# Principles of Engineering System Design Dr T Asokan

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Principles of Engineering System Design Dr T Asokan asok@iitm.ac.in

Principles of Engineering System Design Dr T Asokan asok@iitm.ac.in 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

Asokan T ED 309

FUNCTION FLOW BLOCK DIAGRAM

Asokan T ED 309

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. t1 p1 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 d1d2d3 d4 OK pressed approve approved OK Reject Rejected! (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 d1d2d3 d4 OK pressed approve approved OK Reject Rejected!

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 Deposit 5c Deposit 10 20 Deposit 5 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 Deposit 5 Deposit 10 20 Deposit 5 15 Take Rs 20 bar

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