A Novel Joint Design for Robotic Hands With Humanlike Nonlinear Compliance

Robotic hands are typically too rigid to react against unexpected impacts and disturbances in order to prevent damage. Human hands have great versatility and robustness due, in part, to the passive compliance at the hand joints. In this paper, we present a novel design for joint with passive compliance that is inspired by biomechanical properties of the human hands. The design consists of a compliant material and a set of pulleys that rotate and stretch the material as the joint rotates. We created six different compliant materials, and we optimized the joint design to match the desired humanlike compliance. We present two design features that allow for the tuning of the joint torque profile, namely, a pretension mechanism to increase pretension of the compliant material, and a design of varying pulley configuration. We built a prototype for the new joint by using additive manufacturing to fabricate the design components and built a test-bed with a force sensor and a servo motor. Experimental results show that the joint exhibits a nonlinear, double exponential joint compliance with all six compliant materials. The design feature involving variable pulley configurations is effective in adjusting the slope of joint torque during the joint rotation while the pretension mechanism showed only a limited effect on increasing the torque amplitude. Overall, with its small size, light weight, low friction, and humanlike joint compliance, the presented joint design is ready for implementation in robotic hands.