A helical spring is the most popular SMA shape for an actuator because it can produce more displacement than a
straight SMA wire. Our actuators consist of two SMA springs. One spring extends and raises the pin when heated,
while the other contracts when heated to return the pin to its original position. Thus, the position of pin is based on
tension/compression of two springs while the end-point position defined by using a magnetic latch mechanism.
The force generated by the SMA spring is determined by the constitutive equation proposed by Majima et al [11]. which
contains a relationship between the force and martensite fraction and ξ can be expressed as follows: