BiFlex

A Passive Bimodal Stiffness Flexible Wrist for Manipulation in Unstructured Environments

Gu-Cheol Jeong1, Stefano Dalla Gasperina1, Ashish D. Deshpande1, Lillian Chin2*, Roberto Martı́n-Martı́n3*

1ReNeu Robotics Lab, Department of Mechanical Engineering
2MERGE Lab, Department of Electrical and Computer Engineering
3Robot Interactive Intelligence Lab, Department of Computer Science
The University of Texas at Austin
* Equal advising
Under Review
arXiv CAD Files

Abstract

Robotic manipulation in real-life, human-focused settings is a tricky balancing act, requiring the precision for delicate free-space tasks while also safely managing unexpected contacts. Traditional robotic wrists tend to rely on overly complex controls or bulky designs that still don’t quite hit the mark. That’s why we developed BiFlex—a flexible robotic wrist built with a soft, buckling honeycomb structure that naturally toggles between two modes. In high-stiffness mode, it manipulates household objects with pinpoint accuracy, and in low-stiffness mode, it adapts to external forces. Designed to keep fingertip deflection under 1 cm while supporting up to 500 g, BiFlex integrates with various grippers such as Panda, Robotiq, and BaRiFlex. Our experiments—from surface wiping and precise pick-and-place to using environmental constraints for grasping—show that BiFlex not only simplifies control but also enhances both accuracy and safety in real-world scenarios.

Motivation

Design Concept

BiFlex is a flexible robotic wrist built around a 3D-printed buckling honeycomb structure that passively adapts its stiffness—shifting from the precision of a rigid mode to the safety of soft compliance as needed. A central universal joint ensures smooth, balanced motion, while custom top and base plates let it easily pair with a variety of robotic grippers. Plus, its stiffness threshold is adjustable by altering the honeycomb pillars and middle beam, making BiFlex highly versatile for a range of manipulation tasks.

BiFlex Design

CAD_Design1

CAD design for BiFlex

CAD_Design2

Adaptation in Contact-Rich Interaction: Wiping Test

BiFlex passively switches into compliant mode whenever reaction forces at the fingertip exceed a certain threshold (11 N for the Robotiq gripper), absorbing impacts instead of transferring overload to the arm. We demonstrated this by mounting a 5 mm sponge and wiping 30°-tilted triangular surfaces of increasing height. Thanks to BiFlex, reaction forces stayed under the 15 N safety threshold even on 50 mm peaks—while a rigid wrist already exceeded 15 N on just a 15 mm peak—highlighting BiFlex’s unmatched adaptability in contact-rich tasks. This clear difference underscores BiFlex’s impressive adaptability in such scenarios.

BiFlex

Rigid

Maintaining Accuracy: Pick-and-Place Test

BiFlex is a dual-stiffness wrist offering a rigid mode for high-precision manipulation of objects up to 500 g. To validate its accuracy, we set the internal height of the third shelf in a three-tier cabinet to be 1 cm greater than each object’s height and attempted to place the picked object into that confined space. As intended, BiFlex picked and placed every object under 500 g at its target and then switched to compliant mode when faced with a 600 g load—demonstrating robust impact resistance.

Grasping with Environmental Constraints

In unstructured settings, tiny positioning errors can cause failed grasps or collisions—especially when trying to pick thin objects from a table. Instead of needing perfect positioning, BiFlex's compliant mode safely slides along the table’s surface, turning potential collisions into beneficial guides for accurate grasping. We tested this by having the robot approach a small hex wrench, deliberately setting the target height up to 50 mm below the object’s actual position. BiFlex smoothly adapted to this constraint, keeping reaction forces safely below the 15 N threshold. This demonstrates that BiFlex can naturally handle inaccuracies, turning potential errors into successful, safe grasps.

BiFlex for Any Robotic Hand

BiFlex is designed to be versatile and adapt to various robotic grippers with its modular design and adjustable parameters. We’ve tailored the design for multiple grippers—including Franka Hand, Robotiq, and BaRiFlex—demonstrating its broad applicability and ease of integration in different setups.

Franka Hand

Franka_Compressiob

Robotiq

Robotiq_Compression

BaRiFlex

BaRiFlex_Compression

Theoretical Analysis

We modeled each honeycomb cell as a network of beams and applied Hooke’s law to derive a simple formula for its vertical stiffness (Keq). By applying Euler’s buckling formula to the diagonal beams, we identified the critical load—the point at which the cell suddenly becomes softer when the input force (Fi) surpasses this threshold. This analysis revealed that the tilt angle (γ) largely influences BiFlex’s vertical stiffness, while the beam thickness (b) mainly adjusts the critical buckling load. By tuning these two parameters, BiFlex can be customized to match the specific needs of various robotic end-effectors.

Theoretical_Analysis

BibTeX

@misc{jeong2025biflexpassivebimodalstiffness,
        title={BiFlex: A Passive Bimodal Stiffness Flexible Wrist for Manipulation in Unstructured Environments}, 
        author={Gu-Cheol Jeong and Stefano Dalla Gasperina and Ashish D. Deshpande and Lillian Chin and Roberto Martín-Martín},
        year={2025},
        eprint={2504.08706},
        archivePrefix={arXiv},
        primaryClass={cs.RO},
        url={https://arxiv.org/abs/2504.08706}, 
  }