Browsing by Author "Kachroo, Pushkin"
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- Application of Magnetorheological Dampers for Vehicle Seat SuspensionsReichert, Brian Anthony Jr. (Virginia Tech, 1997-12-03)This study evaluates and provides solutions to the problem of poor subjective feel of seat suspensions that employ magnetorheological (MR) dampers and skyhook control. An Isringhausen seat suspension that had been modified to replace the stock passive damper with a controllable MR damper was used to evaluate the problems and potential solutions. A seat suspension tester was built using materials from 80/20 Incorporated and a hydraulic actuation system from MTS. An HP Dynamic Signal Analyzer was used as the main piece of data acquisition equipment, along with a Pentium PC and National Instruments Data Acquisition card. All of the hardware is installed in a controlled laboratory facility at Virginia Tech's Advanced Vehicle Dynamics Lab. The first task was to analyze the source of the unexpected peak in the acceleration spectrum of the suspended seat. This analysis was accomplished using a combination of pure tone inputs and a Fourier analysis of a simple model of the system. This analysis indicated that the peak is actually three times the resonant frequency of the seat suspension. The analysis also indicates that the frequency components continue at odd multiples of the resonant frequency, however, the third peak is the most noticeable. The third multiple is in the resonant frequency range (4-8 Hz) of the human body, so it was initially blamed for the poor subjective feel of the seat. However, solutions to remove this harmonic were tested without success. The work progressed to a time domain analysis, which eventually led to determining the source of the poor subjective feel. The seat suspension was excited with a variety of inputs. The seat acceleration and damper control current were examined in the time domain to show that the cause of the poor subjective feel is the control signal discontinuities. The control policy was modified to remove the control signal discontinuities and was found to improve the subjective feel of the seat. Finally, several two-degree-of-freedom control policies were implemented and tested. Although the results from this testing are inconclusive, they generated several recommendations for future research.
- Automatic Ultrasonic Headway Control for a Scaled Robotic CarHenry, Richard Douglas (Virginia Tech, 2001-12-18)Intelligent Transportation Systems and supporting technologies have been an active area of research for some time. Human drivers exhibit slower response times and errors in judgment that can have serious adverse affects on traffic flow. These types of errors can be reduced or eliminated from the driving experience by introducing computer control systems into the automotive arena. The purpose of this research was to develop a scale model platform for the rapid prototyping and testing of ITS systems and technologies. Specifically, this body of work was concerned with the development of an automatic headway control system that utilized ultrasonic sensors. This control system was intended to automatically maintain headway distance in an effort to create an adaptive cruise control system for this scale model vehicle. Implementation of such systems could conceivably reduce driver fatigue by removing the burden of maintaining safe following distance from the driver. System dynamics of car-like robots with nonholonomic constraints were employed in this research to create a controller for an autonomous path following vehicle. The application of a working kinematic model describing car-like robotic systems allowed the development of a simple first order controller, as well as a sliding mode controller. Following the development and simulation of these two control laws, the system was applied to the FLASH project scale model vehicle to assess the practical use of the system on a mock highway. A satisfactory result is produced after testing was completed, and the application of such systems to scale model platforms is feasible.
- Autonomous Vehicle Control using Image ProcessingSchlegel, Nikolai (Virginia Tech, 1997-01-27)This thesis describes the design of an inexpensive autonomous vehicle system using a small scaled model vehicle. The system is capable of operating in two different modes: telerobotic manual mode and automated driving mode. In telerobotic manual mode, the model vehicle is controlled by a human driver at a stationary remote control station with full-scale steering wheel and gas pedal. The vehicle can either be an unmodified toy remote-control car or a vehicle equipped with wireless radio modem for communication and microcontroller for speed control. In both cases the vehicle also carries a video camera capable of transmitting video images back to the remote control station where they are displayed on a monitor. In automated driving mode, the vehicle's lateral movement is controlled by a lateral control algorithm. The objective of this algorithm is to keep the vehicle in the center of a road. Position and orientation of the vehicle are determined by an image processing algorithm identifying a white middle marker on the road. Two different algorithm for image processing have been designed: one based on the pixel intensity profile and the other on vanishing points in the image plane. For the control algorithm itself, two designs are introduced as well: a simple classical P-control and a control scheme based on H-Infinity. The design and testing of this autonomous vehicle system are performed in the Flexible Low-cost Automated Scaled Highway (FLASH) laboratory at Virginia Tech.
