Introduction to Industrial Automation

and two START-STOP control positions.

N

C

C

R

 

f

0

b

1

 

b

0

e

f

1

Figure 4.8  The automation circuit extracted from the state diagram of Figure 4.7 for the direct 

on line starting motor with thermal overload protection and two START-STOP control positions.

---

Logical Design of Automation Circuits 

◾

 

125

we would like to introduce the concept of mutual exclusion, which means that the operation of 

one motor should exclude the operation of the other one. Thus, in this case, it means that for 

C

1

 we will have that ‘Turn OFF’ = b

0

+C

2

, as indicated in Figure 4.9. In the same approach, for the 

C

2

 motor, the C

1

 operation signal will be equivalent to the stop of the C

2

 operation. If we apply 

the logical expression (1) for both variables C

1

 and C

2

 we will get that:

 

C

b

C

b C

b C

b C

1

0

2

1

1

0

2

1

1

=

+

+

=

+

⋅

⋅

⋅

(

)

(

)

and

 

C

C s C

2

1

2

=

+

⋅

(

)

The implementation of these logical expressions is displayed in Figure 4.10, where an error exists, 

since the energizing of C

2

 and the operation of the corresponding motor cannot be altered. This is 

due to the fact that, from the moment the relay C

2

 gets energized, this will be in a continuous latch, 

independent of the operation of the sensor. Moreover, the energizing of C

1

 is not possible due to the 

corresponding latch in the first branch of the automation circuit, and thus C

2

 will always be ener-

gized. If the C

2

 variable with the logical expression (1) is applied without the term of the “present 

state of variable”, then we will obtain that:

 

C

C s

2

1

=

⋅

which can be implemented from the third correct branch, indicated in the same figure. Thus, as a 

general design consequence, when the operation signal (or the Turn ON signal) for a state is not 

C

1

=1

C

1

=0

C

2

=0

b

1

b

0

+C

2

C

2

=1

s

C

1