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Presented are a number of novel designs for large-motion shape-memory alloy (SMA) actuators that may minimize the performance constraints associated with such actuators. Shape-memory alloy (SMA) actuators suffer from two performance constraints, namely small strains (epsilon) , such that (epsilon) <EQ 6%, and heat transfer problems in computer- controlled ohmic heating of contractile SMA fiber bundles to induce contraction and/or expansion type linear actuators. Intelligent material systems and structures have become important in recent years due to some potential engineering applications. Accordingly, based on such materials, structures and their integration with appropriate sensors and actuators, novel applications, useful for a large number of engineering applications have emerged. In the present paper a number of conceptual designs and their respective mathematical models are presented for large motion SMA actuators. The dynamic modeling is preceded by a model of a small motion SMA linear actuator. This model considers the dynamic response of contractile fiber bundles embedded in or around elastic springs that are either linear helical compression springs or hyperelastic springs such as rubber-like materials. The proposed theory presents a description of such processes for resilient shape-memory alloy fiber bundles. We consider the fiber bundle of SMA to be either in a serial configuration with a linear tension spring or a parallel configuration circumscribed inside a helical compression spring with flat heads or in parallel with a number of helical compression spring, end-capped by two parallel circular plates with embedded electrodes to which the ends of the SMA fibers are secured. Thus, the fibers can be electrically heated and subsequently contracted to either expand the tension spring or compress the helical compression spring back and forth. Design details are first described. In essence the dynamic behavior of the actuator depends on the frictional interference effects as well as the interaction between the current supplied to the wires and the heat transfer from the wires. Further, a mathematical model is presented to simulate the electro-thermomechanics of motion of such actuators. The proposed model takes into account all pertinent variables such as the strain (epsilon) , the temperature of the fibers T(t) as a function of time t, the ambient temperature T0, the martensite fraction (xi) , the helical compression spring constant k, the frictional effect and the coefficient of friction (mu) and the overall heat transfer coefficient h. Numerical simulations are then carried out and the results are compared with experimental observations of a number of fabricated systems.
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