Rescheduling Manufacturing Rescheduling Manufacturing Systems:Systems:

a framework of strategies, policies, and methodsa framework of strategies, policies, and methods

Vieira, Herrmann and Lin

Manufacturing systemManufacturing system

A manufacturing system is the collection of

operations and processes used to produce a

desired product. J.T.Black

order release

shop floor control

material handling

ManufacturingManufacturing SystemsSystemsThey are complex, dynamic and stochastic

systems

Controlling production activities through dispatching rules, kanban: myopic

production schedule: plans that state when certain

controllable activities should take

place.

Gives shop floor personnel an explicit statement of what

should be done so that managers can measure their

performance.

Production ScheduleProduction Schedule

Help managers and supervisors coordinate activities to

increase productivity and reduce operating costs

Better coordination between production levels

Controls the release of jobs to the shop

Identifies resource conflicts

Identifies periods for preventive maintenance

Determines whether delivery promises can be met

SchedulingScheduling

System has a dynamic, stochastic environment

need to generate high quality schedules react quickly to unexpected events revise schedules in a cost effective manner.

Rescheduling : the process of updating an existing production schedule in response to

disruptions or other changes.

Rescheduling factorsRescheduling factors

Unexpected events cause difficulty in following the schedule precisely, as time passes.

If deterioration in performance is significant, they trigger rescheduling to reduce the impact.

Most common rescheduling Most common rescheduling factorsfactors

Machine breakdownUrgent job arrivalJob cancellationDue date changeShortage of materialsChange in job priorityQuality problemsOver/under estimation of processing timesOperator absenteeism

Actions that suggest Actions that suggest reschedulingrescheduling

OvertimeIn-process subcontractingProcess changeRe-routingMachine substitutionLimited manpowerSetup timesEquipment release

Performance MeasuresPerformance Measures

Schedule efficiency : time-based measures

Schedule stability : stability,nervousness, robustness starting time deviations between the new schedule

and the original schedule. measure of the sequence difference between the two

schedules.

Wu, Storer and Chang (1993)

Cost : earliness , tardiness , WIP minimization

ReschedulingRescheduling costscosts

Computational costs

Setup costs

Transportation costs

A rescheduling frameworkA rescheduling framework

Rescheduling Environments

Rescheduling Strategies

Rescheduling Methods

Rescheduling EnvironmentsRescheduling Environments

Identifies the set of jobs that need to be scheduled

• Static set of jobs

• Dynamic set of jobs

Static rescheduling environmentStatic rescheduling environment

Deterministic • there is a finite set of jobs • no uncertainty about future

Stochastic • some variables are uncertain (random

processing times)

Dynamic rescheduling Dynamic rescheduling environmentenvironment

No arrival variability• Schedule continuously repeated • Single scheduling decision needed

Arrival variability• In a flow shop, steady arrıval rate • Schedule should determine when to

switch producing other classes of products.

Process flow variability• In a job shop

Rescheduling StrategiesRescheduling Strategies

Two common strategies for controlling production in dynamic rescheduling environments:

• Dynamic scheduling

• Predictive-reactive scheduling

Dynamic ReschedulingDynamic Rescheduling

Dispatching rules, pull mechanisms are used.• Decentralized production control methods dispatch jobs using current data• Computational effort may be low or high

Literature of control theoretic models have studies on the control of dynamic manufacturing systems

• If there are no setup times, proposed dispatching rules are based on least slack policy. If there are setup times, finish all jobs in the same class, then move on to other.

Kumar (1994)• There exist idling policies which are good

Chase and Ramadge(1992)

Predictive-reactive reschedulingPredictive-reactive rescheduling

Has two primary steps: generates a production schedule updates the schedule

Rescheduling may occur frequently in a dynamic environment

May be a single revision of the schedule of a stochastic, static environment

Productive-reactive schedulingProductive-reactive scheduling

Iterative process of three steps:

evaluation: evaluates the impact that disruption causedsolution: determines the best rescheduling solutionrevision: updates the schedule or generates a new one

Wu and Li (1995) Three-phase rescheduling scheme:

planning: constructs an initial schedulecontrol: compares the actual progress of operations to the

current schedulerescheduling: constructs a revised schedule

Yamamoto and Nof (1985)

Rescheduling PoliciesRescheduling Policies

Rescheduling policies: periodic event-driven hybrid

Rolling time horizon approaches: periodic and hybrid

Periodic policyPeriodic policy

Rescheduling is done periodically and implemented on a rolling time horizon basis.

Preferable if there is no on-line data

More schedule stability,less schedule nervousness

Following the same schedule although there are significant changes worsens the system performance

What is the optimal rescheduling period?Short interval scheduling

Event-driven policyEvent-driven policy

Rescheduling can happen repeatedly in dynamic systems or it can be a single event to revise a schedule in a static system.

Rescheduling is triggered when Total number of job arrivals reaches a threshold Vieira(2000) Every time a new job arrives Bierwirth and Mattfeld (1999)

Time spent for rescheduling & computations is excessive

Requires fast and reliable electronic data collection

High nervousness, low stability

Hybrid policyHybrid policy

Reschedules the system

Periodically When special events take place

Rescheduling MethodsRescheduling Methods

As part of predictive-reactive scheduling: generate or repair schedules

Schedule Generation Nominal Schedules Robust Schedules

As the level of uncertainty increases, frequent rescheduling becomes more effective in improving the robustness of the schedule

Schedule repairSchedule repair

right shift rescheduling: postpones each remaining operation by the time required

to make the schedule feasible

partial rescheduling: reschedules only the affected operations by the disruption. preserves the initial schedule as much as possible.

complete regeneration : reschedules the entire set of jobs not processed before

rescheduling point including the ones not affected

Whether to repair or rescheduleWhether to repair or reschedule

Reschedule from scratch if the disruption is highly significant.Revise the schedule if the change is less significant.

Two measures to determine the strategy to be used in repair:

Utility: measures the benefit gained by using a particular rescheduling strategy.

Stability: Compares the start and completion times of the schedules. Cowling and Johansson (2002)

Impact of rescheduling policiesImpact of rescheduling policies

Periodic policies are near optimal when order release is periodic.

Rescheduling at the arrival of an urgent job with a tight due date is useful.

Church and Uzsoy (1992)

Higher rescheduling frequencies yields better system performance but increases the number of setups.

Lower frequencies lower the number of setups but increases the cycle time and WIP.

Vieira (2000)

Impact of rescheduling policiesImpact of rescheduling policies

Never reacting to disturbances or reacting to every disturbance are not appropriate. Moderate level of frequency is suggested.

Sabuncuoglu and Karabuk (1999)

Only at tight due date conditions increasing the rescheduling frequency increases the system performance significantly.

Shafaei and Brunn (1999)

Decreasing scheduling frequency is advantageous if setups cost significant amounts.

Herrmann and Delalio (2001)

Future studiesFuture studies

Rescheduling policy must be considered in manufacturing system design.

More research is needed to understand how the interactions between rescheduling policies and other production planning functions affect manufacturing system.

Questions & AnswersQuestions & Answers