Various design modifications were proposed for a torque actuator based on ferromagnetic shape memory alloy composites. These modifications were implemented into the design of the torque actuator, and prototypes were built and measured for their maximum torque, angle of twist, and work output characteristics. The design changes had an effect of increasing one or more of these characteristics with each iteration.
Experimental tests were conducted on a torque actuator based on ferromagnetic shape memory alloy composite and performance data obtained. Based on experimental results of the initial design, several design changes were made to the actuator that resulted in improved torque capability and maximum angle of twist compared to the original design. A full 3-D finite element method model was undertaken to optimize magnetic flux lines generated by the hybrid magnet.
Design of ferromagnetic shape memory alloy (FSMA) based spring actuators is discussed where a variety of design parameters are included, design of FSMA and FSMA composite, that of compact electromagnet, and the mode of deformation of a helical spring. Advantages of FSMA and FSMA composite are simple design, faster actuation speed large axial stroke and magnetic force.
A new first generation torque actuator based on ferromagnetic shape memory alloy composite is designed. The first generation actuator made of Fe bars and TiNi wires is successfully demonstrated. A simple model is proposed for prediction of angle of twist for a given constant load. The optimization of FSMA composite plate is made by a composite modeling under the constraints that the super-elastic SMA plate attains higher stress beyond the onset of the stress-induced martensite transformation while the ferromagnetic plate remains elastic.
A micromechanics approach is proposed to calculate the stress-strain relationship of a polycrystalline Fe-Pd ferromagnetic shape memory alloy. It is modeled as consisting of spherical grains, which are grouped according to their orientations with respect to the loading axis. Therefore, the internal stress and elastic energy are accumulated as straining proceeds due to the strain differences between differently oriented grains. In the present study, the energy dissipation of the interface movement is also considered. Furthermore, a stress-magnetic field-temperature phase transformation diagram is constructed. The magnetic field induced transformation is found to be insignificant based on thermodynamics model. The cases of Fe-Pd and NiMnGa systems are examined for 3D phase transformation diagram.
A new concept of a spring actuator based on the ferromagnetic shape memory alloy (FSMA) is presented. The coil spring made by a FSMA is activated by the attractive magnetic force produced by electromagnets, which is usually not uniform. When the magnetic field is applied, each turn of the spring comes into contact with the neighboring turns one by one, stacking from the turn closer to the yoke of the electromagnet. As a result, entire shrinkage of the spring accompanied by large liner stroke is achieved. This actuator is energy-efficient, since almost all magnet flux originated from electromagnet discharges into the ferromagnetic spring. The performance of the spring actuator, i.e. the output force and stroke, depends on many factors, such as the diameter and the pitch of the spring or the dimension of the cross section of the spring wire, and so on. We processed successfully a spring actuator driven by a hybrid magnet based on the above principle by using polycrystalline FePd alloy. Since the stiffness of the FePd coil spring become softer due to the martensite phase transformation, the movement of the actuator is accelerated during actuation.
Recently, ferromagnetic shape memory alloys (FSMAs) attracted strong attentions due to their fast actuation with relatively large strain. However, the mechanical properties of popular ferromagnetic shape memory alloys are found to be lower than TiNi alloys. A TiNi has high mechanical performances, large transformation strain and stress capability. On the other hand, the speed of TiNi alloys actuated by changing temperature is usually slow. Thus use of a ferromagnetic material is attractive in utilizing it for induction of magnetic force at high speed. If we combine both merits of TiNi and ferromagnetic material, we can design a new ferromagnetic SMA composite. The actuation modes examined here are plate bending and bar torsion, where the stress and strain across thickness direction change linearly with position. The outer layers that are subject to the larger strain are the superelastic TiNi, which sandwich the inner layer of a ferromagnetic material. The force activated in the ferromagnetic layer from the applied magnetic field causes the phase change in the superelastic TiNi layer, i.e. from austenite to martensite, thus the softening of stiffness of TiNi leads to large displacement.
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