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Artificial Intelligence and Soft Computing: Behavioral ... - Arteimi.info

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occurrence of a state, but also in addition coordinates more than one activity<br />

also, concurrently.<br />

24.9.2 Co-ordination by Timed Petri Net Model<br />

Petri nets are graphical models to study parallelism in a program, deadlock<br />

management in operating system, token movements in a data flow graph<br />

<strong>and</strong> reasoning in knowledge based systems [17]. Currently Petri nets are<br />

gaining importance in modeling coordination of a multitasking system.<br />

Thus it has also found applications in coordination of multitasking<br />

activities in mobile robots as well. Ideal Petri nets [4], however, suffer<br />

from one limitation, called conflict problem. The conflict problem is<br />

discussed below with reference to fig. 24.21.<br />

tr1<br />

•<br />

p1 p2 p3<br />

p4<br />

tr2<br />

p5<br />

transition<br />

a token<br />

Fig. 24.21: A Petri net with transition tr1 <strong>and</strong> tr2, places p1, p2, p3, p4, p5,<br />

where p1, p2 are input places of tr1; p2, p3, are input places of tr2.<br />

p4, p5 are output places of tr1 <strong>and</strong> tr2 respectively. Dots denote<br />

tokens in a place.<br />

In this figure, the transition tr1 <strong>and</strong> tr2 could fire if each of them has a<br />

token at all its input places. However, if tr1 fires earlier than tr2, then<br />

according to the notion of Petri nets, p4 will receive one new token <strong>and</strong> p1, p2<br />

will lose their tokens. Consequently, transition tr2 cannot fire as place P2 does<br />

not possess a token. Thus either of these two transitions can fire only in this<br />

system. However none of the transitions tr1 <strong>and</strong> tr2 will fire, unless no<br />

additional condition is imposed regarding the priority of the firing of the<br />

transition. This problem of waiting in a loop is often referred to as time<br />

conflict in Petri nets theory. One way to restore this conflict is to attach time

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