistance. The
piezoelectric plate, per se, comprises a mechanical resonant cavity having
a resonant frequency determined by the effective thickness of the
piezoelectric plate. Therefore, the piezoelectric plate may operate as a
mechanical band-pass filter over a frequency band centered at an acoustic
frequency determined by the thickness of the piezoelectric plate. The
band-pass characteristics of such a filter depend both on the mechanical
characteristics of the piezoelectric material itself and also on the
mechanical transformation of any electrical generating and/or load circuit
connected across the piezoelectric plate, due to the piezoelectric
properties thereof.
By electrically short circuiting the opposite surfaces of a piezoelectric
plate of given thickness, the piezoelectric properties thereof can be
inhibited. This is true because, in accordance with Gauss' law, no net
electric field can exist within a volume defined by two spaced
equipotential surfaces. Thus, a plate of otherwise piezoelectric material
operates as purely acoustic transmission medium when its opposite surfaces
are electrically short circuited.
FIG. 2 shows the mechanical equivalent circuit of the arrangement shown in
FIG. 1. The mechanical equivalent circuit of multi-layered piezoelectric
unit 10 together with switches 28, 30 and 32 comprises switchable resonant
mechanical cavity 34, while mechanical load 36 coupled thereto comprises
the resultant mechanical equivalent of both the transferred load impedance
of electrical circuit 26 connected across the electrical port of device 10
and the direct mechanical load impedance of acoustic medium I and II
connected respectively to the two mechanical ports of unit 10. When all of
switches 28, 30 and 32 are open, the effective depth of resonant
mechanical cavity 34 is at its maximum, a + b + c. In this case, resonant
frequency f.sub.1, shown in graph 40 of FIG. 3, of cavity 34 is relatively
at its lowest. When some, but not all, of the switches 28, 30 and 32 are
closed, the effective depth of cavity 34 is reduced being determined in
accordance with the sum of the thicknesses of only those layers of unit 10
which are not short circuited. Thus, for example, with only switch 28
open, the effective depth of cavity 34 is determined solely by the
thickness a of layer 12. In a similar manner by closing other arrangements
of some, but not all of switches 28, 30 and 32, the effective cavity
depths 34 can be simulated. As there are three layers, the switches can
assume 2.sup.3 or 8 combinations of positions corresponding to eight
cavity depths determined by the respective thicknesses a, b, c, a + b, a +
c, b + c, a + b + c and zero. The resonant frequency of cavity 34 at any
particular time is equal to some value f.sub.j which is determined by the
reduced effective thickness of cavity 34 at that time. This value f.sub.j
is always higher than frequency f.sub.1, obtained when all switches are
open. Graph 46 of FIG. 3 illustrates the resonant frequency f.sub.2 (that
is, j=2) for one particular thickness less than a + b + c. Graph 44 of
FIG. 3 shows that when all of switches 28, 30 and 32 are closed, the
effective thickness of unit 10 is zero. In this last case, unit 10 no
longer operates at a resonant cavity. Instead, with all switches closed,
unit 10 operates only as a length of acoustic transmission medium.
The pass-band characteristics 44 of graph 40 and pass band characteristics
46 of graph 42 of FIG. 3 between its acoustic and electric ports are
determined both by the impedance characteristics of the piezoelectric
material making up the effective cavity portions of unit 10 and the
impedance characteristics of electrical circuit 26 connected across the
electric port thereof as shown in FIG. 1. Thus, unit 10 may be utilized as
a switchably-turnable acoustic filter for selecting any one of a plurality
of frequency sub-bands as an output from unit 10, from incident broad-band
acoustic wave energy applied as an input thereto. Alternatively, where the
incident acoustic wave energy input is within at least a certain one of
the switchable pass-bands, and is absent from another one of the
switchable pass-bands, unit 10 may be employed as a gate which is open
when the switch means thereof are set to provide the certain pass-band and
is closed when the switch means thereof are set to provide the other
pass-band.
In addition to the frequency selective and/or gating functions of unit 10,
the amount of incident acoustic wave energy input within the pass-band
which would normally be transmitted, but instead is actually reflected
and/or absorbed, can also be controlled. More specifically, the effective
relative impedance of unit 10 with respect to the impedance of acoustic
medium I may be adjusted by means of electrical circuit 26 to control the
fraction of input wave energy which is reflected from the upper face of
unit 10. Further, the resistive component of the impedance of electrical
circuit 26 may be adjusted to absorb more or less of the incident acoustic
wave energy input that was not reflected, but entered unit 10. In this
manner, the effective attenuation provided by unit 10 can be controlled.
