RESCHEDULING MANUFACTURING SYSTEMS: A FRAMEWORK OF STRATEGIES, POLICIES, AND METHODS G.Vieira, J. Herrmann E.Lin Jou

1. �RESCHEDULING MANUFACTURING SYSTEMS: A FRAMEWORK OF STRATEGIES, POLICIES, AND METHODS�G.Vieira, J. Herrmann & E.LinJournal of Scheduling, 2003 Presented by: Banu KARAKAYA

2. OUTLINE Definition of Rescheduling in Manufacturing Systems Terminology & Framework Rescheduling Environments & Performance Measures Rescheduling Strategies Rescheduling Policies Rescheduling Methods Theory and Practice Conclusion

3. RESCHEDULING MANUFACTURING SYSTEMS Generate and update production schedules Schedules are plans stating when controllable activities should take place Schedules enable better coordination to -increase productivity -reduce operating costs -control the release of jobs to the shop -to determine whether delivery promises can be met -give a statement of what should be done by shoop floor personel

4. RESCHEDULING MANUFACTURING SYSTEMS Dynamic, stochastic environments Unexpected events or disruptions occur so that both the initial production schedule and system performance is affected Rescheduling must be done in order to minimize the effect of disturbances in the performance of the system

5. DEFINITION OF RESCHEDULING Rescheduling is the process of updating an existing production schedule in response to disruptions or other changes Unexpected events(disruptions)are called rescheduling factors (Dutta, 1990; Dhingra, Musser and Blankenship,1992) -machine failure -urgent(rush or hot)job arrival -job cancellation -due date change -change in job priority -rework or quality problems -over or underestimation of process time

6. TERMINOLOGY & FRAMEWORK Scope of research on rescheduling varies widely No standard classification scheme in the rescheduling literature 3 primary types of studies: -methods for repairing a schedule that has been disrupted -methods for creating a schedule that is robust with respect to disruptions -how rescheduling policies affect the performance of the dynamic manufacturing system

7. TERMINOLOGY & FRAMEWORK The paper presents a framework to understand rescheduling Some common terms are defined throughout the paper Manufacturing system organizes equipment, people and information to fabricate and assemble finished goods shipped to the customers Order release controls a manufacturing system�s input by determining which orders should be moved into production (like order release, input control)

8. TERMINOLOGY & FRAMEWORK Shop floor control determines which operation each person and piece of equipment should do and when they should do it Production schedule specifies the planned start time and end time of each job assigned to the resource Scheduling is the process of creating a production schedule for a given set of jobs and resources Rescheduling is the process of updating an existing schedule in response to disruptions or other changes

9. TERMINOLOGY & FRAMEWORK Framework for understanding rescheduling includes -Rescheduling Environments -Rescheduling Strategies -Rescheduling Policies -Rescheduling Methods

10. RESCHEDULING ENVIRONMENTS identify the set of jobs that need to be scheduled

11. RESCHEDULING ENVIRONMENTS Static rescheduling: finite set of jobs For deterministic, static rescheduling: the specified schedule can be followed without any modifications Stochastic, static rescheduling: -when task processing times are modeled as random variables, the actual start and end times will not match the expected ones a rule or policy is needed for reconciling the error -or uncertainty is not modeled as probability distribution worst-case performance is the key objective

12. RESCHEDULING ENVIRONMENTS Dynamic rescheduling: infinite set of jobs, i.e. jobs continue to arrive over an infinite time horizon a) No uncertainty or variability in the job arrival: -jobs to be processed are known in advance -production schedule is continuously repeated cyclic scheduling b) Arrival variability: in a flow shop -there may exist some uncertainty in job arrivals -but all jobs follow the same route through the manufacturing system -arrival rate is steady

13. RESCHEDULING ENVIRONMENTS c) Process flow variability: often occurs in a job shop -many products, but a limited subset of them are to be produced at any given time product�s arrival process varies greatly -in some situations, no advance information is available about jobs until they arrive -some info. about future arrivals may be known, but it is subject to change because of the disruptions

14. PERFORMANCE MEASURES A rescheduling point is the point in time when a schedule is created or revised The rescheduling period is the time between the consecutive scheduling points Rescheduling frequency is the inverse of the rescheduling period and it measures how often rescheduling is performed It can significantly affect the system performance A lower frequency (longer rescheduling periods) lowers the number of setups, but increases manufacturing cycle time and WIP

15. PERFORMANCE MEASURES (Re)Scheduling stability measures the number of revisions or changes that a schedule undergoes during the execution (Re)Scheduling nervousness is defined as significant changes in MRP plans or instability opposite of scheduling stability (Re)Scheduling robustness measures how much disruptions would degrade the performance of the system when executing the production schedule

