Active Cooling and Strain-Ratios to Increase the Actuation Frequency of SMA Wires

The maximum actuation frequency of an SMA wire does not only depend on the heating and cooling rates but also on the amplitude of strains at which it works. Smaller working amplitudes mean faster working frequencies. SMA-based actuators rarely work throughout their full strain range but they usually work at small amplitudes, which are amplified by means of mechanisms or by the optimised positioning of the wires within the actuator. Furthermore, the nonlinear relationship between the temperature and the strain on SMAs can be used in our favour to increase the actuation frequency of SMA wires by finding the optimum range of temperatures within which the wires must work for a specific strain amplitude. This work presents a method to find the maximum attainable actuation frequency of an SMA wire that is Joule heated and actively cooled while performing at different strain-ratios. The experimental results show that the actuation frequency of an SMA wire can be increased up to three and a half-fold (from 0.8 to 3 Hz) just by making it work within the most appropriate range of temperatures, under similar heating and cooling rate, wire’s diameter and applied stress.