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Chapter 15 Short-term Scheduling

Outline

► Introduction► Loading Jobs: Assignment Method► Sequencing Jobs

► One-machine sequencing► Two-machine sequencing

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INTRODUCTION

4

Introduction Scheduling is the process by which the

production or service plan is implemented. Scheduling determines where and when a job

is to be processed. A scheduling is a timetable for performing

activities, utilizing resources, or allocating facilities.

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Introduction Two issues

Loading: to which machine a job should be assigned

Sequencing: in what order the jobs assigned to a machine should be processed

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Introduction Five printing jobs (1, 2, 3, 4, and 5) are received from local customers and each of them may be done on either of the two high-speed copiers (A and B) in the shop. Scheduling the five jobs calls for a two-step procedure. 1. Determine to which copier each job should be assigned (i.e.,

loading).2. Determine the order in which the jobs assigned to each copier

should be processed (i.e., sequencing).1

2

3

4

5

job copier

A

B

Loading: assign jobs 1, 2 and 4 to copier A;

assign jobs 3 and 5 to copier B

Sequencing: 2 then 1 then 4 on A; 5 then 3 on B

Importance of Short-Term Scheduling▶ Effective and efficient scheduling can be a

competitive advantage▶ Faster movement of goods through a facility means better

use of assets and lower costs▶ Additional capacity resulting from faster throughput

improves customer service through faster delivery▶ Good schedules result in more dependable deliveries

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LOADING JOBS

Assignment Method

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Loading Assignment method:

Quantitative technique for identifying the optimal job-machine pairings

Goal is to allocate one and only one job to one and only one machine such that the total cost is as low as possible

Arrange information into a square matrix called the assignment matrix

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Loading Procedure1. For each row in the original matrix, subtract the smallest number

from each of the numbers

2. For each column in the new matrix, subtract the smallest number from each of the numbers

3. Determine the minimum number of (horizontal or vertical) lines needed to cover all the zeros in the existing matrix. If this is equal to the number of rows (or columns) of the square matrix, go to Step (5); otherwise, go to Step (4)

4. Two sub-stepsa) Subtract the smallest uncovered number from each of the uncovered

numbers in the matrix

b) Add the same smallest uncovered number to every number where two covering lines intersect. Go to Step (3)

5. Start with any row or column containing only one zero (a tie can be broken arbitrarily), match the "job" and the "machine" associated with the zero and then exclude them from further consideration by drawing a horizontal line and a vertical line. Repeat the same process until the complete assignments have been made.

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Example 1The XYZ Corporation produces four products (A, B, C, and D), which can be manufactured by any of the four machines (1, 2, 3, and 4) in the Production Department. Because of expensive set-up costs, each product is usually produced by one and only one machine. Given the following cost data (in dollars), determine the product-machine assignments such that the total cost is minimized. What is the minimum total cost?

1 2 3 4

A 30 50 40 30

B 20 10 30 20

C 30 40 20 20

D 40 30 30 40

Assignment Matrix

Solution

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1 2 3 4

A 30 50 40 30

B 20 10 30 20

C 30 40 20 20

D 40 30 30 40

Assignment Matrix

Step 1: For each row in the original matrix, subtract the smallest number from each of the numbers.1 2 3 4

A

B

C

D

0 20

10

0

10

0 20

101

020

0 010

0 0 10

Step 2: For each column in the new matrix, subtract the smallest number from each of the numbers1 2 3 4

A 0 20 10 0

B 10 0 20 10

C 10 20 0 0

D 10 0 0 10

Step 3: Determine the minimum number of (horizontal or vertical) lines needed to cover all the zeros. If this is equal to the number of rows (or columns) of the square matrix, go to Step (5); otherwise, go to Step (4)

1 2 3 4

A 0 20 10 0

B 10 0 20 10

C 10 20 0 0

D 10 0 0 10

4 lines to cover all zerosMatrix includes 4 linesThus, 4 = 4

Step 5: Starting with any row or column containing only one zero, match the "job" and the "machine" associated with the zero and then exclude them from further consideration. Repeat the same process until the complete assignments have been made1 2 3 4

A 0 20 10 0

B 10 0 20 10

C 10 20 0 0

D 10 0 0 10

A 1B 2

D 3C 4

Total cost = 30 + 10 + 30 + 20 = $90

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Example 2At Valley Hospital in Tyler, TX, nurses beginning a new shift report to a central area to receive their primary patient assignments. Not every nurse is as efficient as another with particular kinds of patients. Given the following patient roster, list of nurses, and estimated task times (in minutes), which nurse should take care of which patient so that the total amount of time needed to complete the routine tasks on this shift will be as small as possible? What is the minimum total time requirement?

