Impact Force Sensing with Magnetostrictive Fe-Ga Alloys

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Fe-Ga alloys (Galfenol) are structural magnetostrictive materials which undergo magnetization changes when subjected to mechanical stress. They are machinable and can withstand large normal or shear stresses. Based on these unique features, this study develops an impact force sensor which consists of an electromagnet, magnetic circuit, cantilevered Fe-Ga alloy beam, and pickup coil. The external impact force generates a stress-induced flux density that is measured by the pickup coil. An axial impact sensor based on a Fe-Ga rod is constructed for comparison. Analytical modeling shows that the sensitivity of the cantilevered beam configuration is 11.27 times higher than that of the rod configuration. Three different geometries, including a rectangular beam, a uniform I beam, and a tapered I beam, are designed and compared. Analytical modeling shows that the tapered I beam exhibits maximum sensitivity. The optimized tapered I beam-based sensor is constructed experimentally and benchmarked against a similar sensor based on a Fe-Ga rod. A nonlinear Levenberg-Marquardt fitting method is used to correlate the input impact force with the resulting flux density variation. Experimental results show that the measurement error is within 5.8% for various impact amplitudes.