Deug-Woo Lee

Various design techniques to reduce cogging torque by controlling energy variation in permanent magnet motors

Using energy method, this paper analytically reveals that the cogging torque can be reduced by controlling N/sub L/th harmonic components of the square of airgap permeance function and flux density function, where N/sub L/ denotes the least common multiple of number of poles and number of slots. As for design tools, it introduces various design techniques to reduce cogging torque with analytical formulation and FEM examples.

Evaluation of cutter orientations in 5-axis high speed milling of turbine blade

Abstract Recently, the development of aerospace and automobile industries has brought new technological challenges, related to the growing complexity of products and the new geometry of the models. High speed milling with a 5-axis milling machine has been widely used for 3D sculptured surface parts. When turbine blades are machined by a 5-axis milling, their thin and cantilever shape causes vibrations, deflections and twists. Therefore, the surface roughness and the waviness of the workpiece are not good. In this paper, the effects of cutter orientation and the lead/tilt angle used to machine turbine blades with a 5-axis high speed ball end-milling were investigated to improve geometric accuracy and surface integrity. The experiments were performed using a lead/tilt angle of 15° to the workpiece with four cutter directions such as horizontal outward, horizontal inward, vertical outward, and vertical inward directions. Workpiece deflection, surface roughness and the machined surface were all measured with various cutter orientations such as cutting directions, and lead/tilt angle. The results show that the best cutting strategy for machining turbine blades with a 5-axis milling is horizontal inward direction with a tilt angle.

Simple and Cost-Effective Fabrication of Highly Flexible, Transparent Superhydrophobic Films with Hierarchical Surface Design

Optical transparency and mechanical flexibility are both of great importance for significantly expanding the applicability of superhydrophobic surfaces. Such features make it possible for functional surfaces to be applied to various glass-based products with different curvatures. In this work, we report on the simple and potentially cost-effective fabrication of highly flexible and transparent superhydrophobic films based on hierarchical surface design. The hierarchical surface morphology was easily fabricated by the simple transfer of a porous alumina membrane to the top surface of UV-imprinted polymeric micropillar arrays and subsequent chemical treatments. Through optimization of the hierarchical surface design, the resultant superhydrophobic films showed superior surface wetting properties (with a static contact angle of >170° and contact angle hysteresis of <3.5°) in the Cassie-Baxter wetting regime, considerable dynamic water repellency (with perfect bouncing of a water droplet dropped from an impact height of 30 mm), and good optical transparency (>82% at 550 nm wavelength). The superhydrophobic films were also experimentally found to be robust without significant degradation in the superhydrophobicity, even under repetitive bending and pressing for up to 2000 cycles. Finally, the practical usability of the proposed superhydorphobic films was clearly demonstrated by examining the antiwetting performance in real time while pouring water on the film and submerging the film in water.

Analysis of harmonic distortion due to uneven magnetic field in a micro-speaker used for mobile phones

Summary form only given. With the advent of 2.5G mobile phone, internet service is to be realized for multimedia data communication. For acoustic parts, a smaller and lighter micro-speaker is also soon to be realized as a MP-3 song player and speakerphone. With uneven magnetic field due to reduced magnetic circuits, harmonic distortion is induced to degrade sound quality. Using FEM, this article shows magnetic flux line of the micro-speaker and uneven exciting force can be predicted as the voice coil vibrates. With mechanical analysis using FEM, surface velocity of the vibrating diaphragm can be determined. The sound pressure level can be also determined using acoustical analysis by assuming diaphragm as a monopole source. For high quality sound generation, harmonic distortion should be minimized by appropriated magnetic circuit design.

A study on micro-grooving characteristics of planar lightwave circuit and glass using ultrasonic vibration cutting

Abstract Micro-grooving of brittle materials such as glass, which are widely used as optical components, was accomplished using a polycrystalline diamond tool with ultrasonic vibration. To determine the cutting characteristics of brittle materials, the machining system with an ultrasonic vibration tool was built. Experiments for micro-grooving of the planar lightwave circuits (PLCs) and glass were performed and their shapes were measured by microscopic photograph. Better groove shapes with low chipping of PLCs and glass were obtained by ultrasonic vibration cutting. These experiments are considered as a possibility for micro-grooving of optical communication components.

Development of a high-efficiency laminated dye-sensitized solar cell with a condenser lens.

Dye-sensitized solar cells have slightly lower photoelectric efficiency than silicon solar cells. Researchers have investigated various ways to address this problem. In this paper, we found that the optimized separation between the condenser lens and the cells was 8 mm. The cell efficiency increased from 2.5% to 8.3% compared to two isolated cells without a lens. If the efficiency of the basic cell can be increased sufficiently, it should be possible to commercialize the product.

Inclined-wall regular micro-pillar-arrayed surfaces covered entirely with an alumina nanowire forest and their improved superhydrophobicity

This paper reports a multiple-scale hierarchically structured superhydrophobic surface that is composed of inclined-wall regular micro-pillar arrays covered entirely with an alumina nanowire forest (ANF) to improve the surface wettability. The multiple-scaled structures were fabricated stably using a simple batch process based on an anisotropic chemical silicon etching process and a subsequent time-controlled anodic aluminum oxide technique. The surface wetting properties of the mono-roughened surfaces with inclined-wall micro-pillar arrays, which are normally in the Wenzel wetting regime, could be transitioned perfectly to the slippery Cassie mode and enhanced greatly in the Wenzel regime in cases of a high- and low-density of the micro-pillars, respectively, by easily amplifying the intrinsic contact angle through the entire coverage of the ANF on the micro-roughened surfaces. The wettability of the proposed multiple-scaled surfaces could also be predicted using analytic surface models and the experimental results agreed greatly with the wetting trends estimated theoretically due to the geometrical regularity of the base micro-structures.

Diamond machining of sinusoidal grid surface using fast tool servo system for fabrication of hydrophobic surface

Ultra-precision diamond machining with piezoelectric-assisted fast tool servo (FTS) was used to produce various free-form surfaces. A low cost, rapid and large area fabrication of uniform hydrophobic surface at room temperature which transfers the FTS fabricated sinusoidal grid surface to the flat film with UV-moulding process was described. A piezoelectric-assisted FTS with high band width of 2 kHz, travel range up to 16 μm and the compact mechanism structure was designed for the sinusoidal grid surface machining and the dynamic performance testing of FTS was described in detail. Machining results indicate that the dimensions of sinusoidal grid change with the variation of the FTS machining condition. Wetting properties of UV-moulded surface were evaluated, the best contact angle was measured to be 120.5° on the sinusoidal grid surface with profile wavelength of 350 μm and peak-to-valley amplitude of about 16 μm.

Microstructure of fast tool servo machining on copper alloy

Abstract The development of the fast tool servo (FTS) for precision machining was investigated. The micron machining performance of a piezoelectric-assisted FTS on copper alloy was evaluated. The results indicate that the quality of the microstructure depends mainly on two important factors: the cutting speed (or spindle speed) and the driving frequency of the FTS. The excessive driving frequency increases the formation of burrs. The effect of the clearance angle of the diamond tool on the microstructure machining precision was also investigated.

Evaluation of characteristics for dye-sensitized solar cell with reflector applied

Dye-sensitized solar cells have slightly lower photoelectric efficiency than silicon solar cells. Researchers have investigated various ways to address this problem. This study improved the efficiency of a dye-sensitized solar cell by re-driving it with a reflector, reusing discarded light after it was absorbed. The reflector increased efficiency by about 50%, by increasing the size of the pattern shape and increasing the distance of the reflector.