- A Bayesian Network Approach to the Self-organization and Learning in Intelligent AgentsSahin, Ferat (Virginia Tech, 2000-08-25)A Bayesian network approach to self-organization and learning is introduced for use with intelligent agents. Bayesian networks, with the help of influence diagrams, are employed to create a decision-theoretic intelligent agent. Influence diagrams combine both Bayesian networks and utility theory. In this research, an intelligent agent is modeled by its belief, preference, and capabilities attributes. Each agent is assumed to have its own belief about its environment. The belief aspect of the intelligent agent is accomplished by a Bayesian network. The goal of an intelligent agent is said to be the preference of the agent and is represented with a utility function in the decision theoretic intelligent agent. Capabilities are represented with a set of possible actions of the decision-theoretic intelligent agent. Influence diagrams have utility nodes and decision nodes to handle the preference and capabilities of the decision-theoretic intelligent agent, respectively. Learning is accomplished by Bayesian networks in the decision-theoretic intelligent agent. Bayesian network learning methods are discussed intensively in this paper. Because intelligent agents will explore and learn the environment, the learning algorithm should be implemented online. None of the existent Bayesian network learning algorithms has online learning. Thus, an online Bayesian network learning method is proposed to allow the intelligent agent learn during its exploration. Self-organization of the intelligent agents is accomplished because each agent models other agents by observing their behavior. Agents have belief, not only about environment, but also about other agents. Therefore, an agent takes its decisions according to the model of the environment and the model of the other agents. Even though each agent acts independently, they take the other agents behaviors into account to make a decision. This permits the agents to organize themselves for a common task. To test the proposed intelligent agent's learning and self-organizing abilities, Windows application software is written to simulate multi-agent systems. The software, IntelliAgent, lets the user design decision-theoretic intelligent agents both manually and automatically. The software can also be used for knowledge discovery by employing Bayesian network learning a database. Additionally, we have explored a well-known herding problem to obtain sound results for our intelligent agent design. In the problem, a dog tries to herd a sheep to a certain location, i.e. a pen. The sheep tries to avoid the dog by retreating from the dog. The herding problem is simulated using the IntelliAgent software. Simulations provided good results in terms of the dog's learning ability and its ability to organize its actions according to the sheep's (other agent) behavior. In summary, a decision-theoretic approach is applied to the self-organization and learning problems in intelligent agents. Software was written to simulate the learning and self-organization abilities of the proposed agent design. A user manual for the software and the simulation results are presented. This research is supported by the Office of Naval Research with the grant number N00014-98-1-0779. Their financial support is greatly appreciated.
