Oliver Holthaus

A comparative study of dispatching rules in dynamic flowshops and jobshops

This paper presents a comparative study on the performance of dispatching rules in the following sets of dynamic manufacturing systems: flowshops and jobshops, and flowshops with missing operations and jobshops. Three new dispatching rules are proposed. We consider a total of 13 dispatching rules for the analysis of the relative performance with respect to the objectives of minimizing mean flowtime, maximum flowtime, variance of flowtime, proportion of tardy jobs, mean tardiness, maximum tardiness and variance of tardiness. First, we carry out the simulation study in flowshops with jobs undergoing processing on all machines sequentially and in jobshops with random routeing of jobs. The results of the study reveal some interesting observations on the relative performance of the dispatching rules in these two types of manufacturing systems. Next, we consider flowshops with missing operations on jobs and jobshops with random routeing of jobs. We observe some interesting results in the sense that the performance of dispatching rules is being influenced by the routeing of jobs and shopfloor configurations.

Efficient dispatching rules for scheduling in a job shop

Abstract We consider in this article the development of new and efficient dispatching rules with respect to the objectives of minimizing mean flowtime, maximum flowtime, variance of flowtime, proportion of tardy jobs, mean tardiness, maximum tardiness and variance of tardiness. We present five new dispatching rules for scheduling in a job shop. Some of these rules make use of the process time and work-content in the queue of the next operation on a job, by following a simple additive approach, in addition to the arrival time and dynamic slack of a job. An extensive and rigorous simulation study has been carried out to evaluate the performance of the proposed dispatching rules against those rules such as the SPT, WINQ, FIFO and COVERT, and the best existing rule. It has been observed that the proposed rules are not only simple in structure, but also quite efficient in minimizing several measures of performance. The important aspects of the results of experimental investigation are also discussed in detail.

Scheduling in job shops with machine breakdowns: an experimental study

Abstract This paper considers the simulation-based analysis of dispatching rules for scheduling in dynamic job shops taking into account interruptions on the shop floor. With respect to flowtime and due date-based objectives, the relative performance of well-known, recently proposed as well as some new dispatching rules is evaluated for different settings of the model parameters. The results of the simulation study reveal that the relative performance of scheduling rules can be affected by changing the levels of the breakdown parameters. For the standard model, where all machines are continuously available, as well as for the models taking into account breakdowns of machines, it is shown that for minimizing mean flowtime the performance of one recently proposed rule (PT+WINQ) is significantly better than the performance of all other rules. Analogously it is shown that for minimizing maximum flowtime, and for minimizing variance of flowtime, one new rule (AT−RPT) is superior to all other rules. With respect to due date-based objectives the relative performance of the analyzed scheduling rules is more sensitive to the percentage of time the machines have failures and the mean time to repair.

Efficient jobshop dispatching rules: Further developments

Many dispatching rules for scheduling in dynamic jobshops have been proposed over many years. The research issues in jobshop scheduling seem to be still open in the sense that no single rule has been found to be the best under all shopfloor conditions even with respect to one single measure of performance. Added to this problem, there are several measures of performance, e.g. the minimizations of mean, maximum and variance of flowtime, percentage of tardy jobs, and mean, maximum and variance of tardiness of jobs. Recent studies have reported the development of more efficient dispatching rules than the popular rules, e.g. SPT, COVERT, MOD and ATC. This study is an attempt to improve some of the recently reported dispatching rules. An extensive simulation study reveals that the improved rules developed in the present study appear to be quite effective in minimizing mean flowtime, and maximum tardiness and variance of tardiness of jobs.

Design of efficient job shop scheduling rules

This paper considers the design and simulation-based analysis of dispatching rules for scheduling in dynamic job shops. Based upon the combination of well-known rules new scheduling rules are developed. For evaluating the relative performance of the dispatching rules the criteria of mean and maximum flowtime, variance of flowtime, mean and maximum tardiness, percentage of tardy jobs and variance of tardiness have been chosen. The results of the simulation study reveal that the simple structured combination rules are quite efficient.

