Biomedical Engineering Reference
In-Depth Information
and down in resonance with the wing-flapping
motion, and any angular velocity of the insect
body, for example, in yaw, results in a bending
moment on the halteres due to gyroscopic
moments. Fine hair or other sensors at the root
of the halteres measure this bending moment
and provide feedback of the angular velocity
to the insect. It has been observed that insects
are unable to fly properly if these halteres are
removed, and therefore they form an integral
part of their flight control and stabilization
system.
Researchers have also been investigating the
sensory mechanisms involved in bat flight. In
addition to the well-known acoustic sensing
mechanisms such as echolocation, bats appear to
employ several other mechanisms. Sterbing-
D'Angelo
et al
.
[100]
investigated the function of
bat hairs as aerodynamic sensors. Tactile recep-
tors associated with the hairs on a bat's wings
can sense airflow in a direction opposite to the
hair growth, identifying the onset of stall and
flow separation. The bat uses this information for
control in various flight regimes. It was experi-
mentally shown that shaving the hair from dif-
ferent areas of a bat's wings significantly altered
their ability to maneuver and avoid obstacles
and increased their flight speed. The exact mech-
anisms involved in this sensing as well as the
processing of the vast amount of information
received are still topics of active research.
Birds also feature several passive sensing
mechanisms. The coverlets on birds, wings,
located near the trailing edge, deploy automati-
cally when the flow over the upper surface of the
wing is stalled. These pop up locally in areas of
separated flow and not only provide feedback to
the bird, but also help in alleviating stall by
increasing the post-stall lift. Bechert
et al
.
[101]
studied this phenomenon and measured the
effect of passive coverlets in stall alleviation on an
airfoil in a wind tunnel. Birds are known to sense
the direction of light polarization as well as the
Earth's magnetic field. Magnetometers are used
in some microflyers for orientation; however,
these are not as sophisticated as the mechanisms
used by birds. A large part of the stabilization and
control mechanisms in birds and insects, from the
point of view of both sensors and algorithms, is
still a topic of active research.
5
.8 FUTURE CHALLENGES
There remain a number of challenges to realizing
a fully autonomous insect-sized or bird-sized
microflyer. The fundamental limits imposed by
the low Reynolds number flight regime neces-
sitate harnessing unsteady aerodynamic mecha-
nisms for achieving efficient flight. The unsteady
mechanisms used by insects and birds have been
studied for a number of years and are still top-
ics of active research. A variety of computational
tools are being developed to model these effects.
These tools must be transitioned into design
analyses to improve and optimize the perfor-
mance of microflyers. In addition, the structural
dynamic models of wings must be developed to
account for aeroelastic behavior that forms a key
part of the aeromechanics of natural flyers.
In terms of stability and control, natural flyers
are highly maneuverable, which comes at the
cost of stability. To replicate this kind of
maneuverability in manmade microflyers
requires high-bandwidth sensors and actuators
as well as robust control algorithms. Each of
these areas requires significant technical
advances for realization of a microflyer with
capabilities comparable to natural flyers. The
enhanced maneuverability must not compromise
the efficiency of flight. Actuators with higher
power density must be developed to power the
microflyers. The power density must be based
not only on the actuator mass alone, but must
include the power supply as well. The size as
well as accuracy and drift behavior of sensors
must be enhanced. In this area, biomimetic
sensors such as optic flow-based sensors are
very promising. Algorithms must be developed
that require low processing capability so that
