Design and analysis of a variable stiffness tendon-driven soft gripper based on twisted tendons

Abstract Due to the inherent limitations of soft grippers, such as low stiffness and low load-bearing capacity, it is necessary to incorporate variable stiffness mechanisms to enhance their performance. Inspired by the twisted string actuator, a variable stiffness actuator is proposed, which alters friction through twisting tendons and generates antagonistic force with the pulling force on the driving tendon to achieve stiffness variation. This method combines the principles of fiber jamming and the antagonistic principle. The effectiveness of the bending deformation and variable stiffness design was validated through finite element simulations. Based on this, a hardware platform for the variable stiffness soft gripper was established, and bending tests and stiffness measurements were conducted on the variable stiffness soft fingers. The experiment results align closely with the finite element simulation results, indicating that the proposed variable stiffness actuator provides 333% stiffness enhancement. Additionally, measurements of the peak grasping force and grasping tests of the variable stiffness soft gripper are conducted. The experimental results demonstrate that this variable stiffness actuator can effectively enhance soft grippers’ stiffness and grasping capability.