DYNAMICS AND CONTROL OF DUAL-ARM SPACE ROBOTS

Abstract

Space robotics has become an escalating field of study. With the development of human space activities in area of space shuttles, spacecrafts and space station; many countries and researchers are paying more and more attention in space robot technology. The typical tasks to be performed by space robots are: deploying or assembling of space platforms, space stations, large antennas, and solar panels; capture of space objects, collection and removal of space debris, catching of uncontrolled satellites etc. The space applications require precise manipulator control. This is a complex task to achieve as the base of the space robot is free-floating in nature and due to the dynamic coupling between the manipulators and the space robot base. The space robot base attitude minimization is necessary in most cases for electrical power generation from solar panels and to retain the communication link with the control station on the earth. The force control at the point of interaction between the arm tips and the environment is another issue. The use of dual arm space robot is preferred in comparison to single arm space robot due to increased dexterity. One can also exploit the dynamic coupling between arms and base to control the attitude of the base of the space robot. The three main aspects of the dual-arm space robot namely, attitude control of the space robot base for a given trajectory of the mission arm, impedance control of dual-arm cooperative space robots and reduced model-based trajectory control of multi-arm space robots were selected as the main areas of study in this thesis. Bond graph was adopted for modeling as it facilitates the system modeling from the physical paradigm, and it is easy to develop various control strategies by modifying the physical paradigm. . For object oriented bond graph modeling, various sub-models are developed and used for modeling of the entire system. The attitude control strategy for two degrees of freedom (DOF) dual-arm planar space robot is proposed. The arm which does useful job is called mission arm while the other arm is called balance arm. The space robot base attitude .control strategy has been derived from the principle of structural controllability and the dynamic coupling between the balance arm and the space robot base. Mission arm uses an overwhelming controller for the trajectory control task. The stability analysis of the overwhelming controller is carried out by Routhian criteria. The control strategy for balance arm reduces the attitude of the space robot base. To demonstrate the effectiveness of the proposed control laws in an experimental system, the prototype of the dual-arm planar space robot has been fabricated. The methodology is extended for the attitude control of the three DOF dual-arm space robot. The methodology for the force control by impedance control at the interaction point between the tips of the dual-arm planar cooperative space robot and floating body is presented. The impedance control of the space robot is achieved by introducing the passive DOF in the controller domain by creating a virtual foundation. The proposed strategy is demonstrated by tacking an example of docking of the floating rigid body by the dual-arm cooperative space robot. Lastly, the reduced model-based trajectory control strategy for the multi-arm space robot system is presented. The validity of strategy is checked by taking different, symmetrical, and skew-symmetrical trajectories for the mission and the balance arm.