CHAPTER I AUTOMATION USING PLC’s-INTRODUCTION

Any industrial process or system basically involves various operations executed in a

particular sequence. If the operations are monitored and controlled by one or more people, it

is known as Manual control.

In case of automation the operations are carried out automatically, using sensors for

monitoring and electronic controllers for controlling. So the system runs automatically once it

is switched on.

As an example for automation a simple water filling system is explained this finds

wide application and can be implemented in every house.

In house we get municipal water (Manjeera) which will be first filled in the underground tank

generally called the Sump. From sump the water has to be pumped to the overhead tank

(OHT) from which the water is distributed to various outlets (taps). Whenever the ‘OHT’ is

empty we have to switch on the motor to pump the water from sump and we also have to

switch off the motor when tank is filled.

Before we switch on the motor we should see whether there is sufficient water in the

sump or else the motor will burn. There are two sensors S1 & S2 are fixed at the OHT to

sense the high and low level respectively. One sensor S3 to sense the minimum water level to

be maintained in the sump to pump water.

The automatic system monitors the sensors and switches the motor on whenever OHT

is empty (minimum level) sensed through sensor S2. The motor switches on only when

sufficient water is there in the sump sensed through sensor S3. The motor switches off

automatically when the tank is filled sensed through sensor S1.

1

S1

S2

Over-head tank

Motor

Fig 1.1: Automatic water filling system

S3

Sump

CHAPTER II PROGRAMMABLE LOGIC CONTROLLERS (PLC’s)

A programmable controller is an industrial computer in which control devices such as

limit switches, push buttons, providing incoming control signals into the unit. The user

program also directs the PLC on how to control field devices like motors, starters, pilot lights

and solenoids.

A formal definition of a PLC comes from the National Electrical Manufactures

Association (NEMA)

“A Programmable logic controller is a digitally operated electronic system, designed

for use in an industrial environment, which uses a programmable memory for the internal

storage of user –oriented instructions for implementing specific functions such as logic,

sequencing, timing, counting and arithmetic to control, to digital or analog inputs and

outputs, various types of machines of process. Both the PLC and its associative peripherals

are designed so that they can be easily integrated into an industrial control system and easily

used in all their indented functions.

2.1 Use of PLC

The PLC is the tool that provides the control for an automated process. Automation will

help a manufacturing facility to:

1. Gain complete control of the manufacturing process.

2. Achieve constancy in manufacturing.

3. Improve quality and accuracy.

4. Working difficult or hazardous environment.

5. Increase productivity.

6. Shorten the time to market.

7. Lower the cost of quality, scrap, and rework.

8. Offer greater product variety.

9. Quickly change over from product to another.

10. Control inventory.

2

+

K1

K1

K2

PB2PB1

K2

I2

O1

+ LAMP

I1PROGRAM

IF I1AND I2

THEN SET O1OTHERWISE RESET O1

PLC

24v

5v

+

K1

K2

+

LAMP

PB2PB1

K1 K2

INPUTSECTION

24v

OUTPUT SECTION

2.2Hard-wired Control Systems

In an industrial environment, electrical controls of many systems are generally wired

with relays and are usually called relay control.

The wiring between the control elements, such as sensors, solenoids etc., through

relays define the control “program”. Any modification to the “program” involves rewiring the

circuit. Therefore a hard-wired control system is constructed only after its program has been

defined.

Hard-wired controls are difficult to modify, when production requirements change

regularly. As an example, consider the relay control circuit shown in figure below:

Fig2.1: Relay control Fig2.2: PLC control

The Input Section consists of switches and sensors. The input devices transfer the

signals to the processing section. The Processing Section, which consists of relay coils and

contacts, determines the relationship between the inputs received and the output required. The

processed signals are transferred to the output section, where the output devices such as

solenoids, lamps or motors are driven.

3

2.3 Programmable Control Systems

An alternative to the hard-wired control system is the programmable control system.

