Chapter 4 scheduling
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CHAPTER 4 : SCHEDULING. Presented by: HAMKA BIN TAIP MOHD SAFUAN BIN ANUAR NUR AMIRA BINTI KHAMIS PUVENDRAN A/L SUNDARAM. OUTLINE. 4.1Introduction 4.2Job Shop Scheduling Terminology 4.3Sequencing Rules 4.3.1 FCFS 4.3.2 SPT 4.3.3 EDD 4.3.4 CR. 1. 4.1INTRODUCTION.

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CHAPTER 4 : SCHEDULING

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Chapter 4 scheduling

CHAPTER 4 :SCHEDULING

Presented by:HAMKA BIN TAIP

MOHD SAFUAN BIN ANUAR

NUR AMIRA BINTI KHAMIS

PUVENDRAN A/L SUNDARAM


Outline

OUTLINE

4.1Introduction

4.2Job Shop Scheduling Terminology

4.3Sequencing Rules

4.3.1 FCFS

4.3.2 SPT

4.3.3 EDD

4.3.4 CR

1


4 1 introduction

4.1INTRODUCTION

What is Scheduling ?

  • Last stage of planning before production occurs

  • Specifies when labor, equipment, facilities are needed to produce a product or provide a service

2


Operations scheduling

Operations Scheduling

  • Scheduling is an allocation decision

  • Scheduling decisions allocate resources over relatively short time periods: a few months, weeks, or hours.

  • The results are time allocations of production resources to different jobs

  • All the orders can be completed while all production resources are utilized with their loads being balance

3


Production management

Production Management

4


Objectives of operations scheduling

Objectives of Operations Scheduling

1. Meet customer due dates

2. Minimize job lateness

3. Minimize completion time

4. Minimize idle time

5. Minimize WIP inventory

6. Minimize the average flow time through the system

7. Reduce setup cost

5


Typical scheduling and control functions

Typical Scheduling and Control Functions

The following functions must be performed in scheduling and controlling a shop floor:

  • Allocating orders, equipment, and personnel to work centers or other specified location-short term capacity planning

  • Determining the sequence of order performance

  • Initiating performance of the scheduled work, commonly termed the dispatching of jobs

  • Shop-floor control, involving

    Reviewing the status and controlling the progress of orders as they are being worked on

    Expediting the late and critical orders

6


Scheduling operations

Scheduling Operations

Operations schedules are short-term plans designed to implement the master production schedule.

  • Operations scheduling focuses on how best to use existing capacity.

  • Often, several jobs must be processed at one or more workstations. Typically, a variety of tasks can be performed at each workstation.

7


Cont scheduling operations

Cont… Scheduling Operations

1.Scheduling in High-Volume System – To obtain a smooth rate of flow of goods or customers through the system in order to get a high utilization of labor and equipment.

Often referred as flow systems; scheduling refer to flow shop scheduling

8


Cont scheduling operations1

Cont… Scheduling Operations

  • Scheduling in Intermediate-Volume System – Typically produce standard outputs. The run sizes are relatively small compared to flow shop.

  • Scheduling in Low Volume System – Job shop scheduling

9


Low volume system

Low-volume, job shop (batch) operations, are designed for flexibility.

Products are made to order and orders usually differ considerably in terms of processing requirements, materials needed, processing time and sequence and setup.

Each product or service may have its own routing (scheduling is much more difficult)

Many service operations have similar issues – how to distribute the workload among work centers and what job processing sequence to use

Low-Volume System

10


Job shop

Job Shop

A job shop is organized by machines which are grouped according to their functions.

11


Sequencing jobs

SequencingJobs

  • Operations schedules are short-term plans designed to implement the sales and operations plan

  • An operation with divergent flows is often called a job shop

    • Low-to medium-volume production

    • Utilizes job or batch processes

    • No similar pattern of movements of jobs from one machine to the next

    • Difficult to schedule because of the variability in job routings and the continual introduction of new jobs to be processed

12


Job shop sequencing

Shipping Department

Raw Materials

Legend:

Batch of parts

Workstation

Job Shop Sequencing

  • Not all jobs are assumed to require exactly the same number of operations, and some jobs may require multiple operations on a single machine.

  • Each job may have a different required sequencing of operations.

