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BACKGROUND OF THE INVENTION
1. Field of the Invention
The subject invention relates generally to pumps and, more particularly, to
a method and apparatus for microscopic scale pumping of a fluid employing
a micromachined electrostatic pumping device.
2. Description of Related Art
As reported in the article "Peristaltic Pumping" by M. Y. Jaffrin and A. H.
Shapiro (1971), peristaltic pumping is a form of fluid transport that
occurs when a progressive wave of area contraction or expansion propagates
along the length of a distensible tube containing a liquid.
Physiologically, peristaltic action is an inherent neuromuscular property
of any tubular smooth muscle structure. This characteristic is put to use
by the body to propel or to mix the contents of a tube, as in the ureters,
the gastrointestinal tract, the bile duct, and other glandular ducts.
Peristalsis is also the mechanism by which roller or finger pumps operate.
Here the tube is passive, but is compressed by rotating rollers, by a
series of mechanical fingers, or by a nutating plate. These devices are
used to pump blood, slurries, corrosive fluids, and foods, whenever it is
desirable to prevent the transported fluid from coming into contact with
the mechanical parts of the pump. Generally the compression mechanism
occludes the tube completely or almost completely, and the pump, by
positive displacement, "milks" the fluid through the tube.
While the prior art has addressed various small electrostatic or
piezo-driven pumps, no truly microperistaltic-type pump has been provided.
Prior art proposals include devices employing triple chambers with
valving, typically implemented with piezodevices. Such systems are not
truly peristaltic.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a miniature pump;
It is another object of the invention to provide a miniature pump
fabricated by micromachining techniques which are applicable to various
substrates and especially those used in semiconductor fabrication; and
It is another object of the invention to provide a micromachined pump which
exhibits true peristaltic action.
These and other objects and advantages are achieved according to the
invention by provision of a micromachined pump including a channel formed
in a semiconductor substrate by conventional processes such as chemical
etching. A number of insulating barriers are established in the substrate
parallel to one another and transverse to the channel. The barriers
separate a series of electrically conductive strips. An overlying flexible
conductive membrane is applied over the channel and conductive strips with
an insulating layer separating the conductive strips from the conductive
membrane. Application of a sequential voltage to the series of strips
pulls the membrane into the channel portion of each successive strip to
achieve a pumping action. A particularly desirable arrangement employs a
micromachined push-pull dual channel cavity employing two substrates with
a single membrane sandwiched between them.
The invention provides a method and apparatus for microscopic scale pumping
of a liquid or vapor fluid. The submicron precision with which
micromachining can define structural dimensions and with which etch stops
can regulate layer thickness enables the fabrication of minutely scaled
structures in which significant and reproducible electrostatic fields are
generated by low voltages. Additionally, the invention provides a method
of facilitating significant convective heat flux by the forced flow of
fluids through microchannels within a solid, as well as many other
advantageous applications hereafter described.
BRIEF DESCRIPTION OF THE DRAWINGS
The objects and features of the present invention, which are believed to be
novel, are set forth with particularity in the appended claims. The
present invention, both as to its organization and manner of operation,
together with further objects and advantages, may best be understood by
reference to the following description, taken in connection with the
accompanying drawings, of which:
FIG. 1 is a perspective view of a micromachined pump according to a first
preferred embodiment;
FIG. 2 is a cross-sectional view of the device of FIG. 1 with voltage
applied;
FIG. 3 is a cross-sectional view of the device of FIG. 1 with no voltage
applied;
FIG. 4 is an exploded perspective view of a dual channel micropump
according to a second preferred embodiment;
FIG. 5 is a top view of a conductive strip layer of a micropump according
to FIG. 4;
FIGS. 6 and 7 are schematic end views illustrating the operation of a
push-pull pump according to the second preferred embodiment;
FIGS. 8 and 9 are partial side cross-sectional views illustrating
sequential application of electrical signals down the channel of a
micropump device according to the first and second preferred embodiments,
respectively;
FIG. 10 is a partial side sectional view of a micropump channel according
to the second preferred embodiment;
FIG. 11 is a schematic block diagram of a low differential pressure gas
delivery system employing a micropump according to a preferred embodiment;
FIG. 12 is a schematic block diagram of a convective heat exchanger
employing a micropump according to a preferred embodiment;
FIG. 13 is a schematic block diagram of a compressor according to a
preferred embodiment;
FIG. 14 is a schematic block diagram of a vacuum pump according to a
preferred embodiment;
FIG. 15 is a schematic block diagram of a fluid delivery system according
to a preferred embodiment;
FIG. 16 is a schematic block diagram of a heat pipe according to a
preferred embodiment;
FIG. 17 is a schematic block diagram of a sterling cycle engine according
to a preferred embodiment;
FIG. 18 is a schematic block diagram of a cryopump system according to a
preferred embodiment;
FIG. 19 is a schematic block diagram of a reaction wheel according to a
preferred embodiment; and
FIGS. 20 and 21 are schematic block diagrams of a valve according to a
preferred embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following description is provided to enable any person skilled in the
an to make and use the invention and sets forth the best modes
contemplated by the inventor of carrying out his invention. Various
modifications, however, will remain readily apparent to those skilled in
the art, since the generic principles of the present invention have been
defined herein specifically to provide a particularly useful and widely
applicable micropump structure.
