Future concepts for ultra-large lightweight space telescopes include the telescopes with segmented silicon mirrors. This paper describes a proof-of-concept inchworm actuator designed to provide nanometer resolution, high stiffness, large output force, long travel range, and compactness for ultraprecision positioning applications in space. A vertically actuating inchworm microactuator is proposed to achieve large actuation travel by incorporating compliant beam structures within a silicon wafer. An inchworm actuator unit consists of a piezoelectric stack actuator, a driver, a pair of holders, a slider, and a pair of polymer beams connected to a centrally clamped flexure beam. Deep reactive ion etch experiments have been performed for constructing the actuator

Dekany, Richard G.

Padin, Steve

Yang, Eui-Hyeok

Caltech Authors - Main

PROCEEDINGS OF SPIE
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Design and fabrication of a large
vertical-travel silicon inchworm
microactuator for the Advanced
Segmented Silicon Space Telescope
Eui-Hyeok  Yang, Richard G. Dekany, Stephen  Padin
Eui-Hyeok  Yang, Richard G. Dekany, Stephen  Padin, "Design and
fabrication of a large vertical-travel silicon inchworm microactuator for the
Advanced Segmented Silicon Space Telescope," Proc. SPIE 4981, MEMS
Components and Applications for Industry, Automobiles, Aerospace, and
Communication II,  (16 January 2003); doi: 10.1117/12.476282
Event: Micromachining and Microfabrication, 2003, San Jose, CA, United
States
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Design and Fabrication of a Large Vertical Travel Silicon Inchworm 
Microactuator for the Advanced Segmented Silicon Space Telescope 
 
Eui-Hyeok (EH) Yang*a, Richard Dekanyb and Steve Padinb 
 
aJet Propulsion Laboratory, California Institute of Technology 
4800 Oak Grove Drive, Pasadena, CA 91109 
*Eui-Hyeok.Yang@jpl.nasa.gov   ph: +1-818-354-7059  fx: +1-818-393-6047 
 
bCalifornia Institute of Technology, Division of Physics, Mathematics, and Astronomy 
M/S 105-24, Pasadena, CA  91125 
 
 
ABSTRACT 
 
     Future concepts for ultra-large lightweight space telescopes include the telescopes with segmented silicon 
mirrors.  This paper describes a proof-of-concept inchworm actuator designed to provide nanometer resolution, high 
stiffness, large output force, long travel range, and compactness for ultraprecision positioning applications in space.  A 
vertically actuating inchworm microactuator is proposed to achieve large actuation travel by incorporating compliant 
beam structures within a silicon wafer.  An inchworm actuator unit consists of a piezoelectric stack actuator, a driver, a 
pair of holders, a slider, and a pair of polymer beams connected to a centrally clamped flexure beam.  Deep reactive 
ion etch experiments have been performed for constructing the actuator.   
 
Keywords:   Advanced Segmented Silicon Space Telescope (ASSiST), Piezoelectric Actuator, Inchworm Actuator, 
Bistable Flexure Beam, Large Vertical Travel 
 
 
1. INTRODUCTION 
 
     Advanced Segmented Silicon Space Telescope (ASSiST) is a novel space telescope concept that derives high 
functionality from passive spacecraft components [1].  We propose to achieve considerable reductions in primary 
mirror mass through the use of thin (~400 micron) silicon wafers, eliminating the need for a heavy, back plane support 
structure.  Rather, we exploit the micromachining capabilities of silicon processing technology to achieve sophisticated 
control of a highly segmented mirror using high-bandwidth, high-stroke actuators.  Ultimately, these actuators can be 
built directly into the mirror segment, resulting in an integrated optics package.  Thus, a single segment can perform 
both the traditional light focusing function of a telescope as well as the control functions, and quite possibly the space 
deployment functions.  Although our long-term vision of silicon-based space telescopes includes even greater 
functionality, such as solar-based power generation, for now we plan to concentrate on the development of robust 
actuators as the initial first step to proving the ASSiST concept.  The basic configuration for ASSiST is presented in 
Figure 1.  The development of MEMS (Micro Electro Mechanical Systems) actuators and sensors enables this concept 
(large segmented telescope mirrors of unprecedented area density). Substantial mass reduction is achieved by 
integrating most of the functionality of these large structures into the individual optical segments.   The conceptual use 
of MEMS actuators in the phasing of a segmented optical system is shown in Figure 2.  We propose a MEMS-based 
actuation system, in which a set of MEMS actuators, such as miniaturized inchworm actuators, controls the optical 
surface figure of the primary optics.   A MEMS-based electrostatic linear inchworm motor has been previously 
reported [2].  These actuators move horizontally on the surface of a silicon wafer, and thus cannot provide vertical 
motion. A mesoscale inchworm actuator has also been developed [3]. However, actuators suitable for the ASSiST 
application require further miniaturization. A thermal inchworm actuation concept has also been presented in 
literature, with 50 µN actuation force and 0.2 µm travel resolution [4].  However, significantly more precision, as well 
as rapid actuation with a larger force is required.  Although a proof-of-concept MEMS-based vertical inchworm 
MEMS Components and Applications for Industry, Automobiles, Aerospace, and Communication II,
Siegfried W. Janson, Editor, Proceedings of SPIE Vol. 4981 (2003) © 2003 SPIE · 0277-786X/03/$15.00
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actuator has been reported [5], to our knowledge, none of the above approaches provide the large vertical travel 
actuation (~ hundreds of microns) that is required.  We propose here a novel concept for a MEMS-based piezoelectric 
inchworm microactuator capable of large vertical travel (~ hundreds microns) with precision step motion.  Our 
inchworm actuator technology is achieved by combining piezoelectric-driving and electrostatic-clamping actuation 
within silicon microstructures.   
 
