Fast slew maneuvers for the High-Torque-Wheels BIROS spacecraft

Abstract

The satellite platform BIROS (Bi-spectral InfraRed Optical System) is the second technology demonstrator of the DLR R&D `FireBIRD' space mission aiming to provide infrared remote sensing for early fire detection. Among several mission goals and scientific experiments, to demonstrate a high-agility attitude control system, the platform is actuated with an extra array of three orthogonal `High-Torque-Wheels' (HTW). For agile reorientation, however, a challenge arises from the fact that time-optimal slew maneuvers are in general not of the Euler-axis rotation type, specially whenever the actuators are constrained independently. Moreover, BIROS' On-Board-Computer (OBC) can only accommodate rotational acceleration commands twice per second. Our objective is therefore to find a methodology to design fast slew maneuvers while considering a highly dynamic plant commanded by piecewise-constant sampled-time control inputs. We do this by considering a comprehensive analytical nonlinear model for spacecraft equipped with reaction wheels and transcribing a time-optimal control problem formulation into a multi-criteria optimization problem which is then solved with a direct approach in a sequential procedure using the trajectory optimization package `trajOpt' of DLR-SR's optimization tool MOPS `Multi-Objective Parameter Synthesis'. Our approach for efficient design of rest-to-rest fast slew maneuvers considers an attitude error whose magnitude is proportional to Euler-axis rotations between current and desired attitudes even for large initial attitude errors. Results based on numerical simulations are presented to illustrate our method.