Rescheduling manufacturing systems:a framework of strategies, policies and

methods

&

Using real time information for effective dynamic scheduling

presented by Zümbül Bulut

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

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.

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.

Importance of scheduling

Schedule is generated

Manufacturing operations begin

Schedule is followed as closely as possible

Underestimation of a job processing time

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

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

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.

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.

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

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 ?

A rescheduling framework• A framework for understanding rescheduling research that

includes

1. Rescheduling environments

2. Rescheduling strategies

3. Rescheduling policies

4. Rescheduling methods

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

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

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

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)

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

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

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

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

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

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

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.

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

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.

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

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

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

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

THANKS!