Elastomer membrane enhanced electrostatic transducer

A transducer having opposed first and second conductive plates for application of an electrical potential difference therebetween. An elastomeric dielectric material such as neoprene rubber is disposed between the plates and in contact therewith. The dielectric material has a plurality of pockets of approximate average depth "d" such that, for a given gas maintained within the pockets at a pressure "P", the product Pd is significantly less than the value required to achieve the minimum breakdown voltage for the gas in the pockets. Alternatively, the elastomeric dielectric material disposed between the plates may take the form of a plurality of strips or nodules which separate the plates by a distance "d" as above.

FIELD OF THE INVENTION 
This application pertains to electrical-to-mechanical transducers. More 
particularly, the application pertains to an electrostatic transducer in 
which an elastomeric dielectric material is disposed between a pair of 
opposed conductive plates across which an electrical potential difference 
is maintained. Slight surface irregularities or pockets in the dielectric 
material facilitate dramatic increases of the electric breakdown field in 
the microscopic gap between the plates and the dielectric material, or in 
the pockets, thereby yielding extremely high electrostatic forces. Very 
thin deposits of dielectric material may alternatively be used to maintain 
a very narrow gap between the opposed plates, thereby also increasing the 
gap breakdown voltage, yielding extremely high electrostatic forces and 
increased compliance of the device. 
BACKGROUND OF THE INVENTION 
A variety of electrical-to-mechanical transducers exist. Familiar examples 
include the electrostatic transducers incorporated in loudspeakers, the 
electromagnetic transducers incorporated in electric gauges and the 
piezoelectric or magnetostrictive transducers used, for example, in 
certain narrow band underwater signalling applications. Conventional 
electrostatic transducers typically utilize the electrostatic force 
generated by applying an electrical potential difference between a pair of 
opposed metal plates separated by an air gap. In an electromagnetic 
transducer, an electric current causes a force to be applied to a wire 
maintained in a magnetic field, thereby moving the wire and whatever it 
may contact. Piezoelectric transducers incorporate certain crystals which 
change their shape, and thus move slightly, in response to an applied 
electric field. Magnetostrictive transducers incorporate certain metals 
which change their shape, and thus move slightly, in response to an 
applied magnetic field. 
For comparison purposes, it is useful to consider transducers having a 
volume of the order of 100 ml. Conventional electrostatic transducers of 
this sort have relatively low mechanical impedance (ranging from about 1 
to about 100 Newton seconds per meter) and are capable of producing only 
relatively small forces (typically about 0.05 to about 0.5 Newtons). The 
mechanical impedance range of electromagnetic transducers is about the 
same as that of conventional electrostatic transducers, although 
electromagnetic transducers are capable of producing forces of about 0.5 
to about 10 Newtons. Piezoelectric and magnetostrictive transducers, on 
the other hand, have extremely high mechanical impedance (ranging from 
about 10.sup.6 to about 10.sup.8 Newton seconds per meter) and generate 
extremely high forces (on the order of about 10.sup.3 to about 10.sup.4 
Newtons). It can thus be seen that there is a conspicuous lack of 
electrical-to-mechanical transducers which, in the 100 ml. size range, 
would have a mechanical impedance on the order of about 10.sup.3 to about 
10.sup.5 Newton seconds per meter and be capable of producing forces in 
the range of about 10 to about 10.sup.3 Newtons. The present invention 
provides an electrostatic transducer which fills this gap in the prior 
art. 
SUMMARY OF THE INVENTION 
In accordance with a first embodiment, the invention provides a transducer, 
comprising opposed first and second conductive plates between which an 
electrical potential may be applied; and, an elastomeric dielectric 
material disposed between the plates and in contact therewith. The 
dielectric material has a plurality of pockets of approximate average 
depth "d" such that, for a given gas maintained within the pockets at a 
pressure "P", the product Pd is significantly less than the value required 
to achieve the minimum breakdown voltage of the gas. The large breakdown 
voltages correspond to high electric fields and correspondingly high 
electrostatic forces. At the same time, the deformability of the 
elastomeric dielectric material, in conjunction with the gas-filled 
pockets, enables the structure to be relatively compliant, thus achieving 
a mechanical impedance in the desired range. 
