4 Petri Nets
4.4 Continuous Petri Nets
4.4.1 Continuous Capacitive Petri Nets
Every place in a continuous capacitive Petri net is provided with a lower and upper limit of marks that it can contain, similar to capacitive Petri nets (see Definition 4.14).
Definition 4.45 (continuous capacitive Petri net)
The tuple , , , G, , , , , is a continuous capacitive Petri net if , , , , , , is a continuous Petri net, the map : → assigns a minimum capacity to every place ∈ , the map : → assigns a maximum capacity
to every place ∈ , and the initial marking must satisfy the condition ∀ ∈ .
The continuous capacitive Petri nets require a redefinition of the activation and firing process (Definition 4.40 and Definition 4.41). A transition in a continuous capacitive Petri net is active if all input places have either a marking greater than their minimum capacities or they are fed by at least one input transition, i.e. the input speed is not zero. Additionally, all output places have either a marking less than their maximum capacities or they are emptied by at least one output transition, i.e. the output speed is not zero. If all input places have a marking greater than their minimum capacities and all output places have a marking less than their maximum capacities, the transition is said to be strongly active.
Definition 4.46 (activation continuous capacitive Petri net)
The tuple , , , , , , , , is a continuous capacitive Petri net. A transition ∈ is active if and only if
∀ ∈ ∶ ∨ ∧ 0
and
∀ ∈ ∶ ∨ ∧ 0 .
It is strongly input active if
∀ ∈ ∶
is also satisfied and otherwise it is weakly input active. It is strongly output active if
∀ ∈ ∶
is also satisfied and otherwise it is weakly output active. If it is strongly input and output active, it is strongly active. If it is weakly input and output active, it is weakly active.
Thereby, ist the set of input places with ∧ 0 and is the set of output places of with ∧ 0.
A strongly active transition fires with maximum speed. However, the speed of a weakly (input/output) active transition has to be decreased according to the input speeds of the input places with markings equal to the minimum capacities and/or according to the output speeds of the output places with markings equal to the maximum capacities.
Definition 4.47 (firing process continuous capacitive Petri net)
The tuple , , , , , , , , is a continuous capacitive Petri net. A strongly active transition ∈ fires with maximum speed . A weakly (input/output) active transition
∈ , not involved in an actual conflict according to Definition 4.48, fires with the instantaneous speed
min min
∈
1
→ → ⋅
∈
,
∈min
1
→ → ⋅
∈
,
min ∈
1
→ ⋅ , min
∈
1
→ ⋅ , .
The firing process of the transitions is described by a negative mark change of all input places expressed by the following differential equation
→ ⋅ ∀ ∈
and a positive mark change of all output places expressed by the differential equation
→ ⋅ ∀ ∈ .
The mark change of the place ∈ can be calculated by the following differential equation ,
where is the balance of place , i.e. the difference between input and output speed.
Example 4.19
Figure 4.26 shows a continuous capacitive Petri net without actual conflicts. Place 2 has a maximum capacity of 100 marks. At time 0, transition 1 becomes strongly active and fires
with the maximum speed 3. Immediately afterwards, 2 becomes strongly active and fires with the maximum speed 2. At time 100, the maximum capacity of 2 is reached 2 100 2 but it is emptied by transition 2. Hence, 1 is weakly output active with the instantaneous speed 2 so that . Then the corresponding mark changes are both zero.
Figure 4.26: Continuous capacitive Petri net without actual conflicts (left) and the mark evolution (right) (Example 4.19)
Example 4.20
Figure 4.27 shows a continuous capacitive Petri net. Place 1 has a minimum capacity of 2 marks and 2 has a maximum capacity of 10 marks. At time 0, 2 is weakly active because the marking of the input place 1 is at the minimum capacity and, additionally, the marking of its output place is at the maximum capacity but both places are fed and emptied, respectively.
Figure 4.27: Continuous capacitive Petri net with a weakly active transition (Example 4.20)
Hence, the speed of 2 has to be slowed down in such a way that and .
The instantaneous speed of 2 is
min 1
1 → 2 1 → 1 , 1
2 → 2 2 → 3 ,
min 2, 1,3 1 and the mark changes are
1 2 1 1
T1 P1
2 T2 P2
10 T3
1 1 1 1
cl(P1)=2 cu(P2)=10
v1=2 v2=3 v3=1
2 1 1 0.
