plc solution book

Contents

1. Introduction 2

2. PLC Architecture 3

3. PLC Advantages 3

4. Modes of operation 4

5. PLC Working 5

6. Programming languages 6

7. Input and output instructions 8

8. Exercises 10

9. Timers 20

10. Counters 27

11. Program control instructions 37

12. Data Manipulation instructions 42

13. Math instructions 45

14. Electrical drawings 47

1

PLC Solution book

Introduction:

This book provides programming solution to the Exercises provided in the PLC

Workbook. This book provides stepwise programming solution, which helps the user to

understand the problem and prepare a logical solution.

The programming language pertains to Ladder logic language that is being used in

Siemens S7-200. The user program is entered in the software package namely Step 7

microwin programming. The program is downloaded to the PLC through serial

communication.

A personal computer is the most commonly used programming device. The

computer makes it possible to carry out programming away from the physical location of

the programmable controller. When the programming is complete, it is saved and

downloaded to the PLC. PLC has an in-built EEPROM in which the program is stored.

Changes in the program can be made and the program can be downloaded to the PLC any

number of times. The program status can be viewed in the software, while the program

gets executed in the PLC.

The computer can be used to document the PLC program. The programmer can

add notes in the ladder diagram, which makes troubleshooting very easier.

The chart status option in the software helps in reading the current value of

registers, timers and counters.

2

this interface is to condition the various signals received from or sent to external field

devices.

The programming device or terminal is used to enter the desired program into the

memory of the processor. The program determines the sequence of operation and

ultimate control of the equipment or machinery. The programming device is connected to

the PLC only while entering or monitoring the program. A personal computer with

appropriate software can act as the program terminal.

There are two ways in which I/O is incorporated into PLC, fixed and modular.

The processor and the I/O are packaged together in Fixed I/O and the I/O terminals are

available but cannot be changed. The number of I/O points can be expanded by buying

additional units of fixed I/O. Modular I/O is divided by compartments into which

separate modules can be plugged. Modular I/O is flexible and the user can mix modules

according to the need. The modules plug into a rack. When a module is slid into the rack,

it makes electrical connection with a series of contacts called backplane, located at the

rear of the rack. The PLC processor is also connected to the backplane and can

communicate with all modules in the rack.

The power supply unit supplies DC power PLC and other modules that may be

connected to the PLC. The power supply for field devices is provided by external

alternating current, in case of large systems.

Advantages of PLC:

PLC is basically designed to operate in industrial environment with wide

range s of ambient temperature and humidity.

PLC's are not affected by electrical noise problems.

3

Hardware and software of the PLC are designed for easy use by the plant

technician and electricians.

PLC is programmed in relay ladder logic making programming simpler.

Troubleshooting is simpler with PLC's as they have been provided with fault

indicators and written fault information displayed on the screen.

PLC Size and Application:

PLC's are divided into three major categories based on the size-

Small size PLC

Medium size PLC

Large size PLC

Small size PLC covers up to 128 I/O and memory up to 128K. These PLC's are

capable of providing simple to advance level of machine control. Medium size

PLC cover up to 2048 I/O and memory up to 32K. Special I/O modules make

Medium size PLC's adaptable to temperature, pressure, flow, weight or other

analog functions commonly encountered in control applications. Large size PLC

cover up to 16,000 I/O and memory up to 2M bytes. Large size PLC's can control

individual production processes to entire plant. The selection of PLC is done

considering the current and future possibility of expansion.

Modes of Operation:

There are three modes of operation

Run mode

Program mode

Remote test mode

Run position:

Places the processor in the Run mode

Executes the ladder program and energizes output devices

Prevents online program editing

4

Program position:

Places the processor in the Program mode

Prevents the processor from scanning or executing the ladder diagram

Allows program entry and editing

Remote position:

Places the processor in the Remote position

Allows online editing

Processor mode can be changed.

PLC working:

The working of a PLC is similar to that of a computer. Once the PLC is placed in

Run mode, the processor reads all the input values and energizes or de-energizes the

outputs according to the user program. This process is known as Scan. The PLC scan

time indicates how fast the controller can react to changes in inputs. Scan time varies

with program content and length. The time required to make a single scan can vary from

1ms to 20ms.

The scan is normally continuous and sequential process of reading the status of

inputs, evaluating the control logic and updating the outputs. The processor examines the

input for a ON condition and when this happens the rung is said to be TRUE. The

processor then sets the output image table bit to 1 for the corresponding output. During

the next cycle, the actual output is turned ON. If the input is turned OFF, the rung

becomes FALSE. The processor then sets the output image table bit to 0 causing the

output device to turn off.

Each instruction entered in a program requires certain amount of time for the

instruction to be executed. Of example, the Processor takes less time to read the status of

an input than it does to read the accumulated value of a counter.

There are two basic scan patterns that PLC manufacturers use to adopt namely the

Horizontal Scan

Vertical Scan

5

In Horizontal scanning system, the processor examines input and output instructions from

the top left to right and then horizontally the next rung and so on.

In Vertical scanning system, the processor examines input and output instruction from the

top left and then vertically down column by column.

