C Rajendran

The problem of scheduling in dynamic conventional jobshops has been extensively investigated over many years. However, the problem of scheduling in assembly jobshops (i.e. shops that manufacture multi-level jobs with components and subassemblies) has been relatively less investigated in spite of the fact that assembly jobshops are frequently encountered in real life. A survey of literature on dynamic assembly jobshop scheduling has revealed that the TWKR-RRP rule is the best one for minimizing the mean flowtime and staging delay, and the job due-date (JDD) rule is the best for minimizing the mean tardiness of jobs. However, the objectives of minimizing the maximum flowtime (and maximum staging delay) and standard deviation of flowtime (and standard deviation of staging delay) are as important as the minimization of mean flowtime and mean staging delay. Likewise, the objectives of minimizing the maximum tardiness and standard deviation of tardiness are also as important as the minimization of mean tardiness. The reason is that the maximum and standard deviation values of a performance measure indicate the worst-case performance of a dispatching rule. The present study seeks to develop efficient dispatching rules to minimize the maximum and standard deviation of flowtime and staging delay, and the maximum and the standard deviation of conditional tardiness of jobs. The dispatching rules are based on the computation of the earliest completion time of a job and consequently determining the latest finish time of operations on components/subassemblies of a job. An extensive simulation-based investigation of the performance evaluation of the existing dispatching rules and the proposed dispatching rules has been carried out by randomly generating jobs with different structures and different shop utilization levels. It has been found from the simulation study that the proposed rules are quite effective in minimizing the maximum and standard deviation of flowtime and staging delay, and the maximum conditional tardiness and standard deviation of conditional tardiness. © 2002 Elsevier Science Ltd. All rights reserved.

In many manufacturing systems, jobs that are completed early are held as finished-goods inventory until their due-dates, and hence we incur earliness costs. Similarly, jobs that are completed after their due-dates incur penalty. The objective in such situations would, therefore, be to meet the due-dates of the respective jobs as closely as possible, and consequently minimize the sum of earliness and tardiness of jobs because earliness and tardiness of jobs greatly influence the performance of a schedule with respect to cost. In addition, a job incurs holding cost from the time of its arrival until its completion. Most studies on scheduling in such manufacturing systems assume unit earliness cost, unit tardiness cost and unit holding cost of a job. However, in reality such an assumption need not always hold and it is quite possible that there exist different costs of earliness, tardiness and holding for different jobs. In addition, most studies on job-shop scheduling assume that jobs are independent and that no assembly operations exist. The current study addresses the problem of scheduling in dynamic assembly job-shops (i.e. shops that manufacture multi-level jobs) with the consideration of jobs having different earliness, tardiness and holding costs. An attempt is made in this paper to present dispatching rules by incorporating the relative costs of earliness, tardiness and holding of jobs in the form of scalar weights. In the first phase of the study, relative costs (or weights for) earliness and tardiness of jobs are considered, and the dispatching rules are presented in order to minimize the sum of weighted earliness and weighted tardiness of jobs. In the second phase of the study, the objective considered is the minimization of the sum of weighted earliness, weighted tardiness and weighted flowtime of jobs, and the dispatching rules are presented by incorporating the relative costs of earliness, tardiness and flowtime of jobs. Simulation studies have been conducted separately for both phases of the current study, the performance of the scheduling rules have been observed independently, and the results of the simulation study have been reported. The proposed rules are found to be effective in minimizing the mean and maximum values of the measures of performance. © 2005 Elsevier Ltd. All rights reserved.

Most dispatching rules for job shop scheduling assume that the cost of holding per unit time is the same for all jobs. Likewise, it is assumed that the cost of tardiness per unit time is the same for all jobs. In other words, it is implied that the holding cost of a job is directly proportional to its flowtime, and the tardiness cost of a job is directly proportional to its positive lateness. These assumptions may not hold good in all situations. Some attempts were made to overcome this deficiency, and a couple of dispatching rules were proposed by considering different weights or penalties for different jobs. However, these dispatching rules assume that the holding and tardiness costs per unit time of a given job are the same, even though these costs may differ from job to job in practice. In this study, we propose dispatching rules by explicitly considering different weights or penalties for flowtime and tardiness of a job. Many measures of performance related to weighted flowtime and weighted tardiness of jobs are considered, and the results of simulation are presented. © 2003 Elsevier B.V. All rights reserved.

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 meas ures 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. © 2000, Taylor & Francis Group, LLC. All rights reserved.

This paper addresses the problem of dispatching in flowshops with bottleneck machines. The presence of bottleneck machines results in the restricted throughput in flowshops. The objective is to develop dispatching rules for scheduling by taking into account the presence of bottleneck machines. The measures of performance are the minimization of total flowtime of jobs, the minimization of the sum of earliness and tardiness of jobs, and the minimization of total tardiness of jobs, considered separately. Many existing conventional dispatching rules and the proposed dispatching rules have been extensively investigated for their performance by generating a large number of problems of various sizes and bottleneck conditions. The results of the experimental investigation show that the proposed dispatching rules emerge to be superior to the conventional dispatching rules. © 2006 Elsevier Ltd. All rights reserved.

This work is an investigation about the relative effectiveness of two approaches to scheduling in flexible flow shops: one approach advocating the possible use of different dispatch-ing rules at different stages of the flow shop, and the other suggesting the use of the same dispatching rule at all the stages of the flow shop. In the latter approach, the dispatching rule contains the information related to both process time and duedate. Both approaches aim at the minimization of measures related to flowtime and tardiness of jobs. This paper essentially is an attempt at exploring the relative effectiveness of these two approaches to scheduling. © 2000, Taylor & Francis Group, LLC. All rights reserved.