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2007

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Articles 2101 - 2130 of 3831

Full-Text Articles in Engineering

Towards The Development Of A Defensive Cyber Damage And Mission Impact Methodology, Larry W. Fortson Jr. Mar 2007

Towards The Development Of A Defensive Cyber Damage And Mission Impact Methodology, Larry W. Fortson Jr.

Theses and Dissertations

The purpose of this research is to establish a conceptual methodological framework that will facilitate effective cyber damage and mission impact assessment and reporting following a cyber-based information incidents. Joint and service guidance requires mission impact reporting, but current efforts to implement such reporting have proven ineffective. This research seeks to understand the impediments existing in the current implementation and to propose an improved methodology. The research employed a hybrid historical analysis and case study methodology for data collection through extensive literature review, examination of existing case study research and interviews with Air Force members and civilian personnel employed as …


Indentifying And Assessing Effective Mechanisms For Technology Transfer, Michael A. Romero Mar 2007

Indentifying And Assessing Effective Mechanisms For Technology Transfer, Michael A. Romero

Theses and Dissertations

The ability to shift knowledge and resources from federal laboratories to industrial and academic partners and vice versa is the primary reason that technology transfer (T2) exists today. Without the cooperation of federal, state, and private agencies working together to resolve today’s technology quandaries, a lot of the breakthroughs experienced today would not exist. This research was focused on uncovering which mechanisms are utilized by scientists and engineers. The research entails uncovering both official and unofficial mechanisms and ascertaining why some methods are preferred over others. It is also a secondary focus, to determine which barriers are impeding …


Quantification Of Risk For Usaf Fire And Emergency Services Flights As A Result Of Shortage In Manpower, Timothy D. Dalby Mar 2007

Quantification Of Risk For Usaf Fire And Emergency Services Flights As A Result Of Shortage In Manpower, Timothy D. Dalby

Theses and Dissertations

The United States Air Force (USAF) is currently experiencing a period of high operations tempo and overseas deployments have become frequent. These deployments will leave home installations short manned. Some amount of risk is incurred by the home installation as a result of the short manning. For an organization, such as an USAF Fire and Emergency Services (FES) flight, whose primary responsibility is the protection of life and property, the incurred risk could be catastrophic. Still no attempt has been made to quantify risk in terms of manpower for the USAF FES flights. The primary purpose of this research was …


Optimizing The Prioritization Of Natural Disaster Recovery Projects, Jason M. Aftanas Mar 2007

Optimizing The Prioritization Of Natural Disaster Recovery Projects, Jason M. Aftanas

Theses and Dissertations

Prioritizing reconstruction projects to recover a base from a natural disaster is a complicated and arduous process that involves all levels of leadership. The project prioritization phase of base recovery has a direct affect on the allocation of funding, the utilization of human resources, the obligation of projects, and the overall speed and efficiency of the recovery process. The focus of this research is the development of an objective and repeatable process for optimizing the project prioritization phase of the recovery effort. This work will focus on promoting objectivity in the project prioritizing process, improving the communication of the overall …


The Chaos Of Katrina: A Nonlinear Analysis Of Federal Logistics Support During Hurricane Katrina Relief Operations, Gerald W. Morris Jr. Mar 2007

The Chaos Of Katrina: A Nonlinear Analysis Of Federal Logistics Support During Hurricane Katrina Relief Operations, Gerald W. Morris Jr.

Theses and Dissertations

This thesis presents a case study of federal logistics support during Hurricane Katrina disaster relief operations. Data from federal contracts covering the first 10 weeks of Katrina are used to measure federal logistics activity. The study investigates whether chaos theory, part of complexity science, can extract information from Katrina contracting data to help managers make better logistics decisions during disaster relief operations. The study uses three analytical techniques: embedding, fitting the data to a logistic equation, and plotting the limit-cycle. Embedding and fitting a logistic equation to the data were used to test for deterministic chaos. The logistic equation and …


Determining Logistics Ground Support Manpower Requirements For A Reusable Military Launch Vehicle, Sydney C. Michalski Mar 2007

Determining Logistics Ground Support Manpower Requirements For A Reusable Military Launch Vehicle, Sydney C. Michalski

