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FIELD OF THE INVENTION The present invention relates to the transmission of acoustic signals
and, more particularly, to transducers for transmitting such signals through air.
BACKGROUND OF THE INVENTION Ultrasonic signals are sound waves of frequencies above the
audible range (usually 20 kHz). For many, if not most, applications involving ultrasound, it is
necessary to generate a well-defined beam. Therefore, in addition to high conversion efficiency,
ultrasound transducers that convert electrical signals into corresponding acoustic signals must
have very good transmission characteristics of directivity. Furthermore, the mechanical
impedance of the transducer should be as close as possible to the impedance of the transmission
Ultrasonic transducers for airborne transmission can be divided into two important classes:
electrostatic crystal devices and piezoelectric crystal devices. In an electrostatic transducer, the
thin film vibrates due to the capacitive effect of an electric field, while in a piezoelectric
transducer, an acoustic signal is generated as the applied potential changes the shape of the
piezoceramic material. Both types of transducers have various operational limitations that greatly
limit their usefulness in some applications. In particular, these operational limitations have
hampered the development of parametric speakers (ie, devices that produce an audible sound
with very good directivity due to non-linear interaction of ultrasound). In parametric systems,
high-intensity ultrasound signals modulated with audio signals are demodulated as they pass
through the atmosphere (i.e., a nonlinear transmission medium), which results in an audible
sound with very good directivity. It is generated.
SUMMARY OF THE INVENTION Piezoelectric transducers usually operate with high efficiency
over a limited bandwidth. In parametric applications, the degree of distortion that appears in the
audio signal is directly related to the available bandwidth of the transducer, so that when using a
narrowband transducer (eg, a piezoelectric transducer), the sound quality is poor Will occur. In
addition, piezoelectric transducers tend to have high acoustic impedance, resulting in poor
radiation efficiency in low impedance atmospheres. Because of this mismatch, most of the energy
applied to the transducer is reflected back to the amplifier (or the transducer itself), generating
heat and wasting energy. Finally, conventional piezoelectric transducers tend to be fragile,
expensive, and difficult to electrically connect.
Conventional electrostatic transducers use metallized polymer films held on a conductive
backplate by direct current (DC) bias. The backplate is provided with depressions that produce an
acousto-mechanical resonance at the desired operating frequency. An alternating current (AC)
voltage applied to a DC bias source alternately increases and decreases the bias, thereby
increasing or decreasing the force drawing the membrane to the backplate. This change has no
effect at all where the surfaces are in contact, but causes the membrane to vibrate above the
depressions. In the absence of sufficient damping, the resonant peaks of the electrostatic
transducer are very sharp and will result in efficient operation at the expense of limited
bandwidth. By damping (eg, roughening the surface of the membrane in contact with air), the
bandwidth will be somewhat extended, but the efficiency will be degraded.
Another technique for broadening the bandwidth of an electrostatic transducer, as described by
Mattila et al., Sensors and Actuators A, 45, 203-208 (1994), is to Varying the depth is to create
different resonances that add up to a wider bandwidth.
The maximum drive power (and maximum DC bias) of the transducer is limited by the strength of
the electric field that the membrane can withstand, and the magnitude of the voltage that the air
gap can withstand.
The largest electric field is generated where the membrane actually contacts the backplate (ie
outside the recess). Films are generally very thin polymer films (polymer films), so even materials
with sufficient withstand voltage can be damaged by charging or punch-through when given very
high voltages. I will wake you up. Similarly, the use of thin films brings the metallized surface of
the thin film so close to the backplate that the electric field across the thin film, and hence the
capacitance of the device, is extremely high, resulting in the need for large drive currents.
Piezoelectric thin film transducers utilize lightweight, flexible membrane materials such as
polyvinylidene fluoride (PVDF) thin films that change shape in response to an applied electrical
potential. This film can be made very light to improve its acoustic impedance match to air,
resulting in efficient ultrasound transmission. In one known configuration, a PVDF thin film is
coated on both sides with a conductive material and placed on a perforated metal plate. The plate
is the top of the otherwise closed space, and the vacuum applied to the space pulls the
membrane into the pores. An alternating voltage source connected between the two metallized
surfaces of the membrane (which act as electrodes separated by a dielectric) causes the PVDF
material to expand and contract and the degree of indentation into the pores It changes, which
generates sound waves. In a related configuration, which is also known, the membrane is not
placed on top of the perforated plate, but below it, and the pressure source replaces the vacuum.
