Henrik Prinzhorn

Project engineer
Practice Areas:
Production planning & control, operation research, simulation of material flow
+49 (0)511 279 76-446


Rising and increasingly volatile energy prices resulting from increased power feeds from renewable sources such as solar and wind energy are confronting manufacturers with new challenges. If these companies procure their power supplies at ?uctuating short-term prices from electricity exchanges or through energy purchasing pools, they can in?uence the result-ing energy costs through production control via its actuating variables while energy consumption remains constant. A form of sequencing that decides at short notice which order will be processed next shows particularly high potential. The energy price-oriented sequencing rule that is introduced in this article prioritises orders with a high energy requirement at times when energy prices are low and gives precedence to orders that require less energy at times when energy prices are high, without neglecting the scheduled completion deadline. However, this sequencing rule can only be applied e?ectively under certain preconditions. These are elaborated in this article by means of a simulation study that will con?rm the way the rule functions.

production planning and control, manufacturing control, sequencing, energy costs

Lot sizing is an important task of production planning and control: basis of lot sizes are order change costs and costs for storage. Models for lot sizing do not consider lot size dependent maintenance costs. However, for a forging company the tool wear is very important, because the tooling costs represent a major part in the production cost. In this article, the deter-ministic lot size model of Andler is extended with lot size dependent maintenance costs. For this purpose, the correlation between lot size and the tool wear is ?rst derived in order to develop a lot size dependent wear function. The linking of a lot size dependent wear function with maintenance costs results in a lot size dependent maintenance cost function, which can be integrated into existing lot size models with a customized total cost function. The validation of the extended lot size model consists of two parts. In the ?rst part, the functionality of the extended lot size model is validated. In the second part, a sensitivity analysis of the lot size is carried out with regard to lot size dependent costs and unit costs.

lot sizing, tool wear, forging industry, sensitivity analysis

In production, product-related error costs can be reduced by focusing on human production factors, such as considering human performance fluctuations during the day, when production planning with respect to job-shop scheduling. In this article, the flexible job-shop scheduling problem is extended by considering product-related error costs and logistic costs. Product-related error costs are increased by over stressing the operative workers. Logistic costs are based on work in process and throughput time. This cost-based definition enables a production plan to be simultaneously optimized in respect of both error and logistic costs. The product-related error costs and flexible job-shop scheduling problem are described mathematically and a memetic algorithm is also presented as an approach. Within the memetic algorithm, the evolutionary process is supplemented with a local search procedure to improve the ability of solutions and repair procedures to rectify infeasible solutions. The influence of product-related error costs on the total costs of a production plan within job-shop scheduling is presented.

flexible job shop scheduling, memetic algorithm, human perfor-mance fluctuation, error costs

In this article, the flexible job-shop scheduling problem is extended by consideration of energy costs which arise owing to the power peak, and further decision variables such as work in process and throughput time are incorporated into the objective function. This enables a production plan to be simultaneously optimized in respect of the real arising energy and logistics costs.

The energy-costs-aware flexible job-shop scheduling problem (EFJSP) which arises is described mathematically, and a memetic algorithm (MA) is presented as a solution. In the MA, the evolutionary process is supplemented with a local search. Furthermore, repair procedures are used in order to rectify any infeasible solutions that have arisen in the evolutionary process. The potential for lowering the real arising costs of a production plan through consideration of energy consumption levels is highlighted.

memetic algorithm, flexible job-shop scheduling, energy-costs, power peak

High temperatures up to 1280 °C and high pressures during the forming opperation lead to strong tool wear in forging processes. Increasing tool wear can lead to very high costs. By experiments conducted at the Institut für Integrierte Produktion in Hanover the correlation between tool wear and lot size in hot forging processes was verfied. The findings will help companies to optimise maintenance procedures and therefore reduce cost in the future.

forging, steel, tool wear, lot size

One research topic of the Institut für Integrierte Produktion Hannover (IPH) is the avoidance of worker overload in production during job shop scheduling to eliminate production errors, improve product quality and reduce cost.

job shop scheduling, human performance curve, error costs

Large-scale products are often assembled according to the job-site principle, meaning the product is not moved through the factory while assembly but located at a fixed position. Though, during the assembly, the area requirements of such a product are constantly changing. On one hand, the product itself is growing with each assembly step, while on the other hand varying areas for storage, machines or working areas are temporarily required. This is an important factor when arranging products to be assembled within the factory. Currently, it is common to reserve a fixed area for each product to avoid overlaps or collisions with other assemblies. Intending to be large enough to include the product and all adjacent areas, this reserved area corresponds to the superposition of the maximum extents of all required areas of the product. In this procedure, the reserved area is usually poorly utilized over the course of the entire assembly process; instead a large part of it remains unused. If the available area is a limited resource, a systematic arrangement of the products, which complies with the dynamic area requirements, will lead to an increased area utilization and productivity. This paper presents the results of a study on the arrangement of assembly objects assuming dynamic, competing area requirements. First, the problem situation is extensively explained and existing research on associated topics is described and evaluated on the possibility of an adaptation. Then, a newly developed mathematical optimization model is introduced. This model allows an optimal arrangement of dynamic areas, considering logical and practical constraints. Finally, in order to quantify the potential of the developed method, some test series results are presented, showing the possible increase in area utilization.

