Optimal low-thrust trajectories to asteroids through an algorithm based on differential dynamic programming

In this paper an optimisation algorithm based on Differential Dynamic Programming is applied to the design of rendezvous and fly-by trajectories to near Earth objects. Differential dynamic programming is a successive approximation technique that computes a feedback control law in correspondence of a fixed number of decision times. In this way the high dimensional problem characteristic of low-thrust optimisation is reduced into a series of small dimensional problems. The proposed method exploits the stage-wise approach to incorporate an adaptive refinement of the discretisation mesh within the optimisation process. A particular interpolation technique was used to preserve the feedback nature of the control law, thus improving robustness against some approximation errors introduced during the adaptation process. The algorithm implements global variations of the control law, which ensure a further increase in robustness. The results presented show how the proposed approach is capable of fully exploiting the multi-body dynamics of the problem; in fact, in one of the study cases, a fly-by of the Earth is scheduled, which was not included in the first guess solution

Manufacturing flow line systems: a review of models and analytical results

The most important models and results of the manufacturing flow line literature are described. These include the major classes of models (asynchronous, synchronous, and continuous); the major features (blocking, processing times, failures and repairs); the major properties (conservation of flow, flow rate-idle time, reversibility, and others); and the relationships among different models. Exact and approximate methods for obtaining quantitative measures of performance are also reviewed. The exact methods are appropriate for small systems. The approximate methods, which are the only means available for large systems, are generally based on decomposition, and make use of the exact methods for small systems. Extensions are briefly discussed. Directions for future research are suggested.National Science Foundation (U.S.) (Grant DDM-8914277

Issues in the modeling and design of material recycling systems

Interest in recycling has surged in recent years due to increasing primary material costs, environmental concerns over material production and disposal, and laws in many countries designed to improve material recycling rates. In response, recycling systems are becoming more complex as increasing material recovery is required from products with complicated material mixtures such as WEEE (Waste Electric and Electronic Equipment) and ELVs (End-of-Life Vehicles). To increase separation system performance and to process complex material mixtures, separation systems are typically organized as highly integrated multi-stage systems. In spite of their cost, the problem of estimating the performance and designing multi-stage separation systems has rarely been tackled from a systems engineering perspective, resulting in poor integration and sub-optimal configuration of industrial multi-stage separation systems. This paper presents a new approach to modeling and analyzing the performance in terms of recovery and grade of multi-stage material separation systems. Individual comminution and separation processes are modeled and integrated into a separation network model that can be studied with analytical techniques. This model can be used to evaluate the performance of multi-stage separation systems under varying operating conditions, supporting decisions related to system configurations. Several basic examples demonstrate the utility of this model for design purposes. Furthermore, several open research challenges in this new research area are highlighted