The effect of magnetic stress and stiffness modulus on resonant characteristics of Ni–Mn–Ga ferromagnetic shape memory alloy actuators

Abstract

The prospect of using ferromagnetic shape memory alloys (FSMAs) is promising for a resonant actuator that requires large strain output and a drive frequency below 1kHz. In this investigation, three FSMA actuators, equipped with tetragonal off-stoichiometric Ni2MnGa single crystals, were developed to study their frequency response and resonant characteristics. The first actuator, labeled as A1, was constructed with low-k bias springs and one Ni–Mn–Ga single crystal. The second actuator, labeled as A2, was constructed with high-k bias springs and one Ni–Mn–Ga crystal. The third actuator, labeled as A3, was constructed with high-k bias springs and two Ni–Mn–Ga crystals connected in parallel. The three actuators were magnetically driven over the frequency range of 10Hz–1kHz under 2 and 3.5kOe magnetic-field amplitudes. The field amplitude of 2kOe is insufficient to generate significant strain output from all three actuators; the maximum magnetic-field-induced strain (MFIS) at resonance is 2%. The resonant MFIS output improves to 5% under 3.5-kOe amplitude. The frequency responses of all three actuators show a strong effect of the spring k constant and the Ni–Mn–Ga modulus stiffness on the resonant frequencies. The resonant frequency of the Ni–Mn–Ga actuator was raised from 450 to 650Hz by increasing bias spring k constant and/or the number of Ni–Mn–Ga crystals. The higher number of the Ni–Mn–Ga crystals not only increases the magnetic force output but also raises the total stiffness of the actuator resulting in a higher resonant frequency. The effective modulus of the Ni–Mn–Ga is calculated from the measured resonant frequencies using the mass-spring equation; the calculated modulus values for the three actuators fall in the range of 50–60MPa. The calculated effective modulus appears to be close to the average modulus value between the low twinning modulus and high elastic modulus of the untwined Ni–Mn–Ga crystal.