A novel design of a semi-active variable stiffness element is proposed, with possible applications in vibration isolation. Semi-active vibration isolators usually use variable dampers. However, it is known from the fundamental vibration theory that a variable spring can be far more effective in shifting the frequencies of the system and providing isolation. Geometry change is a common technique for building variable springs, but has disadvantages due to the complexity of the required mechanism, and slow response due to the inertia of moving parts. In the variable spring introduced here (VS), the stiffness is changed by force control in the links which corresponds to infinitesimal movements of the links, and does not need a change of geometry to provide a change of stiffness. This facilitates a fast response. The proposed VS is a simple prestressed cable mechanism with an infinitesimal mechanism. Theoretically the level of the prestress in the cables can be used to control the stiffness from zero to a maximum value that is only limited by the strength of the links. In this work, the statics, kinematics and stability of the VS are studied, the stiffness is formulated, and possible configurations of the VS are found.

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