Petruzella F.D. Programmable Logic Controllers

Подождите немного. Документ загружается.

Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs Chapter 6 113

Status Indicating Lights —Water pump running light

(green)

- Low water level status light (red)

- High water level status light (yellow)

Figure 6-51 shows a program that can be used to imple-

ment control of the water level in the storage tank. The

latch and unlatch instructions form part of the program.

The operation of the program can be summarized as

follows:

• An internal storage bit is used for the latch and ad-

dress rather than an actual discrete output address.

Both have the same addresses.

• The rung 1 Examine-on instruction addressed

to the off/on switch prevents the pump motor

from starting under any condition when in the off

(open) state.

• In the MAN mode, the rung 1 Examine-on instruc-

tion addressed to the low sensor switch allows the

pump motor to operate only when the low level sen-

sor switch is closed.

• In the AUTO mode, whenever the high sensor

switch is momentarily closed the Examine-on in-

struction of rung 1 addressed to it will energize the

latch coil. The pump will begin running and con-

tinue to operate until the unlatch coil is energized by

the rung 3 Examine-off instruction addressed to the

low sensor switch.

• The pump running status light is controlled by the

rung 4 Examine-on instruction addressed to the

motor output.

• The status bit will remain set to 1 when the pushbut-

ton is released and logical continuity of the latch

rung is lost.

• When the off pushbutton (I:1/1) is momentarily ac-

tuated, the unlatch rung becomes true and the latch

status bit (O:2/5) is reset back to 0 and so the light

is switched off.

• The status bit will remain reset to 0 when the push-

button is released and logical continuity of the latch

rung is lost.

Output latch is an output instruction with a bit-level

address. When the instruction is true, it sets a bit in the

output image  le. It is a retentive instruction because the

bit remains set when the latch instruction goes false. In

most applications it is used with an unlatch instruction.

The output unlatch instruction is also an output instruc-

tion with a bit-level address. When the instruction is true,

it resets a bit in the output image  le. It, too, is a retentive

instruction because the bit remains reset when the instruc-

tion goes false.

The process shown in Figure 6-50 is to be used to con-

trol the level of water in a storage tank by turning a dis-

charge pump on or off. The modes of operation are to be

programmed as follows:

OFF Position —The water pump will stop if it is run-

ning and will not start if it is stopped.

Manual Mode —The pump will start if the water in

the tank is at any level except low.

Automatic Mode —If the level of water in the tank

reaches a high point, the water pump will start so that

water can be removed from the tank, thus lowering

the level.

- When the water level reaches a low point, the pump

will stop .

Figure 6-49 Output latch and output unlatch operation.

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Latch

rung

I:1/0

Unlatch

rung

I:1/1

I:1/0

O:2/5

O:2/

O:2/5

I:1/1

Inputs OutputProgram

L1 L2

OFF

Status bit

Figure 6-50 Process used to control the level of water in a

storage tank.

High sensor switch

Low sensor switch

MAN/AUTO

Pump motor

ON/OFF

Pump

running

Low

level

High

level

pet10882_ch06_095-124.indd 113pet10882_ch06_095-124.indd 113 7/23/10 9:22 PM7/23/10 9:22 PM

114 Chapter 6 Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs

• The low level status light is controlled by the rung

5Examine-off instruction addressed to the low

s ensor switch.

• The high level status light is controlled by the rung

6 Examine-on instruction addressed to the high

s ensor switch.

Figure 6-52 shows a typical I/O module wiring dia-

gram and addressing format for the water level control

program implemented using an Allen-Bradley modu-

lar SLC 500 controller. The chassis power supply has a

relatively small power rating and is used to supply DC

power to all devices physically mounted in the backplane

of the PLC rack. In this application a 24 VDC  eld power

supply is used for the input devices and a 120 VAC  eld

power supply for the output devices. This allows a low-

voltage 24-volt control signal to control 240-volt output

devices. SLC 500 controllers use a rack/slot-based ad-

dress system where the slot location of the I/O modules

in the rack establishes the PLC address. The addresses

for the  eld devices of this particular application are

shown below:

Input module

wiring connections

OFF ON

AutoMan

Low sensor switch

High sensor switch

Man/Auto Low sensor switch

Man/Auto

Latch/Unlatch

Ladder logic program

OFF/ON

Motor

Man/Auto High sensor switch

Latch coil

Man/Auto Low sensor switch

Unlatch coil

Motor

Low sensor switch

High sensor switch

Motor

Output module

wiring connection

Pump running

Low level

High level

Figure 6-51 Program used to implement control of the water level in the storage tank.