Alternatively, in a second embodiment of the invention, the elastomeric 
dielectric material may take the form of small strips or nodules disposed 
between the plates and in contact therewith, thereby separating the plates 
by a distance "d" such that, for a given gas maintained between the plates 
at a pressure "P", the product Pd is significantly less than the value 
required to achieve the minimum breakdown voltage of the gas. 
Advantageously, the elastomeric dielectric material is disposed between 
the plates at a plurality of discrete sites, thus leaving a gas-filled gap 
between and in contact with both plates in regions not occupied by the 
dielectric material. In a particularly preferred embodiment, a first 
plurality of strips of elastomeric dielectric material are disposed 
between the plates in a first direction; and, a second plurality of strips 
of elastomeric dielectric material are disposed between the plates in a 
second direction different from the first direction, thereby increasing 
the compliance of the elastomeric material and decreasing the mechanical 
impedance of the transducer so as to facilitate large displacements in 
response to comparatively small voltages. 
Another particularly preferred embodiment of the invention provides a 
plurality of conductive plates which may be arranged in a stack. An 
electrical potential may be applied between each pair of opposed plates 
comprising the stack. An elastomeric dielectric material is disposed 
between and in contact with each pair of opposed plates comprising the 
stack. The dielectric material separates each of the pairs of opposed 
plates by a distance "d" such that, for a given gas maintained between the 
plates at a pressure "P", the product Pd is significantly less than the 
value required to achieve the minimum breakdown voltage of the gas. 
Advantageously, an electrical insulating material may be disposed between 
each of the plates and the elastomeric dielectric material so as to 
increase the gas breakdown voltage, and to lessen the deleterious effects 
of accidentally exceeding that voltage. 
If the gas is air, and if "P" is normal atmospheric pressure, then "d" is 
preferably about 16 microns or less. 
The elastomeric dielectric material is preferably neoprene rubber. The 
conductive plates are preferably formed of aluminized mylar.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a graph on which transducer force (expressed in Newtons) is 
plotted as the ordinate versus transducer mechanical impedance (expressed 
in Newton seconds per meter) as the abscissa for various 
electrical-to-mechanical transducers having a volume of about 100 
milliliters. As indicated by region 10 on FIG. 1, conventional 
electrostatic transducers have mechanical impedances which vary from about 
1 to about 100 Newton seconds per meter and are capable of producing 
forces of about 0.05 to about 0.5 Newtons. As shown by region 12 on FIG. 
1, electromagnetic transducers exhibit the same range of mechanical 
impedance as conventional electrostatic transducers, but are capable of 
producing forces in a range which is roughly about one order of magnitude 
greater than the force range of conventional electrostatic transducers. 
Piezoelectric and magnetostrictive transducers, on the other hand, have 
extremely high mechanical impedance ranging from about 10.sup.6 to about 
10.sup.8 Newton seconds per meter and are capable of producing forces in 
the range of about 10.sup.3 to about 10.sup.4 Newtons, as illustrated by 
region 14 in FIG. 1. 
It can thus be seen that there is a wide range of mechanical impedance and 
forces which existing electrical-to-mechanical transducers are incapable 
of producing. This gap, illustrated by region 16 in FIG. 1, corresponds to 
an impedance range of about 10.sup.2 to about 10.sup.6 Newton seconds per 
meter and to a force range of about 10 to about 10.sup.3 Newtons. The 
present invention provides an elastomer membrane enhanced electrostatic 
transducer which fits neatly within this gap. That is, the transducer to 
be described exhibits mechanical impedance in the range of about 
0.5.times.10.sup.3 to about 0.5.times.10.sup.5 Newton seconds per meter 
and is capable of generating forces in the range of about 10 to about 
0.5.times.10.sup.3 Newtons. There are a wide range of practical 
applications for which the transducer of the invention is ideally suited. 
