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- Type of Document: Ph.D. Dissertation
- Language: Farsi
- Document No: 55090 (08)
- University: Sharif University of Technology
- Department: Mechanical Engineering
- Advisor(s): Alasty, Aria; Nejat Pishkenari, Hossein
- Abstract:
- It is well-known that helicopters descending fast may enter a region in the velocity space called Vortex Ring State where the blade’s lift differs significantly from regular regions and includes high amplitude fluctuations. These fluctuations may lead to instability and therefore, this region is avoided, typically by increasing the horizontal speed. However, this region is not fully identified for multirotors, which their blades are rigid in contrary of helicopter’s blades which have two degrees of freedom. This project researches this phenomenon in the context of small-scale quadcopters. The region corresponding to the VRS is identified by combining first-principles modeling and wind-tunnel experiments. A model is proposed for the velocity constraints that the quadcopter must meet to avoid these regions. Then, the problem of designing optimal time descend trajectories that avoid the VRS region is tackled. Then the optimal trajectories are implemented on a quadcopter. The flight tests show that by following the designed trajectories, the quadcopter can descend considerably faster than purely vertical trajectories that also avoid the VRS. This method needs horizontal displacement for optimal trajectories which is not proper for some missions like chimney’s inspections. A new control method called Yaw Rate Control (YRC) is proposed to add horizontal velocity on blade disk to avoid entering VRS region and descending fast. The control scheme is designed and implemented on a quadcopter and then flight tests shows that the quadcopter can descend faster than regular methods avoiding VRS and without horizontal displacement. Then, a variable dihedral angle quadrotor is designed and produced to add horizontal air speed by changing the dihedral angle of each arm. This project proposes three different methods to descend fast considering VRS. Finally, an online optimal trajectory design method based on Model Predictive Control (MPC) is presented to make the system robust in external disturbances.
- Keywords:
- Quadrotor ; Trajectory Data ; Control ; Vortex Ring ; Dihedral Angle ; Quadrotor with Dihedral ; Trajectory Design
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