03-09-2012, 01:04 PM
A Small Semi-Autonomous Rotary-Wing Unmanned Air Vehicle (UAV)
A Small Semi-Autonomous.pdf (Size: 2.44 MB / Downloads: 82)
Abstract
Small radio controlled (R/C) rotary-wing UAVs have many potential military and
civilian applications, but can be very difficult to fly. Small and lightweight sensors and
computers can be used to implement a control system to make these vehicles easier to fly. To
develop a control system for a small UAV, an 8-bit microcontroller has been interfaced with
MEMS (Micro-Electro-Mechanical Systems) gyroscopes, an R/C transmitter and receiver,
and motor drivers. A single angular degree of freedom test bed has been developed to test
these electronics and successful pilot-in-the-loop PI control has been achieved for this test
system. A quadrotor with a stability augmentation system that uses these electronics to
control the vehicle has also been developed. The future goals of this research are to
incorporate more sensors to increase the level of autonomy for UAV operation.
Introduction
Small unmanned air vehicles (UAVs) can be deployed at the front lines of combat to provide situational
awareness to small units of troops through real-time information about surrounding areas.1 Small fixed-wing
unmanned and micro air vehicles (such as the Dragon Eye, Aerosonde, Hornet, and Wasp) have become prevalent
and have demonstrated impressive flight abilities and levels of autonomy.2 These UAVs can weigh as little as a few
ounces. However, even the lightest models must fly fairly fast to provide sufficient lift for flight.3 These fixed wing
aircraft also need space to turn and although research has studied their capability to fly in small circles over a
specified area, they are difficult to fly in confined places, such as urban environments and small indoor spaces.3,4
Rotary-wing unmanned air vehicles have the potential to be very useful if they can hover and fly vertically.
VTOL UAVs such as the Fire Scout and Hummingbird currently have the capability to fly autonomously, land in a
specific location and take off again.1,2 Smaller UAVs with these abilities would have many applications, including
flying through buildings for search and rescue or surveillance operations.
MEMS Sensors
Two MEMS (Micro-Electro-Mechanical Systems)
sensors, a gyroscope and an accelerometer, were
interfaced with the PIC microcontroller. A single chip
rate gyro evaluation board (ADXRS150EB Analog
Devices, Inc., www.analog.com) was used to measure
angular velocity. The evaluation board, shown in Fig.
2 weighs three grams and is about 1” long by 0.5”
wide. The sensor is capable of measuring +/-150
degrees/second of angular velocity and includes some
signal conditioning electronics to help preserve the
signal in noisy environments. The bandwidth of the
evaluation board is fixed and set at 40 Hz. The chip
produces an analog voltage output that is proportional
to the angular velocity about the axis normal to the top
surface of the gyroscope package.
Circuit Design and Hardware Integration
A circuit using three individual boards was designed and
fabricated to connect the PIC development board, gyroscopes,
R/C receiver, battery, and electric speed controllers. Figure 5
shows these connections. A freeware version (4.13) of the
Eagle layout editor (http://www.cadsoft.de) was used to create
the circuit board. The circuit board for the UAV was fabricated
by Advanced Circuits (http://4pcb.com).
Three circuit boards were created so that each of the three
gyroscopes could measure angular velocity about a different
perpendicular axis. The three boards were soldered together
perpendicular to each other using right angle male pin headers
(Fig. 6)
Results
The electronics and PI control law were tested using the two-rotor system described in Section III. Two pilot
inputs from the R/C transmitter and receiver were used to determine the desired thrust and pitch rate. A gyroscope
was mounted on the test system to measure the pitch rate. The PI control law was used to calculate the speed of the
two motors needed to maintain the desired angular velocity of the system.
To show the importance of using feedback from the gyroscope, tests were performed to try to keep the test
system level both with and without pitch rate feedback from the gyroscope. The Flight Data Recorder described in
Section III was used to record the pilot input for pitch rate in both cases and outputs from the gyroscope were saved
in the PIC microcontroller’s RAM and printed to a computer monitor after the tests. Figure 9 shows the pitch rates
that were recorded by the gyroscope during these tests. The pitch rates recorded during the test without feedback
control have a much higher range, from −80 to +80 degrees/second, than the test with feedback control (range of
−20 to +20 degrees/second).
Conclusion
A lightweight quadrotor system has been designed with an emphasis on using inexpensive COTS components.
The components have been integrated and pilot-in-the-loop PI control has been successfully implemented on a
single degree of freedom test system using these components. The electronics have been used to develop a stability
augmentation system that will allow manual flight of the quadrotor. The quadrotor was placed on a training stand
with two degrees of freedom, but the flexibility of the quadrotor frame prevented successful flight. More flights will
be conducted in the near future.
This research has demonstrated that low-cost components can be used to create a control system that will allow
manual flight for a quadrotor. These electronics have been developed so that they can be easily integrated with
additional sensors such as cameras, sonar, and optic flow sensors to increase the level of quadrotor autonomy. In
addition, a control algorithm more sophisticated than the current PI control can be used.