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Electromagnetic signals in the ultralow frequency (ULF) range below 3 kHz are well suited for underwater and underground wireless communication thanks to low signal attenuation and high penetration depth. However, it is challenging to design ULF transmitters that are simultaneously compact and energy efficient using traditional approaches, e.g., using coils or dipole antennas. Recent works have considered magneto-mechanical alternatives, in which ULF magnetic fields are generated using the motion of permanent magnets, since they enable extremely compact ULF transmitters that can operate with low energy consumption and are suitable for human-portable applications. Here we explore the design and operating principles of resonant magneto-mechanical transmitters (MMT) that operate over frequencies spanning a few 10 s of Hz up to 1 kHz. We experimentally demonstrate two types of MMT designs using both single-rotor and multirotor architectures. We study the nonlinear electro-mechanical dynamics of MMTs using point dipole approximation and magneto-static simulations. We further experimentally explore techniques to control the operation frequency and demonstrate amplitude modulation up to 10 bits-per-second. We additionally demonstrate how using oppositely polarized MMT modules can permit systems that have low dc-field but do not sacrifice the ac magnetic field produced. [ABSTRACT FROM AUTHOR]

Background. In a country immersed in endless rounds of wars, retained metallic foreign bodies remain a significant dilemma in the daily practice of every Lebanese radiologist. When a shrapnel's hazard is of concern, the decision between performing or refusing a justified MRI exam is not always straightforward. In this small trial, we aimed to better understand the shrapnel's MRI safety by mimicking our daily practice. Methods. Five shrapnel with an incremental increase in their long axis were put in an animal flesh and then introduced into a 3 T magnetic field. The behavior of each shrapnel was concretely assessed by performing before and after magnetic field exposure CT acquisitions. Results. Translation along the z-axis ranged from 0.9 mm to 2.8 mm. Torque angle ranged between 2.8 and 54 degrees with an average of 15.62 degrees. Conclusions. Shrapnel's movements in the magnetic field are not negligible during the acute phase of injury where there is no reinforcing fibroblastic reaction and invite us to reconsider the MRI safety of these metallic foreign bodies. Standard radiographs may be sufficient, but a targeted CT scan may be of better value for a confident decision for assessment of shrapnel position near viscera and major vessels. [ABSTRACT FROM AUTHOR]

We present an analytical and experimental study of a torsional magnetic spring where the restoring torque results from magnetic field interactions between rotating and fixed permanent magnets (PMs). The rotating cylindrical PM, called the rotor, is supported on ball bearings between two fixed permanent magnets, called the stators. Perturbing the rotor from its equilibrium angle induces a restoring magnetic torque whose effect ressembles a torsional spring. Along with this restoring effect, dissipation mechanisms arise from structural viscoelasticity, air, electromagnetic damping, and friction in the ball bearings. To investigate dynamic effect of these restoring and dissipation mechanisms, we construct an experimental setup capable of mechanical, electrical, and magnetic measurements. For various rotor–stator gaps in this setup, we validate an analytical model that assumes viscous and dry (Coulomb) damping during the rotor's free response. Moreover, we force the rotor by an electromagnetic coil into high amplitude oscillations. We observe unusual resonator's nonlinearity: at large rotorstator 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 for low-frequency electromagnetic signal transmission and for designing magneto-elastic metamaterials with tailorable nonlinearity. [ABSTRACT FROM AUTHOR]