Mechanical louvers have frequently been used for spacecraft and instrument thermal control purposes. These devices typically consist of parallel or radial vanes, which can be opened or closed to vary the effective emissivity of the underlying surface. This project demonstrates the feasibility of using Micro-Electromechanical Systems (MEMS) technology to miniaturize louvers for such purposes. This concept offers the possibility of substituting the smaller, lighter weight, more rugged, and less costly MEMS devices for such mechanical louvers. In effect, a smart skin that self adjusts in response to environmental influences could be developed composed of arrays of thousands of miniaturized louvers. Several orders of magnitude size, weight, and volume decreases are potentially achieved using micro-electromechanical techniques. The use of this technology offers substantial benefits in spacecraft/instrument design, integration and testing, and flight operations. It will be particularly beneficial for the emerging smaller spacecraft and instruments of the future. In addition, this MEMS thermal louver technology can form the basis for related spacecraft instrument applications. The specific goal of this effort was to develop a preliminary MEMS device capable of modulating the effective emissivity of radiators on spacecraft. The concept pursued uses hinged panels, or louvers, in a manner such that heat emitted from the radiators is a function of louver angle. An electrostatic comb drive or other such actuator can control the louver position. The initial design calls for the louvers to be gold coated while the underlying surface is of high emissivity. Since, the base MEMS material, silicon, is transparent in the InfraRed (IR) spectrum, the device has a minimum emissivity when closed and a maximum emissivity when open. An initial set of polysilicon louver devices was designed at the Johns Hopkins Applied Physics Laboratory in conjunction with the Thermal Engineering Branch at NASA's Goddard Space Flight Center

Champion, J. L.

Darrin, M. A. Garrison

Osiander, R.

Swanson, T. D.