Although, as is discussed above, the piezoelectric characteristics of short
circuited layers of control device 10 are inhibited, the short circuited
layers may still play a significant role in determining the time delay
characteristics of control device 10 as a transmission medium and, even
more important, the short circuited layers may operate as impedance
transformers inserted between the resonant cavity formed by one or more
unshorted layers and either a respective one of the acoustic mediums
effectively contacting the outer layers of unit 10 or, alternatively,
another resonant cavity formed by one or more other unshorted layers. In
more quantitative terms,
##EQU1##
where, Z is the impedance presented at one face of a short-circuited
layer;
Z.sub.O is the impedance of the layer material itself
Z.sub.R = Z.sub.O /Z.sub.L
z.sub.l is the impedance of the medium in contact with the other face of
the short-circuited layer.
.beta. = 2.pi./.lambda.
.lambda. is the wavelength of the acoustic energy within the layer
T is the thickness of a short circuited layer or layers
One important case is where the thickness T is an odd quarter-wavelength at
the frequency of the acoustic wave energy. In this case equation (1)
reduces to:
Z = Z.sub.O 2/Z.sub.L (2)
the value of impedance Z in this case can be extremely large. For instance,
in the practical case where the propagating medium is water and the layer
material is PZT-4, the relatively low acoustic impedance Z.sub.L of water
(1.5 .times. 10.sup.6 kg/m sec) is transformed to an extremely high value
of acoustic impedance Z (771 .times. 10.sup.6 kg/m.sup.2 sec) by the
intermediate acoustic impedance Z.sub.O of PZT - 4 (34 .times. 10.sup.6
kg/m.sup.2 sec). Thus, an odd quarter-wave thick layer of sufficiently
high intrinsic impedance may be utilized to transform a soft wall into a
rigid wall (or vice versa). Alternatively, such an odd quarter-wave thick
layer may be used to match any impedance Z to a different load impedance
Z.sub.L, when Z.sub.O is chosen to be equal to the geometric means between
Z and Z.sub.L.
A second important case is where the thickness T of the short circuited
layer is an odd number of half-wavelengths. In this second case, equation
(1) reduces to:
Z = Z.sub.L (3)
in this latter case, the short circuited layer has absolutely no effect. It
behaves on impedance as if it has zero thickness (except for a 180.degree.
phase shift and inherent time delay not evident from equation 3),
regardless of the value of Z.sub.O.
Multi-layered piezoelectric unit 10 may also be employed as a transducer
(i.e. to generate an acoustic wave in response to an applied electrical
signal and/or to detect an electric signal in response to an applied
acoustic wave). Efficient electro-acoustic conversion may be accomplished
when the effective thickness of the non-short-circuited layers of unit 10
is an odd number of quarter-wavelengths at an operating frequency equal to
the resonant cavity frequency, if (and only if) one face of the cavity is
rigid and acts substantially as a perfect acoustic mirror. In this case,
acoustic energy can flow easily from the acoustic port defined by the
other face to the electrical port, and vice versa. Furthermore, maximum
conversion efficiency is obtained if electrical circuit 26 incorporates an
appropriate matching network.
Efficient electro-acoustic conversion is also possible when the thickness
of the non-shorted layers composing the resonant cavity is an odd number
of half-wavelengths. In this case, electro-acoustic interaction normally
takes place both between the electric port and each of the acoustic ports,
and between the acoustic ports themselves. However, if desired, it is
possible to restrict energy flow to that between the electrical port and
only one of the acoustic ports, if the other acoustic port is placed in
contact with air to provide an effective acoustic short-circuit. Again,
electro-acoustic conversion efficiency can be enhanced by incorporating an
appropriate matching network in electrical circuit 26.
A special case exists when the thickness of the non-shorted layers is an
even number of full wavelengths. In this case, no energy transfer is
possible. This is because the reflection coefficient is unity at all
ports, so that unit 10 now operates as a band-rejection filter in which
all signals at all ports are rejected.
