A fast-response soft gripper inspired by the mechanics of the hummingbird beak

Biology is a valuable source of inspiration in engineering design. For example, elastic instabilities, which are often used in biological systems to achieve fast movement, can be exploited as possible actuation mechanisms for soft robotics. The hummingbird beak is known for its rapid closing speed (within 10 ms) under sequential actuation inputs, i.e. a twist, rotation, and twist back, applied at the root of the beak. Here, we unveil the mechanical mechanism that leads to the rapid response of the hummingbird beak using the nonlinear finite element method. We present the complete equilibrium manifold of an idealised beak under sequential root twist/rotation/twist, with a focus on the limit point where snap-through occurs. We show that the intermediate rotation input is essential for triggering the snap-through instability, i.e. it is the rotation that stores significant elastic energy that is then converted to kinetic energy. We also investigate the effect of different cross-section profiles on the snap-through mechanics and show that a concave cross-sectional shape with mass concentrated at the top/bottom edges of the beak leads to greater kinetic energy output (equivalent to faster snap-through response). Based on the numerical results, a prototype biomimetic fast-response soft gripper was designed and manufactured. A series of tests were conducted to validate the numerical model, as well as demonstrate the effectiveness of the soft gripper in actuation. The present work sheds light on introducing pre-stress in continuous structures to design fast-response soft robots with low energy input.