  We present an analytical and experimental study of torsional magnetic
mechanism where the restoring torque is due to magnetic field interactions
between rotating and fixed permanent magnets. The oscillator consists of a ball
bearing-supported permanent magnet, called the rotor, placed between two fixed
permanent magnets called the stators. Perturbing the rotor from its equilibrium
angle induces a restoring magnetic torque whose effect is modeled as a
torsional spring. This restoring effect is accompanied by dissipation
mechanisms arising from structural viscoelasticity, air and electromagnetic
damping, as well as friction in the ball bearings. To investigate the system
dynamics, we constructed an experimental setup capable of mechanical,
electrical and magnetic measurements. For various rotor-stator gaps in this
setup, we validated an analytical model that assumes viscous and dry (Coulomb)
damping during the rotor free response. Moreover, we forced the rotor by a
neighboring electromagnetic coil into high amplitude oscillations. We observed
unusual resonator nonlinearity: at large rotor-stator gaps, the oscillations
are softening; at reduced gaps, the oscillations stiffen-then-soften. The
developed reduced-order models capture the nonlinear effects of the
rotor-to-stator and the rotor-to-coil distances. These magnetic oscillators are
promising in low-frequency electromagnetic signal transmission and in designing
magneto-elastic metamaterials with tailorable nonlinearity.