FIG. 4 shows the use of a two-layered piezoelectric unit as a switchable
directional acoustic coupler for acoustic wave energy of a given
frequency. Specifically, two-layers of piezoelectric unit 400 comprise
contiguous left and right piezoelectric layers 402 and 404, each having a
thickness of one-quarter wavelength for acoustic wave energy propagating
therein. Unit 400 further comprises first electrode 406 at the outer face
of left layer 402; second electrode 408 at the outer face of right layer
404, and additional electrode 410 at the interface between layers 402 and
404.
Switch 412, connected between first electrode 406 and additional electrode
410, is selectively operable to short circuit left layer 402. Similarly,
switch 414, connected between second electrode 408 and additional
electrode 410, is selectively operable to short circuit right layer 404.
First and second electrodes 406 and 408 are coupled to an electrical
source and/or load 416 through electrical matching network 418. An example
of matching network 418 is shown in FIG. 4a discussed below. As is known
in the art, an impedance matching network is inserted between two coupled
circuits exhibiting different impedances to ensure maximum power transfer
therebetween. Thus, in the special case where the impedance of block 416
is substantially the same as that of unit 400, matching network 418 may be
omitted.
As shown in FIG. 4, unit 400 is immersed in a water medium (which exhibits
a relatively low impedance Z.sub.L discussed above). The intrinsic
impedance of the piezoelectric material, such as PZT-4, of each of
quarter-wavelength layer 402 and 404 is more than 20 times as high as the
impedance Z.sub.L (as discussed). Therefore, in accordance with the
equation (2), either of these layers, when short circuited, presents a
sufficiently high impedance Z to act as a rigid backing to the
non-short-circuited quarter-wavelength layer. Therefore, this other one of
this quarter-wavelength layers is in a condition to operate as an
efficient electro-acoustic transducer between the water medium at its
outer face and the source and/or load 416 coupled to its electric port.
Thus, with switch 412 closed and switch 414 open and block 416 is a source,
acoustic wave energy is launched only in region 420 of the water medium to
the right to unit 400 and not in region 422 of the water medium to the
left of device 400. Similarly, if block 416 is a load, device 400 absorbs
acoustic wave energy present in region 420, but not in region 422. On the
other hand, if switch 412 is opened and switch 414 is closed, the
situation is reversed, so that acoustic wave energy is launched or
absorbed, as the case may be, from region 422 of the water medium to the
left of unit 400, but not from region 420 to the right of unit 400. In
this manner, unit 400 operates as a directional coupler.
In addition to its operation as a directional coupler, unit 400 may operate
as a gate. Specifically, if both switches 412 and 414 are closed at the
same time, two-layer unit 400 (which has an overall thickness of one-half
wavelength) operates as a substantially transparent transmission medium in
accordance with equation (3) above. This permits acoustic wave energy
present in right region 420 to be transmitted to left region 422, or vice
versa. Finally, if both switches 412 and 414 are open at the same time,
two-layered unit 400 (having an overall thickness of one-half wavelength)
operates as an efficient electro-acoustic transducer for simultaneously
launching acoustic wave energy into or absorbing acoustic wave energy from
both right region 420 and left region 422.
Referring now to FIG. 4a, there is shown the electrical equivalent circuit
of transducer 400 together with a specific example of the circuitry of
matching network 418. As shown, the electrical equivalent circuit 400a is
composed of an L-C series circuit resonant at the nominal operating
frequency. However, the series resonant circuit is shunted by a
capacitance C.sub.O (the capacitance between the electrodes). Matching
network 418 includes a corresponding LC resonant circuit, which has a
parallel L.sub.s - C.sub.s circuit shunting the capacitance C.sub.O of
equivalent circuit 400a. Inductance L.sub.S with the total capacitance
C.sub.s + C.sub.O forms a very high shunting impedance parallel resonant
circuit at the operating frequency of the transducer. Matching network 418
further includes an impedance transformer having a turn ratio between
primary and secondary equal to n:1, the value of the n being selected to
appropriately increase or decrease, as the case may be, the termination
impedance of source 416 to a value equal to the electrical equivalent of
impedance Z.sub.L terminating the acoustic port or ports of device 400.
Blocks 418 and 400a together form a constant K filter.
