Control and Power Supply for Resistance Spot Welding (RSW)

In the automobile industry, Resistance Spot Welding (RSW) is widely used for its low cost, high speed, simple mechanism and applicability for automation. RSW has become the predominant means of auto body assembly, resulting in two to six thousands spot welds performed on each manufactured car. In the North American automobile industry there are approximately 100 billion spot welds, which are done every year. RSW is the joining of two or more metal parts together in a localized area by resistive heating and pressure. Small Scale RSW (SSRSW) is commonly used for medical devices and electronic components, because the welded parts are thinner and smaller compared to common RSW applications, such as automotive applications. According to a study of Edison Welding Institute, 20% of the welding quality issues are the weld schedule or power supply related. Therefore, to contribute to weld quality improvement, the study of different weld schedules or power supplies and control schemes needs to be improved by doing further studies in this area. Thus a novel power supply, which can provide a testing bench for these studies, was designed and developed in 2005 by L. J. Brown and J. Lin. This research study will focus on studying and improving weld power supplies, weld schedules and control modes. One of the goals for this research is to improve the consistency of weld nugget size and strength by using different control parameters, which will be weighted geometric averages of voltage and current. These control parameters are fed back to a Proportional Integral Derivative (PID) controller that is designed to control the Direct Current (DC) power supply for the RSW to come up with the best control parameters that will improve the consistency of the RSW spot welds. Another goal for this research is it to further develop the existing DC power supply that was designed for SSRSW by L. J. Brown, to include tip voltage measurements, and Large Scale Resistance Spot Welding (LSRSW). This goal will lead to build additional weld modules to construct a 6000A welder in the future

Battery Energy Storage Emulation for Power System Applications

The concept of energy storage for power systems has received increasingly more attention in recent decades, and the growing penetration of renewable energy sources has only escalated demand for it. Energy storage systems are excellent for balancing generation and load, for suppressing power fluctuations, and for providing other ancillary services to the grid. The Hardware Testbed (HTB) is a novel converter-based grid emulator created for studying the needs associated with high renewable penetration, but the system currently lacks a battery storage emulator. Thus, this work documents the development of a battery energy storage system (BESS) emulator for the HTB. The BESS emulator includes internal battery models for Lithium Ion, Lead Acid, and Vanadium redox flow battery technologies. The emulated BESS contains a two-stage power electronics interface using a DC-DC converter and a boost rectifier separated by a DC link. Controllers for active power output, reactive power output, and DC link voltage are designed for the power electronics interface, and application-specific control loops for primary frequency regulation, inertia emulation, and voltage support are also added. The models and control for this emulated BESS are implemented on a digital signal processor that controls one voltage source inverter on the HTB as if it were the BESS’s boost rectifier. Consequently, the voltage source inverter mimics the behavior of a BESS at its point of common coupling with the HTB’s power system. The BESS emulator is simulated and then tested experimentally on the HTB, and all of its control functions demonstrate correct operation. The BESS emulator’s primary frequency regulation and inertia emulation functions nearly eliminate the system frequency swing following a step change in load, and the voltage support keeps the BESS terminal voltage at a safer level following the disturbances. These three support functions are concluded to be capable of simultaneous operation, which allows the BESS emulator to support the HTB’s power system in multiple ways at the same time. In the future, the BESS emulator can be used on the HTB to study how battery storage can be used to support renewables and other dynamic power system needs

