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Title: Dual-loop control strategies for high-speed nanopositioning
Author: Altaher, Mohammed T. H.
ISNI:       0000 0004 7961 2973
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2019
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A nanopositioner is an accurate device capable of providing motion within nanometre scale resolution. Resolution and accuracy are the most important performance criteria in a nanopositioner. Numerous applications require such resolution, and accuracy most popularly involves scanning probe microscopy. Piezo-actuators are often used to maintain the desired performance specifications due to their various desirable properties, including but not limited to near infinite resolution, repeatability and fast and frictionless motion. There are key limitations to using piezo-actuators in positioning devices, however, most of which degrade performance. The main limitations are: low stability margin due to sharp resonant peak, nonlinear dynamics (hysteresis) and parameter uncertainty (unmodelled vibration modes). Many traditional control techniques deal with limitations that combine damping and tracking control. As such, damping controllers to damp the dominant vibration mode are used to increase the usable bandwidth by allowing a higher gain in the feedback control law. Tracking feedback control typically uses an Integral (I) or Proportional Integral (PI) to account for nonlinearity and thus to improve the accuracy of the nanopositioner. This method uses a single integrator for tracking; this is not particularly useful in the asymptomatic tracking of the triangular wave used in nanopositioning, however. The traditional control techniques used in nanopositioning produce significant positioning error in the useable scanning area for precise applications. Furthermore, most traditional techniques do not account sufficiently for hysteresis and are reliant on single-loop feedback. This thesis presents an enhanced tracking feedback control by using both linear and nonlinear control methods in a single- and dual-loop feedback control. Integral (I), Proportional Integral (PI) and fuzzy logic controllers are used as tracking controllers in the outer control-loop and an Integral Resonant Controller (IRC) is used as a damping controller in the inner control loop. The control techniques introduced precisely control the nanopositioner in the x and y directions. Tuning producers to find optimal control law parameters using mimicry of the Butterworth filter pattern is obtained. The optimal tuning of this control for higherorder feedback control is problematic due to uncertainty incorporating variable gain affecting stability. It is demonstrated that further modification of the optimal tuning method using low-order feedback control law stability is maintained. A further practical tuning procedure using the trial and error method to maintain stability is proven to achieve superior tracking performance. An integral feedback control law is implemented practically in the outer-loop and this is coupled with the IRC in order to achieve better performance than existing methods. The stability of the proposed control techniques is investigated and a criterion for stability boundaries of the damping and tracking control law is presented. Results are performed experimentally in the presence of plant uncertainty and hysteresis and also using the experimental data recorded and MATLAB simulation modelling. It is demonstrated that the positioning errors are significantly reduced while maintaining robust stability.
Supervisor: Aphale, Sumeet S. Sponsor: University of Aberdeen
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Engineering ; Nanotechnology ; Nanoelectromechanical systems ; Engineering instruments