Jacob Wijngaard

The outpatient appointment system: Design of a simulation study

Abstract This paper describes the model-construction of a simulation study. The purpose of this study was to produce a general method for determining a suitable appointment system for the clinics in the outpatient department of a hospital. The original model contained 11 variables. Investigation of the influence of each variable on patients wainting-time and doctors idle-time showed that a considerable reduction in the number of variables could be achieved. Only 5 variables were finally left in the stimulation. The use of the results of this study in a real-life clinic situation is discussed elsewhere.

Embedding JIT into MRP

Today many companies who are using MRP production control systems are investigating how they can produce some or all of their products using just-in time (JIT) principles. They wonder to what extent MRP can provide support for JIT production. This paper describes how JIT can be embedded into MRP. A three-step framework is described which makes use of MRPs backflushing and phantom features, and permits JIT principles to be used to the furthest extent possible. The JIT benefits that will be reported by MRP, as well as those that remain hidden because of MRP, are outlined. Observations from work with North American and Western European manufacturers are also discussed.

Designing and phasing in just-in-time production systems

Japanese just-in-time (JIT) production systems have been described extensively in books, professional magazines, and academic journals. These systems are so different from the production systems currently in use in North America and Europe that it appears that the only way to implement JIT is to dismantle the current production system on one day and begin a full JIT system the next day. In this paper we describe a three-step process for gradually phasing in JIT. The process begins with a two-bin inventory system, moves to a pull system with kanban, and then to a continuous flow production system. An illustrative example is presented.

Forward recursion for Markov decision processes with skip-free-to-the-right transitions. Part I. Theory and algorithm

We consider a Markovian decision process with countable state space states 0, 1, 2,... which is skip-free to the right a transition from i to j is impossible if j >i + 1. In this type of system it is easy to calculate by forward recursion the maximal total expected reward going from state 0 to state i; the same can be done, of course, for the case where a constant g is subtracted from the one-period reward function g-revised reward. Let -wgi be the maximal total expected g-revised reward going from state 0 to state i. We show that wg· satisfies the average-reward optimality equation. If wg· satisfies a growth condition, then g = g*, the maximal average reward. For all other g, the function wg increases or decreases so fast that this cannot be the case. Thus, in principle the solution wg can be used to check if g g*, which suggests a method for approximating g* and an associated average-return optimal policy. We develop an efficient algorithm based on this idea. In a companion paper we shall show how the algorithm, or modifications of it, can be applied to some special cases, such as control of arrivals to a queue, control of the service rate, and controlled random walks.

Stationary Markovian Decision Problems and Perturbation Theory of Quasi-Compact Linear Operators

In this paper stationary Markov decision problems are considered with arbitrary state space and compact space of strategies. Conditions are given for the existence of an average optimal strategy. This is done by using the fact that a continuous function on a compact space attains its minimum. To prove the continuity of the average costs as function on the space of strategies some perturbation results for quasi-compact linear operators are used. In a first set of conditions the boundedness of the one-period cost functions and the quasi-compactness of the Markov processes are assumed. In more general conditions the boundedness of the cost functions is replaced by the boundedness, on a subset A of the state space, of the recurrence time and costs until A and the quasi-compactness of the Markov processes are replaced by the quasi-compactness of the embedded Markov processes on A.

Decomposition for dynamic programming in production and inventory control

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Local and integral control of workload

textabstractIn most of the literature on performance analysis of production systems, buffer are assumed to be controlled locally. In automated production systems buffers are not always the result of local physical space restrictions, but may also be software- controlled. Software-controlled buffers allow a more efficient use of the production units and a better control of the work in progress. Some of these possible control principles are known from production control and goods flow theory. We present a model for production systems in which the goods flow is continuous. The effect of different control mechanisms on the performance of the system (that is throughput and throughput time) is studied. Examples of control systems are BSC (Base Stock Control), Workload Control and control by local buffers only. It appears that for our model many of these control systems yield equivalent performance measures.

A continuous flow model for three production units in series with buffers

In this paper we study three-stage production lines with intermediate buffers. The goods flow is supposed to be continuous and machine speeds may be different. Production units may be subject to stochastic failure and repair. We calculate the average line production rate for some cases. Furthermore we prove some production lines can be reduced to simpler two-stage ones.

Co-makership in the delivery of packaging materials

In many situations suppliers and customers are looking for ways to improve the co‐ordination of goods flow in both quantitative and qualitative ways. This is often called co‐makership. This article describes a case study of two companies setting up a co‐makership relationship. The relationship is evaluated on criteria like possible improvements, organisational impact and risk of the (increasing) (inter) dependence between the two companies. Both the benefits and the organisational impact of the relationship can be large. The risks of co‐makership are not easy to evaluate.

A Framework for Developing Production Control Systems: A Case in Coffee Roasting/Distributing

Production situations are unique. The way to control production has to be matched to its characteristics. The term production control concept stands for the heart of such a way of control. It is something like a proper view of the production situation; proper in the sense that on one hand the characteristics of the situation, as far as relevant for production control, are sufficiently taken into account, and on the other hand that it is suitable as a basis for developing rules, procedures, performance measures and information systems. A production control concept is the way production control is stratified. To develop a production control concept requires care. Meal [1984] in his plea for a hierarchical approach of production planning and control stressed already the difficulty of the idiosyncracy of production situations. A too standard approach has to be avoided indeed. But by stressing the uniqueness of the situation too much it may get difficult to apply available general production control principles.