- Capacity Modeling of Freeway Weaving SectionsZhang, Yihua (Virginia Tech, 2005-05-09)The dissertation develops analytical models that estimate the capacity of freeway weaving sections. The analytical models are developed using simulated data that were compiled using the INTEGRATION software. Consequently, the first step of the research effort is to validate the INTEGRATION lane-changing modeling procedures and the capacity estimates that are derived from the model against field observations. The INTEGRATION software is validated against field data gathered by the University of California at Berkeley by comparing the lateral and longitudinal distribution of simulated and field observed traffic volumes categorized by O-D pair on nine weaving sections in the Los Angeles area. The results demonstrate a high degree of consistency between simulated and field observed traffic volumes within the various weaving sections. Subsequently, the second validation effort compares the capacity estimates of the INTEGRATION software to field observations from four weaving sections operating at capacity on the Queen Elizabeth Way (QEW) in Toronto, Canada. Again, the results demonstrate that the capacity estimates of the INTEGRATION software are consistent with the field observations both in terms of absolute values and temporal variability across different days. The error was found to be in the range of 10% between simulated and field observed capacities. Prior to developing the analytical models, the dissertation presents a systematic analysis of the factors that impact the capacity of freeway weaving sections, which were found to include the length of the weaving section, the weaving ratio (a new parameter that is developed as part of this research effort), the percentage of heavy vehicles, and the speed limit differential between freeway and on- and off-ramps. The study demonstrates that the weaving ratio, which is currently defined as the ratio of the lowest weaving volume to the total weaving volume in the 2000 Highway Capacity Manual, has a significant impact on the capacity of weaving sections. The study also demonstrates that the weaving ratio is an asymmetric function and thus should reflect the source of the weaving volume. Consequently, a new definition for the weaving ratio is introduced that explicitly identifies the source of the weaving volume. In addition, the study demonstrates that the length of the weaving section has a larger impact on the capacity of weaving sections for short lengths and high traffic demands. Furthermore, the study demonstrates that there does not exist enough evidence to conclude that the speed limit differential between mainline freeway and on- and off-ramps has a significant impact on weaving section capacities. Finally, the study demonstrates that the HCM procedures model the heavy duty vehicle impacts reasonably well. This dissertation presents the development of new capacity models for freeway weaving sections. In these models, a new definition of the weaving ratio that explicitly accounts for the source of weaving volume is introduced. The proposed analytical models estimate the capacity of weaving sections to within 12% of the simulated data, while the HCM procedures exhibit errors in the range of 114%. Among the newly developed models, the Artificial Neural Network (ANN) models performs slightly better that the statistical models in terms of model prediction errors. However, the sensitivity analysis results demonstrate unrealistic behavior of the ANN models under certain conditions. Consequently, the use of a statistical model is recommended because it provides a high level of accuracy while providing accurate model responses to changes in model input parameters (good response to the gradient of the input parameters).
- Characterization, Modeling of Piezoelectric Pressure Transducer for Facilitation of Field CalibrationPakdel, Zahra (Virginia Tech, 2007-05-21)Currently in the marketplace, one of the important goals is to improve quality, and reliability. There is great interest in the engineering community to develop a field calibration technique concerning piezoelectric pressure sensor to reduce cost and improve reliability. This paper summarizes the algorithm used to characterize and develop a model for a piezoelectric pressure transducer. The basic concept of the method is to excite the sensor using an electric force to capture the signature characteristic of the pressure transducer. This document presents the frequency curve fitted model based on the high frequency excitation of the piezoelectric pressure transducer. It also presents the time domain model of the sensor. The time domain response of the frequency curve fitted model obtained in parallel with the frequency response of the time domain model and the comparison results are discussed. Moreover, the relation between model parameters and sensitivity extensively is investigated. In order to detect damage and monitor the condition of the sensor on line the resonance frequency comparison method is presented. The relationship between sensitivity and the resonance frequency characteristic of the sensor extensively is investigated. The method of resonance monitoring greatly reduces the cost of hardware. This work concludes with a software implementation of the signature comparison of the sensor based on a study of the experimental data. The software would be implemented in the control system.
