BiFlex: A Passive Bimodal Stiffness Flexible Wrist for Manipulation in Unstructured Environments

-- Robotic manipulation in unstructured, human-centric environments poses a dual challenge: achieving the precision need for delicate free-space operation while ensuring safety during unexpected contact events. Traditional wrists struggle to balance these demands, often relying on complex control schemes or complicated mechanical designs to mitigate potential damage from force overload. In response, we present BiFlex, a flexible robotic wrist that uses a soft buckling honeycomb structure to provides a natural bimodal stiffness response. The higher stiffness mode enables precise household object manipulation, while the lower stiffness mode provides the compliance needed to adapt to external forces. We design BiFlex to maintain a fingertip deflection of less than 1 cm while supporting loads up to 500g and create a BiFlex wrist for many grippers, including Panda, Robotiq, and BaRiFlex. We demonstrate that BiFlex simplifies control while maintaining precise object manipulation and enhanced safety in real-world applications. Designing robots capable of physical tasks in unstructured environments remains one of the core open problems of modern robotics. Unstructured settings are characterized by their inherent uncertainty that exposes robotic end-effectors to frequent and unpredictable forces. For example, when grasping a flat object or wiping a surface, inaccuracies in the perceived location could lead to the robot missing the target, or creating unexpected and dangerously high reactive forces that could damage the robot.