Browsing by Author "Zhang, Lei"
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Item Access Scheduling and Controller Design in Networked Control Systems(2005-10-05) Zhang, Lei; Hristu-Varsakelis, Dimitrios; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A Networked Control System (NCS) is a control system in which the sensors and actuators are connected to a feedback controller via a shared communication medium. In an NCS, the shared medium can only provide a limited number of simultaneous connections for the sensors and actuators to communicate with the controller. As a consequence, the design of an NCS involves not only the specification of a feedback controller but also that of a communication policy that schedules access to the shared communication medium. Up to now, this task has posed a significant challenge, due in large part to the modeling complexity of existing NCS architectures, under which the control and communication design problems are tightly intertwined. This thesis proposes an alternative NCS architecture, whereby the plant and controller choose to ``ignore'' the actuators and sensors that are not actively communicating. This new architecture leads to simpler NCS models in which the design of feedback controller and communication polices can be effectively decoupled. In that setting, we propose a set of medium access scheduling strategies and accompanying controller design methods that address a broad range of stabilization, estimation, and optimization problems for a general class of NCSs. The performance of the proposed methods is illustrated through a set of simulations and hardware experiments.Item Association of Maternal Medical Conditions and Unfavorable Birth Outcomes: Findings from the 1996–2003 Mississippi Linked Birth and Death Data(2009) Zhang, Lei; Cox, Reagan G.; Graham, Juanita; Johnson, DickThis study aimed to identify factors contributing to high rates of preterm birth (PTB), low birth weight (LBW) and infant mortality in Mississippi while considering both traditional risk factors and maternal medical conditions. The retrospective cohort study used 1996–2003 Mississippi linked birth and infant death files. Multiple logistic regression was used to investigate association between maternal medical conditions and unfavorable birth outcomes. Along with traditional risk factors, hypertension was significantly associated with PTB and LBW. Women with hypertension were about 2.2 and 3.2 times as likely to have PTB and LBW, respectively. Hydramnios/oligohydramnios increased 1.8–4.4 folds of risk for PTB, LBW and infant death and was significantly associated with the unfavorable birth outcomes. Non-Hispanic black women were about 1.5–2.0 times as likely to have an unfavorable birth outcome compared to non-Hispanic white women. Maternal education and prenatal care effect appeared to be modified by maternal race. Certain maternal medical conditions may be contributing to PTB, LBW and infant mortality rates identifying preconception and prenatal healthcare as possible strategies for reducing unfavorable outcomes. Results suggest that different risk profiles for unfavorable outcomes may exist according to maternal race highlighting the need to consider racial groups separately when further exploring the sociodemographic and/or health-related factors that contribute to unfavorable birth outcomes.Item Associations of Fast Food Restaurant Availability With Dietary Intake and Weight Among African Americans in the Jackson Heart Study, 2000-2004.(2011) Hickson, Demarc; Diez Roux, Ana; Smith, Adam; Tucker, Katherine; Gore, Larry; Zhang, Lei; Wyatt, SharonObjectives. We examined the associations of fast food restaurant (FFR) availability with dietary intake and weight among African Americans in the southeastern United States. Methods. We investigated cross-sectional associations of FFR availability with dietary intake and body mass index (BMI) and waist circumference in 4740 African American Jackson Heart Study participants (55.2 612.6 years, 63.3% women). We estimated FFR availability using circular buffers with differing radii centered at each participant's geocoded residential location. Results. We observed no consistent associations between FFR availability and BMI or waist circumference. Greater FFR availability was associated with higher energy intake among men and women younger than 55 years, even after adjustment for individual socioeconomic status. For each standard deviation increase in 5-mile FFR availability, the energy intake increased by 138 kilocalories (confidence interval [CI]=70.53, 204.75) for men and 58 kilocalories (CI=8.55,105.97) for women. We observed similar associations for the 2-mile FFR availability, especially in men. FFR availability was also unexpectedly positively associated with total fiber intake. Conclusions. FFR availability may contribute to greater energy intake in younger African Americans who are also more likely to consume fast food. (Am J Public Health. Published online ahead of print May 5, 2011: e1-e9. doi:10.2105/AJPH.2010.300006).Item COVID-19 and income profile: How communities in the United States responded to mobility restrictions in the pandemic's early stages(Wiley, 2022-11-02) Sun, Qianqian; Zhou, Weiyi; Kabiri, Aliakbar; Darzi, Aref; Hu, Shonghua; Younes, Hannah; Zhang, LeiMobility interventions in communities play a critical role in containing a pandemic at an early stage. The real-world practice of social distancing can enlighten policymakers and help them implement more efficient and effective control measures. A lack of such research using real-world observations initiates this article. We analyzed the social distancing performance of 66,149 census tracts from 3,142 counties in the United States with a specific focus on income profile. Six daily mobility metrics, including a social distancing index, stay-at-home percentage, miles traveled per person, trip rate, work