FIG. 1 illustrates one embodiment of an electrostatically driven
peristaltic pump according to the present invention. A pump channel 13 is
etched into a silicon substrate 15, lined with electrically conductive
strips 21 whose top surfaces are covered with electrically insulating
material 23. The strips 21 are separated from each other by electrically
insulating barriers 25 formed transverse to the channel 13. The channel 13
is then covered by an electrically conductive flexible membrane 17.
With no voltage applied, the membrane 17 is linear in cross-section and
lies over the channel 13, as shown in FIG. 3. By applying a suitable
voltage between the membrane 17 and each of the conductive strips 21 in
succession, the membrane 17 can be electrostatically pulled into the
channel 13, as shown in FIG. 2, at successive positions along the length
"e" of the channel 13, thereby creating a peristaltic pumping action.
The characteristics and performance of the disclosed electrostatic actuated
peristaltic pumps are principally dependent on the properties of the
flexible membrane 17, which may exhibit an elasticity of about 30%. For
low differential pressures and moderate temperatures a graphite
impregnated polyurethane membrane material of thickness 5 .mu.m is
satisfactory. In vacuum applications, surface metallization of
polyurethane membranes is necessary to reduce porosity. Higher voltages,
such as 100 volts, are required to generate the electrostatic forces
necessary to overcome the larger differential pressures, and high
progression rates (500 m/sec) are required to pump nonviscous gases
(vacuum pressures).
FIGS. 4 and 5 illustrate the preferred push-pull dual cavity embodiment of
a microperistaltic pump, where two silicon substrates 115 are placed
together with a single membrane 117 sandwiched between them. The membrane
117 may again be graphite impregnated polyurethane. Between the membrane
117 and each substrate 115 are positioned respective conductive strip
layers 116 and respective insulating layers 118. Each substrate 115
further has a linear conductor pit 119 and a bond metal trench 122 located
adjacent one another and running parallel to a channel 120. While the
thickness of the insulation layer 118 must be of submicron dimensions to
ensure high electrostatic forces on the membrane 117, the channels 120 may
be of millimeter dimensions.
A conductive strip layer 116 is shown in more detail in FIG. 5. The strip
layer includes a number of actuator strip elements 121 which begin at the
top edge of the channel 120 and traverse down the channel 120 and up the
channel to its opposite edge. Thus, the substrate top surface curves down
on either side to form a walled channel 120 having a radiused, concave, or
rounded bottom portion such that no sharp edges are involved. The actuator
strips 121 are rectangular conductor elements lying parallel to one
another, transversely to the channel 120 and laid out down the length of
the channel 120. They may be, for example, 0.1 millimeter in width "w"
such that a group of 200 strips occupies about 20 millimeters. The space
between the elements 121 is filled with insulation provided by an
insulating layer 118 to provide interstrip insulation 3 which insulates
each actuator element 121 from the next element 121. Thinner lead elements
142 lead away from each actuator element 121 to a respective conductive
pad 143 which provides a wire bond pad for establishing electrical
connection to a shift register or other electronic componentry. The
actuator strips 121, leads 142, and pads 143 are preferably formed by
etching a single deposited conductive metal layer such as a gold layer.