Theses and Dissertations

Successful space-based technologies like satellite imagery and GPS have increased military demand for a rapid-response launch capability. AF Space Command's Operationally Responsive Spacelift program was developed to ensure that the AF has the capability to launch a payload into orbit within hours of a tasking notification, and requires development of a new space launch vehicle. The Reusable Military Launch Vehicle (RMLV) is currently in the design phase. The AF Research Laboratory sponsored development of the MILEPOST simulation model in order to assess the turnaround time, and thus responsiveness, of various design alternatives. The focus of this thesis is to improve …


An Analysis Of Robust Workforce Scheduling Models For A Nurse Rostering Problem, Paul K. Tower Mar 2007

An Analysis Of Robust Workforce Scheduling Models For A Nurse Rostering Problem, Paul K. Tower

Theses and Dissertations

Disruptions impacting workforce schedules can be costly. A 1999 study of the United Kingdom's National Health Service estimated that as much as 4% of the total resources spent on staffing were lost to schedule disruptions like absenteeism. Although disruptions can not be eliminated, workforce schedules can be improved to be more responsive to disruptions. One key area of study that has expanded over the past few years is the application of traditional scheduling techniques to re-rostering problems. These efforts have provided methods for responding to schedule disruptions, but typically require deviations to the disrupted schedule. This thesis examines five workforce …


Modeling, Simulation, And Estimation Of Optical Turbulence, Byron P. Formwalt Mar 2007

Modeling, Simulation, And Estimation Of Optical Turbulence, Byron P. Formwalt

Theses and Dissertations

This dissertation documents three new contributions to simulation and modeling of optical turbulence. The first contribution is the formalization, optimization, and validation of a modeling technique called successively conditioned rendering (SCR). The SCR technique is empirically validated by comparing the statistical error of random phase screens generated with the technique. The second contribution is the derivation of the covariance delineation theorem, which provides theoretical bounds on the error associated with SCR. It is shown empirically that the theoretical bound may be used to predict relative algorithm performance. Therefore, the covariance delineation theorem is a powerful tool for optimizing SCR algorithms. …


Critical Infrastructure Rebuild Prioritization Using Simulation Optimization, Namsuk Cho Mar 2007

Critical Infrastructure Rebuild Prioritization Using Simulation Optimization, Namsuk Cho

Theses and Dissertations

This thesis examines the importance of a critical infrastructure rebuild strategy following a terrorist attack or natural disaster such as Hurricane Katrina. Critical infrastructures are very complex and dependent systems in which their re-establishment is an essential part of the rebuilding process. A rebuild simulation model consisting of three layers (physical, information, and spatial) captures the dependency between the six critical infrastructures modeled. We employ a simulation optimization approach to evaluate rebuild prioritization combinations with a goal of minimizing the time needed to achieve an acceptable rebuild level. We use a simulated annealing heuristic as an optimization technique that works …


Improved Hyperspectral Image Testing Using Synthetic Imagery And Factorial Designed Experiments, Joseph P. Bellucci Mar 2007

Improved Hyperspectral Image Testing Using Synthetic Imagery And Factorial Designed Experiments, Joseph P. Bellucci

Theses and Dissertations

The goal of any remote sensing system is to gather data about the geography it is imaging. In order to gain knowledge of the earth's landscape, post-processing algorithms are developed to extract information from the collected data. The algorithms can be intended to classify the various ground covers in a scene, identify specific targets of interest, or detect anomalies in an image. After the design of an algorithm comes the difficult task of testing and evaluating its performance. Traditionally, algorithms are tested using sets of extensively ground truthed test images. However, the lack of well characterized test data sets and …


Prioritizing Satellite Payload Selection Via Optimization, Benjamin S. Kallemyn Mar 2007

Prioritizing Satellite Payload Selection Via Optimization, Benjamin S. Kallemyn

Theses and Dissertations

This thesis develops optimization models for prioritizing payloads for inclusion on satellite buses with volume, power, weight and budget constraints. The first model considers a single satellite launch for which the budget is uncertain and constellation requirements are not considered. Subsequently, we include constellation requirements and provide a more enhanced model. Both single-launch models provide a prioritized list of payloads to include on the launch before the budget is realized. The single-launch models are subsequently extended to a sequence of multiple launches in two cases, both of which incorporate an explicit dependence on the constellation composition at each launch epoch. …


Approximate Analysis Of An Unreliable M/M/2 Retrial Queue, Brian P. Crawford Mar 2007