Nurse 1

Nurse 2

Nurse 3

Nurse 4

Jones 70 110 115 130

Hawkins 10 105 135 50

Becker 30 90 75 35

Sweeney

55 25 15 65

Assignment Matrix

Solution

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N1 N2 N3 N4

J 70 110

115

130

H 10 105

135

50

B 30 90 75 35

S 55 25 15 65

Assignment Matrix

Step 1

N1 N2 N3 N4

J

H

B

S

0 40

45

60

0 95

125

400 6

045

540

10

0 50

Step 2 N

1N2 N3 N4

J 0 30 45 55

H 0 85 125 35

B 0 50 45 0

S 40

0 0 45

Step 3

N1 N2 N3 N4

J 0 30 45 55

H 0 85 125

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B 0 50 45 0

S 40 0 0 453 lines to cover all zerosMatrix includes 4 linesThus, 3 ≠ 4

Step 4(a): Subtract the smallest uncovered number from each of the uncovered numbers in the matrix

N1 N2 N3 N4

J

H

B

S

0 15

25

55

95 5

0

030

50

45

070

0 0 45

Step 4(b): Add the same smallest uncovered number to every number where two covering lines intersect. Go to Step (3).

Go back Step 3 N1 N2 N3 N4

J 0 0 15 25

H 0 55 95 5

B 30 50 45 0

S 70 0 0 454 = 4Step 5

N1 N2 N3 N4

J 0 0 15 25

H 0 55 95 5

B 30 50 45 0

S 70 0 0 45

B N4S N3J N2H N1

Total time = 35 + 15 + 110 + 10 = 170

EX 1 in classCoach Terry Young is putting together a relay team for a 400-meter relay. The four swimmers are Gary Hall (GH), Mark Spitz (MS), Jim Montgomery (JM), and Chet Jastremski (CJ) and each of them must swim 100 meters of breaststroke (BR), backstroke (BA), butterfly (BU), or freestyle (FR). Based on the past records, the coach believes that each swimmer will attain the times (in seconds) given in the following table:

1) To minimize the team’s total time for the race so that it has the best chance to win, which swimmier should swim which stroke? What is the minimum total time for the team to complete the relay

BR BA BU FR

GH 55 55 52 54

MS 53 58 55 52

JM 51 54 55 56

CJ 57 55 56 54

2) Repeat (1) by assuming that it takes Gary Hall and Mark Spitz, respectively, 52 seconds and 54 seconds, to swim 100 meters of freestyle. By how many seconds does the minimum total swimming time differ from the one in (1) in this case?

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Loading Summary of traditional loading problem

The number of jobs is equal to the number of machines

Each of the jobs is processed by any of the machines

The goal is to minimize the total cost Three special loading problems in reality

Case I: unequal numbers of jobs and machines Case II: prohibited job-machine combination Case III: maximization objective

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Loading (special case I) Case I: Unequal numbers of jobs and

machines Solution

A dummy row (job) or column (machine) of zero costs should be introduced to make the assignment matrix a square one

The standard method discussed before should then be applied in the usual manner.

Be careful in interpreting the final optimal solution obtained since the dummy job or machine added does not exist.

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Example 3Four professors are responsible for three projects funded by the National Science Foundation. The table presented below shows the time (in weeks) needed for each professor to finish each project. Determine the optimal assignments as well as the minimum total time requirement.

