This research
describes the first work of using wafer-sized flexible parylene-valved
actuator skin (total thickness ~ 20 _m) for micro adaptive flow control.
The check-valved actuator skin features vent-through holes with tethered
valve caps on the membrane to regulate pressure distribution across
the skin. The skins were integrated onto micro-aerial-vehicle (MAV)
wings that were tested in the wind tunnel for aerodynamic evaluation.
The test result has shown a very significant effect on the aerodynamic
performance. Compare to the reference wings (no actuators), both the
lift and thrust of the check-valved wings are improved by more than
50%. This is the first experimental result to demonstrate that the application
of MEMS actuator skins for flow control is very promising.
From our previous study of MEMS wings [1] for MAV applications, titanium-alloy
wings with parylene membrane were fabricated to fly battery-powered
ornithopters in an unsteady aerodynamics mode. We have learned that
the control of the wingsÕ pressure distribution is very important in
order to achieve an optimal aerodynamic performance. Thus, it is logical
to fabricate devices on the membrane, passive or active, that can regulate
the wing loading. To achieve this, we reported a chip-sized flexible
parylene technology [2]. The more challenging next step, hence this
work, is to make wafer-sized free-standing flexible parylene
actuator membranes or "skins," and integrate these skins onto MEMS wings.
The goal is to distribute actuators on the wings to regulate pressure
distribution during downstroke and upstroke. This concept is illustrated
in Figure 1. The pneumatic actuation relies upon the wing loading during
flapping. For each flapping cycle, the loading either pushes the valve
caps to open, or close to control air movement through the vent holes.
The opening of the valves equalizes the pressure between the upper and
lower wing surfaces. When the valves are closed, the pressure difference
can then exist to affect the aerodynamic performance, i.e., lift and
thrust.
We report
here the successful design and fabrication of a wafer-sized parylene
check-valved skin shown in Figure 2. Figure 3 summarizes the fabrication
process. Photoresist is used as a sacrificial layer. To prevent stiction,
anti-stiction techniques with SAM and BrF3 dry etching of amorphous
silicon are used [3, 4]. Many fabrication challenges and our solutions
will be reported, such as resist bubbles, film wrinkles, and valve stiction.
Examples of these challenges are shown in Figure 4.
The integrated check-valved MEMS wings were tested in a low-speed wind
tunnel as seen in Figure 5. The aerodynamic performance results are
shown in Figure 6. Phase averaging is used to get instantaneous measurements
by collecting the entire lift and thrust history of downstrokes and
upstrokes. It also reduces the noise levels as many flapping cycles
are averaged. The check-valved actuators were mounted onto the wings
in both cases, i.e., the first case of the valves open during downstroke
(close on upstroke) and reversed for the second case. From the test
results, the lift has increased by 31% and 58%, respectively.
More excitingly, the thrust has dramatically increased by almost
50% in both cases. In addition, the data shows that there is a phase
delay between opening and closing of valves. The effect moderates the
extreme lift peaks and in particular the negative lift peak that happens
during the downstroke. The aerodynamic performance is highly dependent
on the formation of the shedding vortex. Interacting with the leading
edge vortex will vary the pressure distribution and alter the aerodynamic
performance. We believe that this delay causes the upstroke peaks to
change and phase shift relative to the unmodified reference wing.
The future work includes the active check-valved electrostatic actuator
skin as shown in Figure 7. The actuators can be actuated using external
voltage to close the valve.
References
T. N. Pornsin-Sirirak, S.W. Lee, H. Nassef, J. Grasmeyer, Y.C. Tai,
C. M. Ho,
M. Keennon, MEMS Õ00, pp. 799-804.
T. N. Pornsin-Sirirak, T. C. Tai, H. Nassef, C. M. Ho, MEMS Õ01, pp.
511-514.
M.R. Houston, R. Maboudian, R. T. Howe, Hilton Head Õ96, pp. 42-47.
T. J. Yao, X. Yang, and Y.C. Tai, Transducer Õ01, pp. 652-655.
