Novel passive Discrete Variable Stiffness Joint (pDVSJ): Modeling, design, and characterization

In this paper, the design and characterization of a novel passive Discrete Variable Stiffness Joint (pDVSJ) is presented. The pDVSJ is a proof of concept of a passive revolute joint with discretely controlled variable stiffness. The current realization is equipped on a passive single-DOF exoskeleton (TELEXOS-I) for future development towards applications in haptic-teleoperation purposed exoskeletons. The key motivation behind this design is the need of instantaneous switching between stiffness levels when applied for Virtual Reality (VR) or Remote Environment (RE) stiffness mapping applications, in addition to the need of low-energy-consumption. Altering the stiffness is achieved by selecting the effective length of an elastic rubber cord. This is realized by creating a new grounding point, thus changing the effective length. Three different levels of stiffness (low, moderate, high) can be discretely selected beside the zero stiffness. The novelty of this work lies in the method used to alter the stiffness of the variable stiffness joint, and the design of Cord Grounding Units (CGUs) which are responsible for selecting the effective length of the elastic cord. The main features of CGU are the fast response and the low-energy consumption. This is achieved through a linear solenoid actuator pushing a cam-cleat which will clutch the motion of the elastic cord, creating a new grounding point. This will limit the length of the cord between the cam cleat and the cord's fitting point on the joint. The solenoid's ON-Time is minimized as the cam cleat's design is passively self-locking the cord through the pulling force between the cord and the cam-cleat. The proposed physical-based model matched the experimental results of the pDVSJ in terms of discrete stiffness variation curves and the stiffness dependency on the behavior of the elastic rubber cord.