Flexure Variable Stiffness Actuators

Series elastic actuators (SEAs) and variable elastic actuators (VSAs) provide shock resistance, energy storage, and stable force control. However, they usually require extra springs, mechanical parts, and transmissions, increasing size, weight, number of moving parts, and reducing the mechanical efficiency. In particular, this mechanical complexity is one of the significant challenges in the design of wearable and scalable force feedback devices. In this article, flexure variable stiffness actuators (F-VSAs), which combine kinematic transmission, elasticity, and stiffness modulation via a network of folding patterns using flexure hinges, are presented. Thus, F-VSAs allow the creation of robots benefiting from the advantages of SEAs and VSAs without hindering form factor or mechanical efficiency. To illustrate the design strategy of F-VSAs, a 4-design-of-freedom (DoF) robot that provides stiffness and force output is presented. An analytical model that estimates the inherent stiffness and the end-effector force output for any given configuration of the folding pattern is proposed. Finally, stiffness modulation and force control of the robot are implemented and good agreement with the predictions from the model is observed. Thus, this novel design strategy allows the creation of compact and scalable robots with stiffness and force output for wearable, rehabilitation, and haptic applications.