These include machine tool actuators and vibrators, alignment preserving 
optical components in laser systems and underwater transducers. 
FIG. 2 is a simplified cross-sectional side view of a conventional 
electrostatic transducer consisting of a pair of opposed metal plates 20, 
22 which are separated a distance "d" by an air gap. If an A.C. voltage 
source 26 is connected across plates 20, 22 to establish an electrical 
potential difference across the plates an electrostatic force is generated 
which causes the plates to oscillate in the directions indicated by double 
headed arrow 28. The magnitude of such oscillation varies in proportion to 
the magnitude of the square of the applied voltage although, as indicated 
by region 10 in FIG. 1, only comparatively small forces can be produced by 
conventional electrostatic transducers. Moreover, there is a maximum 
breakdown voltage of about 10.sup.6 volts per meter beyond which any 
further increase in voltage across plates 20, 22 results in arcing between 
the plates, in which case the transducer fails due to a large increase in 
the flow of electrical current. 
FIG. 8 is a graph which illustrates the relationship between breakdown 
voltage "V", plate separation distance "d" and pressure "P" of the gas 
maintained between the opposed plates of an electrostatic transducer like 
that shown in FIG. 2. The graph shows that for a given cathode material 
(the "cathode" being the plate having the lower voltage) such as 
commercial aluminum, the breakdown voltage V decreases as the product Pd 
decreases, until a minimum voltage "V.sub.min " is reached; and, that the 
breakdown voltage V then increases dramatically as the product Pd 
continues to decrease. It may thus be seen that if the gas pressure P is 
held constant, the breakdown voltage V decreases as the plate separation 
distance d decreases until the aforementioned minimum voltage V.sub.min 
(known as the "Paschen minimum") is reached, but the breakdown voltage V 
then increases dramatically as the plate separation distance d is further 
decreased. As FIG. 8 indicates, the Paschen minimum voltage for air, with 
a commercial aluminum cathode is about 254 volts, and occurs when the 
product Pd is about 1.2 Torr cm. If the gas pressure P is 1 atmosphere 
(i.e. 760 Torr) this corresponds to a plate separation distance d of about 
1.2 Torr cm./760 Torr=1.6.times.10.sup.-3 cm. or about 16 microns. 
It has been recognized that an electrostatic transducer capable of 
measuring small displacements can be made by making d as small as 
possible. [See: W. B. Gauster and M. A. Breazeale: "Detector for 
Measurement of Ultrasonic Strain Amplitudes in Solids", Rev. Sci. Instrum. 
37, 1544-1548 (1966); and, J. H. Cantrell and J. S. Heyman: "Broadband 
Electrostatic Acoustic Transducer for Ultrasonic Measurements in Liquids", 
Rev. Sci. Instrum. 50, 31-33 (1979)]. Unfortunately however, it is very 
difficult to construct a practical electrostatic transducer having a plate 
separation gap "d" of only about 16 microns and the difficulty increases 
as "d" is further decreased (as it must be if an electrostatic transducer 
having higher breakdown voltages is to be produced). Expensive precision 
machining and cumbersome mounting techniques are required which preclude 
the use of such transducers in most practical situations. 
The inventors have discovered that a practical electrostatic transducer 
which exploits the foregoing phenomenon may be easily constructed and 
operated at values of Pd which are significantly less than the value of Pd 
required to achieve the minimum breakdown voltage of the particular gas 
maintained between the transducer plates. The term "significantly" is used 
to imply that the breakdown voltage resulting from a particular value of 
Pd exceeds the minimum breakdown voltage by about 10% or more. 
In accordance with a first embodiment of the invention, an elastomeric 
dielectric material is placed between plates 20, 22 of the FIG. 2 
electrostatic transducer and is maintained in contact with both plates. It 
is of course well known to provide a dielectric material between a pair of 
opposed plates across which a voltage potential difference is maintained 
(as in a conventional capacitor). However, the inventors have discovered 
that if the dielectric material has very slight surface irregularities or 
pockets, and is elastomeric (for example, neoprene rubber), then the 
desired increase in gap breakdown voltage may be achieved, thereby 
facilitating production of transducers having mechanical impedance/force 
characteristics falling within region 16 depicted in FIG. 1, as a result 
of the deformability of the elastomeric dielectric material. 
