Lecture Principle of inventory and material management - Lecture 14

Lecture 14
Production Activity Control (Continued)

Books
•
Introduction to Materials Management, Sixth Edition, J. R. Tony Arnold, P.E., CFPIM, CIRM, Fleming 
College, Emeritus, Stephen N. Chapman, Ph.D., CFPIM, North Carolina State University, Lloyd M. 
Clive, P.E., CFPIM, Fleming College
•
Operations Management for Competitive Advantage, 11th Edition, by Chase, Jacobs, and Aquilano, 2005, 
N.Y.: McGraw­Hill/Irwin.
•
Operations Management, 11/E, Jay Heizer, Texas Lutheran University, Barry Render, Graduate School of 
Business, Rollins College, Prentice Hall


Objectives

•
•
•
•
•
•
•
•

Sequencing
Slack
Johnson's Rule
Monitoring 

Advance planning and scheduling
TOC
Scheduling procedure
Employee Scheduling


Sequencing
•
•
•

Prioritize jobs assigned to a resource
If no order specified use first­come first­served (FCFS)
Other Sequencing Rules
–
–
–
–
–
–
–
–
–

FCFS ­ first­come, first­served
LCFS ­ last come, first served
DDATE ­ earliest due date
CUSTPR ­ highest customer priority
SETUP ­ similar required setups
SLACK ­ smallest slack

CR ­ smallest critical ratio
SPT ­ shortest processing time
LPT ­ longest processing time 


Minimum Slack & Smallest Critical Ratio
•

SLACK considers both work and time remaining
SLACK = (due date – today’s date) – (processing time)

•

CR recalculates sequence as processing continues and 
arranges information in ratio form
time remaining
CR =                       =
work remaining

due date ­ today’s date
remaining processing time

If CR > 1, job ahead of schedule
If CR < 1, job behind schedule
If CR = 1, job on schedule


Sequencing Jobs Through One Process
•

•

•

Flow time (completion time) 
– Time for a job to flow through system
Makespan
– Time for a group of jobs to be completed
Tardiness
– Difference between a late job’s due date and its 
completion time


Simple Sequencing Rules

JOB

PROCESSING
TIME

DUE
DATE

A
B
C
D
E

5

10
2
8
6

10
15
5
12
8


Simple Sequencing Rules: FCFS
FCFS
SEQUENCE

START
TIME

A
B
C
D
E

0
5
15
17
25

PROCESSING COMPLETION DUE
TIME
TIME
DATE

5
10
2
8
6
Total
Average

5
15
17
25
31
93
93/5 = 18.60

10
15
5
12
8

TARDINESS

0
0
12
13
23
48
48/5 = 9.6


Simple Sequencing Rules: DDATE
DDATE
SEQUENCE

START
TIME

C
E
A
D
B

0
2
8
13
21

PROCESSING COMPLETION DUE
TIME

TIME
DATE

2
6
5
8
10
Total
Average

2
8
13
21
31
75
75/5 = 15.00

5
8
10
12
15

TARDINESS

0
0
3

9
16
28
28/5 = 5.6


Simple Sequencing
Rules: SLACK

SLACK
SEQUENCE

START
TIME

E
C
D
A
B

0
6
8
16
21

A(10­0) – 5 = 5
B(15­0) – 10 = 5
C(5­0) – 2 = 3

D(12­0) – 8 = 4
E(8­0) – 6 = 2

PROCESSING
TIME

COMPLETION DUE
TIME
DATE

6
2
8
5
10
Total
Average

6
8
0
8
5
3
16
12
4
21
10
11

31
15
16
82
34
82/5 = 16.40                  34/5 = 6.8

TARDINESS


Simple Sequencing Rules: SPT
SPT
SEQUENCE

START
TIME

C
A
E
D
B

0
2
7
13
21

PROCESSING

TIME

2
5
6
8
10
Total
Average

COMPLETION DUE
TIME
DATE

TARDINESS

2
5
0
7
10
0
13
8
5
21
12
9
31
15

16
74
30
74/5 = 14.80                   30/5 = 6


Simple Sequencing Rules: Summary

RULE

FCFS
DDATE
SLACK
SPT

AVERAGE
COMPLETION TIME

18.60
15.00
16.40
14.80
Best values

AVERAGE
TARDINESS

9.6
5.6
6.8

6.0

NO. OF
JOBS TARDY

3
3
4
3

MAXIMUM
TARDINESS

23
16
16
16


Sequencing Jobs Through Two 
Serial Process
Johnson’s Rule
1.

