Transcript Faster, Better, Cheaper

Nanosatellite Communication
And MEMS Technology
Nick Pohlman
Patrick Schubel
Jeremy Opperer
Introduction to MEMS
ME 381
Robert McCormick School of Engineering and Applied Science
December 2002
Overview
• Changing satellite architecture
– Smaller, distributed systems
– Require RF communication
• MEMS communication devices
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Switches
Antennas
Signal Filters
Phase Shifters
• Completed picosatellite experiment
• Suggestions for future
Faster, Better, Cheaper
• NASA Administrator
Daniel Goldin sought
new methods for space
exploration
• Reduced mass results in significant gain in shrinking
launching cost
• Less expensive to launch small components individually
rather than monolithic device
– Low Earth Orbit (LEO) ~ $10k per kilogram
– Geosynchronous Orbit (GEO) ~ $50k per kilogram
• Situation perfectly suited for MEMS devices
– Low mass, resistant to inertial and vibration damage
– Endurance in high radiation environments
Distributed Satellite Architecture
• Spread component capabilities to separate
vehicles
– Individualized vehicles faster to produce
because of less system integration
– Easily replaceable for component failure
– Eliminate physical hardware connections and
reduce overall mass
Capabilities of DSA
Chandra X-Ray Observatory
• Increase aperture size
for interferometer and
distributed radar
systems
– Hubble, Chandra
limited size due to
launch constraints
– Failures aboard Hubble
are repairable by
humans; Chandra out
of reach
PKS 0637
3C273
CXC
Hubble Space Telescope
Planned DSA Missions
Terrestrial Planet
Finder (JPL)
TechSat 21 Distributed
Radar (AFRL)
Space Technology 5 & 6 (NASA - NMP) –
First to use primarily MEMS components
Consideration for DSA
• Actively control relative positions and velocities
• Robust, reliable feedback from sensors possibly
onboard separate vehicle
• Remote RF communication necessary
– RF comm. requires sender/receiver pair with signal
processing hardware
• MEMS RF devices explored:
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Switches
Antennas
Signal Filters
Phase Shifters
DC-Contact Coplanar Waveguide Shunt Switch
• Switches used for beam
shaping and steering
• RF MEMS switches have
better efficiency and lower
insertion losses than
conventional switches
• Ideal for space:
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Rapid response
Good power handling
Wide bandwidth
Good EM isolation
High open isolation
• May experience stiction and
slow response time
DC-Contact Coplanar Waveguide Shunt Switch
• Force balance can be used to calculate
the electrostatic force
• The restoring force is found from spring
equation
• For deflections greater than 1/3 d, pull in
occurs
• Pull-in voltage not affected by dielectric
layer
• A, 0, V, d, x, and k are the projected area
of the electrodes, permittivity of the free
space, applied voltage, gap between the
line and bridge, deflection of the bridge,
and spring constant respectively
  0 AV 2
FE 
2(d  x)2
8d 3 k
Vp 
27 0 A
DC-Contact Coplanar Waveguide Shunt Switch
S 21  2 Rs Z 0  , L  Rs 
2
2
 2L Z 0  , L  Rs 
2
• Model of RsL
circuit in isolation
• Rc, Rl are contact
resistance and
contact line
resistance
S 21 
2
4Rb2  1 / 2Cb2 
2R
2
b
 Z 0   4 / 2Cb2 
2
• Model of R-C
circuit, insertion
• Capacitors model
coupling between
switch and pull-in
electrodes
DC-Contact Coplanar Waveguide Shunt Switch
• Process similar to other MEMS devices
manufactured by batch lithographic processing
• 1.7 mm PECVD SiO2 grown as sacrificial layer; dimples created by partially
etching 5500 Å; 0.8 mm sputtered Au creates bridge; buffered HF solution
used to remove SiO2
• RSC microrelay: micromachine @ 250 C; SiO2 removed by dry release
etching in oxygen plasma
Hilbert Curve Fractal Antennas
Hilbert Curve Fractal Antennas
Phase shift in each arm and resulting
peak
direction
of arm
theandbeam
Table
2. Phase
shift in each
resulting peak direction of the beam
Case
1
2
3
4
5
6
Elem.
