An Approach to Point-to-Point Control for Direct-Drive SCARA-Type Robots

Abstract

In recent years, attention has been paid to direct-drive for industrial robots which are essential to factory automation. Direct-drive robots, which are directly actuated with no gear reducer, have no backlash, low friction and good mechanical stiffness. This results in the excellent repeatability of positioning, that is, an order of magnitude better than that of gear-drive robots. It is, however, more prominent in the design of control system for direct-drive robots that the arm inertia variation depending on the arm configuration and the Coriolis and centrifugal torques are directly reflected to the motor axes due to the direct coupling.
In this paper, a direct-drive robot with a two degree-of-freedom is developed for planer assembly tasks. The motion of the manipulator arm is constrained within a horizontal plane, so that this type of robot is referred to as a SCARA (Selective Compliance Assembly Robot Arm)-type robot. A balanced manipulator arm, the purpose of which is to cancel the inertia variation and nonlinear effects, is introduced to the direct-drive robot, thus leading to improved control performance as well as simple manipulator dynamics.
In addition, the combination of feedforward compensation and a new approach to positioning control is proposed to perform stable and fast positioning without overshooting.