16. PERFORMANCE MEASURES Measures can be divided into 3 groups: a) Measures of Schedule Efficiency: -time-based measures (makespan, mean tardiness, mean flow- time, average resource utilization etc.) b) Measures of Schedule Stability: -impact of schedule change is a nonregular performance measure -deviation between the revised and initial schedule(such as starting time deviations, sequence difference) -for ex; when searching for more stable and robust schedules, the impact of machine failure is the major concern -as the level of uncertainty , frequent rescheduling becomes more effective to improve the robustness

17. PERFORMANCE MEASURES c) Cost: (economic performance measure for rescheduling) -objective is to minimize the cost of starting jobs too early, WIP inventory and tardiness Computational costs: -costs of computational burden on the computer -costs of investments in the necessary information systems Setup costs: -occur when tooling and fixtures are created or allocated in advance to the schedule Transportation costs: -costs of material handling work to transport jobs -costs of delivering materials earlier than required

18. THE FRAMEWORK Framework for understanding rescheduling includes -Rescheduling Environments -Rescheduling Strategies -Rescheduling Policies -Rescheduling Methods

19. RESCHEDULING STRATEGIES For dynamic rescheduling environments with uncertain job arrivals 1) Dynamic Scheduling: -does not create production schedules -dispatching rules(SPT or EDD) and pull mechanisms(such as Kanban cards) are used to control the production -it can be defined as online scheduling or reactive scheduling -it is related to real-time control because of the decisions made based on the current state of the manufacturing system

20. RESCHEDULING STRATEGIES 2) Predictive-reactive scheduling: -common strategy to rescheduling dynamic manufacturing systems -It has two primary steps: generating a production schedule updating the schedule in response to a disruption to minimize its impact on system performance -It can also be described as an iterative process of three steps: (Wu and Li, 1995) Evaluation step considers the impact that a disruption causes Solution step determines the rescheduling solutions Revision step updates the existing schedules or generates a new one

21. RESCHEDULING POLICIES A rescheduling policy is needed to implement a predictive-reactive scheduling strategy Three types of policies have been studied: a) Periodic Policy: -reschedules the facility periodically and implements the schedules on a rolling time horizon basis -it yields more schedule stability and less nervousness than constant rescheduling -determining the optimal rescheduling period is a difficult task when using this type of policy

22. RESCHEDULING POLICIES b) Event-Driven Policy: -rescheduling can be performed repeatedly in dynamic manufacturing environments or it can be a single event for revising the schedule in a static sytem -for static environments, it is mostly used to reschedule the system when m/c failures occur -for dynamic systems, rescheduling is triggered when the total number of job arrivals reaches a threshold -In the extreme, a new schedule is created every time an event changing the system status occurs

23. RESCHEDULING POLICIES c) Hybrid Policy: -reschedules the system periodically and also when special (or major) events take place -major events: m/c breakdowns, arrival of urgent jobs, job cancellation or job piority changes Determining the impact of rescheduling policies on dynamic environments requires careful study, modeling and analysis of the manufacturing system

24. THE FRAMEWORK Framework for understanding rescheduling includes -Rescheduling Environments -Rescheduling Strategies -Rescheduling Policies -Rescheduling Methods

25. RESCHEDULING METHODS Methods to create or update schedules as part of predictive-reactive scheduling strategy (a most commonly used in practice) a) Generating robust schedules: -attempt to maintain good system performance with simple schedule adjustments -number of papers have proposed methods for creating schedules that are robust with respect to disruptions b)Repairing schedules: -larger deviations occur when unexpected events disrupt the initial schedule schedule repair occurs to react to the disruptions

26. RESCHEDULING METHODS There are 3 common methods used to repair(update) the schedule: Right Shift Rescheduling postpones each remaining operation by the amount of time needed to make the schedule feasible Partial Rescheduling reschedules only the operations affected directly or indirectly by the disruption It preserves the initial schedule as much as possible to maintain schedule stability with less nervousness For example, match-up scheduling Regeneration reschedules the entire set of operations not processed before the rescheduling point, including jobs not affected by the disruption It is also known as complete rescheduling

27. THEORY AND PRACTICE Rescheduling points out that there is a gap between theory and practice of production scheduling Most scheduling results do not consider important characteristics of the environment in which scheduling occurs Researchers do not consider fully the dynamic aspects of the manufacturing system (Re)Scheduling is actually part of a dynamic process

28. CONCLUSION AND FUTURE RESEARCH Since there are no standard definitions of the strategies, policies and methods in the rescheduling literature, this paper aims to describe a framework for understanding rescheduling concept More research is needed -to compare the performance of manufacturing systems under different rescheduling policies and -to understand how interactions between rescheduling policies and other production planning functions(capacity planning, MRP) affect system performance

29. THANK YOU presented by Banu Karakaya

30. �RESCHEDULING MANUFACTURING SYSTEMS: A FRAMEWORK OF STRATEGIES, POLICIES, AND METHODS�G.Vieira, J. Herrmann & E.LinJournal of Scheduling, 2003 Presented by: Banu KARAKAYA