| Summary: | The purpose of this research is to investigate the effects
of different scheduling operating policies in real- time
scheduling. The scheduling policies include due date
assignment method, priority rules, process batch method and
operator reassignment policy. A specific case of
production scheduling-in a hypothetical assembled product
manufacturing system was investigated in this research.
The simulated production system encompasses fifteen work
centres, each containing one to three identical machines
and each machine requires one operator with all operations
being perfectly efficient on all machine types. The
production -system produces finished products as well as
components and sub-assemblies. Orders from outside the
system arrive for service generated according to the
exponential distribution. The orders coming to the system
were classified into "priority" and "standard" orders.
Processing times at each work centre are statistically
independent and uniformly distributed.
A computer simulation technique was chosen as the approach
method. A computer simulation written in DBASE III PLUS
was used to generate the data for analysis. In order to
analyse the behaviour of the simulated production system
with respect to the different performance criteria, a
number of performance measures were selected. These are,
mean tardiness, percent tardy, mean work in progress, mean
machine utilisation, and mean operator utilisation.
The 2x6x2x2 complete factorial is analysed by the
analysis of variance (ANOVA) procedure to statistically
determine whether due date assignment method, priority
rules, process batch method, and operator reassignment
policy or their interaction significantly affect the performance criteria considered. Further analysis to
identify where significant differences in performance occur
is conducted via Duncan multiple comparison test.
Based upon the statistical analysis it was found that the
relative impact of due date assignment method, priority
rule, process batch method, and operator reassignment
policy or their interaction for scheduling policies in real
time scheduling to be dependent upon the measure of
performance considered. In respect of root mean square of
tardiness, the scheduling policies involving the slack time
remaining (STR) priority rule are the most important of
scheduling policies in minimising the tardiness of customer
orders produced by the company. In respect to minimising
the work in progress, there is no dominant level of due
date assignment method, or priority rule or process batch
method or operator reassignment policy. However, the
scheduling policies involving the variable process batch
(VPB) method produce the best result.
The scheduling policies involving the variable process
batch (VPB) method are the best performers in maximising
the utilisation of machine and operator.
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