Resistance Feedback of a Shape Memory Alloy Wire

The identification of a model able to relate the deformation of Shape Memory Alloy (SMA) wire to a state variable easily measured is widely studied in recent years. The electric resistance (ER) of a SMA wire varies depending on its martensite and austenite fraction, so the determination of the biunivocal relationship between the wire shortening and its electric resistance allows “previewing” the position of the SMA wire. In this way the resistance of the SMA wire can be used as feedback variable in a control system. A test bench was first used to determine the above mentioned relationship, and then this relation is used in a fuzzy control system to impose a desired position to a SMA wire and to check the response of the device. This type of control doesn’t need a position sensor, which is a certain advantage in terms of cost, overall dimensions and weight for possible applications. The performances of this position control with resistance feedback are good and adequate for many applications; in particular it will be suitable for all applications which do not dispose of space to place a position sensor—like in aerospace applications or micro manufacturing.