MODELING AND CONTROL OF FLEXIBLE SPACE ROBOTS

Abstract

In case of space robots the position and orientation of the space vehicle will change due to manipulator motion. The motion of the space robots also induces vibrating motions in structurally flexible manipulators. The three main aspects of space robotics, namely trajectory control, model based control, and torque control are addressed in this thesis. Bond graph was selected as a tool for modeling as it provides physical insight and facilitates the system modeling from the physical paradigm itself. It is also easy to develop various control strategies through bond graphs by modifying the physical paradigm. In this thesis bond graph modeling of one arm flexible, two arm rigid flexible and two arm flexible space manipulator are developed. Flexible links are modeled as Euler-Bernoulli beam. A trajectory tracking control method for flexible space robot is presented. The control strategy is based on the overwhelming control concept. The proposed controller is first conceptualized for a single rotational degree of freedom case. Then it is extended for the multimodal case. The stability analysis of the manipulator is carried out by the Routh Hurwitz criterion. Finally we illustrate the application of scheme to one link flexible space manipulator. The efficacy of the scheme is illustrated by simulation and experimental results. This thesis also presents the trajectory control of flexible space robots in work space using the Jacobian and model based controller. To simplify the analysis we have considered only Jacobian of rigid manipulator. For model-based controller one is required to know the model parameters accurately. This is a very difficult task especially if the manipulator is flexible. So a model based controller has been developed, which requires space robot base velocity information and link parameter only. The scheme has been verified for two link rigid-flexible manipulator and two link flexible manipulator. Impedance control is a stable method of providing trajectory and force control in robotic systems. In this thesis a scheme is presented in which the control of impedance at the interface of the flexible space robot end-effector and the space structure is achieved by the introduction of passive degrees of freedom (DOF) in the controller of the robotic system. A simple example of a space robot comprising of a base and two rotational inertias connected by torsional spring and damper are considered. The impedance is shown to depend upon a compensation gain for the dynamics of the passive DOF. An explicit relation between the compensation gain and impedance has been derived.