Structural Design and Stiffness Characteristics of a Passive Variable Stiffness Joint.

Supernumerary robotic limbs(SRLs) are new type of wearable robots. In order to achieve safe and stable operations for the SRLs, a tendon-driven passive variable stiffness joint(pVSJ) integrated into the shoulder joint of the SRLs is presented, which can output different stiffness and improve the safety of human-robot interaction. The stiffness change of the pVSJ is achieved by winding a tendon around pulleys to drive the flexible element to stretch. In the non-operating state, the stiffness characteristics of the pVSJ can be changed by adjusting the pre-tension of the flexible element, the winding method of the tendon, and replacing the flexible element. A mathematical model is established for the tendon-driven pVSJ to reveal its stiffness characteristics. The model is verified through quasi-static experiments. The experimental results show that there is an obvious hysteresis phenomenon during low torque loading. The hysteresis range is 20.8%-32.5%. The angular range of the loading hysteresis of the pVSJ is negatively related to the spring preload, spring stiffness, the number of working branches, and it is positively related to the rope diameter. The unloading process basically conformed to the theoretical model, with a maximum relative error of 3.4%. Therefore, the exact stiffness output can be achieved by first increasing and then decreasing the joint torque to reach the target torque.