A robust, large-force, large-deflection micro balloon actuator for aerodynamic (manoeuvring) control of transonic aircraft has been developed. Using a novel process, high yield linear arrays of silicone balloons on a robust silicon substrate have been fabricated that can deflect vertically in excess of one mm. Balloon actuators have been tested under cyclic conditions to assess reliability. The actuators have been characterized in a wind tunnel to assess their suitability as aerodynamic control surfaces and flight-tested on a jet fighter to assess their resistance to varied temperatures and pressures at high velocity

Grosjean, C.

Ho, C. M.

Hong, W.

Lee, G. B.

Tai, Y. C.

English

Caltech Authors - Main

MICRO BALLOON ACTUATORS FOR AERODYNAMIC 
CONTROL 
C. Grosjean, G.B. Lee*, W. Hong", Y.C. Tai, C.M. Ho* 
Caltech Micromachining Laboratory 
Electrical Engineering 136-93 
Pasadena, CA 91 125, USA 
"Mechanical and Aerospace Engineering 
University of California, Los Angeles 
Los Angeles, CA 
ABSTRACT 
A robust, large-force, large-deflection micro balloon 
actuator for aerodynamic (maneuvering) control of 
transonic aircraft has been developed. Using a novel 
process, high yield linear arrays of silicone balloons on 
a robust silicon substrate have been fabricated that can 
deflect vertically in excess of one 111111. Balloon 
actuators have been tested under cyclic conditions to 
assess reliability. The actuators have been characterized 
in a wind tunnel to assess their suitability as 
aerodynamic control surfaces and flight-tested on a jet 
fighter to assess their resistance to varied temperatures 
and pressures at high velocity. 
INTRODUCTION 
Previously, magnetically actuated surface and bulk 
micromachined flaps have been used for delta wing 
control surfaces [ 11. This is fluid dynamically feasible 
because the similarity in length scales of the actuators 
and the boundary layer before separation at the leading 
edge of a wing allows the use of small surface 
perturbations to generate large aerodynamic control 
moments (pitch, yaw, and roll) through fluid 
amplification. However, these actuators were designed 
only for wind tunnel testing (<50 d s )  and are not 
robust enough for flight tests on real aircraft or high 
speed (>50 d s )  wind tunnel testing as the beams break. 
More robust beams would require very large magnetic 
forces and the resulting structures would still suffer 
from impacts during flight. For practical aircraft, 
requirements for a microactuator control surface then 
have to include robustness (e.g. resistance to particles), 
large deflection and load, low power consumption, 
surface conformability, and small size along the 
thickness of the wing. Pneumatically driven silicone 
membranes [2, 3 ,  41 satisfy the first four requirements, 
but it can be difficult to design robust membranes that 
have small dead areas, as the frame can be very large. 
We have fabricated large deflection silicone membranes 
[4] using anisotropic etching to form a free-standing 
90095, USA 
membrane, but a large area was wasted due to the 
angular nature of the etching. Even if <110> wafers and 
KOH or DRIE were used to form vertical sidewall 
cavities, the resulting structure would be very fragile if 
the membrane area was maximized. Some work has 
been done with photoresist sacrificial layers [3], but we 
have found that acetone attacks our silicone and 
photoresist developer is very slow to undercut large 
(mm scale) structures. We have since developed a new 
process using Parylene C as a release layer for 
fabricating large silicone rubber membranes without the 
use of large bulk micromachined cavities, hence 
resulting in a robust frame. This novel technique takes 
advantage of the poor adhesion between silicone rubber 
and Parylene. 
DESIGN 
The desired geometry of the microactuators for wind 
tunnel testing and flight testing on a scale airfoil model 
were strips two to three inm wide, and stackable end to 
end for a total length as necessary depending on the 
wing. A vertical deflection on the order of one to two 
m was needed for sufficient interaction with the 
boundary layer. Strips of actuators would then be 
Fig. 1 Assembly of Actuator to Backing Strip 
166 
attached to a suitable backing strip and pneumatically 
actuated. It was decided to put the groove for air flow in 
the backing strip so as to make the silicon wafer less 
prone to cracking during fabrication and the final 
actuator strips as robust as possible. 
Earlier work with silicone membranes for valves [4] 