FIELD DEVICE ADDRESS Signiﬁ es

OFF/ON Switch I:2/0 The input module in slot 2 and screw terminal 0

MAN/AUTO Switch I:2/4 The input module in slot 2 and screw terminal 4

LOW SENSOR SWITCH I:2/8 The input module in slot 2 and screw terminal 8

HIGH SENSOR SWITCH I:2/12 The input module in slot 2 and screw terminal 12

MOTOR O:3/1 The output module in slot 3 and screw terminal 1

PUMP RUNNING Light O:3/5 The output module in slot 3 and screw terminal 5

LOW LEVEL Light O:3/9 The output module in slot 3 and screw terminal 9

HIGH LEVEL Light O:3/13 The output module in slot 3 and screw terminal 13

B3:0/0 Internal retentive bit instruction that does not drive a real-word device

pet10882_ch06_095-124.indd 114pet10882_ch06_095-124.indd 114 7/23/10 9:22 PM7/23/10 9:22 PM

Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs Chapter 6 115

Ladder logic program

Low sensor switch OFF/ON

Motor

Man/Auto

Motor G

Low sensor switch

Unlatch coil

Latch/Unlatch

High sensor switch Latch coil

I:2/4

I:2/8

I:2/4

B3:0/0

I:2/4

I:2/4 I:2/8

I:2/4

O:3/1

I:2/8

O:3/5

O:3/9

B3:0/0

I:2/12

B3:0/0

I:2/0 O:3/1

I:2/12

O:3/13

Slots

High sensor switch

Input module

IN 0

IN 2

IN 4

IN 6

IN 8

IN 10

IN 12

IN 14

COM

IN 1

IN 3

IN 5

IN 7

IN 9

IN 11

IN 13

IN 15

COM

Output module

VAC

OUT 1

OUT 3

OUT 5

OUT 7

OUT 9

OUT 11

OUT 13

OUT 15

OUT 0

OUT 2

OUT 4

OUT 6

OUT 8

OUT 10

OUT 12

OUT 14

COM

24 VDC

16 point discrete

input module

240 VAC

16 point discrete

output module

24 VDC

Field device

power supply

⫹DC ⫺DC

240 VAC

Field device

power supply

L2 L1

Power

supply

123456

OFF ON

Man

Motor

Pump running

Low level

High level

Auto

Low sensor

switch

High sensor

switch

Figure 6-52 Water level control program implemented using an Allen-Bradley modular SLC 500 controller.

pet10882_ch06_095-124.indd 115pet10882_ch06_095-124.indd 115 7/23/10 9:22 PM7/23/10 9:22 PM

116 Chapter 6 Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs

Automatic control involves maintaining a desired set

point at an output. One example is maintaining a certain

set-point temperature in a furnace as illustrated in Fig-

ure6-55 . If there is deviation from that set point, an error

is determined by comparing the output against the set point

and using this error to make a correction. This requires

feedback from the output to the control for the input.

The converting of a simple sequential process can be

examined with reference to the process  ow diagram illus-

trated in Figure 6-56 . The sequential task is as follows:

1. Start button is pressed.

2. Table motor is started.

Figure 6-53 Sequential control process.

Source: Photo courtesy Omron Industrial Automation, www.ia.omron.com.

1 - Filling operation 2 - Capping operation

Figure 6-54 Combination control process.

