Katja Schimmelpfeng

Setting inventory levels of CONWIP flow lines via linear programming

This paper treats the problem of setting the inventory level and optimizing the buffer allocation of closed-loop flow lines operating under the constant-work-in-process (CONWIP) protocol. We solve a very large but simple linear program that models an entire simulation run of a closed-loop flow line in discrete time to determine a production rate estimate of the system. This approach introduced in Helber, Schimmelpfeng, Stolletz, and Lagershausen (2011) for open flow lines with limited buffer capacities is extended to closed-loop CONWIP flow lines. Via this method, both the CONWIP level and the buffer allocation can be optimized simultaneously. The first part of a numerical study deals with the accuracy of the method. In the second part, we focus on the relationship between the CONWIP inventory level and the short-term profit. The accuracy of the method turns out to be best for such configurations that maximize production rate and/or short-term profit.

Characteristics of service requests and service processes of fire and rescue service dispatch centers

A sufficient staffing level in fire and rescue dispatch centers is crucial for saving lives. Therefore, it is important to estimate the expected workload properly. For this purpose, we analyzed whether a dispatch center can be considered as a call center. Current call center publications very often model call arrivals as a non-homogeneous Poisson process. This bases on the underlying assumption of the caller’s independent decision to call or not to call. In case of an emergency, however, there are often calls from more than one person reporting the same incident and thus, these calls are not independent. Therefore, this paper focuses on the dependency of calls in a fire and rescue dispatch center. We analyzed and evaluated several distributions in this setting. Results are illustrated using real-world data collected from a typical German dispatch center in Cottbus (“Leitstelle Lausitz”). We identified the Pólya distribution as being superior to the Poisson distribution in describing the call arrival rate and the Weibull distribution to be more suitable than the exponential distribution for interarrival times and service times. However, the commonly used distributions offer acceptable approximations. This is important for estimating a sufficient staffing level in practice using, e.g., the Erlang-C model.

Structuring the Planning Tasks in Biomass-Based Supply Chains

Due to the dependency on crude oil, mitigating, greenhouse gas emissions, energy and food security, support for rural economic development and the effort for environmental sustainability, renewable energy sources become more and more important. This led to an increasing research on biomass-based supply chains in recent years. Conventional supply chains, i.e., the commodity flow including all the stakeholders from the supplier to the end customer, have been studied intensively in the past. Biomass-based supply chains, however, feature different characteristics and uncertainties that have to be considered. In this paper, we identify the differences between the two types of supply chains and elaborate the stakeholders involved in the supply chain process and the different planning tasks structured according to the functional areas. As several possible pathways from feedstocks to different end-products exist, we focus on bio-fuels as the final product. We conclude by reviewing the literature that deals with supply chain optimization using operations research (OR) models to present the relevant planning tasks in biomass-based supply chains.

Verfügbarkeitssicherung hochautomatisierter Produktionssysteme

Aus dem Einsatz flexibler und hochautomatisierter Produktionssysteme resultiert aus primar finanzieller Perspektive eine bedeutende Kapitalbindung (immense Anschaffungsauszahlungen), in produktions- und auftragsbezogener Sichtweise ergibt sich die Forderung nach hoher Verfugbarkeit der Systeme in Abhangigkeit von der Auftragslage. Das Spektrum der Verfugbarkeitssicherung geht daher uber die Aufgabengebiete der klassischen Instandhaltung hinaus. Die letztgenannte umfast — wie schon in Abschnitt 3.2.2.2 erwahnt — samtliche Aktivitaten der Planung, Durchfuhrung und Kontrolle zur Sicherung, Wiederherstellung und Verbesserung der Anlagenfunktionsfahigkeit.1 Dazu zahlen nach der Definition gemas DIN 31051: • die regelmasige Pflege von Anlagenteilen (in Form von Reinigung, Schmierung und ggf. Austausch) als umfassende Wartung zur Bewahrung des Sollzustandes,2 • die Inspektion, bei der in regelmasigen Abstanden der Zustand der Anlagenteile erfast3 und hinsichtlich der Ubereinstimmung mit dem Sollzustand uberpruft wird, sowie • die Instandsetzung, in deren Rahmen Anlagenteile bei storungsbedingten Ausfallen bzw. aufgrund geplanter Masnahmen erneuert werden.