Flow Shop Scheduling
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Transcript of Flow Shop Scheduling
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Operational Research & Management Operations Scheduling
Flow Shop Scheduling
1. Flexible Flow Shop
2. Flexible Assembly Systems (unpaced)
3. Paced Assembly Systems
4. Flexible Flow Systems
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Operational Research & Management Operations Scheduling
Topic 1
Special Case of Job Shops:
Flexible Flow Shops
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Operational Research & Management Operations Scheduling 3
A Flexible Flow Shop with Setups
Stage 1 Stage 2 Stage 3 Stage 4
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Operational Research & Management Operations Scheduling 4
Applications
Very common in applications:
– Paper mills
– Steel lines
– Bottling lines
– Food processing lines
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Operational Research & Management Operations Scheduling 5
Setting for jobj on Machine i
Objectives
Multiple objectives usual
– Meet due dates
– Maximize throughput
– Minimize work-in-process (WIP)
jjTw
ijikiijk aahs ,
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Operational Research & Management Operations Scheduling 6
Generating Schedules
1. Identify bottlenecks
2. Compute time windows at bottleneck stage (release / due dates)
3. Compute machine capacity at bottleneck
4. Schedule bottleneck stage
5. Schedule non-bottlenecks
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Operational Research & Management Operations Scheduling 7
1: Identifying Bottlenecks
In practice usually known
Schedule downstream bottleneck first
Determining the bottleneck
– loading
– number of shifts
– downtime due to setups
Bottleneck assumed fixed
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Operational Research & Management Operations Scheduling 8
2: Identifying Time Window
Local Due date
– Shipping day
– Multiply remaining processing times with a safety factor
Local Release date
– Status sj of job j (# completed stages)
– Release date if sj = l )(lfrbj
Decreasingfunction -determinedempirically
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Operational Research & Management Operations Scheduling 9
3: Computing Capacity
Capacity of each machine at bottleneck
– Speed
– Number of shifts
– Setups
Two cases:
– Identical machines
– Non-identical machines
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Operational Research & Management Operations Scheduling 10
4: Scheduling Bottleneck
Jobs selected one at a time
– Setup time
– Due date
– Capacity
For example ATCS rule
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Operational Research & Management Operations Scheduling 11
5: Schedule Remaining Jobs
Determined by sequence at bottleneck stage
Minor adjustments
– Adjacent pairwise interchanges to reduce setup
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Operational Research & Management Operations Scheduling
Topic 2
Flexible Assembly Systems
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Operational Research & Management Operations Scheduling 13
Classical Literature
Exact solutions
– Simple flow shop with makespan criterion
– Two machine case (Johnson’s rule)
Realistic problems require heuristic approaches
max||Fm C
max2 ||F C
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Operational Research & Management Operations Scheduling 14
Job Shops Flexible Assembly
Each job has an unique identity
Make to order, low volume environment
Possibly complicated route through system
Very difficult
Limited number of product types
Given quantity of each type
Mass production
High degree of automation
Even more difficult!
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Operational Research & Management Operations Scheduling 15
Flexible Assembly Systems
Sequencing Unpaced Assembly Systems
– Simple flow line with finite buffers
– Application: assembly of copiers
Sequencing Paced Assembly Systems
– Conveyor belt moves at a fixed speed
– Application: automobile assembly
Scheduling Flexible Flow Systems
– Flow lines with finite buffers and bypass
– Application: producing printed circuit board
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Operational Research & Management Operations Scheduling 16
Sequencing Unpaced Assembly Systems
Number of machines in series
No buffers
Material handling system
– When a job finishes moves to next station
– No bypassing
– Blocking
Can model any finite buffer situation (pij = 0)
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Operational Research & Management Operations Scheduling 17
Cyclic Schedules
Schedules often cyclic or periodic:
– Given set of jobs scheduled in certain order
Contains all product types
May contain multiple jobs of same type
– Second identical set scheduled, etc.
