An integrated multi-population genetic algorithm for multi-vehicle task assignment in a drift field

This paper investigates the task assignment problem for a team of autonomous aerial/marine vehicles driven by constant thrust and maneuvering in a planar lateral drift field. The aim is to minimize the total traveling time in order to guide the vehicles to deliver a number of customized sensors to a set of target points with different sensor demands in the drift field. To solve the problem, we consider together navigation strategies and target assignment algorithms; the former minimizes the traveling time between two given locations in the drift field and the latter allocates a sequence of target locations to each vehicle. We first consider the effect of the weight of the carried sensors on the speed of each vehicle, and construct a sufficient condition to guarantee that the whole operation environment is reachable for the vehicles. Then from optimal control principles, time-optimal path planning is carried out to navigate each vehicle from an initial position to its given target location. Most importantly, to assign the targets to the vehicles, we combine the virtual coding strategy, multiple offspring method, intermarriage crossover strategy, and the tabu search mechanism to obtain a co-evolutionary multi-population genetic algorithm, short-named CMGA. Simulations on sensor delivery scenarios in both fixed and time-varying drift fields are shown to highlight the satisfying performances of the proposed approach against popular greedy algorithms

Efficient Routing for Precedence-Constrained Package Delivery for Heterogeneous Vehicles

This paper studies the precedence-constrained task assignment problem for a team of heterogeneous vehicles to deliver packages to a set of dispersed customers subject to precedence constraints that specify which customers need to be visited before which other customers. A truck and a micro drone with complementary capabilities are employed where the truck is restricted to travel in a street network and the micro drone, restricted by its loading capacity and operation range, can fly from the truck to perform the last-mile package deliveries. The objective is to minimize the time to serve all the customers respecting every precedence constraint. The problem is shown to be NP-hard, and a lower bound on the optimal time to serve all the customers is constructed by using tools from graph theory. Then, integrating with a topological sorting technique, several heuristic task assignment algorithms are proposed to solve the task assignment problem. Numerical simulations show the superior performances of the proposed algorithms compared with popular genetic algorithms. Note to Practitioners - This paper presents several task assignment algorithms for the precedence-constrained package delivery for the team of a truck and a micro drone. The truck can carry the drone moving in a street network, while the drone completes the last-mile package deliveries. The practical contributions of this paper are fourfold. First, the precedence constraints on the ordering of the customers to be served are considered, which enables complex logistic scheduling for customers prioritized according to their urgency or importance. Second, the package delivery optimization problem is shown to be NP-hard, which clearly shows the need for creative approximation algorithms to solve the problem. Third, the constructed lower bound on the optimal time to serve all the customers helps to clarify for practitioners the limiting performance of a feasible solution. Fourth, the proposed task assignment algorithms are efficient and can be adapted for real scenarios

Efficient Heuristic Algorithms for Single-Vehicle Task Planning With Precedence Constraints

This article investigates the task planning problem where one vehicle needs to visit a set of target locations while respecting the precedence constraints that specify the sequence orders to visit the targets. The objective is to minimize the vehicle’s total travel distance to visit all the targets while satisfying all the precedence constraints. We show that the optimization problem is NP-hard, and consequently, to measure the proximity of a suboptimal solution from the optimal, a lower bound on the optimal solution is constructed based on the graph theory. Then, inspired by the existing topological sorting techniques, a new topological sorting strategy is proposed; in addition, facilitated by the sorting, we propose several heuristic algorithms to solve the task planning problem. The numerical experiments show that the designed algorithms can quickly lead to satisfying solutions and have better performance in comparison with popular genetic algorithms

Tracing blastomere fate choices of early embryos in single cell culture

Blastomeres of early vertebrate embryos undergo numerous fate choices for division, motility, pluripotency maintenance and restriction culminating in various cell lineages. Tracing blastomere fate choices at the single cell level in vitro has not been possible because of the inability to isolate and cultivate early blastomeres as single cells. Here we report the establishment of single cell culture system in the fish medaka, enabling the isolation and cultivation of individual blastomeres from 16- to 64-cell embryos for fate tracing at the single cell level in vitro. Interestingly, these blastomeres immediately upon isolation exhibit motility, lose synchronous divisions and even stop dividing in ≥50% cases, suggesting that the widely accepted nucleocytoplasmic ratio controlling synchronous divisions in entire embryos does not operate on individual blastomeres. We even observed abortive division, endomitosis and cell fusion. Strikingly, ~5% of blastomeres in single cell culture generated extraembryonic yolk syncytial cells, embryonic stem cells and neural crest-derived pigment cells with timings mimicking their appearance in embryos. We revealed the maternal inheritance of key lineage regulators and their differential expression in cleavage embryos. Therefore, medaka blastomeres possess the accessibility for single cell culture, previously unidentified heterogeneity in motility, division, gene expression and intrinsic ability to generate major extraembryonic and embryonic lineages without positioning cues. Our data demonstrate the fidelity and potential of the single cell culture system for tracking blastomere fate decisions under defined conditions in vitro

人造血管在血液透析造瘘的应用及护理

Objective: To study the application of artificial blood vessels to build alternative standard arteriovenous fistula blood vessels for hemodialysis maintenance nursing. Methods: 17 cases of patients underwent artificial blood vessel transplantation, material for ptfe (PTEE), 30 cm in length, inner diameter 5mm, from 2012 to 2013 in Jinan military region general hospital. Artificial blood vessel transplantation is more operated on brachial vein, cephalic vein, median cubital vein and basilic vein. When the anastomosis among artificial blood vessels and brachial artery and vein anastomosis was established, the arterial end was on the inner side. When the anastomosis among brachial vein, median cubital vein and basilic vein was established, the arterial end was on the outside for the extension of vascular access. Results: 15 cases have hemodialysis after two months’ treatment, 1 case have artificial blood vessel fistula after swelling for 4 months, infection of thrombosis occurred one month after the operation in 1 case. Conclusion: Artificial blood vessel show good biocompatibility, high long-term patency rate, blood flow, and convenient puncture point. It builds the “lifeline” for the patients who are unable to establish a good vascular access. It is of great significance to improve the management of artificial blood vessel fistula for maintenance hemodialysis patients.目的  研究应用人造血管造瘘替代标准动静脉血管维持血液透析的护理。方法  本院自2012—2013年对17例血液透析患者进行人造血管移植术，材料为聚四氟乙烯（PTEE）材料，长度30cm，内径5mm。人造血管移植术多选择与肱动脉和头静脉或肘正中静脉、贵要静脉U型吻合。人造血管与肱动脉和头静脉吻合时，动脉端在内侧。但肱动脉与肘正中静脉、贵要静脉吻合时，为延长有效性血管通路，动脉端则在外侧。结果  15例术后2个月成熟顺利实施血液透析，1例持续肿胀4个月后开始应用，1例术后1个月出现感染血栓形成。结论  人造血管具有生物相容性好、长期通畅率高、血流量大、穿刺方便、穿刺部位充足等优点，为自身血管条件差、无法建立良好血管通路的透析患者提供了可靠的血管通路，为维持性血液透析患者建立了“生命线”。加强透析过程中的人造血管内瘘管理对于维持透析患者良好的血管通路具有重要意义