INDEX:

1. INTRODUCTION 1

2. ABOUT PLC 2-16

3. USE OF PLC 17

4. DESIGN HARDWARE OF SYSTEM

17-20

5. START/STOP SWITCH AND TEMPERATURE SET POINT ARRANGEMENT

20-21

6. DESIGN OF SOFTWARE OF THE SYSTEM

21-24

7. RESULT AND CONCLUSION

25

8. REFRENCES 26

1

INTRODUCTION

Programmable logic controllers are extensively used in industries for controlling sequence of

actions of the process. The sequence of process flow is decided for controlling the parameters

like level and temperature. The brain of the system is PLC. Appropriate hardware for interfacing

the process to the controller is developed for controlling the level ant temperature of the process.

For controlling sequence of actions ladder diagram is developed .

Programmable Logic Controller (PLC) is designed to operate in real time environment. It

has been observed that in many industries distributed control system and PLCs are extensively

used. PLC based sequential batch process control (PLCSBC) is a laboratory type setup. This

setup will be useful for demonstrating the use of PLC in sequential control operations in industry

and development of ladder diagram for particular application. The system under consideration is

designed to carry out sequence of events. The process under consideration is Batch process,

which is controlled by PLC.

• PLC USED FOR CONTROLLING SEQUENCE OF OPERATIONS.

• TAKES SIGNAL FROM FIELD AND CONTROLLS PROCESS.

• PLC PROGRAMMING DONE IN HIGH LEVEL LANGUAGE

• FACTORY PROGRAMMABLE

• EXTENSIVELY USED IN INDUSTRIES

• CONTROLLING PARAMETERS---TEMPERATURE,LEVEL

• BRAIN OF THE SYSTEM

• LADDER DIAGRAM---CONTROLLING SEQUENCE OF ACTIONS

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ABOUT PLC:

SCOPE OF STUDY

Study of PLC System at Electrolux, Various Frequency Drives use and their programming,

Power Supply, SCADA Package for MIMIC programs used for data transfer to PLC,

Communication and Networking of PLC.

Following PLC Systems study in respect to their Operating Software, Hardware, and

Programming of Small PLC Logic and necessary modification in existing Programs along with

trouble shooting in PLC System

Siemens PLC with S5 Series

Seimens S7 (Micro Series) PLC Series

Siemens S7 (Modular Series) PLC series

DESCRIPTION OF PLC

PLC refers to programmable logic control as the name suggests it is a computer used to control

the different operations of a machine to which it has been connected.

The different operations to be performed are judged in sequence by the PLC as per the program

installed in the PLC

PLC generally consists of a CPU, input and output ports. A CPU is of 2 type -:

With an i/o device

Without an i/o device

Different hardware is connected to the PLC so as to handle different operations of the machines to

which it has been connected.

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TYPICAL PLC

PARTS OF THE PLCS

CPU Module

This module consists of a central processing unit (CPU) which acts as the brain of the

system and its memory. It has a central processing unit CPU which takes merely .3ms to

process 1024 binary statements and it is equally good at word oriented processing .The

system include a number of CPUs FOR different performance requirements. CPU

executes the user program provides the 5V supply for the 5v supply for the PLC back

plane bus. It communicates with other modes by MPI (multi point interface).

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Input/Output Modules(I/O Modules)

I/O modules allow the PLC to read sensors and control actuators. There exists a wide variety

of I/O module types offered by the PLC manufacturers.

Power Supply Module

Power supply module provides power to the CPU and often provides power to drive sensors and low

power actuators connected to I/O modules. It converts line voltage (230 V ac/115V ac)

into 24v dc which provide power.

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Fig. Power supply Unit

Peripheral Equipments

They are used for purposes such as preparing, storing and loading control programs,system monitoring

and even can be used for communicating with other computers with which the PLC may be

networked.

Some examples can be;

1. Programmer/Monitor (PM) which is used to program instructions and monitor them.

2. Racks and chassis for mounting the other three parts.

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A program is fed to the PLC to make machines operate as per the requirement for the different

operations of the machines.

Advantages of using PLCs over conventional Electrical Relay Panels:-

1. Solid state devices are used for maximum reliability.

2. Logic easily changeable with programming unit, as required.

3. Single panel can be used in multiple applications by mere change of program.

4. Indicator lights provided at major diagnostic points to simplify troubleshooting.

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5. Powerful software features like timers, counters, table operations etc. in conjunction with

optional display to assist trouble shooting.

6. Availability of unlimited number of NO / NC contacts for Inputs / Outputs / Internal logic

coils (flags).

