Sampling-Aware Multi-Rate Combined Control for an Orbital Manipulator

In on-orbit servicing missions using robotic manipulators, certain challenging scenarios require the use of combined control i.e. actuation of spacecraft and the manipulator, to meet mission requirements. The low frequency of the controller of the spacecraft compared to the manipulator can compromise the stability margin of the combined control. In this paper, we first design a combined control strategy to carefully decouple the high-rate manipulator control from the spacecraft's low-rate control. Second, we design a novel discrete controller accounting for the first-order effects of the servicer's low sampling rate. This is realized by augmenting a classical proportional-derivative (PD) control scheme. The operational bounds of the discrete controller are first benchmarked on a one-DoF system and further investigated for performance using a multi-DoF orbital manipulator. The results shed light on the regions of enhanced performance in terms of stability and impulse utilization as a measure of efficiency. Simulation results and hardware-in-theloop experiments are performed to validate the proposed method.