In this version, the alternating current source causes the membrane to protrude into the hole or
to change the degree of protrusion through the hole, and likewise generate a sound.
Although the electro-acoustic properties of these transducers make them suitable for parametric
applications, their practicality remains questionable. In a practical environment from the point of
view of commercialization, being able to maintain sufficient vacuum or pressure for a long period
of time is very unlikely, and a slight leak will cause the transducer to lose sensitivity and
eventually Will fail.
SUMMARY OF THE INVENTION According to a first aspect of the present invention, the
maximum output power of an ultrasound transducer is not limited by the withstand voltage of
the membrane of the transducer. Rather than placing the membrane directly on the surface of
the conductor (which causes the electric field across the membrane to become very large) as in
prior art devices, the membrane is held to a dielectric spacer. The transmission of ultrasound
does not depend on the presence of a strong electric field. Thus, relatively large bias and drive
voltages can be applied across the membrane and the spacer without risking failure. The reason
is that the spacer greatly reduces the electric field to which the film is subjected. Furthermore,
the spacers also reduce the capacitance of the transducer, so the required drive current is
correspondingly reduced, which simplifies the design of the power amplifier.
An acoustic transducer according to this aspect of the invention has a conductive membrane, a
backplate comprising at least one electrode, and a series of indentations arranged between the
membrane and the backplate and arranged in a pattern. A dielectric spacer may be provided, said
depressions forming cavities which respectively resonate at a predetermined frequency. The
recesses may take any suitable form, for example, annular grooves arranged concentrically, a
distribution pattern of cylindrical recesses, etc., which extend partially or completely through the
dielectric spacer be able to. Furthermore, the indentations can vary the depth through the spacer
to form cavities that resonate at different frequencies. That is, different electrodes can be
assigned to each set of indentations that form a depth.
In a second aspect, the invention combines a piezoelectric mode of operation with an
electrostatic mode of operation. An acoustic wave transducer according to this aspect of the
invention comprises a substantially nonconductive piezoelectric film having a pair of oppositely
disposed conductive surfaces, a backplate comprising at least one electrode, a membrane and an
electrode ( Or means for creating a cavity or structure that resonates with the electrodes). For
example, the cavity may be formed by a dielectric spacer having a recess (cylindrical recess or
opening, groove, etc.) and disposed between the membrane and the electrode (or electrodes). A
direct current bias pushes the membrane into the resonating cavity, and an alternating current
source connected across the membrane provides the drive signal.
The transducer is preferably driven with a capacitive transducer at which the resonant frequency
of the transducer resonates with the inductance of the circuit. By doing this, electrical energy can
be transmitted to the transducer very efficiently, so that relatively high carrier (carrier)
frequencies can be easily used. The high efficiency and versatility of the transducers described
herein allow these transducers not only for parametric applications, but also for other ultrasound
applications such as ranging, flow detection, and nondestructive testing. Will also fit. In
parametric applications, multiple transducers can be incorporated into the transducer module,
substantially arranging and / or electrically driving the module to form a large radiation surface
and a large non-linear interaction area.
DETAILED DESCRIPTION OF THE INVENTION The present invention will now be described with
reference to the accompanying drawings. As shown in FIG. 1, an electrostatic transducer module
29 incorporating the present invention may comprise a conical spring 30, which in turn
comprises a conductive electrode unit 32, a number of openings 36. A dielectric spacer 34 and a
metalized polymer membrane 38 are supported. The components 32 to 38 bear the film 38 and
are pressed against the spring 30 by the upper ring 21 insertably engaged with the base member
41 supporting the spring 30. Module 29 is comprised of a plurality of electrostatic transducers
corresponding to each opening 36 of dielectric spacer 34. Specifically, a portion of the film 38 at
the top of each aperture and a portion of the electrode unit 32 at the bottom of the aperture
function as a single transducer, which, among other things, the tension and surface density of the
film 38, the diameter of the aperture. And a resonance (resonance) characteristic that is a
function of the thickness of the polymer layer 34. The varying electric field between each portion
of the membrane 38 and the electrode unit 32 causes that portion of the membrane to deflect in
the direction towards or away from the electrode unit 32. And the frequency of the movement
corresponds to the frequency of the applied electric field.