FIG. 3 is a greatly magnified cross-sectional side view of an electrostatic 
transducer 30 according to the first embodiment of the invention. 
Transducer 30 comprises a pair of thin aluminium plates 32, 34 across 
which an electrical potential difference is maintained by a voltage source 
(not shown). A compressible neoprene rubber dielectric 36 having a 
breakdown voltage of about 2.times.10.sup.7 volts per meter is disposed 
between plates 32, 34 and in contact therewith. The surfaces of dielectric 
36 adjacent plates 32, 34 are very slightly irregular such that, when 
viewed on the microscopic scale shown in the electron micrograph of FIG. 
9, the surfaces exhibit a large plurality of pockets having an approximate 
average depth "d" of about 10 microns each. Accordingly, when dielectric 
36 is disposed between plates 32, 34 there is a corresponding large 
plurality of discrete gaps on the order of about 10 microns between each 
of plates 32, 34 and the adjacent surfaces of dielectric 36. The 
aforementioned pockets would ordinarily be distributed throughout 
dielectric 36, and need not be confined to (or even present on) the 
surface of dielectric 36. 
The slight surface irregularities of dielectric 36 provide, in effect, a 
gap of approximately 10 microns between each of plates 32, 34 and the 
adjacent faces of dielectric material 36. Alternatively, the pockets 
distributed throughout dielectric 36 constitute a large number of 
discrete, localized gaps of about 10 microns each. As discussed above with 
reference to FIG. 8, small gaps of this order of magnitude are capable of 
sustaining relatively high voltages before breakdown occurs. Moreover, 
because the dielectric material is elastomeric, plates 32,34 may oscillate 
significantly in response to the large electrostatic force corresponding 
to the large voltages sustainable by the slight surface irregularities or 
pockets of the dielectric. Dielectric material 36 thus facilitates the 
production of electrostatic forces on the order of the range of forces and 
mechanical impedances indicated by region 16 in FIG. 1. 
The first embodiment of the invention described above and illustrated in 
FIG. 3 is subject to a number of shortcomings. For example, if dielectric 
material 36 is relatively thick in comparison to the average depth d of 
the dielectric surface irregularities or pockets, and if transducer 30 is 
operated with an A.C. voltage, then the effective efficiency of the device 
is decreased. This decrease arises because of the extra power consumed in 
the process of charging and discharging the relatively large volume of the 
dielectric material. Secondly, if the device is connected across a 
constant voltage source, small currents flowing through the dielectric 
surface irregularity or pocket gaps could, after a time, short out the 
electric field in the gaps, thereby reducing the electrostatic force to 
zero. A further shortcoming of such a device is that it could be difficult 
to manufacture inexpensively in large quantities. The foregoing 
shortcomings are overcome by the second and further alternative 
embodiments of the invention illustrated in FIGS. 4, 5, 6 and 7 which will 
now be described. 
FIG. 4 illustrates a transducer 40 having a pair of opposed metal plates 
42, 44 across which an electrical potential difference is maintained by a 
voltage source (not shown). A plurality of strips, beads or nodules 46a, 
46b, 46c, etc. of elastomeric dielectric material are disposed between 
plates 42 and 44 in contact therewith, thereby separating plates 42, 44 by 
a distance "d" such that, for a given gas maintained between plates 42, 44 
at a pressure "P", the product Pd is significantly less than the value 
required to achieve the Paschen minimum breakdown voltage of the gas. 
There are known techniques for rapid application of thin strips or small 
beads of elastomeric material to surfaces, which may be adapted to 
construct the second embodiment of the invention illustrated in FIG. 4. 
Note that in the embodiment of FIG. 4 the thickness of the dielectric 
material is reduced to equal the desired minimum displacement "d" between 
plates 42, 44; thereby facilitating operation of the device at direct 
current voltages (i.e. because the gas is in contact with both plates 42 
and 44, small leakage currents cannot short out the field across the 
gas-filled gap). 
