A simulation optimization approach in production planning of failure prone manufacturing systems

In this paper, the implementation of a new method to control the production rate of manufacturing systems, based on the combination of stochastic optimal control theory, discrete event simulation, experimental design and response surface methodology is outlined. The system under study consists of several parallel machines, multiple-product manufacturing system. Machines are subject to failures and repairs and their capacity process is assumed to be a finite state Markov chain throughout the analytical control model. The problem is to choose the production rates so as to minimize the expected discounted cost of inventory/backlog over an infinite horizon. We first show that, for constant demand rates and exponential failure and repair times distributions of the machines, the hedging point policy is optimal. The structure of the hedging point policy is then parameterized by factors representing the thresholds of involved products. With such a policy, simulation experiments are combined to experimental design and response surface methodology to estimate the optimal control policy. We obtain that the hedging point policy is also applicable to a wide variety of complex problems including non-exponential failure and repair times distributions and random demand rates. Analytical solutions may not be easily obtained for such complex situations.     

Production and set-up control of a failure-prone manufacturing system

The present paper considers a stochastic optimal control problem for a one-machine two-parts manufacturing system, subject to random breakdowns and repairs. The machine under consideration is not completely flexible and hence requires set-up time and cost in order to switch the production from a part type to another. The objective is to find the production plan and sequence of set-ups that minimize the cost function that penalizes inventory/backlog and set-up costs. A continuous dynamic programming formulation of the problem is presented. A numerical scheme is then adopted to solve the obtained optimality conditions equations. The optimal set-up policy. is shown numerically, to be described by a modified hedging corridor strategy. A complete heuristic policy, based on the determination of the boundaries of the corridors that describe the optimal policy in positive and negative areas of the state variables, is developed. The usefulness of such a policy is iJlustrated through experimentation and sensitivity analysis. Extensions to more complex systems are also discussed.     

PREDICTOL: a computer program to determine the thermophysiological duration limited exposures in various climatic conditions

PREDICTOL is a PC program used to determine the thermophysiological duration limited exposures (DLE) in humans, nude or clothed, submitted to various climatic conditions (hot and cold climates) at rest or during a physical exercise. DLE are determined following different standards of the International Standardization Organization (ISO), especially ISO 7933 for hot environment and ISO-TR 11079 for cold environment. The original aspect of this program is that it can be used whatever the climatic conditions. The program presents two modes: an educational interactive mode and a scenario mode. The educational interactive mode demonstrates the thermophysiological effects, expressed as DLE, of different parameter changes (temperature, humidity, wind speed, metabolic heat production by physical exercise, clothing insulation and water vapor permeability). The scenario mode determines DLE for given various linked sequences as encountered in occupational, military or even recreational activities, each sequence being characterized by its climatic conditions, physical activities performed and by physical clothing properties. DLE given by PREDICTOL are correlated to those obtained in various controlled climatic laboratory conditions (r = 0.86; P < 0.001). PREDICTOL is written in Visual Basic 6.0. A "help menu" is provided to explain the use of the program and give information concerning the equations used to calculate both the thermal balance and DLE.     

Children's choice of playground equipment: Development of methodology for integrating user preferences into environmental engineering.

Determined whether preference scaling techniques could be used to aid engineering system design. Children's play equipment was selected as the design area. Paired comparison and rank-order methods were used to determine whether 48 3rd-4th graders had preferences for different types of play equipment; whether these preferences were reliable over time; and whether actual use of play equipment was predictable from the preference scales developed. Photographic stimuli were used for the scaling and were presented to Ss. The frequency of use of play equipment was also determined using time-lapse photography. Results indicate that (a) Ss did have stable and reliable preferences for play equipment and (b) these preferences were correlated with actual usage of the equipment. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Structural studies of the O-specific polysaccharide of Hafnia alvei strain PCM 1206 lipopolysaccharide containing D-allothreonine

The structure of the O-specific side-chain of the Hafnia alvei strain PCM 1206 lipopolysaccharide has been investigated. Methylation analysis, partial acid hydrolysis, FAB-MS/MS and 1H-NMR and 13C-NMR spectroscopy were the principal methods used. D-Allothreonine (D-aThr), amide-linked to the D-galacturonic acid, was identified as a constituent in the polysaccharide and the following structure of a pentasaccharide repeating unit was established: [structure: see text].     

