Coordinated Stability Control Integrating Gait Rhythm Planning and Attitude Feedback for an Underwater Hexapod Robot with Asymmetric Five-Legged Support

Near-seabed contact operations with underwater hexapod robots can require one leg to execute a contact task, which reduces the stability of the remaining asymmetric five-legged support. To address this problem, this study proposes a five-legged asymmetric coordinated stability control method that integrates gait rhythm planning with attitude feedback. The method decouples the left middle leg from support, propulsion and CPG phase evolution to form a “5+1” asymmetric support base. This configuration reduces the periodic influence of task-induced disturbances on body balance. Meanwhile, a five-legged Hopf-CPG rhythm maintains continuous gait under asymmetric support. Low-bandwidth attitude feedback modulation is introduced between the CPG-generated foot trajectory and the inverse-kinematics input to balance attitude correction with foot-contact continuity. Full-degree-of-freedom underwater simulations in Webots show that, at a flow speed of 0.8 m/s, the method reduces combined attitude RMS by 41.52% relative to the no-feedback strategy. Relative to high-gain PD control, it reduces the RMS rate of change of the control output by 71.32%. These results indicate that the method improves input smoothness and compatibility with five-legged gait rhythms, with a moderate trade-off in transient attitude suppression. It therefore offers a coordinated approach to balancing attitude stability and gait continuity under asymmetric operation.