Scheduling
1
Scheduling
• Scheduling: Establishing the timing of the use of equipment, facilities and human activities in an organization • Answering “when” question for activities
JAN
FEB
MAR
APR
MAY
JUN
Build A
A Done
Build B
B Done
Build C
C Done
Build D
Ship
On time!
2
High-Volume Systems
• Flow system: High-volume system with Standardized equipment and activities.
– Assembly line balancing
– Auto, computer industry
• Flow-shop scheduling
– Due to Repetition, scheduling is not a big issue
• Project scheduling
Work Center #1
Work Center #2
Output
3
High-Volume Success Factors
• Process and product design
• Preventive maintenance
• Rapid repair when breakdown occurs
• Optimal product mixes
– Most profit subject to capacities = LP
• Minimization of quality problems
• Reliability and timing of supplies
4
Intermediate-Volume Systems
• Outputs are between standardized highvolume systems and made-to-order job shops • Economic run size:
2 DS
Q0
H
p p D
5
Scheduling Low-Volume Systems
• Low volume systems
– Minimal repetition
– Schedule from scratch every time
• Loading - assignment of jobs to process centers • Sequencing - determining the order in which jobs will be processed
– Sequencing vs. Scheduling
6
Gantt Load Chart
• Gantt chart - used as a visual aid for loading and scheduling – Resources into rows
– Time periods into columns
Work Mon. Tues. Wed. Thurs. Fri.
Center
1
Job 3
Job 4
2
Job 3 Job 7
3
Job 1
Job 6
Job 7
4
Job 10
7
Loading
• Infinite loading: unlimited capacity, like MRP
• Finite loading: consider capacity
• Schedule construction
– Vertical loading
• Load on 1 work center with different jobs at once
– Horizontal loading
• Load operations of 1 job to all work center at once
• Forward scheduling
– Too much wip
• Backward scheduling
– Risky
8
Sequencing
• Priority rules: Simple heuristics used to select the order in which jobs will be processed.
Everything is
#1 Priority
• Job time: Time needed for setup and processing of a job.
• It includes set up time unless setup times are sequence dependent
9
Priority Rules
• FCFS
• SPT
• EDD
•
CR
•
S/O
- first come, first served
- shortest processing time
- earliest due date
- critical ratio
=time remaining / processing time
- slack per operation
=slack remaining / # of operations remaining
• Rush
- emergency
10
Performance measures
• Flow time of a job: Duration of time from a job enters into the system until it leaves
• Lateness of a job: Amount by which completion date exceeds due date. Could be negative.
• Tardiness=max(lateness,0)
• Makespan: total time needed to finish a group of jobs • Average number of jobs until the last is finished:
=Total flow time / Makespan
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Example: Average number of jobs
• Jobs: A and B with processing times 10 each
Number of jobs
2
Average number of jobs
1
A finishes at 10
B finishes at 20 Time
Makespan=20, Total Flow time=10+20
Average number of jobs=30/20
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Example: Sequencing rules
Jobs
Processing time
DD=Due date
A
11
61
B
29
45
C
31
31
D
1
33
E
2
32
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Ex: FCFS
Jobs
Proc.time Flow time
DD
Late Tardy
A
11
11
61
-50
0
B
29
40
45
-5
0
C
31
71
31
40
40
D
1
72
33
39
39
E
2
74
32
42
42
Total
268
202
66
121
Aver.
53.6
40.4
13.2
24.2
14
Ex: SPT to minimize the total flow time
Jobs
Proc.time Flow time
DD
Late Tardy
D
1
1
33
-32
0
E
2
3
32
-29
0
A
11
14
61
-47
0
B
29
43
45
-2
0
C
31
74
31
43
43
Total
135
202
-67
43
Aver.
27.0
40.4 -13.4
8.6
15
Ex: EDD to minimize the maximum lateness
Jobs
Proc.time Flow time
DD
Late Tardy
C
31
31
31
0
0
E
2
33
32
1
1
D
1
34
33
1
1
B
29
63
45
18
18
A
11
74
61
13
13
Total
235
202
33
33
Aver.
47.0
40.4
6.6
6.6
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