PLC Programming Languages:

The term PLC programming refers to the method by which the user

communicates information to the PLC. The three most commonly used language

structures are

Ladder diagram language

Boolean language

Function chart

Ladder Logic Programming:

Ladder diagram language is by far the most commonly used PLC language. Ladder logic

programming is similar to relay ladder wiring making the programming simpler. Ladder

logic programming is basically a symbolic set of instructions used to create the program.

Since the instruction set is composed of contact symbols, ladder logic diagram is also

called Contact Symbology.

The main function of the ladder logic program is to control outputs based on input

conditions. The control is accomplished through the use of Ladder rung. A Rung consists

of set of input conditions represented by contact instructions and an output instruction at

the end of the rung represented by the coil symbol. Each contact or coil is identified with

a unique address. For an output to be activated, at least one left to right path of the

contacts must be closed. A complete closed path is said to have logical continuity. When

logical continuity exists in at least one path, the rung condition is said to be TRUE. The

rung condition is FALSE, if no path has continuity.

Boolean Language Programming:

Boolean language programming incorporates the statement list. Statement list refers to

basic AND, OR and NOT logic gate functions.

6

Function Chart Programming:

The Function chart programming was originally developed in Europe and is

called Grafcet. This method uses a more structured approach. Function chart

programming approach is uses Function blocks (steps and transition units). A function

chart is a pictorial representation of a sequential control process. It shows the possible

paths the process can take and the conditions necessary to go from one block to another.

A major advantage of function chart programming is that block of logic can be

programmed as a module and transition logic ensures that only appropriate software

modules will operate at any given time. Other advantages include simpler programming,

faster scan time, enhanced maintainability and ease of future enhancements.

Interlocks are a major drawback with ladder logic programming. Much of effort is

spend in overcoming the conditions that may occur again and again. Interlocks are

overcome with function chart programming in which the processor scans only the active

steps. Processor does not even scan those parts of the program that are inactive, thus

reducing the number of complex interlocks required.

Some PLC’s support Sequential function chart programming. Sequential function

chart programming is more structured into sections and modules which simplifies

troubleshooting and maintenance.

7

Input Instructions:

Inputs are normally represented by

Normally open contact

Normally closed contact

Normally open contacts are defined as those contacts that are open when no

current flows through the coil, but close as soon as the coil conducts. Normally closed

contacts are closed when the coil is de-energized and open when the coil is energized.

Input devices connected to the Input module are represented as open and closed

contacts. Input devices such as pushbuttons, limit switches. sensors, selector switches

are hardwired to the terminals of the input module. Input devices are configured as

Normally open and close switches as per the manufacturer specification.

The inputs are scanned and their status are updated in the input register table. The

program scan compares the input device status with the programming status and

updates the input register status. The normal state of input does not matter for the

controller. What matters is the user program which is nothing but a logical circuit .If

the contacts in the ladder provide a continuous path from left to right end of the rung,

then the rung is said to be TRUE and the outputs are updated in the output register

table. The table helps in configuring the contacts in the program to obtain the desired

results.

Input device status Programming status Execution status

Normally open switch Normally open Normally open

Normally open switch Normally closed Normally closed

Normally closed switch Normally open Normally closed

Normally closed switch Normally closed Normally open

8

Internal Relay instructions:

Most PLC’s have an area of the memory allocated for internal storage bits. These

storage bits are called internal outputs or internal coils. Internal coil operates in the same

way as any output, however the output is strictly restricted for internal purposes.

The advantage of using internal outputs is that there are many situations where an

output instruction is required in the program, but no physical connection is required.

Internal memory bits can be programmed by the user to perform relay functions without

occupying a physical output.

Output Instructions:

Outputs are normally represented by coil symbol. The PLC output module can

operate a variety of output control devices. These devices include pilot lights, control

relays, motor starters, alarms, solenoids, small motors and horns. In general mostly

outputs are actuators. An actuator is a device that converts electrical signal into linear

mechanical motion. The principle types of actuators are relays, solenoids and motors.

Output latch instruction with a bit level address. When the instruction is true, it

sets a bit in the output image file. It is a retentive instruction because the bit remains Set

even when the latch instruction goes false. In most applications, it is used with an

Unlatch instruction. When the instruction is true, it resets a bit in the output image file. It

is also a retentive instruction because the bit remains Reset even when the latch

instruction goes false

An Electrical relay is a magnetic switch. It uses electromagnetism to switch

contacts. A relay will have one coil and number of normally open and closed contacts.

Thus a number of control devices can be controlled by means of a single relay. Control

relays do not carry heavy currents and voltages. The contacts are usually rated between 5

and 10 Amps with a maximum coil voltage of 120V AC. Contactors are special type of

relays designed to handle heavy power loads. Contactors are designed to make and break

electrical circuits.

9

Entering the Ladder diagram:

The ladder diagram is entered into the PLC by means of a Programming

device. The programming device is usually a PC with appropriate software. The user can

pick the input contacts (NO and NC) and output coil instructions from the programming

window and create the appropriate program for their application. The following pages

will have some virtual applications for which the user can write ladder programs. The

solution for the application is available with the PLC Solution book.

