Wire-driven parallel robotic system and its control for maintenance of offshore wind turbines

This paper describes a wire-driven parallel robotic system for the maintenance of offshore wind turbines. This robotic system vertically climbs on towers or blades, and performs cleaning and inspection using waterjets and phased array ultrasonic testing (PAUT) devices, respectively. For a detailed mechanical design, mechanisms for (1) climbing towers or blades, (2) transition motion between towers and blades, (3) gripping blades, and (4) contact motion between PAUT devices and blade surfaces are proposed so that the robotic system can maneuver on towers and blades. For the motion analysis of the robotic system, nonholonomic constraints are analyzed and utilized in height and attitude control schemes, which are proposed to adapt varying nonholonomic constraints of blade surface and achieve the climbing motion on towers and blades. To validate these control schemes, we conduct experiment and analyze the performance of the proposed control schemes. The results show that the height and attitude control schemes for the robotic system do achieve the control goals and overcome the control challenges.