셀룰로오스 나노피브릴 기반 복합재의 열에너지 저장용 상변화 물질에의 적용

학위논문(박사) -- 서울대학교대학원 : 농업생명과학대학 산림과학부(환경재료과학전공), 2021.8. Hak Lae Lee.Thermal energy storage (TES) has gathered intensive attention in recent decades due to its advantages of ease of use, cost-effectiveness and contribution to energy-saving. Phase change materials (PCMs) are the core part of a TES system, and the thermal energy can be stored and released through the phase transitions of PCMs. However, the drawbacks of PCMs have to be addressed to improve TES efficiency. Cellulose nanofibril (CNF), originated from natural cellulose, has numerous fascinating properties, such as abundant availability, biodegradability, renewability, recyclability and the easiness of various chemical modifications. Furthermore, CNF has light weight, high aspect ratio and high surface area, compared to larger-sized pulp fibers. In this study, CNF and CNF-based composites were prepared and used to overcome the drawbacks of PCMs, aiming at improving their thermal energy storage performance. The thickening effect of CNF, due to the entangled nanofibril networks, was used to solve the phase separation of sodium acetate trihydrate (SAT), a typical drawback of salt hydrate PCM. It was found that adding 0.8% of CNF to SAT increased viscosity, enhanced solid-like rheological behavior, and successfully eliminated phase separation. The use of 3% of sodium phosphate dibasic dodecahydrate (DSP) suppressed the supercooling degree of SAT to 2.1 °C when CNF was used as the thickening agent for a stable phase. Meanwhile, the amphiphilicity of CNF facilitated the dispersion of graphene nanoplatelet (GNP), which was used as a thermal conductivity enhancer. When the prepared CNF/GNP composites were used to prepare SAT-based PCM composites, the supercooling degree was unaffected. Importantly, the prepared PCM composites showed enhanced thermal stability and thermal conductivity compared with that of pure SAT. Considering that CNF ensures the dispersion of nanoparticles, CNF/silver nanoparticles (AgNPs) composites were prepared to solve the supercooling problem of salt hydrate PCM. The synthesized AgNPs were uniformly dispersed by CNF and the prepared CNF/AgNPs were stable when added to SAT. The combined use of CNF/AgNPs composite and DSP further decreased the supercooling degree of SAT to only 1.2 °C. SAT crystals grow on the surfaces of AgNPs, accelerating the crystallization process of SAT. Besides, the CNF/AgNPs composite provided phase stability to SAT, indicating that the presence of AgNPs did not negate the thickening effect of CNF. Owing to the excellent heat transfer performance of AgNPs, the thermal conductivity of the prepared PCM composites also improved, compared to that of the pure SAT. CNF-based foams were prepared to address the leakage problem of paraffin, a widely used organic PCM. The chemical modification of CNF improved the compatibility between the resulting foams and paraffin. These porous foam support greatly stabilized paraffin and prevented its leakage. Due to the light weight and high porosity of these foams, the prepared PCM composites had high fractions of paraffin. Carbon nanotube (CNT)-containing foams were used to enhance the thermal properties of PCM composites, and the uniform dispersion of CNT was achieved by the use of CNF. With the use of silylated CNF/CNF foams, the thermal properties of paraffin-based PCM composites were enhanced, while the high fractions of paraffin and negligible leakage of paraffin were maintained. CNF-based composites (i.e. CNF/GNP and CNF/AgNPs) provided phase stability to salt hydrate inorganic PCMs, CNF/AgNPs suppressed the supercooling, CNF/CNT foams improved the form-stability of paraffin organic PCMs, and these CNF-based composites improved the thermal conductivity of all PCMs. This work shows the potential of CNF-based composites in improving the thermal energy storage performance of PCMs.