Exercise 1:

A 12V DC motor is to be operated by means of a Start switch. The motor is

switched off by means of a Stop switch.

Normally open Start switch to start the motor

Normally closed Stop Switch to stop the motor

A relay to switch on the motor.

Motor connected to NO point of the relay.

12V Motor

START STOP

10

Exercise 2:

A two position single solenoid valve is used to operate a double-acting

cylinder. The cylinder moves in the forward direction when the pushbutton is pressed and

the cylinder retracts when the pushbutton is released.

Normally open pushbutton switch

A Single solenoid coil

A double acting pneumatic cylinder

Forward motion

of piston

Sol A

Exhaust

Start

P

11

Exercise 3:

A two position double solenoid valve is used to operate a double-acting

cylinder. The cylinder moves in the forward direction when the pushbutton is pressed and

the cyliner is in fully retracted position. The cylinder retracts when it has reached the

extended position.

Normally open pushbutton Start switch

A double solenoid coil

Magnetic reed switches A0 and A1 at the retracted and extended position.

A double acting pneumatic cylinder

Exhaust

Sol A

P

Sol B

A0 A1

Start

12

Exercise 4:

A water pump connected to a storage tank is to be controlled by means of a

ON/OFF switch and a selector switch. The selector switch has 2 positions namely Auto

and manual. The level of water in the tank is indicated by means of 2 level sensors fixed

at the low level and high level. The pump is switched ON and OFF by means of a

switch.The pump will start in manual mode, if the level of water in the tank is at any level

except low. When the level of water is high, the pump will start in Auto-mode and will

stop automatically when the water reaches Low level. The selector switch remains in

manual mode by default.

+

ON/OFF switch for pump control

Selector switch for mode control

Normally open high and low level Sensors

Relay for controlling motor

Pilot lamps for Pump running, and level indication.

Pump

motor

Pump

Running

Low

level

High

level

G YR

High level sensor

Low level sensor

13

Exercise 5:

An industrial conveyor unit is driven by means of a motor. The conveyor is

used for transporting materials from one end to the other. The conveyor motor is initiated

by means of a Start switch and a limit switch at the end of the conveyor stops the motor.

An Emergency stop switch is provided to stop the motor in case of any emergency.

Normally open start switch

Normally closed E-stop switch.

Normally open limit switch

A relay for motor control

Pilot lamps to indicate motor running and idle condition.

Package Motor

Stop

R

Run

GLS

E-stop Start

14

Exercise 6:

A simple drilling operation requires the drill press to turn ON only if there is a

material loaded in the vice and the operator has one hand on each of the start switches.

This precaution will protect the drill bit from being damaged and also will ensure that the

operator hands are not in the way of the drill.

Normally open pushbutton switches PB1 and PB2

Normally open part sensor

Relay to operate the drill motor

Drill motor PB1 PB2

Part sensor

15

Exercise 7:

A motorized overhead garage door is to be operated by means of a control

panel that includes the following

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 open

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 light to signal the partially open condition

Yellow door light to signal the fully closed condition

Green door light to signal the fully open condition

A relay for operating the motor

A relay for changing direction of the motor

Up limit

switch

Motor

R G Y

StopUp Down

Down limit

switch

16

Exercise 8:

A continuous filling operation requires boxes moving on a conveyor to be

automatically positioned and filled.

Normally open Start switch for starting the Conveyor

Normally closed Stop switch for stopping the Conveyor

Normally open photo-electric sensor for material sensing

Normally open level sensor for material filling

A relay for operating the Conveyor motor

A Solenoid for material filling operation

Pilot lamps for indicating conveyor status (running and stop) and material

filled status

G

R

Y

Run

Standby

Full

Motor

Photo-electric

sensor

Level sensor Solenoid

Hopper

Start

Stop

17

Exercise 9:

A pump is to be used for filling two storage tanks. The pump is manually

started by the operator from a start/ stop station. When the first tank is full, the control

logic must automatically stop flow to the first tank and direct flow to the second tank

through the use of a level sensor and electric solenoid valves. When the second tank is

Full, the pump must shut down automatically. Indicator lamps are included to indicate

each tank is full.

Normally open start switch for starting the pump motor

Normally closed stop switch for manually stopping the pump

Normally open high and low level sensors for both the tanks

Solenoid coils for directing flow

Indicator lights for indicating tank level.

Pump

motor

G

Tank Full

R

Tank

Empty

High level sensor

Low level sensor

Tank 1

Y

Tank Full

B

Tank

Empty

High level sensor

Low level sensor

Tank 2

Sol A Sol B

Start

Stop

18

Exercise 10:

A temperature control system consists of four thermostats controlling three

heating units. The thermostat contacts are set to close at 50 degree, 60 degree and 70

degree and 80 degree celsius respectively. The PLC program is to be designed in such a

way that at a temperature below 50 degree, three heaters are to be ON. Between 50 deg to

60 deg, two heaters are to be ON. For 60 deg to 70 deg, one heater is to be ON. Above 80

deg, there is a safety shut-off for all three heaters. A pushbutton switch is used to turn ON

the unit.

