Simulation-driven parameter study of concentric Halbach cylinders for magnetorheological robotic grasping

Due to their peculiar property of controlled stiffness and strength under an external magnetic field, magnetorheological (MR) fluids show great potential in developing hybrid robotic grippers that in the future could offer the same versatility as the human hand. This demands for sufficiently strong, compact and switchable magnetic field sources for which permanent magnets are often overshadowed by electromagnets. However, permanent magnets possess higher magnetic flux densities per mass unit, and when assembled in certain ways, they allow to control their joint magnetic field. Within this paper, an extensive parameter study is conducted using grid search for the design of concentric Halbach cylinder assemblies based on finite element simulations. The influence of decisive geometric and material properties on the performance of these magnetic activation mechanisms is studied. These include the magnets’ shapes, sizes and number; the cylinders’ radii and number of pole pairs; and the relative permeabilities of the MR fluid and the grasped object. The performance of a design is measured by a multi-objective function that considers: the mean magnetic flux densities generated in the mechanism’s ON and OFF-state, the magnetic field’s inhomogeneity (i.e. standard deviation) in the ON-state, and the total magnet area for both cylinders. This work concludes by deriving guidelines for the most optimal design of concentric Halbach cylinders for a cylindrical radial bellow gripper.