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.

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.

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.

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.