Closed-loop control of a monolithically 3D nano-printed electromagnetic lens scanner with an integrated Hall sensor

TL;DR

This paper demonstrates a closed-loop control system for a 3D nano-printed optical lens scanner using an integrated Hall sensor, achieving high accuracy and stability by compensating for material hysteresis and drift.

Contribution

It introduces a practical closed-loop control method with integrated sensing for monolithically 3D nano-printed optical microsystems, eliminating the need for additional microfabrication.

Findings

Achieved mean absolute accuracy of 0.86 um

Attained displacement precision of 0.49 um

Effectively suppressed hysteresis and drift

Abstract

3D nano-printing through two-photon polymerization enables monolithic manufacturing of mechanical and freeform micro-optical elements with high inherent alignment accuracy. However, viscoelasticity and temperature-dependent stiffness of the photopolymer lead to hysteresis and drift, which significantly degrade open-loop position accuracy and long-term stability in quasi-static operation. Therefore, closed-loop control with integrated displacement sensing is essential for 3D nano-printed optical microsystems in practical precision positioning applications. Here, we present a closed-loop control system for a such a lens actuator that uses a commercial 3-axis Hall sensor for position tracking. A NdFeB micromagnet encircling the integrated microlens provides both the actuation force and a position-dependent sensing signal. The Hall sensor, located between an anti-Helmholtz-like coil pair that drives the scanner bidirectionally, measures the combined field due to the micro-magnet and the coil pair. Calibration-based subtraction separates the coil field and sensor offset contributions to recover the magnet field and thus the axial magnet displacement. Closed-loop operation yields a mean absolute accuracy of 0.86 um and a precision of 0.49 um over a displacement range of 150 um while eliminating viscoelastic creep, suppressing hysteresis, and minimizing temperature-induced displacement drift under coil self-heating. This sensing approach requires no additional microfabrication steps and provides a practical path toward stable and repeatable positioning for monolithically 3D nano-printed optical microsystems.