Each conductor pit 119 has a conductor channel 123 (FIG. 4) formed therein
of conductive metal which establishes electrical connection to the
membrane 117. The membrane 117 has complementary upper and lower lips 125,
127 (FIG. 4) on respective ends thereof which fit into and mate with a
respective conductor channel 123 to both establish electrical connection
to the membrane 117 and position and hold the membrane 117 in place when
the two substrates 115 are bonded together and hermetically sealed with
the assistance of bond metal placed in the bond metal trenches 122 between
the substrates 115 and the insulation layers 118 as described below. A
ledge 131 is further formed on each substrate 115 parallel to the channel
120 in order to provide for membrane thickness and permit some squeezing
to hold the membrane 117 in position.
Micromachining techniques have evolved from the microelectronics industry.
Both the additive processes of thin film deposition or vapor deposition
and the subtractive processes of chemical or plasma etching are
appropriate for the manufacture of both the channels and pump. The bulk
etching of channels in silicon, quartz, or other suitable substrate,
whether semiconductor, metallic, or otherwise, and its fusing to a mirror
image wafer is one technique of creating a microperistaltic pump. Surface
micromachining may also be deployed where a patterned sacrificial profile
of the channel is created over which the actuator and insulation materials
are deposited.
Isotropic etching techniques are employed in an illustrative implementation
of the micropump to create a smooth, contoured concave channel 120. Once
this channel and other grooves 119, 120, 122 and ledges 131 have been
created, a metal layer of a few hundred Angstroms (.ANG.) in thickness is
vapor or sputter deposited evenly over the whole top surface of substrate
115. An even layer of photoresist is then applied and a photo mask is
thereafter used to define the etch barriers to form the metal actuator
strips 121, leads 142, pads 143, and conductive membrane connection
channels 123 (FIG. 5). The comparatively large depth of field required for
submicron definition of the actuator elements 121 in the channel 120
requires special care.
Following the etching and removal of the photoresist, a vapor epitaxial
deposit of a micron of silicon dioxide, or like insulation material, is
required to form the insulation layers 118. The insulation layers 118
provide the insulation between the actuator strips themselves, the
insulation between the actuator strips 116 and the membrane 117, and the
insulation between the strips 116 and the bond metal to be placed in the
bond metal trenches 122.
After annealing the material to consolidate the insulation layer 118,
another photoresist coating is applied and then another photo mask in
order to define the membrane connection channel 123 and insulation
profile, e.g. to expose the conductive strip connection pads 143. The
final wafer processing step involves the vapor or sputter deposition of a
column of interwafer bond metal in the bond metal trenches 122, for
example, utilizing a shadow mask. The pump die shells or substrates 115
are then cut from their wafer, the flexible membrane 117 placed between
two shells 115, and the assembly clamped together and placed in an oven
until the bond metal melts, pulls the two dies together, and fuses the two
dies 115 together to form a solid structure hermetically sealed down both
sides by the bond metal, such as illustrated in FIG. 6. A typical bond
metal is a mixture of gold and germanium.
Where the membrane 117 is clamped, it is in intimate contact with the thin
insulation layer 118 of both shells 115. Hence, when a voltage is applied
between an actuator element 121 and the membrane 117, an electrostatic
attraction force, proportional to the square of the applied voltage and
the inverse square of the insulation thickness (<1 micron), pulls the
membrane 117 down. The membrane 117 rolls down the surface 144 of the
insulation (FIG. 6), due to the fact that the greatest attractive forces
are generated where distances from conductive strips 121 are the smallest
(i.e. insulation thickness). Conversely, when a voltage is applied to the
strip 121 in the upper shell 115, the membrane 117 rolls up its channel
surface 145. As seen in the cross-sectional view down the channel of FIG.
8, when a neighboring conducting strip 121 is energized the membrane 117
rolls forward (FIG. 8) and down across the activated elements. The
membrane 117 is initially drawn up onto the upper channel surface 145
(FIG. 9) and advanced along the channel 120, then the membrane 117 is
released for several periods (zeros) before the membrane 117 is drawn down
into the lower channel 120 and then rolls down the lower channel surface
144. Thus, a membrane "wall" is placed across the composite channel. By
connecting the actuator elements 121 up to the outputs of a shift register
vial leads 142 and pads 143, a clocked bit stream of "1s" or "0s" apply a
voltage or no voltage with respect to the membrane 117, respectively, to
the actuator elements 121 along the channel 120 in a sequential manner.
This actuation progression provides a miniature peristaltic pump.