2.

3.

4.
5.

List time required to process each job at each process. Set up a one­
dimensional matrix to represent desired sequence with # of slots 
equal to # of jobs.
Select smallest processing time at either process.  If that time is on 
process 1, put the job as near to beginning of sequence as possible.
If smallest time occurs on process 2, put the job as near to the end 
of the sequence as possible.
Remove job from list.
Repeat steps 2­4 until all slots in matrix are filled and all jobs are 
sequenced.


Sequencing
With
Excel


Johnson’s Rule
JOB

PROCESS 1

PROCESS 2

A
B
C
D
E

6
11
7
9
5

8
6
3
7
10

E

A

D

B

C


Johnson’s Rule
E

E

A

5

A

D

D
11

B

C

B

Process 1
(sanding)

C

20

31

38

Idle time
E
5

A
15

D
23

Completion time = 41
Idle time = 5+1+1+3=10

B
30

Process 2
(painting)

C
37

41


Excel for Johnson’s Rule

User inputs processing
times and sequence

Excel calculates
completion times
and makespan

When the set of
jobs is completed


Guidelines for Selecting a Sequencing Rule
•
•
•

•
•
•

SPT most useful when shop is highly congested
Use SLACK for periods of normal activity
Use DDATE when only small tardiness values can be 
tolerated
Use LPT if subcontracting is anticipated
Use FCFS when operating at low­capacity levels
Do not use SPT to sequence jobs that have to be assembled 
with other jobs at a later date


Monitoring
•

•

•

Work package
– Shop paperwork that travels with a job
Gantt Chart
– Shows both planned and completed activities against a 
time scale
Input/Output Control
– Monitors the input and output from each work center


Gantt Chart
Job 32B
Behind schedule

Facility

3
Job 23C

Ahead of schedule

2
Job 11C

Job 12A
On schedule

1

1
Key:

2

3

4

5

6
8
Today’s Date

9

10

11

12

Planned activity
Completed activity

Days


Input/Output Control
PERIOD
Planned input

Actual input
Deviation
Planned output
Actual output
Deviation
Backlog
30

1

2

3

4

65

65

70

70

75

75

75

75

20

10

5

0

TOTAL
270
0
0
300
0
0


Input/Output Control
PERIOD
Planned input
Actual input
Deviation
Planned output
Actual output
Deviation
Backlog
30

1

2

3

4

65
60
-5
75
75
-0
15

65
60
-5
75
75
-0
0

70
65
-5
75
65
-10

0

70
65
-5
75
65
-10
0

TOTAL
270
250
-20
300
280
-20


Excel for Input/Output Control

User inputs
planned and
actual values

Excel calculates deviations
and backlog


Advanced Planning and Scheduling Systems

•

•

Infinite scheduling ­ assumes infinite capacity
– Loads without regard to capacity
– Then levels the load and sequences jobs
Finite  scheduling ­ assumes finite (limited) capacity
– Sequences jobs as part of the loading decision
– Resources are never loaded beyond capacity


Advanced Planning and Scheduling Systems
•

Advanced planning and scheduling (APS)
–
–
–
–

Add­ins to ERP systems
Constraint­based programming (CBP) identifies a 
solution space and evaluates alternatives
Genetic algorithms based on natural selection properties 
of genetics
Manufacturing execution system (MES) monitors status, 
usage, availability, quality

Advanced Planning and Scheduling