- Combat System Modeling:Modeling Large-Scale Software and Hardware Application Using UMLAL-Aqrabawi, Mohammad Saleh (Virginia Tech, 2001-05-09)Maintaining large-scale legacy applications has been a major challenge for software producers. As the application evolves and gets more complicated, it becomes harder to understand, debug, or modify the code. Moreover, as new members are joining the development team, and others are leaving, the need for a well-documented code arises. Good documentation necessitates the visualization of the code in an easy to understand manner. The Unified Modeling Language (UML), an Object Management Group's (OMG) standard, is a graphical modeling language used for specifying, visualizing, constructing, and documenting software intensive artifacts. UML, which has been accepted as an industry standard in November 1997, has aided the design and maintenance of object-oriented legacy applications. While the software developers were building UML models for their existing applications as part of a reverse-engineering process, development of next generation software applications started from the models (forward-engineering process). In the forward engineering process, the system's code is specified and constructed from the UML models, which evolve as the system evolves in order to maintain consistent documentation and visualization of the system. Moreover, UML has the power of hiding unnecessary details of the system by the ability to model its different views. This enables visualizing the system at different levels of hierarchy. This thesis documents how to use UML to model a software-intensive simulation for the combat systems of a fully automated naval "digital ship". This process started with building the use case diagrams based on the system requirements given by the domain experts. Then activity diagrams were used to describe the exact performance of the use cases. The logical view of the system was built using class, interaction, and activity diagrams. Then, the physical view of the system was built using component diagrams. Finally, an example of the code generation process from the UML models was carried out for one of the system components. These models are to be maintained as the application evolves. Using UML has aided in building a well-structured object-oriented application, validating the use cases of the application with the domain experts, visualizing and validating the structure of the source code before writing it, communicating between different members of the development team, and providing an easily understandable documentation of the system.
- Computer Vision Based Analysis of Broccoli for Application in a Selective Autonomous HarvesterRamirez, Rachael Angela (Virginia Tech, 2006-07-25)As technology advances in all areas of society and industry, the technology used to produce one of life's essentials - food - is also improving. The majority of agriculture production in developed countries has gone from family farms to industrial operations. With the advent of large-scale farming, the automation of basic farming operations has increasingly made practical and economic sense. Broccoli, which is still harvested by hand, is one of the most expensive crops to produce. Investing in sensing technology that can provide detailed information about the location, maturity and viability of broccoli heads has the potential to produce great commercial benefits. This technology is also a prerequisite for developing an autonomous harvester that could select and harvest mature heads of broccoli. This thesis details the work done to develop a computer vision algorithm that has the ability to locate the broccoli head within an image of an entire broccoli plant and to distinguish between mature and immature broccoli heads. Locating the head involves the use of a Hough transform to find the leaf stems and, once the stems are found, the location and extent of the broccoli head can be ascertained with the use of contrast texture analysis at the intersection of the stems. A co-occurrence matrix is then produced of the head and statistical texture analysis is performed to determine the maturity of the broccoli head. The conceptual design of a selective autonomous broccoli harvester, as well as suggestions for further research, is also presented.
- Concatenation of Space-Time Block Codes with ConvolutionalCodesAli, Saajed (Virginia Tech, 2004-02-12)Multiple antennas help in combating the destructive effects of fading as well as improve the spectral efficiency of a communication system. Receive diversity techniques like maximal ratio receive combining have been popular means of introducing multiple antennas into communication systems. Space-time block codes present a way of introducing transmit diversity into the communication system with similar complexity and performance as maximal ratio receive combining. In this thesis we study the performance of space-time block codes in Rayleigh fading channel. In particular, the quasi-static assumption on the fading channel is removed to study how the space-time block coded system behaves in fast fading. In this context, the complexity versus performance trade-off for a space-time block coded receiver is studied. As a means to improve the performance of space-time block coded systems concatenation by convolutional codes is introduced. The improvement in the diversity order by the introduction of convolutional codes into the space-time block coded system is discussed. A general analytic expression for the error performance of a space-time block coded system is derived. This expression is utilized to obtain general expressions for the error performance of convolutionally concatenated space-time block coded systems utilizing both hard and soft decision decoding. Simulation results are presented and are compared with the analytical results.