Label solenoid 1

Label

solenoid 2

6.10 Converting Relay Schematics

into PLC Ladder Programs

The best approach to developing a PLC program from a

relay schematic is to understand  rst the operation of each

relay ladder rung. As each relay ladder rung is understood,

an equivalent PLC rung can be generated. This process

will require access to the relay schematic, documentation

of the various input and output devices used, and possibly

a process  ow diagram of the operation.

Most control processes require the completion of several

operations to produce the required output. Manufacturing,

machining, assembling, packaging,  nishing, or transport-

ing of products requires the precise coordination of tasks.

A sequential control process is required for processes

that demand that certain operations be performed in a spe-

ci c order. Figure 6-53 illustrates part of a bottle  lling

process. In the  lling and capping operations, the tasks

are (1)  ll bottle and (2) press on cap. These tasks must

be performed in the proper order. Obviously we could not

 ll the bottle after the cap is pressed on. This process,

therefore, requires sequential control.

Combination controls require that certain operations

be performed without regard to the order in which they

are performed. Figure 6-54 illustrates another part of the

same bottle  lling process. Here, the tasks are (1) place

label 1 on bottle and (2) place label 2 on bottle. The order

in which the tasks are performed does not really matter.

In fact, however, many industrial processes that are not in-

herently sequential in nature are performed in a sequential

manner for the most ef cient order of operations.

Figure 6-55 Automatic control process.

PLC

Furnace

Feedback

Controller

Figure 6-56 Sequential process ﬂ ow diagram.

Package

Limit

switch

Motor

Pilot lights

Stop

Start

pet10882_ch06_095-124.indd 116pet10882_ch06_095-124.indd 116 7/23/10 9:22 PM7/23/10 9:22 PM

Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs Chapter 6 117

Figure 6-58 shows an I/O connection diagram for a

programmed version of the sequential process. Each input

and output device is represented by its symbol and asso-

ciated address. These addresses will indicate what PLC

input is connected to what input device and what PLC

output will drive what output device. The address code, of

course, will depend on the PLC model used. This example

uses SLC 500 addressing for the process. Note that the

electromagnetic control relay CR is not needed because

its function is replaced by an internal PLC control relay.

The hardwired relay schematic for the sequential pro-

cess can be converted to the PLC ladder logic program

shown in Figure 6-59 . In converting the process to a pro-

gram the operation of each rung must be understood. The

pushbuttons PB1, PB2 as well as limit switch LS are all

programmed using the examine-closed (–] [–) instruction

to produce the desired logic control. Also, internal relay

B3:1/0 is used to replace control relay CR. To obtain the

desired control logic, all internal relay contacts are pro-

grammed using the PLC contact instruction that matches

the coil de-energized state. The internal relay imple-

mented in software requires one coil address the contacts

of which can be examined for an ON or OFF condition as

many times as you like.

There is more than one method to correctly design the

ladder logic program for a given control process. In some

cases one arrangement may be more ef cient in terms of

the amount of memory used and the time required to scan

the program. Figure 6-60 illustrates an example of an ar-

rangement of series instructions of a rung programmed

for optimum scan time. The series instructions are pro-

grammed from the most likely to be false (far left) to

the least likely to be false (far right). Once the processor

sees a false input instruction in series, the processor stops

checking the rung at the false condition and sets the out-

put false.

Figure 6-61 illustrates an example of an arrangement

of parallel instructions of a rung programmed for opti-

mum scan time. The parallel path that is most often true is

3. Package moves to the position of the limit switch

and automatically stops.

Other auxiliary features include:

• A stop button that will stop the table, for any reason,

before the package reaches the limit switch position

• A red pilot light to indicate the table is stopped

• A green pilot light to indicate the table is running

A relay schematic for the sequential process is shown

in Figure 6-57 . The operation of this hardwired circuit can

be summarized as follows:

• Start button is actuated; CR is energized if stop but-

ton and limit switch are not actuated.

• Contact CR-1 closes, sealing in CR when the start

button is released.

• Contact CR-2 opens, switching the red pilot light

from on to off.