In the case of the preferred embodiment of a dual channeled pump, dual
shift registers are required where the bit streams are interlaced and
interlocked such that a membrane wall is advanced down channel. By
alternate inversions of the bit streams sequences, multiple membrane
"bubbles" 147 will move down the channel (FIG. 10), pushing the entrapped
fluid in front of each membrane "wall" and pulling the fluid behind each
membrane "wall."
This disclosed pump architecture represents a true two-dimensional analog
of the three-dimensional peristaltic mechanisms that are endemic in living
organisms. It is valveless and impervious to gas bubble entrapment that
has plagued other attempts at miniature pumps. It also does not require
priming and can tolerate the adherence of small foreign articles (small
compared with cavity dimensions) on channel or membrane surfaces. The pump
is self-purging, tending to push everything before the membrane 117 in its
intimate rolling motion across the channel surface. Its performance is
gracefully degraded by the adherence of small foreign particles, with the
membrane 117 still progressing along the channel 120, but with less
attractive force when across the particle due to the greater distance of
that portion of the membrane 117 from the underlying conductive strip 121.
It is presently not certain as to whether an electrostatic peristaltic pump
according to the preferred embodiment can only function with fluids that
are electrically nonconductive. If not, magnetic renditions might be
considered for electrically conductive fluids, but these would be more
complex, require significantly greater amounts of power, and function over
a more restrictive temperature range.
The disclosed pumps have a number of advantages. At micron dimensions small
voltages create high electric fields over small distances which, in turn,
are capable of generating substantial electrostatic forces. Electrostatic
actuators consume no power (fractions of mW at high frequencies) and
function from absolute zero to the eutectic melting temperatures of
interwafer bonding materials.
Several applications for microperistaltic pumps according to the preferred
embodiment have been identified, specifically: low differential pressure
gas pump, forced convective transfer heat exchanger, pneumatic turbine
compressor, vacuum pump, fluid pumps, heat pipe (thermal mass transfer),
compressor for phase interchange heat pump/refrigerator, low vibration
cryogenic fluid pump, fluidic reaction wheel, and high pressure valve.
The simplest application of a microperistaltic pump is a low differential
pressure gas delivery system (FIG. 11). For example, such a pump 151 could
draw gas from an environment of interest and feed it through a gas
analyzer 153. This might be to analyze the ambient air for CO or to search
for gas leaks, or to draw automobile exhaust gas to monitor hydrocarbon
output. A further application might be to sample dust, soot, pollen, or
small insects by drawing air through a filter or array of small channels.
In the application of FIG. 11, the gas flow rate through the pump 151 is
also measured indirectly by knowing the effective cross-section of the
pump 151, the pitch of the conductive strips 121, and the progression rate
of strip excitation (oscillator clocking frequency). The mass flow rate is
also known if the exhaust gas temperature and pressure are measured.
At high flow rates significant convective heat fluxes, and thus intimate
thermal coupling, are achieved by the forced flow of fluids through
microchannels within a solid. A forced convective heat exchanger may thus
be provided as shown in FIG. 12 by micromachined channels 120 in a
thermally conductive material of a pump 151, which channels are
constructed in such a way as to maximize surface area. Fluid flow through
these channels 120 provides effective convective coupling to the channel
surface. Duct size and the flow velocity need to be selected to provide
for optimum heat transfer efficiency between the gas and the pump. This
inventor presently knows of no known existing method of facilitating
significant convective heat flux by the forced flow of fluids through
microchannels within a thermally conductive solid.
Microdimensional solids exhibit small thermal conductive loss, and high
velocity gases traveling through microdimensional channels exhibit small
thermal convective loss. This provides high thermal coupling between solid
and fluid. Microstructured pump-channel implementation on a substrate,
complete with drive electronics, results in a "breathing skin" with a high
thermal transfer coefficient. In a nonvacuum environment, such pumps draw
from still air at the surface and expel away from the surface. The heat
pump is not dependent on density gradients and gravity fields as are
conventional convective heat sinks, etc. They may therefore be used in
space (i.e., shuttle, space station). With many pump-channel cells per
square centimeter, the devices may be bonded to the surfaces of integrated
circuit chips ("hot body" 165) to dissipate their heat directly: no forced
ventilation, no orientation constraints, no noise, and no moving parts.
The pump-channel cell substrate may be bonded to the surface of power
packs or system chassis in large area slabs to remove heat as an
alternative to natural convection heat sinks or ducted air circulation.
