Hybrid Force-Position Control of the Motion Compensated Gangway

Abstract

Motion compensation for personnel transfer has greatly reduced costs and increased accessibility for offshore assets operation and maintenance. Solutions exist using hexapods or gangways that make a stiff connection by clamping to a customized offshore structure. State of the art gangways allow for a stepless workflow and have a rubber tip which does not require specialized landing structures. However, in these solutions contact force variations are high. This results in a reduction of safety due to the loss of contact or slipping motions. Therefore, a system with force sensors in the tip is proposed. This allows for the tip forces to be used in feedback control. Literature on this subject describes hydraulic actuator force control, impedance control, or force control in free moving robotics. Literature fails to describe suitable solutions to the hydraulic force control problem where motions are dictated and force tracking is required. In this thesis the dynamic problem of force-position control with hydraulics is simplified, allowing linear analysis and controller design. A 3D model has been created and used to show the need for force feedback after which an analysis is performed on the actuator dynamics. The root locus method is used to close the force control loop with a proportional-integral controller. This results in a complete force-position control structure which tracks a force setpoint on a moving object. The controller structure has been implemented during sea trials and showed significant improvement in the force control performance. This approach of force control using hydraulic actuation is expected to significantly increase safety in offshore operations.