Electropermanent magnet assembly

EPMs were assembled by hand following Knaian’s design¹⁵. The ferromagnetic poles were cut by wire electrodischarge machining from a 350 µm-thick Hiperco 50 sheet (EFI Specialty Metals, Los Alamitos, CA). This Iron–Cobalt–Vanadium alloy has ideal magnetic switching properties: low coercivity and low A.C. core loss³⁰. These properties enable fast magnetic switching with minimal losses. Furthermore, this material has a high magnetic saturation that allows the poles to magnetize to the full extent of the magnetization of the permanent magnets without saturating. N40 grade Neodymium–Iron–Boron (NIB) rods were used for the hard magnets (Amazing Magnets, Anaheim, CA). These magnets measure 3.2 mm long and 1.6 mm in diameter, have a high coercivity (H_c) of 907 kA/m and a remnant magnetic field (B_r) of 1.25 T. Grade 5 Aluminum–Nickel–Cobalt (Alnico) rods were used for the soft magnets (Magnet Kingdom, Medford, NY). These magnets measure 12.7 mm long and 1.6 mm in diameter, have a low H_c of 50 kA/m and a B_r of 1.25 T. To match the length of the NIB magnets, the Alnico rods were cut and ground lengthwise with a rotary tool. To make the EPM sandwich assembly, the pole pieces and the two magnets were glued in place with Loctite Hysol E60-HP epoxy adhesive (Henkel, Westlake, OH) and hardened overnight at room temperature. Excess glue was then sanded to render a smooth surface for the coil. The coil was made by winding 100 turns of 40 AWG bondable magnet wire (MWS Wire Industries, Westlake Village, CA). Finally, a drop of ethanol was used to activate the bond layer of the wire, after which the EPMs were dried for 15 minutes at room temperature. The activated bond layer fuses the windings together, adding mechanical stability. The completed EPM assembly is shown in Fig. 2b.