7. Very compact and rugged design leading to less downtime.

8. Smaller cabinet size leading to less floor space requirement.

9. Easy installation.

10. Low cost: Hardware costs less than the installed relays.

Difference between PLC and Electrical Relay Panel: -

As described previously, the PLC is an integral unit made up of multiple electronic relays,

timers, counters etc. As far as the sequence execution method is concerned, there exist

fundamental differences between the relay panel and the PLC. All sequences are executed

parallel (at the same time) in the case of relay panel. On the other hand, in the case of PLC, the

execution is sequential and repeatedcyclically, generally known as the PLC scan.

Types of PLC:1. Micro PLC.

2. Modular PLC.

The Micro PLC is a unique product by itself offering the most advanced features in a compact

size. It finds applications in areas like small SPMs, Conveyors, Industrial Washing Machines,

Welding Machines, Screen Printing Machines, Generator control, Small automation processes

etc. to replace relays, timers, counter etc.

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The necessary voltage and current requirements for the internal working of the PLC are

generated by the power supply. The field elements are interfaced to the input or output sections.

Typical input elements are push buttons, limit switches, proximity switches, relay contacts,

selector switches, thumb wheels etc. Typical output elements are solenoid valves, relay coils;

indicator lights, the block diagram of a Programmable Logic Controller (PLC) is shown in Fig. a

The PLC is designed LED display etc. These field elements are selected by OEM or the end user.

The necessary power supply for the input and output elements is built external to the PLC. The

PLC power supply is designed and rated to operate only the internal electronics and not to power

the field elements.

The processor is programmed in a way similar to the hardwired control panels, using

conventional ladder diagrams. The programming of the Micro PLC can be done with software.

Different companies which manufacture PLC wchich are generally used are

1. SEIMENS

2. OMRON

3. ALLEN BRADLEY

PLC are manufactured by the SEIMENS in 2 version they are -:

1. S5

2. S7

Further S7 has 2 versions -:

1. 200 (Micro Series)

2. 300 (Modular Series)

Depending on the number of the input modules connected to the PLC

The languages been used

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1. LAD-ladder

2. Cascade Statement Function CSF

3. STL –statement line

PLC Architecture: -

Basic Building Blocks of the PLC The concept of PLCs evolved from the necessity of

simplifying hardwired control panels. Thus the sensing elements (input elements) and the

actuating elements (output elements) are the only elements that are hardwired to the inputs and

outputs of the PLC. A software program does their interconnection.

Input Elements: -

Input elements are those, which sense the dynamic status of a machine operation or process.

Elements used to give the various commands are also classified as input elements, for e.g. Push

Buttons, Limit Switches, Selector Switches, Proximity Switches, Float Switches, and Pressure

Switches etc. and are directly connected to the inputs of the PLC.

Output Elements: -

Output elements are mainly actuators like, Contactors, Relays, and Solenoid Valves etc.

Indicating lamps, Annunciation lamps are also classified as output elements. These elements are

directly driven through the PLC outputs or in case of high power outputs such as motors through

external contactors.

Flags: -

Flags are just like auxiliary relays used in conventional electrical interlocking systems. These are

similar to output elements as far as internal execution is considered. The vital difference is that,5

elements like Contactors, Relays, and Solenoids etc. cannot be directly interfaced to the flags. It

11

is to be noted that the flags don't have any physical existence. They are just the memory locations

inside the PLC.

Fig.b

Fig.c

The equivalent ladder diagram of a conventional electrical circuit that may be programmed in

PLC is as shown below :

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The PLC is composed of electronic circuit with a microcomputer at the centre. However, it can

be equivalently regarded as an integrated body of ordinary relay, timer, counter etc. The input

relay built in the PLC is driven with the external switch (signal) through the input terminal.The

output relay built in the PLC is provided with internal contacts in addition to the external output

contact.

Considering the conventional electrical circuit and the equivalent circuit of PLC the following

things are to be noted. :

1. The inputs to the PLC can be wired as Normally Open (NO) contact or Normally Closed (NC)

contact. Depending on this, when the input is actuated, the input relay shown in equivalent

circuit of PLC fig. c may turn ON or turn OFF respectively. The normally open or normally

closed contacts of this input relay are used in the PLC ladder diagram. For example PB2 has

been wired as normally closed contact.

2. R1, R2 and R3 shown in Fig. b are used as auxiliary contactors for either interlocking or

multiplexing. The same can be achieved by the PLC software stored in the memory.

3. Since the input relay, output relay, flags etc. are not physical relays but are memory locations,

there is no limitation on the number of normally open or normally closed contacts of these relays

to be used in the ladder. For example the limit switch LS1 has been wired as normally open to

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the PLC. However multiple number of NO and NC contacts of input relay are used to represent

the status of LS1 in the ladder.