Normally open Start switch for starting the unit

Normally open Thermostat Switches

Normally closed E-Stop switch

Relays for switching the Heaters.

Heating

unit -1

Heating

unit -3

Heating

unit -2

Thermostat-1

Thermostat-2

Thermostat-3

Thermostat-4

Reactor tank

E-stop Start

19

Timers:

The most commonly used instruction next to contacts and coils is Timer

instruction. The timing function was previously established by Mechanical timing relay.

The required time delay was established by positioning the needle valve. Thus the set-up

needs to be altered for any timing modification.

PLC timers are output instructions that provide delay for opening and closing

a contact. They are used to activate or de-activate a device after a preset interval of time.

The number of timers that can be programmed depends on the model of the PLC and

have a minimum number of 100.

The advantage of PLC timers is that the timer settings can be altered in the

software by changing the values without having to change any hardware connections.

Secondly, timer accuracy and repeatability are extremely high since it is based on solid

state technology. There are three different types of timers namely the On-delay timer,

OFF delay timer and Retentive timer. The timer instruction is usually associated with the

following terms,

Preset time:

The preset time represents the time duration for the timing circuit.

Accumulated time:

The accumulated time represents the amount of time elapsed from the

moment, the coil was energized.

Timer base:

The intervals that the timers time out are generally referred to as the time base

of the timer. Timers can be programmed with several different time bases: 1s, 0.1s, 0.01s.

The required time delay is determined based upon

Time delay = preset time * timer base

For example, in order to generate a time delay of 50s with timer base of 0.1s, the user

must enter 500 in the preset value.

When the timer rung has logical continuity, the timer begins counting time-

based intervals and times until the accumulated value is equal to the preset value. When

the accumulated time equals the preset time, the output is energized, and the timed output

20

contact associated with the output is closed. The timer contact can be used any number of

times in the program as normally open or normally closed contacts.

ON-Delay Timer:

The On-delay timer operates in such a way that when the rung containing the

timer is TRUE, the timer begins to count. At the end of the timer period, the output is

made active. The ON-delay timer is the commonly used timer. The timer provides an

output only when the timer has timed out completely. If the rung goes FALSE during the

time-out period then the accumulated value resets to zero. This timer is a non-retentive

timer because loss of power flow to the timer causes the timer to reset.

OFF-Delay Timer:

The timer operates in such a way that when the rung containing the timer is

FALSE, the timer begins to count. The timer counts until the accumulated time equals the

programmed preset value. The timer provides an output only when the timer has timed

out completely. If logical continuity is attained before the time-out period, then the

accumulated value is reset to zero. This time is also a non-retentive timer.

Retentive Timer:

A retentive timer accumulates time whenever the device receives power and

maintains the current time even when the power is switched OFF. As soon as logical

continuity is attained, the timer starts counting from the last value. Once the accumulated

value becomes equal to the preset value, the timer contact switches the output. Loss of

power to the timer does not affect the state of the output. The retentive timer must be

reset with a separate signal for the accumulated value to become zero and for the contacts

to return back to the previous state. Thus Retentive timer instruction is always

accompanied by a Reset instruction.

21

Exercise 11:

A simple ladder logic program to control a motor. The motor is initially

switched ON by means of a pushbutton switch. The motor should be automatically

switched off after 2 minutes.

Normally open pushbutton Start switch

A relay to operate the motor

START12V Motor

Exercise 12:

The headlamp of a Car needs to be controlled automatically. The lamp should

be ON immediately after the key is turned on. The lamp should turn OFF automatically

only 2 minutes after the key is turned OFF.

Normally open ON/OFF switch

Pilot lamp

L

ON

OFF

22

Exercise 13:

A simple ladder logic to simulate the operation of a sequential taillight system.

The light system consists of three separate lights. There has to be 1-second time delay

between the activation of each light and a 1-second delay when the lights are OFF. A

simple pushbutton switch switches ON the light system. When the start switch is pressed

momentarily, first light gets ON immediately and then the second light after 1 sec and the

third light after 1 sec. The third light glows for 1 second and all the lights are switched

OFF for 2 seconds. The sequence starts again automatically after 2 seconds. The

operation continues indefinitely until the system is brought to halt by a Stop switch.

Normally open Start switch and a normally closed Stop switch.

Tail lights 1, 2 and 3.

L1 L2 L3

Start Stop

Exercise 14:

When the lights are Turned OFF in a building, an Exit door light must remain ON

for an additional 2 minutes and the parking light are to remain ON for an aditional 3

minutes after the Buliding light goes out. All the lights are initially switched ON by

means of a main power Switch.

Normally open ON/OFF swicth

Buliding light, parking light and exit door light. L L

Building Exit door Parking

light light light

L

ON

OFF

23

Exercise 15:

A certain amount of chemicl solute is to be mixed with a solvent. The solutr

particles are loaded in the tank. The solvent entry into the tank is made possible by means

of a solenoid valve. The mixing action is carried out by means of an agigate motor. The

mixing takes place for about 3 minutes. When mixing is completed, the solution is

drained into a container through a separate solenoid valve. The process is brought to halt

at any time by means of a stop switch. The tank level is indicated by mans of two level

sensors.

