The Use of Variable Stiffness Joints in Adaptive Structures

Adaptive structures are defined here as structures capable of counteracting actively the effect of external loads via controlled shape changes and redirection of the internal load path. These structures are integrated with sensors (e.g. strain, vision), control intelligence and actuators. This paper investigates the use of variable stiffness joints in adaptive structures to achieve large shape changes. Large shape changes are employed as a structural adaptation strategy to counteract the effect of the external load. The structure is designed to ‘morph’ into optimal shapes as the load changes. This way the stress can be homogenized avoiding peak demands that occur rarely. Numerical results show that when large shape changes are considered, material mass (and thus embodied energy) reduction is achieved with respect to both adaptive structures limited to small shape changes and optimised passive structures. Embodied energy savings become substantive when shape changes are allowed to go beyond conventional deflection limits.
However, large shape changes require significant flexibility of the joints because their fixity can affect load-path and shape control. To address this problem, a variable stiffness joint is proposed. During shape/load-path control, the joint reduces its stiffness so that required deformation patterns can be achieved with low actuation energy. After shape control the joint recovers rigidity. Experimental studies are presented to show the potential for application of joints with variable stiffness in adaptive structures.