Unmanned Air Vehicles - Rendezvous and Formation Flight

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I worked on an exciting multi-UAV (unmanned air vehicle) project when I was a graduate student at Aero/Astro department in MIT. The project was sponsored by Draper Laboratory. It was named PCUAV (Parent Child Unmanned Air Vehicle, 1998-2002) to describe the combination of one large and other smaller UAVs. One of the main effort in the project was to demonstrate rendezvous of two UAVs. I worked on the control system development and flight tests.

A series of flight tests was performed using two UAVs which were built in the project. It was demonstrated that each individual aircraft can follow a desired flight path within a position accuracy of 2 meters (based on sensor data) while also tracking the air speed command to within 1 m/s. Based on these capabilities of each aircraft, it was demonstrated that the developed control system can bring the two UAVs from any arbitrary initial positions into a configuration of a tight formation flight, where one vehicle trails the other with a commanded separation of 12 meters while maintaining the relative position error within 2 meters in both horizontal and vertical directions for 85% of the flight time.

Here is the movie of flight data that shows the positions of the two vehicles. You will see the position of the larger aircraft as letter "P" and the smaller one as "M". The larger vehicle trajectory was first maintained on a 500 m diameter circular path. The smaller vehicle finds the larger vehicle and generates its trajectory in such a way that it can join with the other vehicle on the circle in a synchronized fashion - rendezvous. Then, the two vehicles fly along the same circle while maintaining the relative distance - formation flight.

Flight Data Video (335 KB)

 

Here is the video of the two aircraft during the period of formation flight. The separation distance was about 12 meters and the chasing vehicle was commanded to fly 2 meters above the leading vehicle.

Formation Flight Video (275 KB)

 

Aircrafts

Mini Child Vehicle

The smaller aircraft was designed and built mainly by Jason Kepler, one of our team member. This vehicle was named "Mini". The span of the wing is 2.54 meters. The total weight is 9.1 kg including 2 kg of avionics. An internal combustion engine O.S. 0.91 cu.in was used.

 

OHS Parent Vehicle

Outboard horizontal stabilizer (OHS) vehicle configuration was tried for the larger aircraft. It has two horizontal tail surfaces outboard of the main-wing. Two booms extend backward from the tips of the main wing and the horizontal and vertical tail surfaces are located at the end portions of the booms.

The OHS configuration has a unique aerodynamic property. The horizontal tail surfaces experience the upwash of the trailing vortices generated by the main wing because the horizontal tail surfaces are placed outboard and behind the main wings. This feature provides an enhanced stability, which enables an OHS airplane to have its center of gravity positioned further rearward, as compared to conventional airplane. Typically, the OHS can be trimmed in static stable flight with significant positive lift generated by the horizontal tail. Our OHS Parent vehicle has a center of gravity at 55% of the chord from the leading edge of the main wing. And its stability neutral point was estimated at 80% of the chord. The OHS Parent vehicle was built by Francois Urbain and Jason Kepler in the team.

Avionics

Inexpensive, commercial off-the-shelf components were used to construct the onboard avionics. The following is the list of some important avionics components.

- PC104 Computer Stack (CPU module, Analog data module, Utility module, Power supply module)
- GPS : Marconi, Allstar GPS Receiver
- Inertial Sensors (Tokin Ceramic Gyros & Crossbow 3-axis Accelerometers for Mini, Crossbow IMU for OHS)
- Pitot Static Probe for air speed measurement
- Altitude Pressure Sensor (for high frequency estimation)
- Communication : Maxstream 9XStream Transceiver

Avionics box was constructed for each vehicle for quick and easy installation in the field. Below are the photos of the avionics boxes for each aircraft.

Controllers

Here is some descriptions on the controller configurations used on our aircraft. For the control in lateral dynamics, classical aircraft controller configuration was deployed, where a yaw damper with washout filter was implemented using rudder deflection and a bank angle controller was implemented using ailerons. For the longitudinal controller configuration, linear quadratic regulator (LQR) was implemented, where angle of attack, pitch angle, pitch rate, speed, and integration of the speed error and altitude rate error were used for feedback variables and both throttle and elevator were used for actuators. Here is the configuration for the longitudinal controller.

Estimators

Various kinds of complementary filters were used to compensate gyro drifts and GPS delay. Kalman filters were also designed and implemented to overcome the low quality inertial sensors by combining with the aircraft kinematics. Please refer to the related documents below for more information.

Trajectory Tracking Guidance

A new guidance logic was developed for tightly tracking a desired flight path. The nonlinear path-following guidance logic is simple and is derived from geometric and kinematic properties, and has been demonstrated to work better than the conventional aircraft guidance method in waypoint navigation. The law approximates a proportional-derivative controller when following a straight line path, but the logic also contains an element of feed-forward control enabling tight tracking when following curved paths. The logic uses inertial speed in the computation of commanded lateral acceleration and adds adaptive capability to the change of vehicle speed due to external disturbances, such as wind. Please refer to the related documents below for more details.

Path Planning

To assure that the two UAVs perform rendezvous without aborting the procedure it is important to establish a routine for two UAVs to follow every time. A simple and effective routine was developed by Damien Jourdan in the team. First, OHS Parent vehicle is maintained in flight along a 500 meter diameter circle, with no information on the Mini vehicle. It is the Mini vehicle that schedules its path in order to meet the Parent vehicle somewhere on the circle, at a required separation.

Related Documents

"Avionics and Control System Development for Mid-Air Rendezvous of Two Unmanned Aerial Vehicles", Ph.D. Thesis, MIT. February 2004

"A New Nonlinear Guidance Logic for Trajectory Tracking", by Sanghyuk Park, John Deyst, and Jonathan How. AIAA GNC 2004

Other Flight Data

In the PCUAV team led by Prof. John Deyst, there were 6-8 students per year. I thank all the team members who shared their friendship - Francois Urbain, Thomas Jones, Jason Kepler, Damien Jourdan, Richard Poutrel, Sarah Saleh, Alexander Omelchenko...

Prof. John Deyst, Jonathan How, James Paduano, and Dr. Brent Appleby were my advisors for my Ph.D. program in the project. I also learned a lot from Sean George, Vladislav Gavrilets. Thank you all...

Ruane Crummett and Mitch Buckley were flying our aircraft for manual mode. They saved our vehicles numerous times, and thanks to their help we were able to have those fruitful results at the end.

 


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