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custom transmitter (bottom)
There are a wide variety of micro video cameras on
the market today. The challenge for an MAV is to find
a good balance of high image quality, low weight, low
power, and small size. We found a good compromise
with the CMOS video cameras. Table 6 shows the
specifications for the CMOS cameras used on the Black
Widow.
To get the video from the onboard camera to the
ground, we used a radio frequency (RF) transmitter
operating at 2.4 GHz. The transmitter takes the analog
video stream as an input, modulates it using frequency
modulation (FM), and outputs it as a RF signal. 2.4
GHz was used because commercial video receivers and
antennas are readily available. The first generation
COTS video transmitter had moderate performance
because of low power conversion efficiency in the RF
amplifier section. For the final MAV system, an
improved transmitter with higher output power, smaller
size, and lighter weight was developed.
Table 6: CMOS video camera specifications
Black & White Color
Mass (g) 2.2 1.7
Power (mW) 50 150
Resolution
(pixels)
320 x 240 510 x 488
Table 7: Video downlink transmitter specifications
First Generation Final
Mass (g) 3.3 1.4
Power Input (mW) 550 550
Power Output (mW) 50 100
Frequency (GHz) 2.4 2.4
Stability and Control
The small size of an MAV creates several unique
stability and control challenges. Since the mass moment
of inertia scales as the fifth power of the characteristic
dimension, small vehicles tend to have high natural
frequencies of rotational oscillation. Obtaining a stable
video image requires an actively stabilized camera
mount or an actively stabilized aircraft. Therefore high
oscillation frequencies require a control system with
fast processors and fast actuators to stabilize the camera
or the entire MAV. Since wing loading decreases with
decreasing size, small air vehicles are quite susceptible
to gusts. Even small birds (with highly evolved active
control systems) have trouble maintaining steady flight
in extremely turbulent conditions.
The main stability augmentation system used on
the MAV is a yaw damper. Many of the early prototype
MAVs showed a 3 Hz Dutch roll oscillation. The
addition of more vertical tails and the yaw damper
significantly increased the damping ratio. The MAV
has three autopilot modes: dynamic pressure hold,
altitude hold, and heading hold. More autopilot modes
may be added in the future as the system becomes more
advanced, and as new sensors, such as a GPS receiver,
are added. We also developed a data logging system
which can sample 16 channels of data at 20 Hz for 4
minutes. This was used to evaluate and refine the
control system dynamics.
American Institute of Aeronautics and Astronautics
8
Performance
On August 10, 2000 the Black Widow MAV performed
a flight which most likely established several world
records for the MAV class of aircraft. Table 8
summarizes the performance on this flight. The pilot
flew about 90% of the flight "heads-down," which
means he was looking only at the video image and
downlinked sensor data from the MAV.
Table 8: Performance summary for the Black
Widow flight on August 10, 2000
Endurance 30 min
Maximum Range 1.8 km
Maximum Altitude 769 ft
Mass 80 g
Since the Black Widow uses an electric propulsion
system, it is extremely difficult to observe in the air. It
cannot be heard above ambient noise at 100 ft, and
unless you're specifically looking for a 6-inch square
black dot in the sky directly overhead, you can't see it.
It looks more like a bird than an airplane. In fact, we
have seen sparrows and seagulls flocking around the
MAV several times.
Figure 14: Black Widow ground control unit and
cassette launcher shown deployed (above) and
stowed (below)
Ground Control Unit
In addition to the MAV itself, a fully functional MAV
system requires a user-friendly, rugged, and compact
ground control unit (GCU). The Black Widow GCU
evolved through three stages to reach its final form. the
first generation GCU was a collection of off-the-shelf
equipment which was quite bulky, and had to be
assembled at the field. The second generation GCU was
a 15-lb briefcase that contained the MAV, a pneumatic
launcher, a removable pilot's control unit with a 4-inch
LCD display for the downlinked video, and an
automatic tracking antenna. The final GCU (Figure 14)
is built around a Pelican case, which is extremely
rugged, compact, and waterproof. The MAV is stored
　
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