1 phase
Elem. 2
phase
Elem. 3
phase
Elem. 4
phase
0
0
0
0
0
0
0
20
40
60
90
120
0
40
80
120
180
240
0
60
120
180
270
360
[16]
Beam
dir
0
6
13
19
29
38
HCFA radiation characteristics
Case
0
1
2
3
Peak
dir 1
0
18
19
63
Gain
1.56
1.28
1.55
1.74
Peak
dir 2
177
193
195
254
Gain
3 dB width
1.81
0.95
1.33
2.35
83
107
100
92
Signal Filters
•
CPW filter
structure
All RF communication circuits require
at least one filter to pull out a desired
signal or insert one to be transmitted
– Currently done with solid state devices
– Surface Acoustic Wave (SAW) filters or
back-end digital signal processing
•
MEMS offers passive front-end signal
processing capability
– Compact one-chip design
– High fidelity signal handling
– Tunable configuration
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•
CPW Filter
Filter sensitivity and quality factor
(Q) would be greatly increased
Different MEMS filter designs are
possible
– Coplanar waveguide (CPW) layout
on thin GaAs membrane
– Flexural beam resonator
Beam Resonator
Antennas
• Improved performance of system achieved by
integrating antenna design with other
components on same chip
1600 mm
– “Smart” antennas
• Double-folded slot antenna
– 2.5 mm gold deposited on silicon oxide
dielectric membrane
– Cross members placed half wavelength apart for
optimal performance
1600 mm
77 GHz Double-Folded Slot Antenna
• Reconfigurable V-Antenna
– Arms of antenna can be moved independently
with comb-drive actuators
– Structure fabricated using multi-layer surface
micromachining of silicon
– When both arms moved at fixed angle, antenna
can steer beam to focus reception or
transmission
– Adjusting relative angle of arms can modify
shape of beam
17.5 GHz V-Antenna
Phase Shifters
• Phased-array antenna
– Able to transmit or receive signals
from different directions without
being physically re-oriented
– Currently this is done with FET or
diode technology
• Low power consumption but high
signal loss
– MEMS design would cut down on
signal loss, especially at frequency
range 8-120 GHz
– Not as many amplifiers are needed to
boost signal, resulting in power
savings
Phase shifter composed of array of RF-MEMS switches
• Straightforward design: MEMS switches used in place of solid-state components
– Large body of research already exists in phase shifter design and application
– Proper placement of switches is known
• Significant cost benefits
– MEMS-based array could cut cost of complex phase-shifter by an order of magnitude
Picosatellite Mission
• Satellite mission has been completed proving feasibility of MEMS devices in
space
– RF-MEMS switches in picosatellite (< 1 kg)
• Stanford-designed Orbiting Picosatellite Automated Launcher (OPAL)
launched in 2000 released testing platform
– Two tethered picosatellites in LEO containing four RF-MEMS switches in series
• Switches developed by Rockwell Science Center (RSC)
– Each satellite measures 3 x 4 x 1 in3 and weighs less than half a pound
– Actual communication system made with standard radio components
• MEMS switches used only for experiment
Switch Experiment
• RF switches cycled through on and off states at 500 Hz
• During contact time with ground station, test data and statistics downloaded
– Relatively inexpensive test was successful
• Unfortunately, due to difficulties establishing initial contact with the
picosatellites, mission was prematurely ended when power ran out
– If communication system had been composed of MEMS devices, mission could
have been lengthened!
• Future missions are planned: AFRL MightySat 2.1 and beyond
Figure 15. Picosatellite system architecture [21]
Recommendations/Conclusions
• Network of MEMS satellites for continual base
station communication
– Tap into network much like Internet
– Eliminates remote control stations
• MEMS are ideal for reducing cost of space
exploration
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Reduced overall mass (cheaper launch)
Increased efficiency
Adaptability
Robust to space environment
• Faster, Better, Cheaper… and Smaller