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Rescheduling manufacturing systems: a framework of strategies, policies and methods & Using real time information for effective dynamic scheduling. presented by Z ümbül Bulut. Content. Importance of scheduling Definition of rescheduling Literature on rescheduling Rescheduling factors - PowerPoint PPT PresentationTRANSCRIPT
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Rescheduling manufacturing systems:a framework of strategies, policies and
methods
&
Using real time information for effective dynamic scheduling
presented by Zümbül Bulut
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Content
• Importance of scheduling
• Definition of rescheduling
• Literature on rescheduling
• Rescheduling factors
• A rescheduling framework
• Value of real time information
• Rescheduling in theory and practice
• Further research areas
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Example
• A factory that produce notebook
• Several types of notebooks differing in size, # of pages, cover style etc.
• Fixed layout, fixed amount of resources (machines, labor, raw material etc.)
• Objective: minimize the average completion time
• Constraints: - demand,
- due dates,
- resources,
- lot sizes etc.
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Importance of scheduling
• Schedules are plans that state when certain controllable activities should take place
• Schedules provide a coordination of activities to
- increase productivity,
- reduce operating costs,
- identify resource conflicts,
- ensure the raw material availability,
- standardize the production process etc.
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Importance of scheduling
Schedule is generated
Manufacturing operations begin
Schedule is followed as closely as possible
Underestimation of a job processing time
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Definition of Rescheduling
• A dynamic approach for updating an existing production schedule in response to disruptions or real time information
• It is necessary in order to minimize the effect of disturbances or real time information in the system performance
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Literature on rescheduling• No standard classification scheme
• Three primary types of studies:
1. Methods for repairing a schedule that has been disrupted
2. Methods for creating a schedule that is robust with respect to disruptions
3. Studies of how rescheduling policies affect the performance of dynamic manufacturing systems
The paper by Vieira, Herrmann and Lin:
• A framework for understanding rescheduling
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Rescheduling factors• Change the system status and affect performance, trigger
rescheduling
- machine failure,
- urgent job arrivals,
- job cancellations,
- due data change,
- over or underestimation of process time etc.
• Other changes
- overtime,
- process change or re-routing,
- machine substitution,
- equipment release etc.
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Definitions
• Manufacturing system organizes equipment, people and information to fabricate and assemble goods that are shipped to customers.
• Order release controls a manufacturing system’s input by determining which orders should be moved into production.
• Shop floor control determines which operation each person and piece of equipment should do and when they should do it.
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Definitions
• Production schedule specifies for each resource required for production, the planned start time and end time of each job assigned to that 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 production schedule in response to disruptions or other changes
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Questions
• Do I have to include all sets of jobs in the schedule?
• Is it necessary to generate a production schedule?
• When and how I should do rescheduling?
• What should be the procedure ?
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A rescheduling framework• A framework for understanding rescheduling research that
includes
1. Rescheduling environments
2. Rescheduling strategies
3. Rescheduling policies
4. Rescheduling methods
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Rescheduling Environment
• Identifies the set of jobs that need to be scheduled
• 1. Static Rescheduling Environment:
- finite set of jobs
a) Deterministic
- no uncertainty about the future,
b) Stochastic
- some variables are uncertain,
- executing the schedule requires some rule or policy for minimizing the error in the schedule,
- modifications in the schedule may be required,
- partial schedules, leaving the details until appropriate time comes
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Rescheduling Environment
• 2. Dynamic Rescheduling Environment:
- infinite stream of jobs, i.e. jobs continue to arrive over an infinite time horizon
a) No arrival variability:
- jobs to be processed are known in advance
- production schedule is continuously repeated
Cyclic Production Problem
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b) Arrival variability:
- uncertainty in job arrivals,
- all jobs follow the same route through the manufacturing system,
- arrival rate is steady,
- for different types of jobs setups may be required
c) Process flow variability:
- uncertainty in job arrivals,
- no advance information about jobs before they arrive
Rescheduling Environment
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Performance Measures
1. Measures of schedule efficiency:
- generally time-based measures, i.e. makespan, mean-flow time, maximum lateness etc.
