SPT Rule Research Articles

A simulation study for dynamic scheduling in a hybrid assembly/job shop considering the JIT context

This paper is a report of a simulation study that investigates a dynamic approach to scheduling jobs in a hybrid assembly/job shop, with special application to welding shop operations. Both JIT arrival and JIT delivery are considered. Various dispatching rules with different utilization are tested for the following performance measures; mean flow time, percentage of tardy jobs, mean tardiness, sum of mean tardiness and mean earliness. The results indicate that the SPT rule is the best for the mean flow time. The MSLACK rule, that we suggest in this paper, is the best for mean tardiness, the sum of mean tardiness and mean earliness. However, it is not clear which rule is the best for the percentage of tardy jobs. We also investigate the effect of changing the release time on the performance measure.

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.

Flow-time performance of modified scheduling heuristics in a dynamic rescheduling environment

A generic, modular simulation model was constructed using the SLAM II discrete event modeling language to investigate the flowtime performance of a job shop. The findings of earlier research using conventional scheduling heuristics were confirmed and used to validate the design of revised AWINQ and MST heuristics. These heuristics were reformulated to take into account the state of the entire job shop at each rescheduling opportunity. Simulation results indicate that the revised heuristics perform better than the conventional SPT heuristic when job load is moderate. At high loads, the SPT rule was the preferred heuristic.

Sensitivity analysis of list scheduling heuristics

When jobs have to be processed on a set of identical parallel machines so as to minimize the makespan of the schedule, list scheduling rules form a popular class of heuristics. The order in which jobs appear on the list is assumed here to be determined by the relative size of their processing times; well-known special cases are the LPT rule and the SPT rule, in which the jobs are ordered according to non-increasing and non-decreasing processing time respectively. When all processing times are exactly known, a given list scheduling rule will generate a unique assignment of jobs to machines. However, when there exists a priori uncertainty with respect to one of the processing times, then there will be, in general, several possibilities for the assignment that will be generated once the processing time is known. This number of possible assignments may be viewed as a measure of the sensitivity of the list scheduling rule that is applied. We derive bounds on the maximum number of possible assignments for several list scheduling heuristics, and we also study the makespan associated with these assignments. In this way we obtain analytical support for the intuitively plausible notion that the sensitivity of a list scheduling rule increases with the quality of the schedule produced.

Dispatching Procedures for a Flexible Manufacturing Cell in Constant Production Circumstances

Reports development and simulation of real‐time dispatching procedures, for flexible manufacturing cells (FMCs) in constant production circumstances, where a fixed set of part types is processed continuously while maintaining constant ratios between quantities needed of each part type. Five different procedures were tested: the first based on a simple rule, called the largest relative advantage (LRA) rule; two procedures linear programming based (named FMLPP and ALPP); a pull‐type procedure; and the last based on SPT rule. As expected, one of the linear programming based procedures achieved the best results. However, the much simpler LRA based procedure produced similar results, especially in the presence of machine failures. It was shown that, for high processing flexibility, good performance may be achieved by a dispatching procedure using the cell processing flexibility. In contrast to this, in low processing flexibility situations, there is no significant difference between the various scheduling procedures.

Job shop scheduling with alternative machines

SUMMARY In this paper we investigate the problem of minimizing the mean flow time in a general job shop type machining system with alternative machine tool routeings. An analytical formulation of the problem as a mixed integer programming is developed. An efficient algorithm based on this formulation is developed to solve the problem by decomposing it into subproblems that are easier to solve. The algorithm solves large problems in relatively short time. A second algorithm based on the SPT rule is developed and its performance is compared with the first algorithm. A greedy procedure is also developed for the case when a penalty cost is associated with adding alternative machines. Numerical examples are given to demonstrate the use of the above algorithms.

A comparative analysis of the COVERT job sequencing rule using various shop performance measures

The COVERT job shop dispatching rule was tested extensively twenty years ago with impressive results, however, since then it has been included in only one comparative analysis with other sequencing rules, and, reported instances of its application have been infrequent. In this paper, the COVERT rule is examined in detail relative to its applicability, its sensitivity to various operating parameters and performance measures, and its performance compared to several other sequencing rules including truncated SPT rules, dynamic slack rules and modified duedate rules. The performance of COVERT is examined for a variety of tardiness measures. The examination is conducted within the context of a simulation model of a machine-constrained job shop with serial jobs and random routings. The results indicate that COVERT performs well as a sequencing rule and in most instances was superior to the other sequencing rules tested both directly and across varying degrees of due-date tightness.

Openshop and flowshop scheduling to minimize sum of completion times

This paper deals with efficiently solvable special cases of openshop and permutation-flowshop scheduling where the objective function is minimum sum of completion times. Two O(mn) algorithms for openshop scheduling where all operations have equal processing times, are presented. The first constructs a no-wait schedule and the second a schedule where both criteria (sum of completion times and schedule length) take on their minimal values. For permutation-flowshop scheduling where processing times satisfy dominancy and/or ordered relations, SPT rules are proved to be optimal.