Flexibility effects in controlled behaviour of space manipulators

This paper aims to show the relevance of flexibility effects in the modeling of space manipulators dynamics, by investigating a set of possible realistic maneuvers. Design, synthesis and implementation of the control laws should be exploited referring to a model that accurately reproduces the actual plant, in order to plan and safely execute maneuvers such as the grasping and recovery of a target satellite. Space manipulators, generally represented as multibody systems, operate under different environmental conditions, including the dominant gravitational effect, the relevant gravity gradient and perturbing forces and torques such as the differential atmospheric ag. On account of different control laws options, different values of inertial forces can arise and consequently different amplitudes of the elastic displacements can be observed during the motion of the manipulator arms. Actually, the addition of the flexibility effects complicates the analytical model of a spacecraft and, while improving the accuracy of the model itself, it causes a noticeable amount of computational work, which is very unwelcome in a near real-time operations perspective. The choice of introducing the flexibility on the modeling of a space manipulator depends on the type of the maneuver the spacecraft must perform, on the elastic properties and on the geometrical characteristics of the arms. This paper will focus on the relationship between control commands and elastic displacements, assessing the cases when it is possible to neglect them and when, on the contrary, an active vibration damping is needed.