Abstract
This paper presents a motion control scheme for a new concept of omnidirectional aerial vehicle for transportation and manipulation tasks. The considered aerial platform is a novel quadrotor with the capability of providing multi-directional thrust by adding an actuated gimbal mechanism in charge of modifying the orientation of the frame on which the four rotors are mounted. The above mechanical design, differently from other omnidirectional unmanned aerial vehicles (UAVs) with tilted propellers, avoids internal forces and energy dissipation due to non-parallel propellers’ axes. The proposed motion controller is based on a hierarchical two-loop scheme. The external loop computes the force to be applied to the vehicle and the reference values for the additional joints, while the inner loop computes the joint torques and the moment to be applied to the multirotor. In order to make the system robust with respect to the external loads, a compensation of contact forces is introduced by exploiting the estimate provided by a momentum based observer. The stability of the motion control scheme is proven via Lyapunov arguments. Finally, two simulation case studies prove the capability of the omnidirectional UAV platform to track a 6-DoFs trajectory both in free motion and during a task involving grasping and transportation of an unknown object.
Highlights
In recent years, the use of aerial robotic platforms, due to their nearly unlimited motion space, has become popular in many applications scenarios, e.g., inspection and maintenance services [1], data collection and exploration operations [2], and precision agriculture [3]
In order to show the capability of controlling both position and orientation at same time, a trajectory in free-space has been commanded to the vehicle platform; in the latter simulation, the behavior in the presence of a task requiring grasp and transportation of an object has been tested
The orientation of the rotor frame relative to the platform frame can be computed via (1); centrifugal and Coriolis terms have been neglected in control laws (18), (28), and (29); viscous friction has been included in the simulation model but not considered in the control design
Summary
The use of aerial robotic platforms, due to their nearly unlimited motion space, has become popular in many applications scenarios, e.g., inspection and maintenance services [1], data collection and exploration operations [2], and precision agriculture [3]. In order to change its position and/or counteract external disturbances, rotation of vehicle’s body is required: only four degrees of freedom (DOFs) are left to execute the task assigned to the UAV This limitation may become severe in some applications that might benefit from availability of all six DOFs, e.g., manipulation tasks involving contact with the environment [8], and/or limit the disturbance counteraction capabilities of the system. The overactuated platforms require additional motors for tilting the propellers, and this has the negative effect of increasing the system weight and the complexity of the control for handling both vertical and lateral air flows Such a problem can be overcame by adopting hexarotors with tilted propellers, e.g., as in [11], where each rotor is mounted in a fixed configuration rotated about two axes. In [13], an hexarotor with independently tiltable rotors is designed; the tilting angle of the rotors is computed online via a suitable control allocation scheme
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