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

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with the tokens <strong>and</strong> firing delay with the transitions. Such Petri nets that<br />

include the notion of time with tokens are called timed Petri nets. Fig. 24.22<br />

describes a Timed Petri net, where the semantics of the places are presented in<br />

the figure itself. The firing conflict in a Petri net with transitions like tr1 <strong>and</strong><br />

tr2 of fig. 24.21 is avoided by using the following strategy.<br />

Strategy: If the arrival time of the latest token in the input place of one<br />

transition + its firing delay < the arrival time of the latest token in the input<br />

place of the second transition (with common place) + its firing delay, then the<br />

former will fire.<br />

d4,t4<br />

d2,t2<br />

d7,t7<br />

d6,t6<br />

d1,t1<br />

d 1, t1 = Sense surrounding by sensor <strong>and</strong> camera at time t1;<br />

d 2, t2 = Build local map of surrounding from sonar sensor <strong>and</strong> compare with world map<br />

at time t2;<br />

d 3,t3= Image processing <strong>and</strong> matching with image database at time t3;<br />

d 4, t4 = Plan for obstacle free path at time t4;<br />

d 5,t5 = Object is recognized (suppose that is a useful tool) at time t5;<br />

d 6,t6 = Start from node n i at time t6;<br />

d 7,t7 = Stop at node n i+1 at time t7;<br />

d 8,t8 = Start movement to keep the tool in pre specified place at time t8;<br />

d 9, t9 = Place the tool at proper place at time t9;<br />

d8,t8<br />

d3,t3<br />

d5,t5<br />

Fig. 24.22: Timed Petri net showing the coordination of multitasking<br />

mobile robot.<br />

d9,t9

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