Rockwell Automation FactoryTalk Scheduler Users Guide - Manual (Page 46)

F ACTORY T ALK S CHEDULER U SER ’ S G UIDE
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In this simple example, Job A consists of two operations that are defined by sequence
numbers 10 and 20 and must be performed in that order. Operation 20 cannot begin until
Operation 10 is complete. Jobs B and C also contain two operations numbered 10 and 20,
respectively. The processing time for each operation is also shown in the table.
It is important to note that this is merely one of several possible production schedules that
can be generated for this very simple two-resource, three-job problem. In practical
problems involving many different resources and many different jobs, the number of
possible schedules is enormous. We clearly need an effective method for generating a
good schedule that meets our production objectives.
Let’s now turn our attention to the general FCS problem.
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Given: A set of jobs and a limited set of resources
„
Find: A “good” schedule
Note that the objective is defined as determining a “good” schedule rather than an
“optimal” schedule. We state our problem like this for two reasons. First, large scheduling
problems (i.e., those with many resources and many jobs) are typically computationally
challenging, and it’s not possible to achieve an “optimal” solution in a reasonable time.
Second, there are typically several simultaneous objectives (minimize tardiness, minimize
cycle time, maximize resource utilization, etc.), and these are often in conflict. In a
practical setting, therefore, we strive to find a “good” solution that balances these
objectives and can be obtained with minimal computation.
We will describe two fundamentally different approaches to constructing a schedule. The
first is algorithmic, or job-at-a-time sequencing. The second is simulation-based, or
operation-at-a-time sequencing.
Algorithmic sequencing
Algorithmic sequencing is a simple method for schedule construction. The algorithmic
sequencer selects a job and then puts each operation for that job on the planning board. It
Job Sequence Resource Processing Time
A 10 1 3
A 20 2 4
B 10 2 5
B 20 1 2
C 10 1 3
C 20 2 4