English

NASA Technical Reports Server

D R A F T
MEMS LOUVERS FOR TIIERMAL
CONTROL
J. L. Champion, R. Osiander, M. A
Garrison Darrin, T. D. Swanson*
The Johns Hopkins University - Applied
Physics Laboratory, Laurel, MD 20723
"NASA Goddard Space Flight Center, Code
545, Greenbelt, MD 20771
Abstract
Mechanical louvers have frequently been used
for spacecraft and instrument thermal control
purposes. These devices typically consist of
parallel or radial vanes, which can be opened or
closed to vary the effective emissivity of the
underling surface. This project demonstrates
the feasibility of using Micro-Electromechanical
Systems (MEMS) technology to miniaturize
louvers for such purposes. This concept offers
the possibility of substituting the smaller,
lighter weight, more rugged, and les_ costly
MEMS devices for such mechanical louvers. In
effect, a smart skin that self adjusts in response
to environmental influences could be developed
composed of arrays of thousands of
miniaturized louvers. Several orders of
magnitude size, weight, and volume decreases
are potentially achieved using micro-
electromechanical techniques. The use of this
technology offers substantial benef'_s in
spacecraft/instrument design, integration and
testing, and flight operations. It will be
particularly beneficial for the emerging smaller
spacecraft and instruments of the future. In
addition, this MEMS thermal louver technology
can form the basis for related
spacecraft/instrument applications.
The specific goal of this effort was to develop a
preliminary MEMS device capable of
modulating the effective emissivity of radiators
on spacecraft. The concept pursued uses
hinged panels, or louvers, in a manner such that
heat emitted from the radiators is a function of
louver angle. An electrostatic comb drive or
other such actuator can control the louver
position. The initial design calls for the louvers
to be gold coated while the underlying surface is
of high emissivit).. Since the ha.se MEMS
matcri:d, silicon, is transparent in the IR
spectrum the device has a minimum emi&_ivity
when ch)._d ,rod ;t maximum emixqivity when
open. An initial set of polysilicon louver devices
w:t_ designed at the Johns llopkins University
Apl)licd Physics Laboratory in conjunction
with the Thermal Engineering Branch at
NASA's Goddard Space Flight Center
1. Introduction
All spacecraft and the instruments they support
require an effective thermal control mechanism in
order to operate as designed and achieve their
e.,%)ected lifetimes. In an increasing number of
satellites, optical alignment and calibration require
a strict temperature control. Traditionally the
thermal design is part of the spacecraft design
determined by all the subsystems and instruments.
Heat load levels and their location on the
spacecraft, equipment temperature tolerances,
available power for heaters, view to space, and
other such factors are critical to the design process.
Smaller spacecraft with much shorter design cycles
and fewer resources such as heater power, volume,
and surface, require a new, more active approach.
A number of active methods which vary the heat
rejection rate in a controlled fashion are commonly
used to maintain a reasonable thermal equil_durrt
One such method is to cold bias the Slmcecrafl and
use simple electrical resistance make-up heaters to
control the temperature. However, this can require
considerable electrical power, which the spacecraft
may not have available at all times. Another
approach is to employ a radiator connected with
variable conductive heat pipes, capillary pumped
loops, or loop heat pipes. This approach is
effective but adds weight, cost and complexity. In
addition there are ground testing issues with heat
pipes. Another approach is to use mechanical
louvers that can open to ex3_ose a radiative surface
and close to hide it. While functional, traditional
mechanical louvers are bulky, expensive, subject
to damage, and require significant thermal analysis
to evaluate the effect of different sun angles.
In order to meet current and future space science
goals, miniaturized spacecraft with gready reduced
size and mass, short design and build cycles and
restricted resources (power, command,, control,
etc.) are required. Spacecraft in this very small
size range, I0 to 20 kg. will require smaller
thermal control subsystems. Their low thermal
capacitance will subject them to large temperature
swings when either the heat generation rate or the
1-
https://ntrs.nasa.gov/search.jsp?R=19990067639 2020-06-15T21:52:47+00:00Z
\
DRAFT
thermal sink tcmperature changcs Thc ST5
Constellation. for cxamplc, has a requirement to
maintain thcrmal control through extcndcd earth
shadows, possibly over 2 hours long.
2. Variable Emittance Thermal Control
Surfaces
All spacecraft rely on radiative surfaces to
dissipate waste heat. These radiators have special
coatings, typically with low absorptivity and a high
infrared emissivity, that are intended to optimize
