Rockwell Automation FactoryTalk Scheduler Users Guide - Manual (Page 49)

5 • S CHEDULING C ONCEPTS
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Although the idea of constructing a schedule that has no late jobs is appealing, backward
sequencing has some practical limitations, even in cases where a feasible solution is
generated. Backward scheduling shifts all jobs to the right on the planning board so that
they start as late as possible while still meeting the due date. This means that there is no
time buffer in the system, so any disruptions that occur (machine breakdowns, late
material arrivals, etc.) will typically create late jobs. In addition, by postponing the use of
our available capacity and waiting until the last minute to start each job, we give up the
opportunity to consider additional jobs that may arrive later and need to be added to the
schedule. For these reasons, many schedulers prefer a forward sequencer.
Bi-directional or bottleneck sequencing
It is also possible to use an algorithmic sequencer in a bi-directional or bottleneck mode.
In this case, we select an operation somewhere in the middle of the job sequence and
schedule the remaining operations using forward sequencing of the succeeding operations
and using backward sequencing for the preceding operations. This is useful in cases where
we have a bottleneck or highly utilized resource and we want to assign operations to this
bottleneck resource and then load the upstream and downstream operations around it. The
advantage of a bottleneck algorithmic sequencer is that it tends to minimize the cycle
times of all jobs.
In summary, an algorithmic sequencer is a simple and fast method for loading a set of jobs
on the planning board. The schedule is completely determined by specifying the order of
jobs and a rule for selecting among resources.
Algorithmic job selection rules
Algorithmic job selection rules are used to determine the order in which the jobs are
loaded onto the planning board. Most of them are simple ranking rules based on some
attribute of the job. Brief descriptions of the standard algorithmic job selection rules are
given below.
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Bottleneck: Ranking based on a secondary job selection rule. The forward and
backward mode schedules all unallocated jobs with emphasis on those operations that
require a bottleneck resource. The bi-directional mode only schedules jobs that require
an identified bottleneck resource. The user can then schedule the remaining jobs using
any of the available job selection rules.
„
Due Date: Ranking based on the earliest job due date first.
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First-Come, First-Served: Ranking based on the position in which they are entered
into the database.
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Ascending Order Property: Ranking based on the ascending value of a specified
order property. The order property can be numeric or character, and it must be specific