Approximate Analysis Of An Unreliable M/M/2 Retrial Queue, Brian P. Crawford

Theses and Dissertations

This thesis considers the performance evaluation of an M/M/2 retrial queue for which both servers are subject to active and idle breakdowns. Customers may abandon service requests if they are blocked from service upon arrival, or if their service is interrupted by a server failure. Customers choosing to remain in the system enter a retrial orbit for a random amount of time before attempting to re-access an available server. We assume that each server has its own dedicated repair person, and repairs begin immediately following a failure. Interfailure times, repair times and times between retrials are exponentially distributed, and all …


Patterns Of War Termination: A Statistical Approach, Paul D. Robinson Ii Mar 2007

Patterns Of War Termination: A Statistical Approach, Paul D. Robinson Ii

Theses and Dissertations

This research uses an advanced statistical technique to expand upon the current understanding of war termination. Specifically, this thesis addressed questions concerning the most relevant factors toward predicting both the outcomes of interstate wars and the winners of intrastate and extra-systemic wars, within the limitations of the available data. Open-source war data from the Correlates of War Project was analyzed using both binary and multinomial logistic regression techniques. While the Correlates of War Project did not necessarily focus its data collection efforts on those variables historically associated with war termination, it did provide a sufficient number of variables with which …


Mathematical Programming Model For Fighter Training Squadron Pilot Scheduling, Thomas M. Newlon Mar 2007

Mathematical Programming Model For Fighter Training Squadron Pilot Scheduling, Thomas M. Newlon

Theses and Dissertations

The United States Air Force fighter training squadrons build weekly schedules using a long and tedious process. Very little of this process is automated and optimality of any kind is nearly impossible. Schedules are built to a feasible condition only to be changed with consideration of Wing level requirements. Weekly flying schedules are restricted by requirements for crew rest, days since a pilot's last sortie, sorties in the last 30 days, and sorties in the last 90 days. By providing a scheduling model to the pilot charged with creating the schedule, valuable pilot hours could be spent in the cockpit, …


Change-Point Methods For Overdispersed Count Data, Brian A. Wilken Mar 2007

Change-Point Methods For Overdispersed Count Data, Brian A. Wilken

Theses and Dissertations

A control chart is often used to detect a change in a process. Following a control chart signal, knowledge of the time and magnitude of the change would simplify the search for and identification of the assignable cause. In this research, emphasis is placed on count processes where overdispersion has occurred. Overdispersion is common in practice and occurs when the observed variance is larger than the theoretical variance of the assumed model. Although the Poisson model is often used to model count data, the two-parameter gamma-Poisson mixture parameterization of the negative binomial distribution is often a more adequate model for …


Automatic Target Recognition User Interface Tool, David A. Kerns Mar 2007

Automatic Target Recognition User Interface Tool, David A. Kerns

Theses and Dissertations

A computer tool to aid in selecting the best Automatic Target Recognition (ATR) algorithm is developed. The program considers many quantifiers, accepts user-defined parameters, allows for changes in the operational environment and presents results in a meaningful way. It is written for Microsoft Excel. An ATR algorithm assigns a class label to a recognized target. General designations can include "Friend" and "Foe." The error of designating "Friend" as "Foe" as well as "Foe" as "Friend" comes with a high cost. Studying each algorithm's error can minimize this cost. Receiver Operating Characteristic (ROC) curves provide only information on the probabilities given …


Examining Clandestine Social Networks For The Presence Of Non-Random Structure, Joshua S. Seder Mar 2007

Examining Clandestine Social Networks For The Presence Of Non-Random Structure, Joshua S. Seder

Theses and Dissertations

This thesis develops a tractable, statistically sound hypothesis testing framework for the detection, characterization, and estimation of non-random structure in clandestine social networks. Network structure is studied via an observed adjacency matrix, which is assumed to be subject to sampling variability. The vertex set of the network is partitioned into k mutually exclusive and collectively exhaustive subsets, based on available exogenous nodal attribute information. The proposed hypothesis testing framework is employed to statistically quantify a given partition's relativity in explaining the variability in the observed adjacency matrix relative to what can be explained by chance. As a result, valuable insight …


A Future-Based Risk Assessment For The Survivability Of Long Range Strike Systems, William D. Heuck Jr. Mar 2007

A Future-Based Risk Assessment For The Survivability Of Long Range Strike Systems, William D. Heuck Jr.

