Evader's Strategy Research Articles

Anticipated velocity based guidance strategy for wheeled mobile evader amidst stationary and moving obstacles in bounded environment

AbstractThis paper is concerned with a class of pursuit‐evasion game problems amidst stationary and moving obstacles in a bounded environment. We concentrate on evader's strategy taking into account the following challenges: (i) pursuer and evader are nonholonomic wheeled mobile robots and the evader is slower than the pursuer; (ii) pursuer follows a proportional navigation law; and (iii) geometry of the environment is not known to the players, a priori. We propose an efficient evader‐centric anticipated velocity based guidance strategy. Pursuer's trajectory is anticipated at each step by the evader using quadratic polynomial interpolation. The aim of the evader is to escape interception with the pursuer for maximum possible time. To deal with static obstacles, a technique based on a well‐known tangent bug algorithm is presented. While dealing with dynamic obstacles, a recently introduced reciprocal orientation method is employed to avoid collision in situations when the dynamic obstacle also cooperates in the process. In case dynamic obstacles do not participate in the process of collision avoidance, a well‐known velocity obstacle method is employed for planning safe collision‐free paths. Efficiency of the proposed algorithms is analyzed with respect to the interception time and the distance traveled by the players. Copyright © 2014 John Wiley & Sons, Ltd.

A Cooperative Pursuit-Evasion Game for Non-holonomic Systems

-This paper considers a pursuit-evasion game for non-holonomic systems where a number of pursuers attempt to capture a single evader in a bounded connected domain. The problem is challenging because all vehicles have the same manoeuvring capability and are subject to turn radius constraints making them non-holonomic systems. The paper initially presents simple and alternate proofs for results existing in the literature that guarantee capture for holonomic systems. These results that are based on the minimization of safe-reachable area (the set of points where an evader can travel without being caught) are then extended to non-holonomic systems. However, solving such a problem exactly is computationally intractable. Therefore, the paper proposes a computationally efficient algorithm to obtain an approximate solution to the safe-reachable area minimization problem where the pursuers aim to minimize the safe-reachable area of the evader, while the evader chooses control actions to maximize it. Also proposed is an alternative approach that uses a cooperative strategy based on a pure proportional navigation law to capture the evader. In the process, an evader strategy which is superior to those based on the minimization of safe-reachable area is identified. The paper evaluates the proposed algorithms through numerical simulations.

Role of Toll-like receptors in liver health and disease

TLRs (Toll-like receptors), as evolutionarily conserved germline-encoded pattern recognition receptors, have a crucial role in early host defence by recognizing so-called PAMPs (pathogen-associated molecular patterns) and may serve as an important link between innate and adaptive immunity. In the liver, TLRs play an important role in the wound healing and regeneration processes, but they are also involved in the pathogenesis and progression of various inflammatory liver diseases, including autoimmune liver disease, alcoholic liver disease, non-alcoholic steatohepatitis, fibrogenesis, and chronic HBV (hepatitis B virus) and HCV (hepatitis C virus) infection. Hepatitis viruses have developed different evading strategies to subvert the innate immune system. Thus recent studies have suggested that TLR-based therapies may represent a promising approach in the treatment in viral hepatitis. The present review focuses on the role of the local innate immune system, and TLRs in particular, in the liver.

The “metabolovirus” model of hepatitis B virus suggests nutritional therapy as an effective anti-viral weapon

Hepatitis B virus (HBV) is a small DNA virus that targets the liver almost exclusively. Chronic infection with HBV might lead to severe liver-related pathologies including chronic hepatitis, cirrhosis and hepatocellular carcinoma. Based on its enhancer composition, which links nutritional signals that control hepatic glucose and fat metabolism in the liver to HBV gene expression and replication, it appears that the virus has adopted a regulatory system that is unique to the major hepatic metabolic genes. This unique virus-host interaction, mediated by metabolic events in the liver, is designated by us the "metabolovirus model". We hypothesize that by mimicking the expression of key genes implicated in glucose homeostasis, HBV sophisticatedly exploits the host resources to ensure its persistence. Specifically, by recruiting transcription factors and coactivators common to essential hepatic metabolic genes the virus avoids a possible resistance by its host, on the one hand, and ensures a timely and proper response to changes in its environment in terms of metabolic milieu, on the other hand. Furthermore, by coupling its gene expression to the expression of hepatic metabolic genes that fluctuate during the day, we predict a fluctuating nature of HBV gene expression. This can serve the virus in its attempts to escape the host immune system in addition to other immune evading strategies adopted by the virus, such as the secretion of the e antigen. Based on our "metabolovirus model", we suggest new mechanisms to previously unexplained clinical phenomena, such as the observed diversity in disease severity between different geographical areas that differ in nutritional habits. Furthermore, given the up-regulatory effect of food deprivation on HBV gene expression and replication, we suggest that conditions of short-term starvation should be completely avoided by HBV-infected individuals, and dietary recommendations such as the ingestion of complex carbohydrates before sleep should be adopted. Thus, our hypothesis sets the stage for viral manipulation by controlling food intake, and opens additional avenues towards food or nutritional therapy as an effective anti-HBV weapon.

