Song-Yul Choe

The design, fabrication and evaluation of a MEMS PZT cantilever with an integrated Si proof mass for vibration energy harvesting

A microelectromechanical system (MEMS) piezoelectric energy harvesting device, a unimorph PZT cantilever with an integrated Si proof mass, was designed for low vibration frequency and high vibration amplitude environment. Pt/PZT/Pt/Ti/SiO2 multilayered films were deposited on a Si substrate and then the cantilever was patterned and released by inductively coupled plasma reactive ion etching. The fabricated device, with a beam dimension of about 4.800 mm × 0.400 mm × 0.036 mm and an integrated Si mass dimension of about 1.360 mm × 0.940 mm × 0.456 mm produced 160 mVpk, 2.15 μW or 3272 μW cm−3 with an optimal resistive load of 6 kΩ from 2g (g = 9.81 m s−2) acceleration at its resonant frequency of 461.15 Hz. This device was compared with other demonstrated MEMS power generators.

An improved stator flux estimation for speed sensorless stator flux orientation control of induction motors

This paper proposes a programmable low pass filter (LPF) to estimate stator flux for speed sensorless stator flux orientation control of induction motors. The programmable LPF is developed to solve the DC drift problem associated with a pure integrator and an analog LPF with fixed pole. The pole of the programmable LPF is located far from the origin in order to decrease the time constant as speed increases. The programmable LPF has the phase and the magnitude compensator to exactly estimate stator flux in a wide speed range. So, the drift problem is much improved and the stator flux is exactly estimated in the wide speed range. The validity of the proposed programmable LPF is verified by speed sensorless vector control of a 2.2 kW three-phase induction motor.

Dynamic Simulator for a PEM Fuel Cell System With a PWM DC/DC Converter

Polymer electrolyte membrane (PEM) fuel cells typically have low voltage, high current, terminal characteristics that cannot accommodate common electric loads like electric motors or power utility grids. Thus, a dc/dc converter is required to boost the output voltage of these power systems. Furthermore, the terminal characteristics are dependent on loads and operating conditions of the fuel cell system. The continuously changing power demand of an electric load requires dynamically replenishing the air and fuel, by properly maintaining humidity in the cell and efficiently rejecting the heat produced. These factors present important challenges for the design of reliable and durable power systems. We present new dynamic models for a fuel cell system and a pulsewidth modulation dc/dc converter with associated controls and integration. The model for the system consists of three subsystems that include an PEM fuel cell stack, an air supply, and a thermal system. Four different controllers were designed to control the air, the coolant, and the output voltage of the converter, and to optimize the power flow between the fuel cell and the output capacitor. The integrated model with its controls was tested using a real-time simulator that reduced computational time and facilitated the analysis of the interactions between loads and the fuel cell components and also allowed the optimization of a power control strategy. The responses of a static and dynamic load show that the power controls proposed can coordinate two energy sources, resulting in improved dynamics and efficiency.

Analysis of Piezoelectric Materials for Energy Harvesting Devices under High-g Vibrations

We analyzed the miniaturized energy harvesting devices (each volume within 0.3 cm3) fabricated by using three types of piezoelectric materials such as lead zirconium titanate (PZT) ceramic, macro fiber composite (MFC) and poly(vinylidene fluoride) (PVDF) polymer to investigate the capability of converting mechanical vibration into electricity under larger vibration amplitudes or accelerations conditions (≥1g, gravitational acceleration). All prototypes based on a bimorph cantilever structure with a proof mass were aimed to operate at a vibration frequency of 100 Hz. PZT-based device was optimized and fabricated by considering the resonant frequency, the output power density, and the maximum operating acceleration or safety factor. PVDF- and MFC-prototypes were designed to have same resonant frequency as well as same volume of the piezoelectric materials as the PZT prototype. All three devices were measured to determine if they could generate enough power density to provide electric energy to power a wireless sensor or a microelectromechanical systems (MEMS) device without device failure.