4. The flags are internal outputs in the PLC and are used for interlocking. The flags have no

external interface. They are used for temporary storage of certain conditions required in the

ladder ahead.

5. The contacts of output relay can also be used for interlocking in the ladder. This is regardless

of the type of load which is to be connected physically to this output.

User Memory: The Micro PLC is provided with onboard 8K EEPROM. This EEPROM stores

the user program.

PLC Scan: -

STANDARD PLC SCAN CYCLE

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Each PLC operational cycle is made up of three separate parts:

Input scan at which input terminals are read and the input status table is updated,

Program scan at which data in the input status table is applied to the user program,the program is

executed and the output status table is executed

Output scan at which data associated with the output status table is transferred to output terminals.

The PLC scan consists of

1. Input processing

2. Program processing

3. Output processing

When the PLC is performing this scan, it is said to be in RUN mode, else it is said to be in Stop

mode.

1. Input Processing: -

During I/P scan the PLC reads the ON/OFF status of inputs and stores them in the I/P image

memory before execution of program. This information is stored and held valid till the next scan

of input module. No change in status is acknowledged in between scan sequence.

block diagram: input processing

15

2. Program processing: -

The PLC executes the user program taking into consideration the status from image memories

and not that of actual physical elements. Depending upon the logic, the O/P image memory is

updated as the program execution progresses and result is conveyed to the CPU memory for

intermediate storage.

block diagram: program processing

3. Output Processing: -

On completing the execution of all instructions (logic scan), the above stored output image

memory is transferred to output module which will physically change the status of the output on

the PLC module.

block diagram: output processing

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Input/Output Response: -

Once the PLC scan starts, it processes the logic depending upon the status of I/P images. Any

changes in the PLC inputs aftercompletion of input scan will not be accounted for logic scan.

Theses changes in inputs are recognized during subsequent input scan. Thus the I/P response will

be a function of the PLC scan time plus sensing time. Similarly the O/P too will be a function of

scan time of PLC since the O/Ps are functions of the I/P images which get updated during

subsequent PLC scan. The physical O/P response thus will be a function of PLC scan time in

addition to the energizing time of the

O/P relay.

PLC scan

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USE OF THE PLC

PLC is used -:

1. To control different functions done by the machine

2. To provide protection to the people working manually on the machine and to the machine in case of

3. some power failure or some cases of short circuit also.

4. To operate the machine automatically.

Various production equipment that can be connected to PLCs include:

Thermocouples

Strain gauge

Position encoder

Servo valves

Electrical motors

Linear motors

Stepping motors

DESIGN OF HARDWARE OF THE SYSTEM

The basic components of developed system are as follows

1) PROCESS TANK

2) PLC

3) LEVEL CONTROLLER

4) INLET VALVE CONTROLLER

5) STIRRER MOTOR CONTROLLER

6) TEMPERATURE CONTROLLER

7) OUTLET VALVE CONTROLLER

8) PUMP CONTROLLER

9) POWER SUPPLY SECTION

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1.PROCESS TANK:

For experimental setup there are two iron tanks 1)process tank 2) sump tank.

Sump tank is used for circulation of water. In the process tank, two level switches are mounted

and the distance between the low level switch is 300mm.

2.PLC:

PLC is sophisticated controller module used for controlling various operations of the process.

PLC accept the input from the process and according to the ladder diagram

It gives output to the process as per the requirement. The ladder diagram is developed according

to the determined sequence of operations.

3.LEVEL CONTROLLER:

The process tank is required to be filled with water to a certain level. Hence it is required to

control level in the tank. Tank level control unit consists of the following

1) sensor

2) relay driver circuit

Float level sensors are used for sensing the level in the tank. Level floats that are used in the

process are of magnetically coupled type. Whenever level of water goes up to the required level

then high signal is given to the PLC by the float sensor. In turn , PLC turns OFF the inlet valve.

Hence relay-driving circuit is designed for turning OFF the inlet valve. Relay of 24Vdc , 1000

ohm is connected to the output of PLC. According to the level of water in the tank, relay will be

turned ON/OFF by the PLC.

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4.INLET VALVE CONTROLLER:

Solenoid valve is used as inlet valve to fill the water in the process tank. Solenoid valve is

operated from the electromagnetic type of relay of 24Vdc, 1000 ohm that is connected to the

output module of the PLC.

5.STIRRRER MOTOR CONTROLLER:

After filling the tank with water, it is heated using two heaters. Stirrer motor is used for the

uniform heating of the water .Following are the specifications of the motor.

