Summary

Description

Reaction wheel mechanical noise is one of the largest sources of disturbance forcing on space-based observatories. Such noise arises from mass imbalance, bearing imperfections, and other sources. It takes the form of a number of discrete harmonics of the wheel speed, often also with a broadband noise component. Jitter problems can arise when harmonics sweep across observatory modes, and can be exacerbated by gyroscopically coupled spin-rate-dependent wheel structural modes that dynamically amplify the tonal and broadband disturbances. For a well-balanced wheel, higher harmonic forces can be on the same order as the fundamental, therefore when there is a jitter problem it can occur at very low wheel speed. These higher harmonics are generally less well-characterized than the fundamental. The proposed Reaction Wheel Disturbance Model Extraction Software (RWDMES) is a tool for fitting a hybrid physical/empirical model to wheel induced-vibration data. The physical model captures the wheel structure including gyroscopic effects, while the empirical model captures the harmonic forcing and broadband noise. The Phase I effort demonstrated the ability to fit a highly accurate harmonic/broadband/structural model, including 43 harmonics up to 14.63 times the fundamental, to measured wheel disturbance data in a point-and-click environment in about 2 hours. The benefits of the technology include reduced program effort to produce wheel disturbance models, leading to more accurate jitter prediction earlier in a mission. This in turn allows jitter problems to be mitigated at the design stage when changes are relatively inexpensive.