1 2 3

Thompson 50 60 40

Johnson 62 78 36

Nelson 57 74 38

Parkinson 42 66 45

Assignment Matrix

Solution

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1 2 3 4

T 50 60 40 0

J 62 78 36 0

N 57 74 38 0

P 42 66 45 0

Assignment Matrix Step 11 2 3 4

T 50 60 40 0

J 62 78 36 0

N 57 74 38 0

P 42 66 45 0

Step 2

1 2 3 4

T 8 0 4 0

J 20 18 0 0

N 15 14 2 0

P 0 6 9 0

Step 3 1 2 3 4

T 8 0 4 0

J 20 18 0 0

N 15 14 2 0

P 0 6 9 0

4 = 4

Step 5

1 2 3 4

T 8 0 4 0

J 20 18 0 0

N 15 14 2 0

P 0 6 9 0

P 1T 2J 3

Minimum total time = 42 + 60 + 36 = 138 weeks

Note that Professor Nelson will not be responsible for any project since "4" is a dummy project

Dummy machine

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Loading (special case II) Case II: prohibited job-machine combination

A job cannot be processed by a machine for some reason

Solution Replace the cost corresponding to the prohibited job-

machine pair in the matrix with an exceedingly large positive number (e.g., 9,999)

The standard method discussed before should then be applied in the usual manner.

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Example 4Three managers (1, 2, and 3) are to be assigned to three fast-food outlets (A, B, and C). The following table indicates the expense (in dollars) to be incurred for each manager to run each outlet. Suppose that Manager 3 is not willing to work at outlet B due to travel distance. What are the optimal personnel assignments? What is the minimum total expense?

A B C

1 4,000 3,500 3,400

2 4,800 5,000 4,700

3 4,100 3,200 4,400

Assignment Matrix

Solution

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Assignment Matrix

A B C

1 4,000

3,500

3,400

2 4,800

5,000

4,700

3 4,100

3,200

4,400

Manager 3 is not willing to work at outlet B due to travel distance, thus, replace 3,200 with 99,999.

Revised Assignment MatrixA B C

1 4,000

3,500 3,400

2 4,800

5,000 4,700

3 4,100

99,999

4,400

Step 1

A B C

1 600 100 0

2 100 300 0

3 0 95,899 300

Step 2

A B C

1 600 0 0

2 100 200 0

3 0 95,799

300

Step 3

A B C

1 600 0 0

2 100 200 0

3 0 95,799

300

3 = 3

Step 5

A B C

1 600 0 0

2 100 200 0

3 0 95,799

300

3 A1 B2 C

Total cost = 4,100 + 3,500 + 4,700 = $12,300

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Loading (special case III) Case III: maximization objective Solution

Replace every number with the difference between the largest number in the entire matrix and the number under consideration.

The standard method discussed before should then be applied in the usual manner.

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Example 5The following table shows the expected sales volumes (in units) if four salespersons are assigned to four different marketing territories. Determine the optimal assignment plan and the maximum total sales.

1 2 3 4

Mary

1,500 1,400 1,600 1,600

Jim 1,600 1,500 1,450 1,450

Lori 1,550 1,450 1,450 1,350

Tom 1,650 1,500 1,450 1,500

Assignment Matrix

Solution

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Assignment Matrix1 2 3 4

M 1,500

1,400

1,600

1,600

J 1,600

1,500

1,450

1,450

L 1,550

1,450

1,450

1,350

T 1,650

1,500

1,450

1,500

Revised Assignment Matrix1 2 3 4

M 150 250 50 50

J 50 150 200 200

L 100 200 200 300

T 0 150 200 150

Step 1 1 2 3 4

M 100 200 0 0

J 0 100 150 150

L 0 100 100 200

T 0 150 200 150

Step 2 & 3 1 2 3 4

M 100 100 0 0

J 0 0 150 150

L 0 0 100 200

T 0 50 200 150

3 ≠ 4

Step 4 & 3 1 2 3 4

M

J

L

T

50 500 10010

050

200

200

0 0

0 00 0

0 504 = 4

Step 5

1 2 3 4

M 200 200 0 0

J 0 0 50 50

L 0 0 0 100

T 0 50 100 50

M 4L 3J 2T 1

Total sales = 1,650 + 1,500 + 1,450 + 1,600 = 6,200 units

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EX 2 In Class

The following table contains information on the cost to run three jobs on four available machines. Determine an assignment plan that will minimize costs

Machine

A B C D

1 12 16 14 10

Job 2 9 8 13 7

3 15 12 9 11

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SEQUENCING JOBS

One-machine sequencingTwo-machine sequencing

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Sequencing To determine the order in which the jobs

assigned to a machine are to be processed One-machine sequencing Two-machine sequencing

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One-machine sequencing Each of the jobs needs to go through only one

machine in a certain order Applications

Books ("jobs") borrowed by students need to be checked out at the circulation desk ("machine") of a university library.