In the system, a freely programmable electronic controller called Programmable Logic

Controller (PLC) replaces the function of relay portion of the hard-wired control. Figure- 2

depicts the concept of PLC Control as a replacement of relay coil and relay contacts of the

hard-wired control.

The PLC has an input section with a number of inputs designed to accept analog or

digital signals from switches and sensors.

The processor receives the input signals and processes the instructions in the program

with the status of the inputs and sends signals to drive the outputs. The program is written by

the user with aid of a PC or Programming Device and is downloaded in the Controller’s

program memory. The programs are written in the programming language like Ladder

Diagram or Statement List or Function Block Diagram with aid of the Programming software

specific to the PLC. The processor executes the program.

The PLC has an output section with a number of analog or digital outputs to drive the

output devices with its analog or digital signals.

PLCs are available in the market with fixed I/O design or modular design. Some of

the leading manufacturers of PLC are: Allen Bradley, Siemens, Mitsubishi, General Electric,

Omron, ABB, Fanuc etc.

2.4 Comparison of PLCs and Relays

Table 2.1: Comparison of PLC’s and Relays

CHARACTERISTIC PLC RELAY

Construction Easy to program Time consuming

Capable of complicated

Operations

Yes No

Ease of changing the

Control sequence

Very simple Very difficult

Physical size Very compact Bulky

Maintenance Excellent, PLCs rarely fail Poor, if large number of

4

Control bus

Data bus

Address bus

MicroprocessorCPU

ROM

Operating System

RAM

Program & Data

Input Module

Output Module

Fig 2.3: Fundamental design of a microcomputer

contacts and coils have to be

maintained

2.5 Structure of PLC

A PLC is essentially a microcomputer with hardware, firmware and software. The

hardware consists of the actual device technology i.e. the PCBs, ICs, wires, battery, housing

etc. The firmware is the software part, which is permanently installed and supplied by the

PLC manufacturer. This includes the Operating System, which is generally stored in Read-

only-memory (ROM) or in the EPROM. The software is the user program written by the PLC

user. User programs are usually installed in the RAM (Random Access Memory), where they

can be easily modified.

Fig6 shows the fundamental design of a microcomputer. The PLC hardware is based

on a bus system. A bus system is a number of electrical lines divided into Address, data and

Control lines. The Address lines are used to select the address of a connected bus station. The

data lines are to transmit the required information. The control lines are necessary to activate

the correct bus station

2.6 System Components

5

Acknowledge

ments (Limit

Switches & Sensors)

Control elements

(Pushbuttons

Switches)

Input Modules

Central Processing Unit

Output Modules

Final Control

Elements

Relays, Solenoid valves, Lamps

etc.

Program Memory

PLC

Power Supply

The programmable logic controller consists essentially of the input modules, central

processing unit, program memory, output modules and power supply modules.

The inputs supplied by the switches/sensors are passed on to the central processing

unit through input modules. The signals generated by the central processing unit are prepared

by the output modules and passed onto the actuators.

The program is written by using an external programming device (PG) or a PC and

transferred (downloaded) into the program memory. A programming Device (PG) is a hand

held micro-computer with special compact design, suitable for industrial use.

2.6.1 Power Supply

The power supply is necessary to convert 120/230 V AC voltages to 24 V DC

required for the logic circuits of the processor and for the internal power required for the I/O

modules. The power supply can be a separate unit or an integral part of the processor

depending on the manufacturer.

2.6.2 Input/output Section

6

Fig 2.4: Block diagram of a PLC

Programming Device

The Input/output section consists of input and output modules. The input section

converts input voltages of 0 – 24 V DC or 120 – 240 V AC from discrete input devices to

low-level DC voltages (typically 5V) that the processor uses internally to represent status or

condition.

The input section can also convert 4 – 20 mA input signals to low-level DC signals

for the processor. Similarly the output section changes low-level DC signals from the

processor to 120 – 240 V AC or 24 V DC voltages required to operate the discrete output

devices.

