Experimental Investigation Results on the Impact of Gravity During the Characterization of Reaction-Wheel Caused Micro-Vibrations in a Laboratory Setup

Abstract

Spacecrafts operating in orbit benefit vastly from the disturbance-free environment to achieve the designated mission goals, either as remote systems for observation and communication purposes or to conduct missions in micro-gravity. These space systems will require mechanisms to point their payloads to the point of interest, like a star or a patch of land on Earth, or to maintain operation without unnecessarily changing the attitude, like reorienting their solar panels along an orbit. These mechanisms, generally moving components like motors, pumps, or rotating masses for attitude control systems, will generate disturbances, also called micro-vibration or jitter, through the whole space system if not mitigated. It is crucial to characterize the source to understand better the disturbance's origins and the technological possibilities to mitigate the impact it may have later during operation. A common practice to characterize reaction-wheels is to mount them on dynamometers to measure the forces and torques generated over a wide bandwidth, with the rotation axis aligned with the gravity vector. In contrast, another possibility is to perform an integrated test in which the test uses operating reaction-wheels on an integrated or semi-integrated spacecraft suspended from the ceiling in a flight-comparable configuration. However, by combining and applying both methods during the development of spacecraft and mechanisms, there may still be some uncertainties caused by non-linear effects caused by the system complexity of the mechanisms and systems used. In this study, we focused on the impact of changing the gravity-vector direction during the characterization of reaction wheels in a laboratory by comparing the results under varying conditions. We rotated the measurement setup, including the dynamometer and the reaction-wheel, against the gravity vector to measure the generated micro-vibrations. Furthermore, we investigated the existence of behaviors that would only show if geometry- or constraint-caused non-linear behaviors were affected by gravity. Additionally, we evaluated the feasibility of this method for characterizing small and medium-sized reaction wheels. Lastly, this method allows the validation or verification of other simulations and numerical models or tests during a spacecraft's assembly and integration phase. In a broader context, this investigation may show the test possibilities to be considered during the development of both new space systems and actuators, especially as the later ones may get more complex in geometry or design of the components assembly. As such, once the dynamic mass of the actuators gets larger compared to the spacecraft's mass operating these, the impact of even small micro-vibrations may increase in relevance. Finally, evaluating mechanisms under different environmental conditions, such as gravity, as shown in this study, may benefit future missions and spacecraft configurations.