Rockwell Automation FactoryTalk Scheduler Users Guide - Manual (Page 57)

5 • S CHEDULING C ONCEPTS
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The predefined job preference rules are generally used in situations where the characteris-
tic of the job (e.g., priority or cost) is the overriding issue in scheduling, rather than job
due dates or facility utilization. Since these rules ignore due dates, they are typically more
applicable in make-to-stock rather than make-to-order environments.
Minimum Job Tardiness
In many make-to-order environments, the overriding scheduling objective is to ensure that
every job is completed by its assigned due date. The Minimum Job Tardiness rules are
focused on creating a schedule that minimizes the job tardiness. The rules in this category
include Earliest Due Date, Smallest Operation Slack Time, Smallest Order Slack Time,
Critical Ratio, and Actual Critical Ratio.
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The simplest of these rules is the Earliest Due Date. This is a static rule that selects
the job with the earliest due date. Although this rule executes very fast, it is generally
less effective than the other rules in this category for reducing job tardiness.
All of the other rules in this category are dynamic rules based on the calculation of a slack
time. The slack time is defined as the difference between the due date and the earliest job
completion time.
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The Smallest Order Slack Time rule selects the operation whose parent job has the
least amount of slack time. This is based on the premise that this is the job that is most
likely to become late if it is not selected.
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The Smallest Operation Slack Time rule is based on the average slack time per
operation (computed as the slack time for the job divided by the number of remaining
operations) rather than the remaining slack time for the job. The basic idea behind this
form of the slack-time-based rules is that each remaining operation has an inherent
risk, and the job with the smallest slack time per operation is the most critical in terms
of its likelihood of becoming late. Hence, if we were selecting between two jobs with
the same slack time, we would select the job with the largest number of remaining
operations, since this is the highest-risk job in terms of smallest slack time per
operation.
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The last variation of the slack-time-based rules is the Critical Ratio rule. This rule
selects the operation whose parent job has the smallest critical ratio. The critical ratio
is defined as the ratio of the demand time divided by the supply time. The demand
time is defined as the amount of time remaining to complete the order to meet the due
date. The supply time is the remaining work time to complete the order. A ratio less
than one implies the order is already behind schedule, a ratio greater than one implies
the order is ahead of schedule, and a ratio of one implies the order is on schedule.
The Critical Ratio rule is similar in spirit to the Smallest Operation Slack Time rule in that
it uses the slack-time calculation but interprets it differently to account for the remaining