Traversing the Narrow Path: A Two-Stage Reinforcement Learning Framework for Humanoid Beam Walking

Traversing narrow beams is challenging for humanoids due to sparse, safety-critical contacts and the fragility of purely learned policies. We propose a physically grounded, two-stage framework that couples an XCoM/LIPM footstep template with a lightweight residual planner and a simple low-level tracker. Stage-1 is trained on flat ground: the tracker learns to robustly follow footstep targets by adding small random perturbations to heuristic footsteps, without any hand-crafted centerline locking, so it acquires stable contact scheduling and strong target-tracking robustness. Stage-2 is trained in simulation on a beam: a high-level planner predicts a body-frame residual (Delta x, Delta y, Delta psi) for the swing foot only, refining the template step to prioritize safe, precise placement under narrow support while preserving interpretability. To ease deployment, sensing is kept minimal and consistent between simulation and hardware: the planner consumes compact, forward-facing elevation cues together with onboard IMU and joint signals. On a Unitree G1, our system reliably traverses a 0.2 m-wide, 3 m-long beam. Across simulation and real-world studies, residual refinement consistently outperforms template-only and monolithic baselines in success rate, centerline adherence, and safety margins, while the structured footstep interface enables transparent analysis and low-friction sim-to-real transfer.