A review of ring-type rotational MEMS traveling-wave ultrasonic micromotors: design, detection, control, and application validation

The ring-type rotational microelectromechanical systems (MEMS) traveling-wave ultrasonic micromotor has emerged as an important candidate for micro- and small-scale precision rotary actuation because of its thin-film structure, low driving voltage, less electromagnetic interference, and strong potential for system-level integration. In recent years, continuous advances in MEMS wafer processing, lead zirconate titanate (PZT) thin-film deposition and etching, and multi-layer interconnect packaging have driven substantial progress in micro-stator design, output performance, online sensing, motion control, and application validation of ring-type rotational MEMS traveling-wave ultrasonic micromotors (R-MTWUMs). Centered on the framework of design, detection, control, and application, this review systematically summarizes the development of R-MTWUMs. It first outlines their operating principles and representative fabrication processes, and then critically reviews the evolution of micro-stator configurations from substrate supported structures to nodal circle anchored and flexibly supported structures, and further to stator teeth and ball-amplified structures. Key performance metrics, including quality factor, driving voltage, torque, and rotation speed, are comparatively analyzed. On this basis, the review further summarizes the main detection methods used for R-MTWUMs, discusses recent progress in angular position sensing, driving and control, feedback electrode-based mode monitoring, and closed-loop control, and assesses their application potential through the application validation. Finally, this review summarizes major development trends, discusses the remaining challenges in fabrication, mechanism, evaluation, on-chip sensing, and closed-loop control, and outlines future research directions to accelerate the transition of R-MTWUMs from laboratory prototypes to practical engineering systems.