performance under the e.',cpected heat load and
thermal sink environment. Given the dynamics of
the heat loads and thermal environment it is ofen
necessary to have some means of regulating the
heat rejection rate of the radiators in order to
achieve proper thermal balance. The concept of
using a _ed thermal control coating or
surface which can passively or actively adjust its
emissivity in response to such load/environmental
sink variations is a very attractive solution to these
design concerns. Such a system would allow
intelligent control of the rate of heat loss from a
radiator. Variable emissivity coatings offer an
exciting alternative that is uniquely suitable for
micro and nano spacecraft applications.
Variable emittance thermal control coatings have
been under development at NASA-Goddard Space
Flight Center (GSFC) since the mid=1990's. These
coatings change the effective infrared=red
emissivity of a thermal control surface to allow the
radiative heat transfer rate to be modulated upon
command. Three technologies have been under
consideration, electrochromic and electrophoretic
devices, and most recently micro-
electromechanieal &-'vices.
For electrochromic devices, the emittance
modulation is achieved using crystalline
electrochromic materials whose reflectance can be
tuned over a broad wavelength (2 to 40 microns) in
the infrared The electrochromic process is a
reversible, solid-state reduction-oxidation (redox)
reaction. These materials become more reflective
as the concentration of an inserted "alkali metal
(typieaUy lithium) increases. This change is due to
an increase in the electron free density, which
causes the material to undergo a controlled.
transition between an IR transparent wide gap
semiconductor and an IR reflective material. The
elcctrochromic material is typically sandwiched
between rio electrical grids and is also in contact
with an ion-conducting layer that contains the
alkali metal. When a small bias voltage (typically
+/- 1 VDC) is applied, the alkali ions shuttle to one
side or the othcr, thus changing the effective
emissivity of the surface.
Elcctrophorctic devices involve the movcmcnt of
suspended parliclcs (i.e.. very small flakes)
through a fluid under the application of a small
electrical field. The particles carry electric charges
that are acted upon by this field thus causing their
movement through the fluid medium. This
medium is highly absorptive. The particles are
made of. or coated with, a material that has a high
re/lectivity. When an electric field is applied the
flakes are attracted to the electrode and align
themselves with their faces l:m'allel to the surface,
thus displacing the absorptive fluid medium. They
overlap and form an essentially flat surface that is
both a high reflector and spectrally reflective.
When the electric field is reversed the flakes are
drawn to the electrode on the other side of the
highly absorbing fluid medium. The exposed
surface thus becomes highly absorptive. This
process has been demonstrated to be reversible and
should be repeatable for thousands of cycles.
3. Miniature Louvers
Another way to change the emissivity of a surface
is the use of mechanical louvers, where, similar to
the macroscopic, traditional thermal louvers, a
mechanical vane or window is opened and closed
to allow an alterable radiative view to space.
Micro-machining techniques allow the designer to
generate arrays of such structures with feature
sizes on the order of micrometers. This approach
\ /
, /
.__-
Fig. 1: DIFFERE,",_I " CONCEPTS FOR A THEIL\I.M.,
CONTROL LOUVER
2
I) R A I: I
I_ variable¢missi_it,,control ofl_r_ di_tmct
_,d_:mlages over traditional mechanical louvers
,,_,ilh regard to size. vvcighl, mechanical
complexlly, rc..dundancv, and cost. [n addition. Ihc
anal_,;is effort is simpler since the dependence on
sun angle can be eliminated b', having micro
louvers face all directions. This micro louver
approach may offer distinct advantages over the
elcctrochromic and elcctrophorctic approaches
discussed above.
b'(_re I shows throe differcnt conccpts of the
micro louver approach. Micro-clcctro-mcchanical
(MEMS) louvers arc similar to miniature venetian
blinds that can be opened or closed to expose an
undcri_dng thermal control coating. The base
material for the current device is silicon while the
louvers are coated with a metal, in this case gold.
Gold coated surfaces have a very low emissivity,
with values of 0.02 to 0.03 commonly quoted. The
MEMS louver is typically 500 jam on a side and
cover an underlying, high emissivity surface. The
"effective emissivity" of the surface can be
modulated in a controlled fashion by varying either
the open area of the micro-louvers or the total
number of micro-louvers that are opened.
The emittance variability is given by the difference
in area-coverage of the open and closed louver.
This may be on the order of 90 %, which allows
emissivity-variations somewhere between 0.1 and
I)') lhc It_u_crs can Ix: ,ictualcd acit',cl) by
