MOBIUS: A Multi-Modal Bipedal Robot that can Walk, Crawl, Climb, and Roll

Venue: RSS 2026 (Sydney, Jul 13–17) · Session: Humanoids · paper #24 Authors: Alexander Schperberg, Yusuke Tanaka, Stefano Di Cairano, Dennis Hong arXiv: 2511.01774 · program page

Summary compiled from the arXiv paper (v3); all numbers quoted from the paper. Trend context: RSS 2026 survey.

Six hardware snapshots of MOBIUS (Multi-modal Operations Bipedal Intelligent Urban Scout): (a) crawling, (b) bipedal walking, (c) rolling backwards, (d) climbing a children's slide, (e) pull-up while pinch-grasping an aluminum bar, and (f) visual-servo tracking of vertical handles — all with one non-reconfigurable morphology.

Problem

Legged robots typically excel in a single locomotion regime — bipeds, quadrupeds, or specialized climbers relying on adhesion or passive spines — limiting adaptability across continuous and discrete terrain. The authors (MERL, ETH Zurich, UCLA) target a single non-reconfigurable platform that can walk, crawl, roll, and free-climb while also grasping, with no part swapping or topology changes.

Method

MOBIUS is a 10.3 kg biped with four limbs: two 6-DoF arms ending in two-finger spine-enhanced grippers (for manipulation and climbing) and two 4-DoF legs, plus curved back rails for fall protection and rolling. The control stack combines (1) model-free RL locomotion (PPO, two-stage training, POMDP formulation with joystick velocity commands, compared against a model-based MPC baseline), (2) adaptive admittance force control made safe by a Reference Governor that enforces position/velocity constraints with auto-tuning for compliant contact, and (3) a high-level MIQCP planner that autonomously selects locomotion modes to minimize energy subject to reachability and terrain constraints.

Results

In mode-restricted planning tests, crawling achieves the lowest Energy-per-Visited-Cell (5.11 J/cell; CoT 0.104) versus biped (26.89 J/cell; CoT 0.548) and rolling (21.96 J/cell; CoT 0.448); biped motion costs ~104 J/m against 65 J/m for crawling. Hardware trials: climbing a kid's slide succeeded 8/10 (failures from toe entrapment), and full-body pinch-grasp pull-ups from a dead hang succeeded 9/10, with the spine gripper sustaining a dominant 124.2 N shear force on an aluminum bar. The Reference Governor demonstrably suppresses end-effector velocity-limit violations present without it.

Significance

A morphology–planning–control co-design argument: one fixed body plus RL locomotion, safe admittance control, and MIQCP mode selection covers walking, crawling, rolling, climbing, and pull-ups — the first bipedal robot with finger grippers to do all of these per the authors. Relevant to the wiki's Review-Humanoid-VLA and hierarchical-control discussion in Review-System-0-1-2.

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