A place in a continuous capacitive Petri net can have an actual input and output conflict. It has an actual output conflict if the input speed is not sufficient for firing all active output transitions with the instantaneous speed of Definition 4.47. On the other hand, it has an actual input conflict if the output speed is not sufficient for receiving marks from all active input transitions with the instantaneous speed of Definition 4.47. Similar to the continuous Petri nets without capacities, these conflicts can be resolved by priority or sharing. Thereby, it has to be ensured that the solution leads to a positive balance in the case of an output conflict and to a negative balance in the case of an input conflict. Additionally, the preliminary speeds of the involved transitions may not be exceeded.
Definition 4.48 (actual (output/input) conflict, preliminary speed)
The tuple , , , , , , , , is a continuous capacitive Petri net. A place ∈ has an actual output conflict if
and
→ ⋅ ̅
∈
. A place ∈ has an actual input conflict if
and
→ ⋅ ̅
∈
,
whereby
̅ min min
∈
1
→ → ⋅
∈
,
∈min
1
→ → ⋅
∈
, ,
min min
∈
1
→ ⋅ , min
∈
1
→ ⋅ ,
is said to be the preliminary speed of a transition ∈ .
Definition 4.49 (feasible solution continuous capacitive Petri net)
A solution of an actual output conflict of a place ∈ which satisfies the following conditions
0
∀ ∈ ∶ ̅
is said to be feasible; otherwise, it is infeasible.
A solution of an actual input conflict of a place ∈ which satisfies the following conditions
0
∀ ∈ ∶ ̅
is said to be feasible; otherwise, it is infeasible.
The approach of Definition 4.44 has been adapted for continuous capacitive Petri nets to achieve a feasible solution of actual conflicts.
Definition 4.50 (sharing proportional to maximum speeds continuous cap. Petri net) The tuple , , , , , , , , is a continuous capacitive Petri net. An active transition
∈ , not involved in an actual conflict, fires with the speed of Definition 4.47. If a place
∈ has an actual conflict according to Definition 4.48, the speeds of input and output transitions have to be adapted so that the constraints of Definition 4.49 are satisfied.
This is done by sharing proportional to the maximum speeds of the involved input and output transitions. The instantaneous speed of an active transition ∈ which has at least one input place with an actual output conflict and no output places with an actual input conflict is then given by
∈min ⋅ .
Thereby, the expression
∑ ∈ → ⋅
is called output decreasing factor of place . This factor causes that the first condition of a feasible solution is satisfied with 0. The maximum speed is scaled by the minimum decrasing factor of all input places so that the condition 0 is always satisfied. But the preliminary speed could be exceeded for some transitions. Then the speeds of all transitions
∈ for which ̅ are set to the preliminary speed
̅ .
The subset contains all these transitions and the decreasing factor has to be modified by
∑ ∈ → ⋅
∑ ∈ → ⋅ ∑ ∈ → ⋅ .
This factor guarantees that 0 but the premilary speed could be exceeded for some transitions. Then the mentioned procedure has to be performed again.
The instantaneous speed of an active transition ∈ which has at least one output place with an actual input conflict and no input place with an actual output conflict is then given by
∈min ⋅ .
Thereby, the expression
∑ ∈ → ⋅
is called input decreasing factor of place . This factor causes that the first condition of a feasible solution is satisfied with 0. The maximum speed is scaled by the minimum decrasing factor of all input places so that the condition 0 is always satisfied. But the preliminary speed could be exceeded for some transitions. Then the speeds of all ∈
for which ̅ is set to
̅ .
The subset contains all these transitions. Then the decreasing factor has to be modified by
∑ ∈ → ⋅
∑ ∈ → ⋅ ∑ ∈ → ⋅ .
This factor guarantees that 0 but the premilary speed could be exceeded for some transitions. Then the mentioned procedure has to be performed again.
The instantaneous speed of an active transition ∈ which has at least one input place with an actual output conflict and at least one output place with an actual input conflict is then given by
min min
∈ , min
∈ ⋅ .
If this speed exceeds the preliminary speed, the mentioned procedure above has to be performed again.
Example 4.21
Figure 4.28 represents two continuous capacitive Petri nets which only differ in the maximum speed of transition 4. Place 2 of the left Petri net has no actual conflict. Transitions 1 and 2 are weakly output active due to 1 1 , 2 2 , 3 3 but they are all emptied, i.e. , , 0. The instantaneous speeds are given by
̅ min 1
1 → 1 1 → 3 , 1
1 → 2 2 → 4 ,
min 1
1⋅ 1 ⋅ 3,1
3⋅ 1 ⋅ 10.1, 3 3
̅ min 1
2 → 2 2 → 4 , 1
2 → 3 3 → 5 ,
min 1
1⋅ 1 ⋅ 10.1,1
2⋅ 1 ⋅ 1,2 1 2 and, hence,
10.1 1 → 2 ⋅ 2 → 2 ⋅ 91
2.
