A vibration control strategy using variable stiffness joints

Adaptive joints are structural joints made of materials with enhanced transduction properties that can vary their stiffness via solid-state actuation (e.g. thermal, mechanical). In this work, stiffness tuning is used to switch the joint between a ‘locked’ (e.g. a moment connection) and a ‘released’ (e.g. pin) state. Previous work has looked into the feasibility of using variable stiffness joints during shape and force control in order to reduce actuation work. This paper focuses on control of the structure dynamic response to loading. The natural frequency of the structure is tuned to escape dangerous resonance conditions in two ways: 1) a geometric reconfiguration via large shape changes or 2) via the change of stiffness of the joints. Two case studies are considered: 1) an active frame integrated with four actuators fitted on tubular elements which are connected by a shape memory polymer joint 2) a planar truss structure. Experimental tests on the active frame have shown that by varying the length of the linear actuators, large shape changes can be employed to effectively change the natural frequency of the structure. During shape change, the joint stiffness is lowered to ease geometric reconfiguration. For the planar truss case study, simulations have shown that the ‘release’ or ‘locking’ of multiple joints can be employed to change the eigenfrequencies, the more so the higher the eigenmode.