- A continuum Approach to Power system simulationDonolo, Marcos A. (Virginia Tech, 2006-09-04)The behavior of large and tightly interconnected power systems resembles, in certain circumstances, the behavior of a continuously distributed system. This resemblance motivated the derivation of continuum models, which were used to explain and predict disturbance propagation, un-damped power oscillations, and the stability of power systems. In this dissertation, we propose a one-dimensional continuum representation suitable for meshed power systems. Previous continuous representations of meshed power systems used two-dimensional spatial domains. Thus our approach has the potential to provide better resolution for comparable computational burden. It is important to note that, the computational burden required to obtain solutions for PDEs involved in the continuum representation varies notably with the solver implementation. The contributions of this dissertation are: a) Reviewing a previous continuum model and providing a detailed derivation for the one-dimensional version of it. b) Providing and describing in detail a parameter distribution technique adequate for the continuum approach. c) Identifying and documenting limitations on the continuum model voltage calculation. e) Providing a procedure to simulate the behavior of meshed power systems using the one dimensional continuum model. And f) Identifying and applying a numerical PDE solver for the continuum approach.
- Control of Gantry and Tower CranesOmar, Hanafy M. (Virginia Tech, 2003-01-24)The main objective of this work is to design robust, fast, and practical controllers for gantry and tower cranes. The controllers are designed to transfer the load from point to point as fast as possible and, at the same time, the load swing is kept small during the transfer process and completely vanishes at the load destination. Moreover, variations of the system parameters, such as the cable length and the load weight, are also included. Practical considerations, such as the control action power, and the maximum acceleration and velocity, are taken into account. In addition, friction effects are included in the design using a friction-compensation technique. The designed controllers are based on two approaches. In the first approach, a gain-scheduling feedback controller is designed to move the load from point to point within one oscillation cycle without inducing large swings. The settling time of the system is taken to be equal to the period of oscillation of the load. This criterion enables calculation of the controller feedback gains for varying load weight and cable length. The position references for this controller are step functions. Moreover, the position and swing controllers are treated in a unified way. In the second approach, the transfer process and the swing control are separated in the controller design. This approach requires designing two controllers independently: an anti-swing controller and a tracking controller. The objective of the anti-swing controller is to reduce the load swing. The tracking controller is responsible for making the trolley follow a reference position trajectory. We use a PD-controller for tracking, while the anti-swing controller is designed using three different methods: (a) a classical PD controller, (b) two controllers based on a delayed-feedback technique, and (c) a fuzzy logic controller that maps the delayed-feedback controller performance. To validate the designed controllers, an experimental setup was built. Although the designed controllers work perfectly in the computer simulations, the experimental results are unacceptable due to the high friction in the system. This friction deteriorates the system response by introducing time delay, high steady-state error in the trolley and tower positions, and high residual load swings. To overcome friction in the tower-crane model, we estimate the friction, then we apply an opposite control action to cancel it. To estimate the friction force, we assume a mathematical model and estimate the model coefficients using an off-line identification technique using the method of least squares. With friction compensation, the experimental results are in good agreement with the computer simulations. The gain-scheduling controllers transfer the load smoothly without inducing an overshoot in the trolley position. Moreover, the load can be transferred in a time near to the optimal time with small swing angles during the transfer process. With full-state feedback, the crane can reach any position in the working environment without exceeding the system power capability by controlling the forward gain in the feedback loop. For large distances, we have to decrease this gain, which in turn slows the transfer process. Therefore, this approach is more suitable for short distances. The tracking-anti-swing control approach is usually associated with overshoots in the translational and rotational motions. These overshoots increase with an increase in the maximum acceleration of the trajectories . The transfer time is longer than that obtained with the first approach. However, the crane can follow any trajectory, which makes the controller cope with obstacles in the working environment. Also, we do not need to recalculate the feedback gains for each transfer distance as in the gain-scheduling feedback controller.
