Rockwell Automation FactoryTalk Scheduler Users Guide - Manual (Page 48)

F ACTORY T ALK S CHEDULER U SER ’ S G UIDE
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(due date and slack time) focus on reducing the number of late jobs, whereas the priority-
based rule strives to complete the most important jobs as soon as possible.
In some applications, a given operation can be performed on two or more different
resources. For example, a drilling operation might be performed on either of two drills. In
such cases, the schedule in an algorithmic sequencer is first determined by the ordering of
jobs, then by the rule that is used to determine to which resource a given operation is
assigned during the loading process. Again, simple heuristic rules (e.g., assign the
operation to the resource that will complete the operation first) can be used to determine
the schedule.
Backward sequencing
Although a job-at-a-time sequencer typically loads each job working forward in time by
starting with the first operation and continuing through the last operation, it is also
possible to use this same sequencing scheme but reverse the procedure to work backward
in time. In this case, the sequencer begins by loading the job’s last operation to finish at its
due date. It then continues by loading the job’s preceding operation to finish at the start
time of the last operation. This process is continued, working backward in time, until the
first operation for the job is loaded. At this point, the sequencer then selects a new job to
load and repeats the entire process, again starting with the last operation and working
backward in time. It continues in this fashion until all jobs are loaded.
S CHEDULING EXAMPLE 3
The figure below shows a schedule generated by backward sequencing Jobs A, B, and C,
where the vertical line on the right denotes the due date for all three jobs.
The advantage of a backward algorithmic sequencer is that it always generates a schedule
that has no late jobs. However, the schedule may require start times that are infeasible;
that is, the jobs must start before the current time. In essence, a forward job-at-a-time
sequencer fixes the start times for a job and determines the end times (which may violate
the due dates), whereas a backward job-at-a-time sequencer fixes the end times and
determines the start times.
Resource 2
Time
B-10 A-20 C-20
5 10 15
Resource 1 C-10 B-20 A-10