As shown, the electrode units 32 can be divided into individual electrodes 32a below each
opening 36 by suitable etching techniques. The electrodes have individual leads extending from
the electrodes to one or more drive units, as described below. Module 29 can be easily
manufactured using conventional flexible circuit materials, and is therefore low cost. For
example, spacer 34 may be a polymer such as PYRALUX material sold by duPont, and membrane
38 may be metallized MYLAR thin film (also sold by DuPont) can do. If desired, the drive unit
components can be placed directly on the same substrate, eg, tab portion 32b. This structure is
lightweight and can be flexible for easy placement, focusing and / or orientation in an array
The geometry, in particular the depth of the openings 36, can be modified to cover the desired
frequency range, so that the resonance characteristics of the individual transducers of the
module 29 are single acoustic. It will be appreciated that the overall response of the module can
be extended compared to a single transducer with mechanical resonance frequency or an array
of transducers. This can be achieved by using a dielectric spacer 34 consisting of two (or more
than two) layers 34a and 34b, as shown in FIG. The upper layer 34a has a sufficient number of
openings 36a. The lower layer 34b, on the other hand, has a set of apertures 36b that align with
only selected ones of the apertures 36a of the layer 34a. Thus, where the two openings 36a and
36b are aligned, the depth of the opening is deeper than the depth of the opening of layer 34a,
which is above the non-opening portion of layer 34b. The electrode unit 32 has the electrode
32b under the opening of the layer 34b, and has the electrode 32c under the portion with only
the opening of the layer 34a (ie excluding the portion with the opening of the layer 34b) There
is. This provides a first set of transducers with a higher resonant frequency (shallow opening)
and a second set of transducers with a lower resonant frequency (deep opening). These
geometries and configurations can also be created by other processes such as screen printing
and etching.
Different aspects of the construction and operation of module 29 are shown in FIGS. In FIG. 3,
the module 29 has one electrode 32 and the cavities formed by the layers 34a, 34b have
different depths d, d depending on whether the opening 36a is aligned with the opening 36b.
'have. The structure for pushing the membrane 38 into the spacer 34 is not shown in FIG. A
direct current bias source 40 applied to an alternating current source 42 (which generates a
modulated signal for transmission) is connected to both ends of the module 29, ie to the
metallized surface 38m of the electrode 32 and the membrane 38 Ru. The same signal is applied
to all the cavities 36, but their different resonance peaks broaden the bandwidth of the module
29 as a whole.
Alternatively, as shown in FIG. 4, different sets of electrodes 32b, 32c can be connected to
different AC drive signal sources 42b, 42a respectively. Each signal source 42a, 42b is
electrically resonant at the mechanical resonance frequency f1, f2 of the cavity it drives. This
"decoupled multi-resonance" arrangement optimizes response and maximizes power transfer by
pairing each set of resonant cavities with an amplifier tuned to it. Resistors 43a, 43b separate the
electrodes 32b and 32c, but direct current can pass through them (inductors can be used
As mentioned above, in addition to merely changing the transducer's acoustic-mechanical
resonance characteristics, it is also possible to change the electrical resonance characteristics.
For example, multiple electrical resonant circuits can be created by varying the capacitance of
different regions of transducer 29 (eg, by using materials having different dielectric constants for
different regions of spacers 34a, 34b) . This electrical resonance affects the efficiency of power
transfer from the amplifier (ie, as the transducer's impedance closely matches the amplifier's
impedance, more output power is coupled in the transducer and the associated current draw Is
reduced, so that changing electrical resonance in a single transducer can be used regardless of
whether mechanical resonance is also changed, in order to extend the resistance of the
transducer to different amplifier configurations.
The signal sources 42a, 42b can be implemented as shown in FIG. The modulated output signal
(modulated output signal) is sent to a pair of filters 44a, 44b. These filters split the signal into
different frequency bands and send them to a pair of tuned amplifiers 46a, 46b. The amplifier
46a is tuned to f1. That is, the electrical resonant frequency is made equal to the mechanical
resonant frequency of those cavities by the inductance of the amplifier 46a in series with the
measured capacitance across the cavities to which the amplifier 46a is connected. The amplifier
46b is tuned to f2. The filters 44a and 44b may be band pass filters that divide the modulated
signal between f1 and f2, or low pass filters and high pass filters.