- Control of Rotary Cranes Using Fuzzy LogicAl-mousa, Amjed A.; Nayfeh, Ali H.; Kachroo, Pushkin (Hindawi, 2003-01-01)Rotary cranes (tower cranes) are common industrial structures that are used in building construction, factories, and harbors. These cranes are usually operated manually. With the size of these cranes becoming larger and the motion expected to be faster, the process of controlling them has become difficult without using automatic control methods. In general, the movement of cranes has no prescribed path. Cranes have to be run under different operating conditions, which makes closed-loop control attractive.In this work a fuzzy logic controller is introduced with the idea of “split-horizon”; that is, fuzzy inference engines (FIE) are used for tracking the position and others are used for damping the load oscillations. The controller consists of two independent sub-controllers: radial and rotational. Each of these controllers has two fuzzy inference engines (FIE). Computer simulations are used to verify the performance of the controller. Three simulation cases are presented. In the first case, the crane is operated in the gantry (radial) mode in which the trolley moves along the jib while the jib is fixed. In the second case (rotary mode), the trolley moves along the jib and the jib rotates. In the third case, the trolley and jib are fixed while the load is given an initial disturbance. The results from the simulations show that the fuzzy controller is capable of keeping the load-oscillation angles small throughout the maneuvers while completing the maneuvers in relatively reasonable times.
- Control of Rotary Cranes Using Fuzzy Logic and Time-Delayed Position Feedback ControlAl-Mousa, Amjed A. (Virginia Tech, 2000-11-27)Rotary Cranes (Tower Cranes) are common industrial structures that are used in building construction, factories, and harbors. These cranes are usually operated manually. With the size of these cranes becoming larger and the motion expected to be faster, the process of controlling them became dicult without using automatic control methods. In general, the movement of cranes has no prescribed path. Cranes have to be run under dierent operating conditions, which makes closed-loop control preferable. In this work, two types of controllers are studied: fuzzy logic and time-delayed position feedback controllers. The fuzzy logic controller is introduced first with the idea of split-horizon; that is, to use some fuzzy engines for tracking position and others for damping load oscillations. Then the time-delayed position feedback method is applied. Finally, an attempt to combine these two controllers into a hybrid controller is introduced. Computer simulations are used to verify the performance of these controllers. An experimental setup was built on which the time-delayed position feedback controller was tested. The results showed good performance.
- Correlating Computer User Stress and Performance in Both Preferred and Non-preferred ModalitiesCastles, Ricky Thomas (Virginia Tech, 2006-05-08)Most computer interfaces are designed in a one-size-fits-all fashion, which does not account for individual differences in abilities and preferences. Some computer users thrive with one software application while another user may struggle to use the same software. Some people tend to perform very well amidst distraction whereas others have a difficult time concentrating on a primary task when distracting agents are present. Much work has been done in quantifying a person's performance, but it has typically been difficult to quantify how difficult a task was for a person to perform. This thesis looks into the stress exhibited by various computer users while performing tasks in both their preferred and non-preferred modalities. The paper surveys the current physiological methods for analyzing human stress and delineates the hardware and software design and implementation of some of these methods. The physiological data-collecting hardware and software were deployed to collect physiological samples from test subjects engaging in memorization and recollection tasks in both an undistracted and a distracted setting. An analysis of the data shows the correlation between preferred modality and performance of tasks in that modality and other modalities. This analysis also shows the correlation between user arousal level and performance with and without distraction. Individual differences are considered by normalizing the physiological data collected for each subject prior to comparison with other subjects. The work presented herein gives insight into the individual differences of various types of computer users and is a precursor to work in adaptive user interface technology.
