Event-triggered sliding mode control of spinning tether system for fly-around observation

The technology of small fly-around satellites plays an important role in observing the large spacecraft. Traditional small satellite fly-around technology suffers from contradictions between mission sustainability and observation accuracy. To address these challenges, this study introduces a novel tethered fly-around scheme based on a spinning tether system (STS), which can be used for observing large-scale structures such as space stations and defunct spacecraft. This research focuses on the parameter design and stability control of the STS during the fly-around process. First, to describe the large-scale spinning motion of the STS in space, a singularity-adjustable Lagrangian model is developed using the spinning plane coordinate system. Additionally, a flexible lumped model is created for controller validation. Second, the constraints of tether length and spinning rate are analyzed to determine the optimal parameters for fly-around mission. Third, a sliding mode controller is proposed to track the desired spinning motion of sub-satellites. Considering that the STS does not require continuous control during periodic fly-around, an event-triggered control mechanism is introduced to reduce communication consumption. Finally, a comparison of impulse consumption across various fly-around schemes is conducted. Simulation results show that the STS achieves stable fly-around configurations under the proposed control strategy. The two fly-around satellites experience reductions of 86 % and 84 % in control trigger events, respectively. Furthermore, energy consumption analysis shows that tethered schemes reduce impulse by 89.2 % in planar fly-around and 86.5 % in vertical fly-around compared to untethered schemes.