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192 MEMS and Microstructures in Aerospace Applications

beam. The thermal switch has a selectable temperature action such as open or close
on rise with a set point range between 65 and 1758C. According to the material’s
thermal expansion coefficient, heating the beam to a specific temperature causes a
differential elongation of the beam. As the temperature changes from nominal, the
beam deflects toward the transistor source and drain. An applied, adjustable gate
voltage completes this movement and snaps the beam closed when the temperature
reaches the setpoint. Contact of the beam with the substrate completes the circuit,
allowing current to flow from source to drain. Honeywell’s thermal switch can be
used to activate an electrical signal when the switch is activated by a temperature
change, much like a thermostat. Although not designed to modify conduction path,
Honeywell’s MEMS-based heat switch may be used to control the heater operation
for spacecraft active thermal control purpose.

9.4.2 MEMS LOUVERS AND SHUTTERS
Mechanical thermal louvers are active thermal control devices that have been used
to regulate the area of a radiator in response to its temperature. The regulation of
radiator area is achieved by opening and closing of louver blades which are placed
directly in front of the radiator surface. While most commonly placed over external
radiators, louvers may also be used to modulate heat transfer between internal
spacecraft surfaces, or from internal surfaces directly to space through the opening
in the spacecraft wall. 10
Conventional louvers have been used in different forms on many spacecraft,
including Hubble Space Telescope, Magellan, Viking, and Voyager, to control the
amount of cooling for a fixed size radiator. The most commonly used louver
assembly is the rectangular-blade type which is spring-actuated by bimetallic
metals. Hydraulically activated louvers and pinwheel louvers are used less often
today than in the past. Traditional louvers typically provide closed to open effective
2
emissivity variation of 0.1 to 0.6, are 200 to 6000 cm in total area, and have a
2
weight to area ratio of 5 to 10 kg/m . Disadvantages of traditional louver assemblies
for small satellites are the size and weight, and the sensitivity to the solar position.
MEMS shutters and louvers have been suggested very early as a means of
thermal control using MEMS for nano- and picosatellites. 11 The Johns Hopkins
University Applied Physics Laboratory (JHU/APL), together with NASA Goddard
Space Flight Center (NASA/GSFC), has designed, fabricated, and tested a number
of louver designs using the MCNC (now MEMSCAP) MUMPs process. Figure 9.1
shows a 3 4 array of MEMS louvers, each 300 500 mm in size, and
Figure 9.2 shows the infrared (IR) emissivity at 408C at wavelengths between 8
and 12 mm of the MEMS louver array with the louvers closed, partially open, and
open. The open louvers expose the high-emissivity surface. These louvers and a
number of other designs such as shutters and folding structure were prototype
designs of the concept to be flown on NASA/GSFC Space Technology-5 (ST5)
mission as demonstration technology for variable emittance coatings (VEC). ST5
is part of a series of spacecraft in NASA’s New Millennium Program (NMP)
managed by Jet Propulsion Laboratory. The NMP strives to test new spacecraft

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