Referring now to FIG. 5, there is shown an arrangement of the present
invention, which also employs two contiguous quarter-wavelength
piezoelectric layers, which is useful in separating the transmit and
receive functions in an ultrasonic wave pulse-echo imaging system. As is
known in the art, in such a system successive pulses of ultrasonic wave
energy are launched in a liquid medium, such as water, by an
electro-acoustic transducer at a predetermined repetition rate. The
acoustic wave may be focused and/or scanned by suitable means (not shown
in FIG. 5). The acoustic wave is used to probe a region of a given object
and acoustic echos received from the region by the transducer are detected
as the desired signal and applied to a suitable display device, such as a
CRT. By way of example, various embodiments of the resolution pulse-echo
ultrasonic imaging display system are disclosed in copending U.S. patent
application Ser. No. 766,564, filed Feb. 7, 1977 by Mezrich et al., and
assigned to the same assignee as the present invention.
In a pulse-echo imaging system, the desired received signal occurs at a
time delay with respect to the transmitted pulse that is equal to the
round-trip travel time of the ultasonic wave energy between the transducer
and the region of the object being probed. An inherent problem of such a
system is that the transducer also responds to undesirable echos received
both before and after the occurence of the desired received signal. The
conventional solution of this problem is to provide an electronic range
gate between the transducer and the display. This range gate is open only
during the time of occurrence of the desired received signal to prevent
undesirable echoes detected by the transducer from reaching the display
and thereby resulting in spurious display information. However, the energy
within the transducer derived from the undesirable echoes received and
detected thereby must still be dissipated. Such dissipation of energy
cannot take place instantly, but takes time. This is particularly true
because the transducer forms a resonant cavity which is resonant at the
frequency of the acoustic wave energy of the undesirable echoes (as well
as that of the echoes which form the desired received signal). Therefore,
there is a tendency for the transducer to ring in response to the
detection of undesirable echoes. This ringing of undesirable echo energy
normally persists into the time window during which the electronic range
gate is open, thereby reducing the sensitivity and signal-to-noise ratio
with which the desired received signal can be displayed. The arrangement
shown in FIG. 5 overcomes this problem.
Specifically, two-layered unit 500, comprising contiguous top and bottom
piezoelectric quarter-wavelength layers 502 and 504, first and second
electrodes 506 and 508 and additional electrode 510 is intrinsically
identical in structure and function with unit 400 described above.
Similarly, switches 512 and 514, for selectively short-circuiting top
layer 502 and bottom layer 504, are identical in structure and fraction to
switches 412 and 414. However, in FIG. 5, the electrical circuit comprises
source 516, which is coupled solely between first electrode 506 and
additional electrode 510 by matching the network 518, and detector 520,
which is coupled solely between second electrode 508 and additional
electrode 510 by matching network 522. In addition, first mirror 524 is
oriented above top layer 502 and second mirror 526 is oriented below
bottom layer 504. In the specific embodiment shown in FIG. 5, each of
mirrors 524 and 526 is a single, fixed, plane mirror oriented at
45.degree. with respect to the axis of the system. However, modifications
of the mirror sub-system in which each of the mirrors consists of an array
of mirror elements, or in which one or both of the mirrors are curved, or
in which one or both of the mirrors are tilted during each pulse
repetition period, (or including other similar modifications within the
sill of the art) are contemplated by the present invention.
In any case, switches 512, and 514 are operated as range gates.
Specifically, switch 514 is open and switch 512 is closed only during the
time window during which the desired received signal reaches unit 500. At
all other times, switch 512 is open and switch 514 is closed.
Thus, based on the discussion in the connection with FIG. 4, each time that
source 516 applies a pulse to the electric port of unit 500,
unshort-circuited top layer 502 operates as an efficient electro-acoustic
transducer, while short-circuited bottom layer 504 operates as a rigid
wall. The acoustic wave energy transmitted by top layer 502 is reflected
from mirror 524 to provide a transmitted signal 528 which propagates
toward a region of a reflecting object under scrutiny. During a first time
interval extending from the transmission of an acoustic pulse from unit
500 to the arrival of desired received signal 530 thereat, top layer 502
operates as an efficient electro-acoustic transducer for converting the
acoustic energy of undesirable echoes 532a, deflected from mirror 524 back
to top layer 502, into an electrical signal at its port. This electrical
signal is returned to source 516, which now operates as a resistive load
for dissipating the energy derived from undesirable echoes 532a. At the
same time, the acoustic energy of undesirable echoes 532b reflected from
mirror 526 and arriving at bottom layer 504 is substantially totally
reflected therefrom. Most of this acoustic wave energy reflected from
bottom layer 504 during the first time interval ultimately arrives at top
layer 502 before the end of the first time interval. Thus, during the
entire first interval, top layer 502 continuously operates to such up the
dissipate the acoustic energy originally present in both undesirable
echoes in 532a and 532b.