Environmental effects on the coevolution of pursuit and evasion strategies

The game of tag is frequently used in the study of pursuit and evasion strategies that are discovered through competitive coevolution. The aim of coevolution is to create an arms race where opposing populations cyclically evolve in incremental improvements, driving the system towards better strategies. A coevolutionary simulation of the game of tag involving two populations of agents; pursuers and evaders, is developed to investigate the effects of a boundary and two obstacles. The evolution of strategies through Chemical Genetic Programming optimizes the mapping of genotypic strings to phenotypic trees. Four experiments were conducted, distinguished by speed differentials and environmental conditions. Designing experiments to evaluate the efficacy of emergent strategies often reveal necessary steps needed for coevolutionary progress. The experiments that excluded obstacles and boundaries provided design pointers to ensure coevolutionary progress as well as a deeper understanding of strategies that emerged when obstacles and boundaries were added. In the latter, we found that an awareness of the environment and the pursuer was not critical in an evader's strategy to survive, instead heading to the edge of the boundary or behind an obstacle in a bid to `throw-off or hide from the pursuer' or simply turn in circles was often sufficient, thereby revealing possible suboptimal strategies that were environment specific. We also observed that a condition for coevolutionary progress was that the problem complexity must be surmountable by at least one population; that is, some pursuer must be able to tag an opponent. Due to the use of amino-acid building blocks in our Chemical Genetic Program, our simulations were able to achieve significant complexity in a short period of time.

Method for lexicographic suboptimal control of the evader in a conflict problem

An approach is considered that allows generating a suboptimal strategy for the evader in a nonlinear feedback position control problem that can be implemented in real time. The evader does not know exactly what actions the pursuer will take up, and the evader's strategy is selected from the known functionality of both players and worse possible actions of the pursuer.

Life histories of Mediterranean pines

The life history of Spanish pines and their relation to fire as the main disturbance factor in their ecosystems was analysed. The primary ecological attributes studied were the canopy seed bank (onset of cone production, percentage and persistence of serotinous cones), seed and cone morphology, sprouting and bark thickness. Four ecological groups were separated using multivariate cluster analysis and their life-history characteristics are discussed. Serotiny and early flowering in Pinus halepensis and P. pinaster reflect their evader strategy in relation to fire as this character is advantageous to survive frequent crown fires and to attain successful post-fire recruitment. Late flowering and absence of serotinous cones in P. nigra, P. sylvestris and P. uncinata indicate that their natural forest did not evolve under frequent crown fires. P. canariensis and P. pinea appeared in two single groups because of their sprouting capability and their seed size respectively. Intraspecific variation in P. pinaster was also analysed using the same criteria and high variability was found in its life history traits. A group of P. pinaster populations showed high levels of serotiny and thin bark as a possible adaptation to frequent stand-replacing crown fires. In contrast, a group of non- or weakly-serotinous populations seems to have evolved under a low-intensity fire regime where the best fitness corresponds to thick-barked individuals capable of surviving ground fires. Intermediate strategies were also evident in this species and were discussed in relation to the effect of different fire regimes caused by the understorey vegetation.

Pursuit Differential Games with State Constraints

We prove the existence of a value for pursuit games with state constraints. We also prove that this value is lower semicontinuous.

Optimal Evader Strategy in a Positional Group Pursuit Problem with Simple Players Motions

Optimal Evader Strategy in a Positional Group Pursuit Problem with Simple Players Motions

An optimal guidance law for a planar pursuit-evasion game of kind

A coplanar pursuit-evasion game of the kind in the atmosphere between a coasting pursuer and a maneuvering evader of constant speed is considered. For this game the adjoint equations can be integrated analytically starting at the circular target set of the game. This allows to express the optimal strategies of the pursuer and the evader on the boundary of the capture set, as functions of the current and final values of the variables. The evader strategy is a hard turn towards the final line of sight. For the pursuer, the closed form solution allows to synthesize a feedback guidance law for a real-time implementation. The performance achieved by this suboptimal pursuer strategy is substantially superior to proportional navigation and only slightly worse than the optimal.

On closed-loop controls in pursuit-evasion

A hybrid algorithm is introduced in order to generate optimal closed-loop controls in a pursuit-evasion problem. The algorithm is composed of a differential dynamic programming method and a singular perturbation technique. The first mentioned method is used in those parts where the latter is weak. The method so created has an open-loop character. By having the open-loop solution updated as often as possible when new measures of the states have been estimated, the method will be near-optimal closed-loop. The game problem is discussed in terms of optimistic or pessimistic strategies, based on assumptions of the evader's strategy. In practice, the evader he may not do his best. The demonstrative example is in the horizontal plane, formulated as a game of degree. The pursuer uses a variable speed model with throttle and turn controls. The computer requirements can be kept to a reasonable level, and the new algorithm can be implemented on an airborne computer.

Strategy of evasion from many pursuers

AbstractA differential game of evasion with many participants (a game of kind in the terminology of Isaacs1) is considered. One controlled object (the evader) seeks to avoid contact with each of several pursuers. The motion of the evader is subject to a phase constraint. An efficient method (up to the explicit formulae) of constructing the evader's strategy is proposed. This strategy ensures evasion on the infinite time interval and the fulfilment of the phase constraint. The evader's control corresponding to this strategy is always a piecewise‐programme function of time, and the number of programme pieces is finite (there is an upper estimate for this number). There is a lower estimate for the minimal distance between the evader and the pursuers.