Linear active disturbance rejection control with linear quadratic regulator for Stewart platform in active wave compensation system

Abstract

Offshore operations are vulnerable to the vessel motions caused by waves in harsh sea conditions. To compensate for the wave-included motions of the vessel, the shipborne Stewart platform with a gangway mechanism offers an effective means to enhance operator safety and extend the window period for offshore activities. The gangway endures off-center heavy loads and low-frequency vibrations, while the shipborne Stewart platform faces time-varying ship motions caused by waves. In addition, there is a strong motion coupling between the limbs of the Stewart platform. These challenges pose a formidable task in attaining precise control accuracy for wave compensation. In this study, a linear active disturbance rejection control with a linear quadratic regulator is proposed for the shipborne Stewart platform. The original proportional-derivative gain is substituted with a linear quadratic regulator (LQR), thereby effectively addressing the previous challenge of channel parameter tuning. Additionally, a linear extended state observer is devised to enhance system robustness by estimating and counteracting overall disturbance. The proposed controller is designed based on joint-space and its stability is verified using the Lyapunov theory. Simulation results validate that the proposed controller demonstrates superior performance in terms of compensation accuracy, anti-disturbance capability, and decoupling effect compared to the PI and LQR controllers.