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6. PETRI NETS

•These are like state diagrams, except multiple states can be active at the same time.

•Other techniques, such as GRAFCET, are based on Petri nets.

6.1 INTRODUCTION

• Ideal for parallel control problems

Token - Indicates place active (control)

Place - Indicates part of a system state (as before). If there is a token here, the place is valid, or active.

Transition - This will wait until all inputs have a token. Those tokens are destroyed and new tokens are put into all the outputs

e.g. An example of basic operation

The Petri Net (with no tokens)

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Initial State : The tokens are added below, based upon the system

Step 1 : One of the states gives up a token (this is arbitrary and depends on the state)

transition condition fires

Step 2 : Another state gives up a token (again arbitrary)

In this case the transition can fire because both input places now have tokens, we are only waiting for the condition to occur.

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Step 3: Here the conditions of the transition are met. The two input tokens are consumed and a new token is created for the output place.

Step 4: We see the condition for the transition met.

This continues on indefinitely.........

• Basic logic functions are shown below,

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and

exclusive or

regulator

loop/repeat

if/wait

(state transition) synchronize

state machines (only 1 input/output on transitions)

• We can model various logic functions with Petri nets,

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cup removed

filter coffee

10 sec delay

pour coffee

Try:

1.Add a coffee strength selection to the Petri Net.

2.Draw the petri net above using a Parallel Process Flowchart

3.Develop some ladder logic for the petri net diagram

•Petri nets have been used for the modeling, control and validation of the control model [Teng