Theses and Dissertations

The United States Air Force today faces the challenge of allocating development resources to prepare for future force projection requirements. In particular, the Air Force's core competency of Global Attack implies a future capability that can quickly and successfully deliver combat effects anywhere in the world with impunity. Understanding that the future threat environment is dynamic and that continued advancements by adversaries will likely degrade the technical superiority of today's weapon systems, the need arises for a planning model to direct development funding to areas with the greatest probability of successfully defending the strike vehicle of 2035. Examining this problem …


Nonlinear Time-Variant Response In An Avalanche Photodiode Array Based Laser Detection And Ranging System, Michael D. Seal Mar 2007

Nonlinear Time-Variant Response In An Avalanche Photodiode Array Based Laser Detection And Ranging System, Michael D. Seal

Theses and Dissertations

This research effort identifies and models the nonlinear time-variant behavior exhibited by an avalanche photodiode (APD) array based Laser Ranging and Detection (LADAR) system. Based on the original Linear Time-Invariant (LTI) model, the evolution of error in the LADAR signal is examined sequentially from the outgoing pulse through signal digitization. This error evolution shows that the LTI model does not contain a mechanism for causing the observed signal deviations or the failure to meet the Cramer-Rao lower bound for range accuracy. A nonlinear time-variant model is developed based on the interactions of the avalanche photodiodes in the array with the …


Passive Geolocation Of Low Power Emitters In Urban Environments Using Tdoa, Myrna B. Montminy Mar 2007

Passive Geolocation Of Low Power Emitters In Urban Environments Using Tdoa, Myrna B. Montminy

Theses and Dissertations

Low-power devices are commonly used by the enemy to control Improvised Explosive Devices (IEDs), and as communications nodes for command and control. Quickly locating the source of these signals is difficult, especially in an urban environment where buildings and towers can cause interference. This research presents a geolocation system that combines several geolocation and error mitigation methods to locate an emitter in an urban environment. The proposed geolocation system uses a Time Difference of Arrival (TDOA) technique to estimate the location of the emitter of interest. Using sensors at known locations, TDOA estimates are obtained by cross-correlating the signal received …


Stochastic Estimation And Control Of Queues Within A Computer Network, Nathan C. Stuckey Mar 2007

Stochastic Estimation And Control Of Queues Within A Computer Network, Nathan C. Stuckey

Theses and Dissertations

An extended Kalman filter is used to estimate size and packet arrival rate of network queues. These estimates are used by a LQG steady state linear perturbation PI controller to regulate queue size within a computer network. This paper presents the derivation of the transient queue behavior for a system with Poisson traffic and exponential service times. This result is then validated for ideal traffic using a network simulated in OPNET. A more complex OPNET model is then used to test the adequacy of the transient queue size model when non-Poisson traffic is combined. The extended Kalman filter theory is …


Effects Of Prior Aging At Elevated Temperature In Air And In Argon Environments On Creep Response Of Pmr-15 Neat Resin, Joseph L. Broeckert Mar 2007

Effects Of Prior Aging At Elevated Temperature In Air And In Argon Environments On Creep Response Of Pmr-15 Neat Resin, Joseph L. Broeckert

Theses and Dissertations

Creep behavior of PMR-15 neat resin, a polyimide thermoset polymer, aged in air or argon gaseous environments at 288 °C for up to 1000 h was evaluated. Creep tests were performed at 288 C at creep stress levels of 20 and 10 MPa. Creep periods of at least 25 h in duration were followed by recovery at zero stress. Weight loss and growth of the oxidation layer were also monitored and correlated with aging time. The aging time in both air and argon environments had a strong influence on the creep and recovery response of PMR-15 neat resin. Samples aged …


In Vitro Cell Culture Infectivity Assay For Human Noroviruses, Timothy M. Straub, Kerstin Höner Zu Bentrup, Patricia Orosz Coghlan, Alice Dohnalkova, Brooke K. Mayer, Rachel A. Bartholomew, Catherine O. Valdez, Cynthia J. Bruckner-Lea, Charles P. Gerba, Morteza A. Abbaszadegan, Cheryl A. Nickerson Mar 2007

