Dynamics and control of space free-flyers with multiple manipulators

This paper studies the motion control of a multiple manipulator free-flying space robot chasing a passive object in near proximity. Free-flyer kinematics are developed using a minimum set of bodyfixed barycentric vectors. Using a general and a quasi-coordinate Lagrangian formulation, equations of motion for model-based controllers are derived. Two model-based and one transposed Jacobian control algorithms are developed that allow coordinated tracking control of the manipulators and the spacecraft. In particular, an Euler parameter model-based control algorithm is presented that overcomes the non-physical singularities due to Euler angle representation of attitude. To ensure smooth operation, and reduce disturbances on the spacecraft and on the object just before grasping, appropriate trajectories for the motion of spacecraft/manipulators are planned. The performance of model-based algorithms is compared, by simulation, to that of a transposed Jacobian algorithm. Results show that due to the complexity of space robotic systems, a drastic deterioration in the performance of model-based algorithms in the presence of model uncertainties results. In such cases, a simple transposed Jacobian algorithm yields comparable results with much reduced computational burden, an issue which is very important in space.