trip rate, and non-work trip rate, were produced for each census tract using the location data from over 100 million anonymous devices on a monthly basis. Each mobility metric was further tabulated by three perspectives of social distancing performance: “best performance,” “effort,” and “consistency.” We found that for all 18 indicators, high-income communities demonstrated better social distancing performance. Such disparities between communities of different income levels are presented in detail in this article. The comparisons across scenarios also raise other concerns for low-income communities, such as employment status, working conditions, and accessibility to basic needs. This article lays out a series of facts extracted from real-world data and offers compelling perspectives for future discussions.Item DOES NETWORK DENSITY MATTER: ESSAYS ON INTER-FIRM GROUP FORMATION AND PERFORMANCE IMPLICATION(2011) Zhang, Lei; Gupta, Anil K.; Waguespack, David M.; Business and Management: Management & Organization; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation studies inter-firm network formation and performance implication. Different from current network formation literature that focuses on the actor or dyad level, this dissertation examines network formation and its performance implication at the group level. Specifically, I examine: 1) How do inter-firm groups with different levels of density form? 2) When is a firm more likely to participate in a group with mostly unfamiliar firms? and 3) How do group internal and external network structures influence task performances? Using Venture Capital (VC) investments as the research context, I develop novel empirical designs to quantitatively test my theory. In Essay I, to investigate how groups with different levels of density form, I emphasize the path-dependence effect of previous ties among all potential group members and simultaneously examine the formation of all ties in a group. I find that both anticipated environmental adaptation and anticipated internal cooperation are important considerations in a group formation. Taking a firm-focused group perspective, Essay II studies when a firm participates in a group with mostly unfamiliar firms. The empirical results show that the group participation of an unfamiliar firm depends not only on the uncertainty it brings in value creation but also on the uncertainty in value appropriation. Essay III examines the impact of syndicate density and structural holes and finds that both have impacts on the startup company performance. This dissertation enhances our understanding of network formation by bringing in a brand-new perspective, by uncovering group-level antecedents of network formation, by illustrating the impact of concerns in value appropriation, by exploring group dynamics, and by linking network formation behaviors with task performance at the group level.Item Health Impacts of the Built and Social Environments, and Travel Behavior: The Case of the Sunshine State(MDPI, 2022-07-26) Mahmoudi, Jina; Zhang, LeiAs physical inactivity statistics for the U.S. population show an alarming trend, many health problems have been increasing among Americans in recent decades. Thus, identification of the factors that influence people’s physical activity levels and health outcomes has become ever more essential to promote public health. The built envSFironment is among the main factors that impact individuals’ health outcomes. However, little is known about the health impacts of built environment factors at large geographical scales such as those of the metropolitan area of residence. Further, the health impacts of travel behavior such as telecommuting and teleshopping remain unclear. This study uses an ecological model framework to probe the roles of travel behavior and built as well as social environments at different spatial levels in health. Instrumental variable binary probit models have been developed to examine the complex interlinks between measures of travel behavior, physical activity levels, built and social environment characteristics, and individuals’ health outcomes. Findings indicate that built and social environment factors at different spatial levels, including the metropolitan area, are correlated with individuals’ health outcomes. Additionally, the findings suggest that increased levels of telecommuting and teleshopping within communities may lead to unfavorable health outcomes. The findings shed light on the most promising policy interventions that can promote public health through modifications targeting people’s travel choices as well as the built and social environments within urban areas.Item How Urban Form Characteristics at Both Trip Ends Influence Mode Choice: Evidence from TOD vs. Non-TOD Zones of the Washington, D.C. Metropolitan Area(MDPI, 2019-06-20) Nasri, Arefeh; Zhang, LeiUnderstanding travel behavior and its relationship with built environment is crucial for sustainable transportation and land-use policy-making. This study provides additional insights into the linkage between the built environment and travel mode choice by looking at the built environment characteristics at both the trip origin and destination in the context of transit-oriented development (TOD). The objective of this research is to provide a better understanding of how travel mode choice is influenced by the built environment surrounding both trip end locations. Specifically, it investigates the effect of transit-oriented development policy and the way it affects people’s mode choice decisions. This is accomplished by developing discrete choice models and consideration of urban form characteristics at both trip ends. Our findings not only confirmed the important role the built environment plays in influencing mode choice, but also highlighted the influence of policies, such as TOD, at both trip end locations. Results suggest that the probability of choosing