In Vitro Cell Culture Infectivity Assay For Human Noroviruses, Timothy M. Straub, Kerstin Höner Zu Bentrup, Patricia Orosz Coghlan, Alice Dohnalkova, Brooke K. Mayer, Rachel A. Bartholomew, Catherine O. Valdez, Cynthia J. Bruckner-Lea, Charles P. Gerba, Morteza A. Abbaszadegan, Cheryl A. Nickerson

Civil and Environmental Engineering Faculty Research and Publications

Human noroviruses cause severe, self-limiting gastroenteritis that typically lasts 24–48 hours. Because of the lack of suitable tissue culture or animal models, the true nature of norovirus pathogenesis remains unknown. We show that noroviruses can infect and replicate in a 3-dimensional (3-D), organoid model of human small intestinal epithelium. Cells grown on porous collage-coated beads under fluid shear conditions in rotating wall vessel bioreactors differentiate into 3-D architectures resembling both the morphologic and physiologic function of in vivo tissues. Microscopy, PCR, and fluorescent in situ hybridization provided evidence of norovirus infection. Cytopathic effect and norovirus RNA were detected at each …


Nanosecond Near-Spinodal Homogeneous Boiling Of Water Superheated By Pulsed Co/Sub 2/ Laser, Sergey I. Kudryashov, Kevin Lyon, Susan D. Allen Mar 2007

Nanosecond Near-Spinodal Homogeneous Boiling Of Water Superheated By Pulsed Co/Sub 2/ Laser, Sergey I. Kudryashov, Kevin Lyon, Susan D. Allen

Mechanical Engineering - Daytona Beach

The fast boiling dynamics of superheated surface layers of bulk water cavitating under near-spinodal conditions during nanosecond C O2 laser heating pulses was studied using contact broad-band photoacoustic spectroscopy. Characteristic pressure-tension cycles recorded by an acoustic transducer at different incident laser fluences represent (a) weak random oscillations of transient nanometer-sized near-critical bubbles-precursors and (b) well-defined stimulated oscillations of micron-sized supercritical bubbles and their submicrosecond coalescence products. These findings provide an important insight into basic thermodynamic parameters, spatial and temporal scales of bubble nucleation during explosive liquid/vapor transformations in absorbing liquids ablated by short laser pulses in the thermal confinement regime. …


Efficient Algorithms For Machine Scheduling Problems With Earliness And Tardiness Penalties, Guang Feng, Hoong Chuin Lau Mar 2007

Efficient Algorithms For Machine Scheduling Problems With Earliness And Tardiness Penalties, Guang Feng, Hoong Chuin Lau

Research Collection School Of Computing and Information Systems

In this paper, we study the multi-machine scheduling problem with earliness and tardiness penalties and sequence dependent setup times. This problem can be decomposed into two subproblems—sequencing and timetabling. Sequencing focuses on assigning each job to a fixed machine and determine the job sequence on each machine. We call such assignment a semi-schedule. Timetabling focuses on finding an executable schedule from the semi-schedule via idle-time insertion. Sequencing is strongly NP-hard in general. Although timetabling is polynomial-time solvable, it can become a computational bottleneck if the procedure is executed many times within a larger framework. This paper makes two contributions. We …


Studies On The Ion Conductive Performance Of Composite Polymer Electrolyte Containing Nano-Scale Sio2, Lin Ye, Yu-Mei Zhao, Xiao-Wen Zhang, Zeng-Guo Feng, Ying Bai, Feng Wu Feb 2007

Studies On The Ion Conductive Performance Of Composite Polymer Electrolyte Containing Nano-Scale Sio2, Lin Ye, Yu-Mei Zhao, Xiao-Wen Zhang, Zeng-Guo Feng, Ying Bai, Feng Wu

Journal of Electrochemistry

Nano-silicon oxide was doped into a novel comblike polyether,poly(3-(methoxy(triethylenoxy))methyl-3'-methyloxetane) to prepare a series of composite polymer electrolyte.The ion conductivity measurement revealed that the composite polymer electrolyte added with 5% weight fraction of SiO2 has the highest ion conductivity of 7.8×10-5 S/cm at 30℃ and 4.5×10-4 S/cm at 80℃,which is increased 30%~60% as compared to the polymer electrolyte without adding any SiO2.DSC results implied that the nano-silicon oxide inserts into the macromolecule chains to increase the free volume and decrease the glass transition temperature to enhance the ion conductivity.In addition,it seems that the crystallization occurred due …