However, during the entire first time interval short circuited bottom layer
504 is effectively clamped so that it (1) absorbs substantially no
acoustic wave energy and (2) is not capable of ringing. At the end of the
first time interval, the range gate is open so that now top layer 502 is
short-circuited and bottom layer 504 is unshort-circuited for the length
of a time window during which acoustic wave energy of received signal 530
reflected from mirror 526 arrives at bottom layer 504. During this time
window, bottom layer 504 operates as an efficient electro-acoustic
transducer to provide an electrical signal at its electrical port which is
forwarded through matching network 522 to detector 520. The signal
arriving at detector 520 comprises substantially no noise components
derived from undesirable echoes 532 and 532b. Therefore, output 534 of
detector 520, which is forwarded to the display, has a relatively high
sensitivity and signal-to-noise ratio. During a second time interval,
which follows the time window, unit 500 reverts to the conditions
initially present during the first time interval.
FIG. 6 shows a unit 600, comprising two contiguous one-half wavelength
layers 602 and 604, for use as a gated transducer. Specifically, first
electrode 606 is at the outer face of left half-wavelength piezoelectric
layer 602 second electrode 608 is at the outer face of right
half-wave-length piezoelectric layer 604, and additional electrode 610 is
at the interface between layers 602 and 604. A medium, such as air, having
an acoustic impedance of substantially zero, is present at the outer face
of left layer 602. An acoustic medium 612, such as water, having an
acoustic impedance Z.sub.L is present at the outer face of right layer
604. Electrical circuit 614, which may include a source and/or detector,
is coupled across first and second electrodes 606 and 608. Switch 616,
connected between second electrode 608 and additional electrode 610,
selectively short-circuits right layer 604.
When switch 616 is open, additional electrode 610 has no effect and the
total thickness of control device 600 is one full wavelength. As discussed
above, an unshortcircuited piezoelectric layer of an even number of full
wavelengths operates as a band reject filter which isolates each of the
two acoustic ports and the electric port of a piezoelectric unit from each
other. Thus, in this case, the gated transducer formed by unit 600 is in
its open condition, so that an electrical signal from circuit 614 having a
frequency corresponding to one full wavelength of the total thickness of
control device 600 does not launch an acoustic wave in medium 612. At the
same time, an acoustic wave present in acoustic medium 612 does not five
rise to a voltage across first and second electrodes 606 and 608.
The closure of switch 616 short circuits right layer 604, but leaves left
layer 602 unshortcircuited. This results in the opening of the gates
transducer formed by unit 600. Specifically, a signal source within
circuit 614 may now apply a signal between first electrode 606 and
additional electrode 610 solely to unshorted half-wavelength left layer
602, which operates as an efficient electro-acoustic transducer to
generate an acoustic wave. Short-circuited half-wavelength right layer 604
operates as a transmission medium for the acoustic wave propagated to the
right towards acoustic medium 612. No acoustic wave energy is propagated
to the left because the left acoustic port of layer 602 is effectively
short-circuited by the substantially zero acoustic impedance present
thereat.
Referring to FIG. 7, unit 700 is identical to control device 600 in all
respects except that single additional electrode 610 at the interface of
the two layers of FIG. 6 is replaced in FIG. 7 by separate spaced upper
and lower additional electrodes 710a and 710b at the interface between the
two layers in FIG. 7. In addition, separate switches 716a, connected
between upper additional electrode 710a and second electrode 708, and
switch 716b, connected between lower additional electrode 710b and second
electrode 708, selectively short-circuit the upper and lower halves,
respectively, of right layer 704.
