Nested Closed-loop Control of Quasi-Static MEMS Scanners with Large Dynamic Range

This article discusses the challenges in implementing a fully electrical closed-loop control system for high-end applications of quasi-static microscanners, accounting at the same time for wide aperture, high accuracy, and wide bandwidth. The discussion highlights how not only the ringing of the main mode but also the presence of spurious modes and nonlinearity impacts the closed-loop stability and accuracy. A novel control approach based on nested loops is conceived and implemented in the digital domain in a mixed-signal application-specific integrated circuit (ASIC). The developed versatile ASIC is designed to be coupled to different types of micromirrors, with fundamental frequency ranging from few hundred hertz to few kilohertz, by simply adjusting coefficients in the digital domain. The ASIC is here successfully operated with two micromirrors, and results demonstrate sawtooth position control at 28-32 degrees field of view and up to 120-Hz refresh rate, while holding at the same time a tilt angle resolution of 25 m degrees rms (dynamic range of 60 dB) and a linearity error below +/- 1%.