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.
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Taishi Wada, Taishi Wada, Minoru Taya, Minoru Taya, "Spring-based actuators", Proc. SPIE 4699, Smart Structures and Materials 2002: Active Materials: Behavior and Mechanics, (11 July 2002); doi: 10.1117/12.474986; https://doi.org/10.1117/12.474986

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