Iman Shahosseini

Planar Microcoil Optimization of MEMS Electrodynamic Microspeakers

A method for optimizing the planar microcoil of MEMS electrodynamic microspeakers with the aim of maximizing the electroacoustic efficiency is presented. The proposed approach is based on a mixed-model using both analytical models and finite element method (FEM). FEM simulation was used for computing the spatial distribution of the magnetic field created by the permanent magnets, making thus possible to analyze any geometry of permanent magnets. Different configurations of magnets were considered, and for each the planar copper microcoil was optimized while taking into account the technological constraints due to the microfabrication process, the associated electronics and the targeted acoustic power emission. The results showed that the proposed method predicts the force factor in very good agreement with experimental measurements carried out on the micromachined device. Moreover, according to the electro-mechano-acoustic model, these results showed that the optimized microcoil associated to the best magnet configuration increases the electroacoustic efficiency by more than 200% compared to conventional microspeakers.

Ultrasoft Finemet thin films for magneto-impedance microsensors

FeCuNbSiB thin films have been deposited using RF sputtering. Characterizations have shown that oxygen contamination and residual stress are mainly responsible for magnetic hardening. The sputtering and annealing conditions have been optimized and films with coercive field as low as 10 A m-1 (0.125 Oe) have been achieved. In addition, the influence of film thickness on the magnetic properties has been studied. Thus, magnetic field microsensors based on the magneto-impedance effect have been fabricated by stacking up Finemet/copper/Finemet films. The highest sensitivity (4000 V/T/A) is reached for 750 nm thick films. It is in the same range as cm-sized macroscopic devices realized using 20 μm thick ribbons.

Mechanical Amplifier for Translational Kinetic Energy Harvesters

This paper reports the design, optimization, and test results of a mechanical amplifier coupled to an electromagnetic energy harvester to generate power from low- amplitude (±1 mm) and low-frequency (<5 Hz) vibrations in the presence of large static displacements. When coupled to a translational kinetic energy harvester, the amplifier boosts small vibration amplitudes by as much as 4x while accommodating translational displacements of more than 10x of vibration amplitudes. A complete electromagnetic energy harvester using this mechanical amplifier produces 16x improvement in output power (30 mW vs 1.9 mW without amplifier at 5 Hz), and a high power density of 170 μW/cm3.

Cylindrical halbach magnet array for electromagnetic vibration energy harvesters

This paper reports the design, optimization, and test results of a new magnetic structure for kinetic energy harvesters allowing seven-fold increase in power density compared to single-magnet configuration. Electromagnetic energy harvesters with “single cylindrical Halbach array” and “double-concentric Halbach array” magnetic structures composed of NdFeB magnets with 1.4 T residual flux density were fabricated and tested: respectively 5 mW and 15 mW power was measured and generated at low-amplitude (1 mm) low-frequency (<;10 Hz) vibrations. These structures yield respectively a maximum power density of 14 mW/cm³/g² and 26 mW/cm³/g².

Electromagnetic generator optimization for non-resonant energy harvester

This paper reports the modeling, optimization, and experimental results of a non-resonant electromagnetic energy harvesting system. The novelty of this paper is embedded in the use of finite element and theoretical analyses to find the optimum configuration of the generator coil. With the objective of maximizing the output power for a given magnetic structure, the coil is sized in length, thickness, and its relative positioning to magnets. Dynamic tests are also performed on first prototypes to verify theoretical calculations. As a result, 2.2 mW output power was generated at 12 Hz with vibration amplitudes as low as 1 mm. Contrary to narrow bandwidth of resonant energy harvesters, the proposed design remains operational at wide range of excitation frequency (3 - 12 Hz as tested) while requiring no tuning mechanism.

Electrodynamic MEMS: Application to Mobile Phone Loudspeakers

This paper presents an electrodynamic MEMS for mobile phone loudspeaker applications. The whole structure of the loudspeaker is a new conception to reach higher performances than in existing devices: a linear behavior to ensure a high acoustic fidelity and a high efficiency to increase the power autonomy. So, the motor is ironless, constituted of permanent magnet only. Several electrodynamic structures are presented and studied with analytical formulations of the magnetic field. The emissive part is a plane silicon surface, very rigid and light, the suspension is achieved by silicon beams, which are not sensitive to mechanical fatigue, the electroplated copper coil is thick and requires a specialist technique to be deposited. The moving part displacements are in a range far larger than in existing MEMS (600 μ m). The trends for dimensioning the structure are investigated and prototypes realized and tested, with NdFeB ring magnets. As a result, the 70 dB SPL at 10 cm bandwidth reaches up to 100 kHz, and the behavior is particularly linear.

Magnetic Properties of NiFe and FeCuNbSiB Thin Films: Effect of Thickness on Static Permeability

The magnetic static permeability of sputtered FeCuNbSiB and electrodeposited NiFe patterns with thicknesses varying from 0.1 to 8 μm has been studied. The permeability varies proportionally with the pattern size/thickness ratio, like for uniformly magnetized patterns. Different hypothesis on this variation have been investigated and a critical analysis favors a demagnetizing effect. The comparison between experimental results and the theoretical value of the demagnetizing factor of squares plates confirms MOKE imaging, i.e. large magnetic domains but a non-uniform magnetization in the samples.

High acoustic performance MEMS microspeaker

In this paper, a theoretical approach for the electrodynamic motor optimization has been presented. The analytical simulations of the electroacoustic efficiency were validated with an experimental measurements were we have seen a very good agreement. The same assembled device was characterized in an anechoic chamber, where we have detected an SPL around 80 dB for 0.5 W.

Acoustic vs electric power response of a high-performance MEMS microspeaker

This paper presents the characterization of the acoustic performance of an electrodynamic MEMS microspeaker and the influence of magnets position, which allows halving device size while intensifying the force factor. This work is based on the previously reported MEMS microspeaker dedicated to handheld electronics. A vacuum-formed seal has been designed and applied to enhance acoustic performance at low frequencies. The efficiency of the electroacoustic conversion is increased by a factor 5-10 compared to conventional microspeakers and low harmonic distortions were observed. Measurements of the acoustic power versus electrical input power were carried out in an anechoic room within the hearing range.