- Demand Management in Evacuation: Models, Algorithms, and ApplicationsBish, Douglas R. (Virginia Tech, 2006-07-31)Evacuation planning is an important disaster management tool. A large-scale evacuation of a region by automobile is a difficult task, especially as demand is often greater than supply. This is made more difficult as the imbalance of supply and demand actually reduces supply due to congestion. Currently, most of the emphasis in evacuation planning is on supply management. The purpose of this dissertation is to introduce and study sophisticated demand management tools, specifically, staging and routing of evacuees. These tools can be used to produce evacuation strategies that reduce or eliminate congestion. A strategic planning model is introduced that accounts for evacuation dynamics and the non-linearities in travel times associated with congestion, yet is tractable and can be applied to large-scale networks. Objective functions of potential interest in evacuation planning are introduced and studied in the context of this model. Insights into the use of staging and routing in evacuation management are delineated and solution techniques are developed. Two different strategic approaches are studied in the context of this model. The first strategic approach is to control the evacuation at a disaggregate level, where customized staging and routing plans are produced for each individual or family unit. The second strategic approach is to control the evacuation at a more aggregate level, where evacuation plans are developed for a larger group of evacuees, based on pre-defined geographic areas. In both approaches, shelter requirements and preferences can also be considered. Computational experience using these two strategic approaches, and their respective solution techniques, is provided using a real network pertaining to Virginia Beach, Virginia, in order to demonstrate the efficacy of the proposed methodologies.
- Deploying an ITS Warning System for No-Passing Zones on Two-Lane Rural RoadsEl Zarif, Jamal A. (Virginia Tech, 2001-06-28)A new safety application, as part of ITS Advanced Rural Transportation System (ARTS), has been developed to be deployed on a two-lane rural road (Route 114), in Southwest Virginia. The route segment under study is subject to significant head-on accidents, as a result of two main conditions: 1- Illegal passing maneuvers crossing solid yellow line, and 2- A short passing sight distance due to the road vertical profile. The main objective of this research is to design a video detection-based warning system by installing an affordable and efficient system on the vertical crest curve on Route 114, capable of performing the following two main functions: 1.Detect vehicles that attempt to violate the no-passing zone restriction (i.e. when crossing into the opposing direction). 2.Warn the drivers violating the restriction in order to discourage them from continuing their maneuvers. System architecture as well as detailed system design was developed. A system simulation was conducted with the use of a special software program written with MATLAB. The simulation was applied for both "with" and "without" the system cases. The simulation runs showed that the system could virtually eliminate all head-on collisions, should violators obey the early warning messages displayed. Several sensitivity tests were made for different scenarios. Finally, the viability of the system was evaluated from economic point of view. The financial analysis revealed high economic indicators.
- Design of Adaptive Vibration Control Systems with Applicaion to Magneto-Rheological DampersSong, Xubin (Virginia Tech, 1999-11-11)The design of nonlinear adaptive control systems for reducing vibration transmission in applications such as transportation systems is discussed. The systems studied include suspension systems, such as those used in vehicles, employing nonlinear magneto-rheological (MR) dampers that are controlled to provide improved vibration isolation. Magneto-rheological dampers use a novel class of smart fluid whose apparent viscosity changes as it is exposed to a magnetic field. The developed adaptive control scheme is designed to deal with the nonlinearities and uncertainties that commonly arise in most suspension applications. Some of the nonlinearities that are considered include time-varying characteristics, displacement-dependent effects, and hysterisis damping of magneto-rheological dampers. The uncertainties include mass and stiffness variations that can commonly occur in a suspension system. A number of nonlinear analytical models are developed and used in numerical simulation to evaluate the validity and effectiveness of the developed adaptive controllers. Further, the results of the numerical study are used in an experimental evaluation of the controllers on a seat suspension for heavy vehicles. The analytical and experimental evaluation both indicate the effectiveness of the proposed adaptive control technique in controlling vibration transmission in the presence of both system nonlinearities and uncertainties. The manuscript will provide a detail account of the modeling, dynamic analysis, adaptive control development, and testing that was performed throughout this study.