The I/O section can be divided into two categories, namely fixed I/O and modular I/O.

The fixed I/O type has built-in inputs and outputs along with power supply and CPU. In the

modular I/O type, plug–in type units called I/O modules or signal modules (SMs) are added

as per the control requirement. Analog modules are used to process analog signals and digital

modules are used to process digital signals. The maximum number of I/Os that can be used is

PLC or CPU specific.

Analog input modules convert electrical signals corresponding to process values (e. g.

temperature) into digital values that can be processed by the CPU and analog output modules

convert digital values from the CPU into analog manipulated variables.

2.6.3 Input Modules

The function of an input module is to convert incoming signals into signals which can be

processed by the PLC and pass these to the central processing unit.

7

Signal to processing unitError

voltage detection Signal delay

Input signal

Output signalAmplifier

Short-circuit

monitoring

Signal from the CPU

The input module contains an error voltage recognition facility, which is activated,

when the input voltage exceeds specified tolerance limits. A trigger circuit with signal delay

ensures that momentary interference peaks are suppressed. Opto-couplers separate internal

and external circuits so that interference is not able to penetrate the PLC through conductive

lines. The input modules also contain LEDs which indicate the status of the input signal.

2.6.4 Output Modules

The function of an output module is to convert the PLC signals into signal suitable for

the actuators

Fig 2.6: Block diagram of output module

8

Fig 2.5: Block diagram of input module

ALU

Accumulator

Command register

Program Counter

Arithmetic unit Control unit

Control busControl bus

Address bus

The opto-couplers are used to isolate the CPU signals from the external circuits in

order that no interference is received through conductive connections. The output signals

need to be amplified so that the final control elements and actuators which require additional

current can be directly connected to the output. The short-circuit-monitoring facility is to

protect the CPU from any short-circuit in the output.

2.6.5 Central Processing Unit

The CPU is the decision-maker or the brain of the system. It is a micro-processor

based system designed to execute the user programs. It can perform various arithmetic

functions, data manipulation and communication between local input/output section,

remotely located I/O sections and/or other networked PLCs.

2.7 Software

The processor of the programmable controller executes the application program

written by the user with the aid of a PC or a PG and stored in controller’s program memory.

The user programs are written with the programming software suitable for the specific PLC.

For example, STEP 5 or STEP 7 is the programming software for the Siemens PLCS and RS

Logic 500 is the programming software for the Allen Bradley PLCs.

9

Data bus

Fig 2.7: Block diagram of CPU

Table 2.2: STEP 7 three methods of representation can be used to translate a program

into user program.

Ladder Diagram (LAD) Graphical representation of control task using relay symbology.

Function Block Diagram (FBD) Graphical representation of control task using logic boxes

familiar from Boolean algebra.

Statement List (STL) Textual representation of control task using mnemonic

abbreviations.

2.8 Programming in general

The PLC programming language called “Ladder Diagram” uses symbolic

representation of switches and output devices to describe the operation of the system. The

Ladder Diagram (LAD) has two vertical sides. The left side is assumed to be connected to the

positive voltage source and right side to the zero volt. Between these vertical sides are the

horizontal current paths or rungs. Symbols representing various program elements such as

contacts and coils etc. are placed on the rungs in accordance with the operating sequence.

2.8.1 NO contact and NC contact (LAD)

LAD has two kinds of contacts for scanning bit operands:

NO contact &

NC contact

Table 2.3: Symbols of NO and NC

10

PB1

K1“0”

“0”

How an NO Contact works?

K1“1”

“1”PB1

Activated

Normally open contact

(NO)Scans for signal state “1”

Normally closed contact

(NC)Scans for signal state “0”

2.8.1.1NO contact (LAD):

Power flows through the NO contact, if the scanned “binary operand” has the

signal state “1”.