rcnlolc conlrol or r_lsSi_.cly using smart fccdLrack
such as he-morph dc,.iccs The small li:att,rc sizes
m;.ikc Ihcsc k)u_.cr conccpl_ COlll_lliblc _,,ith
miniah,rc _l_]cccral'l.
4. Prototype MEMS Louver
The development of the louver concept can be
dividcd into thrcc difl'crcnt tasks: The louver
design, the louver l_brication, and the actuation.
Our carlv efforts focused on thc design of the
louvers for l_abrication using thc MCNC Multi-user
MEMS process (MUMPs). Two generations of
protobpe MEMS louvers have been developed.
Both sets of the polysilicon de_4ces were designed
at APL. fabricated at the MCNC Technology
Applications Center under the MUMPs program,
and subsequently released and tested at APL..
This first iteration of MEMs louvers was
fabdcated at the MEMS Technology Applications
Center, Research Triangle Park, NC under the he
Multi-User MEMS Processes. or MUMPs, and
subsequently released and tested at APL. The
second generation louver designs include
improvements to the hinge designs, corrugated
louver structures, and gold coating on the louvers.
For the design of the hinges, a rapid prototyping
stereo lithograph)' technique was used, where the
design was "printed" as a plastic mold. This
• . ._..,,r-
'" :S:: 7'!
ffig. 2." ARRAY OF MEMS LOUVERS WITtl MANUAL
ACT_ "AT1ON FOR SETS OF TWO EACtl
['Tg, 3.HINGE DESIGN FOR TtIE _IEMS I,OUVERS IN
Fsg.2
I)l( kl I
allo,,,,cdllav,'_tnlltchmgc dc_ignto bc dclc,,:Icdat
,i_irI_',;I:J_¢lx:I'orcunning lhc rcI_Ir.,clvhmg
MISMP_ processing|railSc,.crul_cI_of louvers
h;.r,,c 13,¢¢tI grouped Iogclhcr Io :_IIo,_v for
mcasurcments of en'tissb.il',,variationusing an
inI'r_Jrcdimagcrv,,tlha closefocusattachment.All
Iouvcrswere designed for illanualactualion.A
scanning electron microscope (SEM) image of a
group of louvers is shown in./(__ure 2. This image
shows an array of six dual-sets of louvers togcthcr
with the structure to alloys' manual actuation. The
corrugated structures in the louvers are visible as
well as the gold coating and the etch holes. In
ordcr to prevent bending from residual stress after
the gold coating, the louvers were coated in small.
non-overlapping 20x20 _xm areas. After the
release only a small amount of bending was
observed on the louvers which was reduced even
further by thinning the gold coating to 100 nm.
Figure 4 shows SEM images of the louvers in the
semi-open and open condition.. The other
concepts depicted in Figure 1 were also developed
and prototypes for these devices are shown in
Figures 5 and 6. The Figure 5 shows
three "sliders" next to each other, each about 0.5 x
1.5 mm in size. The actuation was intended to be
manual. The sliders also are gold coated and have
corrugated structures for support. Figure 6 shows
a "folding louver" in the closed position. All SEM
pictures were taken after the louvers were released,
and any residual polysilicon was removed from the
back surface. Although _nt at the IR
v,:i,,clcnglhs of interest. Ihc sdtcon _;ub,,tr:tlc tlsclf
iS II()I',;tlll;IhJc ;IS ;1 r:ldi;lli_ c surlht:c duc to its low
CIIIISSIklI_. ;llld high rcllccli',ilx llcncc, after
removal o1 the silicon st,bqr, ltc b_ ctchanl during
D_st processing these louvers _ill bc placed on a
surface suitable tbr radiati_ c he:it toss.
5. Infrared Emissivity Measurements
Although thc louvers arc nol mountcd on a
radiator, infrared images taken at room
temperature in the 8-12 _m wavclcngth range
allow a reasonable demonstration of their
pcrformancc. Infrared imagcs _vcrc takcn using a
Mikron Scanncr. which scans the imagc with two
mirrors onto a HgCdTe detector with a spatial
resolution of 320x240 pLxels. Using a close-focus
attachment, the image resolution was in the order
of 2 _m per pLxel, way below the resolution limit
given by the wavelength. Calibration was
performed on an emissivity standard, and the room
temperature background was subtracted. An
emissivity image of the materials on the dies is
shown in Figure 7. The structures are the gold-
coated "sliders" on the lefL the silicon structures
(for the manual actuation) in the middle, and the
SiN subsrtate coating on the left. The emissivity
varies from 0.3 for the SiN to 0 (0.02 is the
literature value _ for gold). The same
measurements were performed for the louver
arrays in the closed and open position. The
emissi_Sty images are shown in Figures 8 (array
closed) and 9 (array partially open). The average
fog. 4." SEM IXl.-_GEOF [.OUVERS IN Till.: SF.MI-OPFN .M\'DOPEN l,osrrloN. TtlE WIDTH OF EACll I.OI.%'ER IS
500 _x[
() R A !: I"
C7
Fig. 5: SEM IMAGE OF A "SLIDER", ARRAY, EACH
0.5.X 0.75 ,_n_I
Fig. 6: SEM IMAGE OF A "FOLDING LOUVER"
emissivity [2 for the louver area changes from 0.1
in Figure 8 to 0.2 in Figure 9, according to
=0.3* Ao+(n-r_)*A_, where At is the uncovered
area, n is the number of louvers with area A_, and
n¢ is the number of closed louvers. 0.3 is the
emissivity for the SiN background. Experimental
setup improvements will allow measurements to
be taken at increased temperatures with reduced
