Design and Control of a Novel 3-DOF Flexible Robot, Part 2

This paper aims to present the design of a practical controller for a flexible, industrially competent, robot arm. The language used is that of a practicing control engineer, with emphasis on the physical in sights useful to the designer during the design process. This contrasts with most experimental results presented in the literature, which are usually for one- and two-link planar arms and are amenable to closed-form analysis and simulation. Such arms also manifest limited performance with respect to rigid-body manipulators, and underline the difficulties in extending controller design to more than two flexible links. Furthermore, experimenters invariably rely on the efficacy of the control algorithm, rarely exploiting their understand ing of control theory to add mechanical design features to ease the control task. The work presented in this paper centers on a three-link (spatial) flexible and industry-sized prototype arm, the Rotabot, and the purpose is to demonstrate that the control task can be simplified by using control-theory-driven mechanical design. The special de sign features of the Rotabot facilitating ease of arm dynamic control are presented in a companion paper, Part 1. The final control task, despite the care taken with the arm design, was hampered by driver nonlinearities such as torque ripple, Coulombic friction, and pure time delays within the commutation circuitry of the motors. The use of direct drive means that drive nonlinearities, which tend to have high bandwidths, are closely coupled to the vibration characteristics of the arm—a problem that tends not to arise in geared robots. These nonideal drive characteristics had to be dealt with by the controller. Simulation and experimental results are presented to demonstrate the feasibility of controlling such an arm and pave the way for a new practical research path on flexible manipulators.