Fig 2.8: NO contact

2.8.1.2 NC contact

11

+

PB2PB1

24v

+LAMP

+

PB2PB1

24v

+ LAMP

PLC

For example, figures (a) and (b) shows the connections to control a lamp (L)

connected to the output with address Q 0.0 using two pushbuttons PB1 and PB2 connected to

the inputs with addresses I 0.0 and I 0.1 either in AND logic or in the OR logic. The

programs written in Ladder logic are also shown.

Chapter IIISIEMENS PLCs

12

PLC

Fig 2.9: AND logic Fig 2.10: OR logic

The concept of fully integrated automation describes a revolutionary new way of

combining the worlds of manufacturing and process engineering. All hardware and software

components are integrated into one single system: ‘SIMATIC’

The name SIMATIC was used as the synonym for SIEMENS programmable

controllers of S5 family. Now, the name ‘SIMATIC’ stands for fully integrated automation.

This complete integration is made possible by the universal compatibility offered by the

SIMATIC S7 system.

SIMATIC S7 consists of the following three types of programmable logic controllers

classified according to their performance range.SIMATIC S7 200 SIMATIC S7 200 is a compact micro PLC designed for application having

the lowest performance range.

SIMATIC S7 300 SIMATIC S7 300 is a modular mini controller designed for applications

having intermediate performance range.

SIMATIC S7 400 SIMATIC S7 400 is designed for applications having a high performance

range.

Fig 3.1: Simatic Programmable controllers

3.1 SIMATIC S7 300 Programmable ControllersAn S7 – 300 Programmable Controller is made up of the following hardware components:

Table 3.1: Hardware components of S7-300

13

RacksAccommodate the modules and connect them to each other.

Power Supply

module (PS)

Provides the internal supply voltages. Converts the power system voltage (230V

AC) to 24 V DC for S7 300 and for external loads.

Central Processing

Unit (CPU)

Stores and processes the user program. Supplies the S7 300 backplane bus with

5V; communicates with other nodes in an MPI network via the MPI interface.

Interface Modules

(IMs)

Connects the racks to one another.

Signal Modules

(SMs)

Adapt the signals from the system to the internal signal level or control actuators

via digital or analog signals.

Function Modules

(FMs)

Execute complex or time-critical and memory intensive processes independently

of the CPU, for example positioning or closed-loop control.

Communication

Processors (CPs)

Establish the connection to subsidiary networks (subnets).

SubnetsConnect Programmable Controllers to each other or to other devices.

PROFIBUS bus

cable with bus

connector

Interconnects stations on an MPI or PROFIBUS subnet.

3.2 System Memory

System memory contains the addresses that are accessed through the program.

Addresses may be, for example, inputs used to scan the signal states of PBs/ Limit switches,

or outputs.

The system memory on a CPU contains the following address areas:

Table 3.2: Symbols of address areas in CPU

14

Inputs I Images of digital input modules

Outputs Q Images of digital output modules

Bit memory M Stores of information accessible throughout the whole

program

Timers T Locations used to implement waiting and monitoring

times

Counters C Locations for up and down counting

Temporary local data L Locations used as dynamic intermediate buffers (L

stacks)

CHAPTER IVBASIC PROCEDURE USING STEP 7

Step 7 projects can be created in two different orders:

15

Designing the solution to the automation task

Creating a project

Configuring the hardware

Transferring the program to the CPU and debugging

Creating a program

Creating a program

Configuring the hardware

Fig 4.1: Basic procedure for creating programs

4.1 Projects in STEP 7

In STEP 7, a project contains all the programs and data necessary for an automation

solution, regardless of the number of CPUs involved and how they are networked. Thus, a

project is not just limited to a user program used for a particular programmable module;

instead, it contains several user programs used for many programmable modules, which are

all stored together under a common project name.

With STEP 7, a system can be structured into projects. A project contains the entire

database for an automation solution. Creating a project or project structure is an essential

prerequisite for working with STEP 7

16

Option 2Option 1

Fig 4.2: Simatic manager

4.2 Project structureA project structure can also be created easily using the project wizard as explained below:

The default setting starts the STEP 7 Wizard, which assists you when creating a STEP

7 project. The project structure is used to store and arrange all the data and programs

in order.

