Torsional SMA actuator with adjustable parameters of working cycle

Abstract The aim of the present paper was a design of the torsional SMA actuator capable of stiffness variation alongside thermally responsive operation within tightly defined upper and lower temperature bounds and a study of the influence of the actuator parameters on the recoverable strain and work output variation during thousand thermal cycles under load conditions mimicking industrial use. A prototype SMA torsional drive was successfully developed, incorporating an induction heater specifically selected for its capability to achieve rapid, localized volumetric heating (> 300 °C/min) for automated thermal cycling with precise temperature control. Current frequency of 210 kHz was selected to ensure uniform sample heating (penetration depth ≈ sample radius) with rapid heating/cooling rates. Angle measurements were conducted via an integrated digital inclinometer. Variable stiffness was implemented by adjusting the mass attached to the lever, thereby enabling tunable mechanical behavior of the drive. Influence of thermal cycling with different cooling/heating rates and temperature ranges on the variation in the actuator parameters (recoverable strain, recovery stress and work output) was investigated. It was shown that the high cooling/heating rates achievable with induction heating exhibited no significant impact on the NiTi SMA drive’s performance compared to low-rate cycles, confirming the viability of this method for industrial use. This finding supports the integration of induction-based systems into drive designs for enhanced operational efficiency. Experimental results confirm that methodologies initially validated under limited cycling conditions can be effectively extended to prolonged thermal cycling (over 1,000 cycles). Confining thermal cycles to incomplete martensitic transformation ranges markedly improved the long-term stability of SMA drive parameters by suppressing degradation mechanisms, thereby preserving functional integrity across extended cycling.