Normally open Start switch

Normally closed Stop switch

Normally open level sensors

Solenoid valves for Solvent entry and Solution exit

A relay to operate the agitate motor

Start

Stop

Motor

Full Level sensor

Empty Level sensor

Sol A

Sol B

Solute particles

24

Exercise 16:

A simple process involves pumping fluid from Tank A to Tank B. When the Start

button is momentarily pressed, the pumping action begins. The pump stops when the

Stop button is pressed. The over-ride pressure is set with two pressure switches. The two

pressure switches closes 5 seconds after the pump is switched ON. In case, either of the

switch opens, the pump shuts off and cannot be switched on again for 2 minutes. The

pressure switches, in case of low pressure indicating that fluid level is low in Tank 1.



Normally open Pressure switches

A relay for operating the pump.

Tank B Tank A

PS1

PS2

Pump

motor

Start

Stop

25

Exercise 17:

A ladder logic diagram to simulate the working of Traffic light in one direction.

The traffic light system is controlled automatically and the system works continuously

unless there is a power failure due to low battery.

Red – 30 seconds On

Green- 25 seconds On

Amber- 5 seconds On

R G Y

26

Counters:

The most commonly used instruction next to contacts, coils and timers is

Counter instruction. The counting function was previously established by Mechanical

Counters. Every time, the actuator lever moves, the counter attached to it adds one

number, while the actuating lever returns automatically to its original position.

Electronic counters can count up, count down or can be combined to count up

and count down. The counter instruction is similar to timers except the fact that counters

do not operate on internal clock but are dependent on external or program sources for

counting. There are two different types of counters namely

Up counter

Down counter

The counter instruction is associated with a preset value and accumulated value.

The Up counter increments its accumulated value by 1 each time the counter rung makes

a False to True transition. When the accumulated value equals the preset value, the output

is energized and the counter output is closed. The counter contact can be used many

number of times in the program as NO or NC contact. A counter Reset instruction is

always used in conjunction with the counter instruction. Up counters are always reset to

zero and the down counters are reset to the Preset value. Reset instruction forms a part of

the Counter instruction in many PLC’s. Counter instructions are retentive because

whatever count was contained in the counter at the time of power shutdown will be

restored to the counter on power-up condition. However, the counter may reset, if the

reset condition of the counter is activated at the time of power shutdown.

Up Counter:

The count Up instruction is an output instruction whose function is to increment

its accumulated vale on false to true transition. Thus it can be used to count False to True

transitions of an input instruction and trigger an event after a required number of counts.

The Up counter increments by 1 for every negative to positive transition. The reset

condition resets the counter to zero.

Preset value is the set point of the counter and it has a range from –32768 to

32767. The accumulated value is the current count based on the number of times the

27

Rung has gone from negative to positive transition. The accumulated value has the same

range has the preset value. The accumulated value counts past the preset value instead of

stopping at the preset value. Therefore the counter must be reset in-order to bring it back

to zero. Any number of counters can be used in a PLC program (about 256 counters). The

counters are provided with a number for easier programming.

Down Counter:

The down counter decrements by 1 each time the rung changes from False to True

state. Down counter normally counts from the preset value down to zero. This counter

also requires a reset condition. The reset condition resets the counter to its Preset value.

Normally, a down counter is used in conjunction with a up counter to form a Up/Down

counter.

Cascading counters:

Sometimes, it may be necessary to count events that far exceed the maximum

number allowed for a counter instruction. This can be accomplished by means of

interconnecting two counters. This is called Cascading. For example, when it is required

to count up to 250000, we can make use of 2 counters for achieving this count. Both

counters can have a Preset value of 500. Whenever counter 1 reaches 500, its output

contact resets the counter 1 and increments counter 2 by one. When the second counter

completes 500, a total count of 250000 would have been attained.

28

Exercise 18:

A conveyor is used for transporting some cases in an industrial application. The

cases are packed in a wooden box. When 50 cases have reached the end of the conveyor,

the conveyor stops automatically and the package is closed. A start button is used to start

the conveyor motor. A proximity sensor detects the case moving over the conveyor. The

conveyor can be started and stopped manually without loss of the accumulated count. The

counter is reset externally by means of a counter reset switch.



Normally open Counter reset switch

Normally open Proximity sensor

A relay to operate the conveyor motor.

Proximity

sensor

Start

Stop

Counter

reset switch

Case

12V Motor

29

Exercise 19:

A conveyor line transports materials coming to final packaging. Each package

must contain 10 parts and the box is closed by means of a solnoid operation. A display is

provided to indicate the number of materials coming for final packaging and the number

of package that has been completed. A pushbutton is provided to restart counting at the

beginning of the day. A proximity sensor detects the material over the conveyor

Normally closed pushbutton reset switch

Normally open photo-electric sensor

A solenoid valve for closing the package

Photo-electric

sensor

Solenoid

Material

Package

Counter

reset switch 00 00

Display unit

No. of

materials

No. of

packages

30

Exercise 20:

A procees monitoring system continuously displays the number of in-process

parts. Before starting, the system is completely empty of parts and the counter is reset by

means of a reset switch. There are two photo-electric sensors at the entry and at the end

of the assembly line. The infeed (photo -electric sensor at the entrance) counts the

incoming material and the outfeed sensor (photo -electric sensor at the exit) counts the

outgoing material.

