9,407 research outputs found

    Optimal Partial Harvesting Schedule for Aquaculture Operations

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    Abstract When growth is density dependent, partial harvest of the standing stock of cultured species (fish or shrimp) over the course of the growing season (i.e., partial harvesting) would decrease competition and thereby increase individual growth rates and total yield. Existing studies in optimal harvest management of aquaculture operations, however, have not provided a rigorous framework for determining "discrete" partial harvesting (i.e., partially harvest the cultured species at several discrete points until the final harvest). In this paper, we develop a partial harvesting model that is capable of addressing discrete partial harvesting and other partial harvesting using impulsive control theory. We derive necessary conditions of the efficient partial harvesting scheme for a single production cycle. We also present a numerical example to illustrate how partial harvesting can improve the profitability of an aquaculture enterprise compared to single-batch harvesting and gradual thinning. The study results indicate that well-designed partial harvesting schemes can enhance the profitability of aquaculture operations.Partial harvesting, impulsive control theory, aquaculture., Livestock Production/Industries, C61, Q22,

    On an SEIR Epidemic Model with Vaccination of Newborns and Periodic Impulsive Vaccination with Eventual On-Line Adapted Vaccination Strategies to the Varying Levels of the Susceptible Subpopulation

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    This paper investigates a susceptible-exposed-infectious-recovered (SEIR) epidemic model with demography under two vaccination effort strategies. Firstly, the model is investigated under vaccination of newborns, which is fact in a direct action on the recruitment level of the model. Secondly, it is investigated under a periodic impulsive vaccination on the susceptible in the sense that the vaccination impulses are concentrated in practice in very short time intervals around a set of impulsive time instants subject to constant inter-vaccination periods. Both strategies can be adapted, if desired, to the time-varying levels of susceptible in the sense that the control efforts be increased as those susceptible levels increase. The model is discussed in terms of suitable properties like the positivity of the solutions, the existence and allocation of equilibrium points, and stability concerns related to the values of the basic reproduction number. It is proven that the basic reproduction number lies below unity, so that the disease-free equilibrium point is asymptotically stable for larger values of the disease transmission rates under vaccination controls compared to the case of absence of vaccination. It is also proven that the endemic equilibrium point is not reachable if the disease-free one is stable and that the disease-free equilibrium point is unstable if the reproduction number exceeds unity while the endemic equilibrium point is stable. Several numerical results are investigated for both vaccination rules with the option of adapting through ime the corresponding efforts to the levels of susceptibility. Such simulation examples are performed under parameterizations related to the current SARS-COVID 19 pandemic.This research was supported by the Spanish Institute of Health Carlos III through Grant COV 20/01213 and by the Spanish Government and European Commission through Grant RTI2018-094336-B-I00 (MCIU/AEI/FEDER, UE)

    State-of-the-art in Power Line Communications: from the Applications to the Medium

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    In recent decades, power line communication has attracted considerable attention from the research community and industry, as well as from regulatory and standardization bodies. In this article we provide an overview of both narrowband and broadband systems, covering potential applications, regulatory and standardization efforts and recent research advancements in channel characterization, physical layer performance, medium access and higher layer specifications and evaluations. We also identify areas of current and further study that will enable the continued success of power line communication technology.Comment: 19 pages, 12 figures. Accepted for publication, IEEE Journal on Selected Areas in Communications. Special Issue on Power Line Communications and its Integration with the Networking Ecosystem. 201

    Dynamically Stable 3D Quadrupedal Walking with Multi-Domain Hybrid System Models and Virtual Constraint Controllers

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    Hybrid systems theory has become a powerful approach for designing feedback controllers that achieve dynamically stable bipedal locomotion, both formally and in practice. This paper presents an analytical framework 1) to address multi-domain hybrid models of quadruped robots with high degrees of freedom, and 2) to systematically design nonlinear controllers that asymptotically stabilize periodic orbits of these sophisticated models. A family of parameterized virtual constraint controllers is proposed for continuous-time domains of quadruped locomotion to regulate holonomic and nonholonomic outputs. The properties of the Poincare return map for the full-order and closed-loop hybrid system are studied to investigate the asymptotic stabilization problem of dynamic gaits. An iterative optimization algorithm involving linear and bilinear matrix inequalities is then employed to choose stabilizing virtual constraint parameters. The paper numerically evaluates the analytical results on a simulation model of an advanced 3D quadruped robot, called GR Vision 60, with 36 state variables and 12 control inputs. An optimal amble gait of the robot is designed utilizing the FROST toolkit. The power of the analytical framework is finally illustrated through designing a set of stabilizing virtual constraint controllers with 180 controller parameters.Comment: American Control Conference 201

    Methods and tools for preliminary low thrust mission analysis

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    The aim of the present thesis is to develop new methods that are useful for a space mission analyst to design low thrust trajectories in the preliminary phases of a mission study, where the focus is more on exploring various concepts than on obtaining one optimal transfer. The tools cover three main axes: generating low thrust trajectories from scratch, improving existing low thrust trajectories and exploring large search spaces related to multiple gravity assist transfers. Stress is put on the computational efficiency of the tools. Transfer arcs are generated with shaped based approaches, which have the advantage of having the ability to reproduce close to optimal transfers satisfying time of flight constraints and varied boundary constraints without the need for propagation. This thesis presents a general framework for the development of shape-based approaches to low-thrust trajectory design. A novel shaping method, based on a three-dimensional description of the trajectory in spherical coordinates, is developed within this general framework. Both the exponential sinusoid and the inverse polynomial shaping are demonstrated to be particular two-dimensional cases of the spherical one. The pseudo-equinoctial shaping is revisited within the new framework, and the nonosculating nature of the pseudo-equinoctial elements is analysed. A two-step approach is introduced to solve the time of flight constraint, related to the design of low-thrust arcs with boundary constraints for both spherical and pseudo-equinoctial shaping. The solutions derived from the shaping approach are improved with a feedback linear-quadratic controller and compared against a direct collocation method based on finite elements in time. Theoretical results are given on the validity of the method and a theorem is derived on the criteria of optimality of the results. The shaping approaches and the combination of shaping and linear-quadratic controller are tested on four case studies: a mission to Mars, a mission to asteroid 1989ML, to comet Tempel-1 and to Neptune. The design of low thrust multiple gravity assist trajectories is tackled by an incremental pruning approach. The incremental pruning of reduced search spaces is performed for decoupled pairs of transfer legs, after which regions of the total search space are identified where all acceptable pairs can be linked together. The gravity assists are not powered therefore the trajectory is purely low thrust and the transfer arcs are modelled by shaping functions and improved with the linear quadratic controller. Such an approach can reduce the computational burden of finding a global optimum. Numerical examples are presented for LTMGA transfers from Earth to asteroid Apollo and to Jupiter
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