XXL products, dynamic area arrangement, layout planning

In production, product-based failure costs can be reduced by focusing the production factor „human“. Therefore, human performance fluctuations during the course of day have to be considered in the production planning and control. This paper presents an approach for quality-orientated flexible job shop scheduling, taking into account human performance fluctuations during the day.

production planning and control, performance curve, quality

Assembling large-scale products involves frequent process interruptions why in order to reduce the impact of interruptions, a short-term response is necessary to reduce delivery delays and idle times of resources. An approach to challenge this represents the flexibility of a production system. Regarding the environment of large-scale product assembly, it has to be shown which potentials of flexibility are able to be used in a short-term manner.

assembly, production management, disruption management

Considering production planning and control, lot sizing is a very important task. Lot sizes are usually resorted to lot size dependent order and storage costs. However, models for lot sizes ignore the aspect of lot size dependent maintenance costs. For forging companies the tool wear has a great economic importance, because the tooling costs represents a major factor in production costs.

production planning and control, lot sizing, process stability

Assembling large-scale products, such as airplanes, locomotives, or wind turbines, involves frequent process interruptions induced by e.g. delayed material deliveries or missing availability of resources. This leads to a negative impact on the logistical performance of a producer of xxl-products. In industrial practice, in case of interruptions, the identification, evaluation and eventually the selection of an alternative order of assembly activities (‘assembly alternative’) leads to an enormous challenge, especially if an optimized logistical decision should be reached. Therefore, in this paper, an innovative, optimization model for the identification of assembly alternatives that addresses the given problem is presented. It describes make-to-order, large-scale product assembly processes as a resource constrained project scheduling (RCPS) problem which follows given restrictions in practice. For the evaluation of the assembly alternative, a cost-based definition of the logistical objectives (delivery reliability, inventory, make-span and workload) is presented.

assembly sequencing, logistical objectivs, optimization modell

Disassembling of large-scale products (e. g. wind energy plants, crane and conveyor systems which are known as XXL-Products) becomes increasingly important, as the operating time is limited. The challenge is to optimize the positioning of the complex and expensive disassembling on the operation site on the one hand and the costly transportation of modules to the disassembling factories. For this reason the location and allocation problem will be pictured in a mathematical model within the research project “DemoNet”. This model supports companies to create a XXL-disassembly-network. The research hypothesis assumes: a disassembly under ecological, economical and logistical aspects constitutes the optimum. The extension of a location planning tool forms the basis of the mathematical optimization model for the disassembly of XXL-Products. For solving the problem a genetic algorithms will be used. The result helps companies to arrange disassembling networks for XXL-Products efficiently.

dismantling planing, dismantling networks, impact model

Assembling large-scale products involves frequent process interruptions. To reduce the duration of an assembly stop, the WZL of RWTH Aachen University and the IPH Hannover developed an approach which is able to identify alternative assembly plans and to evaluate them on the basis of the logistic performance objectives. Stock, capacity utilization, lead time and adherence to schedule are transferred into costs and assembly plans are thus comparable based on one parameter.

production planning and control, assembly, logistical objectives

The assembly of large-scale products is usually organized in the construction-site principle. For each assembled product, the area requirement increases with assembly time. So for each product a defined area is reserved, that corresponds to the maximum expansion of the product. As the existing area is limited, the single products are competing for it. This results in an increased area requirement and a poor use of area. Therefore time-variable and competing area requirements should be considered in the facility layout planning.

layout problem, time dynamics, competing areas

Assembling large-scale products involves frequent process interruptions induced by e.g. delayed material deliveries or missing availability of resources. Our approach for identifying alternative assembly sequences by analyzing the product structure and process dependencies allows for continuing with the assembly process in case of interruptions and therefore increases the process efficiency.

large-scale product, adaptive assembly, alternative assembly process

By leveraging the production factor "human" it is possible by taking into account human performance fluctuations during the day to reduce product-related defect costs and increase quality. To this end, the integration of human performance variations in methods of production planning and control must be carried out. This article introduces a method by which a quality-oriented job shop schedule can be created, taking into account human performance fluctuations.

produktion planning, job shop scheduling, human performance fluctuation, optimization problem

A xxl-product is defined as product, whose production costs increase disproportionately in relation to further growth of a characteristic product feature. Existing models of supply chain design are not designed for xxl-products. For this reason a method was developed to create a supply chain for a xxl-product. The supply chain design works in due consideration of the most relevant xxl-product specific features. The created supply chain can be evaluated for their logistical quality. Therefore a comparison between different supply chain variants is possible.

xxl-product, supply chain design, logistical objectives

The assembly of complex, large-scale products is often characterized by poor date availability, limited process standardization, frequent product modifications and flexible usage of resources, which often leads to interruptions in the assembling process. Therefore, in a research project between the IPH Hannover and the WZL of RWTH Aachen University, a methodology for assembly processes is developed which allows the identification and evaluation of process alternatives in case of interruptions. For the evaluation of those process alternatives logistical objectives are used.

xxl-product, adaptive assembly scheduling, interruption assembly process, logistical objectives

Research projects