demonstrated that large deflection (1.5 mm) was 
achievable with small square silicone membranes, but 
they underwent plastic deformation and failed rapidly at 
high pressures. A first generation of micro balloon 
actuators was fabricated with dimensions as shown in 
Fig. 2. These actuators could actuate about one mm 
vertically before undergoing severe deformation at the 
edges as shown in Fig. 3 .  They also failed fairly often 
by losing adhesion at the edges, especially at the sharp 
corners. Initial wind tunnel tests also showed that a 
larger vertical deflection was desirable and it was 
USi lcone Rubber, 120 vm 
Parylene C, 1.6 p m 
1.4'" I- 8.6 mm+ 
Fig. 2 First (2 mm) Balloon Actuator Design 
Fig. 3 Plastic Deformation of 1'' Generation Actuator 
decided that a 3 tnm wide actuator strip was desirable. 
The design was revised in three ways (Fig. 4) in t e r m  
of increasing reliability, both of the silicone itself, and 
the adhesion to the silicon frame. The width of exposed 
silicon on either side of the Parylene release layer was 
increased from 0.3 mm to 0.35 mm. The corners of the 
release layer were rounded to decrease stress from the 
boundary conditions, and one inlet hole was removed 
while the remaining two were moved closer together to 
equalize the pressure exerted on the membrane. 
This design has performed fairly well, but some small 
changes are still necessary. After extended cycling of 
Silicone Rubber, 120 p m 
Parylene C, 1 6 pm 
3.0," .Q r; m m d  
' 2.3'" l- 8.6 mm--i 
Fig. 4 Second (3 mm) Actuator Design to Reduce 
Plastic Deformation and Improve Adhesion 
the mcmbranes, they fail along one edge with a pinhole 
implying that the silicone at the edges is undergoing 
high stress. A revised design would include changes in 
the boundary condition along the edge of the membrane 
to reduce the sharpness of the angle formed during 
actuation. 
FABRICATION 
The crucial fabrication hurdle is releasing a large area 
silicone membrane without damaging the material itself. 
Most typical sacrificial materials (oxide, silicon, 
photoresist, metal) are incompatible as the releasing 
etchants are harmful to silicone rubber after long term 
exposure. 
While working with composite Parylene/silicone 
membranes, we discovered that silicone rubber has very 
poor adhesion to Parylene. Thus, a patterned layer of 
Parylene can be used as a release layer to form large 
area silicone membranes without the complications 
associated with etching a sacrificial layer. 
The process is shown in Fig. 5. Starting with a <loo> 
silicon wafer, 0.5 pm of LPCVD silicon nitride is 
deposited on both sides. The back side is patterned 
using RIE, and cavities are etched anisotropically using 
KOH forming 300 pm square free-standing membranes. 
1.6 pm of Parylene C is deposited on the front side and 
patterned using O2 plasma. Without removing the 
photoresist on the Parylene, the front side is roughened 
with SF6/02 plasma. The photoresist is stripped and 100 
to 150 pm of MRTV-1 silicone rubber is spin-coated on 
the front side. The wafers are cured and the composite 
nitride/Parylene membranes are removed from the 
backside with SF6/02 plasma followed by 0 2  plasma. 
Application of pressurized gas to the inlet holes frees 
the silicone membranes with very high yield. 
Strip Nitride and Parylene C 
Release Membranes with Compressed Air 
Fig. 5 Process Flow for Silicone Balloon Actuators 
167 
The first devices made used MRTV-I silicone rubber 
diluted with silicone fluid (8% by volume); a common 
practice to lower the viscosity. This mixture, while 
easier to spin coat with good results, degraded the 
mechanical properties of the cured silicone. Later 
devices used MRTV-1 diluted with a reactive silicone 
with superior results. 
Tensile ElonAation, , _" % ".,".llll .......i.....-"- i 1,000 
Younx's " Modulus [4] --"" t TemDerature Ranee. "C 7 3 E - 2 0 0  
Table 1 Mechanical Properties of MRTV-1 Silicone 
MECHANICAL TESTING 
Completed actuators were tested after fabrication to 
determine the pressure-deflection relationship using the 
experimental apparatus in Fig. 6. It was found that 
approximately 3 psi of pressure was necessary to free 
the silicone membranes from the substrate prior to 
testing, and any time that the membranes were allowed 
to "stick" to the substrate, a similar pressure was 
necessary, i.e. there was stiction between the silicone 
and Parylene. For the proposed usage as an aircraft 
control surface, this is not an issue and insures that the 
