Study of MRI-compatible Notched Plastic Ultrasonic Stator with FEM Simulation and Holography Validation

TL;DR

This work addresses the challenge of MRI-compatible actuation by designing a notched Ultem 1000 plastic stator for a piezoelectric ultrasonic motor. By coupling finite element modeling with time-averaged and stroboscopic digital holography, the authors validate traveling-wave generation and rotor drive while reducing magnetic field distortions. Key findings include $\pm 250\ \mathrm{nm}$ displacements at $100\,\mathrm{V}_{pp}$, a settling time of $\approx 3.4\ \mathrm{ms}$, and FEM–holography agreement within $12.6\%$, with insights into dual-resonance effects and manufacturing-induced asymmetries. The results demonstrate a viable, MRI-safe actuator design with improved rotary performance for intraoperative MRI-guided procedures.

Abstract

Intra-operative image guidance using magnetic resonance imaging (MRI) can significantly enhance the precision of surgical procedures, such as deep brain tumor ablation. However, the powerful magnetic fields and limited space within an MRI scanner require the use of robotic devices to aid surgeons. Piezoelectric motors are commonly utilized to drive these robots, with piezoelectric ultrasonic motors being particularly notable. These motors consist of a piezoelectric ring stator that is bonded to a rotor through frictional coupling. When the stator is excited at specific frequencies, it generates distinctive mode shapes with surface waves that exhibit both in-plane and out-of-plane displacement, leading to the rotation of the rotor. In this study, we continue our previous work and refine the motor design and performance, we combine finite element modeling (FEM) with stroboscopic and time-averaged digital holography to validate a further plastic-based ultrasonic motor with better rotary performance.

Figures (9)

• Figure 1: Dimensions of notched stator design, 22 notches with 1.59mm by 1mm were added to the stator based on the flat design.
• Figure 2: Notched stator design. (Left) CAD drawing of the notched stator. (Middle) FEM meshing of the notched stator. (Right) The notched stator made by Ultem plastic was fixed during the testing stage.
• Figure 3: configuration and results of multiple point probe monitoring the out-of-plane displacement. The motor settling time is around 3.4ms, where the inner, middle, and edge circumference reaches the maximum amplitude around $\pm$30nm, $\pm$60nm, and $\pm$100nm respectively.
• Figure 4: Plastic flat stator and digital holographic system on an optical table set up for quantitative imaging with nanometer resolution in full-field. Image imported from carvalho_2020.
• Figure 5: Notched plastic stator time-averaged excitation patterns versus simulation eigenmodes.