3. KINDS OF DECISIONS
Sequencing
Jobs which are more important go first, i.e., jobs which don’t
require much of the machine’s time, should be given preference.
Releasing
There are a number of motives for not releasing raw material to
the production floor until just before it is needed.
Routing
Lotting
4. INPUT-OUTPUT CONTROL
We can determine from Input-Output Control whether the
amount of work flowing through a work center is more or less
than the capacity.
GANTT CHART
The Gantt Chart (Henry L. Gantt, 1916) is a useful tool to help a
production manager monitor the activities of a production
operation.
In addition, in presenting activities according to time scale, a
manager can see if operation movements can be made.
5. INFINITE AND FINITE LOADING
In infinite loading approach, jobs are assigned to work centers
considering the capacities of work centers.
In finite loading approach, jobs are allocated as per the capacities
of the work centers.
FORWARD AND BACKWARD SCHEDULING
In forward scheduling, the job is assigned to the work centers in
the earliest slot so as to complete it as early as possible.
In backward scheduling, the due date of completion is
considered first and from it, the latest start date is calculated
using lead time so as to meet the due date.
FLOW SHOP
In a flow shop, a schedule which minimizes make span time also
minimizes other objectives, that is, it minimizes the sum of
machine idle time and the sum of job waiting time.
6. TECHNIQUES FOR SEQUENCING
FIRST COME, FIRST SERVED (FCFS)
FCFS means that the next job to be processed is the one that
arrived first in the waiting line.
THE EARLIEST DUE DATE (EDD)
EDD implies that the next job to be processed is the one that has
the earliest date when the finished job is promised to the client.
LAST ARRIEVED, FIRST PROCESSED (LAFP)
LAFP implies that the next job to be processed is the one that
arrived last.
7. Continued…
THE SHORTEST PROCESSING TIME (SPT)
SPT implies that the next job to be processed is the one that has the
least time necessary to complete. The philosophy here is to get the
smallest jobs over quickly, which gives a physiological impression that
one is being more productive.
THE LEAST SLACK TIME (LST)
LST implies that the next job processed is the one that has the least
amount of slack time.
Slack time/Float time => Difference between the time the job has been
promised to be completed and the promising time to complete the job.
8. CRITICAL RATIO
Critical ratio (CR) implies that the next job to be processed is the
one that has the lowest critical ratio.
CR = Time to the due date/Remaining process time.
CR<1 : The job will be finished after the promised delivery date.
LEAST CHANGEOVER COST
The least changeover cost implies that the next job processed is
the one that involves least machine changeover, or setup time.
It tries to minimize the cost of changeover by considering those
jobs that has similar machine settings.
9. EXAMPLE:-
Six jobs are received for processing and their
processing times and customer delivery dates are given
in table.
Job Sequence Production Time Delivery Date
A 2 4
B 5 18
C 3 8
D 4 4
E 6 20
F 4 24
10. FIRST COME FIRST SERVE SEQUENCE
(1)
Job
Sequence
(2)
Production
Time
(3)
Delivery
Date
(4)
Flow Time
(5)
Lateness
(4) – (3)
A 2 4 2 0
C 5 18 2+5 = 7 0
D 3 8 7+3 = 10 2
F 4 4 10+4 = 14 10
B 6 20 14+6 = 20 0
E 4 24 20+4 = 24 0
11. SHORTEST PROCESSING TIME RULE
(1)
Job
Sequence
(2)
Production
Time
(3)
Delivery
Date
(4)
Flow Time
(5)
Lateness
(4) – (3)
A 2 4 2 0
B 3 8 2+3 = 5 0
C 4 4 5+4 = 9 5
D 4 24 9+4 = 13 0
E 5 18 13+5 = 18 0
F 6 20 18+6 = 24 4
12. CRITICAL RATIO RULE
(1)
Job
Sequence
(2)
Productio
n Time
(3)
Delivery
Date
CR (4)
Flow Time
(5)
Lateness
(4) – (3)
D 4 4 1.00 4 0
A 2 4 2.00 6 2
C 3 8 2.67 9 1
E 6 20 3.33 15 0
B 5 18 3.60 20 2
F 4 24 6.00 24 0
13. AVERAGE FLOW TIME
Rules Average Flow Time Rank
FCFS (2+7+10+14+20+24)/6 = 12.83 Hours 2
SPT (2+5+9+13+18+24)/6 = 11.83 Hours 1
CR (4+6+9+15+20+24)/6 = 13.00 Hours 3
AVERAGE NUMBER OF JOBS IN THE SYSTEM
Rules Average Numbers of Jobs Rank
FCFS (2*6+5*5+3*4+4*3+6*2+4*1)/24 = 3.21 2
SPT (2*6+3*5+4*4+4*3+5*2+6*1)/24 = 2.96 1
CR (4*6+2*5+3*4+6*3+5*2+4*1)/24 = 3.25 3
14. AVERAGE JOB LATENESS
Rules Average Flow Time Rank
FCFS (0+0+2+10+0+0)/6 = 2.00 Hours 3
SPT (0+0+5+0+0+4)/6 = 1.50 Hours 2
CR (0+2+1+0+2+0)/6 = 0.83 Hours 1
15. THE n-JOB/m-MACHINE FLOW SHOP PROBLEM
Minimize machine idle time
Palmer method (1965)
DES heuristic (1977)
NEH heuristic (1988)
CDS procedure (1970)
16. 1. MINIMIZE IDLE TIME METHOD
The basic principle is to schedule the jobs in such a way as to minimize the idle time
of the machines.
We list machine on columns and jobs on rows.
Job i can start on the machine k only if the following two conditions are satisfied:
(1) Job is available, i.e., it has been processed on the previous machine, machine k-1.
(2) Machine k is available, i.e., the machine has already completed work on the
previous job.
The idle time of the machine k is given by the completion time of job i on the
previous machine (row i, column k-1) minus completion time of job i-1 in machine k
( row i-1, column k)
18. Example:-
Find the summation of processing time for each job and arrange the jobs in
the ascending order of their sum. Schedule the job with minimum sum in
the first position and calculate the completion time on each machine.
The ascending job order is 6,2,5,3,4,1. So, the first job to be sequenced is job 6.
Jobs
Machine
1 2 3 4 (Pij)
1 25 45 52 40 162
2 7 41 22 66 146
3 41 55 33 21 150
4 74 12 24 48 158
5 6 15 72 52 146
6 12 14 22 32 80
19.
20. The next step is to develop a table to determine which job should
follow job 6.
Job
Machine
1 2 3 4 Total machine idle time
Confirmed
2 19 67 89 155
Test jobs
5 7 26 15 82 72 161 52 213 161-155 = 6
3 41 60 55 122 33 155 21 176 122-89 = 33
4 74 93 12 105 24 129 48 203 (93-67)+(105-89) = 42
