C Rajendran

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.

A flowshop is a manufacturing environment with unidirectional flow of jobs through the various machines that are arranged in the order of job processing. A flowline-based manufacturing system (FBMS) is characterized by the flowline-based layout of machines with the machines arranged in the order of processing of jobs, but with the possibility of some or all jobs having missing operations on some machines. The significance of non-permutation schedules (NPS) and their effect on improving the schedule performance measures, in the context of FBMS, is highlighted in this paper. A simple heuristic procedure to derive NPS from a given permutation schedule is proposed. Results of the extensive computational experimentation, with makespan as the primary criterion and total flowtime as the secondary criterion, are presented. The results show that the proposed heuristic procedure for generating non-permutation schedule set is effective and improves the schedule performance measures with minimal computational effort. © 2002 Elsevier Science Ltd. All rights reserved.

Most studies on scheduling in dynamic job-shops assume that the holding cost of a job is given by the flowtime of the job and that the tardiness cost of a job is given by the tardiness of the job. In other words, unit holding and unit tardiness costs of a job are assumed. However, in reality, such an assumption need not hold, and it is quite possible that there are different costs for holding and tardiness 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 (manufacturing multilevel jobs) with the consideration of different holding and tardiness costs for different jobs. An attempt is made to develop efficient dispatching rules by incorporating the relative costs of holding and tardiness of jobs in the form of scalar weights. The primary objective of scheduling considered here is the minimization of the total scheduling cost consisting of the sum of holding and tardiness costs. The performance of the scheduling rules in minimizing the individual components of total scheduling cost is also observed. The results of an extensive simulation study on the performance of different dispatching rules show that the proposed rules are effective in minimizing the means and maximums of the primary measure, and are quite robust with respect to different job structures and experimental settings.

A flowline-based manufacturing system is a manufacturing environment where machines are arranged in accordance with the order of processing of jobs, with all jobs having an identical and unidirectional flow pattern through the machines; however, some or all jobs may have missing operations on some machines. In several practical situations the setup times of jobs are separable, significant and sequence-dependent. The problem of scheduling in such a flowline-based manufacturing system is considered with the focus on the development of non-permutation schedules. The deficiency of using the existing set of recursive equations in developing the timetable for permutation schedules is first highlighted, and a correct and modified set of recursive equations to take account of the missing operations properly is formulated. A simple heuristic procedure to derive non-permutation schedules from a given permutation sequence is proposed subsequently. Through extensive computational experimentation, it is shown that the proposed heuristic procedure yields solutions of good quality.

The flowline-based manufacturing system (FBMS) is a manufacturing environment wherein machines are arranged in the order in which jobs undergo operations, and jobs have an identical and uni-directional flow pattern. However, some or all jobs may not require processing on some machines. Scheduling of jobs in such a FBMS is considered in this paper with the focus on the development of non-permutation schedules. The significance of non-permutation schedules in improving the schedule performance measures, in the context of FBMS's, is first brought out. A heuristic procedure to derive non-permutation schedules from a given permutation schedule is subsequently proposed. The minimization of total flowtime of jobs is considered as the objective in the first instance, followed by the consideration of the minimization of total weighted flowtime of jobs. An extensive computational experimentation has been performed to evaluate the effectiveness of the proposed heuristic in improving the performance measures. Further, the performance of the proposed heuristic in generating non-permutation schedules has been compared with that of the benchmark dispatching rules namely, the SPT and SWPT rules.