M.Sc Thesis


M.Sc StudentBen Yosef Eyal
SubjectNumerical Investigation of PMN-PT for Twisting
Actuators
DepartmentDepartment of Mechanical Engineering
Supervisor PROF. David Elata
Full Thesis textFull thesis text - English Version


Abstract

This research considers the use of piezoelectric PMN‑PT for the fabrication of a twisting actuator. PMN‑PT is a single‑crystalline, ferroelectric material with great piezoelectric coupling coefficients. These coefficients are significantly higher than those of most of the common piezoelectric materials used in industry, such as PZT. Therefore, PMN‑PT is a leading candidate to replace PZT in piezoelectric actuators and sensors.

Piezoelectric materials are often used to actuate micromirror devices, using a motion conversion scheme. The actuator converts the deflection induced to the piezoelectric material into a tilting motion of the mirror. In previous works, our research group designed a polycrystalline PZT beam actuator which responds in pure twist, using angled interdigitated electrodes, enabling a better actuation scheme for micromirror devices.

In this research, we wish to examine the applicability of PMN‑PT as a material for pure torsion beam actuators. To this end, we analyze its advantages and limitations, specifically in terms of its low coercive field and dielectric strength. Then, we build two‑ and three‑dimensional finite elements models of bending and torsion beams, and find the optimal operating scheme for each device.

Using a self-written iterative solution scheme, we found the optimal actuation voltage inducing maximum deflection of our bending beam. Notice that this scheme considers the re‑poling effect in ferroelectric materials. In addition, this optimal voltage is found to be considerably lower than the voltage causing dielectric breakdown (i.e. when the applied electric field exceeds the dielectric strength of the material).

Our simulations predict that our torsion beam will respond in pure torsion. The twisting angle increases with increasing actuation voltage, and the maximal twisting angle at the end of the beam is 0.609° for poling and actuation voltages of 12 [V] each. However, all our simulations are quasi‑static, so under dynamic actuation of the twisting actuator, the amplitude of the twisting angle is expected to be larger than the static response, roughly by the quality factor of the system. Typical values of the quality factor of piezoelectric MEMS devices are in the range of 100. Therefore, we expect that under dynamic actuation, our twisting actuator can achieve twisting angles in the scale of tens of degrees. These values are comparable with the twisting angles achieved by current commercial micromirror devices.