4,1 Dynamic scheduling
5. RESCHEDULING METHODS
The distinction between generating a schedule and repairing a schedule can seem a bit arbitrary since, in practice, schedulers may not start from scratch when generating a schedule. They often update an existing schedule, removing the operations and orders that are complete, adding new orders and operations, and resequencing operations as needed.
5.1 Generating robust schedules
Generating robust schedules is an important part of rescheduling. As discussed before, a robust schedule achieves good performance even though changes occur. This involves two key objectives:
1. Reduce the probability that something bad will occur.
2. Reduce the impact if something bad does occur.
The first aspect has not been studied in the traditional scheduling literature, which treats disruptions as exogenous random variables.
However, it is clear that scheduling can have affect the chance of a disruption happening. A robust schedule will assign sensitive operations to machines or workers that are more reliable.
From a traditional scheduling perspective, the only way to achieve the second objective is to include idle time in the schedule. This idle time serves as a buffer that can absorb delays due to longer-than-expected setup or processing times or machine failures. The idle time also provides openings that a new, urgent job can use if one arrives. However, when a disruption occurs, one can react by bringing additional resources into service. These may be backup equipment or personnel who were assigned to other tasks. Some factories have highly-skilled and versatile personnel who can be sent to the rescue when a serious disruption occurs. McKay and Wiers (2004) list other practical actions that schedulers can take to generate robust schedules. They suggest that schedulers identify potential problems as early as possible, regularly meet with individuals who may have information about future changes, schedule a number of small batches (instead of one large one), expedite learning by scheduling difficult operations first, avoid troublesome operations during a period with little slack, do simple operations after a process change, do work earlier than scheduled, keep the most powerful and flexible resources as free as possible (so that they're free if needed), and maintain backup plans for the most critical problems that could occur.
5.2 Repairing schedules
The methods available to repair a schedule that is no longer feasible depend upon the scheduling system in place. If the scheduler generates schedules manually, it is unlikely that he will have time for complete regeneration when reacting to a disruption. Delaying the affected operations (while pulling ahead work where possible) may be the only choice. On the other hand, a computer-based scheduling system that is closely integrated with the shop floor and manufacturing execution system is likely to have up- to-date information about the state of the machines, operators, and jobs.
Thus, it can use its scheduling engine and completely regenerate the schedule.
Of course, as mentioned before, informal schedule repair occurs automatically as operators and foremen deal with disruptions on their own.
Operators and foremen typically respond to a disruption by finding something else to do. There is usually some order that is waiting for processing and can be done today. If the original schedule is not precise, such actions can be reasonable and don't even affect the schedule. However, this type of reaction makes a more precise schedule even more infeasible and leads to more confusion and miscommunication.
6. S U M M A R Y A N D C O N C L U S I O N S
The rescheduling framework presented by Vieira et al. (2003) provides a way to understand the topic of rescheduling. Unlike that paper, which reviewed numerous papers that describe specific approaches in each area of the rescheduling framework, this chapter addresses more practical issues associated with rescheduling and gives some indication of how different rescheduling strategies and methods can be used in practice.
The objective of this chapter is to help those who are trying to improve production scheduling systems by highlighting the importance of rescheduling and the key concepts associated with rescheduling.
Rescheduling is done both formally and informally in practice. Informal rescheduling is a good thing if it reflects the ability of the shop floor personnel to meet production goals even when disruptions occur. However, if it reflects the fact that the formal scheduling system is unable to predict problems and plan responses to them, informal rescheduling is a symptom of a dysfunctional production scheduling system.
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