In the molecular-beam epitaxy (MBE) process precise control over thickness, composition, and doping profiles are critical for end device performance. This article addresses the problem of accurately modeling and contr...
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In the molecular-beam epitaxy (MBE) process precise control over thickness, composition, and doping profiles are critical for end device performance. This article addresses the problem of accurately modeling and controlling the flux sources (effusion cells) used in MBE. A first-principles nonlinear dynamic model of an effusion cell is derived, investigated, tuned, and used for designing advanced closed-loop feedback schemes. The model is a coupled set of driven nonlinear ordinary differential equations. It provides the transient and steady-state time-dependent response of the cell to a given (heater power) input. Further, it provides output predictions of the produced molecular flux and the temperature read by a thermocouple built into all modern effusion cells. We show how to tune parameters in the model from actual experimental data using a nonlinear least squares identification algorithm. Simulations show excellent agreement with experimental data and empirical experience including reproduction of the well-known shutter transient event when the thermocouple temperature is regulated with a proportional-integral-derivative (PID) controller. The model further can be used to explore interesting effects. For example, the magnitude and duration of a shutter transient under PID regulation is different for gallium versus indium charged cells. This leads to a thin indium rich initial layer in the growth of InGaAs. The tuned model is used to investigate the efficacy of nested PID control designs and, finally, is used in an advanced feedback controller we design using differential geometric design methods of nonlinearcontrol theory. Simulations show our advanced nonlinearcontroller eliminates flux transients over a wide range of operating conditions without the need for any recalibration or adjustment and provides superior tracking. The latter feature of superior tracking will become increasingly important as future devices call for new growth regimes and methods such as
Describes a PC-based vision system that can be used to detect moving objects from a mobile robot. An image processing board equipped with an MPEG motion estimation processor calculates a sparse but robust optic flow i...
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Describes a PC-based vision system that can be used to detect moving objects from a mobile robot. An image processing board equipped with an MPEG motion estimation processor calculates a sparse but robust optic flow in real-time. An algorithm to evaluate this kind of optic flow has been realized in software. It determines relevant motion parameters and a simple scene interpretation in terms of moving object regions. The image processing board and the algorithm are presented in some detail; the performance of the system is demonstrated by experiments.
Consolidation of fiber reinforced metal matrix composite preforms at high temperatures seeks to eliminate matrix porosity while minimizing fiber microbending/fracture and the growth of reaction products at the fiber-m...
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Consolidation of fiber reinforced metal matrix composite preforms at high temperatures seeks to eliminate matrix porosity while minimizing fiber microbending/fracture and the growth of reaction products at the fiber-matrix interface. This paper evaluates to what extent uncertainty in geometry and material parameters affects the final microstructural attributes of the composite. Then several design methods and feedback control designs for robust regulation of consolidation are presented. Efficacy of the methods and designs are illustrated by exploring the design of process schedules for the Ti-6Al-4V/SCS-6 titanium matrix composite with variability in the matrix creep properties.
This paper deals with self-localization of a mobile robot on the condition that no a-priori knowledge about the environment is available. The applied method features to be accurate, robust, independent of any artifici...
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This paper deals with self-localization of a mobile robot on the condition that no a-priori knowledge about the environment is available. The applied method features to be accurate, robust, independent of any artificial landmarks and feasible with such a moderate computational effort that all necessary tasks can be executed in real-time on a standard PC. The perception system used is a panorama laser range finder (PLRF) which takes scans of its present environment. A modified Dynamic Programming (DP) algorithm provides pattern matching and pattern recognition on the preprocessed panorama scans and thereby renders a qualitative fusion of the sensor data. For an exact quantitative estimate of the robot's current position, a robust localization module is employed. The knowledge gained about the environment along that way is stored in a self-growing, graph based map which combines geometrical information and topological restrictions. Preliminary experiments in a common office environment proved the reliability and efficiency of the system.
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