Temperature-Gradient-Based Partial Activation of Wet SMA Actuators for Displacement Output Linearity

Shape Memory Alloy (SMA) actuators have output non-linearities, posing challenges for control in robotic applications. As an alternative to model- and controller-based compensation, in this paper, with the objective of enhancing the linearity of the SMA output to make it possible to use conventional control algorithms, we present a novel physical compensation method. This method applies a temperature gradient to activate and deactivate specific longitudinal sections of the SMA. Adjusting the gradient's inflection point position allows control over the active/inactive length ratio, ideally linearly tied to the total actuator output. By using this activation technique, improving the output linearity of the SMA is possible. An actuator based on this concept was built and tested. The results of the experiment showed improved open-loop output linearity, time constant, and dead time compared to a typical SMA temperature controlled wet activation, which allowed us to combine the approach with a PID controller and displacement feedback to achieve a resolution of 2.6%, comparable to a typical pneumatic cylinder position control system. The concept presented brings about position control of SMA actuators without characterization experiments or temperature feedback and simplifies its control strategies.