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News Services Consulting Customer Support Scheduling Technology Research Professional Place
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Any calculated schedule of future events, no matter how robust or precise, is subject to subsequent unforeseen occurrences and circumstances that tend to progressively erode its accuracy. This inevitable process is called schedule decay. Unforeseen occurrences can be divided into two groups:
Signal events necessitate a schedule regeneration which is typically accomplished with PROSPAC's recovery scheduling feature. White noise events, on the other hand, require a different approach. The three most important characteristics of any schedule presented to a shop floor are
White noise, as might be supposed, bears adversely on schedule stability. While it is not the only contributor to instability, it is perhaps the most significant and the most difficult to manage without PROSPAC's assistance. A stable schedule is one whose Machine Load Lists maintain their accuracy between periodic schedule regenerations in the absence of any signal events. While it is practically impossible to attenuate white noise, PROSPAC has features that preserve schedule stability in spite of high white noise levels. Those events that comprise white noise include:
In order to manage schedule decay, a technique must first be found to measure either it or its causes. Then it is necessary to utilize methods that reduce its rate. To this end, the PROSPAC approach equates minimum schedule decay with stable schedules. Schedule stability is achieved by minimizing the impact of white noise. PROSPAC provides a quantitative way to measure schedule stability, called schedule creep. The successful management of schedule decay is achieved when schedule creep is reduced to zero or very close to it. Schedule creep is a number that arises out of comparing two schedules: a reference schedule and a target schedule. The reference schedule can be the immediately preceding periodic schedule or some earlier schedule generation that has particular significance. The target schedule is typically the current schedule. Those in production control, who use the Shop Work Order Status Report, can easily observe schedule creep on individual shop work orders. It is sometimes alarming when key shop work orders are scheduled to complete one or more days later each time a periodic schedule is regenerated. Such a schedule is not stable! PROSPAC not only provides a way to measure schedule stability, it also provides features that mitigate the effects of white noise. These include:
These features, once enabled and parameters declared, function automatically each time PROSPAC's scheduling "engine" is used.
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