  • Operations scheduling of shop floor usually means job shop scheduling;

Diagram of a Manufacturing Job Shop Process

13


Cont sequencing jobs

Cont…. SequencingJobs

An operation with line flow is often called a flow shop

  • Medium- to high-volume production

  • Utilizes line or continuous flow processes

  • All jobs follow the same paths from one machine to the next

  • Tasks are easier to schedule because the jobs have a common flow pattern through the system

14


Flow shop sequencing

Flow Shop Sequencing

15


Method of shop floor planning and control for job shop

Method of Shop Floor Planning and Control for Job Shop

1.Gantt Chart

  • Used for variety of purposes related to loading and Scheduling

  • The purpose is to organize and visually display the actual or intended use of resources in a time framework.

16


Cont method of shop floor planning and control for job shop

Cont……Method of Shop Floor Planning and Control for Job Shop

2.Loading

  • Refer to the assignments of jobs to processing (work) centers.

  • Loading decisions involve assigning specific jobs to work centers and to various machines in the work centers.

    3.Scheduling

  • Forward scheduling and Backward scheduling

17


Cont method of shop floor planning and control for job shop1

Cont……Method of Shop Floor Planning and Control for Job Shop

1.Gantt Charts

  • Developed in the early 1900’s by Henry Gantt

    1.1Load charts

    -Illustrates the workload relative to the capacity of a resource

    - Shows job schedule by employee

18


Cont method of shop floor planning and control for job shop2

Cont……Method of Shop Floor Planning and Control for Job Shop

1.2 Progress charts:

- Illustrates the planned schedule compared to actual performance

- Brackets show when activity is scheduled to be finished. Note that

design and pilot run both finished late and feedback has not started yet.

19


2 loading

2. Loading

Cont……Method of Shop Floor Planning and Control for Job Shop

2.1 Infinite loading:

  • Ignores capacity constraints, but helps identify bottlenecks in a proposed schedule to enable proactive management

    2.2 Finite loading:

  • Allows only as much work to be assigned as can be done with available capacity – but doesn’t prepare for inevitable slippage

20


Cont method of shop floor planning and control for job shop3

Cont……Method of Shop Floor Planning and Control for Job Shop

3.Scheduling

3.1 Forward Scheduling

  • Scheduling ahead from a point in time

  • If the issue is “How long will it take to complete this job?”

    3.2 Backward Scheduling

  • Scheduling backward from a due date

  • If the issue is “When the latest the job can be started and still be completed by the due date?”

21


Chapter 4 scheduling

Start processing when order is received regardless of due date

Schedule the job’s last activity so it is finished right before the due date

22


4 2 job shop scheduling terminology

Job A

M1

M2

Job B

M3

M4

4.2Job Shop Scheduling Terminology

  • Parallel processing versus sequential processing

    • Sequencing Processing: the m machines are distinguishable, and different operations are performed by different machines.

    • Parallel processing: The machines are identical, and any job can be processed on any machine.

Job A

M1

M2

Job B

M3

M4

  • M1, M2, M3, and M4 are different;

  • Job A has 2 operations which should be processed on different Machines: M1and M2;

  • Job B has 3 operations which should be processed on different Machines: M3, M2 and M4;

  • M1, M2, M3, and M4 are identical;

  • Jobs A and B can be processed on any one of the 4 machines

23


Cont job shop scheduling terminology

Cont… Job Shop Scheduling Terminology

2Flow time

  • The flow time of job i is the time that elapses from the initiation of that job on the first machine to the completion of job i.

  • The mean flow time, which is a common measure of system performance, is the arithmetic average of the flow times for all n jobs

Mean Flow Time=(F1+F2+F3)/3

Machines

M1

Job 1

Job 2

Job 3

M2

Job 1

Job 2

Job 3

F1: FT of Job 1

Time

F2: FT of Job 2

F3: FT of Job 3

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Cont job shop scheduling terminology1

Machines

M1

Job 1

Job 2

Job 3

M2

Job 1

Job 2

Job 3

F1: FT of Job 1

Time

F2: FT of Job 2

F3: FT of Job 3

Cont… Job Shop Scheduling Terminology

3. Make-span

  • The make-span is the time required to complete a group of jobs (all n jobs).

  • Minimizing the make-span is a common objective in multiple-machine sequencing problems.