In the preview you can toggle the view of the project structure being created on and

off. To move to the next dialog box, click next.

Select the appropriate CPU (say, CPU 313C-2DP, CPU 314 etc.).

Every CPU has certain properties, say with regard to its memory configuration or

address areas etc. That is reason for selecting the CPU before starting the program.

The MPI address (Multipoint Interface) is required in order for the CPU to

communicate with the programming device or PC. The default setting for the MPI

address is 2.

Select the Organization Block OB1.

OB1 represents the highest programming level and organizes other blocks in the S7

program.

Select one of the programming languages: Ladder Logic (LAD), Statement List

(STL), or Function Block Diagram (FBD).

The programming languages can be changed at a later stage.

Double-click to select the suggested name in the “project name” field, say “Getting

started”.

17

Click Make to generate the new project. The SIMATIC Manager will open with the

window for the Getting started” project

If you create project without the STEP 7 Wizard, you must create each directory

within the project yourself.

Fig 4.3: Project structure

4.3 Working with Symbol Editor

Navigate in the project window until S7 Program (1) is reached. Double-click to open

the Symbols component and the symbol table is opened. The symbol table currently only

consists of the predefined Organization Block (OB1). Click the Cycle execution and

18

overwrite it with “Main Program”. Enter “PB_1” and the corresponding address, say I4.0 in

row 2.Complete the entries by pressing Enter. Similarly complete the entries in row 3 & 4

with PB_2 and LAMP along with their addresses respectively. In this way symbolic names

can be assigned to all the absolute addresses of the inputs and outputs in the program.

4.4 Programming a series circuit in Ladder Logic

In the following section, the steps to program a series circuit (AND) in Ladder Logic

(LAD) are explained.

In the same way, insert a second normally open (NO) contact and a coil from the tool

bar. The addresses of the NO contacts and the coil are still missing in the series circuit.

Enter the symbolic name “PB_2” for the second NO contact and “LAMP” for the coil.

Save the block, if there are no more symbols shown in red. Symbols are shown red, if they do

not exist in the symbol table, or if there is a syntax error.

19

Select the current path for the first element.

Click the NO contact in the tool bar.

Click the ??.? Sign and enter the symbolic name “PB_1”.

Confirm with

You can also insert the symbolic name directly from the symbol table. Click the?Sign

and then the menu command Insert > Symbol. Scroll through the pull-down list until you

reach the corresponding name and select it. The symbolic name is added automatically.

20

CHAPTER VINTRODUCTION TO PNEUMATICS

5.1 Pneumatics – Definition

The study of pneumatics deals with systems operated with air or other gaseous media.

The term “pneumatics” is derived from the Greek “pneuma”, meaning wind or breath. Hence

pneumatics may be defined as the study of movement of air. Pneumatics is concerned with

the use of compressed gaseous fluids to provide power or to control power.

5.2 Compressed air for transmitting power

Fig 5.1: A simplified pneumatic system

A simplified pneumatic system with only three blocks is given in figure 2.1. In

industry, the pneumatic medium generally used for transmitting power is the highly

compressible air. Since gaseous substances are compressible in ratios of reduction in volume

to increase in pressure, a compressor is used as a source of energy. The compressed air is

then prepared or treated in several stages to remove various contaminants present in it and

stored in a tank called receiver tank. Other factors of concern at this stage are the distribution

of air, regulation of pressure and lubrication of compressed air.

The compressed air medium is then used to do work in a controlled manner by being

allowed to expand back to the atmospheric pressure. The work done in this expansion is

transmitted to a load surface such as a piston or a vane, which will be moved by the

21

Power source

ControlValve

Actuator(Cylinde

Compressed air Air

expanding air with force equal to the pressure acting on the surface area (Force = Pressure x

Area).

