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Petri Nets A Tutorial Based on: Petri Nets and Industrial Applications: A Tutorial.

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Presentation on theme: "Petri Nets A Tutorial Based on: Petri Nets and Industrial Applications: A Tutorial."— Presentation transcript:

1 Petri Nets A Tutorial Based on: Petri Nets and Industrial Applications: A Tutorial

2 Petri Net Intro. Often used for description of distributed systems Provide a graphical notation for stepwise processes – Choice – Iteration – Concurrent execution Has exact mathematical definition of their execution semantics 2

3 Petri Nets Basics Bipartite directed graph – Made of: Places Transitions Directed Arcs Tokens 3

4 Petri Nets Basics Places – Circles – Input place to a transition if there is a directed arc connecting the place to a transition – Output place to a transition if there is an arc connecting a transition to the place 4

5 Petri Nets Basics Transitions – Bars (box) – Represent events Tokens – Dot in places – Indicate if a condition is true or false Petri Net Marking – defines current state of modeled system, distribution of tokens. 5

6 Petri Nets Formal Notation PN = {P,T,I,O,M 0 } where – P={p 1,p 2,…,p n } is a finite set of places – T={12,t1,…,tn} is a finite set of transitions, P ᴜ T ≠ {} and P ∩ T = {} – I: (P X T) → N is an input function that defines directed arcs from places to transitions, N is set of nonnegative integers – O: (P X T) → N is an output function that defines directed arcs from transitions to places – M 0 : P→N is the initial marking 6

7 Petri Nets Formal Notation Formalize this Petri Net PN = {P,T,I,O,M 0 } 7

8 Petri Nets Formal Notation PN = {P,T,I,O,M 0 } P={p1,p2,p3} T={t1} I(p1,t1)=2 O(p2,t1)=2 O(p3,t1)=1 M 0 (p1)=2 8

9 Petri Nets Token Flow Rules Enabling Rule – t:T, t is enabled if each input place p of t has at least the same number of tokens as weight of directed arc 9

10 Petri Nets Token Flow Rules Firing Rule – A) Enabled transition may or may not fire depending on additional interpretation – B) Firing of an enabled transition t, removes an equal number of tokens from each input place as the weight of the transition, and puts tokens in each output place. 10

11 Petri Net Properties Reachability – a marking M i is said to be reachable from M 0 if there exists a sequence of transitions firings which transforms a marking M 0 to M i. Boundedness and safeness – Petri net is k- bounded if the number of tokens in a p is always less than or equal to k. A Petri Net is safe it k =1. Conservativeness – Petri Net is conservative if number of tokens is conserved. 11

12 Petri Net Properties 12 Boundedness and Conservativness

13 Petri Net Analysis The Coverability Tree – Enumerate all possible markings reachable from the initial state. – Algorithm: Let M 0 be root of tree, tag as ‘new’ While ‘new’ markings exist: – Select a ‘new’ marking – If M is identical to another marking, tag as ‘old’ – If no transitions are enabled in M, then M is ‘terminal’ – For every transition in t enabled in M: » Obtain the marking M’, result from firing » If a token value can increase indefinitely place ‘ω’ » Introduce M’ as a node, tag as ‘new’ draw arc from M to M’ 13

14 Petri Net Analysis 14 Boundedness and Conservativness

15 Petri Net Uses 15 Uses: Software Development Industrial Engineering Chemistry


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