Page 174 - ExtendSim User Guide

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148 Queueing
Sorting items using the Queue Equation block
Least Dynamic Slack model
The example model Least Dynamic Slack illustrates the improvement in on-time performance that can be achieved by sequencing orders by least dynamic slack instead of first in, first out ordering. The two models are identical, except the top model uses Queue Equation blocks with
Least Dynamic Slack model
least dynamic slack calculations and the bottom model uses Queue blocks and typical FIFO ordering. In the model, the equations in the Queue Equation blocks calculate the dynamic slack for each item. The item with the smallest dynamic slack (least amount of time before being late) will be selected first. As seen on the plot which has been cloned onto the model work- sheet, on-time performance is higher using least dynamic slack (top line) compared to FIFO (bottom line).
Minimizing setup
In some systems, setup time (the changeover from one product to another) can add significant delay to the processing of items. If this is the case, it may be useful to process the same item type until there is no longer any of that item type in the queue. Only when a particular type of item has been exhausted will another type of item be processed. Giving priority ranking in a queue to the same type of product that has just exited the queue reduces the number of setups or changeovers between products. Like least dynamic slack, minimizing setup time is another type of queue ranking rule.
Minimize Setup model
The model Minimize Setup compares the Product attribute for each item in the Queue Equa- tion block to the Product attribute on the last item to leave the queue. The first item with its Product attribute value equal to the item that has just exited is released first. If no item in the queue can be found with an attribute value that matches the last exited item, the first item in the queue is selected. The plot shows the effects of this rule: the queue builds up initially until it can combine enough batches together to gain an efficiency from minimizing the setup time.
Discrete Event
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