The resonant cavity of the module 29 does not have to have a circular cross section as shown.
Alternatively, annular grooves (square, V, circular, etc.) arranged concentrically in spacer 34,
which may be of a different cross-section (eg square, rectangular or other polygonal shape) It can
also take the form of Alternatively, it may have other volumetric shapes suitable for the chosen
application (or the desired manufacturing method). The electrodes of the backplate for driving an
array of concentrically slotted transducers are shown in FIGS. 6 and 7. Here, the conductive
pattern of the electrode unit 52 is composed of rings 53, 55 and 57, so that grooves of different
depths can be driven individually. The spacing of the rings and the relative phase of the applied
signal can be selected to shape the ultrasound beam emitted from the transducer module.
The proper groove depth for the desired operating frequency can be easily obtained without
undue experimentation. In the case of a thin film of cotton density σ (kg / m 2) and a square
groove of depth h (m), the resonance frequency f 0 is expected to have the following values.
[Equation 1]
Here, c is the velocity of sound in air, and ρ 0 is the density of air.
(The equation for non-square grooves is similar to the above equation). The resonant frequency is
also affected by film tension, groove width, and DC bias. Thus, for a transducer having a resonant
frequency of 65 kHz and a thin film with an areal density of σ = 0.0113 kg / m 2, the hole /
structure depth h is 74 μm (3 mils). Depending on the depth of this cavity, for example, if a
capacitance of 500 pF is produced, an inductance of 12 mH (usually the secondary side of the
transformer) is chosen to achieve a resonance of 65 kHz.
In the case of this transducer, a reasonable bandwidth for efficient driving is 10 kHz (ie 60-70
kHz). Thus, it may be desirable to utilize a second set of transducers having a resonant frequency
of 75 kHz to broaden the effective output bandwidth. Using the same design approach, a 56 μm
(2 mil) feature depth is required to achieve 75 kHz resonance.
8 and 9 show an array of transducer modules in an alternative configuration. In FIG. 8, each
module has a hexagonal outer shape, whereby the modules are closely packed. In FIG. 9, the
modules are square-shaped and again the modules are closely packed. This arrangement is well
suited to the generation of multiple beams and the orientation of phased-array beams. It should
be noted that in all previous transducer implementations, electrical crosstalk between the
electrodes can be reduced by placing so-called "guard tracks" between the electrodes. It should
also be understood that transducers with multiple electrical resonances (not necessarily acousticmechanical resonances) can be used to increase the amplification efficiency over a wide band.
The above transducer implementations are electrostatic in nature. As shown in FIG. 10, a
dielectric spacer approach can be used with the piezoelectric film. In this case, the transducer
module 60 comprises a piezoelectric (eg, PVDF) membrane 62, a conductive back plate 64, and a
dielectric spacer 66 having an opening 68, which resonates through the opening 68. A cavity is
formed. Again, the cavities 68 do not have a single depth, but may vary in depth, and the
backplate 64 is a series of electrodes aligned with the respective cavities of the cavities 68. It can
consist of
Film 62 is preferably dielectric in nature and is metallized on its top and bottom surfaces. A
direct current bias provided by circuit 70 is connected between back plate 64 and the conductive
top surface of membrane 62, thereby forcing the membrane into cavity 68. This provides a
reliable mechanical (mechanical) bias to the membrane 62, whereby the membrane is connected
to the drive circuit 72 connected across the membrane 62 in a conventional piezoelectric
transducer driving manner. Can act in a linear fashion to generate an acoustic signal in response
to the electrical output of the Thus, the membrane is held in place by electrostatic (electrostatic)
forces but is driven piezoelectrically. As mentioned above, the DC bias circuit 70 can include
components that separate it from the AC drive circuit 72.
Alternatively, the mechanical shape of the membrane displacement can be used as a substitute,
or to increase the DC bias. For example, the membrane can be formed or tensioned mechanically
such that the membrane is drawn into the cavity 68. Piezoelectrically induced contraction and
expansion displaces the biased membrane and generates an acoustic signal.
