The various relationships that are possible between the mechanical properties of single actuators and the overall mechanism (in this case a human arm with or without a prosthetic elbow) are discussed. Graphical and analytical techniques for describing the range of overall limb stiffnesses that are achievable and for characterizing the overall limb stiffness have been developed. Using a biomimetic approach and, considering energetic costs, stability and complexity, the implications of choosing passive or active implementations of stiffness are discussed. These techniques and approaches are particularly applicable with redundant (agonist -antagonist) actuators and multiple degrees of freedom. Finally, a novel biomimetic approach for control is proposed.
The relationship between the stiffness properties of actuators configured in an agonist-antagonist configuration and the stiffness of the joint is simple but not intuitively obvious. In this paper, this relationship is investigated and it is shown that the stiffness properties (hardening or softening) of the joint depend on the second derivative of the individual actuator stiffness with respect to actuator displacement. Numerical examples are given to illustrate the relationship and two practical examples, in which the actuators stiffness arises from Belleville washers or from an air piston, are analyzed.
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