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Title: Novel shear force scanning modes enabled with digital implementations of robust nano-precision controllers and estimators
Author: Zhang, Kaiqiang
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2019
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The research primarily focuses on the development of novel instrument and control (I&C) for a Transverse Dynamic Force Microscope (TDFM) system, which is a special Probe Microscope enabling the specimens morphology on nanometre scale. The TDFM genuinely allows accurate specimen morphology in non-contact manner, whereas typical Atomic Force Microscopes (AFMs) image the specimen topography via physical contact and therefore may cause damage to delicate specimens. The novel I&C design realises a digital control solution using field programmable gate array (FPGA) boards, which have high computational power running at fast implementation rates (e.g. 4 MHz). This novel system integrates a previous TDFM mechanical design with a novel cantilever-specimen separation sensing mechanism, an online reconfigurable digital control programme, etc. An offline optimisation method is developed to enhance the robustness of the digital control programmes to perform dynamic scans of various samples. The novel cantilever-specimen separation sensing mechanism exploits the cantilever oscillation amplitude in relation to the cantilever-specimen distance. The sensing method improves the sensing range up to ∼6 nm in contrary to the previous range of ∼2 nm. The nonlinear dynamics of the vertical positioning systems for the cantilever, which have not been sufficiently studied before, are investigated. Robust H∞ control algorithms are designed for two nonlinear vertical positioning systems. Two developed closed-loop systems are implemented and practically achieve robust cantilever positioning performance at 1 nm accuracy. Significantly, the thesis provides the first interaction force reconstruction method for real-time scanning in practice. The force reconstruction algorithm is developed employing a sliding mode observer using FPGA. The digital design and implementation are presented. Practical assessments show that the force reconstruction can identify the variation of the low interaction-force at sub-nN level in real-time. Therefore, the mechanical properties, including viscosity and elasticity, of the scanned specimen surface can be reconstructed. The developed TDFM system integrates these novel features collectively and enables three functional modes, i.e. contact mode, non-contact mode, and force-scan mode, at 1 nm precision avoiding physically cantilever-specimen contacts. The contact and non-contact modes are two specimen scanning modes that allow imaging nano-structure of delicate specimens. Specifically, the non-contact mode is focused and considered more powerful in the thesis. The completely novel force-scan mode reconstructs the cantilever-specimen interaction force allowing to analyse the specimen surface properties for various practical purposes, e.g. physics, chemistry, medicine.
Supervisor: Antognozzi, Massimo Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available