Introduction to Industrial Automation

4.3.5   Components’ State Diagram Method

Some types of automation circuit design problems may be solved by composing a state diagram 

different from that presented in Section 4.3.1. Any automated industrial system consists of a num-

ber of digital active components (by means of ON-OFF states) or devices, as explained in Section 

1.1. Such devices may be relays, coils, solenoids, auxiliary relays, lights, timers, etc. Each one of 

the system’s digital components is represented in the diagram by a corresponding state when it 

is in the ON state. Since such a state is not a system state but the ON state of the corresponding 

component, it is denoted as the C

i

 state of the i

th

 component, instead of the notation S

i

. Hence, 

the diagram contains as many states as there are digital components of the industrial system. 

Furthermore, the diagram contains one more state, which corresponds to the common rest state 

of all components, which is also a system state, where all devices are OFF and it is denoted as S

0

. 

The further composition of the diagram is based on the following rules:

  1. A pair of transitions must be introduced between S

0

 and each state C

i

: One for Turn ON and 

another for Turn OFF of the state C

i

.

  2. The logic of the system operation, derived from the corresponding specifications, is allocated 

on the transitions as conditions for firing them.

  3. It is permitted to have two or more synchronous transitions. Hence, two or more states C

i

 

may exist simultaneously in an energized situation.

  4. The ON or OFF mode of a state C

i

 may be a condition of a transition.

  5. If two or more components are identical from the point of view of the operation’s conditions, 

then they are expressed by one common state.

  6. After completion of the diagram, the logical formula (1) is applied for each state C

i

. Hence, 

there is no need for the introduction of auxiliary variables and further coding assignments.

N

R

b

1

 

b

0

C

1

C

1

C

2

C

2

C

2

d

d

T