Michael Gevelber

Dynamics and control of the Czochralski process. I: Modelling and dynamic characterization

Abstract A nonlinear dynamic model of the Czochralski process, valid throughout the batch growth cycle is derived for use in designing an improved process controller. The model is a lumped element representation of the major system components, and simulates the dynamic system response to disturbances and system inputs. The linearized model is used to determine the system eigenstructure, revealing the system stability, transient response constants, and coupling. Significant results for control design include the identification of the basic time varying nature of the eigenstructure and the disturbances acting on the system, and identification of mechanisms that affect the transient system characteristics. In addition, the growth dynamic effects of liquid encapsulation, low thermal gradient schemes, and magnetic fields are discussed.

Diagnostics and Control in the Thermal Spray Process

The plasma-spray process features complex plasma-particle interactions that can result in process variations that limit process repeatability and coating performance. This paper reports our work on the development of real-time diagnostics and control for the plasma spray process. The strategy is to directly monitor and control those degrees of freedom of the process that are observable, controllable and affect resulting coating properties. This includes monitoring of particle velocity and temperature as well as the shape and trajectory of the spray pattern. Diagnostics that have been developed specifically for this purpose are described along with the demonstration of a closed loop process controller based on these measurements.

Dynamics and control of the Czochralski process II. Objectives and control structure design

Abstract A new control system design is proposed for the Czochralski process in order to improve crystal quality, particularly GaAs. An expanded set of formal control objectives are proposed based on a considerations of defect formation, segregation, and the process coupling. A control structure is developed that meets the control objectives and the restrictions posed by the batch related disturbances, system coupling, and the dynamic characteristics of the measurements, outputs, and inputs.

Advanced control design considerations for the Czochralski process

Abstract A new control system design is proposed for the Czochralski process. A set of control objectives is proposed to meet the materials objectives and limitations posed by the coupling of the process (i.e., heater, melt, meniscus, interface, crystal and environment). Attention is given to the requirements for gallium arsenide growth including thermal stress in the growing crystal. Analysis of the process dynamics indicates the disturbances and dynamic features that must be considered in the controller design.

Dynamics and control of the Czochralski process. IV: Control structure design for interface shape control and performance evaluation

Abstract Analysis of a low order model of the Czochralski process reveals the performance limitations in controlling both the crystal diameter and interface shape for both feedforward and feedback structures. A new control structure is proposed that maintains crystal diameter, interface shape, and thermal gradients throughout the batch growth cycle. Performance of alternative control structures are evaluated.

Dynamics and control of the Czochralski process III. Interface dynamics and control requirements

Abstract An expanded set of control objectives related to achieving line defect, crystal shape, and segregation requirements is proposed for Czochralski crystal growth. Consideration of the thermal stress near the interface yields an operating regime specification in terms of interface shape. A lumped order model of the process is proposed that captures the dominant nonlinear dynamics while relating important control objectives to conventional and advanced actuators. The dynamic characteristics of the interface shape and input/outputs that limit the achievable controller performance are identified.

The effect of relative humidity and evaporation rate on electrospinning: fiber diameter and measurement for control implications

This paper presents an experimental study of the influence that relative humidity and evaporation rate have on the electrospinning process in terms of fiber diameter, process measurements, and selection of operating regime (applied voltage and flow rate) for polyethylene oxide/water (aqueous) solutions and poly(vinylpyrrolidone)/alcohol (non-aqueous) solutions. Poly(vinylpyrrolidone) alcohol solutions are studied to understand the separate influence of relative humidity and evaporation rate. Correlations are developed that relate measurable process parameters (jet diameter, charge density) as well as relative humidity and evaporation rate to fiber diameter. In addition, the influence that relative humidity has on selection of operating regime to achieve desired fiber diameter and maximum production rate is presented.

Solidification modeling of plasma sprayed TBC: Analysis of remelt and multiple length scales of rough substrates

A two-dimensional, finite-element model based on an enthalpy formulation, was developed to simulate a splat solidifying on a rough substrate (with an idealized, sinusoidal-shaped roughness) capturing the multiple-length scales seen in real coatings as well as different aspect ratios. The model was used to study the effects of substrate temperature, splat temperature, and roughness characteristics on the onset and extent of remelt. Remelt is studied since it is indicative of local heat transfer conditions and might explain observed coating properties. Multiple splats were simulated using the two-dimensional model for short-time cooling coupled to a one-dimensional model for long-time cooling to predict substrate temperature rise prior to subsequent splat impacts. The presence of roughness promoted substrate remelting at conditions under which no remelting was observed for a smooth surface, suggesting that substrate roughness is an important parameter to include in splat solidification studies. The effects of splat temperature and substrate temperature on remelt were consolidated into a single nondimensional parameter, which captured a number of critical phenomena including characterization of the onset of remelt with a nondimensional remelting point.

Towards closed-loop control of CVD coating microstructure

The objective of our work is to develop a measurement-based feedback system can enhance the ability to achieve coating microstructure, compensate for process variations (disturbances) and improve the capability of transferring process recipes to different systems. Successful development of an appropriate control structure (i.e. selection and connection of measurements to specific inputs), however, requires an explicit understanding of the process dynamics.

Electrospinning Nanofibers: Measurements, Dynamics, and Control Strategy Development

Electrospinning produces submicron fibers for a variety of applications using a wide range of polymers. Achieving the desired fiber diameter, maximizing productivity, and minimizing variation are important production objectives. This paper addresses several important areas needed to develop a general electrospinning control approach including: developing a correlation between measurements, process conditions, and the resulting fiber diameter, developing a method to determine an operating regime that meets manufacturing objectives, and identifying process dynamics for controller design.Copyright