S. Choi

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Papers by S. Choi

Development of the Omni Directional Intelligent Navigator

IEEE Robotics & Automation Magazine, Mar 1, 1995

Page 1. SK Choi, J. Yuh, and GY Takashige Development of the Omni-Directional Intelligent Navigat... more

Two-dimensional self-patterning of PbTiO3 on a Nb–SrTiO3 (001) surface using atomic force microscope lithography and hydrothermal epitaxy

Applied Physics Letters, Apr 23, 2007

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Experimental study on fine motion control of underwater robots

Advanced Robotics, 2004

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Fault-tolerant system design of an autonomous underwater vehicle ODIN: An experimental study

International Journal of Systems Science, 1999

This paper describes the design and implementation of a fault-tolerant system for Omni-Directiona... more

A Novel Iterative Algorithm for Recovering Shape and 3D Information using Photometric Stereo with Point Light Sources in Attenuating and Scattering Media

International journal of computer applications, Dec 18, 2014

When photometric stereo technique is used to recover shape and 3D information of an object from m... more When photometric stereo technique is used to recover shape and 3D information of an object from multiple images, it is common to assume that the light sources being used are collimated. When the light sources that are being used are actually point light sources, as in robot applications for weld seam inspection or underwater imaging, such an assumption causes significant error. The error increases further when the imaging system is deployed in an attenuating and scattering media. In such situations, a purely analytical solution is not possible. Current work proposes a novel iterative algorithm for recovering shape and 3D information in such situations.

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Effect of film density on electrical properties of indium tin oxide films deposited by dc magnetron reactive sputtering

Journal of vacuum science & technology, Sep 1, 2001

Tin-doped indium oxide (ITO) films were fabricated using dc magnetron reactive sputter deposition... more Tin-doped indium oxide (ITO) films were fabricated using dc magnetron reactive sputter deposition of an In 10 wt. % Sn alloy target in an Ar and O2 gas mixture. To understand the relationship between the electrical properties of ITO films and O2 partial pressure, the resistivity, carrier concentration and mobility, film density, and intrinsic stress in the films were measured as a function of O2 partial pressure. It was found experimentally that an ITO film having both maximum conductivity and carrier mobility has a film density close to its theoretical density. For the ITO film with density close to theoretical, the mean free path was the same as the grain size (=columnar diameter). This indicated that the electrical characteristics of the ITO films discussed here depend strongly on the grain size. However, for the ITO films with density lower than theoretical, the mean free paths were smaller than the grain size. It is suggested that electron scattering at pores and voids within the grain is the major obstacle for electron conduction in the ITO films having a lower density. The measurements of intrinsic stress indicate that the density of ITO films is determined by bombardment of oxygen neutrals on the growing film.

Conductivity switching characteristics and reset currents in NiO films

Applied Physics Letters, Feb 28, 2005

Conductivity switching phenomena controlled by external voltages have been investigated for vario... more Conductivity switching phenomena controlled by external voltages have been investigated for various NiO films deposited by dc reactive sputtering methods. Pt∕NiO∕Pt capacitor structures with top electrodes of different diameters have showed increasing off-state current with the diameter of a top electrode and nearly the same on-state current independent of the diameter. Local conductivity switching behaviors have been observed in a series structure consisting of two Pt∕NiO∕Pt capacitors with different resistance values. By reasoning out conductivity switching mechanisms from the switching characteristics and introducing multilayers consisting of NiO layers with different resistance values, we have reduced the reset current by two orders of magnitude.

Experimental Study of Advanced Controllers for an Underwater Robotic Vehicle Thruster System

Intelligent Automation and Soft Computing, 1999

ABSTRACTMany underwater robotic vehicles (URVs) use propeller-type thrusters driven by DC motors.... more ABSTRACTMany underwater robotic vehicles (URVs) use propeller-type thrusters driven by DC motors. The thruster system is known to be nonlinear and time varying. Robust control of the thruster system is crucial to navigate and achieve high performance. This paper presents experimental results of five different thruster control systems based on on-line neural network (NN) control, off-line NN control, fuzzy control, adaptive-learning control and PID control. These control systems were initially designed or trained for the thruster system with no fin. Then each controller was implemented for the thruster system modified with different size fins. The fins changed the system hydrodynamics, especially the drag force. In this paper, each control system is briefly described and its practical advantages and disadvantages are discussed with experimental results.