 Figure 1: Spacecraft formation flying geometry for ASSiST.  Starlight entering from the top-right is collected by a Schmidt 
corrector mirror (bottom) then focused by the F/10 primary mirror (top).  Also shown are two large sunshades (at left), which may 
be inflatable.  The instrument bus resides behind the corrector mirror. 
 
 
Figure 2: The role of MEMS technology in segmented mirror actuation is depicted.  In this example two actuators are shown, one 
controlling relative segment height (or piston) and the other controlling the relative dihedral angle between segments. 
 
 
2. PRIMARY MIRROR AND ACTUATION 
 
2.1 Primary Mirror Configuration 
 
     The current design for the ASSiST primary mirror consists of a sheet composed of 30 cm diameter, 400 µm thick, 
hexagonal silicon wafers. The mirror is made into hexagonal rafts of approximately 100 segments, each mounted on a 
deep frame to provide stiffness on 3 m scales. Conventional actuators (e.g. motor-driven screws) adjust inter-raft 
Actuators 
for height control 
Actuators 
for angle control
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piston and tilt about the raft edges to control spatial scales larger than a raft.  This is equivalent to the control of a 
ground-based segmented mirror telescope, but the raft frames are much simpler and lighter than a ground-based mirror 
cell. Autonomous control of each raft is an obvious hierarchy that simplifies the overall control of the mirror.  The 
segments within a raft are mounted on 10 mm deep silicon frames, which would limit the segment warp and provide 
mounting surfaces for the actuators. The frames also distribute actuator forces to reduce local deformation (actuator 
“print-through”). Control forces would be applied between adjacent segments, with a control range of a few times 10 
µm to accommodate misalignments during segment assembly.  The forces required to correct warps of 10 µm over a 
segment are on the order of 1 mN, which is well within the capabilities of the actuators described in this paper. The 
yield strength of the actuators allows acceleration of the telescope up to 10-7 rad s-2 (cf. 10-4 rad s-2 for a continuous 3-
m diameter, 200 µm thick silicon sheet), making  pointing changes of order 1 rad hr-1 possible. 
 
     A key element of the success of this concept relies heavily on the ability to develop very precise, very stiff, high 
bandwidth actuators using MEMS processing techniques.  While these actuators may be used in the classical sense to 
push against some form of back plane, MEMS technology makes possible an interesting option of phasing a mirror 
without the need for a heavy backing structure.  Rather, segment tip/tilt would be maintained through forces conducted 
within the plane of individual segments, while segment piston, which is purely a relative motion, would be maintained 
through forces normal to the segments.  Thus the figure control actuators provide a unique adaptive structural linkage 
between the many individual segments. Furthermore, an important capability is added because of the small-scale sizes 
of MEMS actuators; typically tens of microns, they are capable of operation at bandwidths of tens of kilohertz thus 
providing significant mechanical stiffness for the large segmented ASSiST mirrors.  Although these same scales size 
limit the forces that MEMS actuators can exert, this limitation is not severe when positioning very thin (and thus low 
moment of inertia) segments in Space. 
 
