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

TL;DR

BiFlex addresses safe, precise manipulation in unstructured settings by introducing a passive, bimodal stiffness wrist realized with a buckling honeycomb structure. The design achieves high stiffness for accurate free-space motion and low stiffness upon contact without actuators, enabling simpler control and safer interactions with various grippers. An analytical model links geometry to stiffness and buckling behavior, and extensive experiments demonstrate consistent buckling points, reduced contact wrenches, and successful wiping and pick-and-place of sub-500 g objects. The work presents a practical, scalable embodiment that can be readily integrated across different robotic hands to enhance manipulation in uncertain environments.

Abstract

Robotic manipulation in unstructured, humancentric 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 validate BiFlex under several real-world experimental evaluations, including surface wiping, precise pick-and-place, and grasping under environmental constraints. We demonstrate that BiFlex simplifies control while maintaining precise object manipulation and enhanced safety in real-world applications.

Figures (10)

• Figure 1: (top) The BiFlex wrist, featuring a honeycomb structure, mounted on a robotic arm and attached to a robotic gripper. (bottom) Typical manipulation sequence using the BiFlex wrist. When grasping in unstructured environments, the wrist transitions from high- to low-stiffness states upon contact with the table surface to exploit it as a constraint. Illustration adapted from kanitz_compliant_2018.
• Figure 2: The desired bimodal behavior ensures high stiffness during precision mode (red) and low stiffness during compliant mode (green), with a smooth transition at the buckling point (yellow).
• Figure 3: (left) Structural configuration of the BiFlex wrist design. The top plate and base frame are customizable to accommodate different robotic arms and grippers. (right) Cross-sectional and frontal views of the honeycomb structure.
• Figure 4: Frontal view (left) and free-body diagram (right) of one buckling honeycomb module. $\mathbf{F_i}$ represents the compression force applied to each single honeycomb module.
• Figure 5: Honeycomb structure designs corresponding to the three different grippers. Each design is tailored to the specific gripper, with the angle $\gamma$ and diagonal beam width $b$ adjusted to achieve the desired buckling point.