Design and control of stewart platform
Uluçay, Özer (2006) Design and control of stewart platform. [Thesis]
Demand on high precision motion has been increasing in recent years. Since performance of today's many mechanical systems requires high stiffness and accurate positioning capability, parallel manipulators gained popularity. Their superior architecture provides better load capacity and positioning accuracy over the serial ones. In this work, a popular parallel manipulator, Stewart Platform, has been studied. Stewart Platform is a positioning system that consists of a top plate (moving platform), a bottom plate (fixed base), and six extensible legs connecting the top plate to the bottom plate. This work includes design, analysis, control and testing of a complete positioning system. In order to achieve better accuracy over commonly used universal joints, magnets and spherical joints have been employed in the architecture. Flexure joints have been also analyzed to achieve higher precision levels. Because they eliminate friction and backlash, flexure joints would give better results than universal and spherical joints. Therefore, a parametric study for optimum hyperbolic flexure joints has been also presented. Desired performance of the proposed platform is six degrees of freedom with 500 nm minimum incremental motion. System uncertainties and inherent nonlinearities have been eliminated using sliding mode control. This type of control methodology allows stability in the presence of parametric uncertainties and external disturbances. Although analyses indicate system capability of less than 1 nm resolution, real performance is typically lower due to manufacturing imperfections and measurements errors. The proposed model has been analyzed and tested through simulations, and verified via experiments on a designed and constructed sample Stewart Platform. Laser measurements have been used to measure positional accuracy. System has demonstrated positional accuracy better than the desired 500 nm target performance value.
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