Principles of Engineering System Design

Principles of

Engineering System Design

Dr T Asokan

[email protected]

Principles of

Engineering System Design

Dr T Asokan

[email protected]

GRAPHICAL MODELLING TECHNIQUES

BEHAVIOR MODELING Method used for exploring the dynamics of the system.

Address the discrete-event behavior, which is behavior that is triggered by the occurrence of specific events.

• Control, activation, and termination of system functions

• Ex: FFBD ( Functional Flow Block Diagrams), Behavior diagrams, Petrinets

FFBDs provide a hierarchical decomposition of the systems function and show a control structure that dictates the order in which the function can be executed at each level of the decomposition.

• Series, Concurrent, selection, and multiple exit

Petri Nets• First introduced by Carl Adam Petri in 1962.• A diagrammatic tool to model concurrency and

synchronization in distributed systems.• Used as a visual communication aid to model

the system behaviour.• Based on strong mathematical foundation.• Captures precedence relations and structural

interactions of potentially concurrent and asynchronous events.

Asokan T ED 309

A Petri Net Specification ...• consists of three types of components: places

(circles), transitions (rectangles) and arcs (arrows):– Places represent possible states of the system (P= p1,p2,

….)– Transitions are events or actions which cause the change

of state (T=t1,t2…); And– Every arc simply connects a place with a transition or a

transition with a place.

t1p1

p2

The state of PN is defined by the marking.

Asokan T ED 309

A Change of State …• is denoted by a movement of token(s)

(black dots) from place(s) to place(s); and is caused by the firing of a transition.

• The firing represents an occurrence of the event or an action taken.

• The firing is subject to the input conditions, denoted by token availability.

t1

Asokan T ED 309

A Change of State• A transition is firable or enabled when there

are sufficient tokens in its input places.• After firing, tokens will be transferred from

the input places (old state) to the output places, denoting the new state.

Asokan T ED 309

Example: EFTPOS System (A Petri net)

Initial

1 digit 1 digit 1 digit 1 digit

d1 d2 d3

d4

OK

OKpressed

approve

approved

OK OK OKOK

RejectRejected!

(EFTPOS= Electronic Fund Transfer Point of Sale)

Asokan T ED 309

EFTPOS System

• Scenario 1: Normal – Enters all 4 digits and press OK.

• Scenario 2: Exceptional – Enters only 3 digits and press OK.

Asokan T ED 309

Example: EFTPOS System (Token Games)

Initial

1 digit 1 digit 1 digit 1 digit

d1 d2 d3

d4

OK

OKpressed

approve

approved

OK OK OKOK

RejectRejected!

Asokan T ED 309

Example: Vending Machine

• The machine dispenses two kinds of snack bars – Rs 20 and Rs 15.

• Only two types of coins can be used – Rs 10 coins and Rs 5 coins.

• The machine does not return any change.

Asokan T ED 309

Example: Vending Machine (A Petri net)

5

Take Rs 15 bar

Deposit 5

0

Deposit 10

Deposit 5

10

Deposit 10

Deposit5c

Deposit 1020

Deposit5

15

Take Rs 20 bar

Asokan T ED 309

Example: Vending Machine (3 Scenarios)

• Scenario 1: – Deposit 5, deposit 5, deposit 5, deposit 5, take Rs

20 snack bar.• Scenario 2:

– Deposit 10, deposit 5, take Rs15 snack bar.• Scenario 3:

– Deposit 5, deposit 10, deposit 5, take Rs20 snack bar.

Asokan T ED 309

Example: Vending Machine (Token Games)

5

Take Rs 15 bar

Deposit 5c

0c

Deposit 10

Deposit 5

10

Deposit 10

Deposit5

Deposit 1020

Deposit5

15

Take Rs 20 bar