Introduction to Industrial Automation

duration and type.

N

C

1

R

C

1

C

2

START

e

1

 

STOP

e

2

C

1

Figure 4.26  Automation circuit for Example 4.10 based on the empirical designing.

S

0

C

2

C

3

C

1

s

x

s

x

T

C

2

Figure 4.27  The component state diagram for Example 4.10.

---

140

 

◾

  Introduction to Industrial Automation

The next step of circuit design is to write the logical formula (1) for all states C

i

, as follows:

 

Turn ON C

x

Turn OFF C

s

Turn ON C

x

Turn OFF C

1

1

2

=

=

=

22

3

3

2

=

=

=

T

Turn ON C

s

Turn OFF C

C

 

C

s x C

C

T x C

C

C s C

C s C

1

1

2

2

3

2

3

2

3

=

+

=

+

=

+

=

+

(

)

(

)

(

)

(

)

For the operation of the active components we have,

 

M

C M

C T C

1

1

2

2

3

=

=

=

,

,

By converting the above equations to an automation circuit, we obtain the circuit shown in Figure 4.28.

The contacts e

1

 and e

2

 of the thermal overload relays are added in the circuit a posteriori, since 

there is no reason to increase the difficulty in writing Boolean equations. The Boolean equation for 

state C

3

, where the timer T is activated, also introduces a simple contact without delay. Usually, the 

timers do not offer simple contacts without delay. This necessitates the introduction of two distinct 

devices, the relay C

3

 and the timer T.

THIRD SOLUTION BASED ON THE SYSTEM STATE DIAGRAM METHOD

The system of the drilling and milling machine tool has two operation states, except that of the so-

called “rest state”. The first one corresponds to the operation of both motors M

1

 and M

2

, while the 

second one to the operation only of the motor M

2

. The timer T is activated simultaneously with M

2

. 

N

C

1

R

C

1

C

2

START (x)

e

1

 

STOP ( s )

e

2

C

2

START (x)

T

C

3

C

3

STOP (s)

C

2

T

(M

1

)

(M

2

)

Figure 4.28  Automation circuit for Example 4.10 based on the component state diagram of 

Figure 4.27.

---

Logical Design of Automation Circuits 

◾

 

141

The three-state diagram is shown in Figure 4.29, where the signals “x” and “s” correspond to the 

START and STOP buttons as previously. The code assignment of the auxiliary variables A and B 

present a simultaneous change of them from the state S

2

 to state S

0

. The violation of this basic rule, 

concerning the state diagram synthesis, can be bypassed by introducing the pseudo-state S

3

, keep-

ing the same signal T for both its transitions. For the code assignment shown in Figure 4.29, the 

following equations can be derived:

 

Turn ON A s

Turn OFF A

Turn ON B x

Turn OFF

=

=

=

B

TB

A

,

,

B

=

TA

Applying logical formula (1) to the two auxiliary variables we obtain,

 

A TB sB A

T B sB A

B TA xA B

T A xA B

=

+

=

+

+

=

+ =

+

+

(

)

(

)

(

)(

)

(

)(

)

For the operation of the two motors and the timer we have,

 

M

AB

1

=

 

M

AB AB B

2

=

+

=

 

T AB

=

By converting the above equations to an automation circuit, we obtain the circuit shown in Figure 4.30.

A careful inspection of the circuit can lead to the conclusion that the power relay M

2

 is identical 

to the auxiliary relay B. Therefore, the relay B can be omitted and its contacts may be substituted 

by corresponding ones of relay M

2

.

The comparison of the three automation circuits shown in Figures 4.26, 4.28, and 4.30, 

derived by three different methods, leads to the following significant comments. First of all, 

it should be highlighted that all the three automation circuits are different, with each one 

having a different total number of contacts and auxiliary relays, while all the three circuits 

are operating correctly, which is also the most important demand when building industrial 

automations.

x

S

0

M

1 

= ON

M

2 

= ON

S

1

S

2

S

3

s

T

01

11

10

M

1 

= OFF

M

2 

= ON

T = ON

T

AB

00