FIG. 5 illustrates a further alternative embodiment of the invention 
comprising a transducer 50 having a pair of opposed metal plates 52, 54 
across which an electrical potential difference is maintained by a voltage 
source (not shown). A first plurality of strips 56a, 56b, 56c, etc. of 
elastomeric dielectric material are disposed between plates 52, 54 in a 
first direction. That is, strips 56a, 56b and 56c have longitudinal axes 
perpendicular to the plane of the paper. A second plurality of strips of 
elastomeric dielectric material, only one of which; namely, strip 58a is 
visible in FIG. 5, are disposed between plates 52, 54 in a second 
direction which is different than the first direction. That is, strip 58a 
and the other strips comprising the second plurality of strips have 
longitudinal axes which are closer to the plane of the paper. The 
embodiment of FIG. 5 may be fabricated by utilizing known techniques to 
rapidly apply thin elastomeric beads to each of plates 52 and 54, 
following which the plates may be aligned with the axes of the beads so 
applied at an angle to each other. This minimizes the contact area between 
the dielectric material on the two plates 52, 54. The compliance of the 
elastomeric material is thus increased, resulting in reduced mechanical 
impedance. This feature is desirable when large displacements are needed 
in response to comparatively small voltages across plates 52, 54. 
FIG. 6 illustrates a still further embodiment of the invention comprising a 
transducer 60 having a pair of opposed metal plates 62, 64 across which an 
electrical potential difference is maintained by a voltage source (not 
shown). An electrical insulating material 66 such as mylar or metal oxide 
is applied over each of the opposed surfaces of plates 62, 64. A plurality 
of strips, beads or nodules 68a, 68b, 68c, etc. of elastomeric dielectric 
material are then disposed between the opposed layers of insulating 
material. (FIG. 6 illustrates the use of beads or nodules of elastomeric 
material as shown in FIG. 4, but overlapping strips of elastomeric 
material could also be used as shown in FIG. 5.) Insulating material 66 
serves to increase the breakdown voltage of the gasfilled gap maintained 
between insulating layers 66 by the elastomeric dielectric material. Since 
the gap is bounded by insulating material, electrical breakdown occurs in 
accordance with a process known as "electrodeless breakdown" or "external 
electrode breakdown". There is some evidence that the minimum breakdown 
voltage of a gas obtained via electrodeless breakdown exceeds that which 
is obtained when the gas is allowed to contact the electrodes [see: D. 
Friedmann, F. L. Curzon and J. Young: "A New Electrical Breakdown 
Phenomenon in Gas-Filled Insulating Bulbs", Appl. Phys. Lett. 38, 414-415 
(1981)]. Increased breakdown voltage is desirable because transducer 60 
could then produce larger electrostatic forces than those attainable in 
the absence of insulating material 66. Moreover, this reduces the risk of 
transducer failure by preventing arcing between plates 62, 64. Also, by 
ensuring that the average conductivity of insulating material 66 exceeds 
that of the gas, one can still maintain operation at constant voltages, 
without leakage through the air gap reducing the resulting electrostatic 
force. 
The minimum breakdown voltage may also be increased by maintaining an 
electronegative gas such as carbon dioxide, sulphur hexafluoride or oxygen 
in the gap between plates 62, 64. Mixtures of electronegative and 
non-electronegative gases are expected to be particularly useful because 
the high breakdown voltage characteristics of electronegative gases could 
then be exploited in combination with the larger Pd values which 
characterize the Paschen minimum voltages of non-electronegative gases, 
which in turn implies that rougher surfaced dielectric materials (i.e. 
materials having surface pockets deeper than about 16 microns) could be 
used. The following table provides the Paschen minimum voltage (expressed 
in volts) and corresponding Pd values (expressed in Torr cm.) for three 
electronegative gases (carbon dioxide, sulphur hexafluoride and oxygen) 
and for one non-electronegative gas (air): 
______________________________________ 
Paschen Min. 
