Biomedical Engineering Reference
In-Depth Information
move faster through the field of view of the
observer than objects that are farther away. For
an observer moving at a constant velocity
through a tunnel, the optical flow will be larger
as the tunnel becomes narrower. Researchers
observed that honeybees flying through tunnels
to a food source significantly overestimated the
distance flown, compared to when they flew
through an open environment
[92, 93]
. It was
concluded that the honeybees measure distance
by integrating the optical flow across their eyes,
and the higher optical flow created by the nar-
row tunnel walls resulted in an increased esti-
mate of the distance flown.
In Chapter 9, Chahl and Mizutani discuss the
use of optical flow for biomimetic sensing. The
optical flow technique is being investigated by
several researchers for application on microflyers,
using CMOS sensors, special lenses, and dedi-
cated electronics to minimize processing require-
ments
[94]
. For example, Barrows
et al
.
[95]
developed an optical flow sensor that they incor-
porated into a commercially available hobby
indoor helicopter and demonstrated stable hover
in a fixed location using feedback from the sensor.
Garratt and Chahl
[96]
described an optical flow-
based terrain-following system for an unmanned
helicopter. They designed and constructed a sys-
tem consisting of a downward-looking camera
and hardware to compute the optical flow.
The system was installed on an 80 kg
Yamaha RMAX helicopter as well as on a
smaller 8 kg electric helicopter. Flight testing
demonstrated that the system could accurately
measure the height of the aircraft above
ground; combined with global positioning sys-
tem (GPS) measurements, the system was able
to estimate height above terrain with an accu-
racy of 7.5% at a flight speed of 5 m/s. Other
discussions of the physical principles that
form the basis of optical flow and descriptions
of several physical implementations can be
found in Refs.
[97, 98]
.
The halteres on insects are highly developed
angular velocity sensors
[99]
. They vibrate up
Antennas
and hairs
Compound eyes
Ocelli
Halteres
FIGURE 5.27
Different types of sensors on a typical insect
[97]
.
FIGURE 5.28
Compound eyes on an insect
[97]
.
stabilize flight as well as avoid obstacles.
Optical
flow
relies on the measurement of the rate of
change of an image, which is directly related to
the velocity of the image sensor with respect to
the object being imaged. Consider a camera or
an observer moving along a straight line at a
constant velocity, imaging an object located
along a direction perpendicular to the direction
of motion. The closer the object is to the line of
motion of the observer, the larger the angle it
subtends. Therefore, closer objects appear to
