Performance comparison of virtual cellular manufacturing with functional and cellular layouts in DRC settings

This study investigates the performance of virtual cellular manufacturing (VCM) systems, comparing them with functional layouts (FL) and traditional, physical cellular layout (CL), in a dual-resource-constrained (DRC) system context. VCM systems employ logical cells, retaining the process layouts of job shops. Part family-based scheduling rules are applied to exploit the benefits of group technology while retaining the flexibility and functional synergies of the job shop. Past studies of VCM have been based entirely on single-resource-constrained (SRC) systems, i.e. as purely machine-limited systems, assuming that resources such as labour and tooling do not restrict the output. However, given the fact that labour forms a second major constraining resource, and many of the advantages associated with cellular manufacturing are derived from labour flexibility, it becomes necessary to extend the research to DRC systems. In this study, we assume several levels of labour flexibility in all three systems, in addition to other relevant factors such as lot size, set-up reduction, and labour assignment rules. It is shown that VCM can outperform efficiently operated FL and CL in certain parameter ranges, as preliminary research has shown so far. However, it is shown that CL tends to outperform both VCM and FL in the parameter ranges customarily advocated for CL, namely, low lot sizes, adequate levels of set-up reduction, cross training of workers, and worker mobility within cells.     

Comparing functional and cellular layouts: A simulation study based on standardization

In the last decade, over two dozen simulation studies have focused on comparing cellular and functional layouts. The results reported by these studies vary widely, however. This remains true even when the key performance measure is flow time. These variations reflect the disparate manufacturing and operating environments, as well as differences in parts demands, set-up economies, overall loads and other factors. This work attempts to reduce the sources of variation due to different operating assumptions while retaining the variability associated with differences in part mix and demand characteristics. Instead of focusing on a single data source, this study uses a test bed of six problems extracted from the literature and ensures they share the same operational rules. The simulation results show that conversion to CMS can reduce flow times (relative to the job shop configuration) consistently across all data sets, provided the same operating rules and ranges for key parameter are used. We investigate the reduction in flow time while controlling for the key factors of set-up reduction, overall load on the system and batch size. We also assess the benefits of using transfer batches as a further factor in reducing flow time. Our overall conclusion is that set-up reductions in cells can overcome pooling losses, even under the conservative assumptions where batch size remain unchanged and the material transport times in the job shop are assumed to be negligible.     

CDMA分层蜂窝移动通信系统的性能分析

分析了CDMA分层蜂窝移动通信系统(HCMS)网络覆盖特性,在此基础上提出了一种由宏小区和分布式天线系统(DAS)组成的双层HCMS,即宏小区/DAS结构,并将其与CDMA宏小区/微小区双层蜂窝系统作了比较.网络覆盖特性分析表明, 该网络结构更有利于降低网络建设和功率控制的复杂性;信干比仿真结果表明,该网络结构改善了系统的上下行链路的信干比,从而有利于提高CDMA蜂窝移动通信系统的系统容量.     

Performance evaluation of a thermal Doppler Michelson interferometer system

The thermal Doppler Michelson interferometer is the primary element of a proposed limb-viewing satellite instrument called SWIFT (Stratospheric Wind Interferometer for Transport studies). SWIFT is intended to measure stratospheric wind velocities in the altitude range of 15-45 km. SWIFT also uses narrowband tandem etalon filters made of germanium to select a line out of the thermal spectrum. The instrument uses the same technique of phase-stepping interferometry employed by the Wind Imaging Interferometer onboard the Upper Atmosphere Research Satellite. A thermal emission line of ozone near 9 microm is used to detect the Doppler shift due to winds. A test bed was set up for this instrument that included the Michelson interferometer and the etalon filters. For the test bed work, we investigate the behavior of individual components and their combination and report the results.     

Performance evaluation of a multi-product system under CONWIP control

Analytical models of multi-product manufacturing systems operating under CONWIP control are composed of closed queuing networks with synchronization stations. Under general assumptions, these queuing networks are hard to analyze exactly and therefore approximation methods must be used for performance evaluation. This research proposes a new approach based on parametric decomposition. Two-moment approximations are used to estimate the performance measures at individual stations. Subsequently, the traffic process parameters at the different stations are linked using stochastic transformation equations. The resulting set of non-linear equations is solved using an iterative algorithm to obtain estimates of key performance measures such as throughput, and mean queue lengths. Numerical studies indicate that the proposed method is computationally efficient and yields fairly accurate results when compared to simulation.     

