Design and implementation of a sliding-mode disturbance observer for robotic manipulators with unbalanced rotating payloads

Robotic manipulators with unbalanced payloads rotating at nearly constant speeds can often be found in automated manufacturing tasks, such as drilling, grinding and deburring. In this paper, a sliding-mode disturbance observer (SMDOB) is designed for such systems that are subject to unknown periodic disturbances due to unbalanced rotating payloads. Stability of the closed-loop system, in which the SMDOB is in combination with a nominal controller, is addressed. Besides, this paper presents an adaptation algorithm for automatic switching-gain adjustments. To alleviate chatter, this algorithm is able to decrease switching gains according to practical operating conditions of robotic manipulators. An experimental study on a two-link direct-drive robotic manipulator was conducted. The experimental results are reported in this paper with a comparative study, realistically showing the efficacy of the proposed system.