Biomedical Engineering Reference
In-Depth Information
5.3.2 Energy Storage
The majority of manmade microflyers rely on
stored electrical energy for flight in the form of
batteries. The energy storage capacity of bat-
teries is often the major bottleneck in terms of
flight endurance. Batteries have a significantly
lower energy density than hydrocarbon fuels. In
addition, batteries have typically been limited
in terms of the continuous current that can be
drawn from them, which constrains the maxi-
mum power that they can supply to the electric
motors driving the microflyers. Recently, there
have been large improvements in the energy
density as well as maximum current draw of
batteries. Specifically, the introduction of lithium
polymer batteries has revolutionized the field of
remotely piloted aircraft and has brought these
vehicles within the reach of a vast number of
hobbyists.
Figure 5.2
shows a comparison of
the energy density of different battery chemis-
tries. Note that although LiIon batteries have
the highest energy density, they are limited in
terms of the current that they can supply. LiPoly
batteries can supply several times their charg-
ing current and hence they have the highest
power density, which makes them ideal for use
in microflyers. Some other battery chemistries
and are the subject of active research. The
structural arrangement of an insect or bird
wing also accommodates specialized func-
tions such as wing folding. Therefore, it is not
straightforward to replicate the structure of a
natural wing to obtain the desired structural
couplings.
Another important consequence of scaling
down a mechanical assembly is the effect on
hinges, linkages, and bearings. Conventional
hinges based on rotary joints suffer from frictional
losses as well as loss of precision due to play
between the fixed and rotary members. The
relative losses increase as the size of the hinge
decreases. Flexure hinges are ideally suited to
small precision assemblies due to their lack of
moving parts, repeatability, and absence of
friction
[8]
. Several microflyers rely on flexure-
based mechanisms to power their flapping flight.
These often feature simplified kinematics to
focus on specific degrees of freedom, rather than
exactly copy mechanisms that occur in nature.
As an example, the wing joint of a honeybee
contains a number of extremely complicated
shapes, linkages, and muscle attachments
[9]
.
Although the functions of each of these features
has been mapped out, it is very challenging to
replicate the shape and dynamic characteristics
of each of these components in a mechanical
assembly.
For example, Wood
[10]
developed a robotic
insect of 60 mg mass using a smart composite
microstructure consisting of rigid carbon fiber
reinforced prepegs sandwiching a thin polyimide
layer that acts as a flexure. The wing joints
included three degrees of freedom, out of which
only one was controlled by an actuator and the
other two responded passively. Similar flexure
joints have also been used to fabricate the entire
resonant thorax mechanism of microscale insects
[11]
. Compliant drive mechanisms for actuating
wing flapping have also been fabricated using
an injection-molding process that combines a
soft flexural material with a stiff structural
material
[12]
.
FIGURE 5.2
Nominal energy density of different battery
chemistries. LiIon—lithium ion, LiPoly—lithium polymer,
NiZn—nickel zinc, NiMH—nickel metal hydride, NiCd—
nickel cadmium.
