A Rotating-Permanent-Magnet Array for ULF Through-the-Sea Magnetic Communications

To realize magnetic induction (MI) communication through seawater, a rotating-permanent-magnet array (RPMA) operating in the ultralow frequency (ULF) band is proposed. The array consists of
$5\times $
5 cylindrical Nd–Fe–B magnets driven by servo motors. To maximize the transmitting magnetic moment, the motors are controlled to rotate synchronously through network communication. The design of the magnet and array configuration is implemented by analyzing the torque and magnetic moment. Magnetic forces between the array magnets are analyzed by using the equivalent static magnetic dipole model. Permalloy sleeves are used to reduce the influence of magnetic force on the synchronization of magnets. The total transmitting magnetic moment of the RPMA is 360 A
$\cdot ~\text{m}^{2}$
, and its power efficiency is 14.4 times higher than the conventional transmitting system using coils. Binary frequency shift keying (BFSK) modulation is realized by switching the rotating speed of the magnets and verified by an indoor test. The equivalent orthogonal horizontal magnetic dipole (HMD) model of the array is used, and the magnetic field distribution in layered media is simulated using FEKO. The calculated MI is 100 fT in the position with a distance of 1 km and a depth of 5 m in the sea. The results of the through-the-sea experiment show that the magnetic field of the RPMA can be detected in a range of 500 m by magnetic inductive sensors immersed at a depth of 5 m in seawater and agree well with the simulated results. Transmission bit rates of 10 and 5 b/s can be achieved in the positions of 60 and 110 m.