A Posture Control Algorithm Based on Position Inverse Kinematics for Hexapod Robots on Slope Terrains

Hexapod robots traversing unstructured slope terrains often suffer from premature foot-ground collision and postural instability due to unknown topography gradients and body tipping moments. To enhance terrain adaptability without relying on exteroceptive sensors, this paper proposes a position inverse kinematics (PIK) posture control framework for hexapod robots on slope terrains. The proposed algorithm dynamically establishes the support plane model using foot contact positions of primary support legs and estimates the slope gradient solely through onboard proprioceptive feedback (IMU and joint encoders). To mitigate foot impact and accommodate touchdown uncertainty, a composite swing foot trajectory is designed by coupling a sixth-order polynomial lift with a uniform linear landing. Crucially, a periodic floating-base coordinate reset strategy is introduced to decouple the stance phase kinematic mapping from absolute global localization, allowing attitude and body height adjustment. The proposed approach is validated through both MATLAB/Simulink and ADAMS dynamic co-simulations across composite slopes and physical prototype experiments on a small electric hexapod robot, HexWalker II. Experimental results demonstrate that the robot successfully navigates physical terrains of 10° uphill and 10° downhill while maintaining continuous body-slope parallelism and a positive static stability margin. Quantitatively, the comprehensive mean absolute error (MAE¯) and comprehensive maximum absolute error (MaxAE¯) for attitude tracking reach 0.555° and 1.782° during downhill locomotion, representing a 26.3% and 36.2% reduction over uphill performance, respectively.