• Contact CR-3 closes, switching the green pilot light

from off to on.

• Contact CR-4 closes to energize the motor starter

coil, starting the motor and moving the package to-

ward the limit switch.

• Limit switch is actuated, de-energizing relay

coilCR.

• Contact CR-1 opens, opening the seal-in circuit.

• Contact CR-2 closes, switching the red pilot light

from off to on.

• Contact CR-3 opens, switching the green pilot light

from on to off.

• Contact CR-4 opens, de-energizing the motor starter

coil to stop the motor and end the sequence.

Figure 6-57 Relay schematic for the sequential process.

PB1

Start

L1 L2

PB2

Stop

Limit

switch

Control

relay

CR-2

CR-3

CR-4

CR-1

PL1

Stop

PL2

Run

Motor

starter coil

O:4/2

OutputsInputs

LS - Limit switch

L2L1

PB1 - Start

Motor starter

coil

O:4/1

PL1 - Stop

I:3/0

I:3/2

I:3/1

PB1 - Stop

O:4/3

PL2 - Run

Figure 6-58 I/O connection diagram.

pet10882_ch06_095-124.indd 117pet10882_ch06_095-124.indd 117 7/23/10 9:22 PM7/23/10 9:22 PM

118 Chapter 6 Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs

placed on the top of the rung. The processor will not look

at the others unless the top path is false.

Figure 6-62 shows a hardwired jog control circuit that

incorporates a jog control relay. The operation of the cir-

cuit can be summarized as follows:

• Pressing the start pushbutton completes a circuit for

the CR coil, closing the CR1 and CR2 contacts.

• The CR1 contact completes the circuit for the M

coil, starting the motor.

• The M maintaining contact closes; this maintains

the circuit for the M coil.

• Pressing the jog button energizes the M coil only,

starting the motor. Both CR contacts remain open,

and the CR coil is de-energized. The M coil will

not remain energized when the jog push button is

released.

Figure 6-63 shows a PLC program equivalent of the

hardwired relay jog circuit. Note that the function of the

control relay is now accomplished using an internal PLC

instruction (B3:1/0).

Figure 6-59 Sequential process PLC ladder logic program.

Input module

wiring connections

PB1

PB2

I:3/2

I:3/1

I:3/0

B3:1/0

I:3/1

Ladder logic program

I:3/2 B3:1/0

PB1 PB2 LS

Internal

relay

O:4/2

PL1

O:4/3

PL2

O:4/1

O:4/2

Output module

wiring connection

PL1

O:4/3

PL2

Instruction most

likely to be FALSE

Instruction least

likely to be FALSE

Figure 6-60 Series instructions programmed for optimum

scan time.

Path most likely to be TRUE

Less likely

Least likely

Figure 6-61 Parallel instructions programmed for

optimum scan time.

Control relay

H1 H3

H2 H4

X1 X2

120 V

Start

Jog

CR2

CR1

Stop

Figure 6-62 Jog circuit with control relay.

Source: Photo courtesy IDEC Corporation, www.IDEC.com/usa, RR Relay.

pet10882_ch06_095-124.indd 118pet10882_ch06_095-124.indd 118 7/23/10 9:22 PM7/23/10 9:22 PM

Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs Chapter 6 119

I:3/0

B3:1/0

I:3/1 B3:1/0

I:3/2

B3:1/0

I:3/1 O:2/2

O:2/2

Ladder logic program

Start

L1 L2

Inputs Output

Stop

Internal

relay

Jog

Stop M

I:3/2

Start

Stop

I:3/0

I:3/1

Figure 6-63 PLC program equivalent of the hardwired relay jog circuit.

6.11 Writing a Ladder Logic Program

Directly from a Narrative Description

In most cases, it is possible to prepare a ladder logic program

directly from the narrative description of a control process.

Some of the steps in planning a program are as follows:

• D e  ne the process to be controlled.

• Draw a sketch of the process, including all sensors

and manual controls needed to carry out the control

sequence.

• List the sequence of operational steps in as much

detail as possible.

