Introduction to Industrial Automation

Figure 7.28  The machine (C) operates when sensor contact S

1

 is open. (a) Conventional circuit 

and (b) implementation of (a) with PLC.

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Basic Programming Principles of PLCs 

◾

 

303

Subsequently, the automation circuit of Figure 7.30a will be considered, where the contained 

two branches have conflicting logic. This means that when the C

1

 relay is activated, it is not pos-

sible to activate the C

2

 and C

3

 relays due to the sensor S

1

. Thus, to activate the C

1

 relay, the S

1

 

sensor should be activated, while in order to activate the C

2

 and C

3

 relays, the S

1

 sensor should 

be deactivated, two conditions that cannot coexist. In particular, it is desired that the relay C

1

 is 

activated when the switching contacts (in the corresponding branch) S

1

 and S

3

 are closed and the 

contact S

2

 also remains closed. In the same way, the relays C

2

 and C

3

 need to be activated when 

the S

1

 and S

3

 contacts, in the second branch, remain closed and the contact S

2

 is also closed. It is 

obvious that when the contact S

1

 in the first branch is closed, then the corresponding contact S

1

 in 

the second branch will open, since these are two different contacts of the same sensor (S

1

). Figure 

7.30b shows the connection of the I/O devices to the PLC in order to implement the circuit as a 

programmable automation. The indicated status of the switching contacts applies to the case of 

non-activated sensors. The C

2

 and C

3

 relays, connected in parallel to the conventional circuit, are 

PLC

INPUTS

OUTPUTS

0 V

C

Q 0.2

I 1.0

 

I 1.4

STOP 

START

R

 

N

C

C

 

 

PLC

INPUTS

OUTPUTS

0 V

Q 0.2

I 1.0

 

I 1.4

PROGRAM

I 1.0

I 1.4

Q 0.2

Q 0.2

PROGRAM

I 1.0

I 1.4

Q 0.2

Q 0.2

PROGRAM

I 1.0

I 1.4

Q 0.2

Q 0.2

PROGRAM

I 1.0

I 1.4

Q 0.2

Q 0.2

PROGRAM

I 1.0

I 1.4

Q 0.2

Q 0.2

PLC

Inputs

Outputs

+24 V DC 

0 V

+24 V DC 

Q0.2

I1.0

STOP

 

START

I1.4

 

 

PLC

Inputs

Outputs

0 V

C

+24 V DC 

0

 V

+24 V DC

Q0.2

I1.0

STOP

 

START

I1.4

OR

Program

I1.0

I1.4

Q0.2

Q0.2

Program

I1.0

I1.4

Q0.2

Q0.2

Figure 7.29  The conventional START-STOP automation circuit for the LAD program for NC and