AN INTEGRATED COMPUTATIONAL & EXPERIMENTAL STUDY OF STIFFNESS BEHAVIOR OF 6-DOF PLATFORM TOP PLATE SUBJECTED TO VARIOUS JOINT & LOADING CONDITIONS

Abstract

Research and development of various parallel mechanism/manipulator applications in engineering are now being performed more and more actively in every industrial field. Simulation driven designs are used for the efficient development of these high precision devices. Accurate prediction of these simulations depend upon the fact that how closely actual conditions are incorporated in the analysis. Stiffness is a key property for parallel platforms as it is directly affects the positional accuracy of system. It is directly related to deformations in mechanisms. These deformations in parallel platforms have adverse effects on static and fatigue strength, reduces wear resistance & efficiency in terms of frictional losses. Thus compromising the dynamic stability and accuracy of the system. Modeling the static and dynamic stiffness is the lead up for optimizing the mechanical structure, which in turns aides in development of efficient control system. Overall stiffness of the platforms is not only driven by the stiffness of the links but also by contact stiffness of joints. For large heavily loaded manipulators, top plate presents itself as critical component; as it shares a major percentage of the cumulative mechanism's weight. Consequently, top plate stiffness will dominate the overall stiffness of platform throughout the regular workspace. This makes the operational accuracy and safe working of the system largely dependent on accurate modeling of top plate's stiffness. In this thesis, an accurate stiffness modeling methodology is proposed for special application of 6-DOF platforms i.e. subjected to heavy loads. Effect of joint contact conditions on both static and dynamic stiffness behavior of platform's top plate is analyzed. Top plate was designed using simplified loading and joint contact conditions. A prototype was developed using this design and failed when tested on operating conditions. Detailed design was carried out using actual loading conditions and introducing the realistic joint contact formulation. Resulting stiffness parameters obtained were found to be in good agreement with experimental results. The findings of this work are used as an additional index to find an optimum compromise between a lightweight design and the stiffness performance of top plate by performing multi-objective structural optimization. Design of the top plate was optimized for size and shape in order to minimize the mass in motion while guaranteeing a desired stiffness through-out a v regular workspace of the mechanism. Stiffness behavior of optimized structure matched the experimental results when developed and tested.