Design and control of agile automated CONWIP production lines

In this article, we study the design and control of manufacturing cells with a mix of manual and automated equipment, operating under a CONWIP pull protocol, and staffed by a single agile (cross-trained) worker. For a three-station line with one automated station, we fully characterize the structure of the optimal control policy for the worker and show that it is a static priority policy. Using analytical models and extensive simulation experiments, we also evaluate the effectiveness of practical heuristic control policies and provide managerial insights on automation configuration design of the line. This characterization of the worker control policy enables us to develop managerial insights into the design issues of how best to locate and concentrate automation in the line. Finally, we show that, in addition to ease of control and greater design flexibility, the CONWIP protocol also offers higher efficiency and robustness than does the push protocol. © 2008 Wiley Periodicals, Inc. Naval Research Logistics 2009Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/61525/1/20325_ftp.pd

PREDICTING THE SUMMER TEMPERATURE OF SMALL STREAMS IN SOUTHWESTERN WISCONSIN 1

One of the biggest challenges in managing cold water streams in the Midwest is understanding how stream temperature is controlled by the complex interactions among meteorologic processes, channel geometry, and ground water inflow. Inflow of cold ground water, shade provided by riparian vegetation, and channel width are the most important factors controlling summer stream temperatures. A simple screening model was used to quantitatively evaluate the importance of these factors and guide management decisions. The model uses an analytical solution to the heat transport equation to predict steady-state temperature throughout a stream reach. The model matches field data from four streams in southwestern Wisconsin quite well (typically within 1°C) and helps explain the observed warming and cooling trends along each stream reach. The distribution of ground water inflow throughout a stream reach has an important influence on stream temperature, and springs are especially effective at providing thermal refuge for fish. Although simple, this model provides insight into the importance of ground water and the impact different management strategies, such as planting trees to increase shade, may have on summer stream temperature.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74032/1/j.1752-1688.2005.tb03714.x.pd

Identifying efficient solutions via simulation: myopic multi-objective budget allocation for the bi-objective case

Simulation optimisation offers great opportunities in the design and optimisation of complex systems. In the presence of multiple objectives, there is usually no single solution that performs best on all objectives. Instead, there are several Pareto-optimal (efficient) solutions with different trade-offs which cannot be improved in any objective without sacrificing performance in another objective. For the case where alternatives are evaluated on multiple stochastic criteria, and the performance of an alternative can only be estimated via simulation, we consider the problem of efficiently identifying the Pareto-optimal designs out of a (small) given set of alternatives. We present a simple myopic budget allocation algorithm for multi-objective problems and propose several variants for different settings. In particular, this myopic method only allocates one simulation sample to one alternative in each iteration. This paper shows how the algorithm works in bi-objective problems under different settings. Empirical tests show that our algorithm can significantly reduce the necessary simulation budget

Trends in parameterization, economics and host behaviour in influenza pandemic modelling: a review and reporting protocol.

BACKGROUND: The volume of influenza pandemic modelling studies has increased dramatically in the last decade. Many models incorporate now sophisticated parameterization and validation techniques, economic analyses and the behaviour of individuals. METHODS: We reviewed trends in these aspects in models for influenza pandemic preparedness that aimed to generate policy insights for epidemic management and were published from 2000 to September 2011, i.e. before and after the 2009 pandemic. RESULTS: We find that many influenza pandemics models rely on parameters from previous modelling studies, models are rarely validated using observed data and are seldom applied to low-income countries. Mechanisms for international data sharing would be necessary to facilitate a wider adoption of model validation. The variety of modelling decisions makes it difficult to compare and evaluate models systematically. CONCLUSIONS: We propose a model Characteristics, Construction, Parameterization and Validation aspects protocol (CCPV protocol) to contribute to the systematisation of the reporting of models with an emphasis on the incorporation of economic aspects and host behaviour. Model reporting, as already exists in many other fields of modelling, would increase confidence in model results, and transparency in their assessment and comparison

On the Introduction of an Agile, Temporary Workforce into a Tandem Queueing System

We consider a two-station tandem queueing system where customers arrive according to a Poisson process and must receive service at both stations before leaving the system. Neither queue is equipped with dedicated servers. Instead, we consider three scenarios for the fluctuations of workforce level. In the first, a decision-maker can increase and decrease the capacity as is deemed appropriate; the unrestricted case. In the other two cases, workers arrive randomly and can be rejected or allocated to either station. In one case the number of workers can then be reduced (the controlled capacity reduction case). In the other they leave randomly (the uncontrolled capacity reduction case). All servers are capable of working collaboratively on a single job and can work at either station as long as they remain in the system. We show in each scenario that all workers should be allocated to one queue or the other (never split between queues) and that they should serve exhaustively at one of the queues depending on the direction of an inequality. This extends previous studies on flexible systems to the case where the capacity varies over time. We then show in the unrestricted case that the optimal number of workers to have in the system is non-decreasing in the number of customers in either queue.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47647/1/11134_2005_Article_2441.pd

A framework for effective optimization via simulation

Issued as final reportNational Science Foundatio

Efficient simulation techniques for comparing constrained systems

Issued as final reportNational Science Foundation (U.S.

