Bioinspired Quadrupedal Rock-Climbing Robot with Heterogeneous Fore–Hind Grippers and Knee Parallel Elastic Actuators

Robot rock climbing on natural terrain requires large and well-directed attachment forces on irregular surfaces, utilizing limited onboard energy. This paper presents the Multimodal Adaptive Rock-Climbing Robot Version 2 (MARCBot V2), a human-inspired quadrupedal climber with heterogeneous spiny grippers and minimalist knee Parallel Elastic Actuators (PEAs). The forelimb gripper uses underactuated fingers, Linearly Constrained Compliant Spines (LCCSs), and passive compliance. This design better aligns attachment-force directions with climbing loads, increasing tolerance to local surface geometry. The rubber-padded support mode enables walking on level ground. The hindlimb gripper utilizes a lightweight toe–heel layout, enabling toe-hook/heel-hook support to better resist slip and pitching during transitions and pitch changes. We designed a rubber-cord PEA using a Neo–Hookean model and multi-objective optimization. The PEA effectively reduces peak motor power and mitigates knee joint overload without adding complex mechanisms. Basalt tests report mean shear/normal forces of 62.2/25.9 N (maximum 76.2/31.3 N) for the forelimb. System demos demonstrate transitions from 0 to 70° and vertical climbing. PEA tests reveal a mean power reduction from 48.74 to 45.18 W (estimated 7.3% reduction in cost of transport).