Session: 16-01-01: Poster Session: NSF-Funded Research (Grad & Undergrad)

Paper Number: 99599

99599 - A Novel Variable Stiffness Compliant Robotic Link Based on Discrete Variable Stiffness Units for Safe Human-Robot Interaction 

Variable stiffness manipulators balance the trade-off between manipulation performance needing high stiffness and safe human-robot interaction desiring low stiffness. Variable stiffness compliant links provide a solution to enable this flexible manipulation function in human-robot co-working scenarios. In this paper, we propose a novel variable stiffness link based on discrete variable stiffness units (DSUs).

A DSU is a parallel guided beam that can adjust stiffness discretely by changing the cross-sectional area properties of the hollow beam segments. The on/off mode of the block changes the second area of the moment of inertia of the beam leading to stiffness varying. A linear actuator is fixed to the solid segment on the side of the DSU near the pedestal end of the arm. When the solid block is pulled into the cavity, called on mode, where the DSU has relatively high stiffness and accuracy.  For the OFF mode, the stroke is extruded, and the solid block is pushed out of the cavity of the parallel beam, there is no block inside the cavity. The Tri-DSU consists of three DSUs connected in series, so each DSU can be adjusted individually to achieve the eight stiffness modes of the entire system. When all DSUs are in off mode, the Tri-DSU is in off-off-off (FFF) mode, and the system has the lowest stiffness. When two DSUs are in off mode and another DSU is in on mode, the Tri-DSU may be in off-off-on (FFN) mode, off-on-off (FNF) mode, or on-off-off (NFF) mode. When two DSUs are in on mode and the other two DSUs are in off mode, the Tri-DSU may be in on-on-off (NNF) mode, on-off-on (NFN) mode, or off-on-on (FNN) mode. In these three cases, the stiffness of Tri-DSUs is relatively high. When all DSUs are in on mode, the Tri-DSU is in on-on-on (NNN) mode, the system has the highest stiffness. Theoretically, the Tri-DSU in such a parameter configuration can achieve 57 times the variation in stiffness.

To optimize the design, stiffness analysis of the DSU and Tri-DSU under various configurations and forces was performed by a derived theoretical model compared with finite element analysis (FEA). The analytical stiffness model is derived using the approach of serially connected beams and superposition combinations. It works not only for thin-walled flexure beams but also for general thick beam models. 3-D printed prototypes were built to verify the feature and performance of the Tri-DSU in comparison with the FEA and analytical model results. It’s demonstrated that our analytical model can accurately predict the stiffnesses of the DSU and Tri-DSU within a certain range of parameters. stiffness relationships, of Tri-DSU under eight configurations. For each configuration, the error between the FEA and the theoretical values is less than 3%. For all seven configurations except the NNN mode, the errors between FEA and experimental values are less than 20%. For NNN mode, the error between FEA. The actual stiffness variation rate of Tri-DSU can reach 31 times, which is an innovative breakthrough.

The developed variable stiffness link method and analytical model are extendable to multiple DSUs with different sizes and parameter configurations to achieve modularization and customization. The advantages of the stiffness change mechanism are rapid actuation, simple structure, and compact layout. These methods and results provide a new conceptual and theoretical basis for the development of new reconfigurable cobot manipulators, variable stiffness structures, and compliant mechanisms. In the future, we will apply the DSU concept to variable stiffness actuators and grippers.

Presenting Author: Jiaming Fu Purdue University

Presenting Author Biography: Jiaming Fu is a second year Ph.D. student in the School of Engineering Technology at Purdue University, West Lafayette, IN. His major advisor is Dr. Dongming Gan. He got Master’s degree in Mechanical Engineering from Columbia University in 2019 and foucus on soft robot research there. He got Bachelor’s degree in Mechanical Engineering from Florida Institute of Technology in 2017. He has published 8 international journal and conference papers so far. For now, his research interest is on discrete variable stiffness mechinnisms, including discrete variable stiffness units, actuatosr, link, and gripper.

Authors:

Jiaming Fu Purdue University
Han Lin Purdue University
Ziqing Yu Purdue University
Dongming Gan Purdue University