Analysis of guidance and control laws for orbiting multibody manipulators

The high risk and cost of manned missions calls for a renovated interest in automatic missions and in space robotics. The analysis of guidance and control performance of space manipulators needs an intensive campaign of numerical simulations carried out by means of a suitable mathematical model, taking into account different forces such as gravity and gravity gradient, elastic forces, possibly the air drag and of course the inertial forces. The model should allow for an efficient solution of the inverse kinematics, exploited to attain a desired final state. At the same time, the model should preserve the correctness of the solution which is seriously affected in the specific case by numerical problems, due to the different scales of the phenomena occurring: gravity, related to the orbital radius; gravity gradient, related to the manipulator overall dimensions; and flexibility, related to the characteristics of each link. The paper, after a short recall of a proper dynamic model of a multi-body system solving these numerical issues, will investigate in detail the GNC loop architecture. The working scheme is based on the subdivision of the proposed mission in a number of operational phases, each one characterized by different issues. For example, the deployment of the robotic arm and the recovery of the target load is accomplished via a path planning optimization algorithm for the evaluation of the reference motion, realized by means of two possible control laws, i.e. the classical proportional derivative controller and the feedback linearization technique. The grasping or docking phase requires instead the online measurements of the relative distance between end effector and target. The performances of the GNC loop are assessed in terms of the accuracy of the final state achieved, of the time required to complete the maneuver and of the power consumption needed by the actuators. Several simulations are presented in the paper for different cases, all of them supposing a Low Earth Orbit maneuver.