Integrating Identification and Regulation: A Foreign-Load Adaptive Balance Control Framework for Biped Robots via a Hierarchical Hip-Ankle Strategy

Bipedal robots are increasingly deployed in tasks where stability under load is critical. However, control methods relying solely on state feedback struggle to maintain stability under unknown foreign loads, as dynamics induced by the load directly affect balance critical states in intrinsically unstable bipedal systems. This paper proposes a foreign-load adaptive balance control framework for bipedal locomotion based on online load parameter estimation and composite model update. Load dynamic parameters are estimated online using an optimal method and embedded into the controller's internal 'robot with load' composite model to continuously update balance-relevant dynamics. Based on this representation, a hierarchical hip-ankle coordination strategy is adopted, where the hip provides proactive load compensation and the ankle regulates reactive posture stabilization and ground compliance. Simulation and hardware experiments demonstrate stable dynamic bipedal locomotion under heavy foreign loads with unknown dynamic parameters, validating improved balance robustness, load adaptability, and posture regulation for real-world applications. Note to Practitioners - Bipedal robots performing transportation tasks in industrial environments must identify unknown foreign loads during motion while maintaining stable locomotion. This work addresses this problem by updating the composite 'robot with load' balance model using online load estimates. By treating the carried load as part of the robot dynamics, the proposed framework enables proactive regulation of the combined system rather than passive disturbance compensation. The updated model informs a coordinated hip-ankle strategy, where the hip compensates for foreign-load-induced effects and the ankles maintain postural stability and ground compliance. This enables reliable walking, turning, off-axis carrying, and uneven-terrain traversal under heavy, unmodeled foreign loads, improving the operational reliability of bipedal robots in material handling and related industrial applications.