Rockwell Automation FactoryTalk Scheduler Users Guide - Manual (Page 54)

F ACTORY T ALK S CHEDULER U SER ’ S G UIDE
48
• •
•
•
•
determines which operation to load. This rule is the key factor that determines the quality
of the resulting schedule.
R ESOURCE GROUPS AND QUEUES
Before discussing the rule mechanisms in FactoryTalk Scheduler, we must first discuss
the concept of Resource Groups and Queues. Each of the resources in the system may be
assigned to one or more resource groups. These resource groups are used to represent
situations where an operation can be performed on one of several alternative resources.
Each resource group is assigned to one and only one queue that holds an ordered set of
operations that are waiting to be loaded onto some member of the resource group. A
queue holds operations that are in contention for resources within the resource group and
that are subject to the same OSR. For example, a queue might be managed using a Critical
Ratio rule, in which case, all operations within this queue are selected using this same
rule.
T YPES OF OPERATION SELECTION RULES
There are two basic types of OSR’s supported in FactoryTalk Scheduler: standard and
user-coded logic.
Standard rules are hard-coded into FactoryTalk Scheduler. The user simply selects one of
these rules from the menu of available rules. There are more than 20 standard rules built
into FactoryTalk Scheduler. Different rules focus on different objectives; e.g., the Critical
Ratio rule tends to minimize tardiness. These rules can be further classified as static or
dynamic selection rules.
The static selection rules provide a simple sorting mechanism for ordering all waiting
operations in the queue. These rules do not require re-evaluation after each simulation
time advance. The parameters used to perform the operation selection are fixed. An
example is the Earliest Due Date rule, since the due date never changes during
sequencing. The first operation in the queue (as established by the static sort rule) is
allocated to a waiting resource. Since the rule always selects the first waiting operation,
this rule executes very fast.
In a dynamic selection rule, each operation in the queue is examined every time the rule is
invoked. Therefore, we make our selection based on the current status of the job and the
system. This mechanism fully accounts for any changes that occur as the result of time
advances or events. An example is the Smallest Operation Slack rule since the operation
slack value will change as a result of time advances during sequencing. Since dynamic
selection rules require re-evaluation of each operation in the queue after each simulation
time advance, it is slower than a static sorting rule.
User-coded logic rules provide a very powerful and flexible rule mechanism. User-coded
logic rules are programmed by the user in Microsoft
® Visual Basic ® or other OLE-
compliant language. In this case, the user explicitly searches the operations in the queue