Feedback Linearized Joint Torque Control of a Geared, DC Motor Driven Industrial Robot

Abstract

Implementation of advanced model based robot control algorithms necessitates that joint actuators be capable of generating com manded joint torques. This capability permits compensation of gravity torques, for example. Relatively recent work reported in the robotics literature has focused on development of load torque sensing and control of robots actuated with permanent magnet DC motors and harmonic drive gear reducers. This development in troduces the possibility of the retrofit of a large class of small- to midsized industrial robots with advanced model based controllers, which require that the commanded torque signal be reproduced at each robot joint. In this paper, the authors establish theoretically that the direct application of computed torque control to a geared, DC permanent magnet actuated robot with local joint load torque controllers does not lead to decoupled dynamics, as is the case with direct drive robots. The resultant dynamics are not decoupled due to the interaction of the computed torque control with the inner joint torque control loops. To achieve decoupled system behavior, a modified computed torque control law is developed that leads to decoupled system dynamics. Experimental results are presented that compare the modified computed torque control with the stan dard computed torque control. The experiments are conducted on a commercial 6-DOF robot that has been retrofitted with joint torque control on the first three joints. Tests have been conducted at a con trol update rate of 1000 Hz. The experimental results illustrate that the modified computed torque controller performance is somewhat better than the conventional computed torque control approach.