Willem H.M. Zijm

Models for warehouse management: Classification and examples

In this paper we discuss warehousing systems and present a classification of warehouse management problems. We start with a typology and a brief description of several types of warehousing systems. Next, we present a hierarchy of decision problems encountered in setting up warehousing systems, including justification, design, planning and control issues. In addition, examples of models supporting decision making at each of these levels are discussed, such as distribution system design, warehouse design, inventory management under space restrictions, storage allocation, and assignment and scheduling of warehouse operations.

Generic planning and control of automated material handling systems

This paper discusses the problem to design a generic planning and control architecture for automated material handling systems (AMHSs). We illustrate the relevance of this research direction, and then address three different market sectors where AMHSs are used, i.e., baggage handling, distribution, and parcel & postal. The research in this paper is heavily motivated by a collaboration between the authors and a major global company supplying AMHSs. We analyze requirements from practice for a generic control architecture, and then review the literature to investigate whether these practical requirements have been met. From this confrontation of theory with practice, we conclude that many practical issues are not yet covered in the current literature. We take the initiative to define a research direction in concrete terms, pinpoint problems to work on, and propose an agenda for future research. Moreover, we take a step to propose a concept control architecture.

Capacity planning and lead time management

In this paper we discuss a framework for capacity planning and lead time management in manufacturing companies, with an emphasis on the machine shop. First we show how queueing models can be used to find approximations of the mean and the variance of manufacturing shop lead times. These quantities often serve as a basis to set a fixed planned lead time in an MRP-controlled environment. A major drawback of a fixed planned lead time is the ignorance of the correlation between actual work loads and the lead times that can be realized under a limited capacity flexibility. To overcome this problem, we develop a method that determines the earliest possible completion time of any arriving job, without sacrificing the delivery performance of any other job in the shop. This earliest completion time is then taken to be the delivery date and thereby determines a workload-dependent planned lead time. We compare this capacity planning procedure with a fixed planned lead time approach (as in MRP), with a procedure in which lead times are estimated based on the amount of work in the shop, and with a workload-oriented release procedure. Numerical experiments so far show an excellent performance of the capacity planning procedure.

Service differentiation in spare parts supply through dedicated stocks

We investigate the option of keeping dedicated stocks at customer sites in addition to stock kept at some central location as a tool for applying service differentiation in spare parts supply. We study the resulting two-echelon system in a multi-item setting, both under backordering and under the use of emergency shipments (equivalent to lost sales for the inventory system). In an extensive computational experiment, we show that dedicated stocks have significant added value: the approach results in clear savings compared to a strategy of providing all customers with uniform service. Furthermore, the savings found with dedicated stocks are close to those with critical level policies, with dedicated stocks usually being much easier to implement in practice.

The Integration of Process Planning and Shop Floor Scheduling in Small Batch Part Manufacturing

In this paper we explore possibilities to cut manufacturing leadtimes and to improve delivery performance in a small batch part manufacturing shop by integrating process planning and shop floor scheduling. Using a set of initial process plans (one for each order in the shop), we exploit a resource decomposition procedure to determine schedules to determine schedules which minimize the maximum lateness, given these process plans. If the resulting schedule is still unsatisfactory, a critical path analysis is performed to select jobs as candidates for alternative process plans. In this way, an excellent due date performance can be achieved, with a minimum of process planning and scheduling effort.

A Decision Support System for Ship Maintenance Capacity Planning

In this paper, the basic framework and algorithms of a decision support system are discussed, which enhance process and capacity planning at a large repair shop. The research is strongly motivated by experiences in a project carried out at a dockyard, which performs repair, overhaul and modification programs for various classes of navy ships. We outline the basic requirements placed upon order acceptance, process planning and capacity scheduling for large maintenance projects. In subsequent sections a number of procedures and algorithms to deal with these requirements, in particular a procedure for workload-based capacity planning, a database system to support process planning are developed, as well as a resource-constrained project scheduling system to support work planning at a more detailed level. The system has been designed to support decision making at the Navy Dockyard in particular, however, we believe that, due to its generic structure, it is applicable to a wide range of project-based manufacturing and maintenance environments.

Process planning for a modular component placement system

This paper discusses the design of a hierarchically structured process planning system for a printed circuit board assembly line. Basically, the assembly system consists of a series of independently operating placement modules, connected by a carrierless conveyor system. Both the modules and the conveyor system are fully automated, while the complete system is also under centralized computer control. We show how the process planning problem is naturally decomposed into a series of hierarchically coupled subproblems, each of a combinatorial nature. Models for all subproblems are developed and solution techniques are briefly indicated. Numerical results for a number of industrial cases are also discussed

Building holarchies for concurrent manufacturing planning and control in EtoPlan

The increasing versatility in order characteristics calls for planning and control systems that are able to evolve in time. Traditional hierarchical systems are usually based on a function-oriented static control structure in which all orders and products are handled similarly. Due to the dynamically changing characteristics of manufacturing environments these static control structures are not suitable anymore. Hence, a concept description and a prototype implementation for concurrent manufacturing planning and control (EtoPlan) based on multiple and temporary hierarchies (holarchies) are presented. The alternative control structure allows among other things to bridge the gap between process planning and production planning. The EtoPlan order planning method explicitly models the uncertainty in the information due to incompleteness of process planning information and shop floor randomness.

Selecting parts for additive manufacturing in service logistics

PurposenFor more than ten years, the value of additive manufacturing (AM) for after-sales service logistics has been propagated. Today, however, only few applications are observed in practice. In this paper, possible reasons for this discrepancy are discussed and a method is developed to simplify the identification of economically valuable and technologically feasible business cases.nnDesign/methodology/approachnThe approach is based on the Analytic Hierarchy Process (AHP) and relies on spare part information that is easily retrievable from the company databases. This has two advantages: first, the approach can be customized towards specific company characteristics, and second, a very large number of spare parts may be assessed simultaneously. A field study is discussed in order to demonstrate and validate the approach in practice. Furthermore, sensitivity analyses are performed to evaluate the robustness of the method.nnFindingsnResults provide evidence that the method allows a valid prioritization of a large spare part assortment. Also, sensitivity analyses clarify the robustness of the approach and illustrate the flexibility of applying the method in practice. More than 1000 positive business cases of AM for after-sales service logistics have been identified based on the method.nnOriginality/valuenThe developed method enables companies to rank spare parts according to their potential value when produced with AM. As a result, companies can evaluate the most promising spare parts first. This increases the effectiveness and efficiency of identifying business cases and thus may support the adoption of AM in after-sales service supply chains.

Multi-resource scheduling of an FMC in discrete parts manufacturing

In this paper we develop a scheduling procedure for a Flexible Manufacturing Cell (FMC), consisting of a number of basically identical machining centres and operating as part of a larger component manufacturing shop (basically a hybrid job shop). Each job on the FMC is subject to release and due dates; the objective of the scheduling procedure is to minimize the maximum lateness, subject to both machine and cutting tool capacity constraints.