Feedback linearized joint torque control of a geared, DC motor driven industrial robot

Implementation of advanced model based robot control algorithms necessitates that joint actuators be capable of generating commanded 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 introduces 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 nor 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 standard 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 control 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.