The function of a pneumatic system can be summed up in the following simple terms:

Applying a force to a gaseous fluid like air and transmitting pneumatic pressure throughout

the fluid and converting the stored energy back into mechanical force before work can be

done.

5.3 Use of Pneumatics for automation

As explained above Pneumatics is basically energy system i.e. compressed air

converted to force and displacement to get work done. But the widely used energy is

electrical energy. So the question arises why electrical energy can’t be used for automation. It

can definitely be used, but the basic motion we get is rotational and when we require linear

motion, we have to convert rotary motion to linear by mechanical conversion. But in case of

Pneumatics the basic motion is linear and high speeds in the range of 4.5m/sec to 6m/sec can

be achieved. Hence Pneumatics is used widely for low cost automation.

Pneumatics finds application in industry sectors ranging from automotive

manufacturing to on-board commercial vehicles, from rail applications to printing and

textiles, from food packaging to process industries, from the electronic sector to medical care

and in thousands of other specialised industries.

5.4 Pneumatic system structure

A pneumatic system can be broken down into a number of levels representing

hardware and signal flow. The various levels form a control path for signal flow from the

signal (input) side to the work (output) side.

22

ACTUATING DEVICES(OUTPUTS)

PROCESSING ELEMENTS

(PROCESSOR SIGNALS)

INPUT ELEMENTS(INPUT SIGNALS)

ENERGY SUPPLY(SOURCE)

FINAL CONTROL ELEMENT

Fig 5.2: Pneumatic system structure

5.5 Energy supply ( )

Air in a pneumatic system must be clean and dry to reduce wear, at the required pressure

(regulated) and perhaps lubricated.

5.6 Pneumatic Actuators

The actuator group includes various types of linear and rotary actuators of varying size and

construction. Actuators can be broken down into the following groups:

Linear Actuators

Single acting cylinder

Double acting cylinder

Rotary Actuators

Vane type

Air motors

23

5.6.1 Single acting cylinder

With single acting cylinders, compressed air is applied on only one side of the piston

face. The other side is open to atmosphere. The cylinders can perform work in only one

direction. The return movement of the piston is effected by the application of an external

force or by a built-in spring as shown in figure below.

Fig 5.3: single acting cylinder

5.6.2 Double acting cylinder

With double acting cylinders, compressed air is applied on both sides of the piston

face. The cylinders can perform work in both directions as shown in figure below.

Fig 5.4: Double acting cylinder

5.6.3 Rotary actuator

The rotary actuator is compact with high torque ratings. The force is transmitted to the

drive shaft by a rotary vane. The range of angular movement is adjustable with end stops. The

angle can be adjusted between 0 º and 180 º.

Symbols

24

Table 5.1: symbols of types of cylinders

Single acting cylinder

Double acting cylinder

Rotary actuator

5.7 Pneumatic valves

A valve is used as a power valve for supply of working air to the actuator. A valve is also

used to alter, generate or cancel signals for the purpose of sensing, processing and

controlling. Valves can be divided into a number of groups according to their function:

Directional control valve

Non-return valve

Flow control valve

Pressure control valve

The directional control valve controls the passage of air signals by generating, canceling

or redirecting signals.

5.7.1 Representation of valves

A valve is described by:

Number of ports -2 way, 3 way, 4 way, 5 way etc.

Number of positions -2 positions, 3 positions etc.

Method of actuation -Manual, Mechanical, Pneumatic, Electrical.

Method of return actuation -spring returns etc.

25

Table 5.2: Representation of valves

3/2 directional control valve

(Normally closed)

3/2 directional control valve

(Normally open)

4/2 directional control valve

5/2 directional control valve

5.7.1.1 Flow control valveFlow control valves (Throttle valves) influence

the volumetric flow of the compressed air, in both

directions.

