Actuation and design of two-dimensional curved hard-magnetic soft bilayers

Abstract

Hard-magnetic soft actuators (HMSAs) has gained prominence for its application in magnetically steerable soft robots, such as robotic grippers. Compared to straight HMS beam, the magneto-mechanical response of continuous and discontinuous curved multilayered HMSA under external magnetic fields is more complex. Based on the finite deformation theory and nonlinear theory of hard magnetoelasticity, this paper develops a theoretical model for a residually stressed curved HMS bilayer induced by pre-stretch under external magnetic fields. It is found that the bending angle of the bilayer can be adjusted by tuning the pre-stretch, thickness ratio and shear modulus ratio. This bending angle influences the magneto-mechanical response of the bilayer, thereby affecting the set of points reachable at the free-end of the bilayer (i.e., the workspace). The theoretical model's accuracy is validated through comparisons with experimental and simulation results. The study finds that as the pre-stretch increases, the thickness ratio and shear modulus ratio of the bilayer decrease, leading to opposite trends between workspace, maximum curvature and weight. Furthermore, based on the theoretical model, the Non-dominated Sorting Genetic Algorithm-II (NSGA-II) is adopted to address the multi-objective optimization problem of HMS bilayer. The Pareto fronts of the three-objective optimization containing weight, maximum curvature and workspace are obtained. This work provides the theoretical guidance for designing and optimizing magnetically soft robots in future applications.