Optical control and manipulation of diamagnetically levitated pyrolytic graphite

When levitating above an alternating-pole permanent magnet array, pyrolytic graphite can be displaced by asymmetric diamagnetic forces resulting from optically-induced, localized temperature changes and the thermal dependence of pyrolytic graphite’s magnetic susceptibility. In this paper, we present methods for addressable, automated position control of levitating pyrolytic graphite samples acting as milli-robots for small-scale assembly and manipulation applications using optical actuation and machine vision techniques. A simple control method is presented that harnesses interactions between complex magnetic fields and small-scale thermomagnetic and optical material properties. A user may dictate arbitrary desired milli-robot positions to a closed loop control system, which automatically detects and actuates milli-robots to the desired positions and works to maintain them there. Hardware requirements and considerations are discussed along with software calibration, image processing, and control methodologies. Results from experiments demonstrating automated position control of pyrolytic graphite milli-robots of various sizes levitating over various permanent magnet array configurations are presented. Sequential control of multiple milli-robots in close proximity without work surface segmentation is also demonstrated successfully. The observed effect of milli-robot size to magnet array grid spacing ratio on milli-robot in-plane motion is discussed, concluding that larger ratios result in smoother and faster motion control due to relative decreases in magnetic barrier forces on the pyrolytic graphite milli-robot and minimized separation between minimum free energy positions.