Fig5.5: flow control valve

5.7.1.2 Throttle relief valveThis is also called one-way flow control valve. In this valve air flow is throttled in one

direction only and the air can flow only through regulated cross-section. In the opposite

direction, the air can flow freely through the open check valve are used for speed regulated of

pneumatic cylinders.

Fig 5.6: Throttle relief valve

26

No. of portsNo. of positions

CHAPTER VIELECTRO-PNEMATICS

Electro-pneumatics is the result of integration of pneumatic and electrical

technologies. In such a system the electrical signals in the control part have to be

converted into actions by the pneumatic power part. The control part may consist of

sensors and pushbuttons for providing input signals. This information is passed to the

processing device such as a relay for processing the received signals. The processor

then drives the output stage of the system through the solenoid valves. The solenoid

valve is electrically actuated with a pneumatic output which is connected to the

actuator.

Table 6.1: System components

ACTUATORS

CONTROL ELEMENTS

PROCESSORS

SENSORS

ENERGY SUPPLY

6.1 Solenoid

If a conductor is formed into the shape of one turn of coil and a current is

passed through it, an electromotive force (EMF) is generated. By increasing the

number of turns, the magnetic field is strengthened and the EMF is increased. The

EMF is greatly increased, if the wire is formed around an iron core. There is an

interesting property for the solenoid which is used in solenoid valves to get the

actuating force. An iron core which is displaced from the centre of the coil has the

27

tendency to move towards the centre of the coil when a current is drawn through the

coil, thus providing a linear motion. This linear motion can be used to actuate the

solenoid

Fig 6.1: Solenoid

6.2 Switches and pushbuttons

A switch is a device for making a circuit or breaking a circuit. A switch is primarily

distinguished by the type of contact:

Table 6.2: Switches

Normally open contact

(NO)On actuation, enables energy flow

Normally closed contact

(NC)On actuation, disables energy flow

Change-over contact

(CO)A combination of NO and NC contacts

6.2.1 Reed switch

28

Soft iron core

Reed switches are also known as magnetically actuated proximity switches. In

its basic form the reed switch has a contact pair fused into an inert gas filled tube and

encapsulated in epoxy resin.

Reed switches are commonly used to sense the positions of pneumatic

cylinder piston rods and angle of rotation of the shaft in rotary actuators.

Fig 6.2: Reed switch

29

CHAPTER VIIAUTOMATIC STAMPING UNIT

Prefabricated parts are stamped automatically on a special pneumatic unit as

shown below. The parts are pushed form feed magazine into operating position

against a stop by Feed Cylinder 1 where they are clamped. Stamping Cylinder 2 then

extends slowly and stamps the part. After stamping, cylinder 2 & cylinder 1 return

one after the other. The parts are ejected (pushed in to the out bin) by the Ejector

Cylinder 3 and then returns back.

Fig 7.1: Automatic stamping unit

30

Cylinder 3

Cylinder 2

Cylinder 1

7.1 Construction

The system consists of three cylinders, namely

i. Feed cylinder or clamping cylinder

ii. Stamping cylinder

iii. Eject cylinder.

These cylinders (special cylinders with a magnetic band on the piston) have

Reed switch sensors fixed on them. Reed switches are also known as magnetically

actuated proximity switches. Each cylinder is with 2 reed switch sensors, one at the

piston side S1, other at Rod side S2, likewise S3, S4 for stamping cylinder and S5, S6

for the Eject cylinder respectively. Each cylinder is driven by the double solenoid

valve. As shown in the figure each valve has two coils. When we apply 24V to the

coil Y1, Y3, Y5 the valve changes its position and allows the air compressed air from

P to A port through which the air enters piston side port of the cylinder. At the same

time the rod side port of the cylinder which is connected to port B of the valve, is

routed to exhaust port S through inner path of the valve and air is released to exhaust

from the cylinder. Hence the cylinder piston along with rod moves forward and

known as forward stroke.

Similarly when the voltage signal is applied to Y2, Y4, and Y6 the valve

changes its position and allows the air from P to B port and releases air from port A

through exhaust port R. Hence piston moves back along with Rod is known as Return

stroke.