Normally open infeed sensor(photo -electric sensor at the entrance)

Normally open outfeed sensor(photo -electric sensor at the exit)

Normally closed Counter reset switch.

Counter

reset switch

Outfeed sensor Infeed sensor

Material

processing station

Infeed

(Raw parts)

Outfeed

(Finished parts)

00

Display unit

Material in

process

31

Exercise 21:

A ladder logic program to generate a Real time clock. The clock displays

Seconds, minutes and hours. The real time clock application is combined with a pressure

generator unit. Constant pressure is to be maintained inside the boiler unit. The clock is

initiated at the time of switching on the boiler unit. When the pressure falls below the set

level, the clock stops automatically and the pressure is set again. Thus the time of failure

can be determined.

Normally open pressure switch

A display unit for time display

A relay to operate the Boiler

00 00

Display unit

Hour Minute

00

Seconds

Pressure switch

Boiler

32

Exercise 22:

An automatic stacking operation is carried out after final inspection. Materials

coming after final inspection from Conveyor 1 are dropped over another stationary

conveyor. When 15 pieces have been stacked, the second conveyor starts moving to the

despatch section and the first conveyor is stopped. The second conveyor movement is

carried out for just 25 seconds during which it reaches the end. The process starts again

automatically after the second conveyor has reached the end. A photo-electric sensor

fixed in the second conveyor is used for counting the pieces.



Normally photoelectric sensor

Relays to operate both the conveyor motors.`

Start

Stop

Conveyor 1

Conveyor 2

Stack

Photoelectric

sensor

33

Exercise 23:

A ladder logic program to generate extremely long time delay for switching on a

Lamp. An ON/OFF switch is used to switch on a lamp after 1,00,000 seconds.


Pilot lamp

ON

OFF

L

Lamp

Exercise 24:

A typical servomotor with encoder feedback is used for running a Conveyor

motor. A pushbutton start switch is used to start the Conveyor motor. Two limit switches

are fixed at the start and end positions of the conveyor. The motor needs to be stopped at

an intermediate position A, approximately 10 mm from the starting position. The Encoder

pulse is read in a counter. The motor generates 2 pulses for every mm. After a time delay

of 10 seconds, the motor should start again and stop at the end position. An Emergency

stop switch is used to stop the operation at any time and to reset the counter.


Normally closed E- Stop switch

A relay to operate the motor.

E-stop Start

LS 1

Encoder

LS 2

Forward A

34

Exercise 25:

A quality inspection process requires 1 part out of 1000 parts moving over the

assembly line to be sent for quality control. A start/switch is used to turn the conveyor On

and Off. A proximity sensor counts the parts as they move over the conveyor. When 1000

is reached, the counter output activates the Gate solenoid to be open which facilitates

material to switch over to the inspection line. The gate solenoid resets automatically after

2 seconds and the counter also gets reset. The process continues again in the same

manner. A reset button is provided to manually reset the counter.



Normally open Reset button.

Normally open proximity sensor

Gate solenoid coil

A relay to operate the Conveyor motor

Material

inspection

Counter

reset button

Proximity

sensor

Start

Stop

Gate

Solenoid

35

Exercise 26:

A company that makes electronic assembly kits needs a counter to count and

control the number of resistors placed into each kit. The controller must stop the spool at

a pre-determined amount of resistors, say 100. A worker on the floor will then cut the

resistor strip and place in it the kit. A start/ stop switch is provided to turn th spool motor

ON and OFF. A through sensor is used to count the number of resistors. A display is

provided to count the grand total made in a day. Manual reset buttons are provided for the

counters that are used



Normally open through beam sensor

A display unit

A relay to operate the spool motor.

Normally closed reset buttons

Through beam sensor

Spool motor drive

00

Display unit

No. of

Resistors

Counter

reset button

Counter

reset button Start

Stop

36

Program Control Instructions:

The use of program control instrcutions in a program can shorten the time

required to complete a program scan. Portions of the program not being utilized at any

particular time can be jumped over. The program control instructions allows for greater

flexibility and greater efficiency in the program scan. Most PLC’s have Label, Jump, For-

Next and Subroutine instruction under program control instruction set.

Jump Instruction:

As in computer programming, it is sometimes desirable to jump over certin

program conditions, if certain condition exists. The jump instruction is an output

instruction. The advantage to the jump instruction is the ability to reduc processor scan

time by jumping over certain instructions that are not intended for the machine at that

instant. The important uses include

The programmable controller can hold more than one program and

scan only the program appropriate to operator requirements.

Sections of the program can be jumped over when a production fault

occurs.

