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Operation overlap occurs when a succeeding operation is scheduled to begin before a preceding operation is complete. Overlap constraints are actually a subset of operation lagging constraints. For purposes of this discussion we will define operations 100 and 110 as two successive operations on the same routing. Let us further define the beginning schedule of the run time of the first piece of the work order quantity as the operation's "head", and the completion time of the last piece of the work order quantity as the operation's "tail". (Note that the scheduled setup time immediately precedes an operation's head.) PROSPAC's scheduler uses three types of lagging constraints to position operation 110 with respect to operation 100: Head-to-head (HH), tail-to-tail (TT), and head-to-tail (HT). Operation overlap employs the HH and TT lagging constraints. In general the HH lagging constraint is employed where operation 110 runs slower than operation 100, that is, "a fast feeding a slow". The scheduler uses the HH protocol to position the head of operation 110, as close as constraint parameters will permit, after operation 100's head since it "knows" that operation 100 can produce parts faster than operation 110. Similarly the TT lagging constraint is employed where operation 110 runs faster than operation 100, where we have "a slow feeding a fast". The scheduler uses the TT protocol to position the tail of operation 110, as close as constraint parameters will permit, after operation 100's tail since it "knows" that it must delay the start of operation 110 to avoid having intermittent idle time as a result of running out of work. Of course the HT lagging constraint necessarily prohibits operation overlap. Operations in a routing are either enabled or disabled for overlap. By default, overlap is disabled when a new operation is created. All lagging constraints are controlled by a series of parameters. Some are fixed while some are set dynamically by the adaptive feedback control mechanisms that cause the scheduler to generate optimum schedules. One of the most extreme cases of operation overlap ever observed by Heuristima engineers was in a (subcontractor) shop that manufactured the Main Landing Gear Door Actuator for the Lockheed L-1011 "Tristar". The routing for this part, at the time, had more than 110 operations. The subject of observation was a work order for 16 parts. There were a few times when it was possible to walk out on that shop floor and see all 16 parts simultaneously in work on 16 successive operations on 16 different machines. |
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