Performance evaluation of single-sludge reactor system treating high-strength nitrogen wastewater

In the single-sludge reactor system treating high-strength nitrogen wastewater (similar to anaerobically pretreated piggery wastewater), the NH4(+)-N removal efficiencies (98-82%) are higher than total nitrogen removal efficiencies (71-43%). The mixed liquor recycle ratio only imposes a slight effect on total nitrogen removal efficiency. The alkalinity change data could be used for monitoring and control of the reactor system. To evaluate the performance of the single-sludge reactor system, a simplified nitrification-denitrification model (with nitrification capacity, denitrification capacity, and denitrification potential concepts) and a graphically analytical technique are proposed. It turns out that ammonia nitrification and total nitrogen removal efficiencies are strongly dependent on the process load and reactor configuration, and an optimal operating condition requires a proper match between nitrification and denitrification.     

Extension of dual analysis to 3-D problems: evaluation of its advantages in error estimation

The dual analysis is one of the first methods developed to evaluate the discretization error of the finite elements computations. It is based on the comparison of two solutions of a given finite element problem, one of them being of the classical displacement type, the second being of the equilibrium type. This method has proved its efficiency in the frame of elastostatic computations for which it leads to an upper bound of the global error in energy norm. Due to the use of a stiffness matrix, this method is very easy to implement in a standard finite element program and it is computationally very efficient.     

Citation contexts as a data source for evaluation of scholarly consumption

Scientometrics - In recent years, large datasets of citation contexts from research publications have become available for scientometric studies. Such citation contexts contain different...     

Performance evaluation of an auction-based manufacturing system using coloured Petri nets

The next generation of manufacturing systems is assumed to be intelligent enough to make decisions and automatically adjust to variations in production demand, shop-floor breakdowns etc. Auction-based manufacturing is a control strategy in which various intelligent entities in the manufacturing system bid themselves, accept bids and make selections among the bids available based on a heuristic. This paper deals with the simulation modelling and performance evaluation of a push-type auction (negotiation) based manufacturing system embedded in a pulltype production system using coloured Petri nets. Three different models of an auction-based manufacturing system have been discussed. This methodology helps in developing systems for real-time control, anticipation of deadlocks, and evaluation of various performance metrics like machine utilization, automated guided vehicle (AGV) utilization, waiting times, work in process (WIP) etc. Various decision-making rules were identified for the real-time control of auction-based manufacturing systems.     

Performance evaluation of a flexible machining/assembly system and routing comparisons

This paper discusses the performance evaluation of the flexible machining/assembly systems (FMS/FAS) of a central server type, and gives a comparative consideration of a fixed, dynamic versus an ordered-entry routing rule. First, the steady-state equations are given, and the system throughput is obtained. Next, the system configurations of FMS/FAS are numerically discussed on the basis of system throughput. Finally, the superiority of an ordered-entry routing rule is numerically discussed for development of routing theory.     

Performance evaluation and simulation of a new absorbent for an absorption refrigeration system

In this article the employment of an alternative absorbent used in absorption refrigeration cycles to replace the absorbent currently employed in this kind of enginnes, lithium bromide, has been studied. The alternative system consits of absorbent (LiBr:CHO₂K=2:1 by mass ratio) and refrigerant (H₂O). To compare the performance of both systems, a program to simulate theoretical absorption cycles from empirical data has been developed. By means of the program, the efficiency of the absorption cycles have been evaluated. Also the influence of changes in the operation conditions on the performance of each cycle has been analysed. The study allows an insight into the internal parameters and into the possible behaviour for more severe conditions than those studied.     

Evaluation of Home Elevator Performance Based on Extension Fuzzy Theory

It is quite difficult as to how to choose an appropriate home elevator, because the factories manufacturing home elevator are numerous and the models of production are different. Therefore, this article combines fuzzy math with extension theory on the basis of matter element theory, extension set theory and correlation function, constructs a classical matter element and a segment field matter element by classifying home elevator performance and making a performance index membership zero dimension, and establishes an extension fuzzy evaluation model of home elevator performance levels. The example in this article shows that the established method is able to evaluate actual home elevator performance.     

Performance evaluation of photolithography cluster tools

The photolithography cluster tool is typically the most expensive tool set utilized in the production of semiconductor wafers and is often selected as a fabricator bottleneck. Modeling such a tool as a serial processing cluster tool, we deduce measures of tool performance. Queueing models reveal that the mean cycle time in the presence of a Poisson arrival process is related to the parallelism inherent in the system configuration. As a consequence, the normalized mean cycle time behavior has a different form than that of the standard single server queue. The process time of a lot and the throughput are evaluated in the presence of disruptions common in practical manufacturing environments. For multiple products with different process rates, it is shown that the throughput is not influenced by the order in which the lots are sequenced.     