The jump instruction is always accompanied by a Label instruction. The label is

the target of the jump and its always TRUE. If the jump coil is energized, all logic

between the jump and label instruction is bypassed and the processor continues scanning

after the LBL instruction.

Label instruction:

The label instruction is used to identify the ladder rung that is the target of the

jump instruction. The label address number must match that of the Jump instruction with

which it is used. The label instruction does not contribute to logical continuity and for

practical purposes, its always TRUE. Input conditions are not scanned in the portion

between jump and label and the outputs remain in their last state. Any timer opr counter

within the jump area do not operate. A single Label instruction can be used as destination

for multiple jumps. Its also possible to jump backward in the program. Care should be

taken that the scan does not remain too long in the loop. The processor has a Watchdog

37

timer that sets the maximum allowable time for a total program scan. If this time is

exceeded, the processor indicates a fault and shutdown occurs.

Sub-routine Instruction:

As in computer programming, it is possible to escape from the main program and

go to a program Subroutine to perform certain functions and return to the main program.

When certain function has to be performed several times, the Subroutine can be an useful

option. The subroutine will be performed when the rung containing the Call Subroutine is

true. The exit from the Subroutine is always returned to the rung following the call

subroutine instruction. The subroutine is returned by RET instruction. This RET

instruction is optional and the program control returns automatically to the main program.

For- Next instruction:

For-Next instruction is very useful, when a particular process has to be

performed “n” number of times. The program scan completes the portion between For

and Next. The index value increments everytime the loop is executed and when the index

value becomes equal to final value, the loop is terminated. The Next instruction marks the

termination of FOR loop.

Immediate Input and Output instructions:

The immediate input and output instructions interrupt the normal program scan to

update the input image table file with current input data or update an output with the

current output image table file data. These instructions are intended to be used in areas

where timing is critical.

The immediate input instruction is a special version of Normally open and

Normally closed instruction to read an input condition before the I/O update is

performed. This instruction is used with critical input devices that require updating in

advance of I/O scan. The immediate input instruction is most useful, if the instruction

associated with the critical input device is at the middle or toward the end of the program.

38

Even though the immediate input instruction sppeds up the updating of bits, its scan time

interruption increases the total scan time of the program.

The immediate output instruction is a special version of the output energize

instruction that is used to update the status of an output device before the I/O update is

performed. The immediate output instruction is used with critical output devices that

require updating in advance of the I/O scan.

END instruction:

The Conditional END (END) instruction terminates the main user program based

upon the condition of the preceding logic.The Conditional END instruction can be used

in the main program, but cannot be used in either subroutines or interrupt routines.

STOP instruction:

The STOP instruction terminates the execution of the program immediately by

causing a transition of the CPU from RUN to STOP mode.If the STOP instruction is

executed in an interrupt routine, the interrupt routine is terminated immediately, and all

pending interrupts are ignored. The rest of the program is scanned and the transition from

RUN to STOP mode is made at the end of the current scan.

Watchdog instruction:

The Watchdog Reset instruction allows the CPU system watchdog timer to be re-

triggered. This extends the time that the scan is allowed to take without getting a

watchdog error.

39

Exercise 27:

A certain mixing operation is carried out in a chemical Reactor unit. The

temperature of the unit is maintained by means of a thermostat. When the temperature

falls below 100 deg, heater is switched ON. The level of water in the unit is maintained

by means of a Level sensor. When the level falls below, pump is switched On for 5

minutes and then switched OFF. The pressure level in the tank is maintained by means of

a pressure switch. When the pressure level exceeds a certain limit, pressure switch opens

and an Alarm is activated immediately and all the other outputs left in the previous state.

The chemical solution is drained OFF at this stage, which brings back the pressure to

normal level. The entire unit is shut down by a ON/OFF switch.


Normally open Thermostat

Normally open Level sensor

Normally closed pressure switch

A relay to operate the Pump

A relay to operate the heater

A relay to operate the solenoid controlling the Drain valve.

An Alarm

Heater

Level sensor

Thermostat

Pressure sensor

Reactor tank

Solenoid valve

Discharge tank

Alarm

ON

OFF

40

Exercise 28:

A conveyor used for transporting beads is fixed with a weight sensor.. The

weight sensor has a pre-determined value of weight, which makes sure that the weight of

the material over the conveyor does not exceed the limit. The materials are transported to

the Conveyor from a hopper via a solenoid valve. When the weight limit exceeds the

preset value, alarm light flashes and the solenoid is de-energized. When the material is

removed from the conveyor, the alarm light returns to its normal state. An E-stop switch

is programmed to stop the unit immediately.


Normally closed E-stop switch

Normally open Weight sensor

A relay to operate the Conveyor motor

A solenoid

A flashing alarm light

FL Flashing

Alarm

Motor

Weight sensor

Solenoid

Hopper

E-stop Start

41

Data Manipulation Instructions:

Data manipulation involves transferring data and operating on data with math

Functions, data conversion, data comparison and logical operations. Data manipulation

instructions enable the PLC to take some of the qualities of a computer system. Most

PLC’s are equipped with the capability to manipulate data. Each instruction requires two

or more words of data memory for operation. The words of data memory are referred to

as Registers.

