Continuous workload control order release revisited: an assessment by simulation

Order release is a key component of the Workload Control concept. Jobs do not enter the shop floor directly – they are retained in a pre-shop pool and released in time to meet due dates while keeping work-in-process within limits or norms. There are two important groups of release methods: continuous methods, for which the workload falling to a specified level can trigger a release at any moment in time; and, periodic release methods, for which releases take place at fixed intervals. Continuous release methods in general have been shown to outperform periodic release methods. Yet, there is incongruence in the results presented in the literature on the relative performance of the various continuous release methods. We use a job shop simulation model to examine the performance of continuous release methods from the literature and find that the contradictory results are explained by the different rules applied to sequence jobs in the pool – a factor neglected in previous work. Finally, a new breed of continuous release methods has recently emerged, but these have not been compared with prior approaches. Therefore, we also examine these methods and show that they significantly improve overall performance, although this is to the detriment of jobs with large processing times.     

Discontinuities in citation relations among journals: self-organized criticality as a model of scientific revolutions and change

Scientometrics - Using 3-year moving averages of the complete Journal Citation Reports 1994–2016 of the Science Citation Index and the Social Sciences Citation Index (combined), we analyze...     

Rapid method for low cycle fatigue properties: thickness effect on the fatigue crack initiation life of welded joints

Welded assemblies are commonly used in the shipbuilding industry. Because of the combination of stress concentration and cyclic loading, welded joints could be a critical area for fatigue damage. Thus, knowing stress and strain histories at the critical points of the structure is necessary, particularly when a confined plasticity occurs, to determine the fatigue life of welded assemblies. To avoid time‐consuming nonlinear finite element analyses (FEA), simplified estimation methods of the elastic–plastic strain/stress can be used. In a previous work, an approach to estimate stress state at critical points was developed and employed in the case of double‐notched specimens. The present paper focuses on welded joints in order to validate this strategy with the aim to estimate the fatigue crack initiation life of T‐joints. To go further, a parametric approach has been adopted to take into account the local geometries of welded joints and to determine the constraint operator without any FEA. The results predicted by this approach are compared with experimental fatigue results.     

Workload control in dual-resource constrained high-variety shops: an assessment by simulation

Workload Control (WLC) seeks to align capacity with demand, where capacity is typically assumed to be restricted by a single constraint – machine capacity. In practice, however, shops are often restricted by dual resource constraints: labour and machines. This study, therefore, uses simulation to investigate the performance of WLC in Dual Resource Constrained (DRC) high-variety shops with fully interchangeable labour. By considering several environmental factors and different labour assignment and dispatching rules, it is demonstrated that the order release function of WLC maintains its positive impact on performance in a DRC shop under different staffing levels. The positive effect of considering labour availability at release, as proposed in previous research, could not, however, be confirmed. Thus, the original release method can be applied if labour is fully interchangeable. In terms of labour assignment, we show that a distinct assignment pattern that differs between upstream and downstream stations improves performance if the routing is directed. Meanwhile, dispatching plays a less important role but creates important interaction effects with the assignment rule. Finally, the results suggest that increasing the service rate is a better response to the reduction in capacity that results from labour absenteeism than lowering the input frequency of work.     

Optical computing: introduction by the feature editors

This feature issue of Applied Optics: Information Processing contains 19 papers on Optical Computing. Many of these papers are expanded versions of presentations given at the Optical Society of America's Sixth Topical Meeting on Optical Computing held in Salt Lake City, Utah, in March 1995. This introduction provides a brief historical account of the series of optical computing meetings and a brief review of the papers contained in this special issue.     

Photorefractive processing for large adaptive phased arrays

An adaptive null-steering phased-array optical processor that utilizes a photorefractive crystal to time integrate the adaptive weights and null out correlated jammers is described. This is a beam-steering processor in which the temporal waveform of the desired signal is known but the look direction is not. The processor computes the angle(s) of arrival of the desired signal and steers the array to look in that direction while rotating the nulls of the antenna pattern toward any narrow-band jammers that may be present. We have experimentally demonstrated a simplified version of this adaptive phased-array-radar processor that nulls out the narrow-band jammers by using feedback-correlation detection. In this processor it is assumed that we know a priori only that the signal is broadband and the jammers are narrow band. These are examples of a class of optical processors that use the angular selectivity of volume holograms to form the nulls and look directions in an adaptive phased-array-radar pattern and thereby to harness the computational abilities of three-dimensional parallelism in the volume of photorefractive crystals. The development of this processing in volume holographic system has led to a new algorithm for phased-array-radar processing that uses fewer tapped-delay lines than does the classic time-domain beam former. The optical implementation of the new algorithm has the further advantage of utilization of a single photorefractive crystal to implement as many as a million adaptive weights, allowing the radar system to scale to large size with no increase in processing hardware.     

