Thermoplastic forming of bulk metallic glass for surgical blade edges: A robust control approach

In corneal surgery, higher quality blades result in shorter recovery time, less trauma to the eye, and reduced possibility of repercussion. Bulk metallic glass (BMG) due to its high strength, hardness, and biocompatibility at room temperature is an economically viable alternative material for manufacturing surgical blades. Thermally-assisted micro-molding and micro-drawing process enables making surgical-grade knife blade cutting edges from BMG samples. In manufacturing of multi-faceted bulk metallic glass knife edges, the control of both mold temperature and velocity of the drawing actuator is critical to the thermoplastic deformation, which ultimately determines the quality of the blade edges. In addition, it is necessary to develop drawing velocity profile that can accommodate various shapes of surgical blades ensuring the consistency in the manufacturing of good quality surgical blades. To regulate temperature of the molding dies, we propose an electric resistance heating system, where the current through the resistor is accurately controlled using a buck converter. A nested inner-current outer-temperature control system for high-fidelity temperature regulation is proposed and implemented. For current control, we use switch-averaged electrical model together with appropriate feedback-linearization techniques to address the complex nonlinear dynamics that describe a buck converter. The outer-temperature control design ensures temperature regulation of a heater by prescribing real-time reference power that the current-control loop should provide. The challenge of nonlinear relationship between the required reference power and the regulated current is addressed by exploiting the high-bandwidth of the inner-current loop. The control objectives of regulation performance and robustness to modelling uncertainties of both current and temperature regulation are solved using an optimal control (H∞) framework. Experimental results demonstrate that our control design achieved the required molding temperature of 673 ± 0.03 K at steady state. The velocity of drawing actuator is also critical for manufacturing BMG cutting edges. The reference drawing velocity profile is obtained based on the filament stretching process model and knowledge of the thermoplastic forming map of BMG. The reference drawing velocity profile identifies and covers two stages of the drawing process: the initial transient stage before the extensional viscosity stage is fully developed, and the extensional viscosity stage. A control design is synthesized and implemented to satisfy the objectives of the drawing velocity and its implementation feasibility. A H∞ control framework is used to achieve control objectives of regulation performance and robustness to modeling uncertainties. The proposed controller shows a significant improvement over other controllers including PID controllers in terms of robustness to uncertainties and tracking performance. Good quality blades with 90° straight edges are manufactured with consistent straightness and blade edge radius. A mechanistic approach is proposed to determine drawing velocity profiles for several shapes of the blade edges. In this study, the velocity profile is obtained from an appropriate analysis and understanding of how filament stretching process and blade geometry factor in regulating flow stress during the blade edge formation. To demonstrate the feasibility of the proposed approach, H∞ controller is used to facilitate the tracking of velocity profile for 45° straight and crescent blade edges. Qualitative assessment of manufactured surgical blades shows consistency of the high degree of repeatability in terms of straightness, included angle, and edge radius. Finally, a feedback-based approach for generating velocity profiles is presented, where real-time measurements or estimations of blade diameter are used. This feedback-based design enables real-time correction of deviations from the desired geometry. Simulation results are presented that demonstrate the efficacy of this design.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Dancholvichit, Nattasit

Contributors dc:contributor

• Kapoor, Shiv G
• Salapaka, Srinivasa M
• Shao, Chenhui
• Beck, Carolyn L

• manufacturing
• process control
• robust control
• metallic glass
• mechanistic approach
• thermoplastic drawing

Statement dc:rights

• Copyright 2022 Nattasit Dancholvichit

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/115522

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F115522