Page 162 - Orlicky's Material Requirements Planning

CHAPTER 7 Processing Logic 141

Action Cycles

Many changes may occur in the same inventory record on the same day, in which case
the timing of open orders appears to require revision several times that day, even though
the changes may have a mutually canceling effect. The inventory planner’s reaction to
change, however, can be decoupled from the rate at which individual changes occur and
are processed by the system. The most common method of dampening reaction to change
is simply to delay such reaction. In practice, this takes the form of periodic action cycles
on the part of the inventory planner. He or she need not react to the continuous stream of
individual changes but can let them accumulate for some period of time.
The system can provide output of action requests on a cyclic basis. Some action mes-
sages typically would be generated in a batch once a day. Most requests for normal order
action (e.g., release of shop orders and purchase requisitions) belong in this category.
Different action cycles apply to various types of actions depending on their purpose.
Thus due dates for all open shop orders may be reevaluated once per shift so as to main-
tain the validity of shop priorities. For certain types of messages (e.g., premature suppli-
er deliveries), a weekly cycle would be sufficient. Other types of messages, however,
should be generated without any delay because corrective action time is critical. For
example, an open purchase order may become a candidate for cancellation as a result of
changed requirements. A 24-hour delay in reacting to the new situation can make the dif-
ference between being or not being able to cancel. Other examples of situations that call
for reaction without delay are excessive scrap and a significant downward adjustment of
on-hand inventory following a physical count. When major changes in the MPS are being
processed or following regular periodic issues of the MPS, all action-request output
should be suppressed until the entire change has been processed completely by the sys-
tem. This type of change may affect thousands of records, and the status of an inventory
item may change several times during the processing of such a change.
Planning cycles and action cycles are established on a more or less arbitrary basis.
Delaying action on available information does dampen reaction to change, but delay
obviously cannot be prolonged indefinitely. Under any action cycle, once delay is ter-
minated, subsequent changes still can invalidate the action taken. As a general rule, it is
better to act with less delay under a system capable of frequent—or continuous—replan-
ning, reevaluation, and revision of previous action than to tolerate unresponsiveness by
operating on long planning and action cycles. An MRP system offers a range of respons-
es from zero delay to weekly and monthly cycles. The relative promptness of reaction to
change should be a function of the type of change in question. This means that it can be
transformed whenever its user is ready into an online communications-oriented inven-
tory management system without a change in approach, reeducation, and fundamental
system overhaul.