A compliant mechanism with variable stiffness achieved by rotary actuators and shape-memory alloy

The aim of this article is to study the consequences of the active stiffening of a compliant mechanism on the workspace created by the deformation of its structure. In connection with recent soft robotics research integrating shape-memory alloys (SMAs), the variation in stiffness over time is here obtained by the thermal activation of a nickel–titanium SMA spring. The workspace is created by the deformation (in the strength of materials sense) controlled by two rotary actuators acting on a structure comprising two angled flexible beams. In addition to a natural variation in the elasticity modulus of the SMA component during its thermal activation, its shape reconfiguration adds a structural deformation modifying the workspace. The existence of a common area between the workspaces of the mechanism corresponding to the non-activated and activated modes of the SMA is preserved. Several compliance maps are determined from measurements using a laser tracker targeting a given position of the loaded structure. The impact of SMA pre-stretch on stiffness variability is compared to that of a change in Young’s modulus. Variations in the stiffness distributions between the two modes reveal interesting properties (stiffness sign inversion, anisotropy) for the future optimal design of compliant mechanisms with high versatility, associating the spatial positions of the effector with variable stiffness values.