Transcript Rescheduling Manufacturing Systems

Rescheduling
Manufacturing Systems
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Ali Koç
March 3, 2003
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Outline
Brief Definition and Some Terminology
Rescheduling Framework
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Strategies
Policies
Methods
Conclusion
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Terminology
Rescheduling, process of updating an
existing production schedule in response
to disruptions
Disruptions (Rescheduling Factors)
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Machine Failure
Urgent Job Arrival
Job cancellation
Due date change
Operator Absenteeism
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Change in Job Priority
Delay in Arrival
Rework or Quality Problems
Over or under estimation of
processing times
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Terminology
 Manufacturing System
 Order Release, controls the input, determining
their order
 Shop floor control, determines the jobs for each
person and equipment and corresponding time
 Production schedule, the planned start and finish
times of each job assigned to a particular
resource
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Terminology
 Scheduling, creating production schedules and
rescheduling, updating an existing one
 Rescheduling framework, consists of rescheduling
environment, rescheduling strategies,
rescheduling policy and rescheduling methods
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Rescheduling Framework
Rescheduling environment, dynamic vs.
static environment
Rescheduling strategy, dynamic vs.
predictive-reactive scheduling
Rescheduling policy, when and how
rescheduling is done
Rescheduling method, generate and
update production schedule
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Rescheduling Framework
Rescheduling Environment
Static (finite set of jobs)
Deterministic
Stochastic
Dynamic (infinite set of jobs)
Deterministic
Arrival Process flow
Variability variability
Dynamic rescheduling environments are
most suitable to manufacturing systems
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Rescheduling Framework
Rescheduling Strategies (any environment with variability)
Dynamic (no schedule)
Dispatching rules Control-theoretic
Predictive-reactive (generate and update)
Rescheduling Policies)
Periodic
Event-driven
Hybrid
Rescheduling Methods (for predictive-reactive)
Schedule Generation
Nominal
Schedules
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Robust
Schedules
Schedule Repair
Right-shift
Partial
Complete
Rescheduling Rescheduling Regeneration
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Rescheduling Environment
Static, deterministic
Static, stochastic (worst-case analysis, if
uncertainty not modeled)
Dynamic, deterministic, yields cyclic-scheduling
Dynamic with uncertainty in job arrivals
Dynamic with job arrivals and flow variability
 Presence of additional capacity, subcontracting,
overtime, capacity buffers etc.
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Some other terms
Scheduling point, time when a scheduling
decision is made
Scheduling period / frequency
Scheduling stability, number of revisions
that a schedule undergoes during
execution (nervousness)
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Performance Measures
Time-based measures of schedule
efficiency (make-span, tardiness, average
resource utilization)
Schedule stability, not an issue in static,
deterministic rescheduling environments
Cost-based measures
 Computational costs
 non-recurring
 recurring
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 Setup costs
 Material handling costs
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Rescheduling Strategies
Dynamic Scheduling
 do not use scheduling policies, uses current
information to dispatch the jobs
 tradeoff utility, measure of improvement,
against stability, measure of nervousness
 three types of actions upon information arrival:
no move, repair and reschedule.
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Dynamic Scheduling
 Repair, local adjustment of the current schedule
 Reschedule, global adjustment of the current
schedule, cost of computation and nervousness
Dispatching rules
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SPT, EDD, FIFO etc.
batching incentive when setup exists
not appropriate in dynamic manufacturing environments
some other policies such as idling policies, dynamic
cyclic policies
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Dynamic Scheduling
 Utility, a measure of improvement such as,
decrease in total completion time
 Stability, a measure of nervousness, such as,
total change in start times and completion times
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Utility & Stability vs. time of arrival information
and/or change in the current system
Decide on repair or reschedule
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Predictive-Reactive
Scheduling
Evaluation step, generate a robust
schedule, evaluating the impact that a
disruption causes
Solution step, determine rescheduling
solutions enhancing the current
performance
Revision step, update the existing
schedule or generate a new one
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Rescheduling Policies
Periodic policy
 update periodically, implement on a rolling
horizon basis
 more schedule stability
 difficult to find optimum period
 requires less data acquisition
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Rescheduling Policies
Event-driven rescheduling policy
 reschedule system upon disruptions such as
machine failures
 requires fast and rapid data collection
 more system nervousness
Hybrid rescheduling policy
 reschedule periodically
 react to major events such as cancellations,
machine failures etc.
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Rescheduling Policies
(findings)
 Periodic policy near optimal when order release is
periodic
 Rescheduling frequency significantly affect the system
performance
 Lower frequency decreases setup, and increases cycle time and
WIP
 Higher frequency, more reactive but increases setup and
nervousness
 Frequency is more effective when due dates are tight
 Frequent rescheduling is more beneficent as uncertainty
increases
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Rescheduling Methods
Right shift rescheduling, postpones each
remaining operations
Partial rescheduling, schedules only
operations affected by the disruption
 Matchup scheduling, reschedule all the jobs before a
matchup point
 If point too large, use integer programming or
dispatching rules
Regeneration, reschedule the entire jobs
before the rescheduling point
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Notes and Conclusion
Rescheduling studies decrease the gap
between theory and practice of scheduling
Dynamic scheduling environment is the
appropriate one for real life systems.
Repair or reschedule considering the
tradeoff utility with stability
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Notes and Conclusion
Cost of rescheduling policies depends on
frequency of rescheduling
Implementation of rescheduling policy
depends on information acquisition
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Notes and Conclusion
Predictive-Reactive is the most popular
strategy and needs further study
More research on the interaction of
rescheduling policies with other
production planning decisions is needed
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Q/A
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