Design, Modeling and Control of a Variable Stiffness Elbow Joint

New technological advances are changing the way robotics are designed for safe and dependable physical human-robot interaction and human-like prosthesis. Outstanding examples are the adoption of soft covers, compliant transmission elements, and motion control laws that allow compliant behavior in the event of collisions while preserving accuracy and performance during motion in free space. In this scenario, there is growing interest in variable stiffness actuators (VSAs) [1]. Herein, we present a new design of an anthropomorphic elbow VSA based on an architecture we developed previously [2]. A robust dynamic feedback linearization algorithm is used to achieve simultaneous control of the output link position and stiffness. This actuation system makes use of two compliant transmission elements (CTEs), characterized by a nonlinear relation between deflection and applied torque. The main parameter of the CTEs were determined using a Matlab\ANSYS APDL optimization routine, whereby ANSYS was tasked to solve the 1D model of the beam while Matlab managed the optimization procedure in batch. The optimal parameters were then verified using a 3D FEA simulation and confirmed through experimental test. A Static feedback control algorithms have been proposed in literature considering purely elastic transmission [3]; however, viscoelasticity is often observed in practice. This phenomenon may harm the performance of static feedback linearization algorithms, particularly in the case of trajectory tracking. To overcome this limitation, we propose a dynamic feedback linearization algorithm that explicitly considers the viscoelasticity of the transmission elements, and validate it through simulations and experimental studies. The results are compared with the static feedback case to showcase the improvement in trajectory tracking, even in the case of parameter uncertainty. Figure 1 shows all the phases of the work that have been carried out.