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Schriber and Stecke [55] provided a simulation study of an FMS that investigated the effect of various modellings. They
demonstrated successfully the validity of the theoretical results and tested the sensitivity of parameters such as the number of AGVs, the number of buffers, and the level of WIP. They used SPT as a dispatching rule, and two major performance measures were machine utilisation and production rate. Schriber and Stecke [56] further extended their previous study and provided a comparison of FIFO and SPT scheduling rules, but they did not extend the number of performance measures.
Denzler and Boe [57] used simulation to study a dedicated FMS by using actual data of routeing and operation times. The model was comprised of 16 CNC machines and pallet loading for the investigation of part-dispatching rules. The influence of the number of pallets and different scheduling rules on machine utilisation was demonstrated, which was the only performance measure used in the model. Results indicated that FMS performance was significantly affected by the choice of scheduling rules.
Co et al. [58] investigated the influence of queue length on five sequencing rules, which were FCFS, SPT, LWKR, TWK, and NXQL. A computer simulation model was developed to evaluate the performance of the alternative sequencing rules listed above, under various system configurations. The simul- ation model was written in SIMAN and contained n part types and m single server stations. Each part type followed a fixed routeing sequence, which defined the number of operations required to complete each job, the sequence of machines to visit, and the related processing times. The only performance measure reported was mean flow-time. The authors confirmed that the influence of the sequencing rules on the performance of the system, even in the case of short queue length, should not be ignored. No due-date-based rules and related perform- ance measures were used in the simulation model.
Chryssolouris et al. [59] compared the system performance of a manufacturing system with different dispatching rules using a simulation model with only one decision point. Four dispatching rules and four performance measures were employed. The performance measures included mean flow- time, number of task orders completed, average WIP, and mean tardiness. They did not conclude which rule dominated the others for all performance measures.
Choi and Malstrom [60] described the use of a simulator to evaluate work scheduling rules in an FMS. An FMS was modelled using actual data. The model consisted of a miniature closed-loop system with eight NC machines, five robots, one washing station, and I/O queues. The combinations of seven part selection rules with four machine centre selection rules were investigated. Part selection rules were RANDOM, FSFS, EDD, SPT, SLACK, S/PT, and VALUE. Machine selection rules were RANDOM, FMFS, NINQ, and WINQ. Scheduling rules were evaluated using the following performance measures: actual and relative system effectivity, total and average travel- ling time, actual production output, achievement rate, total and average manufacturing throughput time, total and average wait- ing time, imminent operation work content, and total and average production lateness. The authors concluded that the SLACK/WINQ and SPT/WINQ scheduling rules dominated the other major decision rules for the due-date- and flow-time- based criteria.
Slomp and Gaalman [61] proposed three scheduling pro- cedures. All procedures executed four functions, but the sequences of execution were different. The first function sched- uled workstations and used the earliest possible moment rule. The second function scheduled transport devices and used the earliest moment rule and, in the case of a tie, the least moving time was used. The third one scheduled operators and used the earliest moment rule and, in the case of a tie, the operator who needed the least walking time was chosen. The last function scheduled operations and employed four dispatching rules consisting of SPT, SPT.TOT, SPT/TOT, and EFTA. The simulation model of a case study was used including two workstations, one loading station, and one unloading station. There was one input buffer with a capacity of two jobs in front of each workstation. Only two performance measures, makespan per part and mean flow-time, were used in the model. In addition to the case study, several simulation tests were carried out to demonstrate the behaviour of the procedures developed. In each simulation, scheduling started with an empty system, but the author did not mention how the initial bias (i.e. the transient state) of the results was eliminated. Results showed that the EFTA rule performed worse than SPT/TOT regarding both measures of performance. The main shortcoming is that due dates were completely ignored in this model.