Robust Control of Underactuated Manipulators: Analysis and Implementation

Underactuated manipulators are robot manipulators composed of both active and passive joints. The advantages of using such systems reside in the fact that they weight less and consume less energy than their fully-actuated counterparts, thus being useful for applications such as space robotics. Another interest reside in the reliability or fault-tolerant design of fully-actuated manipulators. If any of the joint actuators of such a device fails, one degree of freedom of the system is lost. It is usual in this situation to simply brake the failed joint and try to resume the task with less degrees of freedom available [7]. Following the methodology proposed in this work, the passive (failed) joint can still be controlled via the dynamic coupling with the active joints, and so the system can still make use of all of its degrees of freedom originally planned. The methodology proposed in this paper uses the dynamic coupling between the passive and the active joints 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.