Figure 4.28: Continuous capacitive Petri nets without actual input conflict (left) and with actual input conflict (right) (Example 4.21)
The mark changes are
1 1 → 1 ⋅ 1 → 3 ⋅ 0
2 1 → 2 ⋅ 2 → 2 ⋅ 2 → 4 ⋅ 0.6
3 2 → 3 ⋅ 3 → 5 ⋅ 0.
However, 2 of the right Petri net has an actual input conflict due to
̅ min 1
1 → 1 1 → 3 , 1
1 → 2 2 → 4 ,
min 1
1⋅ 1 ⋅ 3,1
3⋅ 1 ⋅ 7.5, 3 2.5
̅ min 1
2 → 2 2 → 4 , 1
2 → 3 3 → 5 ,
min 1
1⋅ 1 ⋅ 7.5,1
2⋅ 1 ⋅ 1,2 1 2 7.5 ≱ 3 ⋅ ̅ 1 ⋅ ̅ 8.
The resolution of this conflict is performed by sharing proportional to the maximum speeds of the involved transitions 1 and 2. At first, the input decreasing factor of 2 is calculated
1 → 2 ⋅ 2 → 2 ⋅
7.5 3 ⋅ 3 1 ⋅ 2
15 22. Then the speeds of 1 and 2 are decreased by this factor
⋅ 15
22⋅ 3 2 1 22 ̅
⋅ 15
22⋅ 2 1 4
11≰ ̅ .
But the calculated speed of 2 exceeds the preliminary speed so that a feasible solution is achieved by setting the instantaneous speed to the preliminary speed
̅ 1
2
and the factor for decreasing the speed of 1 has to be recalculated by 7.5 1 ⋅
3 ⋅
7 9
⋅ 21
3. The mark changes are
1 1 → 1 ⋅ 1 → 3 ⋅ 2
3
2 1 → 2 ⋅ 2 → 2 ⋅ 2 → 4 ⋅ 0
3 2 → 3 ⋅ 3 → 5 ⋅ 0.
Example 4.22
Figure 4.29 shows a continuous capacitive Petri net. Transitions 4 and 5 are weakly active because the input places have markings equal to the minimum capacities and the output
places have markings equal to the maximum capacities; but the input places are fed and the output places are emptied.
Figure 4.29: Continuous capacitive Petri net with actual input and output conflict (Example 4.22)
The preliminary speeds
̅ min min 1
1 → 4 1 → 1 , 1
2 → 4 2
→ 2 , min 1
4 → 4 4 → 6 , 1
4 → 5 5
→ 7 , min min 3,2.5 , min 3,2 , 3 2
̅ min min 1
2 → 5 2 → 2 , 1
3 → 5 3
→ 3 , min 1
5 → 5 5 → 7 , 1
5 → 6 6
→ 8 , min min 7.5,2 , min 6,2 , 2 2 reveal that 2 has an actual output conflict
7.5 2 → 4 ⋅ ̅ 2 → 5 ⋅ ̅ 8
and that 5 has an actual input conflict
6 4 → 5 ⋅ ̅ 5 → 5 ⋅ ̅ 8.
The resolution of these conflicts is performed by sharing according to Definition 4.50. The decreasing factors
2 → 4 ⋅ 2 → 5 ⋅
15 22
T6
T7
T8 P4
10
P5 10
P6 10
v6=3
v7=6
v8=2 1
1
2 cu(P4)=10
cu(P5)=10
cu(P6)=10 T1
T2
T3
P1 2
P2 2
P3 2
T4
T5 1
2
1
3
1
1 1
v1=3
v2=7.5
v3=1
v4=3
v5=2 1
3
1
2 cl(P1)=2
cl(P2)=2
cl(P3)=2
4 → 5 ⋅ 5 → 5 ⋅
6 11 lead to the instantaneous speeds
min , ⋅ 1 7
11 ̅
min , ⋅ 1 1
11 ̅ which is a feasible solution. The corresponding mark changes are
1 1 → 1 ⋅ 1 → 4 ⋅ 1 4
11
2 2 → 2 ⋅ 2 → 4 ⋅ 2 → 5 ⋅ 11
2
3 3 → 3 ⋅ 3 → 5 ⋅ 10
11
4 4 → 4 ⋅ 4 → 6 ⋅ 1 4
11
5 4 → 5 ⋅ 5 → 5 ⋅ 5 → 7 ⋅ 0
6 5 → 6 ⋅ 6 → 8 ⋅ 1 9
11.