Design, dynamic modelling and experimental validation of a new three-degree-of-freedom flexible arm

Abstract

This article describes the design, dynamic modelling and experimental validation of a new three-degree-of-freedom flexible arm which is suitable for industrial applications. The arm was designed on the assumption that all its mass was concentrated at the tip. In keeping with this, the arm was built with very lightweight links, and all the actuators were located at its base. A second feature is a special mechanical configuration that approximately decouples radial tip motion from angular tip motions; i.e. the first actuator produces an azimuthal movement, the second actuator produces an elevation movement and the third produces an approximately radial movement. From the dynamics point of view, this new mechanism also decouples the forces transmitted by the actuators to the tip of the arm, i.e. the torque generated by each actuator produces a tip force which is orthogonal to the forces produced by any of the other actuators. On the assumption that all the mass is concentrated at the tip, a compliance matrix can be used to model the oscillations of the structure. Then, dynamics of the arm becomes very simple (a lumped single mass model instead of the usual distributed mass model), and its control system can be significantly simplified: minimum sensing effort is required (only motor and tip measurements are needed), and PID controllers combined with a dynamic inversion subsystem can be used. These design specifications have been experimentally validated on the prototype we have built. Experimental identification of the dynamics has confirmed the validity of the assumption of a single mass concentrated at the tip. A flexible manipulator of simplified dynamics has been built and tested which is much lighter than equivalent standard industrial robots.