Biomedical Engineering Reference
In-Depth Information
Gradients or steps of intensity between the
sky and ground can be used to stabilize an
aircraft or ground vehicle. The sky is much
brighter than the ground in the blue through
ultraviolet wavelengths
[34]
under almost all
daylight conditions in almost all environments.
Notable exceptions are environments covered
in snow. In the simple case, consider ultravio-
let- or blue- sensors looking laterally outward
on the right and left sides of a vehicle. Simply
balancing the left and right intensities will
ensure that both sensors are looking at the hori-
zon. A representative implementation of this
concept is shown in
Figure 9.11
(right), with a
photodiode looking into each quadrant of the
horizon. In the vertical plane, the sensors are
arranged to look outward, with the center of
the field of view aligned with the horizon. A
variation of this system has been proposed by
Stange in studies of dragonflies
[30]
and by
Taylor in studies of locusts
[35, 36]
, where the
arrangement is equivalent to that of
Figure 9.11
(left) with no rear-looking ocellus and a wide-
field-of-view median ocellus.
The proposed control law was that the air-
craft (or insect) would tilt its head or body away
from the darker side. In an ideal environment
with a level horizon, the consequences of this
control law are that the head and ultimately the
wings would be level. Consider an aircraft fly-
ing toward the observer. When the aircraft is
rolled left, as in
Figure 9.12
(top), the right sen-
sor sees more sky and the left sees less. In the
case of a sensor that sees a bright sky and dark
ground, the right sensor will have a higher out-
put than the left. When the aircraft is rolled right
as in
Figure 9.12
(middle), the imbalance between
left and right sensors reverses. When the craft is
flying with wings level, as in
Figure 9.12
(bot-
tom), the sensors register the same output
signal.
The same rule may be applied on the pitch axis
for a device that looks forward and backward; the
resulting equilibrium point is with the fuselage
oriented horizontally. Clearly, the combination of
FIGURE 9.10
The field of view of the dragonfly ocellar
system: left lateral, right lateral, and median ocelli, mapped
onto the view sphere. The measurements were made by
observing the points that showed tapetal retro-reflections in
each ocellus from an external light source. The hot-scale of
color from white (maximum) to black (minimum) indicates
the intensity of the retro-reflection.
view 120° wide by 150° high. The forward-look-
ing median ocellus has a horizontal field of view
of approximately 160° and a vertical field of view
of 40°
[29]
. Behind each ocellus is a retina contain-
ing green- and ultraviolet-sensitive photorecep-
tors
[30]
.
The dynamic and static properties of the
ocelli have been known for some time due to
work by Stange and others
[30-32]
. Individual
L-neurons on the retina of median ocelli have
been shown by Berry
et al.
[33]
to have receptive
fields of as little as 12°, allowing spatial informa-
tion to be preserved for possible higher-order
processing in the brain. For the purposes of bio-
mimcry, we have considered the well-known
intensity-based responses of dragonfly ocelli
originally mapped by Stange
[30]
. The full func-
tionality of the ocellar visual system of the drag-
onfly has not yet been determined.