2.2 Mechanical Issue 
 
     The ASSiST primary has no mirror cell to provide 
reaction forces for the segment actuators. Although 
this scheme reduces the mass of the primary, position 
control forces must be applied between segments. 
Each segment edge requires piston and tilt actuators to 
uniquely determine the shape of the mirror. The 
construction of piston actuators alone is relatively 
straightforward, however the addition of tilt actuators 
requires a combination of hinges and linear actuators 
(see Fig. 3). The hinges have to be fairly stiff in piston 
mode, and so serial operation of hinge and piston 
actuators is preferred. Such operation is achieved by 
mounting the piston actuators on beam flexures, which 
provide the hinge function.  The tilt actuators are also 
mounted on flexures to accommodate piston and tilt motions. Flexures are integrated with the MEMS actuators, or 
included as separate sub-assemblies, which might be more appropriate for a prototype mirror. 
 
 
3. INCHWORM STRUCTURES FOR LARGE TRAVEL ACTUATION 
 
3.1 Design 
 
     The inchworm actuator motion is facilitated via sequential actuation of actuation elements. The inchworm actuator 
is essentially a linear motor with extremely fine resolution.   The design proposed in this paper consists of a 
piezoelectric stack actuator, a silicon driver, a pair of holders, a slider, and a pair of polymer beams connected with a 
centrally-clamped bi-stable flexure beam as shown in Figure 4.  All the parts except for the piezoelectric stack and the 
polymer beams are made from a 350 µm thick silicon-on-insulator (SOI) wafer.  The estimated clamping force is 1 N.  
pivot 
Actuator for angle adjustment 
Mirror segment 
Mirror segment 
Figure 3: Segment configuration for the edge actuation concept. 
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Since the motion step size (step resolution) is critically dependent on the smoothness and straightness of the sidewalls, 
a robust fabrication process for smooth, vertical sidewalls needs to be developed.   
 
     The ultimate device is expected to have the 
following specifications: 
 
• Operation up to 1 kHz 
• Operation at temperatures at or below 
77K with 15-20% room temperature 
efficiency  
• 250 µm total travel with nanometer-level 
precision 
• Electrostatic holding force between 
segments of approximately 1 N (@ 
100V) 
• Mean steady-state power consumption 
at100 mW per actuator 
 
     Our initial efforts have focused on performing 
Deep Reactive Ion Etching (DRIE) experiments 
for the microfabrication of the MEMS inchworm 
actuators.  Several technical issues were 
encountered during the course of these 
experiments.  (See the SEM images shown in 
Figure 5.)  Additional process development is required in order to address these issues.  In particular, a robust DRIE 
process needs to be developed for obtaining smooth, vertical sidewalls required for nanometer step resolution for the 
inchworm actuation.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 5:  SEM micrographs of the microfabricated actuator structures.  The DRIE challenges, such as “footing”, over-etching and 
non-uniform etching, remain to be overcome. 
 
3.2 Bi-stable Beam for Optimized Electrode Gap 
 
: Over-etching 
and non-uniform 
etch profile 
: Footing 
Slider 
Holder 
Driver 
1.5 mm 
 Flexure beam 
T-bar 
Polymer 
beam 
Figure 4:  Top view of the actuator 
 
PZT  
actuator 
 
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     The minimum width of trenches etched in silicon via the DRIE process is determined primarily by the achievable 
aspect ratio of the etching process, which is typically 25:1. Thus, trench widths on the order of 15~25 microns, are 
obtained for silicon microstructures fabricated from 350~500-micron-thick wafers. The use of such wide trenches for 
electrostatic clamping could result in defective actuation with attendant high voltage operation. Therefore, a centrally 
clamped, bi-stable flexure beam is incorporated in the design in order to reduce the gap between the slider and the 
driver/holder. The flexure beam connected to the slider is designed to snap towards the driver, via a mechanical 
buckling process, and thereby reduce the initial gap (for the electrostatic actuation) between the slider and 
driver/holder.  A considerable amount of work remains to be done in order to optimize the design and fabrication 
process for this type of flexure beam structure.  Estimates of total travel greater than 250 µm have been obtained for 
polymeric materials such as Teflon, Parylene and Silicone rubber (low Young’s moduli) when used for the compliant 
beam structures.  
 
