Enhanced Motion Control Concepts on Parallel Robots

Abstract

During the last years parallel robots have found their way into industrial applications. Though the ratio of workspace to designspace is usually worse compared to their serial counterparts, parallel robots are superior in terms of stiffness, accuracy and high-speed operation. This chapter takes the development into account and focuses on control concepts of parallel robots used for handling and assembly. To exploit these features, an effective control system is inevitable. Since the nonlinearities of parallel structures are not negligible, control schemes have to include a precise dynamic model. This chapter presents several approaches of model-based control laws and discusses their characteristics, in theory as well as in implementation. All discussed concepts operate on a uniform interface that takes a fully specified trajectory of position, velocity and acceleration in Cartesian space. This design of the interface can be considered as a minor restriction, since trajectories for high-speed operation usually are defined to be jerk limited (C-continuous) to reduce mechanical stress of the robot. The chapter starts with a brief description of the discrete modeling scheme, afterwards a compact formulation of the robots dynamics is derived. Several control schemes using this model are presented, which can be classified into two major groups depending on the usage of the robot model as feedback or feedforward type. Based on linearization techniques the controllers for each axis are designed independently within a linear framework. The control algorithms are augmented by disturbance observers to reduce distortion of trajectory and tracking error. Besides these classical approaches, nonlinear concepts such as sliding mode are used for control. Using a boundary layer concept and adding discontinuities to the control law ensures global asymptotic tracking with robustness against model uncertainties and disturbances. Chattering formally associated with sliding mode can be coped with modification of the control law by using continuous sliding surfaces. On contrary to the first approaches it is inherently based on nonlinear design. Considering properties of parallel robots the control schemes of described approaches are designed. Explicit design rules are given at hand and discussed. For experiments the concepts are implemented on a planar parallel robot. The unified approaches of modeling and control guarantee transfer to more complex robots. Evaluation of the results starts with a general comparison of control concepts. The effect of the design parameters on closed-loop system dynamics is analyzed theoretically, paying www.intechopen.com Automation and Robotics 18 special attention to robustness and performance as essential characteristics. To substantiate the statements of the theoretical analyses, experimental results are presented and evaluated with respect to different aspects. Cartesian distortion, tracking error, drive torques and their impact are of major concern. Finally, an overall categorization is given at hand, featuring application hints for each design concept and pointing out specific drawbacks and advantages.