Practical if insignificant setup time
– Low inventory cost
– Easy to implement
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Operational Research & Management Operations Scheduling 18
Minimum Part Set
Suppose L product types
Let Nk be target number of jobs of type k
Let z be the greatest common divisor
Then
is the smallest set with ‘correct’ proportions
Called the minimum part set (MPS)
* 1 2, ,..., LN N NN
z z z
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Operational Research & Management Operations Scheduling 19
Defining a Cyclic Schedule
Consider the jobs in the MPS as n jobs
Let pij be as before
A cyclic schedule is determined by sequencing the job in the MPS
Maximizing TP = Minimizing cycle time
*
1 1
1 L L
k kl l
n N Nz
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Operational Research & Management Operations Scheduling 20
Minimizing Cycle Time
Profile Fitting (PF) heuristic:
– Select first job j1
Arbitrarily
Largest amount of processing
– Generate profile:
– Determine which job goes next
1, ,1
ij h jh
X p
i
i
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Operational Research & Management Operations Scheduling 21
PF: Next Job
Compute for each candidate job
– Time machines are idle
– Time job is blocked
– Start with departure times:
2 1 1
2 2 1
2 2
1, 1, 1 2,
, 1, 1,
, 1,
max ,
max , , 2,..., 1
j j j
i j i j i i j
m j m j m
X X p X
X X p X i m
X X p
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Operational Research & Management Operations Scheduling 22
Nonproductive Time
Calculate sum of idle and blocked time
Repeat for all remaining jobs in the MPS
Select job with smallest number
Calculate new profile and repeat
2 1, ,
1
m
i j i j ii
X X p
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Operational Research & Management Operations Scheduling 23
Discussion: PF Heuristic
PF heuristic performs well in practice
Refinement:
– Nonproductive time is not equally bad on all machines
– Bottleneck machine are more important (OPT philosophy)
– Use weight in the sum (see example)
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Operational Research & Management Operations Scheduling 24
Discussion: PF Heuristic
Basic assumptions
– Setup is not important
– Low WIP is important
Cyclic schedules good
Want to maximize throughput
Minimize cycle time
PF heuristic performs well
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Operational Research & Management Operations Scheduling 25
Additional Complications
The material handling system does not wait for a job to complete
Paced assembly systems
There may be multiple machines at each station and/or there may be bypass
Flexible flow systems with bypass
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Operational Research & Management Operations Scheduling
Topic 3
Paced Assembly Systems
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Operational Research & Management Operations Scheduling 27
Paced Assembly Systems
Conveyor moves jobs at fixed speeds
Fixed distance between jobs
– Spacing proportional to processing time
No bypass
Unit cycle time
– time between two successive jobs
– linebalancing
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Operational Research & Management Operations Scheduling 28
Grouping and Spacing
Attributes and characteristics of each job
– Color, options, destination of cars
Changeover cost
– Group operations with high changeover
Certain long operations
– Space evenly over the sequence
– Capacity constrained operations (criticality index)
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Operational Research & Management Operations Scheduling 29
Objectives
Minimize total setup cost
Meet due dates for make-to-order jobs
– Total weighted tardiness
Spacing of capacity constrained operations
– Pi(L) = penalty for working on two jobs L positions apart in ith workstation
Regular rate of material consumption
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Operational Research & Management Operations Scheduling 30
Grouping and Spacing Heuristic
Determine the total number of jobs to be scheduled
Group jobs with high setup cost operations
Order each subgroup accounting for shipping dates
Space jobs within subgroups accounting for capacity constrained operations
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Operational Research & Management Operations Scheduling 31
Example
Single machine with 10 jobs (with 2 attributes)
Each job has a unit processing time
Setup cost (color)
If there is a penalty cost (sunroof)
11 kjjk aac
22 kj aa
2 ( ) max(3 ,0)
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Operational Research & Management Operations Scheduling 32
Example Data
Job 1 2 3 4 5 6 7 8 9 10
ja1 1 1 1 3 3 3 5 5 5 5
ja2 0 1 1 0 1 1 1 0 0 0
jd 2 6
jw 0 4 0 0 0 0 4 0 0 0
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Operational Research & Management Operations Scheduling 33
Grouping
Group A: Jobs 1,2, and 3
Group B: Jobs 4,5, and 6
Group C: Jobs 7,8,9, and 10
Best order: A B C (according to setup cost)
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Operational Research & Management Operations Scheduling 34
Duedate
Grouped Jobs
A B C
Job 1 2 3 4 5 6 7 8 9 10
ja1 1 1 1 3 3 3 5 5 5 5
ja2 0 1 1 0 1 1 1 0 0 0
jd 2 6
jw 0 4 0 0 0 0 4 0 0 0
Order A C B
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Operational Research & Management Operations Scheduling 35
Capacity Constrained Operations
A C B
Job 2 1 3 8 7 9 10 5 4 6
ja1 1 1 1 5 5 5 5 3 3 3
ja2 1 0 1 0 1 0 0 1 0 1
jd 2 6
jw 4 0 0 0 4 0 0 0 0 0
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Operational Research & Management Operations Scheduling
Topic 4
Flexible Flow Systems
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Operational Research & Management Operations Scheduling 37
Flexible Flow System with Bypass
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Operational Research & Management Operations Scheduling 38
Flexible Flow Line Loading Algorithm
Objectives
– Maximize throughput
– Minimize work-in-process (WIP)
Minimizes the makespan of a day’s mix
– Actually minimization of cycle time for MPS
Reduces blocking probabilities
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Operational Research & Management Operations Scheduling 39
Flexible Flow Line Loading Algorithm
Three phases:
– Machine allocation phase
assigns each job to a specific machine at station
– Sequencing phase
orders in which jobs are released
dynamic balancing heuristic
– Time release phase
minimize MPS cycle time on bottlenecks
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Operational Research & Management Operations Scheduling 40
1: Machine Allocation
Bank of machines
Which machine for which job?