열에너지 저장(TES) 기술은 사용이 용이할 뿐 아니라 비용 절감, 에너지 절약 측면에서 장점을 지니기에 최근 많은 주목을 받아왔다. 상변화물질(PCMs)은 TES 시스템의 핵심 요소로써, 상변화물질의 상 변이에 따라 열에너지가 저장되고 방출된다. 그러나, TES 시스템의 효율을 높이기 위해서는 PCMs의 단점들을 개선해야한다. 셀룰로오스 나노피브릴(CNF)은 천연 셀룰로오스로부터 유래된 생분해가 가능한 물질로서, 공급원이 다양하며, 재생과 재사용이 가능할 뿐 아니라 다양한 화학적 개질이 가능하다는 등 수많은 매력적인 특징을 지니는 물질이다. 게다가, CNF는 큰 사이즈의 펄프 섬유들에 비해 가볍고, 종횡비가 크고, 높은 표면적을 지니고 있다. 본 연구는 CNF와 CNF 기반 복합재를 준비, 활용하여 PCMs의 단점을 극복하고자 하였으며, 열에너지 저장 성능을 향상시키는 것을 목적으로 하였다. 나노섬유의 고유한 네트워크 형성능력에 의해 발생하는 CNF의 증점효과를 이용하여 염수화물 PCM의 하나인 아세트산나트륨 삼수화물(SAT)의 전형적인 상 분리 현상의 단점을 개선하였다. SAT에 0.8%의 CNF를 첨가하는 것으로 점도와 저장탄성계수를 향상시키고, 상변화를 방지할 수 있었다. CNF가 증점제로 사용되어 안정된 상이 유지될 때, 3%의 인산나트륨 이염기성 십이수화물(DSP)의 사용은 SAT의 과냉각 정도를 2.1 °C 억제하였다. 한편, CNF의 양친성은 열전도성 향상제로 사용된 그래핀 나노판 (GNP)의 분산을 용이하게 하였다. CNF/GNP 복합재가 SAT 기반 PCM 복합재를 제작하는 데 사용되었을 때, 과냉각 정도는 영향 받지 않았다. 더욱이, 제작된 PCM 복합재는 순수한 SAT에 비해 향상된 열안정성과 열 전도성을 보였다. CNF가 나노 입자들의 분산을 촉진한다는 점을 고려하여, CNF/은 나노입자 (AgNPs) 복합재가 염수화물 PCM의 과냉각 문제를 해결하기 위해 활용되었다. CNF는 합성된 AgNPs의 분산균일성을 향상시켰으며, 제작된 CNF/AgNPs 복합재를 SAT에 첨가하였을 때 높은 안정성을 보였다. CNF/AgNPs 복합재와 DSP를 함께 사용함으로써 SAT의 과냉각 정도를 단 1.2 °C로 더 감소시킬 수 있었다. AgNPs의 표면에 SAT 결정이 자라남에 따라, SAT의 결정화 과정이 촉진되었다. 게다가, CNF/AgNPs 복합재는 SAT에 상 안정성을 부여하였고, 이는 AgNPs의 존재가 CNF의 증점 효과를 방해하지 않는다는 것을 의미한다. AgNPs의 우수한 열 전도 성능은 순수한 SAT에 비해 제작된 PCM 복합재의 열전도성을 향상시켰다. 널리 쓰이는 유기물질 기반의 PCM인 파라핀의 누출 문제를 해결하기 위해 CNF 기반의 폼을 활용하였다. CNF의 화학적 개질은 제작되는 폼과 파라핀 간 화합을 증진시켰다. 이 다공성 폼 지지대는 파라핀을 매우 안정화시킴으로써 이의 누출을 방지하였다. 이 폼들은 가볍고 높은 공극률을 나타내기에, 제작된 PCM 복합재는 높은 비율로 파라핀을 탑재할 수 있었다. PCM 복합재의 열 특성을 향상시키기 위해 탄소 나노튜브 (CNT)를 함유하는 폼을 활용하였고, CNF를 활용하여 CNT를 균일하게 분산시킬 수 있었다. 실란화 CNF/CNF 폼을 이용하여, 파라핀 기반의 PCM 복합재의 열 특성을 향상시키고, 높은 파라핀 함량을 유지시키면서도 누출을 방지할 수 있었다. CNF 기반의 복합재 (즉, CNF/GNP와 CNF/AgNPs)는 무기물질 기반의 염수화물 PCM에 상 안정성을 부여하고, CNF/AgNPs는 과냉각을 억제하며, CNF/CNT 폼은 파라핀 유기 물질 기반의 PCM의 폼 안정성을 향상시키고, CNF 기반의 복합재들은 모든 PCM들의 열전도성을 향상시켰다. 본 연구는 CNF기반의 복합재가 상변화물질의 열에너지 저장 성능을 향상시킬 수 있는 잠재력이 있음을 보여주었다.Chapter 1 Introduction 1 1. Introduction 2 1.1 Energy crisis and energy storage 2 1.2 Thermal energy storage 4 1.2.1 Sensible heat storage 4 1.2.2 Latent heat storage 6 1.3 Introduction of PCMs 9 1.3.1 Organic PCMs 12 1.3.2 Eutectic PCMs 14 1.3.3 Inorganic PCMs 15 1.4 Barriers of PCMs for efficient TES 19 1.4.1 Phase separation of salt hydrates 20 1.4.2 Supercooling of salt hydrates 21 1.4.3 Thermal conductivity insufficiency of PCMs 24 1.4.4 Poor form-stability of paraffin 26 1.5 The potential of CNF to improve the performance of PCMs 28 2. Objectives 34 3. Literature review 36 3.1 Sodium acetate trihydrate for TES 36 3.2 Solutions to eliminating the phase separation 39 3.3 Nucleating agents for supercooling suppression 43 3.4 Ways to enhance the thermal conductivity 47 3.5 Methods to improve the form-stability of paraffin 52 Chapter 2 Preparation and application of CNF/GNP composites in salt hydrate PCM 56 1. Introduction 57 2. Experimental 61 2.1 Materials 61 2.2 Methods 61 2.2.1 Preparation of CNF 61 2.2.2 Preparation of CNF/GNP