A six degrees of freedom active isolator based on Stewart Platform for space applications
- Paper number
IAC-05-C2.2.01
- Author
Mr. More Avraam, University of Brussels, Belgium
- Coauthor
Mr. Bruno De Marneffe, University of Brussels, Belgium
- Coauthor
Prof. André Preumont, University of Brussels, Belgium
- Coauthor
Dr. Iulian Romanescu, University of Brussels, Belgium
- Year
2005
- Abstract
Introduction
The future interferometric space missions will require more and more stringent conditions and higher order of accuracy. The sensitive optical payloads need to be sufficiently isolated from the disturbances produced by the reaction wheel assembly and the cryocoolers while having the ability to be slewed and positioned successfully in quasi-static conditions. In order to achieve this goal, an active vibration isolator based on a Stewart platform has been designed, manufactured and tested at the ULB.
Technical description
The six-axis vibration isolator consists of six independent legs orthogonal to each other, connecting a base plate (attached to the disturbance source) and an upper plate (attached to the payload); this architecture is known as a Stewart platform. Being very flexible, the isolator cannot be tested on ground without special equipments for gravity compensation (introducing disturbances on the measurements).The isolator has the form of a Stewart platform with cubic architecture. Each of the six legs consists of an axial spring acting in parallel with an electromagnetic actuator (voice coil), while a force sensor measures the total force in the leg. A decentralized feedback controller connects the force sensor to the current in the electromagnetic actuator in order to realize a "sky-hook" damper. Such a controller allows a high frequency attenuation rate of -40dB per decade, combined with a low overshoot at the corner frequency. Such a behavior cannot be achieved via a passive isolator.
The legs of the platform are the key elements of the system. They govern the isolator corner frequency, as well as the efficiency of the control system and the undesirable, performance-decreasing, local dynamics of the hexapod. These local dynamics were at first underestimated. Previous experiments have however shown that they must be taken into account during the design phase in order to obtain good performances over a large frequency range. A new platform implementing radical changes in this design, lowering the moving mass and the number of components has thus been implemented and tested during a parabolic flight campaign in October 2004.
Measurements in micro-gravity have shown that this new design leads to much better performance of the overall system. These changes have moreover made the leg much more resistant, which is crucial as the isolator has to resist the loads that occur during launch.
- Abstract document
- Manuscript document
IAC-05-C2.2.01.pdf (🔒 authorized access only).
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