An energy harvesting device combined with a giant magnetoimpedance (GMI) sensor is presented to analyze low frequency vibrating systems. An electromagnetic harvester based on magnetic levitation is proposed for the electric power generation. The device is composed of two fixed permanent magnets at both ends of a cylindrical frame, a levitating magnet acting as inertial mass and a pick-up coil to collect the induced electromotive force. At the resonance frequency (10 Hz) a maximum electrical power of 1.4 mW at 0.5 g is generated. Moreover, an amorphous wire was employed as sensor nucleus for the design of a linear accelerometer prototype. The sensor is based on the GMI effect where the impedance changes occur as a consequence of the variations of the effective magnetic field due to an oscillating magnetic element. As a result of the magnet’s periodic motion, an amplitude modulated signal (AM) was obtained, its amplitude being proportional to mechanical vibration amplitude (or acceleration). The sensor’s response was examined for a simple ferrite magnet under vibration and compared with that obtained for the vibrational energy harvester. As a result of the small amplitudes of vibration, a linear sensor response was obtained that could be employed in the design of low cost and simple accelerometers.
In this work a sensitive micrometric non-contact position sensor based on the Giant MagnetoImpedance effect (GMI) is analyzed. A nearly zero magnetostrictive CoFeSiBCr wire was employed as sensor nucleus. The sensing principle is based on the changes in the high frequency electric impedance, Z, of the soft magnetic element as a function of the relative position of a permanent magnet generating a non-uniform magnetic field along the wire´s axis. The sensor sensitivity is analyzed in terms of the magnetic field gradient and wire's length. The comparison between the sensing response of a single wire element and a long wire (12 cm in length) with different voltage contacts along its axis is performed. Higher micrometric sensitivities are achieved in wires with a certain critical length. A slight enhancement of the sensor sensitivity is found under the single wire configuration below the critical wire length. These results are interpreted as the contribution of the characteristic closure domain structure at the sample ends in these soft magnetic wires. Finally, the application of the sensor for the detection of the daily micrometric trunk shrinkage variations in a lemon tree is presented. The results indicate that this type of magnetic sensors can be easily implemented in the agricultural sector, providing a low cost and sensitive detection technique regarding water monitoring purposes.
Active thrust magnetic bearings provide an axial force to balance the moving parts of machines. However, most devices produce null or unbalancing passive forces. Furthermore, reported designs usually feature very small axial and radial gaps. This paper presents a thrust actuator for wide axial gaps that produces both passive and active restoring axial forces. It features a long biconical rotor and a stator housing a single winding and two permanent magnets. Simulations are done using finite-element-analysis (FEA) and compared to magnetic circuit analysis and experimental results from a prototype with a diameter of 48 mm and 20 mm axial displacement.
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