composites 62 2.2.3 Preparation of PCM composites 62 2.2.4 Phase stability and fluidity tests 63 2.2.5 Rheology tests 63 2.2.6 Determination of supercooling degrees 64 2.2.7 Transmission electron microscopy 65 2.2.8 Field-emission scanning electron microscopy 65 2.2.9 Fourier transform-infrared spectroscopy 66 2.2.10 X-ray diffraction 66 2.2.11 Differential scanning calorimetry 66 2.2.12 Thermogravimetric analysis 67 2.2.13 Measurement of thermal conductivity 67 2.2.14 Thermal reliability tests 68 3. Results and discussion 69 3.1 Effectiveness of CNF in eliminating phase separation 69 3.2 Screening of suitable nucleating agents 73 3.3 Incorporation of CNF/GNP to fabricate PCM composites 76 3.4 Morphology and structures of PCM composites 82 3.5 Thermal properties of PCM composites 86 4. Summary 94 Chapter 3 Preparation and application of CNF/AgNPs composites in salt hydrate PCM 95 1. Introduction 96 2. Experimental 100 2.1 Materials 100 2.2 Methods 100 2.2.1 Preparation of CNF 100 2.2.2 Synthesis of CNF/AgNPs composites 100 2.2.3 Preparation of PCM composites 101 2.2.4 Transmission electron microscopy 102 2.2.5 Measurement of particle size and its distribution 102 2.2.6 UV-visible spectroscopy 102 2.2.7 Determination of silver content 103 2.2.8 Measurement of supercooling degrees 103 2.2.9 Phase stability and fluidity tests 104 2.2.10 Field-emission scanning electron microscopy 104 2.2.11 Fourier transform-infrared spectroscopy 104 2.2.12 X-ray diffraction 105 2.2.13 Differential scanning calorimetry 105 2.2.14 Thermogravimetric analysis 105 2.2.15 Measurement of thermal conductivity 106 2.2.16 Thermal reliability tests 106 3. Results and discussion 107 3.1 Characterizations of CNF/AgNPs composites 107 3.2 Effect of CNF/AgNPs composites on supercooling suppression 112 3.3 Phase stability and physical structures of PCM composites 117 3.4 Chemical structures of PCM composites 122 3.5 Thermal properties of PCM composites 125 4. Summary 133 Chapter 4 Preparation and application of CNF/CNT composite foams in organic PCM 134 1. Introduction 135 2. Experimental 138 2.1 Materials 138 2.2 Methods 138 2.2.1 Preparation of CNF 138 2.2.2 Preparation of CNF/CNT composites 139 2.2.3 Preparation of foams 139 2.2.4 Measurement of density and porosity of foams 140 2.2.5 Measurement of water contact angles 141 2.2.6 Preparation of foam-stabilized PCM composites 141 2.2.7 Transmission electron microscopy 142 2.2.8 Field-emission scanning electron microscopy 142 2.2.9 Leakage tests 142 2.2.10 Fourier transform-infrared spectroscopy 143 2.2.11 X-ray diffraction 143 2.2.12 Differential scanning calorimetry 144 2.2.13 Thermogravimetric analysis 144 2.2.14 Measurement of thermal conductivity 144 2.2.15 Heat transfer analysis 145 2.2.16 Thermal reliability tests 145 2.2.17 Test of PCM composite in solar-thermal-electricity system 146 3. Results and discussion 147 3.1 Characterizations of SCNF foams and their paraffin-stabilization effect 149 3.2 Characterizations of SCNF/CNT foams and their paraffin-stabilization effect 157 3.3 Physical and chemical structures of PCM composites 165 3.4 Phase change and thermal properties of PCM composites 169 3.5 Thermal reliability and practical application 175 4. Summary 183 Chapter 5 Overall conclusions and future work 184 1. Performance improvement of inorganic salt hydrate PCM 186 2. Performance improvement of organic PCM 188 3. Future work 190 References 192 초 록 231박