Data transfer instructions involves the transfer of the contents from one word to

another. Data transfer instructions are Output instructions. The MOVE instruction is used

to copy the value in one word to another word. The instruction copies data from a source

word to a destination word.

Data compare instructions are input instructions. Data comparison instructions are

useful in cases such as

To start an action or process when the counter value at a specific value.

To verify that an input device’s data (Analog input) is within the range.

To verify that the parts are within tolerance limit.

Data comparison instruction includes a wide range of instructions such as

Greater than

Lesser than

Equal to

Not equal to

Greater than or equal to

Lesser than or equal to

42

Exercise 29:

Three different types of products are run on a conveyor line. The storage rack has

room for only 300 boxes of product A or 175 boxes of product B or 50 boxes of product

C. A photoelectric switch fixed over the conveyor counts the number of objects. A

selector switch is provided to select the product label that is being transported. A pilot

lamp is used to indicate the FULL status of the Storage rack. The three switches help in

setting the preset value for the counter. Manual reset button is provided to reset the

counter

Normally open Photoelectric switch

Normally open switches A, B and C

Pilot lamp

Photoelectric

switch

AForward

A

Lamp

C

BL

Counter

reset button

43

Exercise 30:

A simple ladder logic program to switch on a lamp 5 seconds after the Start

pushbutton is pressed. The light should glow for another 30 seconds and then switch OFF

automatically.


A pilot lamp

L

Start

Exercise 31:

A certain Pneumatic operation requires operation of four solenoids in a time delay

sequence. A single pushbutton is switch is used to switch On the system. As soon as the

Start button is pressed, Solenoid A gets energized. After 5 seconds, Solenoid B gets

energized and C gets energized 10 seconds later and Solenoid D is energized 20 seconds

later. A stop button is used to stop the sequence at any time.



Four Solenoid valves

Start StopSol A Sol B Sol C Sol D

44

Math Instructions:

Math instructions enable the PLC to replicate some of the qualities of a computer

system. The Math functions allow the processor to perform arithmetic functions on

values stored in Memory word. The basic math functions include Addition, Subtraction,

Division and Multiplication. These math functions can be performed on two numbers or

values stored in memory locations.

Addition instruction:

The ADD instruction performs the addition of two values stored in memory

locations. Addition is performed when the rung containing the Function block is TRUE.

When performing addition care should be taken to ensure that the result is within the

range that the data table can take.

Subtraction instruction:

The SUBTRACT instruction is similar to ADD instruction and it subtracts the two

values stored in memory locations. Subtraction is performed when the rung containing

the Function block is TRUE. When the second number is larger than the first number, a

negative number results.

Multiplication instruction:

The MULTIPLY instruction multiplies the two values stored in memory

locations. Multiplication is performed when the rung containing the Function block is

TRUE. When the multiplication result is larger, the value is stored in a Double word.

Divide instruction:

The DIVIDE instruction divides the two values stored in memory locations.

Multiplication is performed when the rung containing the Function block is TRUE. The

result of division is stored in two words. The first word stores the integer part and the

second word stores the decimal fraction.

45

Exercise 32:

A certain operation requires a Light to come on only when the Sum of two

counters is zero. First counter starts counting when the limit switch 1 is closed. Similarly,

second counter starts counting when limit switch 2 is closed. A reset button is provided to

reset the counters.

Normally open Limit switches

Normally closed reset button

A pilot lamp

LReset

buttonLS 1 LS 2

Exercise 33:

A filling application requires 5 kg(preset) of raw material to be filled in a vessel.

When the start button is pressed, the fill solenoid is activated and the filling light is also

turned ON. When the weight reaches 5 kg, the solenoid is de-energized and the filling

pilot lamp is switched off. The Full pilot lamp gets turned ON when 50 kgs has been

reached. If by chance, the weight exceeds a tolerance limit of 20 grams, an Alarm is

energized and stays On until the excess weight is removed.


Normally closed stop switch

Normally open weight transducer( Analog input)

Sol A Start Stop

Fi

Alarm

Fu

Weight

transducer

46

Exercise 34:

A certain application requires temperature display in Fahrenheit. The input

temperature is recorded in Celsius degree. The output Fahrenheit temperature is displayed

in a display unit. The thermostat records the input temperature.

Normally open Thermostat

A display unit

Display unit Thermostat

00 F

Exercise 35:

An industrial Oven needs automatic control of upper and lower set point limits.

The upper and lower limits are set automatically at +- 1% regardless of the set point

value. The set point temperature is adjustable b y means of a thumbwheel switch. An

analog thermocouple module monitors the current temperature. The heater is switched On

when the temperature falls below the low point and the heater is turned Off when the

temperature reaches the upper limit. A red pilot lamp turns On when the lower limit is

reached and a green pilot lamp turns On when the upper limit is reached.

Normally open On/Off switch

Normally open thermocouple

Pilot lamps and a relay to operate heater

Thumbwheel switch

OvenL H

Thermocouple

ON

OFF

Heater

47

plc solution book

Documents