Nak-Sun Choi

Comparison of Three Modeling Methods for Identifying Unknown Magnetization of Ferromagnetic Thin Plate

This study presents three different magnetization models for identifying unknown magnetization of the ferromagnetic thin plate of a ship. First, the forward problem should be solved to accurately predict outboard magnetic fields due to the magnetization distribution estimated at a certain time. To achieve this, three different modeling methods for representing remanent magnetization (i.e., magnetic charge method, magnetic dipole array method, and magnetic moment method) were utilized. Material sensitivity formulas containing the first-order gradient information of an objective function were then adopted for an efficient search of an optimum magnetization distribution on the hull. The validity of the proposed methods was tested with a scale model ship, and field signals predicted from the three different models were thoroughly investigated with reference to the experimental data.

Composite First-Order Reliability Method for Efficient Reliability-Based Optimization of Electromagnetic Design Problems

This paper proposes a composite first-order reliability analysis method to effectively perform the reliability-based optimization of electromagnetic (EM) design problems. The proposed method utilizes both of two different ways, reliability index approach (RIA) and performance measure approach (PMA), for reliability analysis of probabilistic constraints. However, each performs a distinct function: the first merely checks the feasibility status of probabilistic constrains, and the second evaluates the failure probability of only active constraints selected from the feasibility identification. Such a unique scheme can substantially enhance computational efficiency during optimization process. The proposed method is applied to two EM design problems, and its numerical efficiency and accuracy are examined by comparison with existing methods.

Efficient Methodology for Optimizing Degaussing Coil Currents in Ships Utilizing Magnetomotive Force Sensitivity Information

This paper presents an efficient methodology for optimizing degaussing coil currents in ships to minimize the anomaly of underwater magnetic fields due to the hull magnetization induced under the Earths magnetic field. To achieve this, first, the shielding effect of the hull on the underwater fields is thoroughly examined. Then, for a fast search of optimum degaussing currents, a sensitivity formula of an objective function with respect to the magnetomotive force is adopted. The feature of the proposed method is that it does not require any numerical field analyses to assess an objective function during optimization process providing experimental field data for each coil are given. The validity and effectiveness of the method has been tested with a model ship.

Sampling-Based Sensitivity Approach to Electromagnetic Designs Utilizing Surrogate Models Combined with a Local Window

This paper proposes a sampling-based optimization method for electromagnetic design problems, where design sensitivities are obtained from the elaborate surrogate models based on the universal Kriging method and a local window concept. After inserting additional sequential samples to satisfy the certain convergence criterion, the elaborate surrogate model for each true performance function is generated within a relatively small area, called a hyper-cubic local window, with the center of a nominal design. From Jacobian matrices of the local models, the accurate design sensitivity values at the design point of interest are extracted, and so they make it possible to use deterministic search algorithms for fast search of an optimum in design space. The proposed method is applied to a mathematical problem and a loudspeaker design with constraint functions and is compared with the sensitivity-based optimization adopting the finite difference method.

A Single-Loop Strategy for Efficient Reliability-Based Electromagnetic Design Optimization

This paper proposes a single-loop optimization strategy for efficient reliability-based electromagnetic design in the presence of uncertainty. Unlike conventional reliability-based optimization methods, optimization and reliability assessment are totally decoupled from each other during the overall design process. To achieve the desired design feasibility, constraint boundaries of an original design problem are shifted to feasible directions determined by the reliability information of a previous design. After all, the proposed single-loop method for reliability-based optimization performs a serial design of deterministic optimization with shifting constraints and reliability analysis. The proposed method is tested with a mathematical problem and the TEAM Workshop Problem 22, and its numerical accuracy and efficiency is examined by comparison with existing methods.

Assessment of Statistical Moments of a Performance Function for Robust Design of Electromagnetic Devices

This paper proposes a statistical probability approach to evaluate the quality loss function of electromagnetic design problems, which is expressed in terms of the first two statistical moments, mean and variance. A univariate dimension reduction method is employed to calculate the statistical moments of a performance function and their sensitivities accurately, and efficiently. Finally, the method is integrated into the robust design optimization algorithm. The proposed method explores an optimum design with statistical information so that it can provide a more accurate solution than other robust design methods. The proposed method is tested with a mathematical model and a blushless DC motor, and its numerical accuracy and efficiency are examined by comparison with existing methods.

Optimal Design of a RFID Tag Antenna Based on Plane-Wave Incidence

In order to increase the detection range and radar cross section (RCS), the patch shape of a UHF RFID tag antenna is optimized using continuum design sensitivity analysis. To maximize RCS and reflect the practical RFID tag operating condition, the design goal is set considering not only the matching characteristics of the antenna but also the induced voltage at the RFID chip terminal when the plane-wave is incident upon the antenna. The complex impedance of the RFID chip is taken into account by defining a lumped port at the chip location. The design results show that the induced voltage at the chip has increased more than 32% after the optimization.

Optimization of Degaussing Coil Currents for Magnetic Silencing of a Ship Taking the Ferromagnetic Hull Effect Into Account

This paper presents an efficient methodology for optimizing degaussing coil currents in a ship with ferromagnetic hull to minimize magnetic field anomaly underwater. For fast search for an optimum, a magnetomotive force sensitivity formula is adopted. The feature of the method does not require any numerical field analyses to assess an objective function during optimization process providing experimental field data on each coil with respect to the reference magnetomotive force are given. Especially, the shielding effect of the hull on degaussing fields is thoroughly investigated. The validity of the proposed method has been tested with a model ship.

Design Optimization of Waveguide Filters Using Continuum Design Sensitivity Analysis

This paper presents a new methodology for design optimization of dielectric waveguide filters based on the continuum design sensitivity analysis in conjunction with standard electromagnetic analysis codes. To achieve this, first, an analytical sensitivity formula in the frequency domain is systematically derived by exploiting the augmented Lagrangian method, material derivative concept and adjoint variable method. Then unified program architecture integrating several engineering software packages into a design tool is proposed for optimum design of high-frequency devices. A 3-D dielectric resonator used in waveguide filters has been tested to prove the validity of the proposed method.

Sequential Design Method for Geometric Optimization of an Electrothermal Microactuator Based on Dynamic Kriging Models

This paper proposes a sequential optimization methodology for designing an electrothermal polysilicon actuator in the presence of a fabrication tolerance. In the proposed method, a deterministic optimum is first sought from an initial design, and then a reliability-based robust design is obtained launching at the deterministic point. This serial design strategy can enhance numerical efficiency through minimizing the use of computationally expensive reliability-based design optimization. To effectively perform the robust design of very complex multiphysics problems, elaborate surrogate models based on the local window concept are exploited comprehensively. The proposed method is applied to an electrothermal polysilicon actuator with seven design random variables, and then three different nominal designs are examined in terms of maximum deflection, consumed power, and confidence level.