Real-time target alignment system for high-power laser operations using a hybrid mechanism

Karim, Shah (2020) Real-time target alignment system for high-power laser operations using a hybrid mechanism. PhD thesis, University of Nottingham.

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To utilise the full potential of high-power lasers, such as for fusion energy applications, large-scale facilities need to operate at high-repetition rates. The challenge of positioning and aligning a target at the laser beam focus with an accuracy of few micrometres (typically ± 4 μm) is significant, but it is a fundamental requirement for a high-power laser interaction to ensure that targets are reproducibly accessible to the highest intensities available. This requirement represents a serious problem for a high-repetition rate laser system in which a fresh target has to be positioned and aligned at the laser beam focus at a rate of at least 0.1 Hz (with plans for 10 Hz or higher in future). Research presented in this thesis is the only known comprehensive study to focus on the demonstration of a real-time target alignment system to fulfil the requirements for the application of high-power high-repetition rate laser operation.

High-accuracy target alignment for high-repetition rate operation means that a real-time position compensation method is required to maintain the target’s reference position and plane throughout the operation. Achieving this requirement with a real-time target alignment (position and orientation) system comes with challenges, which can be broadly categorised in two areas: 1.capability development of the elements which are used for the real-time target alignment control system, and 2. design of the real-time target alignment control system. In this research, the performance improvement issues of the target’s position control system, in terms of its capability of delivering high-accuracy target positioning and orientation, were addressed before focusing on the development of the real-time position and orientation control system.

The position control system used in this research for the target alignment is a five degree-of-freedom hybrid kinematic mechanism, comprising both serial and parallel kinematic mechanisms. Although parallel and hybrid mechanisms have recently received much attention, and research is underway to extend their use in industrial applications, comprehensive studies of their design, kinematics, dynamics and error sources are lacking. For accuracy improvement of a hybrid or parallel mechanism, a high level of complexity, as compared to a serial mechanism, arises mainly due to the very high number of error parameters required to describe the mechanism kinematically. For solving the problem, this research presents a method for developing a simplified error model for a complex spatial mechanism with closed loops. The model was used to demonstrate a practical calibration procedure to significantly improve the positional accuracy of a hybrid or parallel mechanism. The procedure outlines effective strategies for carrying out simple measurements to determine the error parameters and compensating for the positional deviations of target with simple steps using software-based compensation technique.

For the real-time target alignment system, the design of the control system is based on an Abbe-compliant, in-process position measurement system of targets, employing a plane mirror interferometer and the hybrid mechanism. An error model was developed for the kinematic error analysis of the mechanism (dynamic influences, e.g. vibration, were not considered) associated with the high-repetition rate process to determine the position feedback information of the target during a high-repetition rate process. The model was also used to identify the effects of the non-collocation of the target and the measurement point of the interferometer on the control system’s performance - a challenge for the real-time position control of targets. The behaviour of the control system was investigated with the error model and experimental data. It was found that the controller’s position compensation scheme can be ineffective due to erroneous position feedback for the non-linear position information representing the non-collocated measurement point and the actual target. To solve the problem, an angular compensation technique was proposed for high-accuracy, real-time position and orientation control of targets.

The findings of this research are valid for wider applications. For example, 1. The method of simplifying the error model and the strategies developed for the calibration and compensation procedure can be used for other types of parallel or hybrid mechanisms, 2. The design principle of the real-time control system, the error model and the position and orientation compensation strategies can be applicable to the design of positioning systems requiring highly accurate position control of the end-effector.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Leach, Richard
Piano, Samanta
Keywords: High-power high-repetition rate lasers, parallel mechanisms, hybrid mechanisms, real-time target alignment, error model, kinematic calibration, in-process measurement system
Subjects: T Technology > TA Engineering (General). Civil engineering (General) > TA1501 Applied optics. Phonics
T Technology > TJ Mechanical engineering and machinery
Faculties/Schools: UK Campuses > Faculty of Engineering
UK Campuses > Faculty of Engineering > Department of Mechanical, Materials and Manufacturing Engineering
Item ID: 63459
Depositing User: Karim, Shah
Date Deposited: 07 Dec 2023 11:54
Last Modified: 07 Dec 2023 11:54

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