- Design of Switched Reluctance Motors and Development of a Universal Controller for Switched Reluctance and Permanent Magnet Brushless DC Motor DrivesVijayraghavan, Praveen (Virginia Tech, 2001-11-15)Switched Reluctance Machines (SRMs) are receiving significant attention from industries in the last decade. They are extremely inexpensive, reliable and weigh less than other machines of comparable power outputs. Although the design principles of the machine are available as a concatenation of many different sources, the need for a unified, step-by-step design procedure from first principles of electromagnetics is an absolute requirement. This dissertation discusses a procedure that can be applied by engineers with a basic background in electromagnetics. Subsequent to the design of the machine, existing finite element software can do the analysis of the machine. However, this is a laborious process and the need for an analytical method is preferable to verify the design procedure before the final verification by finite elements. The analytical procedure as well as a procedure to calculate iron losses is also developed in this dissertation. A prototype machine has been developed as an example of the design process and an existing prototype is analyzed to verify the analysis procedure. The similarities between the SRM and the Permanent Magnet Brushless DC Machine (PMDBC) beg the consideration of the development of a converter that can be used to drive either machine. One such converter has been developed in this dissertation. The design of the drive for both the machines is seen to be very similar. As a consequence, a universal controller that can be used to operate both machines has been developed and implemented with a DSP. Simulations and experimental correlation for both drives have been presented.
- Development of Reduced-Order Flame Models for Prediction of Combustion InstabilityHuang, Xinming (Virginia Tech, 2001-09-21)Lean-premixed combustion has the advantage of low emissions for modern gas turbines, but it is susceptible to thermoacoustic instabilities, which can result in large amplitude pressure oscillations in the combustion chamber. The thermoacoustic limit cycle is generated by the unsteady heat release dynamics coupled to the combustor acoustics. In this dissertation, we focused on reduced-order modeling of the dynamics of a laminar premixed flame. From first principles of combustion dynamics, a physically-based, reduced-order, nonlinear model was developed based on the proper orthogonal decomposition technique and generalized Galerkin method. In addition, the describing function for the flame was measured experimentally and used to identify an empirical nonlinear flame model. Furthermore, a linear acoustic model was developed and identified for the Rijke tube experiment. Closed-loop thermoacoustic modeling using the first principles flame model coupled to the linear acoustics successfully reproduced the linear instability and predicted the thermoacoustic limit cycle amplitude. With the measured experimental flame data and the modeled linear acoustics, the describing function technique was applied for limit cycle analysis. The thermoacoustic limit cycle amplitude was predicted with reasonable accuracy, and the closed-loop model also predicted the performance for a phase shift controller. Some problems found in the predictions for high heat release cases were documented.
- Diffusion Tensor Imaging: Evaluation of Tractography Algorithm Performance Using Ground Truth PhantomsTaylor, Alexander James (Virginia Tech, 2004-05-13)Diffusion Tensor Magnetic Resonance Imaging (DT-MRI), also known as Diffusion Tensor Imaging (DTI), is a unique medical imaging modality that provides non-invasive estimates of White Matter (WM) connectivity based on local principal directions of anisotropic water diffusion. DTI tractography estimates are a macroscopically sampled description of underlying microscopic structure, and are therefore of limited validity. The under-sampling of underlying white matter structure in DTI data gives rise to Intra-Voxel Orientational Heterogeneity (IVOH), a condition in which white matter structures of multiple different orientations are averaged into a single DTI voxel sample, causing a loss of validity in the diffusion tensor model. Fast Marching Tractography (FMT) algorithms based on fast marching level set methods have been proposed to better handle the presence of IVOH in DTI data when compared to older Streamline Tractography (SLT) methods. However, the actual performance advantage of any tractography algorithm over another cannot be conclusively stated until a ground truth standard of comparison is developed. This work develops an optimized version of the FMT algorithm that is dubbed the Front Propagation Tractography (FPT) algorithm. The FPT algorithm includes unique approaches to the speed function, connectivity estimation, and likelihood estimation components of the FMT framework. The performance of the FPT algorithm is compared against the SLT algorithm using ground truth software phantom data and human brain data. Software phantom ground truth experiments compare the performance of each algorithm in single tract and crossing tract structures for varying levels of diffusion tensor field perturbation. Human brain estimates in the corpus callosum yield qualitative comparisons from inspection of 3D visualizations. A final area of exploration is the construction and analysis of a ground truth physical DTI phantom manifesting IVOH.