• Write the ladder logic program to be used as a basis

for the PLC program.

• Consider different scenarios where the process se-

quence may go astray and make adjustments as needed.

• Consider the safety of operating personnel and

make adjustments as needed.

The following are examples of ladder logic programs

derived from narrative descriptions of control processes.

Figure 6-64 shows the sketch of a drilling process that

requires the drill press to turn on only if there is a part

present and the operator has one hand on each of the

start switches. This precaution will ensure that the opera-

tor’s hands are not in the way of the drill.

The sequence of operation requires that switches 1

and 2 and the part sensor all be activated to make the

drill motor operate. Figure 6-65 shows the ladder logic

program required for the process implemented using an

SLC 500 controller.

EXAMPLE 6-1

Inputs

Ι:3/5

Sensor

Ι:3/6

Ι:3/4

Ladder logic program

Motor

contactor

PB1 PB1

Sensor

Motor

contactor

PB1

PB2

Ι:3/4 Ι:3/5 Ι:3/6

O:4/0

Output

Figure 6-65 Drilling process PLC program.

PB1

Drill

motor

Switches

PB2

Part sensor

Figure 6-64 Sketch of the drilling process.

pet10882_ch06_095-124.indd 119pet10882_ch06_095-124.indd 119 7/23/10 9:22 PM7/23/10 9:22 PM

120 Chapter 6 Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs

A motorized overhead garage door is to be operated au-

tomatically to preset open and closed positions. The ﬁ eld

devices include one of each of the following:

• Reversing motor contactor for the up and down

directions.

• Normally closed down limit switch to sense when the

door is fully closed.

• Normally closed up limit switch to sense when the

door is fully opened.

• Normally open door up button for the up direction.

• Normally open door down button for the down

direction.

• Normally closed door stop button for stopping the

door.

• Red door ajar light to signal when the door is partially

open.

• Green door open light to signal when the door is fully

open.

• Yellow door closed light to signal when the door is fully

closed.

The sequence of operation requires that:

• When the up button is pushed, the up motor contac-

tor energizes and the door travels upward until the up

limit switch is actuated.

• When the down button is pushed, the down motor

contactor energizes and the door travels down until

the down limit switch is actuated.

• When the stop button is pushed, the motor stops.

The motor must be stopped before it can change

direction.

Figure 6-66 shows the ladder logic program required for

the operation implemented using an SLC 500 controller.

EXAMPLE 6-2

Figure 6-66 Motorized overhead garage door PLC program.

Input devices

(shown in unactuated

condition)

Output devices

Up limit

Door up

Ι:3/0

Ι:3/7

Program

O:4/3

O:4/0

Door ajar

Door down

Stop door

Ι:3/4 Ι:3/5

Down limit

Ι:3/1

Ι:3/4

Ι:3/5

Ι:3/7

O:4/4 Ι:3/0

Stop Motor upUp

Ι:3/0 O:4/0Ι:3/1

Up limit Door ajarDown limit

Ι:3/0

O:4/1

Up limit

Door open

O:4/3

Motor up

Down

Intrlock Up limit

O:4/4

Ι:3/1

O:4/2

Down limit

Door closed

Motor down

O:4/4

Motor down

Ι:3/7 Ι:3/5 Ι:3/4 O:4/3 Ι:3/1

Stop Down Up

intrlock Down limit

O:4/1

Door open

O:4/2

Door

closed

O:4/3

Motor

contactor

O:4/4

Motor

contactor

pet10882_ch06_095-124.indd 120pet10882_ch06_095-124.indd 120 7/23/10 9:22 PM7/23/10 9:22 PM

Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs Chapter 6 121

Figure 6-67 shows the sketch of a continuous ﬁ lling opera-

tion. This process requires that boxes moving on a con-

veyor be automatically positioned and ﬁ lled.

The sequence of operation for the continuous ﬁ lling op-

eration is as follows:

• Start the conveyor when the start button is momen-

tarily pressed.

• Stop the conveyor when the stop button is momen-

tarily pressed.

