This paper is the first attempt to successfully design efficient approximation algorithms for the single-machine weighted
flow-time minimization problem when jobs have different release dates and weights equal to their processing times under the
assumption that one job is fixed (i.e., the machine is unavailable during a fixed interval corresponding to the fixed job).
Our work is motivated by an interesting algorithmic application to the generation of valid inequalities in a branch-and-cut
method. Our analysis shows that the trivial FIFO sequence can lead to an arbitrary large worst-case performance bound. Hence,
we modify this sequence so that a new 2-approximation solution can be obtained for every instance and we prove the tightness
of this bound. Then, we propose a fully polynomial-time approximation algorithm with efficient running time for the considered
problem. Especially, the complexity of our algorithm is strongly polynomial. 相似文献
This paper addresses the deadlock-free scheduling problem in Flexible Manufacturing Systems. An efficient deadlock-free scheduling algorithm was developed, using timed Petri nets, for a class of FMSs called Systems of Sequential Systems with Shared Resources (S 4 R). The algorithm generates a partial reachability graph to find the optimal or near-optimal deadlock-free schedule in terms of the firing sequence of the transitions of the Petri net model. The objective is to minimize the mean flow time (MFT). An efficient truncation technique, based on the siphon concept, has been developed and used to generate the minimum necessary portion of the reachability graph to be searched. It has been shown experimentally that the developed siphon truncation technique enhances the ability to develop deadlock-free schedules of systems with a high number of deadlocks, which cannot be achieved using standard Petri net scheduling approaches. It may be necessary, in some cases, to relax the optimality condition for large FMSs in order to make the search effort reasonable. Hence, a User Control Factor (UCF) was defined and used in the scheduling algorithm. The objective of using the UCF is to achieve an acceptable trade-off between the solution quality and the search effort. Its effect on the MFT and the CPU time has been investigated. Randomly generated examples are used for illustration and comparison. Although the effect of UCF did not affect the mean flow time, it was shown that increasing it reduces the search effort (CPU time) significantly. 相似文献
Because of the risks associated with exposure to metallic particles, efforts are being put into controlling and reducing them
during the metal working process. Recent studies by the authors involved in this project have presented the effects of cutting
speeds, workpiece material, and tool geometry on particle emission during dry machining; the authors have also proposed a
new parameter, named the dust unit (Du), for use in evaluating the quantity of particle emissions relative to the quantity of chips produced during a machining
operation. In this study, a model for predicting the particle emission (dust unit) during orthogonal turning is proposed.
This model, which is based on the energy approach combined with the microfriction and the plastic deformation of the material,
takes into account the tool geometry, the properties of the worked material, the cutting conditions, and the chip segmentation.
The model is validated using experimental results obtained during the orthogonal turning of 6061-T6 aluminum alloy, AISI 1018,
AISI 4140 steels, and grey cast iron. A good agreement was found with experimental results. This model can help in designing
strategies for reducing particle emission during machining processes, at the source. 相似文献
Bone substitutes are more and more used in bone surgery because of their biologic safety, clinic efficiency and facility to synthesize. Bone substitutes with active osteogenic properties, associating biomaterials with organic macromolecule components of the extracellular matrix (protein, GAG) are recommended. Nevertheless, we should have a simple technique to control interactions between proteins and the material. Natural coral and nacre have been found to be impressive bone graft substitutes. In this work, we characterize nacre and coral powder using energy dispersive X-ray analysis (EDX). We used electrochemical impedance spectroscopy (EIS) and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy to evaluate bovine serum albumin (BSA) as model protein, adsorbed to these biomaterial surfaces. In order to understand the nacre/coral-protein interfacial compatibility, it is necessary to investigate the wettability. 相似文献
Intelligent Service Robotics - The problem of task allocation in a multi-robot system is the situation where we have a set of tasks and a number of robots; then each task is assigned to the... 相似文献
Vehicular ad-hoc networks play an important role in providing safety on the road. Vehicles generate and exchange emergency and control messages to avoid dangerous situations. According to IEEE 1609.4 standard, all these messages share the same control channel interval even if the emergency messages are with a highest priority. Besides that, network’s characteristics, such as vehicle density and high mobility, would make the diffusion of emergency messages a challenging task. Especially with the absence of acknowledgements and retransmission. Network coding could be seen as a solution where a block of data packets could be sent on the same transmission process. Firstly, we analyze the messages’ dissemination, and we propose a thoroughly model for success probability according to the emergency messages’ generation probabilities. Further, we propose a new network coding access scheme. The emergency messages take the priority in the channel access and it could be retransmitted several times. Thus, the scheme deals with the emergency messages’ loss and latency. NS-3 simulations show that our model increases emergency messages’ reliability.