Control of an Electrohydraulic Stewart Platform Manipulator as Vessels Motion Simulator

This work considers the use of Stewart Platforms with hydraulic actuators as a tool for developing motion control algorithms in open-sea applications, such as cargo transfers between ships and floating platforms, whose main goal is to attenuate the undesired relative oscillations caused by waves and tides. The Stewart Platform is one of the most representative examples of parallel manipulators, comprising two parallel bases connected by six linear actuators. The coordinated movement of these actuators allows both bases to have relative motions with six degrees of freedom, with superior precision and load/weight ratios when compared to serial manipulators. In this context, this work is motivated by the ongoing development of two coupled hydraulically driven Stewart Platforms. The first one is intended to emulate the movement of a ship deck under the influence of sea waves, while the second one, positioned on top of the first, will be used as a compensation device for controlling the motion imposed by the first one. This paper focuses on describing the design considerations of the lower platform. In the first part of the paper, we describe the main features of this research project. Then, we focus on its current development stage, which regards the mathematical modeling of the first platform and the design of its corresponding control algorithm. The proposed controller takes into consideration the manipulator’s mechanical and the actuators’ hydraulic dynamics, using the computed-torque control approach [9,10]. Lyapunov theory is used to guarantee the closed loop stability of the system. The control performance is verified by means of computer simulations results, which indicate that the proposed strategy yields good performance in leading the first platform to emulate a large range of motions that are compatible with vessel heave movements.