Fabrication and characterization of SMA film actuator array with bias spring for high-power MEMS tactile display

A MEMS-type SMA thick-film actuator (5 × 5 array) with a pitch of 2 mm was designed and fabricated for high-power vibrational stimuli to human skin. The deformable part (1.08 mm × 1.52 mm) of the SMA actuator was initially deformed to 320 μm to provide vibrational motion. The SMA actuator device was successfully fabricated without fractures in an SMA meandering actuator and microheater circuit when the SMA actuator was deflected to the initial displacement (320 μm). The surface temperature of the meandering-shaped SMA actuator successfully rose to about 65 °C, the shape recovery temperature, in a short time by high-pulse voltage heating (32 V) to the SMA. However, it was found that a cooling time longer than 99 ms was needed to completely cool down owing to poor heat conduction. Although the SMA actuator generated 38–40 mN of force by DC current heating (~72 mW), by periodic pulse-driving above 15 Hz, where human skin has high sensitivity, the generated force drastically decreased by 1/4 owing to a poor thermal response during the pulse-off time. The completed SMA tactile display with a bias-spring mechanism generated a displacement amplitude of 23 μm at a frequency of 1 Hz. However, the amplitude decreased to 10–13 μm at 15 Hz, which is slightly above the human perception limit