Magneto-Mechanical Transmitters for Ultralow Frequency Near-Field Data Transfer

Abstract

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 humanportable 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.

Frequency Response and Eddy Current Power Loss in Magneto-Mechanical Transmitters

“…Similar promising solutions using magneto-mechanics have also been proposed for improvements in ULF receivers [13]- [15]. Interestingly, the scaling laws for such magneto-mechanical systems reveal that, instead of using one large magnetomechanical oscillator, subdivision of the magnetic dipole to an array of smaller oscillators provides a major scaling advantage for increasing frequency and for reducing the power requirements [5], [6]. At the same time, the technical complexity of the system does increase since we now must consider the mutual magnetic interactions within the array.…”

“…where µ 0 = 4π × 10 −7 H/m is the vacuum permeability, m r, i and m r, n are the magnitudes of the magnetic dipole moments of the i th and n th rotors, while d ni is the centerto-center distance between the dipoles. This dipole model is often used [5], [6], [16], [17] to study the resonances of magneto-mechanical systems. However, as the distance between the rotors decreases, this model becomes a poor predictor of the MMRA dynamics due to near field effects.…”

“…The biggest challenges with ULF systems are the significant power requirement [2], [3] and the need for large antenna structures that cannot be easily transported [4]. Recently, an innovative approach has emerged for the ULF transmitter challenge [5]- [12], in which the large magnetic field is produced by a fixed magnetic dipole, while the ULF carrier frequency generation is achieved through either torsional mechanical oscillation or rotation of this dipole. The generated field is non-radiating and should be considered in a quasi-static manner.…”

“…The generated field is non-radiating and should be considered in a quasi-static manner. It has been shown that this approach can reduce the transmitter power requirements from a few megawatts to tens of watts, and the transmitter size from a few kilometers to tens of centimeters [5], [6]. Similar promising solutions using magneto-mechanics have also been proposed for improvements in ULF receivers [13]- [15].…”

“…At the same time, the technical complexity of the system does increase since we now must consider the mutual magnetic interactions within the array. Even though there is an increase in the degrees of freedom, only the mechanically synchronized mode (i.e., the in-phase mode) is of interest since it has the highest resonant frequency and produces the largest magnetic signal [5], [6]. Presently, however, the models for frequency prediction and power consumption for magnetomechanical resonators are not sufficiently advanced, especially in the context of the complexity added by mutual interactions in arrays, near-field effects, eddy current loss, and nonlinearity.…”