Research On Space Large Misalignment Tolerance End-effector And Its Soft Capture Strategy

On-Orbit Servicing is a leading and promising technology for space exploration.It is applied to the structure assembly, dialy maintance, system upgrading, refuelingspace vehicle, replacement of compenents and orbit transfer and so on. It is helpfulfor extending life of space vehicles and recovering the space vehicle in malfunction.The space manipulator with dexterity and multifunction is the predominant methodfor On-Orbit Servicing. Moreover, the construction and maintenance of lagre spaceinfrastructure will become frequent as the development of space exploration. Thelarge space manipulators will be imminently imployed to these construction andmaintenance, because the Extravehicular Activities of astronauts are insufficient. Inordert to promote the manipulation efficiency and reliability of large spacemanipulator, many end-effector design schemes and capture strategies are presentedand studied in this paper. Finally, an end-effector prototype with capabilities ofpromoting the reliability and rapidity of on-orbit capture is developed.Basing on the characteristics of large space manipulator, the design guidelingswith large misalignment tolerance, soft capture and hard connecting for spaceend-effector schemes are proposed. And the basic requirements are largemisalignment tolerance and soft capture. According to these two requirements, manydesign schemes are presented. The three fingers-three petals and the steelcable-snared end-effector which are designed in detail are most representative andfeasible. According to the comparison of basic requirements through theory analysisof misalignment tolerance and the ADAMS dynamic simulation analysis, the steelcable-snared end-effector is superior to the three fingers-three petals end-effector.The tendon-sheath system is employed as the transmission mechanism of the capturesubassembly. It maximizes the capture space with strict limitation of outsideenveloped diameter. Moreover, the flexibility of the tendon-sheath system willenhance passive compliance of the capture subassembly to promote the soft capturecapability. The cross section with foursquare shape of the end-effector is proposed,and it will minimize the enveloped diameter and maximize the capture space.An end-effector prototype with cable-snared mechanism is developed. Itscapture space is determined by the precondition of large misalignment tolerancecombined with requirements of light-weight and small-volume. The modularizationdesign of actuator systems and sensor collocation are presented, and the features ofthe tendon-sheath are analysed. And then the tendon-sheath system with multi-curvatures are modeled and analysed. Therefore, the transmission route and material of the tendon-sheath system are designed and chosen. The outside shell dimensionand the misalignemnt tolerance performance are obtained by the analysis of thecapture space. The sensor collocation is beneficial to the intelligence andindependence of payload capture. There actuator units of the end-effector aremodularization design, and all of the three actuator units are fixed on the outsideshell of the end-effector, it is useful for the replacement and repair on orbit. Thedeployment of latching loop is analysed. And the dynamics models of the captureand rigidizing loop are modeled to offer a theory support for capture strategies.The contact model of the end-effecor is proposed based on the capture principleof the end-effector. And the motion of the free-floating target is analysed. Themathematical model of the relationship between the force/moment of the wrist jointand the motion of the free-floating target is constructed, according to the contactmodel of capture subassembly and the force/moment information of the wrist joint.The dynamics parametes identification method is obtained based on this model. Themodel and method of dynamics parameters identification is vital to the captureoperation of free-floating targets of which the dynamics parameters are unknown.It is difficult for the micro-gravity simulation device with high cost to simulatethe mass and inertia parameters of the free-floating target on ground. Morever, thedynamic parametes are important for the capture operation. Therefore, the dynamicssimulation is the main method to study the capture operation and capture strategy.The capture field with variable stiffness is presented, according to the flexibility andstiffness change as the rotating ring rotates. The dynamics model and virtualprototype of the capture subassembly are proposed by a discrete model of threecapture cables in ADAMS. The end-effector performance of large misalignment andsoft capture are validated by capture simulation of free-floating target with differentparameters. And the capture mode and strategy are also studied by simulations tosearch for the capture strategy to promote the capture efficiency and reliability.Therefore, the capture strategy with trajectory tacking control is presented for themanipulation of fixed payload and self-relocation of the large manipulator. Amicro-gravity simulation decive is developed to test the capability of misalignmenttolerance and carry out the capture experiment. The performance of the end-effectorprototype meets the requirements, and it is validated by the dynamics simulations inADAMS software and the experiments on the micro-gravity simulation decive.