Unified Force-Position Hybrid Control for a 3-DOF Serial Offshore Gangway: A Motion Prediction and Kinematic Decoupling Approach

Aiming at a 3-DOF (slew–luff–telescopic) serial gangway, a unified synergistic force-position hybrid control framework is proposed. All three degrees of freedom adopt an identical dual-loop nested structure consisting of an outer-loop active disturbance rejection position controller and an inner-loop admittance force controller. Task differentiation is achieved solely by setting different force targets: constant-force contact for the telescopic axis, and zero-force following for the luffing and slewing axes. A Kalman filter-based vessel motion state estimation and multi-step prediction-fusion strategy is designed, reducing the equivalent delay from 350 ms to 85 ms and the pitch estimation root-mean-square error (RMSE) from 1.82° to 0.65°. The kinematic orthogonality between the telescopic direction and the luffing/slewing directions is rigorously proved, enabling force/position subspace decoupling. Simulation results verify the effectiveness of the proposed method. Results show that the predicted force-control RMSE in the telescopic direction is 16.80 N, 34.7% lower than that without prediction; the predicted tracking RMSE in the luffing direction is 0.50° with a compensation efficiency of 89.37%, an improvement of 7.2% over the non-prediction scheme; and the predicted tracking RMSE in the slewing direction is 2.77° with a compensation efficiency of 85.62%, an improvement of 11.8%. The proposed control method substantially enhances the accuracy, compliance, and safety of the force-position cooperative control of the serial gangway.