Auto-tuning PI controller for surface tracking in atomic force microscopy - A practical approach

Abstract

Correct tuning of feedback gains is important for AFMs to cope with uncertainties of the system dynamics coming from a large range of different samples, cantilevers and scan parameters. For state of the art AFMs gains have to be adjusted manually by the operator. The typical operator such as biologist, physicist or material scientist may not have detailed knowledge about control engineering. In order to increase usability and acceptance an easy and intuitive approach is needed. The method presented in this paper is based on the commonly applied manual tuning strategy for AFM-imaging and copies the behavior of an experienced user. By spectral analysis ringing of the feedback loop is detected when feedback gains are increased beyond the stability margins. Increasing gains is stopped when an 1/f stop criteria is reached. The algorithm is successfully tested in simulation and practical AFM topography measurements with different cantilever - sample combinations, demonstrating that auto-tuning can be applied to achieve imaging performance close to ideal settings.