PhD Research

Abstract:

Rotating components such as reaction wheels and momentum wheels on spacecraft generate micro-vibrations up to 250 Hz during on-orbit operations. While the use of Gough-Stewart Platforms (GSPs) for six degrees of freedom (DOF) vibration isolation has been proposed, a conventional GSP lacks dynamic isotropy, hindering effective vibration attenuation. A dynamically isotropic mechanism, characterized by equal or nearly equal first six natural frequencies is useful for effective vibration isolation, as it enables the attenuation of the first six modes of vibration effectively from a sensitive payload. Consequently, a dynamically isotropic Modified Gough-Stewart Platform (MGSP) where the first six natural frequencies are equal has been investigated in this work to redress this limitation in conventional GSPs.

This work deals with the design of an MGSP for dynamic isotropy. The analytical expressions for the design variables in their explicit form are derived from a geometry-based approach, which in turn results from the properties of two triangles facilitating straightforward design. The approach accommodates various payload configurations, including variable center of mass and mass/inertia properties. Validation through finite element software ANSYS ensures similar natural frequencies across six modes, crucial for effective isolation. Practical viability is enhanced by incorporating flexural joints and structural damping in the design. A prototype of the MGSP featuring flexural joints was tested for a 10 kg dummy payload at the Indian Space Research Organization (ISRO) vibration test facility, and it yielded experimental outcomes in close agreement with the finite element analysis results — the first six natural frequencies were close to the expected 29 Hz and vibration isolation of about 22 dB/octave. The close agreement among analytical, finite element, and experimental outcomes underscores the efficacy of our design approach and the suitability of an MGSP for micro-vibration isolation applications in spacecraft.

Our study extended the geometry-based approach to include the design of dynamically isotropic MGSPs with more than six legs. In practical scenarios, the preference for a higher leg count (>6) stems from the need to distribute heavy loads across multiple actuators or to enhance fault tolerance capabilities. This can facilitate an economical approach to modifying previously developed dynamically isotropic 6-6 MGSPs into dynamically isotropic configurations with more than six legs with enhanced payload capacity.