Vibration and position control of piezoelectric tube scanners for fast atomic force microscopy

Research Doctorate - Doctor of Philosophy (PhD)The performance of piezoelectric tube scanner in Atomic Force Microscope (AFM) is limited by vibrations and nonlinearities exhibited by the piezoelectric material such as hysteresis and creep. The aforementioned limitations restrict the use of the piezoelectric tube scanner for fast and high resolution operations. As such, this thesis presents several ways of improving the speed and accuracy of piezoelectric tube scanner for the use in Atomic Force Microscopy. In this thesis, two types of feedback control approaches are designed and implemented experimentally in order to improve the performance of piezoelectric tube scanners. The first approach uses strain voltage signal induced in the piezoelectric tube to measure of high frequency displacements of the scanner. Together with capacitive sensor, the use of strain voltage signal allows the closed-loop bandwidth to be increased for fast scans without the additional sensor noise otherwise contributed by the capacitive sensor during fast operation of the scanner. In the second approach, a Positive Position Feedback (PPF) control scheme is implemented on a commercially available AFM to compensate for scan-induced vibration and cross-coupling of its piezoelectric tube scanner. As a result of the implementation of the PPF control scheme, the scanning speed is doubled in comparison to the scanning speed obtained from the standard controller supplied with the commercial AFM. Finally, a spiral scanning method is comprehensively described and evaluated for the use in AFM. Two modes of spiral scanning method, Constant Angular Velocity (CAV) and Constant Linear Velocity (CLV) modes, are presented and compared with the widely used raster scanning method. The use of the spiral scan in CAV mode is shown to allow the scanning speed to be increased very high, approaching the mechanical bandwidth of the scanner. The use of the spiral scan in CLV mode allows scanning of samples to be done at linear velocity, a property shared with the raster scan