This study investigates how the placement and excitation frequency of piezoelectric actuators embedded in a soft silicone haptic thimble influence displacement patterns on the human fingertip. A finite element model (HapThimb) was developed in COMSOL Multiphysics by extending the DigiTip (Serhat & Kuchenbecker, 2021) model with a 4 mm thick Ecoflex 30 layer and four tangentially acting actuators positioned on the bottom, front, and both sides of the fingertip. The model simulates both free and forced vibrations to identify resonance modes and actuator-specific deformation patterns.
Free vibration analysis revealed that the addition of the thimble significantly reduced natural frequencies, with the first eigenmode shifting from 103.5 Hz (bare finger) to 45 Hz (with thimble).
Moreover, the number of observed modes increased, reflecting the thimble’s contribution to the complex dynamic behaviour of the system. Forced vibration analysis across the frequency range of 1–260 Hz revealed that actuator location has a strong effect on both the amplitude and spatial distribution of displacements. The bottom actuator yielded the highest local response (53.8 μm at 185 Hz), while the front actuator produced weaker, and localised responses. The side actuators, activated in-phase, resulted in the broadest and most versatile vibrational patterns, exciting multiple finger regions with peaks up to 41.4 μm. These findings highlight the importance of actuator placement in achieving desired tactile effects. The results inform design strategies for wearable haptic devices by identifying configurations that maximise vibrational efficiency and spatial selectivity. ... Read more