silicone surface stays relatively conformal when not 
actuated. 
Pressure Regulator 
% 
Three-Wav 
Particle Release Solenoid Vaive 
Fig. 6 Pressure vs. Deflection Apparatus 
Filter Valve 
Static Testing 
Due to the excessive plastic deformation of the first 
generation of actuators, extensive testing was limited to 
the second generation. Fig. 7 shows three sets of data 
for the same actuator taken at different times. The first 
(and lowest) trend line was taken directly after 
fabrication. At the low pressures used, no plastic 
0 2 4 6 a n 
pressure (psi) 
Fig. 7 Deflection vs. Pressure for the Same Actuator 
After Various Experiments 
deformation was noted visually, or as a rapid slope 
change in the curve. The actuator was then inflated with 
a wide range of pressures up to 12 psi to observe the 
behavior of the membrane (and generate the series in 
Fig. 8). A second set of data was taken and plastic 
deformation was observed at IO+ psi although the "dog 
bone" shape exhibited by the first generation actuators 
wasn't noticed. The actuator was then tested past 15 psi 
and more plastic deformation was observed. A third set 
of data was then taken. At higher pressures, the 
deflection increased 350 pm or more over the first set of 
data indicating permanent stretching of the silicone 
membrane and the "dog bone" shape was observed. 
Cyclic Testing 
To determine the reliability of the actuators under 
continuous use, eight actuators were cycled using a 
three-way solenoid valve at frequencies from one to two 
Hz at a pressure of 5.1 psi which was chosen to be out 
of the plastic regime. Pressure vs. deflection curves 
Fig. 8 Various States of Inflation for a 3 mm Actuator 
168 
1000 I I 
900 
800 
700 
h 
v 5 600 
.- 500 
2 400 
300 
c 
0 
0) 
200 
100 
0 
0 1 2 3 4 5 6 7  
pressure (psi) 
Fig. 9 Typical Deflection vs. Pressure Data for a 
Cycled Silicone Membrane Actuator 
were taken after 0, 2000, 5000, and 11000 cycles for 
each actuator. Deflection was recorded from an initial 
pressure of 0.2 psi to take any slack out of the 
membrane and provide a consistent starting point. 
Actuators failed at 4500, 5000, 7100, 7900, and 10000 
cycles leaving three actuators after 11000 cycles. In 
each case failure was preceded by a slight peeling of the 
silicone at the edges of the Parylene release layer (at the 
point of highest stress during actuation) and failure 
consisted of a pinhole along the center of the long edge 
of the actuator. As the pressure was sufficiently low, 
none of the failures were due to bursting membranes. 
Fig. 9 shows the pressure vs. deflection trend for one of 
three actuators that survived past 11,000 cycles 
(although the trend is similar for the actuators that 
failed). From 0 through 5,000 cycles, the curves are 
almost identical with a slight stretching occurring after 
2,000 cycles. After 11,000 cycles, all three actuators 
showed signs of the silicone membrane losing adhesion 
at the edges and the higher deflection is partly due to 
this effect. The shape of the pressure vs. deflection 
curve doesn't change indicating that cyclic stress below 
the plastic region does not induce plasticity in the 
membrane which might lead to early membrane failure. 
Q 49' B" D 
I l l  nml 
"' \L_l 
Gratin2 A 
Rerum Crcuit Fan Rmm - 
Fig. 10 UCLA Wind Tunnel 
WIND TUNNEL TESTING 
To verify the feasibility of using micro balloon actuators 
for control of a macro aerodynamic device such as a 
delta-wing aircraft, wind tunnel testing was performed. 
Theoretically, the similarity in scale of the actuators and 
the thin boundary layer before separation near the 
leading edge should allow the use of small perturbations 
to change the separation point. The development of 
leading edge vortices, which contribute a significant 
portion of the total lift at high angles of attack, are 
modified by this control. By manipulation of the vortex 
asymmetries, moments for wing motion control can be 
generated. 
Facility and Delta Wing Configuration 
An open-return wind tunnel was used for aerodynamic 
testing at UCLA (Fig. 10). The test-section is 91 cm by 
91 cm by 730 cm long. The maximuin operating speed 
of the wind tunnel is 45 mls .  
Leading Edge 
\ 
Actuator Along Leading 
Edge at Various Angles 
Fig. 11 Schematic of Delta Wing Model 
A delta wing model (Fig. 1 I )  with a swept angle of 
56.5" and chord length of 30 cm was employed. The 
delta wing model has round leading edges formed by a 
rod on which the micro balloon actuators are mounted. 
By rotating the rod, the position of the actuators can be 
