Kinematic Design, Dynamic Modelling and Control Studies of Revolute Planar Parallel Manipulator with Secondary Considerations

Abstract

Parallel manipulators, due to their several intrinsic advantages such as high payloadability, good stiffness characteristics and accuracy are widely used in various application areas including machine tools, motion simulators, micro robots, medical devices and at many other places. Main drawbacks in parallel manipulators are their relatively small workspace resulting from closed-loop configurations. This leads to low manoeuvrability with large number of singular regions. The motion of end-effector is further affected by various factors such as joint clearances, link flexibilities etc. During trajectory tracking task, several unknown external disturbances and internal uncertainties affect the endeffector motion considerably. Various techniques for improvising the kinematic and dynamic characteristics are usually first tested on planar manipulators due to their simplicity and ease of control.

Present work focuses on the kinematic and dynamic analyses and control studies of 3-RRR planar parallel manipulator to improve the design and stability characteristics by considering various practical aspects. A user graphics interface is first developed for obtaining various kinematic characteristics. Forward and inverse kinematic solutions of the manipulator are obtained by a neural network model and via an optimization approach based on genetic algorithms. Further, in order to enhance the kinematic performance characteristics such as dexterity, stiffness and payload, constrained optimization-based formulation is proposed and various geometric design alternatives are reported. A 3PRRR (three active prismatic and revolute joints) kinematically redundant manipulator is selected and the optimal locations of three redundant prismatic joints are obtained by minimizing the active-joint torques without loss of workspace. Using three degree of freedom spherical joints a reconfigurable spatial parallel manipulator is designed with a maximum nine degrees of freedom to move in space. Its kinematic studies are illustrated in detail. Effects of joint clearances resulting from manufacturing tolerances, assembly process and wear and tear are accounted on the kinematic characteristics of a fully actuated 3-RRR manipulator and a method of stabilization is illustrated with a kinematically redundant 3-PRRR manipulator. Dynamic equations of manipulator are derived from Euler-Lagrangian mechanics for a rigid link system and inverse dynamics are validated with ADAMS solutions. By considering link flexibility, a similar set of equations is developed in terms of stiffness, mass and Coriolis (nonlinear internal forces) matrices and dynamic characteristics including natural frequencies and dynamic response are obtained. In order to minimize the dynamic reaction forces at the active joints, desig of reactionless mechanism is illustrated with counter weights. Optimal locations of their mass centres and angular positions are obtained by minimizing the reaction forces and torques at the ground joints. Trajectory control simulations of manipulator are conducted with different control schemes and a disturbance-observer based control scheme based on fuzzy and adaptive neuro-fuzzy inference systems (ANFIS) is proposed under different types of disturbance loads. A scaled prototype of the manipulator with servo drives is fabricated to understand the singular regions, open-loop control behaviour and dynamic characteristics including natural frequencies. Detailed summary of observations in this comprehensive study of the manipulator has paved the roots for some future directions.