This paper proposes a trajectory optimization method considering gravitational orbit-attitude coupling (GOAC) for asteroid landing. First, by modeling the spacecraft as a rigid body rather than a point mass, and using the polyhedral method to describe the irregular gravitational field of the asteroid, GOAC is fully embedded into the 6 degree-of-freedom (DOF) dynamic model of the spacecraft. Second, attitude control, which is assumed to consume electric energy, is introduced into the optimization, the indirect method is adopted to formulate the 6-DOF fuel-optimal control problem, and the boundary conditions are modified to expand the search space of the problem. Third, a two-phase homotopic approach is proposed to realize a smooth continuation from the 6-DOF energy-optimal problem to the fuel-optimal problem. Finally, a numerical example is presented to demonstrate the feasibility of the proposed method, and the comparison results show that the obtained 6-DOF optimal trajectory can guarantee the fuel-optimality of the landing mission under GOAC. INDEX TERMS Asteroid, landing mission, orbit-attitude coupling, trajectory optimization.
This paper proposes a coordinated hybrid force/position control strategy of robonaut performing object transfer operation. Firstly, the constraint relationships between robonaut and object are presented. Base on them, the unified dynamic model of the robonaut and object is established to design the hybrid force/position control method. The movement, the internal force and the external constraint force of the object are considered as the control targets of the control system. Finally, a MATLAB simulation of the robonaut performing object transfer task verifies the correctness and effectiveness of the proposed method. The results show that all the targets can be control accurately by using the method proposed in this paper. The presented control method can control both internal and external forces while maintaining control accuracy, which is a common control strategy.
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