Micelle-assisted bilayer formation of cetyltrimethylammonium bromide thin films studied with combinatorial spectroscopic ellipsometry and quartz crystal microbalance techniques

We report on a combinatorial approach to study the formation of ultra-thin organic films using in-situ spectroscopic ellipsometry and quartz crystal microbalance methods. In contrast to the quartz crystal microbalance, which is sensitive to the total mass attached to the surface, including coupled and entrapped solvent, spectroscopic ellipsometry only measures the amount of adsorbent on the surface. By using these two techniques in tandem, we define and determine the solvent fraction of cetyltrimethylammonium bromide thin films adsorbed onto a gold-coated quartz crystal. Cetyltrimethylammonium bromide thin films grown from aqueous solutions above the critical micelle concentration reveal critical phases in thickness and porosity evolution. We relate these effects to the mechanisms of formation and removal and the structure of cetyltrimethylammonium bromide films, which we determine to have systemic defects due to the presence of micelles.     

Effects of soluble surfactants on the Langmuir monolayers compressibility: A comparative study using interfacial isotherms and fluorescence microscopy

Langmuir monolayer isotherms and fluorescence microscopy (FM) techniques have been used to study the effect of two soluble surfactants on the methyl octadecanoate monolayer's compressibility at the air/water interface. The combination of these two techniques allows one to bridge the mechanical and morphological properties of the monolayer at different surfactant subphase concentrations. Our results show that the presence of sodium dodecyl sulfate (SDS) or dodecyltrimethylammonium bromide (DTAB) affects the monolayer elasticity differently. In addition, the outcome of this study emphasizes the role of the cationic and anionic surfactants on the monolayer compressibility. In fact, their effect was found to be primly depending on the monolayer thermodynamic situation. The isotherms of the monolayers at different surfactant concentrations underneath the monolayer preserve the characteristics behavior of the monolayer as imaged by FM. The calculated monolayer compressibility shows two different trends depending on the monolayer pressure and the surfactant type. A decreasing compressibility as a function of SDS concentration was found at pressure π = 5 mN/m, while no noticeable effect was found due to DTAB. At π = 10 mN/m both surfactants convert the monolayer from rigid to soft monolayer. Such characteristic behavior of the monolayer has been confirmed by FM.     

Optoelectronic implementation of a 256-channel sonar adaptive-array processor

We present an optoelectronic implementation of an adaptive-array processor that is capable of performing beam forming and jammer nulling in signals of wide fractional bandwidth that are detected by an array of arbitrary topology. The optical system makes use of a two-dimensional scrolling spatial light modulator to represent an array of input signals in 256 tapped delay lines, two acousto-optic modulators for modulating the feedback error signal, and a photorefractive crystal for representing the adaptive weights as holographic gratings. Gradient-descent learning is used to dynamically adapt the holographic weights to optimally form multiple beams and to null out multiple interference sources, either in the near field or in the far field. Space-integration followed by differential heterodyne detection is used for generating the system's output. The processor is analyzed to show the effects of exponential weight decay on the optimum solution and on the convergence conditions. Several experimental results are presented that validate the system's capacity for broadband beam forming and jammer nulling for linear and circular arrays.     

Performance optimization of spectroscopic process analyzers

To increase the power and the robustness of spectroscopic process analyzers, methods are needed that suppress the spectral variation that is not related to the property of interest in the process stream. An approach for the selection of a suitable method is presented. The approach uses the net analyte signal (NAS) to analyze the situation and to select methods to suppress the nonrelevant spectral variation. The empirically determined signal-to-noise of the NAS is used as a figure of merit. The advantages of the approach are (i). that the error of the reference method does not affect method selection and (ii). that only a few spectral measurements are needed. A diagnostic plot is proposed that guides the user in the evaluation of the particular suppression method. As an example, NIR spectroscopic monitoring of a mol-sieve separation process is used.