background radiation. (the discussion immediately
above is cortfusing to me)
Hence, a change in the effective emittance of a
sttrface has been successfiaUy demonstrated.
Significant improvements to the data quoted above
appear possible once certain design improvements,
such as a better radiative substrate and denser
packaging, can be instituted.
6. Louver actuation for thermal control
For a successful application of the louver concept
for spacecraft thermal control, an actuation
mechanism "has to be identified which allows the
highest individual louver control possible with a
minimum of space necessary. Note. that all the
space covered by the actuation is not active and
presents an emissi_lty bias. Highly individual
louver control provides the best accuracy in setting
the emissivity and further allows increased control
I I i
0.0 0.i 0.2 0.3
Figure 7.EMISSIVITYLMAGE (2X 3 MM)OF CLOSED,
GOLD COATED "SLIDERS" (LEFT), Sl STRUCTURES,
AND BARE SIN.
of the spatial emissivity variations. Further, low
power consumption and zero power in a static
condition are required for small spacecraft
applications. Several actuation mechanisms have
been considered and in part. implemented. One
implementation was the use for an electrostatic
comb drive. While this is a low power, reliable
and straight-for_vard designed MEMS acttmtion
mechanism _-, disadvantages arise due to the large
area requirement and from a space-craft
DRAFT
pcrspcctivc,relativelyhighdrivingvoltages.In
addition,static hargingof the surface from space
radiation could bc an issue. Anothcr mechanism
used in some prototype louvers is a "hcatuator ''_,
which does not have the high voltage requirement
but still takes up a lot of area on the louver chip.
Both actuation mechanisms are solutions where
the actuation devices could be placed outside of
the active area above the radiator, but in this case
individual control of the louvers will be difficult.
A different actuation mechanism is under
consideration for the next generation. It involve
coating of the actuation structures with a metal
different than gold to create a bi-morph, which can
is heated electrically to generate actuation in
connection with the change due to different
thermal expansion. Such an actuation mechanism
could be used in a smart way, where the surface
temperature directly controls the louver actuation.
Similar in function could be the use of shape
memory alloy coatings such as Titanol for the
actuators 4.
7. Reliability Aspects
There are many reliability issues surrounding the
extended use of MEMS devices for spacecraft
applications. 5 The louvers must survive through
the launch and operate in the harsh environment of
space. In addition, the effects of pre-launch
storage must also be taken into consideration. A
non--e:,daaustive list of the of MEMS reliability
concerns includes: stiction, ground contamination,
fatigue including radiation, wear, and vibrational
loading induced. An extended evaluation of these
issues is currently under study and only a brief
overview is given here.
modulation in the spacecraft's' heat rejection mtc.
Specifically, the ST5 Constellation spacecraft will
undergo an appro._matcly 2-hour or longer eclipse
during which time the instruments must su_'ivc
and possibly operate. Given their small capacity
for power storage by batteries and low thermal
capacitance, the best strategy will be to "close off"
their radiator area and radically reduce their heat
loss rate. One recent Aerospace study 6 predicted
heater power savings of 50 to 90°/, and a nearly 4:1
reduction in component temperature variations. In
addition to the obvious weight and power savings,
the technology of MEMS louvers for thermal
control would greatly simplify spacecraft design
and qualification testing and also allow adaptive
response to changing power levels or unexpected
thermal environments once on-orbit.
Acknowledgements
The program was funded by NASA under contract
.EETLY. Thanks go to R.L. Edwards and J. Layton
and .EET( for their help on processing of the
MZfMPs dies.
References
i IR Handb_k
2 Comb-drive reference
3Heatuator reference
4 Shape Memory Alloy Reference
s JPL 99-1
s Aerospace Study
8. Conclusions
To date, two generations of prototype MEMS
louvers have been developed clearly demonstrating
the feasibility of using arrays of devices for
miniaturized satellite thermal control. Successful
actuation of the initial devices and the results of
preliminary emissivity testing indicated the
validity of the hinged louver concept for thermal
control applications. After verification of space
qualification of the louvers, the next step will be to
fly one or more very small prototypes in a standard
calorimeter as experiments on upcoming
spacecraft.
Numerous future NASA missions such as the ST5
Constellation, will undergo significant changes in
its thermal emironment and will require means of


MEMS Louvers for Thermal Control

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