varied over the entire leading edge area. A six 
component force/moment transducer (AMTI Inc.) was 
used to measure aerodynamic forces and moments. This 
transducer was used to record changes in the 3-axis 
torques caused by the micro balloons. Data was 
digitized and processed by a computer. 
Pitching, Yawing, and Rolling Moment Control 
To investigate the rolling moment generated by the two 
generations of actuators (2 mm and 3 mm wide strips 
with respective vertical deflections of 1 inm and 1.6 
mm), actuators were positioned and actuated along one 
leading edge from the apex to the wing tip. Fig. 12 
represents the effects of the 2 min and 3 mm actuators 
169 
15 
h - 10 C 
a 
I 5 5  
VI 
S 
0 
K 
s o  
0 20 40 60 80 100 120 140 
Angle from Bottom Surface (") 
Fig. 12 Rolling Moment vs. Angular Location 
on rolling moment at an angle of attack (AOA) of 2.5". 
The data shows that 14% and 25% increments of the 
rolling moment can be obtained by two-sided actuation 
if we normalize these results to the experimentally 
obtained moment generated by a single leading-edge 
vortex. 
In addition to rolling moments, we can also generate 
pitching and yawing with the same actuation. In order to 
compare the data with that generated by conventional 
control surfaces, like rudders, we plot the data for the 3 
mm actuators as pitching, yawing, and rolling moment 
coefficients (Fig. 13). The pitching, yawing, and rolling 
moment coefficients, C,,, Cy, and C,, are defined in the 
following way: 
0.025 I I 
c 
C 
W .- g 0.02 
0 
0 
E 0.015 
W 
E 5 0.01 
0 
2.OE+05 4.OE+05 6.OE+05 8.OE+05 
Reynolds Number 
Fig. 13 Moment Coefficients vs. Reynolds Number at 
an Angle of Attack of 2.5" 
d 
where AM,,, AM,,, AM, are the changes in the pitching, 
yawing, and rolling moments produced by micro 
actuation. Q, S, b, and c are dynamic pressure, wing 
area, trailing edge length, and distance from the apex to 
the centroid of the wing respectively. From Fig. 13, we 
can see that pitching, yawing, and rolling moment 
coefficients are not sensitive to Reynolds number. The 
maximum pitching moment coefficient is about 0.02. 
This value is large enough for most flight control. 
Flight Testing 
Several strips of 2 mm actuators (first generation) were 
attached to a small section of F-15 fighter jet wing and 
flight tested to assess their resistance to temperatures 
and pressures seen at high velocity. The section of wing 
(Fig. 14) also contained shear stress sensors which will 
be used in conjunction with the actuators for future 
flight control. The aircraft achieved a maximum speed 
of 0.9 Ma and temperatures fluctuated from -9 OF to 
110 O F .  It was found that all the MEMS devices 
survived the harsh flight test several times, which 
demonstrates the robustness of the devices. 
Fig. 14 Balloon Actuators and Shear Stress Sensors 
Attached to a Section of F- 15 Wing 
CONCLUSION 
A novel, high yield fabrication process has been 
developed for silicone rubber balloon actuators. Arrays 
of large-deflection, large-force actuators for 
aerodynamic control have been fabricated. Balloon 
actuators have been tested under cyclic conditions to 
assess long-term reliability. Actuator strips mounted on 
the leading edge of a delta wing model have been 
characterized in a wind tunnel and the results indicate 
that they are suitable for use as aerodynamic control 
surfaces. Several strips of actuators have been flown 
successfully on an F-1.5 fighter jet demonstrating the 
170 
durability of the actuators to elements encountered in 
transonic flight. 
ACKNOWLEDGEMENTS 
This work is supported under Navy Contract N6600 I - 
97-C-8610. The authors would like to thank Xuan-Qi 
Wang for help with Parylene deposition and Trevor 
Roper for fabrication assistance. 
REFERENCES 
[ l ]  C. Liu, T. Tsao, and Y. C. Tai, “Out of Plane 
Permalloy Magnetic Actuators for Delta-Wing 
Control,” MEMS 1995, Amsterdam, The 
Netherlands, pp. 7- 12 (1 995). 
[2] C. Vieider, 0. Ohman and H. Elderstig, “A 
Pneumatically Actuated Microvalve with a 
Silicone Rubber Membrane for Integration with 
Fluid Handling Systems,” Transducers 1995, 
Stockholm, Sweden, pp. 284-286 (1995). 
[3] L. Bousse, E. Dijkstra, and 0. Guenat, “High- 
Density Arrays of Valves and Interconnects for 
Liquid Switching,” Hilton Head 1996, Hilton Head 
Island, SC, pp. 272-275 (1996). 
[4] X. Yang, C. Grosjean, Y. C. Tai, and C. M. Ho, “A 
MEMS Thermopneuinatic Silicone Membrane 
Valve,” MEMS 1997, Nagoya, Japan, pp. 1 14-1 18 
(1997). 
171 


