Biomedical Engineering Reference
In-Depth Information
from the photodiodes on the left side. If the sun
is not within the field of view of the pair of sen-
sors, then
G
l
(
t
)
can be treated as constant, i.e.,
UV
UV
(9.11)
O
l
(
t
) =
U
l
(
t
) −
G
,
Σ
UV
UV
and changes in
O
l
are driven only by changes in
U
l
. Should the sun be visible to the sensor pair,
the output becomes
O
l
(
t
)
=[
U
l
(
t
)
+
S
]−
(
G
+
S
)
=
U
l
(
t
)
−
G
,
Roll Command
(9.12)
FIGURE 9.15
A simple implementation of the ocelli using
only ultraviolet light will fail under some circumstances.
where
S
is the change in intensity caused by the
sun, a constant and almost equal in the green
and ultraviolet bands for our purposes. There
are other bright objects in the sky, such as clouds,
which diffuse sunlight, and these also can be
considered as a common-mode signal
S
.
We refer to this method as
spectral opponency
due to the subtraction of the two spectral
bands to extract the feature of interest, the
horizon.
The signal that drives the actuator on the axis
orthogonal to the view directions of the two sets
of photodiodes is produced from the difference
of the signals from either side; thus,
Reliance on the green band for stabilization
would fail due to lack of reliable contrast. Reli-
ance on the ultraviolet band alone would be sat-
isfactory under some circumstances but would
fail in others. For example, in
Figure 9.15
, where
the sun is within the field of view of one ocellus,
the aircraft will bank to turn toward the sun.
For the purposes of horizon stabilization in
fair weather, the green band can be seen as a
sun
detector
, contributing little to the distinction
between sky and ground but removing biases
caused by the sun. The effect is shown graphi-
cally in
Figure 9.16
.
Considering the signals received by the two
calibrated point-light sensors on one side of the
aircraft, we get
A
(
t
) =
O
l
(
t
) −
O
r
(
t
),
(9.13)
where
A
(
t
) is the processed actuator signal. This
processing eliminates the effects of green light
intensity on the output signal yet also varies in
amplitude with the absolute level of ultraviolet
light in the environment (which may complicate
control system design). The basic function of the
(9.10)
O
l
(
t
) =
U
l
(
t
) −
G
l
(
t
),
where
O
l
(
t
)
is the left-side output signal, whereas
U
l
(
t
)
and
G
l
(
t
)
are the ultraviolet and green signals
FIGURE 9.16
The environment measured by a green-sensitive sensor is illustrated on the left; in the center is the same
measurement by the ultraviolet sensor. With the correct weighted sum of the two sensors, the effect of the sun can be
removed.
