Damping and tracking controllers for nanopositioning systems: applications for high-speed scanning probe microscopy

Nanopositioning systems are electromechanical devices that provide displacement with atomic- scale accuracy over a range of 10 to 1000 um. They are essential in a variety of scientific and industrial applications, such as in biological cell microinjection, nanoassembly and scanning probe microscopy. The wide application range introduces challenges for the design and control of nanopositioning systems, where the goals are a long travel range, high resonance frequency, high resolution and high bandwidth. However, as these goals are contradictory, compromises are inevitable. The first section of the study focusses on the control design methodologies that improve the perfor- mances of nanopositioning systems. The main limitation is due to the lightly damped resonance modes in the system which results in a low gain margin. A new damping control scheme is proposed to deal with the lightly damped resonance modes. This is paired with a Structured PI controller, and the tracking performance outperforms a standard PID controller when tested on a commercial objective lens positioner. Periodic references are commonly applied to nanopositioning systems. Hence, the tracking per- formances of such references are of interest. Improvements to Repetitive Control (RC), a popular controller for period reference, are described. RC schemes are normally paired with inverse and ro- bustness filters to maximise the tracking performance. A new method for synthesising the inverse filters is presented which is simpler, consistent and more efficient than the existing methods in the literature. In addition, the design process of the robustness filter is at present ad-hoc. This process is replaced with an automated process based on a convex optimisation and an uncertainty model. Experimental results are presented throughout to clarify and validate the concepts presented. The second section of the study focuses on scanning methods for scanning probe microscopy. The scanning method is shown to directly affect the accuracy of the positioning and the image quality. Hence, the scanning methods in the literature are reviewed and quantitatively analysed for performance. The relationships between the imaging quality and the scanning method are also examined. The theoretical predictions are accompanied by simulation and experimental results from a commercial atomic force microscope.