Desacyl-ghrelins påverkan på han- och honråttors alkoholkonsumtion

Alkoholberoende är en av de vanligaste psykiatriska sjukdomarna i världen. Det är en komplex neuropsykiatrisk sjukdom som orsakar mycket lidande, hög sjukdomsbörda samt betydande dödlighet. Trots detta finns det bara fyra läkemedel på marknaden och som ofta endast har en begränsad effekt. Sedan millennieskiftet har man börjat studera kopplingen mellan aptitreglerande hormoner och alkoholberoende. Detta eftersom att man bl.a. har hittat receptorer för aptit-reglerande hormoner i belöningscentra i hjärnan. Sammanfattningsvis tyder forskningen på att det finns ett överlapp mellan reglering av belöning från mat och alkohol. Ett av hormonerna som har studerats är ghrelin – ett hormon som bl.a. ökar aptiten – och har setts öka alkoholintag och belöning. Nyligen har man börjat studera desacyl-ghrelin (DAG) – en av ghrelins nedbrytningsprodukter – som till viss del har effekter motsatta till ghrelin. Därför ville vi undersöka om DAG påverkar alkoholintag hos han- och honråttor. Dessutom ville vi undersöka om DAG påverkar den motoriska aktiviteten i han- och honråttor. Vi gjorde två experiment. I ett av experimenten hade vi både han- och honråttor som fick tillgång till en alkohol-vattenlösning tre dagar i veckan under 12 veckor. Efter dessa 12 veckor delades de in i grupper där varje grupp fick en injektion av en av tre doser DAG eller saltlösning. Vi mätte därefter alkohol-, vatten- och matintag efter 1, 4 och 24 timmar. I det andra experimentet hade vi han- och honråttor som fick en injektion av en av tre doser DAG eller saltlösning innan vi mätte deras motoriska aktivitet i aktivitetsboxar. I det första experimentet såg vi först ingen skillnad på alkoholintag när vi tittade på alla doser. Däremot, när vi jämförde den högsta dosen med saltlösning såg vi en minskning i alkoholintag mellan 1:a och 4:e timmen hos honorna och mellan 4:e och 24:e timmen hos hanarna. I det andra experimentet såg vi ingen skillnad i den motorisk aktiviteten beroende på dos. Detta säger oss att vi såg en minskning i alkoholintag och den uppmätta minskningen beror inte på skillnad i motorisk aktivitet, t.ex. sedering – som gör att råttorna blir trötta och då dricker mindre. Denna studien är dock inte designad för att ge oss ett fullständigt svar på hur DAG påverkar alkoholintag i råttor - den ger oss en basal och relativt grov förståelse för DAGs effekter på alkoholintag. Sammanfattningsvis så har vi fått resultat som tyder på att DAG minskar alkoholintag i han- och honråttor. Av detta drar vi slutsatsen att DAG är en substans som är värd att studera vidare för en mer utförlig förståelse av dess effekter samt för att belysa om DAG är en potentiell farmakologisk behandling mot alkoholberoende.Since the new millenium, multiple studies have investigated the appetite-regulating peptide ghrelin’s effect on alcohol intake and reward in rodents. Particularly, ghrelin receptor (growth hormone secretagogue receptor 1a) antagonists have been of interest as they decrease alcohol intake, reward and conditioned place preference in rodents. Recently, attention has been drawn to one of ghrelin’s metabolites – des-acyl ghrelin (DAG) – a peptide previously thought to be inert. Preclinical studies have reported DAG to have multiple effects, some of which are opposing to ghrelin. As the role of DAG in relation to reward and addiction is unknown, we wanted to study DAGs effect on alcohol intake in rats. We therefore chose to (1) examine the effects of an acute injection with DAG on alcohol intake in male and female rats and (2) investigate how an acute injection with DAG influences locomotor activity in alcohol-naïve male and female rats. After 12 weeks of voluntary alcohol consumption, 24 male and 24 femalre rats were injected with multiple different doses of DAG. When analysed with one-way ANOVA, no significant difference was noted in alcohol intake. A t-test was performed for the highest dose of DAG (1 mg/kg) and revealed a significant decrease in alcohol intake and preference for both male and female rats. No differences were observed in the locomotor activity experiment at any given dose when analysed with one-way ANOVA. These findings suggest that DAG decreases alcohol intake in male and female rats and that this decrease is not due to changes in locomotor behaviour. We suggest that DAG should be considered a candidate for further preclinical studies in search of novel treatments for alcohol use disorders

Optimal Control of Noncollaborative Servers in Two-Stage Tandem Queueing Systems

We consider two-stage tandem queueing systems with dedicated servers in each station and a flexible server that is trained to serve both stations. We assume no arrivals, exponential service times, and linear holding costs for jobs present in the system. We study the optimal dynamic assignment of servers to jobs assuming a noncollaborative work discipline with idling and preemptions allowed. For larger holding costs in the first station, we show that (i) nonidling policies are optimal and (ii) if the flexible server is not faster than the dedicated servers, the optimal server allocation strategy has a threshold-type structure. For all other cases, we provide numerical results that support the optimality of threshold-type policies. Our numerical experiments also indicate that when the flexible server is faster than the dedicated server of the second station, the optimal policy may have counterintuitive properties, which is not the case when a collaborative service discipline is assumed. (C) 2014 Wiley Periodicals, Inc