Introduction to Industrial Automation

Figure 7.65  The moving up-staircase is controlled by a PLC.

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Basic Programming Principles of PLCs 

◾

 

335

T

+24 V DC

Stand-by

ON

Stand-by

OFF

Stand-by

indication

0 V

C

Alarm

stop 1

Alarm

stop 2

e

d

1

d

2

d

1

d

2

d

1

Stop

control

panel  

PC

T

Momentary

operation 

PC = photocell, e = thermal overload switch

Figure 7.66  Conventional automation circuit of the moving up-staircase shown in Figure 7.65.

OR

AND

M3.0

M5.0

I1.6

Q0.1

M4.0

I2.6

AND

OR

I1.2

I1.4

I2.0

I2.2

I2.4

M3.0

AND

M3.0

M5.0

T1

OFF-delay

40 sec

M5.0

LAD

I1.0

M3.0

M3.0

M4.0

M4.0

I1.2

I1.4

I2.0

I2.2

I2.4

M3.0

I2.6

M3.0

M5.0

40 sec

OFF-delay

T1

M5.0

M3.0

Q0.1

I1.6

Q0.4

PLC

Inputs

Outputs

0 V

+24 V DC 

0 V

+24 V DC

I1.0

I1.4

C

Q0.4

I1.2

I2.0

I2.4

I2.2

I2.6

Q0.1

e

PC

Stop

(panel)

Stand-by

ON

Stand-by

OFF

Alarm

stop 1

Alarm

stop 2

Moment

operation

Stand-by

indication

(M)

I1.6

FBD

I1.0

M3.0

AND

M3.0

Q0.4

M4.0

OR

Figure 7.67  Connection status of I/O devices for implementation in a PLC and the required 

programs in LAD (left) and FBD (right) languages.

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336

 

◾

  Introduction to Industrial Automation

Counting and numerical calculations. Figure 7.68 shows two conveyor belts that move similar 

products from one point to another in an industrial manufacturing process. On each conveyor 

belt, the transported objects are counted by a corresponding photocell (PC

1

 and PC

2

). We will be 

dealing with only a part of the whole operation of the conveyor belts and therefore only part of 

their automation logic. Thus, it is supposed that the two conveyor belts have been put into opera-

tion based on some conditions and a corresponding set of instructions that do not need to be 

analyzed further in this example. Specifically, it is desired:

  1. When the number of objects on a conveyor belt exceeds 500, the conveyor stops.

  2. When the difference between the numbers of transferred objects on the two conveyor belts 

exceeds 200, a light indicator h has to be activated without stopping the conveyor belts.

  3. The current sum of the counted objects on both conveyors needs to be stored into a data 

block (DB) as a numeric value renewed in each scan cycle.

The required Boolean language program for this section of the entire operation and automa-

tion of the two conveyors is the following:

•••

S

Q0.0

S

Q0.4

A

I1.0

CU C1

A

I1.3

CU C2

L

C1

L

‘500’

≥ 

R

Q0.0

L

C2

L

‘500’

≥ 

R

Q0.4

OPN  DB12

L

C1

L

C2

+

T

DBW3

L

C1

L

C2

> 

JC

BIG1

<

JC

BIG2

BIG1: L 

C1

L

C2

–

T

MW100

L

MW100

L

‘200’

> 

=

Q0.2

JU

EN D

BIG2: L

C2

L

C1

–

T

MW200

L

MW200

L

‘200’

>

=

Q0.2

EN D: BE

DB12

0:

1:

2:

3: “C1+C 2”

4:

PC

1

PC

2

(M

1

)

(M

2

)

PLC

Inputs

Outputs

0 V

+24 V DC

0 V

+24 V DC

I1.0

C

1

Q0.2

I1.3

Q0.0

PC

2

(M

1

)

(M

2

)

PC

1

C

2

Q0.4

h

Figure 7.68  Belt conveyors for transferring products and the PLC controlling their operation.

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Basic Programming Principles of PLCs 

◾

 

337

Temperature control. Figure 7.69 shows a temperature sensor (thermocouple), which measures 

the temperature of a body. The output of the sensor is an analog voltage (±50 mV) and is connected 

to a corresponding analog input (Ch. 1) of the PLC. Based on the measurement made by the 

sensor, it is desired to control the body temperature by activating and deactivating the heater C, 

(ON-OFF control). In particular, it is desired to keep the temperature at 300 °C with an accept-

able variation of 2%, and therefore to keep it between the limit values of 297 °C and 303 °C. This 

means that the heater will operate (ON) for t ≤ 297 °C and will stop (OFF) for t ≥ 303 °C. In order 

to proceed, it is important to know how to read (in which memory address) the analog input value 

in the particular PLC being utilized, and where it is stored. For this example, it is assumed that 

the analog input value (Ch. 1) is stored in the MW 100 memory word and the layout is turned on 

with an ON-OFF switch (input I1.1). The Boolean language program will include the following 

instructions:

L

MW100

L

‘297’

≤ 

=

M0.2

L

MW100

L

‘303’

≥ 

=

M0.3

A

I1.1

AN M0.3

A(

O

M0.2

O

Q0.1

)

=

Q0.1

BE

At this point it is necessary to clarify the following issue. In the above program, it has been 

implicitly assumed that by executing the instruction L MW100, a numeric value, which expresses 

the temperature in degrees Celsius, is loaded into the register. This is not really the case, since the 

temperature sensor, in reality, applies to the analog input voltage, and the numerical value of this 

voltage is stored in the memory. Therefore, the comparison that follows in the program refers to 

dissimilar things. In order for the comparison to be correct, it is necessary to make the so-called 

“analog value scaling”. The scaling procedure means converting the voltage values to equivalent 

temperature values, or vice versa, and thus to have comparable sizes. As a rule of thumb, the first 

solution is preferred because humans understand the engineering units (values) of physical size 

Ch.1

Analog

outputs

± 50 mV

PLC

Digital

inputs

Digital

outputs

0 V

+24 V DC 

0 V

+24 V DC 

C

I1.1

Ch.2

Q0.1

ON-OFF

switch

Analog

inputs

Ch.1

Ch.2

Object

Thermocouple

Shielded cable

Heater

Heater