A parallel manipulator is a closed-loop mechanism in which the end-effector is connected to the base by at least two independent kinematic loops. A general description of these types of manipulators is explained with examples and applications. With multiple closed loops, stiffness of the manipulator is typically improved because the multiple leg connectors sustain the payload in a distributive manner. Off-late the re-configurability of the platforms gain more research interest among researchers for its increasing practical applications in industries. The commercial hexapods that are available in the market are mission specific with no choice offered between structural rigidity and dexterity to use the same platform for other applications.
In this chapter, an effort is made to characterize the parameters for developing a reconfigurable Stewart platform for contour generation and vibration isolation applications. The limited treatment of the platform characteristics leads to the lack of an efficient methodology for determining the optimum geometry for this task. A solution is provided through the formulation of dimensionless parameters in combination with a study on the generic parameters like configuration. The variable geometry approach for the reconfiguration of Stewart platforms is adopted in detail for four different platforms, and a generic approach is formulated after studying different parameters.
A stiffness model developed for contour generation application is used in tandem with this generic approach to identify the trajectory with maximum stiffness for complex contours. An extensive study on the effect of the identified parameters on the performance and characteristics of Stewart platform for both the applications are performed. Simulations, in order to study different contours are performed to obtain the trajectory with maximum stiffness. The configuration best suited for contour generation also is identified and the data set required to move the tool for a specific trajectory is also identified. The effect of the identified dimensionless parameter on the system performance is also studied. The parameters are tested for vibration isolation application also and the results are used in designing the test rig.
The Stewart platform test rig developed is fed with the dataset obtained for different trajectories and error analysis is performed to validate the simulation results. An algorithm is developed on the basis of kinematic path control for conducting the experiments to find the tracking performance. The results are used to establish the importance of dimensionless parameters for reconfiguration of Stewart platform. It is proposed that this methodology could be adopted for any application to develop a complete set of design tool for any new reconfigurable Stewart platform. Experimentations to identify the natural frequency of the developed Stewart platform were performed to ascertain the frequency used for simulation studies. A novel concept of Multilevel reconfigurable Stewart platform is introduced to overcome difficulties and also for effective performance in both the applications.
Keywords: Stewart platform, reconfigurable, parallel manipulator, robot, characterization, contour generation, trajectory, vibration isolation, cubic configuration, hexapod, joint vector, maximum stiffness.
