Robust Control of Underactuated Manipulators: Analysis and Implementation

Abstract

Underactuated manipulators are robot manipulators composed of both active and passive joints. The advantages of using such systems reside in the fact that they weigh less and consume less energy than their fully-actuated counterparts, thus being useful for applications such as space robotics. Another interest resides in the reliability or fault-tolerant design of fully-actuated manipulators. If any of the joint actuators of such a device fails, an entire operation may have to be aborted because of the loss of one or more degrees of freedom. The methodology proposed in this paper uses the dynamic coupling between the passive joints and the active joints, and controls the active ones in order to bring the passive joint angles to a desired set-point. Therefore, the control law and the performance of the system are completely dependent on the dynamic model. Since it is difficult to obtain the exact dynamic model of the system in general, considerable position errors and even instability can result in some cases. In this paper, we propose a variable structure controller to provide the system with the robustness necessary to perform tasks regardless of the modelling errors. Case studies are provided as a mean of illustration.