High performance raster scanning of atomic force microscopy using Model-free Repetitive Control

The image quality of an atomic force microscope highly depends on the tracking performance of the lateral X-Y axis scanner. To reduce the requirement for accurate system models, this article describes a method based on Model-free Repetitive Control (MFRC) for high performance control of fast triangular trajectories in the X-axis while simultaneously achieving coupling compensation from the X-axis to Y-axis. The design and stability analysis of the MFRC scheme are presented in detail. The tracking results are experimentally evaluated with a range of different load conditions, showing the efficacy of the method with large variations in plant dynamics. To address the coupling from the X-axis to the Y-axis while tracking the non periodic staircase trajectories, a pre-learning step is used to generate the compensation signals, which are combined with the baseline Proportional-Integral (PI) control in a feedforward manner in real-time implementations. This approach is also applied to address the problem of longer convergence if needed. Experimental tracking control and coupling compensation are demonstrated on a commercially available piezo-actuated scanner. The proposed method reduces the root-mean-square coupled tracking errors in Y-axis from 191.4 nm in open-loop control or 194.6 nm with PI control, to 2.8 nm with PI+MFRC control at 100 Hz tracking frequency, which demonstrates the significant improvement achieved by the proposed method.

Automated summary

This article describes a method based on Model-free Repetitive Control (MFRC) for high performance control of a fast triangular trajectory in the X-axis of an atomic force microscope. The method achieves coupling compensation and reduces the requirement for accurate system models.