Hybrid Position/Force Control of Direct Drive Robots by Disturbance Observer in Task Coordinate Space

Abstract

Since direct drive (DD) motors directly drive their loads without reduction gears or ball screws, high stiffness and little friction loss are expected. However, DD motors are more sensitive to the disturbance and the parameter variation than motors with reduction gears. This paper proposes a simple and high performance hybrid position/force control of robots based on disturbance compensation by using the disturbance observer in task coordinate space. The disturbance observer linealizes system of robot manipulators in task coordinate space and realizes acceleration control. To realize the strict acceleration control, the disturbance observer whose input is a position signal by simple computation, works as if it were a disturbance detector. The inverse kinematics can be simplified, because the disturbance observer in task coordinate space compensates not only the disturbance but also the error due to the simplification of the inverse kinematics. The new strategy is applied to a three-degrees-of freedom direct drive robot. The robust and simple hybrid position/force control is realized experimentally.