A fast ant-colony algorithm for single-machine scheduling to minimize the sum of weighted tardiness of jobs

The problem of scheduling on a single machine is considered in this paper with the objective of minimizing the sum of weighted tardiness of jobs. A new ant-colony optimization (ACO) algorithm, called fast ACO (FACO), is proposed and analysed for solving the single-machine scheduling problem. By considering the benchmark problems available in the literature for analysing the performance of algorithms for scheduling on a single machine with the consideration of weighted tardiness of jobs, we validate the appropriateness of the proposed local-search schemes and parameter settings used in the FACO. We also present a comparison of the requirements of CPU time for solving the single-machine total-weighted tardiness problem by the FACO and the existing algorithms.

Decomposition approaches for solving the integer one-dimensional cutting stock problem with different types of standard lengths

Abstract This paper considers the integer one-dimensional cutting stock problem with different types of standard lengths and the objective of cost minimization. Decomposition approaches based on the well-known column-generation technique for exactly solving the continuous relaxation of the problem and specific methods for separately solving the arising residual problems are proposed. For evaluating the performance of the different approaches, more than 12,000 randomly generated test problems have been solved. The computational experiments prove the effectiveness of one of the proposed decomposition procedures with respect to the computing effort and the quality of the solutions obtained.

New dispatching rules for scheduling in a job shop — An experimental study

We present two new dispatching rules for scheduling in a job shop. These rules combine the process-time and work-content in the queue for the next operation on a job, by making use of additive and alternative approaches. An extensive and rigorous simulation study has been carried out to evaluate the performance of the proposed dispatching rules compared with those by the SPT rule, the WINQ rule, a random rule based on the SPT and WINQ rules, and the best existing rule. The important aspects of the results of the experimental investigation are also discussed in detail.

A study on the performance of scheduling rules in buffer-constrained dynamic flowshops

Most studies on scheduling in manufacturing systems using dispatching rules deal with jobshops, while there are only few reports dealing with dynamic flowshops. It is known that the performance of many dispatching rules in dynamic jobshops is different from that in dynamic flowshops. Moreover, many research reports assume that there are no buffer constraints in the shop, and even those reports dealing with buffer-constrained shops present the evaluation of existing dispatching rules for unconstrained shops in the context of buffer constraints with the consideration of a limited number of objectives of scheduling. In this study, we deal with the problem of scheduling in dynamic flowshops with buffer constraints. With respect to different time-based objectives, the best dispatching rules for scheduling in unconstrained shops have been identified from the existing literature. In addition, two new dispatching rules specially designed for flowshops with buffer constraints are proposed. All dispatching rules under consideration are evaluated in dynamic flowshops with buffer constraints on the basis of an extensive simulation study covering different levels of buffer constraints, shop load or utilization, and missing operations in flowshops. The proposed rules are found to perform better than the existing dispatching rules in buffer-constrained flowshops with respect to many measures of performance.

On the best number of different standard lengths to stock for one-dimensional assortment problems

Abstract This paper considers the one-dimensional assortment problem (1D-AP) which includes the determination of the number of different sizes of standard lengths to be maintained as inventory and to be used to fulfill a set of cutting orders. Keeping two or more types of stock lengths in inventory is generally a better practice with respect to the total material cost, when compared to a single type of stock lengths. However, a greater variety in the number of different stock lengths results in increased complexity of operation, stocking and handling. The purpose of this paper is therefore to evaluate the possible savings in the total material cost which can be realized by using an assortment with two or more types of stock lengths, compared to an assortment with a single type of stock lengths. By investigating a large number of problem instances of different classes of the 1D-AP, it is shown that there exist problem classes for which it is possible to realize substantial savings in the total material cost by using an assortment with two, three or four types of standard lengths. However, the results of the computational study also reveal that there are other problem classes for which it is not possible to realize savings of more than 0.5%, even when an assortment with five or more different stock sizes can be used.