Flight test results for microgravity active vibration isolation system on-board Chinese Space Station

Microgravity research is crucial because Earth's gravity obscures subtle effects like capillarity and van der Waals forces that become dominant in space. These effects influence fluid behavior, heat transfer, and more, making microgravity environments essential for understanding fundamental fluid physics. Many fluid physics experiments are conducted on space stations, but vibrations from the station itself can interfere with delicate experiments. Therefore, vibration isolation systems are necessary to maintain the desired microgravity level for research. The International Space Station (ISS) already employs various systems such as the Active Rack Isolation System (ARIS) and Microgravity Vibration Isolation Subsystem (MVIS). This paper focuses on the MAVIS system aboard the Chinese Space Station (CSS), a new system designed to improve the stability of the microgravity environment within the FPR, enabling more precise and reliable scientific experiments in the field of fluid physics and related areas such as space material preparation and space biological processes.

Previous research highlights the impact of vibrations and g-jitter on diffusion-controlled processes in microgravity environments. Studies on the ISS, such as the Influence of Vibrations on Diffusion in Liquids experiment, demonstrated that imposed vibrations can influence diffusion kinetics. The acceleration levels required for various fluid physics experiments vary widely, ranging from 1–100 µgo for phase change experiments to 10,000 µgo for rheology of non-Newtonian fluids. Existing vibration isolation systems on the ISS, including ARIS and MVIS, have been successful in mitigating some of these disturbances. However, the design and implementation of effective vibration isolation systems remain a challenge, especially considering the complexities of space environments and the need for high levels of vibration attenuation.

The MAVIS system consists of a stator fixed to the FPR and a floater supporting payloads, connected only by umbilicals. Non-contact electromagnetic actuators, accelerometers, and displacement transducers control vibration isolation. The system addresses the challenge of umbilical stiffness by incorporating stiffness correction into the controller design. The study used multiple coordinate systems (geocentric equatorial inertial, CoM orbit, spacecraft-fixed, stator-fixed, and floater-fixed) to model MAVIS dynamics, applying both Newton-Euler equations and rigid body composite motion characteristics. A nonlinear model was linearized for controller design and analysis. Two control strategies were employed: Single-loop displacement-based control (SDC) and two-loop impulse-averaging acceleration-based and displacement-based control (TIADC). The SDC controls floater position relative to the stator, while the TIADC includes acceleration feedback for superior microgravity performance. The controller utilized PID control, with parameters determined based on desired performance and the closed-loop transfer function, incorporating typical frequency elements for effective vibration attenuation. The parameters were systematically tuned using a flowchart-driven process involving several iterations to meet pre-defined targets. Both strategies considered the umbilicals' impact on performance. The impact of umbilical stiffness on system bandwidth was assessed by comparing microgravity accelerations of the stator and floater under free levitation. The study also investigated the interaction between translational and rotational control. In-orbit tests lasted 13 days, involving self-checks, control algorithm testing, and testing in both microgravity and vibration excitation modes. Data analysis involved comparing measured accelerations of the stator and floater, evaluating control forces and torques, and examining the system's frequency response under different operating conditions.

The in-orbit tests validated the effectiveness of the MAVIS system. In the microgravity mode, MAVIS achieved a microgravity level of 1–30 µgo in the 0.01–125 Hz frequency range, surpassing requirements for many microgravity experiments. Disturbances above 2 Hz were attenuated by more than tenfold. In the vibration excitation mode, the system generated a minimum vibration acceleration of 0.4091 µgo at 0.00995 Hz and a maximum of 6253 µgo at 9.999 Hz, demonstrating its ability to generate controlled vibrations of specific frequencies and amplitudes. The tests revealed that umbilical stiffness significantly impacts the system's vibration isolation performance and that the pretensioning force and torque of the umbilicals are approximately equal and opposite to the output force and torque from the controller under steady-state closed-loop control. Analysis of the control algorithm showed that TIADC generally outperformed SDC, particularly at higher frequencies. The use of a negative coefficient for the proportional term of the PID controller in the displacement control loop proved effective in improving microgravity acceleration levels. The coupled nature of translational and rotational motion highlighted the need for integrated control system design.

The successful in-orbit testing of MAVIS demonstrates the feasibility of achieving high-precision microgravity environments on the CSS. The results confirm the effectiveness of the control strategies and system design, highlighting the importance of considering umbilical stiffness and the coupled nature of translational and rotational motions. The achieved microgravity levels and vibration generation capabilities significantly enhance the capabilities of the FPR, enabling a wider range of microgravity experiments. These findings contribute to the advancement of microgravity research and pave the way for future improvements in vibration isolation technologies for space-based experiments.

The MAVIS system, successfully tested aboard the CSS, provides a stable microgravity environment for conducting fluid physics experiments. Its dual control strategies effectively handle microgravity and vibration excitation modes. Future research could focus on further minimizing the impact of umbilical stiffness and exploring advanced control algorithms to achieve even higher levels of isolation.

The study's scope is limited to the MAVIS system on the CSS. The generalizability of the findings to other vibration isolation systems or different space environments may require further investigation. The umbilical stiffness modeling could be refined by considering more advanced methods to account for its nonlinear and hysteretic characteristics.