Extending Quadrotor Motion Capability by Centrally Coaxial Tilting Rotors

Motion of the typical quadrotor is restricted since it employs only four fixed rotors and thus underactuated. We extend its capability by introducing the coaxial tilting rotors that can generate thrust force with lateral component. They are installed at the center of the body frame with a gimbal mechanism to control the thrust direction. The original and new rotors together produce control force and torque that make attitude stabilizing, tilting, and sideway translation possible. Based on contraction analysis, we propose a nonlinear PID controller for this new UAV to follow admissible desired trajectories. The new special integral error term addresses modeling uncertainties, loss of thrust force, and disturbances. Moreover, we apply optimization technique to allocate rotor thrusts considering the gimbal dynamics and rotor speed limit. Simulation and experiments are conducted to compare the effectiveness of the special integral error, validate the range of achievable motion, and demonstrate the tracking robustness and motion tracking on a 3D-Lissajous curve.