A Petri Net model of the uncontrolled plant is shown in Figure
22, and the meaning of the places is given in Table 4
below. Notice that we have modeled the system such that machine 3
loads one part at a time. This makes the model general enough so that
we may consider different loading specifications for machine 3 without
changing the plant model.
Figure 22: Petri Net Model for 3-Machine Example
We can see in the PN model that both safety specifications (
Safety 1 and Safety 2), as well as Mutual Exclusion, are
already satisfied. The last three specifications, as mentioned
earlier, are sequential specifications, and it should be noted that,
with the above PN model, it is not possible to describe these
specifications in terms of the markings of this net. Once again, we
are lacking a formal procedure to handle sequential specifications
with Petri Nets. A possible solution to the Different Types
for M3- specification is shown in Figure 23.
Figure 23: Petri Net Supervisor for 3-Machine Example