Supply voltage -230V

Phase -single phase

Current - 1 amp

RPM - 1500

PLC turns ON/OFF the motor depending on the sequence of the program developed for PLC.

When stirrer motor is operated then corresponding LED on the front panel of the control box will

glow.

6.TEMPERATURE CONTROLLER:

It is required to control the temperature of water in the process tank. It consists of following

units.

1) sensor

2) signal conditioning circuit

3) relay driver circuit

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RTD PT-100 is used to sense the temperature of water in the process tank. Resistance is

converted into equivalent voltage depending upon the temperature using WSB. The voltage is

then amplified and is given to relay driver circuit. Whenever temperature of water in the process

tank goes up to the set point , relay gets energized which turns OFF the heaters. The LED, on the

front panel of the control box, indicates status of heater 1 & 2.

7.OUTLET VALVE CONTROLLER:

Solenoid valve is used as outlet valve to drain the water from the process tank when temperature

of the water reaches to the set point. Solenoid valve is operated from the electromagnetic type of

relay of 24Vdc, 1000 ohm that is connected to the output module of the PLC.

8.PUMP CONTROLLER:

When water is heated to the require temperature, it will be taken out from the process tank. This

is carried out with the help of pump. Pump will start only after outlet valve is opened i.e when

the batch process is completed and is started using the relay connected to the output module of

PLC.

START/STOP SWITCH AND TEMPERATURE SET POINT ARRANGEMENT:

Start/stop switches and set point knob are provided on the front panel of the control box. Start

switch starts the process and stop switch stops the process. The temperature set point is provided

with the help of set point knob control box also consists of LED indicators to observe the online

status of process.

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Figure: HARDWARE DIAGAM OF THE SYSTEM

DESIGN OF SOFTWARE OF THE SYSTEM :

The PLC is programmed to provide system operation in a manner illustrated, in simple form in

flow chart. The whole software is grouped into following parts.

1.INITIALIZATION AND MONITORING OF POWER:

When reset switch is turned ON the self check of ROM & RAM of process is executed. If self

check is carried out successfully then the PLC checks whether start switch is pressed .If pressed

then process starts.

22

Figure 1 PLC WITH I/P & O/P

2.CONTROLLER PROGRAM FOR LEVEL CONTROL:

PLC checks for the high level switch. If the water level is below high level switch, timer starts.

After 5 seconds , inlet valve is opened and outlet valve remains closed. When the water level in

the tank reaches up to the high level switch, inlet valve will be turned OFF and outlet valve will

remain closed. This completes the level of control loop.

3.CONTROLLER PROGRAM FOR HEATER CONTROLLER:

Two heaters are provided for heating the water in the process tank. The rotary is switch of the

front panel provides temperature set point. When the water temperature in the process tank is less

than that of set point of temperature provided then it would turn on both the heaters. Heaters will

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be turned OFF when the temperature in the process tank reaches upto the set point of

temperature.

Figure 2 LADDER DIAGRAM

4.CONTROLLER PROGRAM FOR STIRRER MOTO CONTROLLER:

Stirrer motor will be turned ON by the PLC for 10 seconds and OFF for 10 seconds. When both

the heaters are turned ON stirrer motor is turned so as to provide uniform heating to the water in

the process tank.

5.CONTROLLER PROGRAM FOR OUTLET VALVE CONTROLLER:

When the batch process of heating the water is completed then for circulating the water, outlet

valve is opened by the PLC.

24

6.CONTROLLER PROGRAM FOR PUMP CONTROLLER:

When outlet valve is opened the pump is started by the PLC so as to circulate the water and feed

it in the sump tank.

FLOWCHART:

Figure 3 FLOW CHART

RESULTS AND CONCLUSION:

25

PLC checks water level in the tank and if water level is low, it opens the inlet valve.

Similarly it has been observed that rest of sequence of events like turning the heaters 1 & 2 ON,

timer operation, checking whether set point is reached or not ,opening of the outlet valve etc.,

work as per the requirement of the process. Different interlocks incorporated in the ladder

diagram works satisfactorily. Hence it can be seen the PLCSBC is an ideal application. The PLC

SBC is an application of PLC. PLC and its related circuits provide the logic diagnostic and

options so as to provide a safe, reliable and versatile system to control various operations of the

process. The system is cost competitive and is applicable for industrial applications.

Hence, it can be concluded that the system designed and developed set up works

satisfactorily and can be used for demonstrating an application of PLC

REFERENCES

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1.WWW.IEEE.COM

2.WWW.WIKIEPEDIA.COM

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