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

Abstract

Magneto-mechanical transmitters offer a compact and low-power solution for the generation of ultra-low frequency (ULF) magnetic signals for through-ground and throughseawater communications. Resonant arrays of smaller magnetomechanical transmitters are particularly interesting in this context as the physical scaling laws allow for the increase of operating frequency and reduce the power requirements for ULF signal generation. In this work, we introduce a generalized model for accurate prediction of frequency and mode shape in generalized magneto-mechanical resonator arrays (MMRAs) that accounts for near-field magnetic interactions as well as magnetically induced nonlinearity. Using experiments, we demonstrate that our predictive capability is significantly improved compared against simplified dipole approximations. We additionally model the eddy current losses internal to the array and find that they are in agreement with experimental observations.

Simultaneous Maximization of Voltage and Power Efficiencies in Magneto-Mechanical Transmitters

“…We can find this resonance condition using Eq. 3, with the simplification that the damping coefficient β of the MMT is very small for most MMTs [5], [6], [14]. The denominator of FPRW(ω) is then dominated by the coil resistance R c , implying that the shape of FPRW(ω) closely matches the shape of H θ/I (ω), which exhibits a resonance as we will see in the following paragraph.…”

“…Under approximation that the angular motion of the MMRA dipole is relatively small, we can expand the sinusoidal using Taylor expansion and ignore the higher-order dependencies and simply write B M M RA (t) = A θ θ(t) and B c (t) = A I I(t) where A θ and A I are coefficients that describe the relationship between the magnetic fields and the current and rotor motion, whose values have been discussed in Ref. [14] and Ref. [15], respectively.…”

“…Since the permanent magnetic dipole brings the power requirement for generating the magnetic field to exactly zero, the power dissipation now only appears in the driving mechanism [5], the mechanical losses in the suspension system [13], and the eddy current losses in the magnetic materials [14]. Notably, as the mechanical oscillation frequency decreases, the associated mechanical loss and eddy current loss also decrease rapidly.…”

“…A typical oscillatory MMT consists of a drive coil and a magneto-mechanical resonator array (MMRA) [14] as shown in Fig. 1a.…”

“…The resulting mechanical motion of the permanent dipoles produces a time-varying magnetic field at all points in space around the MMT, whose amplitude is directly related to the oscillatory amplitude of the rotors [5]. The driving frequency ω of the source is always set to be close to the mechanical resonant frequency of the MMRA [5], [6], [14] in order to achieve higher voltage efficiency, which can be expressed in terms of the time-varying magnetic field signal strength at the receiver normalized to the voltage level applied on the drive coil. We can also define power efficiency as the time-varying magnetic field signal strength normalized to the power dissipated in the MMT system.…”