The influence of an extrinsic interfacial layer on the polarization of sputtered BaTiO3 film

Applied Physics Letters, May 16, 2005

As an origin of degradation of remnant polarization in Pt/ BaTiO 3 / Pt capacitor structure, an i... more As an origin of degradation of remnant polarization in Pt/ BaTiO 3 / Pt capacitor structure, an interfacial layer formed at the interface of BaTiO 3 film and a Pt bottom electrode is considered. BaTiO 3 films were deposited on two types of bottom electrodes ͑La 0.5 Sr 0.5 CoO 3 and Pt͒ by the radio frequency magnetron sputtering method and both capacitors showed a microstructural similarity with strong preferred orientations. However, a Pt/ BaTiO 3 /La 0.5 Sr 0.5 CoO 3 capacitor exhibited a saturated hysteresis loop with the remnant polarization ͑2P r ͒ of 6 C/cm 2 , and for the Pt/ BaTiO 3 / Pt structure, the polarization-voltage curve revealed a linear dielectric characteristic. From a cross-sectional high-resolution transmission electron microscope analysis of the Pt/ BaTiO 3 / Pt capacitor showing the linear dielectric property, an interfacial layer with an amorphous structure as well as a multidomain structure in the interior of the BaTiO 3 film were observed. It is concluded that the interfacial layer might help degradation of polarization and its origin can be classified as being extrinsic.

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An Underwater Vehicle Monitoring System and Its Sensors

Springer eBooks, Sep 3, 2007

This paper describes a virtual collaborative world simulator, DVECS (Distributed Virtual Environm... more This paper describes a virtual collaborative world simulator, DVECS (Distributed Virtual Environment Collaborative Simulator), for underwater robots and its underwater vehicle, SAUVIM (Semi-Autonomous Underwater Vehicle for Intervention Missions). DVECS is used for testing unmanned underwater vehicles (UUVs) of both real and simulated worlds where interaction and cooperation of other real and simulated vehicles, obstacles, situations, conditions and disturbances in a hybrid, synthetic, virtual environment can be observed without physical intervention. This virtual system can be used to determine: (1) the optimal performance and criteria for the cooperating vehicles and its relative application; (2) the determination of the advantages and disadvantages of collaborative application tasks between multiple UUVs; and (3) the optimal communication links between the cooperating vehicles and its remote control stations. DVECS is used as a monitoring system.

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Influence of residual stress and film thickness on crystallographic orientation in Al thin films deposited by bias sputtering

Journal of vacuum science & technology, Nov 1, 1998

Al thin films with various residual stress and film thickness were deposited by bias sputtering. ... more Al thin films with various residual stress and film thickness were deposited by bias sputtering. Residual stress and relative intensity of the (200) to (111) plane of Al thin films were measured by x-ray diffraction. When film thickness is fixed at 1 μm below the residual stress of ∼80 MPa, Al thin films have a (111) orientation, but above this stress, a (100) orientation. If Al thin films are in the high residual stress of ∼145 MPa, crystallographic orientation changes to a (100) from a (111) orientation as film thickness increases to 2 μm from 1 μm. However, Al thin films with the residual stress of ∼25 MPa have a (111) orientation regardless of film thickness. Total energy for Al thin films of a (100) and a (111) orientation was calculated as a function of residual stress and film thickness. These experimental results could be explained by the minimization of the sum of strain energy and surface energy in the films.