Basic idea: workload balancing
Use LPT dispatching rule
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Operational Research & Management Operations Scheduling 41
2: Sequencing
Basic idea: spread out jobs sent to the same machine
Dynamic balancing heuristic
For a given station, let pij be processing time of job j on ith machine
Let and (the total workload)
n
jiji pW
1
n
jiWW
1
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Operational Research & Management Operations Scheduling 42
Dynamic Balancing Heuristic
Let Sj be the jobs released before and including job j
Define
Target
1,0 jSk i
ikij W
p
Wpppjj Skk
n
k
m
iik
Sk
m
iikj //
1 11
*
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Operational Research & Management Operations Scheduling 43
Minimizing Overload
Define the overload of the ith machine
The cumulative overload is
Minimize
/ij ij j io p p W W
*
j j
ij ik ik j ik S k S
O o p W
1 1
max , 0n m
iji j
O
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Operational Research & Management Operations Scheduling 44
3: Release Timing
MPS workload of each machine known
– Highest workload = bottleneck
– MPS cycle time Bottleneck cycle time
Algorithm
– Step 1: Release all jobs as soon as possible, then modify according to
– Step 2: Delay all jobs upstream from bottleneck as much as possible
– Step 3: Move up all jobs downstream from the bottleneck as much as possible
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Operational Research & Management Operations Scheduling 45
Example
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Operational Research & Management Operations Scheduling 46
Data
Jobs 1 2 3 4 5
'1jp 6 3 1 3 5
'2jp 3 2 1 3 2
'3jp 4 5 6 3 4
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Operational Research & Management Operations Scheduling 47
Machine Allocation
Jobs 1 2 3 4 5
jp1 6 0 0 3 0
jp2 0 3 1 0 5
jp3 3 2 1 3 2
jp4 4 5 0 3 0
jp5 0 0 6 0 4
6 35 3 1
6 45 4 3
max2 ||solutionsP C
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Operational Research & Management Operations Scheduling 48
Workload
From this table we obtain
each row each column
51
10
12
11
9
9
5
4
3
2
1
W
W
W
W
W
W
11
9
8
10
13
5
4
3
2
1
p
p
p
p
p
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Operational Research & Management Operations Scheduling 49
Overload
With job 1 as first job of sequence
11
21
31
41
51
136 9 3.7151130 9 2.2951133 11 0.2051134 12 0.9451130 10 2.5551
o
o
o
o
o
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Operational Research & Management Operations Scheduling 50
Overload Matrix
With job 1 to 5 as first job of sequence
3.71 -1.76 -1.41 1.41 -1.94-2.29 1.24 -0.41 -1.59 3.060.20 -0.16 -0.73 1.06 -0.370.94 2.65 -1.88 0.88 -2.59
-2.55 -1.96 4.43 -1.76 1.84
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Operational Research & Management Operations Scheduling 51
Dynamic Balancing
3.71 0.00 0.00 1.41 0.000.00 1.24 0.00 0.00 3.060.20 0.00 0.00 1.06 0.000.94 2.65 0.00 0.88 0.000.00 0.00 4.43 0.00 1.84
4.84 3.88 4.43 3.35 4.90
First Job
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Operational Research & Management Operations Scheduling 52
Selecting the Second Job
Calculate the cumulative overload
where
1
*11 1 1 1
1{4,1}
0.43 9
(3 6) 0.43 9
5.12
kk S
kk
O p W
p
43.051/)139(
51//}1,4{
*1
k
kSk
k pWpj
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Operational Research & Management Operations Scheduling 53
Cumulative Overload
1
2
3
5
( 5.12, 3.88, 1.26, 1.82, 4.32)
( 0.35, 0.36, 0.90, 3.52, 3.72)
( 0.00, 2.00, 0.33, 1.00, 2.67)
( 0.53, 1.47, 0.69, 1.71, 0.08)
i
i
i
i
O
O
O
O
Selected next
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Operational Research & Management Operations Scheduling 54
Final Cycle
Schedule jobs 4,5,1,3,2
Release timing phase
– Machine 4 is the bottleneck
– Delay jobs on Machine 1, 2, and 3
– Expedite jobs on Machine 5
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Operational Research & Management Operations Scheduling 55
Final Sheet
Flexible Manufacturing Systems (FMS)
– Numerically Controlled machines
– Automated Material Handling system
– Produces a variety of product/part types
Scheduling
– Routing of jobs
– Sequencing on machines
– Setup of tools
Similar features but more complicated