transit and non-motorized modes is higher for trips originating and ending in TOD areas. However, the magnitude of this TOD effect is larger at trip origin compared to destination. Higher residential and employment densities at both trips ends are also associated with lower probability of auto and higher probability of transit and non-motorized mode choices.Item Impact of the Built Environment Measured at Multiple Levels on Nonmotorized Travel Behavior: An Ecological Approach to a Florida Case Study(MDPI, 2020-10-24) Mahmoudi, Jina; Zhang, LeiResearch continues to reveal the benefits of nonmotorized travel modes such as walking and bicycling. Therefore, identification of the factors that nurture these activities is essential in developing sustainable urban planning policies and designs. Among those factors are the built environment characteristics of the place of residence. To date, research on the role of the built environment in nonmotorized travel has focused on neighborhood-level factors. However, people do not stay within their neighborhoods; they live and work at a regional scale and travel to various destinations and distances each day. Nonetheless, little is known about the impact of built environment factors at larger spatial scales on nonmotorized travel behavior. Guided by the principles of the ecological model of behavior, this study investigates the role of the built environment at hierarchical spatial scales in nonmotorized travel behavior. Multilevel Structural Equation Models have been developed to comprehensively examine the complex links between the built environment and individuals’ nonmotorized travel. Findings indicate that built environment factors at multiple spatial scales can influence nonmotorized travel behavior. Thus, to promote walking and bicycling, more effective policies are those that include multilevel built environment and land use interventions and consider the overall physical form of urban areas.Item Prenatal Care Utilization in Mississippi: Racial Disparities and Implications for Unfavorable Birth Outcomes.(2009) Cox, Reagan G; Zhang, Lei; Zotti, Marianne E; Graham, JuanitaThe objective of the study is to identify racial disparities in prenatal care (PNC) utilization and to examine the relationship between PNC and preterm birth (PTB), low birth weight (LBW) and infant mortality in Mississippi. Retrospective cohort from 1996 to 2003 linked Mississippi birth and infant death files was used. Analysis was limited to live-born singleton infants born to non-Hispanic white and black women (n = 292,776). PNC was classified by Kotelchuck's Adequacy of Prenatal Care Utilization Index. Factors associated with PTB, LBW and infant death were identified using multiple logistic regression after controlling for maternal age, education, marital status, place of residence, tobacco use and medical risk. About one in five Mississippi women had less than adequate PNC, and racial disparities in PNC utilization were observed. Black women delayed PNC, received too few visits, and were more likely to have either "inadequate PNC" (P < 0.0001) or "no care" (P < 0.0001) compared to white women. Furthermore, among women with medical conditions, black women were twice as likely to receive inadequate PNC compared to white women. Regardless of race, "no care" and "inadequate PNC" were strong risk factors for PTB, LBW and infant death. We provide empirical evidence to support the existence of racial disparities in PNC utilization and infant birth outcomes in Mississippi. Further study is needed to explain racial differences in PNC utilization. However, this study suggests that public health interventions designed to improve PNC utilization among women might reduce unfavorable birth outcomes especially infant mortality.Item Stabilization of Networked Control Systems under Feedback-based Communication(2004) Zhang, Lei; Hristu-Varsakelis, Dimitrios; Hristu-Varsakelis, Dimitrios; ISRWe study the stabilization of a networked control system (NCS) in which multiple sensors and actuators of a physical plant share a communication medium to exchange information with a remote controller. The plant's sensors and actuators are allowed only limited access to the controller at any one time, in a way that is decided on-line using a feedback-based communication policy. Our NCS model departs from those in previous formulations in that the controller and plant handle communication disruptions by ``ignoring'' (in a sense that will be made precise) sensors and actuators that are not actively communicating. We present an algorithm that provides a complete and straightforward method for simultaneously determining stabilizing gains and communication policies and avoids the computational complexity and limitations associated with some previously proposed models. We introduce three feedback-based scheduling policies that quadratically stabilize the closed-loop NCS while achieving various objectives related to the system's rate of convergence, the priorities of different sensors and actuators, and the avoidance of chattering.Item Stabilization of Networked Control Systems: Designing Effective Communication Sequences(2004) Zhang, Lei; Hristu-Varsakelis, Dimitrios; Hristu-Varsakelis, Dimitrios; ISRThis paper discusses the stabilization of a networked control system (NCS) whose sensors and actuators exchange information with a remote controller over a shared communication medium. Access to that medium is governed by a pair of periodic communication sequences. Under the model utilized here, the controller and plant handle communication disruptions by ``ignoring' (in a sense to be made precise) sensors and actuators that are not actively communicating. It is shown that under mild conditions, there exist periodic communication sequences that preserve the reachability and observability of the plant, leading to a straightforward design of a stabilizing feedback controller.