Repetitive Learning Sliding Mode Stabilization Control for a Flexible-Base, Flexible-Link and Flexible-Joint Space Robot Capturing a Satellite

During the process of satellite capture by a flexible base–link–joint space robot, the base, joints, and links vibrate easily and also rotate in a disorderly manner owing to the impact torque. To address this problem, a repetitive learning sliding mode stabilization control is proposed to stabilize the system. First, the dynamic models of the fully flexible space robot and the captured satellite are established, respectively, and the impact effect is calculated according to the motion and force transfer relationships. Based on this, a dynamic model of the system after capturing is established. Subsequently, the system is decomposed into slow and fast subsystems using the singular perturbation theory. To ensure that the base attitude and the joints of the slow subsystem reach the desired trajectories, link vibrations are suppressed simultaneously, and a repetitive learning sliding mode controller based on the concept of the virtual force is designed. Moreover, a multilinear optimal controller is proposed for the fast subsystem to suppress the vibration of the base and joints. Two sub-controllers constitute the repetitive learning sliding mode stabilization control for the system. This ensures that the base attitude and joints of the system reach the desired trajectories in a limited time after capturing, obtain better control quality, and suppress the multiple flexible vibrations of the base, links and joints. Finally, the simulation results verify the effectiveness of the designed control strategy.

Thin current sheets as key structures in space plasma

<p>Plasma structures with extremely small transverse size (named thin current sheets or TCSs) have been discovered and investigated by spacecraft observations in the Earth's magnetotail, then in other planetary magnetospheres and the solar wind. Their formation is related with complicated dynamic processes in collisionless space plasma near the magnetic reconnection regions. The proposed models describing TCSs in space plasma, based on the assumption of a quasi-adiabatic proton dynamics and magnetized electrons were successful. Various modifications of the initial equilibrium allowed describing such current sheets as the system of current sheets where the central sheet is supported by magnetized electron drifts, and the external sheets are supported by quasi-adiabatic protons and sometimes oxygen ions. Such current configurations are shown to have properties that are completely different from the well-known Harris model, particularly the multiscale structure, embedding and metastability. The structure and evolution of TCSs under the tearing mode as well as the related paradox of complete tearing mode stabilization in configurations with a nonzero normal magnetic field component is highlighted.</p><p>This work is supported by the Russian Science Foundation grant № 20-42-04418.</p>

Modelling effects of edge density fluctuations on electron-cyclotron current drive used for neoclassical tearing mode stabilization

In this work we study the undesired effects of electron density fluctuations (in the form of blob structures which may exist in the edge region of tokamak plasmas) to the electron-cyclotron wave propagation and current drive in connection to the efficiency of neoclassical tearing mode stabilization. Our model involves the evaluation of the driven current in the presence of density perturbations, by using a combination of a wave solver based on the transfer matrix and electromagnetic homogenization methods for the propagation part prior to and inside the region of these structures (where standard asymptotic propagation methods may not be valid due to the short-wavelength limit breakdown), with a ray tracing code including island geometry effects and current drive computation for the propagation past the perturbed region. The computed driven current is input into the modified Rutherford equation in order to estimate the consequences of the wave deformation (driven by the density fluctuations) to the mode stabilization.