Voltage Pd 
______________________________________ 
carbon dioxide 488 .45 
sulphur hexafluoride 
507 .24 
oxygen 446 .8 
air 260 .6 
______________________________________ 
FIG. 7 illustrates yet another embodiment of the invention which, like the 
embodiment of FIG. 6, may be constructed by using alumininized mylar in 
continuous sheet form. The thin layer of aluminium deposited on the mylar 
serves as electrically conductive plate material for construction of 
transducers generally similar to those shown in FIGS. 4, 5 or 6. Thin 
beads, strips or nodules of elastomeric material may be applied to the 
aluminized mylar surface as explained above. The sheet of aluminized mylar 
may then be cut into a large number of individual plates which may then be 
stacked one on top of the other to construct a multilayer transducer 70 as 
shown in FIG. 7. As may be seen, transducer 70 includes a plurality of 
plates 72a, 72b, 72c, etc., each separated by a layer 74a, 74b, etc. of 
electrically insulating mylar. An electrical potential difference is 
maintained across the plates by a voltage source (not shown). The 
elastomeric material applied to the aluminized mylar serves as a 
compressible dielectric disposed between and in contact with each pair of 
opposed plates comprising the stack. Although FIG. 7 illustrates the use 
of overlapping strips 76a, 76b, 76c, etc. of elastomeric material as shown 
in FIG. 5, those skilled in the art will understand that strips, beads or 
nodules of elastomeric material could also be used as shown in FIG. 4. 
Furthermore, a layer of insulating material could also be disposed between 
each pair of opposed plates and the elastomeric dielectric material which 
separates the plates, as described above with reference to FIG. 6. 
As in the embodiment of FIG. 5, the dielectric material 76a, 76b, 76c, etc. 
separates each of the pairs of opposed plates 72a, 72b, etc. comprising 
the stack by a distance "d" such that, for a given gas maintained between 
the plates at a pressure "P", the product Pd is significantly less than 
the value required to achieve the Paschen minimum breakdown voltage of the 
gas. The resultant transducer is capable of generating very large 
displacements, due to the cumulative effect of the displacements generated 
by each of the opposed pairs of plates comprising transducer 70. 
There are a wide variety of practical applications for elastomer membrane 
enhanced electrostatic transducers constructed in accordance with the 
invention. As one example, the invention facilitates the production of an 
inexpensive, highly controllable device for generating small scale motions 
at forces falling within region 16 shown in FIG. 1. This may have 
application for example, in the control of machine tools in which fast, 
accurate, minute movements of a cutting tool are required. This is 
conventionally done with large, expensive hydraulic controls which are 
typically not very accurate when dimensions measured in thousandths of 
inches are to be accommodated. 
The geometry of the transducer is readily adjusted to match its acoustic 
impedance to that of water. Therefore, transducers constructed in 
accordance with the invention may be directly coupled to water and are 
well suited for use in sonar underwater signalling applications, over a 
wide frequency band. Conventionally, in comparison, piezoelectric 
transducers are used in underwater sonar signalling applications but they 
are only capable of accommodating a very narrow band of frequencies 
centered on the resonant frequency of the particular piezoelectric crystal 
material utilized. 
As will be apparent to those skilled in the art in the light of the 
foregoing disclosure, many alterations and modifications are possible in 
the practice in this invention without departing from the spirit or scope 
thereof. For example, in order to increase the available range of suitable 
dielectric materials, elastomeric materials may be combined with other 
essentially rigid (i.e. non-elastomeric) dielectric materials to produce 
composite dielectric structures which retain much of the deformability of 
elastomers and are thus still capable of exploiting the phenomenon 
outlined above to yield transducers exhibiting force and mechanical 
impedance characteristics falling within, or even beyond, region 16 shown 
in FIG. 1. The rigid dielectric portion could be applied to the conductive 
plates by painting, spraying, vacuum deposition, or other known 
techniques. Accordingly, the scope of the invention is to be construed in 
accordance with the substance defined by the following claims.