Where multiple membrane "bubbles" are moving down the channels the pump
exhibits a multi stage characteristic where the differential pressures
across each membrane "bubble" may be cascaded cumulatively across the
pump.
At high flow rates and moderate pressures the pump 151 may function as a
compressor (FIG. 13). Such a compressor may drive a pneumatic turbine 167
enabling a useful class of small rotational mechanisms. Examples of these
mechanisms are dental drills, miniature gyroscopes, rotating shutter
systems, etc. Such a compressor may also be used to inflate small
structures or via an isolation membrane or syringe plunger to dispense
precise volumes of drugs or reagents.
At high membrane progression rates and high differential pressures the
device may function as a vacuum pump 151 for evacuating a chamber 169
(FIG. 14). In such applications surface metallization of polyurethane
membranes would be necessary to reduce porosity. Higher voltages would
also be required to generate the greater electrostatic forces that are
required to overcome the larger differential pressures and high
progression rates required (500 m/sec) to pump nonviscous gasses (vacuum
pressures).
For vapor phase pumps the pitch of the strip actuators may also be
progressively increased to provide a staged pressure build up along the
channel. This compression build-up would be particularly beneficial where
gas/liquid phase changes occur.
The growing trend in biotechnology toward automation and miniaturization of
components and reagent consumption is elevating interest in
micro-fluidics, particularly the need for physically small pumps, valves,
and mixing chambers. Microfabricated "lab-on-a-chip" instruments are
emerging for conducting electrophoresis, radiography, protein sequencing,
DNA diagnostics, and genotyping that require sample and reagent delivery
systems capable of regulating volumes in the 10-1000 nanoliter range.
Miniature biosensors and drug delivery systems are other arenas requiring
microfluidic pumps, valves, pipes, and vessels. FIG. 15 shows a pair of
micromachined peristaltic pumps 151 arranged to deliver a reagent from a
reservoir 171 and a sample liquid specimen from a supply source 170
through micro-machined delivery channel sections 172 to a reaction chamber
173. The reaction chamber 173 may output to a detector 174.
In configurations like FIG. 15, such pumps 151 can deliver and measure
minuscule volumes of "incompressible" liquids and at precisely determined
times or time intervals, for example, by actuating the membrane at times
recorded in the memory of a programmed digital processor or computer. The
precision with which volumes can be measured (or delivered) by the
disclosed microperistaltic pump is that associated with a single stepped
advance of an actuator strip. This minimum volume is thus defined by the
product of the channel cross-section and actuator pitch. For example, a
relatively large channel, by micromachining standards, with a
cross-section of 0.5 mm.sup.2 and an actuator pitch of 0.1 mm has a
minimum volume displacement of 50 nanoliters. By micromachining standards,
this is a large pump.
In continuous flow microreaction cells, separate pumps may be used for each
reagent and respectively run at clocking rates that are proportional to
the required concentration ratios. In batch mode operation, specific
volumes of reagents may be metered by providing sufficient clock pulses to
deliver the necessary number of minimum volume displacements. When the
pump is operated in the static or intermittent mode the "across channel"
membrane functions as a valve. If reaction cells input ports are directly
coupled to pumps the membrane "valves" can isolate the cell against
appreciable back pressure and for an indefinite period between successive
deliveries of metered volumes of reagent.
Effective thermal conductivity of active heat pipes is markedly superior to
that of the best passive thermal conductive materials. A micropump 151 may
be used to circulate a fluid between a thermal source 181 and sink 183 as
shown in FIG. 16, effectively transferring heat both within the
circulating medium by thermal mass transfer and between the medium and the
source and sink by improved convective transfer. At high flow rates
significant convective heat fluxes, and thus intimate thermal coupling,
are achieved by the forced flow of fluids through micro-channels within a
solid.