By opening both switches 716a and 716b or closing both switches 716a and
716b unit 700 operates in a manner identical to that described in
connection with FIG. 6 to launch or receive any acoustic wave in region
712. However, the arrangement of FIG. 7 has the further ability to
selectively launch or receive an acoustic wave solely in the upper or
solely in the lower half of region 712 by selectively closing either (but
not both) switch 716a or switch 716b.
The aforesaid copending U.S. patent application Ser. No. 766,564, in FIGS.
6 and 6a thereof, shows a space-divided embodiment of a scanning
ultrasonic source and detector for providing real-time scanning of a
target area to be displayed by appropriate imaging electronics. An
improved transducer for use in such a space-divided embodiment is shown
herein in FIG. 8. Unit 800 forms this improved transducer.
Specifically, unit 800 comprises two contiguous one-half wavelength layers
802 and 804. A set of m spaced vertical electrodes 806-l . . . 806-m are
at the outer face of rear layer 802, second electrode 808 is at the outer
face of front layer 804, and a set of n additional electrodes 810-l . . .
810-n is at the interface between layers 802 and 804.
Parallel-to-serial converter 811 comprises a set of load resistances
R.sub.l . . . R.sub.m connected between corresponding ones of first
electrodes 806-l . . . 806m and a point of reference potential.
Parallel-to-serial converter 811 further includes a set of amplifiers
811-l . . . 811-m which respectively amplify any signal developed across
the corresponding one of load resistance R.sub.l . . . R.sub.m. The
outputs A.sub.l . . . A.sub.m from the set of amplifiers is applied in
parallel to a set of storage means, (not shown). These storage means are
then serially read out, as described in the aforesaid U.S. patent
application Ser. No. 766,546. In addition, the set of first electrodes
806-l . . . 806-m are coupled to the point of reference potential through
corresponding ones of a set of inductances L.sub.l . . . L.sub.m.
Pulse generator 813 is coupled between second electrode 808 and the point
of reference potential. Further, a set of steering gates 816, composed of
switches S.sub.l . . . S.sub.n, is connected between second electrode 808
and corresponding ones of additional electrodes 810-l . . . 810-n to
short-circuit a corresponding horizontal linear segment of front layer
804.
Rear layer 802, per se, together with the sets of first electrodes 806-l .
. . 806-m and the set of additional electrodes 810-l . . . 810-n
correspond with the single-layered transducer shown in FIGS. 6 and 6a of
the aforesaid U.S. patent application Ser. No. 766,564. As disclosed
therein, the set of vertical electrodes and the set of horizontal
electrodes define m times n cross points. The region of layer 802 defined
by each individual cross point is capable of operating as a discrete
electro-acoustic transducer element.
The operation of the embodiment of FIG. 8 employs the principles discussed
above in connection with FIGS. 6 and 7 hereof. Specifically, only the
particular horizontal row of transducer elements X.sub.i associated with
the then closed one S.sub.i of switches S.sub.l . . . S.sub.n are then
effective as efficient electro-optic transducers, because for these
particular horizontal rows of transducer elements front half-wavelength
804 is then in a short-circuited condition. However, all the remainder of
the m times n transducer elements, associated with all the then open ones
of switches S.sub.l . . . S.sub.n, are ineffective at this time. This is
because unshortcircuited half-wavelength front layer 804 together with
unshortcircuited half-wavelength rear layer 802 form a full-wavelength
cavity for these remainder of the the transducer elements. The electric
port and the two acoustic ports of each remainder transducer element
associated with an open switch are, therefore, decoupled from each other
for the reasons discussed above in connection with the full wavelength
acoustic cavities.
Each of the set of inductances L.sub.l . . . L.sub.m has substantially the
same given value. This given value is selected to form a resonant circuit
at the operating frequency with the shunt capacitance between second
electrode 808 and the corresponding one of each one of the first
electrodes 806-l . . . 806-m. This prevents the relatively low shunt
capacitance reactance from bypassing the signal applied to or derived from
each then-effective transducer element associated with the closed switch
S.sub.i.
The prior art transducer employed in FIGS. 6 and 6a of the aforesaid U.S.
patent application Ser. No. 766,564 employs only a single piezoelectric
layer corresponding to rear layer 802, and omits front layer 804, second
electrode 808 and the set of inductances L.sub.l . . . L.sub.n. In other
respects, the structure of this | | |
