Inverse control and stabilization of free-flying flexible robots

SUMMARY The question of control and stabilization of flexible space robots is considered. Although, this approach is applicable to space robots of other configurations, for simplicity, a flexible planar two-link robot, mounted on a rigid floating platform, is considered. The robotic arm has two revolute joints and its links undergo elastic deformation in the plane of rotation. Based on nonlinear inversion technique, a control law is derived for controlling output variables describing the position and orientation of the platform and the joint angles of the robot. Although, the inverse controller accomplishes reference trajectory tracking, it excites the elastic modes of the arm. For the vibration suppression, three different stabilizer are designed. Using linear quad-ratic optimal control theory, a composite stabilizer for stabilization of the rigid and flexible modes and a decoupled flexible mode stabilizer are designed for regulating the end point of the robot to the target point and vibration suppression. Stabilization using only elastic mode velocity feedback is also considered. For large maneuvers, first the inverse controller is active, and the stabilizer is switched for regulation when the motion of the robot lies in the neighborhood of the terminal equilibrium state. Simulation results are presented to show that in the closed-loop system including the inverse controller and each of the stabilizers, trajectory tracking and stabilization of elastic modes are accomplished. 1. INTRODUCTION In future space missions, space robots will play important role in the deployment or retrieval of payloads and will be used for the repair and construction of space structures. These robots have long flexible arms. Space robots have complex dynamical behavior. Lack of an inertially-fixed base on which the manipulators are mounted and control-structure interaction pose many difficulties in control system design. In recent years, researchers have focused attention on the problem of dynamics and control of space robots. Research has been done related to free-flying robots as well to space robots for docking and intercept maneuvers. 1–8 In references 10 and 12 the perturbation technique is used to derive a control law. The control problem is divided into one for the