Softening nonlinear-stiffness elastic mechanism with continuous adjustability for human-robot interaction force control

Human-robot interaction of human augmentation robots presents a considerable challenge in achieving accurate and robust interaction force control. This paper proposes a novel softening nonlinear elastic mechanism with continuous adjustability (SNEMA) to address this challenge. The SNEMA achieves softening stiffness behavior through a nonlinear mapping relationship between the lengths of the diamond diagonals. This unique stiffness profile, featuring high stiffness for low output and low stiffness for high output, strikes a balance between the output force range and force resolution. Moreover, the continuous and convenient adjustment of the stiffness profile is realized by utilizing two antagonistic linear springs, enabling optimal stiffness matching for different output force ranges. Bench tests were conducted to validate the stiffness modeling and evaluate the force tracking and interaction performance of the developed SNEMA. Experimental results demonstrate the capability of the SNEMA to achieve precise force control and good collision safety in human-robot interaction. The proposed SNEMA is finally deployed on the Centaur robot to demonstrate its advantages in practical application.