Thermal transfer capacity is further enhanced by the absorption or
dissipation of latent heat generated from gas/liquid or liquid/gas phase
transitions. These phase transitions can be orchestrated by pumping where
compression liquefies and evacuation vaporizes. The Carnot vapor
compression cycle defines such a heat engine. In a micromachined version
of a closed loop sterling cycle as shown in FIG. 17, the whole engine, or
a serial cascade of sterling engines, may be fabricated from two fused
wafers. A miniature peristaltic pump 151 draws refrigerant vapor and
compresses it into its liquid phase. The liquid, heated from the liberated
latent heat, is then cooled by convective transfer into the surrounding
substrate microchannel 181 and onto a highly thermally conductive heat
exchanger 185 created in the substrate. Another microchannel 188 conducts
the cooled liquid refrigerant to an expansion nozzle 190 in a thermally
isolated cold pad 186, where the refrigerant expands into its vapor phase,
drawing the latent heat of evaporation from the cold pad 186. This cold
vapor is conveyed in yet another microchannel 187 to the inlet port 191 of
the miniature peristaltic pump 151. The peristaltic pump 151 exhibits very
low vibration, as it has no reciprocating parts, but instead has a very
low mass membrane that rolls across the surfaces of the channels, e.g.
144, 145. A micromachined version is thus ideally suited to cooling long
wavelength infrared detectors. Equipped with low porosity and low
temperature membranes, micromachined peristaltic pumped Carnot engines are
capable of cooling imaging detectors from room temperature to around
70.degree. K.
It is questionable whether a micromachined peristaltic pump could generate
the hundreds of PSI required of the hydrogen cycle cryogenic heat
exchangers. The need for tight thermal coupling between conventional
cryogenic pumps and imaging objectives compromises the level of pump
vibration that an objective can endure. However, as shown in FIG. 18, a
small peristaltic pump 151 can provide the tight thermal coupling between
a cryopump 201 and an objective (infrared detector) 203 without vibration
coupling. To operate at cryogenic temperatures a polymer membraned
peristaltic pump would be required.
Conventional reaction wheels consist of electric motor driven fly wheels
where the precession generated by fly wheel momentum changes, due to
changes in angular velocity, provide corrective forces for stabilizing
space craft. There are no known existing methods of producing micron scale
motors or rotational members for incorporation in "micro" reaction wheels.
A closed circular version 204 of a micromachined peristaltic pump may be
manufactured as shown in FIG. 19, complete with convenient electrical
interfacing, to provide an extensive range of flow rates. The entire
circular channel is lined with radially-arrayed conductor elements, e.g.
121, and driven by a shift register 208 and cooperating oscillator/power
supply 207, 209. The circulation of a dense fluid in a smooth contoured
isotropically etched circular channel of the pump 204 mimics the function
of a reaction wheel. Precession forces now result from changes in fluid
flow rates within the channel or channels.
The large electrostatic forces generated on a membrane across a thin layer
of insulation has other applications aside from a peristaltic pump. If the
membrane cross-section is small, then significant pressures are required
to separate the membrane from or prevent the membrane attaching to the
channel surface. Multiple strip conductors can increase withhold pressures
and reduce propensity toward valve leakage. With 100 volts across
insulation between the membrane and conductive strip actuators and a small
membrane cross-section, a "normally open" valve, FIGS. 20 and 21, capable
of withholding hundreds of PSI pressure may be realized. The closing
pressure limit is determined by the effective area of the membrane and the
electrostatic force generated across the thin insulation layer. The valve
ports 211, 213 may be at either end of a pump-like channel 215,
perpendicular to a channel and perforating a conductive strip or one
perpendicular and one parallel. Because the valve actuator is
electrostatic and draws no current, little power, aside from maintaining
potential, is required to keep the valve closed.
To summarize, some of the advantages and areas of application of the
invention are as follows:
1. The miniature peristaltic pump can be used to transport fluids (or
vapors) over an extensive range of flow rates.
2. The suggested implementation of the pump doubles as a positive
displacement flow meter, thus the mass flow rate and angular momentum can
be measured directly.
3. Microstructured pump-channel implementation on a substrate, complete
with drive electronics, results in a microreaction wheel.
4. Many of these channels may be organized in concentric circles and some
of them operated in opposite directions to maximize momentum change.
5. The electrostatic actuator mechanism requires potential but no current
thus it requires little power and therefore generates virtually no heat.
There are no orientation constraints, no noise and no moving parts.
6. The pump-channel cells need be of the largest diameter possible for
maximum angular momentum. In a fully integrated space craft architecture
the inner area of the substrate would be used for electronics or other
MEMS devices.
Those skilled in the art will appreciate that various adaptations and
modifications of the just-described preferred embodiment can be configured
without departing from the scope and spirit of the invention. Therefore,
it is to be understood that, within the scope of the appended claims, the
invention may be practiced other than as specifically described herein.
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