Of course sometimes you don't mind - when scheduling school classes, you can be pretty sure that a good deterministic schedule will work fine. On the other hand, sometimes ignoring uncertainty is just impossible and this has been recognized - in airline operations for instance, disruptions occur all the time.
In manufacturing, the situation seems as bleak as in aviation industry, even though manifestations of uncertainty may be less spectacular and perhaps because of this, the uncertainty aspect is less stressed. In my current manufacturing OR adventure, I have encountered in principle two related views on uncertainty:
- Handling disruption is what manufacturing managers are for. Scheduling tools can help (among other things) to make sure that day-to-day planning and scheduling is done appropriately when nothing weird happens.
- The factory should have certain amount of well-managed buffers (e.g. overtime possibilities, surplus stocks) which can help them get back to normal when an unexpected event occurs.
As for the buffers, a question is how to put the phrase "well-managed" in more concrete terms. Given my amount of such and such uncertainty, what is my "optimal" amount of such and such buffers?
Now this is an area I want to learn about, so I can't flood you with a discussion of existing approaches... Still, I know there is this "robust scheduling" stuff. Incorporating uncertainty directly, e.g. by taking processing times to be random variables, seems just too complex to swallow, at least for systems with more than 2 machines...Still, under the label of Stochastic optimization one can find, among other approaches, simulation of possible scenarios - a schedule is made for each - and from these what-if schedules, a final schedule can be obtained by taking kind-of-expected-value. The idea of running a scheduling algorithm for each scenario seems off-putting from the computer-time perspective; still, the concept seems attractive.
Well, enough said for today...
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