Patients ("jobs") are treated by a doctor ("machine") in the local hospital.

Passengers ("jobs") check in at an airline's ticket counter ("machine") in the airport.

Research manuscripts ("jobs") are typed by a secretary ("machine").

Tasks ("jobs") are performed by a worker ("machine") in a manufacturing plant

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Objectives – minimize m: total time needed to process the set of jobs ACT: average completion time (or mean flow time) AJL: average job lateness AWT: average waiting time NLJ: number of late jobs

One-machine sequencing

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Dispatching (or priority) rules FCFS: First come, first served, i.e., the job that

arrives first is processed first EDD: The job with the earliest due date is

processed first SPT: The job with the shortest processing time is

processed first

One-machine sequencing

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Example 6

Five machines in a shop have failed and will be repaired by a maintenance mechanic. The estimated repair times (in days) along with the due times (in days) for the machines are given below. Assume that the machines were down in the order shown in the table. Apply each of the FCFS, EDD, and SPT rules to dispatch the jobs and then compute m, ACT, AJL, AWT, and NLJ for each of the resulting sequences

Machine A B C D E

Repair time

4 7 2 6 3

Due date 15 16 8 23 9

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Machine A B C D E

Repair time

4 7 2 6 3

Due date 15 16 8 23 9

Solution

(1) FCFS: First come, first served

Job(i)

Processing time for job i (Pi)

Waiting time of job

i (Wi)

Completion time of job

i (Ci)

Due time for job i

(di)

Lateness of job

i (Li)

total

A

B

C

D

E

4

7

2

6

3

15

16

8

23

9

Sum of the processing time of

preceding jobswi = p1 + p2 + ... +

pi-1

0

4

4 + 7 =11

4 + 7 + 2 =13

4 + 7 + 2 + 6 =19

waiting time plus its processing timeci = wi + pi

4

7 + 4 = 11

2 + 11 = 13

6 + 13 = 19

3 + 19 = 22

Li = max {ci - di, 0}

0

0

13 – 8 = 5

0

22 – 9 = 1322 47 69 18

m = 22 days

ACT = 69/5 = 13.8 days

AJL = 18/5 = 3.6 days

AWT = 47/5 = 9.4 days

NLJ = 2 machines

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Machine A B C D E

Repair time

4 7 2 6 3

Due date 15 16 8 23 9

Solution(2) EDD: The job with the earliest due date is processed first

Job(i)

Processing time for job i (Pi)

Waiting time of job i

(Wi)

Completion time of job i (Ci)

Due time for job i

(di)

Lateness of job i

(Li)

total

C

E

A

B

D

Order the due date from the smallest to the largest: 8 (C), 9 (E), 15 (A), 16 (B) and 23 (D)

8

9

15

16

23

2

3

4

7

6

0

2

2 + 3 = 5

2 + 3 + 4 = 9

2 + 3 + 4 + 7 = 16

2 + 0 = 2

3 + 2 = 5

4 + 5 = 9

7 + 9 = 16

6 + 16 = 22

0

0

0

0

0

22 32 54

m = 22 days

ACT = 54/5 = 10.8 days

AJL = 0/5 = 0.0 days

AWT = 32/5 = 6.4 days

NLJ = 0 machines

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Machine A B C D E

Repair time

4 7 2 6 3

Due date 15 16 8 23 9

Solution(3) SPT: The job with the shortest processing time is processed first

Job(i)

Processing time for job i (Pi)

Waiting time of job i

(Wi)

Completion time of job i (Ci)

Due time for job i

(di)

