Introduction to Industrial Automation

ditions (Application 4.4.3).

---

Logical Design of Automation Circuits 

◾

 

153

 

Turn OFF C

S S p

S S p

3 A 0, B

3 A

=

+

=

=

7 3

1

6 2

 

 

( )

( )

==

=

=

=

=

+

+

1, B 1

3 A , B 0

3

A

S S p

S S p t

5 1

1

4 0

0

 

 

( )

( )

,, B

p AB p AB p AB p tAB p BA p Bt p B

p BA

=

=

+

+

+

=

+

+

=

+

0

3

3

3

3

3

3

3

3

pp B p t p B p A p t p B A t

3

3

3

3

3

3

+

=

+

+

=

+ +

(

)

Applying logical Equation (1) to the A, B, and C auxiliary variables, we obtain:

 

A p B C t s

A

p

BCt

s

A

1

=

+ +

+

=

+

+









1

1

1

(

)(

)

(

)

(

)

 

B p C A t s

B

p

CAt

s

B

2

2

2

=

+ +

+

=

+

+





(

)(

)

(

)

(

)

2

 

C p B A t s

C

p

BAt

s

C

=

+ +

+

=

+

+

(

)









3

3

3

3

(

)

(

)

(

)

The implementation of the previous logical expressions gives us the automation circuit shown 

in Figure 4.41 for the case of three machines, where it is obvious that this can be easily expanded 

and generalized for the n machine case.

The automation circuit shown in Figure 4.41, although it operates well, presents a basic dis-

advantage. The pressing of pushbutton STOP of the last machine must happen simultaneously 

N

R

 

A

p

1

B

A

s

1

C

t

 

B

p

2

C

B

s

2

A

t

 

C

p

3

B

C

s

3

A

t

Figure 4.41  Automation circuit for application in Section 4.4.3 based on the state diagram of 

Figure 4.40.

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154

 

◾

  Introduction to Industrial Automation

with the activation of the sensor. This requirement, specifically for the case of central heating, is 

not acceptable, since each inhabitant can’t wait for when the sensor will be energized in order to 

press the STOP pushbutton. In addition, each inhabitant does not have information about the 

state of the sensor, if it has already been energized, or when it is going to be energized. In fact, the 

condition “p

i

 

⋅

 t” for the STOP of the last machine in the state diagram means that the button “p

i

” 

must be pressed simultaneously with the sensor “t” activation, something that was intentionally 

stated in order for the reader to establish the improvement of the automation circuit succesively. 

What is desired in this case is the STOP signal, caused by the press of the button, is saved in the 

“memory” of the automation circuit and when the sensor stops the last machine is realized. One 

approach to solve this problem may be the addition of one more state per machine in the state dia-

gram of Figure 4.40. This new state will represent the condition “STOP has been pressed and the 

sensor awaits” where the electrovalve continues to be open. With three new states in the already 

complex diagram of Figure 4.40, someone has to write the Turn ON and Turn OFF expressions 

of the auxiliary variables again, applying the logical formula (1) and extracting the new automa-

tion circuit. A second approach, based on empirical design knowledge, is to create the “memory” 

element for saving an instant signal, based on the self-latching principle described in Section 3.1.1. 

According to this principle, in the world of automation, one auxiliary relay constitutes a memory 

with a size of 1 bit, when it is energized by a momentary pushbutton signal, and remains energized 

through its self-latching contact. Therefore, we need three more auxiliary relays d

i

 (i=1, 2, 3) than 

N

R

A

p

1

B

A

s

1

C

t

d

1

A

d

1

d

1

B

p

2

A

B

s

2

C

t

d

2

B

d

2 

d

2

C

p

3

B

C

s

3

A

t

d

3

C

d

3 

d

3

Figure 4.42  The improved automation circuit for application in Section 4.4.3 by adding the