Structural classification of Quillaja saponins by electrospray ionization ion trap multiple-stage mass spectrometry in combination with multivariate analysis, proof of concept

MS² and MS³ spectra of thirteen previously identified saponin structures from Quillaja saponaria Molina were obtained by electrospray ionization ion trap multiple-stage mass spectrometry (ESI-ITMSⁿ) in positive ion mode. The spectra were analysed by partial least squares regression-discriminant analysis (PLS-DA) yielding a method that can classify the spectra according to saponin structure. For this method a referencing of the m/z axis was essential to obtain a correlation between peaks in the spectra and common structural elements. The MS² and MS³ spectra could be classified according to the structures of the oligosaccharides at C-3 and C-28 of the corresponding saponins. Terminal residues with the same mass, such as β-d-Apif and β-d-Xylp, could also be distinguished. The PLS-DA method was applied on Quillaja saponins separated from a commercial bark extract by reversed-phase chromatography.The method outlined here is applied on a specific group of Quillaja saponins, but can be used on other types of glycoconjugates. Combination of this technique with chromatography would give a very powerful tool for rapid identification of components in complex mixtures.     

Control policy simulation based on machine age in a failure prone one-machine,one-product manufacturing system

This paper deals with the production planning problem of a flexible manufacturing system (FMS). This problem is within the class of problems which are usually faced during the life cycle of FMS. There are basically two approaches used to solve this problem: analytical and simulation. The objective of this paper is to develop a methodology for determine the control policy for a given FMS by using the combination of both analytical and simulation approaches. The production rate control of a one-machine one-product system is presented to illustrate our approach. The contribution of the analytical part of the proposed model consists of generating a near-optimal controller based on the concept of an age-dependent hedging point policy. This sub-optimal policy is used as input of the simulation part and the improvement of operating conditions (control policies) achieved with a constant demand rate. The proposed model includes also a stochastic demand and lot sizes in production which consists of similar products batches with bulk arrivals. The robustness of the controller against both demand rates and production types has been validated.     

Steroid saponins from Solanum laxum

Two new steroid saponins, named laxumins A and B, were isolated from the ethanolic extract of the aerial parts of Solanum laxum. These compounds were characterized, using mainly NMR spectroscopy, mass spectrometry and chemical methods, as (23S,25S)-spirost-5-en-3 beta, 15 alpha, 23-triol 3-O-{beta-D-glucopyranosyl-(1-->2)- beta-D-glucopyranosyl-(1-->4)-[alpha-L-rhamnopyranosyl-(1-->2)]-beta-D- galactopyranoside} and 3-O-{beta-D-glucopyranosyl-(1--> 4)-alpha-L-rhamnopyranosyl-(1-->2)]-beta-D-galactopyranoside}, respectively.     

Economic evaluation of small-scale photovoltaic hybrid systems for mini-grid applications in far north Cameroon

A comparison between photovoltaic hybrid systems (PVHS), standalone photovoltaic (PV) and standalone diesel generator options is performed using the net present value (NPV) technique. A typical village mini-grid energy demand of 7.08 kWh/day is considered in the computation of energy costs and breakeven grid distances. A first sensitivity analysis is conducted using remote diesel prices of 0.8 €/l, 0.98 €/l, 1.12 €/l, 1.28 €/l with a PV module cost of 7.5 €/Wp. A second sensitivity analysis is also done using PV module costs of 5.25 €/Wp, 6 €/Wp, 6.75 €/Wp, 7.5 €/Wp with a diesel price of 1.12 €/l. The energy cost for the diesel option was found to be 0.812 €/kWh at a diesel fuel price of 1.12 €/l. The sensitivity analyses showed that minimum energy costs were attained in PVHS at renewable energy fractions in the range 82.6–95.3%. In the second sensitivity analysis the energy costs and breakeven grid distances were found to be in the ranges 0.692–0.785 €/kWh and 5.1–5.9 km respectively. For a PV module cost of 5.25 €/Wp, the lowest energy cost for the PVHS option was 0.692 €/kWh at a final renewable energy fraction of 95.3% with the diesel generator hours being 37 h compared to 2075 h in the standalone diesel generator option. Consequently, a 30% reduction in custom duties and taxes on imported PV modules and sub-systems would increase the use of small-scale and climate friendly PV mini-grids in remote areas of far north Cameroon that have an annual insolation of at least 5.55 kWh/m²/day.