Biomimetic Robotics - Engineered Biomimicry - page 83

side of this centerline, the tips of the hair cells
are oriented in opposite directions, facing the
centerline in the utricle and turning away from
the centerline in the saccule. Hair cells of par-
ticular type are located in abundance near the
centerline. Due to the opposite orientation of the
tips of the hair cells and the curvature of the
centerline in both otolith organs, they are sensi-
tive to multiple directions of linear acceleration
over a center-frequency bandwidth. Yet, because
they are immersed in a fluid medium, they func-
tion as integrating accelerometers over high fre-
quencies. This feature, which represents a form
of compliance, provides these sensors with a
relatively large operational bandwidth.
Located within the inner ear three semicircu-
lar canals, which are fluid-filled membranous
ovals oriented in three different planes, function
like rate gyroscopes. At the base of each is a
region called the ampulla , which contains hair
cells called the crista that are affected by move-
ment. These hair cells also respond to movement
in a manner similar to a set of integrating accel-
erometers. Again, the primary feature of the
semicircular canals is their compliance.
Another form of compliance is also a primary
feature in the case of touch sensors. In fact, on
observing many biological sensors, we find that
a primary underlying feature is that they are
essentially compliant in some way. When robotic
manipulators are performing noncontact or
unconstrained tasks, there is no need for compli-
ance. The need for compliance arises when con-
tact tasks that impose severe constraints due to
the geometry of the local environment are being
performed. When the robot attempts to meet
these constraints, there is generally a need for it
to be compliant with the geometry or other con-
straints imposed by the local environment.
For example, a minimally invasive approach
to robotic surgery uses three-dimensional imag-
ing techniques coupled with tiny robotic manip-
ulators to perform delicate urological surgery,
including prostatectomy (removal of the pros-
tate gland to treat cancer). It relies on making

microscopic incisions with accuracy and control
and subsequently cuts and manipulates tissue
through the tiny holes. Thus, successful comple-
tion of the tasks depends on the robot being able
to meet the compliance requirements, which in
turn requires that the compliance be monitored
and controlled.
Since the compliance requirements depend to
a large extent on the forces and moments gener-
ated by contact, there is a need to monitor and
control the forces and moments generated by
contact. Sensors with the ability to self-monitor
and control the forces and moments generated
by contact can therefore be considered biomi-
metic sensors.
There is yet another important smart feature
associated with certain biological sensors. Every
biological organism generates noise, but only
some species such as bats have highly sensitive
sensors capable of sensing the self-generated
noise [8] . Furthermore, because these organisms
are endowed with memory, they can compare
the sensed noise with a record of the noise
sensed at an earlier instance. This ability to com-
pare and correlate the measurements empowers
these organisms with an interesting kind of per-
ception. Thus, bats have acoustic sensors that act
like sonar and allow them to perceive an obsta-
cle in the dark. The key features are the sensors'
sensitivity, the presence of memory, and an ele-
ment of built-in self-signal processing, a topic
that is broadly discussed in Section 4.2.3 .
We are now in a position to consider the special
features associated with biomimetic actuators.
Probably the best examples of biological actuators
are the muscles in the human body. Biological
actuators such as muscles are complex systems
involving a variety of feedback mechanisms and
pathways that facilitate both voluntary and invol-
untary responses to stimuli. Moreover, they gen-
erally incorporate a class of biomimetic sensors
and so possess all the features of biomimetic sen-
sors discussed earlier in this section. Muscles
work in pairs, called agonists and antagonists , to
facilitate the coordinated movement of a joint.