A Resonant Piezoelectric S-Morph Actuator With a Large Vibration Force Operating at a Haptic Frequency in a Hexahedral Smart Watch With a Small Form Factor

We developed a resonant piezoelectric S-morph actuator (RPSA) with a large vibration force operating at a haptic frequency for the application of hexahedral smart watches with a small form factor. We optimized the piezoelectric S-morph composed of two piezoelectric layers and one elastic layer to minimize the tilting vibration, which causes noise and reduces the vibration force. We also designed the thickness and length of the piezoelectric S-morph in such a way that the proposed RPSA has the first natural frequency in the range of the haptic frequency even under the small form factor of smart devices. We calculated the displacement and bending angle of the piezoelectric S-morph in the static state and compared the results to the piezoelectric unimorph using a theoretical equation. Furthermore, we utilized finite element analysis to calculate the natural frequency, output displacement, vibration force at the S-morph, and unimorph modes of the proposed RPSA. Finally, we verified the proposed design method as well as the proposed RPSA by comparing the simulated natural frequency and output displacement with measured values obtained in the vibration experiments of the prototype RPSA. The prototype RPSA generates a maximum output displacement of 620 μm at a haptic frequency of 185 Hz with a small tilting vibration.