Design, Fabrication and Characterization of Layered Jamming Bistable Composite Structures for Assistive Robotics 57 Fig. 5 Morphing behavior of two layered bistable composite structure jammed at full vacuum pressure: (a) before cycling loading, (a) after cycling at 1 Hz for 10 cycles., (c) after reducing vacuum to below -25 kPa, (d) deforming the specimen to -12 mm and drawing a full vacuum, (e) after unloading, (f) after reducing the vacuum to below -25 kPa. Conclusions We have recently characterized layered jamming structures consisting of layered bistable laminated composites, in order to understand how layered jamming can be used to create novel lightweight morphing structures for various applications, such as assistive robotics. The bistable laminated composite is capable of transitioning between two stable shapes and can be combined together or with passive elements through layered jamming, the bistable behavior can be controlled using vacuum pressure, resulting in morphing behavior that is similar to a shape memory material. Specimens were fabricate from two composite bistable plates made from carbon fiber/epoxy lamina, with two different layups to induce different levels of curvature. A stiff 3D printed structure was attached to one side of the plate with the lower curvature in order to prevent the surface from deforming and therefore inducing a reverse bending moment that could reverse the structure from the second stable state back to the original stable state upon unloading. It was observed that as the vacuum pressure increased, the hysteresis increased, as stiffness after the instability initiated due to slip at the interface. A similar trend was observed upon unloading. However, as the pressure increased to full vacuum, the load would reach 0 N upon unloading, and the specimen would stay in a permanently deformed state at the end of 10 cycles. The specimen would then exhibit similar cycling behavior without the instability as it would if the specimen had the vacuum drawn after it had been deformed to -12 mm without a vacuum. The change in the minimum load observed after the instability is initiated upon loading and unloading correlated to the increase in loading that would be required to overcome the frictional force, which is proportional to the vacuum pressure, at the interface between the plates. This vacuum-controlled stiffness allows for novel implementation for impedance control of assistive robotics, where itis possible to control the bistable response to adjust to the desired needs of a patient. The cyclic behavior in the permanently deformed state also correlated to model predictions without the instability. The permanently deformation could be recovered by reducing the vacuum pressure below -25 kPa. This enabled the structure to exhibit morphing behavior similar to a shape memory material, where it could be used to control the interactions with a patient for assistive robotic applications, such as exoskeletons, where it can activate either a permanent decoupling of the structure from the patient, or a permanent grasping force.
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