Magnetostrictive Vibration Sensor based on Iron-Gallium Alloy
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Magnetostrictive Vibration Sensor based on Iron-Gallium Alloy Supratik Datta and Alison B. Flatau Alfred Gessow Rotorcraft Center Department of Aerospace Engineering University of Maryland College Park, MD 20742 ABSTRACT This work characterizes magnetostrictive single crystal Fe84Ga16 (Galfenol) as static and dynamic sensor in bending mode. Galfenol patch bonded to a cantilevered aluminum beam was characterized for static and dynamic loading of the beam. A figure of merit has been defined and sensor parameters have been obtained for different stress conditions and Galfenol patch thickness. Issues related to application in static and dynamic sensing have been discussed. INTRODUCTION According to the Oxford English dictionary, a sensor is “a device which detects or measures some condition or property, and records, indicates, or otherwise responds to the information received.” Sensing of mechanical quantities like force, torque, strain, position or acceleration with high accuracy and sensitivity within a wide range and obtaining this data in an electrical form which can be conveniently processed and stored is of primary concern for many engineering applications that affect our day-to-day life. Most magnetostrictive sensors are based on the stress (σ) dependency of change in flux density (B) at a given bias field (H = Ho) which is known as Villari effect [1] and can be approximated by the linearized constitutive equation 1. B = d *σ + µ H (1) Here the magnetomechanical constant d* and permeability µ are functions of both σ and H. Magnetostrictive sensors have been commercially used as position and torque sensors and can be used for health monitoring of structures [2,3]. They can also be plated as thin films thus motivating their application as MEMS sensor. Alloys which exhibit large magnetomechanical coupling are generally brittle and can only be used in compression while the more ductile alloys (e.g. iron and nickel) have low coupling and hence poor sensitivity. Unlike crystalline materials, amorphous Metglas is both ductile and has a large coupling coefficient, with an exceptionally high figure of merit in sensing [4,5]. However, its metastable structure affects its sensing performance over time due to loss in its anisotropy and magnetostriction. Investigations by Kellogg [6] and Atulasimha [7] on iron-gallium alloys (Galfenol) have shown sensitivity (d*) as high as 30x10-9 TPa-1 and linear characteristics between B and σ for an axial compressive stress range of around 40 MPa along with a tensile modulus of 65GPa. These results motivated further investigation of Galfenol as static and dynamic strain sensor in bending mode [8]. In this work a Galfenol patch is bonded on to an aluminum beam and biased with a permanent magnet. When the beam is loaded, the rigidly attached patch strains along with the beam. This produces some stress in the patch and changes its flux density which is picked up by a Hall sensor and converted to a corresponding voltage signal.
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