Research On Tension Control Of The Tethered Combination After Target Capture By A Space

Space tethered-net system is a novel concept with the advantages of good fault-tolerance, long operating distance, little influence on the platform. Recent research attention has been paid to the space tethered-net system. The space tethered-net system involves subjects such as mechanics, mechanical engineering, electrical engineering, control engineering and material engineering. However, many scientific problems existed to be solved in system design, system model, and system control. The capture process of space tethered-net system includes tracking& pointing, approaching, ejecting net, closing net and stabilization maneuvers, meanwhile with following actions such as towing and deorbiting after capture. The target is connected to the platform by the tether, the control of tether length and tension is implemented by the tension control mechanism mounted on the platform which can accomplish the deployment, retrieval and state maintenance of target. It is considered to exchange the momentum of the two rigid bodies and avoid the impact, swing and winding problems by tension control mechanism to ensure capture tasks to be finished efficiently. This dissertation aiming at the above-methtioned tension problems, takes the tethered combination after capture as the research object, and the main research contents and achievements are as follows.(1) A multifunctional and high-performance tether tension control mechanism is proposed and developed. The possible impact, swing and winding problems during the system stabilizing from two decentralized systems to a combination system are emphatically demonstrated. To fulfill the control requirements, the tension control mechanism should be with the measurement of attitude angles, the measurement of the length of tether, the regulation and measurement of tether tension. A design scheme of tension control mechanism with high precision is proposed and the tension tracking test and other static and dynamic performance tests are studied.(2) General nonlinear dynamics of the tethered combination system in the post-capture phrase are established with consideration of the attitudes of two spacecrafts and the quadratic nonlinear elasticity of the tether, and the coupling effect of libration and vibration is analyzed. The motion law of the tethered combination in the deorbiting process with different disturbances is simulated and discussed. The approximate analytical solutions of the dynamics for a simplified model are obtained through the perturbation method. The condition of the inter resonance phenomenon is given which will provide theoretical basis for the space tethered-net capture system.(3) A tension and torque coordinated control strategy for solving the impact and winding problems caused by the velocity contrast of the platform and target is developed. General electromechanical model of tethered combination in the platform-fixed frame is established, taking the attitude of target, the electromechanical system of tension control mechanism and the elasticity of the tether into consideration. The simulation results show that the strategy is effective for two conditions:the motion of the target is translation and the motion of the target is translation and rotation. The ground-based flotation platform is set up, and the experiment results show the designed controller can prevent impact effectively.(4) An in-plane swing damping method is proposed. An in-plane swing damping method is proposed which is inspired from the swing impacted. First, the swing system under constant gravity is studied according to the dynamic similarity, and the results of simulation and experiment show the method is very effective in swing damping under constant gravity. Then, the method is extended to the space tethered system, the simulation results validate that the method is applicable, too.(5) The longitudinal nonlinear characteristics and torsional dissipation characteristics of Kevlar tether is studied and a passive despun method of target is proposed. The Kevlar tether exhibits strong nonlinear characters through the tensile test designed, which can be fitted accurately by the cubic function and power function. The tensile test also shows obvious hysteresis effect of the tether, and the Kawabata stress-strain model is used to describe this effect. The torsional experiment is developed, and the relationship between the torsional stiffness and damping of tether and tether length and tension is mainly studied. A passive despun method of target is proposed, which is validated by the experiment designed.