2. Measures of schedule stability:
- deviations between the revised and initial schedules
(starting time deviations, sequence differences etc.)
- number of revisions or changes,
- effect of constant change in the schedule ( nervousness)
- amount of disruptions that would degrade the performance of the system (robustness)
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Performance Measures
3. Cost:
a) Computational Costs:
- costs of investment in the necessary information system
- cost of computational burden, maintenance, upgrades etc.
b) Setup costs:
- cost of all allocations made in advance to the schedule
c) Transportation costs:
- costs of material handling,
- cost of delivering the material earlier than required
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Rescheduling Strategies
1. Dynamic Scheduling:
- no production schedule,
- on-line scheduling whenever new information arrives,
- dispatching rules: jobs are sorted by some criteria
- pull mechanisms: kanban cards and constant WIP order release
- Closely related to Real-Time Control: decisions are made based on the current state of the manufacturing system
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Rescheduling Strategies2. Predictive-Reactive Scheduling:
- common strategy to reschedule dynamic systems,
- an iterative procedure that has two steps:
1. Generate a production schedule
2. Update the schedule in response to disruptions
a) Evaluation
b) Solution
c) Revision
- a rescheduling policy is needed to implement a predictive-reactive scheduling strategy
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Rescheduling Policies
1. Periodic rescheduling policy:
- reschedule the facility periodically and implement the schedules on a rolling time horizon basis
- more schedule stability and less nervousness
2. Event-driven rescheduling policy:
- a single event causes a revision in the schedule
- in extreme case revision is made every time an event alters the system status
3. Hybrid rescheduling policy:
- rescheduling the system periodically and also when special event take place
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Rescheduling Methods1. Generating robust schedule:
- an attempt to maintain good system performance with simple schedule adjustments
- disruptions do not affect the system a lot
2. Repairing schedules:
- schedule repair occurs as the operators react to the disruptions
a) Right-shift schedules
b) Partial rescheduling
c) Regeneration scheduling
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Decisions “Use the information that the processing time of a job is
underestimated to
- improve estimated values of some parameters
- improve the scheduling decisions”
- trade off the quality of the revised schedule against the production disturbance which result from changing the planned schedule
- the measure of improvement: utility
- the measure of disruption: stability
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DecisionsUtility:Benefit which may be gained by using a particular
rescheduling strategy
Stability:Amount of change in the original schedule
- arrival time of the information must be considered
before rescheduling is performed,
- response time must be acceptable,
- revised schedules must be appropriate.
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Utility Function- Objective: minimize the average completion time
- n jobs which enter to the production floor according to SPT rule
- Utility of information:
U(S1, S2, E, t) = the difference in the makespan between the schedule S1 that ignores real time event E that arrive at time t and the schedule S2 which uses it.
U((p1,p2,….pn), k, pk’, t) = Sum |pi-pi’| / n
pi: processing time of i. job
pi’: new processing time of the i. job
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Stability Function
Stability Measure:
S(S1, S2, E, t) = the sum over all jobs of the absolute change of start and finish times divided by number of jobs.
S((p1,p2,….pn), k, pk’, t)=
Sum(|Ci-Ci’| + | (Ci-pi) - (Ci’-pi’)|) / n
Note: In determination of these functions only those jobs
that effected by the schedule are considered, already
processed jobs are ignored.
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Results
The course of actions that should be taken is determined by the resulting effect of the change on the average completion time and the effect upon the schedule stability
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Further decisions
• Relative importance of utility and stability
• Strategy for handling the real time information:
1. Do nothing
2. Repair: localized changes
3. Reschedule: significant system wide changes
4. Conclude infeasibility
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Theory and Practice• Most scheduling results do not consider important
characteristics of the environment in which scheduling occurs,
• Scheduling models and algorithms are unable to make use of real time information, which is widely available from process control computers and other monitoring systems,
• Researches should consider fully the dynamic aspects of the manufacturing system
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Further Research Areas
• Comparison of different rescheduling policies
• Interactions between rescheduling and other production planning functions (capacity planning, material requirements planning)
• Application of the results on other types of dynamic, stochastic decision-making systems
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THANKS!