Concentric Helical Axial Spring Tunable Stiffness Mechanism: Analytical Modeling, Design Optimization and Experimental Validation

Abstract

The human foot modulates its stiffness in different anatomical regions in order to attain greater efficiency and performance. The purpose of this study is to develop an adaptable robotic foot by biomimicking the human foot’s arch, horizontal tie and its ability of varying its stiffness. In this research we describe the design, modeling, development and optimization of the robotic foot with a novel Tunable Stiffness Mechanism (TSM) which acts as a horizontal tie. The TSM is made up of two concentric helical springs and the tunable stiffness feature is obtained by changing the parallel/series configuration of these springs. A flexible fiberglass composite arch is selected, and Multiobjective Design Optimization (MDO) is used to maximize the stiffness range and effectiveness of the system. Analytical modeling results closely match the experimental validation of both the tunable axial stiffness behavior of the TSM and tunable stiffness foot.