• Energize the run status light when the process is

operating.

• Energize the standby status light when the process is

stopped.

• Stop the conveyor when the right edge of the box is

ﬁ rst sensed by the photosensor.

• With the box in position and the conveyor stopped,

open the solenoid valve and allow the box to ﬁ ll. Filling

should stop when the level sensor goes true.

• Energize the full light when the box is full. The full light

should remain energized until the box is moved clear

of the photosensor.

Figure 6-68 shows the ladder logic program required for

the operation.

EXAMPLE 6-3

Figure 6-67 Sketch of the continuous ﬁ lling operation.

Run

Standby

Full

Level

switch

Photo

switch

Motor

Start

Solenoid

Hopper

Stop

Start

Photo

Level

Stop Start

Run

Ladder logic program

Run

Standby

OutputsInputs

Motor

Solenoid

Run

Standby

Full

Level Photo

Full

Photo Run

Level Photo

Motor

Run

Solenoid

Full

Figure 6-68 Continuous ﬁ lling operation PLC program.

pet10882_ch06_095-124.indd 121pet10882_ch06_095-124.indd 121 7/27/10 10:13 PM7/27/10 10:13 PM

122 Chapter 6 Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs

1. Explain the basic operating principle of an electro-

magnetic control relay.

2. What is the operating difference between a nor-

mally open and a normally closed relay contact?

3. In what ways are control relay coils and contacts

rated?

4. How do contactors differ from relays?

5. What is the main difference between a contactor

and a magnetic motor starter?

6. a. Draw the schematic for an across-the-line AC

magnetic motor starter.

b. With reference to this schematic, explain the

function of each of the following parts:

i. Main contact M

ii. Control contact M

iii. Starter coil M

iv. OL relay coils

v. OL relay contact

7. The current requirement for the control circuit of a

magnetic starter is normally much smaller than that

required by the power circuit. Why?

8. Compare the method of operation of each of the

following types of switches:

a. Manually operated switch

b. Mechanically operated switch

c. Proximity switch

9. What do the abbreviations NO and NC represent

when used to describe switch contacts?

10. Draw the electrical symbol used to represent each

of the following switches:

a. NO pushbutton switch

b. NC pushbutton switch

c. Break-make pushbutton switch

d. Three-position selector switch

e. NO limit switch

f. NC temperature switch

g. NO pressure switch

h. NC level switch

i. NO proximity switch

11. Outline the method used to actuate inductive and

capacitive proximity sensors.

12. How are reed switch sensors actuated?

13. Compare the operation of a photovoltaic solar cell

with that of a photoconductive cell.

14. What are the two basic components of a photoelec-

tric sensor?

15. Compare the operation of the re ective-type and

through-beam photoelectric sensors.

16. Give an explanation of how a scanner and a de-

coder act in conjunction with each other to read a

bar code.

17. How does an ultrasonic sensor operate?

18. Explain the principle of operation of a strain gauge.

19. Explain the principle of operation of a thermocouple.

20. What is the most common approach taken with re-

gard to the measurement of  uid  ow?

21. Explain how a tachometer is used to measure rota-

tional speed.

22.

How does an optical encoder work?

23. Draw an electrical symbol used to represent each of

the following PLC output control devices:

a. Pilot light

b. Relay

c. Motor starter coil

d. OL relay contact

e. Alarm

f. Heater

g. Solenoid

h. Solenoid valve

i. Motor

j. Horn

24. Explain the function of each of the following

actuators:

a. Solenoid

b. Solenoid valve

c. Stepper motor

25. Compare the operation of open-loop and closed-

loop control.

26. What is a seal-in circuit?

27. In what is the construction and operation of an

electromechanical latching relay different from a

standard relay?

28. Give a short description of each of the following

control processes:

a. Sequential

b. Combination

c. Automatic

CHAPTER 6 REVIEW QUESTIONS

pet10882_ch06_095-124.indd 122pet10882_ch06_095-124.indd 122 7/23/10 9:22 PM7/23/10 9:22 PM