Performance assessment and uncertainty evaluation of a portable NaI-based detection system used for thyroid monitoring

This work aims at assessing the performance of a portable detection system, equipped with an NaI(Tl) scintillation detector for in vivo thyroid monitoring, which was properly calibrated using an anthropomorphic neck phantom. The anthropomorphic physical phantoms commonly used for the efficiency calibration of in vivo counters often present certain limitations regarding the geometry and the activity distribution. Therefore, the feasibility of these detection systems for in vivo monitoring should be assessed whenever possible. To accomplish this assessment, patients to whom (99m)Tc and (123)I marked radiopharmaceuticals have been administered in the framework of nuclear medicine diagnostic procedures were monitored. As the biokinetic models of the administered radiopharmaceuticals are known, the time-dependent activity functions in the critical organs after administration are easily quantified. The measured activities in the thyroid using the NaI(Tl) scintillation detector were compared with the estimated activities using the biokinetic models, in order to reach conclusion about the applicability of the portable scintillation counter for in vivo thyroid monitoring. The state-of-the-art Monte Carlo computer program PENELOPE and two voxel phantoms (male and female) were used to evaluate the overall uncertainties influencing the thyroid monitoring. A computational parametric study was performed to quantify the influence of several parameters in the activity quantification (neck-detector distance, thyroid shape, thyroid size and overlying tissue thickness), which allowed one to gain insight and to better understand the discrepancies between the calculated and measured activities.     

Performance evaluation of an optoelectronic oscillator

Phase noise measurements of an optoelectronic oscillator (OEO) at frequencies less than 10 Ha from the carrier (10.6 GHz) as well as the measured Allan variance are presented for the first time. The system has a measured single-side-band (SSB) phase-noise of -123 dB/Hz at 10 kHz from the carrier and a sigma(y)(tau)=10(-10) for an integration time between 1 and 10 seconds. The importance of amplifier phase-noise and environmental fluctuations in determining the noise of the oscillator at these low Fourier frequencies is verified experimentally and analyzed using a generalized model of noise sources in the OEOs. This analysis then allows prediction of the oscillator performance from measured parameters of individual components in the system.     

Preparation and evaluation of charged solid lipid nanoparticles of tetrandrine for ocular drug delivery system: pharmacokinetics,cytotoxicity and cellular uptake studies

In this study, tetrandrine-loaded cationic solid lipid nanoparticles (TET-CNP) and solid lipid nanoparticles (TET-NP) were prepared by the emulsion evaporation-solidification at low temperature method. The particle size, zeta potential, and entrapment efficiency of TET-CNP and TET-NP were characterized. The results showed that the TET-CNP and TET-NP had average diameters of (15.29?±?1.34) nm and (18.77?±?1.23) nm with zeta potentials of (5.11?±?1.03) mV and (?8.71?±??1.23) mV and entrapment efficiencies of (94.1?±?2.37)% and (95.6?±?2.43)%, respectively. In vitro release studies indicated that the TET-CNP and TET-NP retained the drug entity better than tetrandrine ophthalmic solutions (TET-SOL). In the pharmacokinetics studies, the AUC values of TET-CNP and TET-NP were 1.96-fold and 2.00-fold higher than that of TET-SOL (?p?C_max values of TET-CNP and TET-NP were 2.45-fold and 2.53-fold higher than that of the TET-SOL (p?相似文献   

Nanotopography: cellular responses to nanostructured materials

Several in vitro and in vivo experiments have shown that nanostructured materials, which mimic the nanometer topography of the native tissues, improve biocompatible responses, and result in better tissue integration in medical implants. Understanding various aspects of nanotopography is extremely important for better designs of these devices. In this review paper, recent progress in the fabrication, characterization, biological responses, and application of nanostructured materials are discussed. Specifically, materials such as ceramics and polymers used to manufacture nanostructured surfaces are briefly introduced. Techniques for fabrication and characterization of nanostructured materials are also explored. Cellular responses such as morphology, alignment, adhesion, proliferation, and profiles of gene expression of various cell types after their exposure to nanofeatured materials are particularly reviewed. Finally, the paper briefly discusses some application of nanostructured materials including those in biosensor and tissue engineering fields.     

Performance evaluation of sandwich panels subjected to bending compression and thermal bowing

The engineering performance of sandwich panels with expanded polystyrene foam core and steel or aluminium faces is evaluated in this paper. Such panels are usually used in semi-structural building applications with an insulating function. Bending, compression and thermal bowing experiments are conducted on these panels in the laboratory and their results are shown to conform in general to design values determined by current building codes and commercial practices. In edge-wise compression tests failure by column buckling has never occurred and localised face wrinkling is the usual failure mode. The adhesion between the polystyrene core and the metal skin as well as the location of the polystyrene joint in the panel are shown to have significant effects on the integral performance of the sandwich panels.