PERT & CPM is a very important topic in GATE Mechanical Engineering and other engineering exams from the subject of Industrial Engineering.
Network scheduling is a technique used for planning and scheduling large projects in the fields of construction, maintenance, fabrication and purchasing of computer systems etc. It is a method of minimizing trouble spots such as production,delays and interruptions by determining critical factors and coordinating various parts of overall job.
There are two basic planning and control techniques that utilize a network to complete a predetermined project or schedule. These are Programme evaluation Review Technique(PERT) and Critical path method(CPM).
A project is defined as a combination of interrelated activities,all of which must be executed in a certain order for its completion.
The work involved in a project can be divided into 3 phases corresponding to the management functions of planning ,scheduling and controlling.
Planning:This phase involves setting the objectives of the project as well as assumptions to be made .It also involves the listing of tasks or jobs that must be performed in order to complete a project under consideration. In this phase ,in addition to the estimates of costs and duration of various activities,the manpower ,machines and materials required for project are also determined.
Scheduling:This consists of laying the activities according to their order of precedence and determine the following :
[i] The start and finish times of each activity
[ii] The critical activities on which activities require special attention.
[iii] The slack and float for the non critical paths.
Controlling: This phase is exercised after the planning and scheduling .It involves the following :
[i] Making periodical progress reports
[ii] reviewing the process
[iii]Analysing the status of the project
[iv] Making management decisions regarding updating ,crashing, and resource allocation
To understand the network techniques one should be familiar with a few basic terms of which both CPM and PERT are special applications:
Network: It is graphic representation of sequentially connected arrows and nodes representing activities and events in a project. Networks are also called arrow diagrams
Activity: An activity represents some action and is a time consuming effort necessary to complete a particular part of the overall project .Thus each and every activity has appoint of time where it begins and a point where it ends.
It is represented in the network by an arrow.
Here A is called activity
Event: The beginning and end points of an activity are called events or nodes .Event is point in time and does not consume any resources. It is represented by a numbered circle. The head event called jth event always has a number higher than the tail event which is also called ith event.
Merge and Burst Events: It is not necessary for an event to be the ending event of only one activity as it can be the ending event for two or more activities. Such an event is defined as merge event.
If the event happens to be beginning event of two or more activities ,it is defined as burst event.
Preceding, succeeding and concurrent activities: Activities that must be completed before a given event can occur are termed as preceding activities.
Activities that cannot be accomplished until an event has occurred are termed as succeeding activities.
Activities that can be accomplished concurrently are known as concurrent activities.
This classification is relative which means that one activity can be preceding to a certain event and the same activity can be succeeding to some other event or it may be a concurrent activity with one or more activities.
Dummy Activity: Certain activities which neither consume time nor resources but are used simply to represent a connection or link between the events are known as dummies. It is shown in the network by dotted line. The purpose of introducing dummy activity is:
[i] To maintain the uniqueness in numbering system as every activity may have a distinct set of events by which activity can be identified.
[ii] To maintain a proper logic in the network
Following are common errors in a network construction:
Looping(Cycling) In a network diagram a looping error is also known as cycling error. Drawing an endless loop in network is known as error of looping .A loop can be formed if an activity is represented by going back in time.
Dangling: To disconnect an activity before completion of all activities in the network diagram is known as dangling.
If a dummy activity is only activity emanating from an event and can be eliminated it is known as redundancy.
In the next set of short notes, we will look at Rules of network construction.