Biomimetic Robotics - Engineered Biomimicry - page 82

characteristics of living organisms. A smart
structure can be considered to be biomimetic
when it is a smart structural system containing
multifunctional parts that can perform sensing,
control, and actuation as well as a primitive
analog of a biological organism. Typically, a
biomimetic smart structure is an integration of
sensors, actuators, and a control system that is
able to mimic a biological organism in perform-
ing many desirable functions, such as synchro-
nization with environmental changes, self-repair
of damage, etc.
A typical example of a smart structure is a
composite structure with embedded piezocer-
amic layers capable of acting as piezoelectric
actuators [6] . The piezoelectric effect is a prop-
erty of certain materials including certain ceram-
ics, such as lead zirconate titanate (PZT), and
polymers, such as polyvinylidene flouride
(PVDF), that cause it to generate a stress in
response to an applied voltage. The stress gener-
ated in turn causes a particular type of strain that
may then be employed either to bend or to alter
the shape of a structure. Moreover, one is able to
minutely deform a structure at specific locations
in a controlled and relatively fast manner, thereby
facilitating their use in high-precision position-
ing applications. Although these materials they
exhibit the property of hysteresis, methods of
compensation have been developed so they can
be controlled in a predictive way. For detailed
exposition of the dynamics of smart structures,
see Kim and Tadokoro [2] and Vepa [7] .
Another typical example of a smart material
that can be embedded in a structure is furnished
by a class of electrostrictive materials that are
ferroelectric materials such as lead-magnesium-
niobate (PbMgN). Electrostrictive and magneto-
strictive materials are capable of directly
transforming an electric or magnetic stimulus
into a small mechanical displacement response
with little or no hysteresis. Consequently, these
materials are used in high-speed and high-pre-
cision positioning applications. However, their
response to the stimulus is not linear and the

presence of even a small amount of hysteresis is
difficult to compensate. Yet, they are used in
high-temperature applications because they are
able to perform beyond the Curie temperature
in a temperature domain where piezoelectric
actuation is not possible. They are particularly
suitable for acoustic noise or sound generators
and for generating large forces with low applied
voltages.

4.2.2 Biomimetic Sensors and Actuators
Biomimicry refers to mimicry of life—that is, to
imitate biological systems. The term is derived
from bios , meaning life, and mimesis , meaning
imitation. Biomimicry concerns the design of
biologically inspired mechanical devices. One is
then interested in isolating the special features
associated with biomimetic sensors. At first
sight, sensors associated with human physiol-
ogy, such as the eye, motion sensors within the
inner ear, and touch sensors, seem to be like con-
ventional pinhole cameras, accelerometers and
gyroscopes, and pressure sensors, respectively.
And yet a closer examination of the eye reveals
that it is able to alter the focal length of the
lens, whereas a pinhole camera cannot. Thus,
the human eye can be considered a compliant
sensor.
The saccule and the utricle in the inner ear
function as inclinometers, which are essentially
accelerometers measuring a component of the
acceleration in a preferred direction. Yet, because
the saccule and the utricle are immersed in a
fluid medium, they function as integrating accel-
erometers over a high-frequency band. The sac-
cule is primarily located in a vertical plane, while
the utricle is primarily oriented horizontal with
respect to the horizon. Both organs have sensory
membranes called the maculae . The maculae are
covered by the otolithic membrane. The primary
sensors in these organs are the macular hair cells
that project into the otolithic membrane. These
hair cells are oriented along a centerline that
curves through the center of the macula. On each