Make-span of the 3 jobs

25


Cont job shop scheduling terminology2

Tardiness

of Job i

Completion

time of Job i

Due date

of Job i

Completion

time of Job i

Due date

of Job i

Cont… Job Shop Scheduling Terminology

4. Tardiness and lateness

  • Tardiness is the positive difference between the completion time and the due date of a job.

  • Lateness refers to the difference between the job completion time and its due date and differs from tardiness in that lateness can be either positive or negative.

  • If lateness is positive, it is tardiness; when it is negative, it is earliness

Lateness>0---Tardiness

Lateness<0---Earliness

When the completion of Job is earlier than due date, the tardiness is 0

26


4 3 sequencing rules

4.3Sequencing Rules

FCFS (first come-first served)

  • Jobs are processed in the sequence in which they entered the shop;

  • The simplest and nature way of sequencing as in queuing of a bank

SPT (shortest processing time)

  • Jobs are sequenced in increasing order of their processing time;

  • The job with shortest processing time is first, the one with the next shortest processing time is second, and so on;

EDD (earliest due date)

  • Jobs are sequenced in increasing order of their due dates;

  • The job with earliest due date is first, the one with the next earliest due date is second, and so on;

27


Cont sequencing rules

Cont… Sequencing Rules

CR (Critical ratio)

  • Critical ratio is the remaining time until due date divided by processing time;

  • Scheduling the job with the smallest CR next;

Current time

Remaining time of Job i

Due date of Job i

Processing time of Job i

CRi=Remaining time of Job i/Processing time of Job i

=(Due date of Job i-current time)/Processing time of Job i

  • CR provides the balance between SPT and EDD, such that the task with shorter remaining time and longer processing time takes higher priority;

  • CR will become smaller as the current time approaches due date, and more priority will given to one with longer processing time;

  • For a job, if the numerator of its CR is negative ( the job has been already later), it is naturally scheduled next;

  • If more than one jobs are later, higher priority is given to one that has shorter processing time (SPT).

28


Cont sequencing rules1

Cont… Sequencing Rules

  • Example 5.1

  • A machine center in a job shop for a local fabrication company has five unprocessed jobs remaining at a particular point in time. The jobs are labeled 1, 2, 3, 4, and 5 in the order that they entered the shop. The respective processing times and due dates are given in the table below.

  • Sequence the 5 jobs by above 4 rules and compare results based on mean flow time, average tardiness, and number of tardy jobs

29


1 first come first serve fcfs

Mean Flow time=268/5=53.6

Average tardiness=121/5=24.2

No. of tardy jobs=3.

1. First Come First Serve (FCFS)

1 11 61 0

2 40 45 0

3 71 31 40

4 72 33 39

5 74 32 42

Totals 268 121

30


2 shortest processing time spt

Mean Flow time=135/5=27.0

Average tardiness=43/5=8.6

No. of tardy jobs=1.

2. Shortest Processing Time (SPT)

4 1 1 33 0

5 2 3 32 0

1 11 14 61 0

2 29 43 45 0

3 31 74 31 43

Totals 135 43

31


3 earliest due date edd

Mean Flow time=235/5=47.0

Average tardiness=33/5=6.6

No. of tardy jobs=4.

3. Earliest Due Date (EDD)

3 31 31 31 0

5 2 33 32 1

4 1 34 33 1

2 29 63 45 18

1 11 74 61 13

Totals 235 33

32


4 critical ratio cr

4. Critical Ratio (CR)

Current time should be reset after scheduling one job

33


Cont 4 critical ratio cr

Mean Flow time=289/5=57.8

Average tardiness=87/5=17.4

No. of tardy jobs=4.

Cont … 4. Critical Ratio (CR)

Both Jobs 4 and 5 are later, however Job 4 has shorter processing time and thus is scheduled first; Finally, job 1 is scheduled last.

34


Sequencing rules summary

Sequencing Rules——Summary

Discussions

  • SPT results in smallest mean flow time

  • EDD have the minimum tardiness (42, 43, 18, and 31 for the 4 different rules)

35


Exercise

Exercise

There are five sheet metal jobs are waiting to be assigned at a company work centre.Their work (processing) time and due dates are given below. Then, determine the sequence of processing according to (FCFS,SPT,EDD and CR)

Figure 1: Jobs are assigned as below:

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