Preparation And Lithium Storage Performance Of Nano-Structural Sno2/Carbon Nanotubes Composite Electrodes, Hong-Bing Wei, Ling Huang, Xiao-Bin Wu, Fu-Sheng Ke, Jin-Shu Cai, Xiao-Yong Fan, Fang-Zu Yang, Shi-Gang Sun Feb 2007

Preparation And Lithium Storage Performance Of Nano-Structural Sno2/Carbon Nanotubes Composite Electrodes, Hong-Bing Wei, Ling Huang, Xiao-Bin Wu, Fu-Sheng Ke, Jin-Shu Cai, Xiao-Yong Fan, Fang-Zu Yang, Shi-Gang Sun

Journal of Electrochemistry

The Nano-structural SnO2/Carbon Nanotubes Composites were prepared in a solution of SnCl2,ethylene glycol(EG),purified MWCNTs and sodium dodecyl sulfate(SDS) at 160 ℃.The phase structure and surface morphology of the composites were measured by XRD HRTEM and SEM.The results indicate that there are high dispersions and high loadings of SnO2 nanoparticles on MWCNTs.Electrochemical lithium storage performance was studied preliminarily on the obtained samples.MWCNTs modified with SnO2 nanoparicles were shown to have a higher electrochemical reversible capacity than that of MWCNTs during the charge and discharge process.MWCNTs can accommodate the larger structure strain of the active materials …


Effect Of 2,2'-Dipyridyldipyridyl And K4Fe(Cn)6 On Electroless Copper Plating Using Glyoxylic Acid As Reducing Agent, Dan-Dan Shen, Fang-Zu Yang, Hui-Huang Wu Feb 2007

Effect Of 2,2'-Dipyridyldipyridyl And K4Fe(Cn)6 On Electroless Copper Plating Using Glyoxylic Acid As Reducing Agent, Dan-Dan Shen, Fang-Zu Yang, Hui-Huang Wu

Journal of Electrochemistry

The electroless copper plating was studied in a bath consisting of glyoxylic acid as reducing agent,Na2EDTA as complexing agent,both 2,2'-dipyridyl and K4Fe(CN)6 as additives,focused on the influence of the additives on the deposition rate,as well as on the composition,structures and surface morphology of the deposits.It was found that adding an optimum amount of 2,2'-dipyridyl and K4Fe(CN)6 to the bath will not only improve the stability of the bath,but also increase the deposition rate by 100 %.The simultaneous addition of both additives causes the deposits to bright appearance and the change in morphology.The …


The Influence Of Electrolytic Conditions On Synthesis Of Feo42-, Yan Zhou, Wei-Hua Yang, Dong-Qin Bi, Ting-Ting Huang, Zhen Sun, Quan-Feng Guo Feb 2007

The Influence Of Electrolytic Conditions On Synthesis Of Feo42-, Yan Zhou, Wei-Hua Yang, Dong-Qin Bi, Ting-Ting Huang, Zhen Sun, Quan-Feng Guo

Journal of Electrochemistry

The FeO42-was synthesized using iron lattice as anode and the solid K2FeO4 was also prepared.Influence of electrolytic conditions,such as stir,ultrasonic,temperature and current density on the current efficiency were studied.The results showed that preferable ferrate(VI) current yield was obtained in a current density of 10 mA·cm-2,temperature 30 ℃ and 14 mol/L NaOH as electrolyte under at stirring effect.


Variable Frequence Switching Power Supply Used To Aluminum Foil Corrosion, Xiang-Dong Zhuo, Lan Sun, Guo-Qing Qian, Chang-Jian Lin Feb 2007

Variable Frequence Switching Power Supply Used To Aluminum Foil Corrosion, Xiang-Dong Zhuo, Lan Sun, Guo-Qing Qian, Chang-Jian Lin

Journal of Electrochemistry

A new variable frequence and multi-wave power supply used to aluminum foil corrosion is introduced. The switching supply power supply controlled by the microcontroller and Variable Frequence in low band and multi-wave, big power supply controlled by the electronic switch bridge for changing phase were designed. This power supply does not need bulky and cumbersome transformer, and the frequency at lower limit is not limited, even lower than 1Hz.