| Shape memory alloys have become increasingly popular for use in many engineering fields, including aerospace, robotics, and biomechanics. A major research focus is the application of Nitinol shape memory alloy wire as an actuator. While position and force control of shape memory alloy actuator wires has been successfully demonstrated in the past, most control algorithms have been developed using position feedback. Recently, it has been shown that there exists a significant correlation between the electrical resistance and strain of the material. This correlation has been used to model the strain as a function of the electrical resistance for use in predicting the actuator position for control purposes. However, the influence of applied stress as well as the presence of a third microstructure phase (R-phase) make modeling of the resistance-strain correlation difficult since hysteretic effects become more substantial. This thesis presents new models of the resistance-stress-strain behaviour of shape memory alloy for use in actuator position control. Characterization of the material behaviour was performed through experimental analysis, and used to develop the models based on empirical curve fitting. The models were then validated through simulation as well as application in a simple PID position control algorithm. Furthermore, two different alloys were investigated: a Ni-Ti alloy called Flexinol which exhibits significant hysteresis due to the presence of R-phase, as well as a Ni-Ti-Cu alloy which shows negligible hysteresis. |
