Mechatronics

Chapter 4

Robust Control of Atomic Force Microscopy ¹

The atomic force microscope (AFM) is an instrument used for acquiring images at nanometer scale. Obtaining better image quality at higher scan speed is a research area of great interest in the control of an AFM. Improving the dynamic response of the scanning probe in the vertical direction and the dynamic response of the scanning motion in the lateral plane are the two major areas of application of advanced control methods to an AFM. The uncertainties inherent in the models of AFM vertical and lateral direction motion stages dictates the application of robust control methods. In this chapter, robust control methods are applied to AFM, treating first the vertical direction and then the lateral plane.

4.1. Introduction

Improvement in the AFM stage dynamics is achieved either by designing stages with higher bandwidth or by designing more sophisticated controllers rather than in the PI, PID, or PIID types of controllers that are most commonly used in practice. A robust repetitive controller is used for the vertical direction as it can reject higher frequency disturbances due to the periodic part of the surface topography in AFM much better than a conventional controller. Besides increasing the scan speed, it is also important that the phase lag can be compensated using repetitive control, with the knowledge of the surface topography from the previous period by introducing appropriate phase advance into the controller. Next, a ...