Spiral Scanning With Improved Control for Faster Imaging of AFM

Abstract

One of the key barriers to an atomic force microscope (AFM) achieving high scanning speeds is its use of the traditional zig-zag raster pattern scanning technique. In this paper, we consider the use of a high-speed spiral imaging technique with an improved multi-input multi-output (MIMO) model predictive control (MPC) scheme with a damping compensator for faster scanning by an AFM. The controller's design is based on an identified MIMO model of the AFM's piezoelectric tube scanner (PTS) and it achieves a higher closed-loop bandwidth, significant damping of the resonant mode of the PTS, and reduces the cross-coupling effect between the PTS's axes. The spirals produced have particularly narrow-band frequency measures which change slowly over time, thereby making it possible for the scanner to achieve improved tracking and continuous high-speed scanning rather than being restricted to the back and forth motion of raster scanning. To evaluate the performance improvement using this proposed control scheme for spiral scanning, an experimental comparison of its scanned images with those of the open-loop condition is performed. Experimental results show that, by using the proposed method, the AFM's scanning speed is significantly increased up to 180 Hz.