Combined Remote Sensing Methods for Active Charge Control Using an Electron Beam in a Dynamic Space Environment

Active charge control of a target object allows a servicing spacecraft to control the electrostatic forces and torques experienced by the neighboring crafts. Driving the potentials to zero eliminates these perturbations for safer proximity and rendezvous operations while maintaining non-zero potentials yields non-contract forces, allowing the servicer to manipulate the motion of the target. The latter is interesting as a servicer with the ability to contactlessly move target objects is extremely beneficial for active debris removal. Closed loop control of a target's potential and methods for remotely sensing a target's electrostatic potential (the Secondary Electron and X-ray Methods) have been investigated, but only preliminary work has integrated both methods together. While prior work demonstrates offline sensing methods, this paper develops a new method for sensing a target's potential that is suitable for real-time potential measurements. The solution is demonstrated in active charge control vacuum chamber experiments. It is shown that, for this new Dual Method, by using the X-ray Method to inform the secondary electron method, quick and accurate potential control is achieved, combining the positive attributes of the two methods. Vacuum ultra-violet lights are also used to apply a constant current bias and evaluate both the controller and remote sensing methods in a non-ideal and dynamic charging environment. The inclusion of this photo-electron current is shown to not alter the ability of the control to converge, but does affect beam energies required to achieve a specific target potential.