Optimization and Experimental Verification for the Antagonistic Stiffness in Redundantly Actuated Mechanisms: A Five-bar Example

Abstract

A closed-chain mechanism having redundancy in force domain can produce an effective spring effect by proper internal load distribution. The so-called antagonistic stiffness is provided by redundant actuation in conjunction with nonlinear geometric constraints. The objective of this study is twofold. The first one is to propose a methodology for optimal kinematic design of antagonistic stiffness and the second one is to verify the phenomenon of the antagonistic stiffness through experimentation. In this work, a five-bar mechanism that can modulate antagonistic stiffness by using redundant actuators is employed as an exemplary system. The optimal structure of the five-bar mechanism that can maximize efficiency in generation of antagonistic stiffness is evaluated and analyzed. A measure of stiffness isotropy and the gradient property of the measure are employed as the performance indices in optimization. To deal with multi-criteria based design, a composite design index based on max-min principle of fuzzy theory is used as an objective function. Two optimization results are obtained. Furthermore, a five-bar mechanism has been developed for experimental verification. The phenomenon of antagonistic stiffness has been successfully demonstrated by free vibration and indirect force control experimentation. (C) 2004 Elsevier Ltd. All rights reserved.