3.3 Actuation Mechanism 
 
     The inchworm actuation mechanism works as follows (See Figure 6.): 
 
1) The centrally clamped flexure beam is made to snap down to the 2nd stable position by means of an external 
probe.  The snap down process makes the T-bar push the slider toward the driver. The polymeric beam is 
compliant enough to accommodate the shear strain required for this action. The snap down insures that the 
gap between the slider and the driver is within a couple of microns. 
2) The slider is then electrostatically clamped to the driver.  The driver is subsequently pushed upward by the 
piezoelectric stack actuator directly bonded to the driver.  The slider also moves upward with the driver. 
3) The slider is then electrostatically clamped by the fixed holders.  Next, the slider is released from the 
driver.  The released driver moves downward via piezoelectric actuation, while the slider remains clamped 
to the fixed holders. 
4) Vertical inchworm actuation is achieved by repeating these steps (step 2-3).  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 6:  Actuation sequence of the inchworm actuation.  1-2) The slider is electrostatically clamped to the driver.  The driver is 
moved upward (out of the page) by the piezoelectric stack actuator bonded to the bottom of the driver.  3) The slider is then 
clamped with the holders.  The slider is subsequently released from the driver.  The separated driver moves downward (into the 
page) via piezoelectric actuation, while the slider is clamped to the fixed holders. 4) The slider is clamped once again to the driver 
while the slider is still clamped to the holders.  Next, the slider is released from the holders. 5) The slider is then moved upward by 
the driver. Inchworm actuation is achieved by repeating these steps (step 2-5). 
Slider 
Holder 
Driver 
Moving 
up with 
driver 
(Electro-
static 
clamping
Clamped 
with 
holder & 
driver 
restored 
Clamped 
with 
driver 
Moving 
up with  
driver 
Up Down
1 2 
3 4 5 
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4. CONCLUSIONS 
 
     One of the key drivers for segment and actuator concepts for ASSiST is the segment configuration for the primary 
and corrector mirrors.  Depending on segment geometry, assuming 300 mm diameter wafer technology, ASSiST the 
total number of segments required are between 10,000 and 15,000.  Thus, MEMS actuation technology is absolutely 
essential in order to realize large structures such as the 30 m diameter primary mirror.  A MEMS-based vertical 
inchworm microactuator has been proposed to achieve the large travel actuation required for this concept.  Compliant 
polymer beams and bi-stable flexure beams have been designed to achieve the inchworm structure with large vertical 
travel actuation.  The inchworm actuator motion can be created through sequential activation of actuation elements.   
The MEMS inchworm actuator discussed in this paper will be capable of a very large stroke of about 250 microns, 
with a resolution of a few nanometers, and will occupy only about 2 mm3, and weigh only about 10 milligrams.  The 
ultra lightweight large stroke actuator technology proposed in this paper opens up a whole new paradigm in micro-
actuation technology, and will enable the ultra lightweight actuated mirrors for future generations of space telescopes. 
 
 
ACKNOWLEDGEMENTS 
 
     The Jet Propulsion Laboratory, California Institute of Technology carried out the research described in this paper 
under a contract with the National Aeronautics and Space Administration.   
 
 
REFERENCES 
 
1. R. Dekany, et al., "Advanced Segmented Silicon Space Telescopes (ASSiST)," SPIE Int’l symp. On Astronomical 
Telescopes and Instrumentation, Adaptive Optical System Technologies II, 22-28 August 2002, Waikoloa, Hawaii, 
USA. 
2. R. Yeh, et al., "Single Mask, Large Force, and Large Displacement Electrostatic Linear Inchworm Motors," IEEE 
Micro Electro Mechanical Systems (MEMS) 2001 Conference, Interlaken, Switzerland, Jan. 21-25, 2001, p. 260. 
3. Q. Chen, et al., " Mesoscale Actuator Device with Micro Interlocking Mechanism," IEEE Micro Electro 
Mechanical Systems (MEMS) 1998 Conference, Heidelberg, Germany, Jan. 1998, p. 384. 
4. H.N. Kwon et al., "Characterization of a Micromachined Inchworm Motor with Thermoelastic Linkage 
Actuators," IEEE Micro Electro Mechanical Systems (MEMS) 2002 Conference, Las Vegas, Jan. 2002, p. 586. 
5. S. Konishi, et al., "Vertical Motion Microactuator based on the Concept of ECLIA," IEEE Micro Electro 
Mechanical Systems (MEMS) 2002 Conference, Las Vegas, January 2002, p. 602. 
6. J. Qiu, et al., "A Centrally-Clamped Parallel-Beam Bistable MEMS Mechanism," IEEE Micro Electro 
Mechanical Systems (MEMS) 2001 Conference, Switzerland, Jan. 2001, p. 353. 
 
 
 
 
 
 
 
 
 
 
 
 
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Design and fabrication of a large vertical travel silicon inchworm microactuator for the Advanced Segmented Silicon Space

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Design and fabrication of a large vertical travel silicon inchworm microactuator for the Advanced Segmented Silicon Space

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