Siemens P01-07 manual Industry Automation and Drive Technologies - SCE, Uncertain structure

Industry Automation and Drive Technologies - SCE

A particular control engineering problem is the possibility to generate -by unfavorably using jumps and branches- faulty step sequences. We are distinguishing three possible cases.

–Uncertain sequence: A step sequence contains a structure whose availability is not ensured through the defined sequential performance.

–Partially stuck: A step sequence contains an internal loop that is not exited. Although the steps within this loop are executed, the steps outside the loop are not. This makes parts of step sequence unavailable.

–Totally stuck: A step sequence contains a structure for which no permissible step enabling condition exists. In this case, the step sequence remains permanently in one state and all other steps are unavailable.

Such structures are not permitted in step sequences and have to be excluded with corresponding design methods. Figure 3 shows examples of two step sequences with impermissible structures.

In the left sequence we can’t ensure that Step S6 is available since the alternative branch after Step S3 prevents -when transition t3 is enabled- that the parallel branch is merged again in transition t4. For that reason, this sequence is uncertain. The right sequence, on the other hand, is executed exactly once and then stops at Step S4. Since Step S2 is not active in this state, the parallel branch can no longer be merged in transition t3. It is totally stuck; Step S5 is unavailable.

S 1
t 1
S 2	S 3
	t 2	t 3
	S 4	S 5
t 4		t 5
S 6		S 7
t 6		t 7

S 1
t 1
S 2	S 3
	t 2
	S 4
t 3

S5

t 4

Uncertain structure Illegal structure

Figure 3: Uncertain and illegal structures

TIA Training Manual	Page 4 of 41	Module P01_07
Status: 12/2010		PCS 7 for Universities