Experimental Study on a Learning Control System with Bound Estimation for Underwater Robots

Springer eBooks, 1996

ABSTRACT

Domain structure in a micron-sized PbZr1−xTixO3 single crystal on a Ti substrate fabricated by hydrothermal synthesis

Applied Physics Letters, Apr 20, 2004

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Hydrodynamic and thruster model validation for Autonomous Underwater Vehicles

From Pontryagin’s Maximum Principle to the Duke Kahanamoku Aquatic Complex; we develop the theory... more From Pontryagin’s Maximum Principle to the Duke Kahanamoku Aquatic Complex; we develop the theory and generate implementable time efficient trajectories for a test-bed autonomous underwater vehicle (AUV). This paper is the beginning of the journey from theory to implementation. We begin by considering pure motion trajectories and move into a rectangular trajectory which is a concatenation of pure surge and pure sway. These trajectories are tested using our numerical model and demonstrated by our AUV in the pool. In this paper we demonstrate that the above motions are realizable through our method, and we gain confidence in our numerical model. We conclude that using our current techniques, implementation of time efficient trajectories is likely to succeed

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Increasing Underwater Vehicle Autonomy by Reducing Energy Consumption

by Ryan N. Smith and S. Choi

In this paper, we concern ourselves with finding a control strategy that minimizes energy consump... more In this paper, we concern ourselves with finding a control strategy that minimizes energy consumption along a trajectory connecting two given configurations. We develop an algorithm, based on our previous work with the time optimal problem, which provides implementable control strategies that are energy efficient. We find an interesting correlation between the duration of these trajectories and the optimal duration. We present the algorithm, control strategy and experimental results from our test-bed vehicle.

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Design and implementation of time efficient trajectories for autonomous underwater vehicles

by Ryan N. Smith and S. Choi

This paper discusses control strategies adapted for practical implementation and efficient motion... more This paper discusses control strategies adapted for practical implementation and efficient motion of autonomous underwater vehicles (AUVs). For AUVs we would like efficiency in both the measured time and the energy consumption, the mission dictating the weight to put on each of these cost. As a first approach to this problem, we focus in this paper on time minimization.

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Towards Practical Implementation of Time Optimal Trajectories for Underwater Vehicles

by Ryan N. Smith and S. Choi

In this paper, we are concerned with the practical imple- mentation of time optimal numerical tec... more In this paper, we are concerned with the practical imple- mentation of time optimal numerical techniques on underwater vehicles. We briefly introduce the model of underwater vehicle we consider and present the parameters for the test bed ODIN (Omni-Directional Intelligent Navigator). Then we explain the numerical method used to obtain time optimal trajectories with a structure suitable for the implementation. We follow this with a discussion on the modifications to be made considering the char- acteristics of ODIN. Finally, we illustrate our computations with some experimental results.

format_quoteReducing parameters in NLP formulation yields a simpler model for thrust strategy, needing about 100 parameters versus tens of thousands in OCP.format_quote

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Real-Time 3D Sonar Image Recognition for Underwater Vehicles

The recognition of objects and environment are one of keys to automate underwater tasks. This pap... more The recognition of objects and environment are one of keys to automate underwater tasks. This paper presents a predictor based image recognition method using high-resolution sonar images. They are very sensitive to sonar&amp;amp;amp;#39;s view point changes. The proposed predictor predicts an object images depending on the view point. Using the object model, it modifies a template for optimal recognition. To

A virtual collaborative world simulator for underwater robots using multidimensional, synthetic environment

IEEE International Conference on Robotics and Automation

Collaborative Simulator) is used for testing of unmanned underwater vehicles (UUVs) where both re... more Collaborative Simulator) is used for testing of unmanned underwater vehicles (UUVs) where both real and simulated vehicles can interact and cooperate in a hybrid, synthetic, virtual environment. This virtual reality system can be used to determine: (1) the optimal performance and criteria for the cooperating vehicles and its relative application; (2) the determination of the advantages and disadvantages of collaborative application tasks between multiple UUVs; and (3) the optimal communication links between the cooperating vehicles and its remote control stations. It also utilizes a virtual reality projection and polarized eyewear systems, which immerses the user for optimal visualization.

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Design of a semi-autonomous underwater vehicle for intervention missions (SAUVIM)

Abstract As the research in the autonomous underwater vehicle (AUV) field intensifies and the nec... more

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