As shown in FIG. 11, separate piezoelectric drivers and electrostatic drivers can be utilized. Thus,
while the piezoelectric driver 72a is connected across the membrane 62 as described above, the
electrostatic driver 72b is connected between the metallized top surface of the membrane 62 and
the backplate 64, similar to the DC bias circuit 70. Be done. As a result, piezoelectric and
electrostatic forces are used together to drive the membrane 62. Depending on the orientation of
the membrane 62, the drivers 72a, 72b can be driven in phase or out of phase (this causes the
forces to reinforce each other rather than to weaken each other). Thus, when the drive voltage
generated by the AC source 72a swings in the positive direction, electrostatic forces pull the
membrane 62 towards the backplate 64 (which is maintained at a high DC bias voltage as shown
in the figure) At the same time, the piezoelectric driver 72b expands or contracts (thins) the
membrane 62. When the voltage generated by the driver 72a swings in the negative direction,
the attracting electrostatic force weakens, and the piezoelectric driver 72b assists this process by
contracting or expanding (thickening) the film 62.
Conversely, the forces do not mutually reinforce one another, for example, so as not to operate
selected parts of the transducer in order to redirect the signal, so as to deliberately counteract
piezoelectricity. Drivers and electrostatic drivers can be operated.
As shown in FIG. 12, in another embodiment, the use of an electric field replaces the vacuum
utilized in prior art devices to pull the membrane through the holes towards the backplate.
The transducer module 80 of FIG. 12 includes a piezoelectric film 62 metallized on the top and
bottom surfaces, which further includes a perforated top plate 82 (which may be conductive or
nonconductive). I am in contact with Similar to conventional transducer modules, the top plate
82 is spaced above the back plate 64 by sidewalls 84. The direct current bias provided by the
circuit 70 is connected between the backplate 64 and the conductive surface of the membrane
62, thereby drawing the membrane 62 into the opening 86 of the plate 82. This provides a
reliable mechanical bias to the membrane 62, whereby the membrane acts linearly to generate
an acoustic signal in response to the electrical output of the piezoelectric drive circuit 72. be able
The structure shown in FIG. 10 is further simplified by using an electrically conductive, grooved
(eg V-shaped) grooved metal backplate instead of the spacers and backplates shown. It can be In
this case, the groove acts the same as the gap of the spacer, and the DC biased backplate (or the
mechanical configuration as described above) draws the membrane 62 into the groove.
All of the above transducer embodiments can be used not only for transmission but also for
reception, and it is often possible to mount the drive circuitry and associated circuitry directly on
the substrate of the transducer. I emphasize that there is.
Thus, it will be appreciated that the inventor has developed an improved ultrasound transducer
which eliminates the limitations recognized in the prior art.
The terms and expressions used herein are used as terms of description and not limitation, and
equivalent features illustrated and described in using such terms and expressions, It is not
intended to exclude any of its parts. However, it will be apparent that various modifications are
possible within the scope of the present invention.
Brief description of the drawings
1 is an exploded view of an electrostatic transducer module incorporating the present invention.
2 shows a variant of the transducer module of FIG. 1 configured for multiple resonant frequency
3 is a partial schematic side view showing the configuration and operation mode of the
transducer module shown in FIG. 1 or 2.
4 is a partial schematic side view showing a different configuration and operation mode of the
transducer module shown in FIG. 2 from FIG.
5 is a schematic diagram of a drive circuit used in the embodiment shown in FIG.
6 shows a typical electrode configuration.
FIG. 7 shows another representative electrode configuration.
FIG. 8 shows an arrangement of representative transducer modules.
FIG. 9 shows another exemplary transducer module arrangement.
FIG. 10 is a partial schematic side view of a hybrid transducer utilizing a piezoelectric drive and
resonance with a DC bias.
11 is a partial schematic side view of a hybrid transducer driven by both electrostatic and
FIG. 12 shows the configuration of the improved piezoelectric transducer.
Explanation of sign
29 Transducer Module 32 Electrode 34 Dielectric Spacer 36, 68 Opening (Cavity) 38 Film 40, 70
DC Bias Source (DC Bias Circuit) 42, 72 AC Source (AC Drive Circuit) 64 Back Plate
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