Novel Metamaterials and Their Applications in Subwavelength Waveguides, Imaging and Modulation

The development of metamaterials has opened the door for engineering electromagnetic properties by subwavelength artificial atoms , and hence accessing new properties and functionalities which cannot be found among naturally occurring materials. In particular, metamaterials enable the flexibility of independently controlling the permittivity and permeability to be almost any arbitrary value, which promises to achieve deep subwavelength confinement and focusing of electromagnetic waves in different spectrum regimes. The next stage of this technological revolution will be focused on the development of active and controllable metamaterials, where the properties of the metamaterials are expected to be tuned by external stimuli. In this sense, some natural materials are also promising to provide the tunable capability, particularly in the near infrared and terahertz domains either by applying a voltage or shining light on the materials. The objective of this dissertation is to investigate novel metamaterials and explore three important applications of them: subwavelength waveguiding, imaging and modulation. The first part of this dissertation covers the theory, design and fabrication of several different types of metamaterials, which includes artificially designed metamaterials and some naturally existing materials. The second part demonstrates metal gratings functioning as designer surface plasmonic waveguides support deep subwavelength surface propagation modes at microwave frequency. The third part proposes multilayered metal-insulator stack as indefinite metamaterial that converts evanescent waves to propagating waves, hence deep subwavelength image can be observed. The fourth part explores the tunability of several natural materials - gallium (Ga), indium tin oxide (ITO) and graphene, and demonstrates electro-optical (EO) modulators based on these materials can be achieved on nano-scale. The final part summarizes the work presented in this dissertation and also discusses some future work for photodetection, photovoltaics, and modulation

Real-time digital signal processing system for normal probe diffraction technique

Ultrasonic systems are widely used in many fields of non-destructive testing. The increasing requirement for high quality steel product stirs the improvement of both ultrasonic instruments and testing methods. The thesis indicates the basics of ultrasonic testing and Digital Signal Processing (DSP) technology for the development of an ultrasonic system. The aim of this project was to apply a new ultrasonic testing method - the Normal Probe Diffraction method to course grained steel in real-time and investigate whether the potential of probability of detection (POD) has been improved. The theories and corresponding experiment set-up of pulse-echo method, TOFD and NPD method are explained and demonstrated separately. A comparison of these methods shows different contributions made by these methods using different types of algorithms and signals. Non-real-time experiments were carried out on a VI calibration block using an USPC 3100 ultrasonic testing card to implement pulse-echo and NPD method respectively. The experiments and algorithm were simulated and demonstrated in Matlab. A low frequency Single-transmitter-multi-receiver ultrasonic system was designed and built with a digital development board and an analogue daughter card to transmit or receive signals asynchronously. A high frequency high voltage amplifier was designed to drive the ultrasonic probes. A Matlab simulation system built with Simulink indicates that the Signal to Noise Ratio (SNR) can be improved with an increment of up to 3dB theoretically based on the simulation results using DSP techniques. The DSP system hardware and software was investigated and a real-time DSP hardware system was supposed to be built to implement the high frequency system using a rapid code generated system based on Matlab Simulink model and the method was presented. However, extra effort needs to be taken to program the hardware using a low-level computer language to make the system work stably and efficiently

