Conventional mechanisms provide a defined mobility, which expresses the number of degrees of freedom of the mechanism. This allows the system to be driven by a low number of control outputs. This property is virtually retained in the case of compliant mechanisms with lumped compliance, which are obtained by replacing the conventional hinges by solid-state ones. Compliant mechanisms with distributed compliance have, in general, an infinite number of degrees of freedom and therefore cannot guarantee defined kinematics. In this paper the concept of compliant mechanisms with selective compliance is introduced. This special class of compliant mechanisms combines the advantages of distributed compliance with the easy controllability of systems with defined kinematics. The task is accomplished by introducing a new design criterion based on a modal formulation. After this design criterion has been implemented in an optimization formulation for a formal optimization procedure, mechanisms are obtained in which a freely chosen deformation pattern is associated with a low deformation energy while other deformation patterns are considerably stiffer. Besides the description of the modal design criterion and the associated objective function, the sensitivity analysis of the objective function is presented and an application example is shown.

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If a structural system has to be subjected to high loads and large geometry changes, according to the state of the art, articulated systems with discrete actuators are used, with the articulated systems consisting of stiff members connected by hinges. As an alternative, smart structures technologies can supply solutions based on the deliberate use of structural flexibility and on distributed actuation. In order to assess the advantages, which can be expected from such solutions, a thorough comparison must be made between the properties of compliant mechanisms and the conventional ones. A crucial aspect of this comparison, on which this contribution is focused, is the impact on the system's structural weight.The first part of the paper deals with the relevance of weight penalty effects in shapeadaptable systems, with a special focus on airfoil shape control. A quantitative analysis of weight penalty effects in pin-jointed articulated structures follows. The criterion on which this analysis is based allows a characterization of general validity, not restricted to a particular example, and can be applied to other mechanisms or hinge architectures, providing a sound way of assessing the lightweight potential of a given concept and allowing a consistent comparison between different design philosophies. An extension to weight penalty effects in compliant systems shows a higher degree of complexity and could not be addressed in the same detail in this study. Anyway, some peculiar aspects are discussed in the final part of the paper, which can serve as a basis for future developments in this sense. In particular, the dependence of weight penalty effects on the system's range of motion as well as on the loaddependence of the mechanism's kinematics is addressed.Even if the presented results can be of direct significance to the designer of conventional articulated mechanisms, the primary relevance of this work is to be seen in the long term. Its main target is to provide a basis for the analysis of the potential offered by the compliant mechanisms and smart materials for the realization of light shape-adaptable structures and to give an impulse to research efforts aiming at developing suitable optimization procedures as well as formulating proper design rules for such kind of systems.

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