Magnetic Actuator in Space and Application for High Precision Formation Flying

摘要

Electromagnetic (EM) actuators in space applications are not a new idea but they are most of the time associated to low Earth orbit missions, where the on-board magnetic moment interacts with the Earth magnetic field. More recently EM actuators have been studied in the context of formation flying as a way to generate inter-spacecraft force and torque in order to control the formation geometry (relative position and attitude of the spacecrafts). With respect to other possible actuators (like FEEP and cold-gas thrusters), EM actuators do not require propellant (hence ensuring longer lifetime) and do not cause contamination problems, but are effective only in a limited range of distances (due to the magnetic field decreasing as 1/r(exp 3)). In this paper we consider a setup where a spacecraft (called the hub) generates the magnetic field while the other spacecrafts (called the flyers) modify their on-board magnetic moment in order to generate the desired force and torque. We show that if the magnetic field generated by the hub is constant it is not possible to generate any combination of force and torque (there are forbidden directions, independently of the magnitude of the desired force and torque) and we present a possible solution based on a rotating field. The main idea is to have a time- varying magnetic field generated by the hub (chosen as a rotating magnetic dipole) and time-varying magnetic moments on the flyers. The variation law on each flyer's magnetic moment is computed in order to obtain the desired force and torque on average (over one rotation period). With such approach, the instantaneous force and torque may differ from the desired ones, these discrepancies being "filtered out' by the spacecraft inertia.