Dynamic decoupling and control of a direct-drive manipulator

Direct-drive manipulators are highly sensitive to the linear and nonlinear inertial interactive forces when the actuators are mounted between adjacent links. This paper presents effective design methods for articulated direct-drive arms. A new approach is developed for arm linkage design. The goal is to simplify the controller design by reducing the effects of complicated manipulator dynamics and to improve the control performance. A method for designing an arm linkage with small inertial load changes and small interactive forces is developed by analyzing the kinematic structure, actuator locations and the mass properties. This design methodology achieves a decoupled and invariant manipulator inertia tensor in joint space. As a result, each actuator experiences completely decoupled dynamics as well as a constant load which is independent of the arm configuration. This dynamic behavior is particularly desirable for manipulators performing high-speed high-accuracy trajectory control tasks. These design guidelines are applied in the development of the articulated 3 d.o.f M.I.T direct-drive arm. The manipulator, with a decoupled and nearly invariant arm inertia tensor, shows a substantial improvement in control performance.