Vibration-Induced Changes in the Contact Resistance of High Power Electrical Connectors for Hybrid Vehicles

Relatively little is known about the fretting mechanism of high power connectors used in hybrid vehicles, even though the vehicles are widely being introduced to the market. This paper experimentally investigates the fretting mechanisms of silver-plated high power connectors caused by vibrations. In order to emulate operational and environmental effects, a test stand was designed that is capable of measuring electrical contact resistance (ECR), relative displacement and connector temperature. The experimental results show that the variation of ECR of connectors subject to vibration is primarily due to periodic changes of contact area caused by relative motion between the contact interfaces, rather than other fretting corrosions. This finding was reinforced by observing a good correlation between relative motion and the increase of ECR under vibration. When a vibration stops, the ECR decreased to a value that is slightly larger than the original value. A surface analysis shows no obvious corrosion until the coating is worn away. In addition, the effect of high current on the fretting mechanism is also investigated.

A Multiphysics Finite Element Model of a 35A Automotive Connector Including Multiscale Rough Surface Contact

Electrical contacts influence the reliability and performance of relays, electrical connectors, high power connectors, and similar systems, and are therefore a key region which needs to be considered. In the current study, a new inclusive multiphysics (involving mechanical, electrical, and thermal fields) finite element model (FEM) of a 35A automotive connector has been developed. The contact resistance is predicted using a multiscale rough surface contact method and is embedded in the multiphysics FEM. The coupled connector model is solved to obtain stresses, displacements, contact pressures, electrical and thermal contact resistances, voltage, current density, and temperature distributions. It appears that the current flows mostly through very small regions that are usually near the contacting surfaces in the connector, thereby suggesting that the available conducting material can be more efficiently used by developing optimized connector designs. Through analytical calculations and experimental measurements of temperature rise (ΔT or change in temperature) for the cable and the connector, it is believed that a large portion of the temperature rise in actual 35A connectors is due to the Joule heating in the supply cables. The model is a powerful tool that can be used for the basic connector characterization, prototype evaluation, and design through various material properties, and surface finishes.

The Optimal Design and Analysis of Piezoelectric Cantilever Beams for Power Generation Devices

With the rapid development of wireless remote sensor systems, battery is becoming the limiting factor in the lifetime of the device and miniaturization. As a way to eliminate battery in the system, the conversion of ambient vibration energy has been addressed. The piezoelectric cantilever beam with a proof mass was exploited for energy conversion since it can generate large strain and power density. The design of cantilever beams was optimized through numerical analysis and FEM simulation at higher acceleration condition. The investigated parameters influencing the output energy of piezoelectric bimorph cantilevers include dimensions of cantilever beam and proof mass. The resonant frequency and robustness of cantilever structure were also considered for enhancing power conversion efficiency and implementing devices at high acceleration condition. The power density generated by the optimized piezoelectric device was high enough (> 1200 μW/cm 3 ) to operate microsensor systems. However, high stress near clamping area of cantilever beam could lead to the fracture at high acceleration condition.

Piezoelectric Energy Harvesting Device in a Viscous Fluid for High Amplitude Vibration Application

The fragile nature of ceramic piezoelectric cantilevers limits their ability to withstand high acceleration amplitudes in vibration energy harvesting applications. This study reports a potential solution, which is vibrating the fragile power generator in a viscous fluid. The changes in resonant frequency, Q-factor, damping ratio, and power output of a bimorph piezoelectric cantilever prototype, as well as the relationship of power output vs the vibration acceleration were investigated. The broadened application frequency spectrum and the increased maximum operational acceleration demonstrate the potential of the bimorph piezoelectric cantilever operating in a viscous fluid for high amplitude vibration applications.

Static and dynamic analysis of Li-polymer battery using thermal electrochemical model

A dynamic model of a micro cell for Li-polymer battery based on the thermal electrochemical principle is developed to analyze static and dynamic performances. The micro cell model is one dimensional and coupled with two-dimensional macro cell thermal model to construct a pouch type battery. Firstly, the model is used to analyze both static and dynamic performances that include overpotentials, current distribution, potential distribution, and concentration of ions through the

Modeling and Analysis of Vibration-Induced Changes in Connector Resistance of High Power Electrical Connectors for Hybrid Vehicles

High power connectors used in hybrid vehicles are exposed to vibrations that cause changes in connector resistance. When vibration starts, the connector resistance increases temporarily and oscillates. When vibration stops, the connector resistance returns to a value that is similar to the original state. In this paper, finite element models are developed to analyze this phenomenon and compared with experimental results. A two-dimensional finite element model was developed to predict the motions at any location of the connector system. A contact spring present between the female and male parts of the connector is modeled in three dimensions and used to analyze the time response. The analysis shows that the relative displacement is closely related to the changes of connector resistance during vibration, and the models can be used to improve connector design and ensure better performance and reliability.