Robust nanomanipulation control based on laser beam feedback

This paper reports a control strategy of a micro-gripper based on two AFM tips for manipulation at micro/nano scale. It is composed of dual micro/nano manipulators in order to handle and to maintain a microsample through the focus of a X-ray or laser beam for material characterization and analysis. The main idea is to control and to drive in a robust way the micro/nanomanipulators by focusing the beam on the center part of the handled micro-object. To this aim, the maximum intensity of the laser beam is measured in real-time by a four-quadrant photodiode sensor. As the sample under consideration here is a superparamagnetic microsphere of 8.2 μm (focusing laser spot less than few μm²), the laser tracking system is very sensitive to light intensity variations, mechanical vibrations, microhandling force perturbations and thermal relaxation of magnetic microsamples. First, we propose to compensate the laser beam variations by estimating the position of the laser beam using a particle filter (PF) algorithm. Then, a robust control strategy based on H_∞ controllers ensures a robust microhandling task under the focus of the laser beam whatever the external perturbations involved and parametric model uncertainties. The dual manipulators are controlled cooperatively by combining the different actuator dynamics to track a laser beam with nanometer precision. Finally, experimental results demonstrate the robustness of the microhandling task using the proposed robust control scheme.