The application of artificial neural networks to interpret acoustic emissions from submerged arc welding

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Automated fusion welding processes play a fundamental role in modern manufacturing industries. The proliferation of joint geometries together with the large permutation of associated process variable configurations has given rise to research into complex system modelling and control strategies. Many of these techniques have involved monitoring of not only the electrical characteristics of the process but visual and acoustic information. Acoustic information derived from certain welding processes is well documented as it is an established fact that skilled manual welders utilise such information as an aid to creating an optimum weld. The experimental investigation presented in this thesis is dedicated to the feasibility of monitoring airborne acoustic emissions of Submerged Arc Welding (SAW) for diagnostic and real time control purposes. The experimental method adopted for this research takes a cybernetic approach to data processing and interpretation in an attempt to replicate the robustness of human biological functions. A custom designed audio hardware system was used to analyse signals obtained from bead on mild steel plate fusion welds. Time and frequency domains were used in an attempt to establish salient characteristics or identify the signatures associated with changes of the process variables. The featured parameters were voltage / current and weld travel speed, due to their ease of validation. However, consideration has also been given to weld defect prediction due to process instabilities. As the data proved to be highly correlated and erratic when subjected to off line statistical analysis, extensive investigation was given to the application of artificial neural networks to signal processing and real time control scenarios. As a consequence, a dedicated neural based software system was developed, utilising supervised and unsupervised neural techniques to monitor the process. The research was aimed at proving the feasibility of monitoring the electrical process parameters and stability of the welding process in real time. It was shown to be possible, by the exploitation of artificial neural networks, to generate a number of monitoring parameters indicative of the welding process state. The limitations of the present neural method and proposed developments are discussed, together with an overview of applied neural network technology and its impact on artificial intelligence and robotic control. Further developments are considered together with recommendations for future areas of research

Capabilities of the Investment Casting process for producing meso/micro metal castings using Rapid Prototyping manufacturing routes

This thesis examines the capabilities of different Rapid Prototyping (RP) manufacturing processes for producing sound metallic parts incorporating features in the micrometre range using the Investment Casting (IC) process. RP has been growing in the past twenty years and is nowadays widely employed in the area of precision investment casting since the technology offers the possibility of manufacturing wax patterns which can be directly implemented into investment casting. Owing to the steady improvements of the technology, some of the recently developed RP building machines offer the possibility of manufacturing small parts incorporating micro-features. In this work, a detailed description of the accuracy and capabilities of the IC process regarding its potential for producing sound meso/micro components is given using two types of conventional RP machines. The results of this analysis are then compared through a benchmarking study with a recently developed RP process suitable for the direct manufacture of ceramic moulds. The different technological chains are compared regarding their overall accuracy, surface finish, the amount of structural defects present in the castings and their relative production costs and lead-time. Finally, the potential of the investment casting process for manufacturing sound micro-castings with high aspect ratio is approached from a structural point of view. Through a metallographic analysis study, the research investigates the size-scale effect of cast micro-components upon their microstructure and the subsequent changes in their mechanical properties

NASA's Microgravity Technology Report, 1996: Summary of Activities

This report covers technology development and technology transfer activities within the Microgravity Science Research Programs during FY 1996. It also describes the recent major tasks under the Advanced Technology Development (ATD) Program and identifies current technology requirements. This document is consistent with NASA,s Enteprise for the Human Exploration and development of Space (HEDS) Strategic Plan. This annual update reflects changes in the Microgravity Science Research Program's new technology activities and requirements. Appendix A. FY 1996 Advanced Technology Development. Program and Project Descriptions. Appendix B. Technology Development