Lateness of job i

(Li)

total

C

E

A

D

B

Order the repair time from the smallest to the largest: 2 (C), 3 (E), 4 (A), 6 (D) and 7 (B)

8

9

15

23

16

2

3

4

6

7

0

2

2 + 3 = 5

2 + 3 + 4 = 9

2 + 3 + 4 + 6 = 15

2 + 0 = 2

3 + 2 = 5

4 + 5 = 9

6 + 9 = 15

7 + 15 = 22

0

0

0

0

6

22 31 53

m = 22 days

ACT = 53/5 = 10.6 days

AJL = 6/5 = 1.2 days

AWT = 31/5 = 6.2 days

NLJ = 1 machine

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Solution

Applying the SPT rule leads to the sequence with the minimum ACT and AWT

Applying the EDD rule leads to the sequence with the minimum AJL and NLJ

The FCFS rule should be used if fairness is the major concern

m ACT AJL AWT NLJ

FCFS 22 13.8 3.6 9.4 2

EDD 22 10.8 0 6.4 0

SPT 22 10.6 1.2 6.2 1

Summary

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EX 3 in ClassProcessing time (including setup times) and due date for six jobs waiting to be processed at a work center are given in the following table. Determine the sequence of jobs, the average flow time (ACT), average job lateness (AJL), average waiting time (AWT), and the number of late jobs (NLJ) at the work center, for each of these rules: FCFS, SPT and EDD. (assume jobs arrived in the order shown)

Job A B C D E F

Processing time (day)

2 8 4 10 5 12

Due date 7 16 4 17 15 18

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Two-Machine Sequencing Each of the jobs to be processed has to go

through two machines in the same order Applications

Athletes ("jobs") competing in the Olympic Games must be subject to a series of two drug tests ("two machines") after each event

Cars ("jobs") in a garage must be sanded ("first machine") and then painted ("second machine")

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Objectives Minimize the total time needed to process the set of jobs

Johnson's rule Step 1: Select the job that has the shortest processing

time among all processing times on both machines. If the time is associated with the first machine, then the job should be processed first. If the time is associated with the second machine, however, then the job should be processed last.

Step 2: Schedule the job identified in Step (1) and then eliminate it along with its processing times on both machines. If all of the jobs have been scheduled, stop; otherwise, go to Step (1) and work toward the center of the sequence

Two-Machine Sequencing

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Example 7Hirsch Products, Inc., produces certain computer connectors, which first require a shearing operation and then need a punch press operation. The company currently has orders for five jobs with estimated processing times (in days) given below:

Job P Q R S T

Shearing 3 4 12 7 2

Punch press

5 1 6 10 4

Questions1. Determine the order in which the jobs should be processed to

minimize the total time needed to process the set of jobs.2. Based on the sequence obtained in Part 1 above, construct a

time-phased bar chart (or Gantt chart) for both operations3. What is the total idle time on both operations?

Solution Job P Q R S T

Shearing 3 4 12 7 2

Punch press

5 1 6 10 4

Sequencing

1. Determine the order in which the jobs should be processed to minimize the total time needed to process the set of jobs.

QT P RS

2. Based on the sequence obtained in Part 1 above, construct a time-phased bar chart (or Gantt chart) for both operations

T P S R QShearing

Punch press T P S R Q

2 5 12

24

28

2 6 11

12

22

24

30

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3. What is the total idle time on both operations?There is no idle time on "Shearing"

Idle time on "Punch Press" = (2 - 0) + (12 - 11) + (24 - 22) = 5 daysTotal idle time = 0 + 5 = 5 days.

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EX 4 In Class

A group of six jobs is to be processed through a two-machine flow shop. The first operation involves cleaning and the second involves painting. Determine a sequence that will minimize the total completion time for this group of jobs. Processing times are as follows:

Job A B C D E F

Work Center 1

6 4 8 2 6 12

Work Center 2

5 3 9 7 8 15

Questions1. Determine the order in which the jobs should be processed to

minimize the total time needed to process the set of jobs.2. Based on the sequence obtained in Part 1 above, construct a time-

phased bar chart (or Gantt chart) for both operations3. What is the total idle time on both operations?