Micro Balloon Actuators for Aerodynamic Control

A Pneumatically Actuated Microvalve with a Silicone Rubber Membrane for Integration with Fluid Handling Systems,” Transducers

C. Liu, T. Tsao, and Y. C. Tai, “Out of Plane Permalloy Magnetic Actuators for Delta-Wing Control,” MEMS 1995, Amsterdam, The Netherlands, pp. 7- 12 (1 995).

C. Vieider, 0. Ohman and H. Elderstig, “A Pneumatically Actuated Microvalve with a Silicone Rubber Membrane for Integration with Fluid Handling Systems,” Transducers 1995, Stockholm, Sweden, pp. 284-286 (1995).

HighDensity Arrays of Valves and Interconnects for Liquid Switching,”

L. Bousse, E. Dijkstra, and 0. Guenat, “HighDensity Arrays of Valves and Interconnects for Liquid Switching,” Hilton Head 1996, Hilton Head Island, SC, pp. 272-275 (1996).

Out of Plane Permalloy Magnetic Actuators for Delta-Wing Control,”

X. Yang, C. Grosjean, Y. C. Tai, and C. M. Ho, “A MEMS Thermopneuinatic Silicone Membrane Valve,” MEMS 1997, Nagoya, Japan, pp. 1 14-1 18 (1997).

https://authors.library.caltech.edu/601/1/GROmems98.pdf

Micro Balloon Actuators for Aerodynamic Control

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

Caltech Authors - Main