7.2 Control circuit

The control is implemented through a 6 step sequence as explained below.

Step1: Feed cylinder forward stroke A+, the cylinder brings the component to be

stamped from the feed magazine and clamps (holding the component tightly against

wall on the opposite side as shown in figure).

Step2: Stamping cylinder forward stroke B+, the stamping cylinder rod mounted

vertically comes down and stamps the component.

31

Step3: Stamping cylinder backward stroke B-, the stamping cylinder rod goes back to

home position.

Step4: Feed cylinder backward strike A-. The feed cylinder rod goes back to home

position leaving the stamped component.

Step5: Eject cylinder forward C+. The eject cylinder moves forward and pushes the

stamped component in the out bin.

Step6: Eject cylinder backward strike C-. The cylinder goes back to the home

position.

Fig 7.2: Displacement-step diagram Table 7.1: Assignment list

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Clampin

g

Cylinder

A

1

0

Feed

CylinderB

1

0

Ejector

CylinderC

1

0

PB1 I0.0

PB2 I0.1

S1 I1.0

S2 I1.1

S3 I1.2

S4 I1.3

S5 I1.4

S6 I1.5

Y1 Q0.0

Y2 Q0.1

Y3 Q0.2

Y4 Q0.3

Y5 Q0.4

Y6 Q0.5

7.3 Ladder program explanation

The type of program used in PLC is ladder program. The control ladder

program contains 6 networks for 6 steps in serial order as mentioned above (6 steps).

Each network contains only memory element i.e. nothing but set/reset flip-flop.

Network 1:

Set control: A+On the set side we have 2 NO contacts first contact activated by start button

and 2nd contact activated by sensor S5 which senses the backward stroke of eject

cylinder. This is the last step C- in the 6 step sequence. By checking this so we are

making sure that the previous cycle is completed. This has to be done essentially

before starting ,because sometimes the system may stop in the middle of the sequence

either due to mechanical problem (due to misalignment of sensors) or due to electrical

or pneumatic power failure. The system should not start unless previous cycle is

completed for safety reasons. So the system has to be brought back to initial state.

This is done by giving Mechanical Reset (MR) to bring back all the cylinders to home

position, followed by electronic Reset(ER) to bring back all the memory elements to

reset condition low state.

So when start button is pressed and S5 is activated by eject cylinder completing back

stroke C-, the signal passes to set side of memory M0.0 and sets it i.e. makes output

Q0.0 high and drives feed cylinder to forward strike.(A+)

Reset control:This is activated by the NO contact driven by M0.1 i.e. when the next step is

activated. So step1 memory is reset when step2 memory is set. This is necessary as

once feed cylinder makes forward stroke, the output has to be switched back to low

state, for 2 reasons. Firstly to bring back the initial condition for next cycle and

secondly to allow backward stroke A- when the particular o/p is activated. The 2nd

‘NO’ in parallel is activated by ‘Electronic reset input’ (PB1) which is used to reset

all the outputs to initial condition to start a fresh cycle.

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Network 2:

Set control: B+This is driven by 2 NO contacts. 1st NO contact is activated by M0.0 i.e. when

previous step is executed. This makes sure that the sequence is maintained properly.

The 2ndNO is activated by S2 which is activated only when feed cylinder forward

stroke is executed successfully.

So only when previous step output is activated AND sensor S2 activated by execution

of forward stroke by feed cylinder, the signal passes to SET side of M0.1 and sets

output HIGH and drives Q0.2, and forward stroke of stamping cylinder B+ is

executed.

Reset control:This is activated when the next step output M0.2 is activated in the sequence

or when the electronic reset push button (PB2-I0.1) is pressed.

Network 3:

Set control: B- when the previous step outputM0.1 is activated and sensor S4 is activated by

forward stroke of the stamping cylinder, the signal passes to set side of M0.2 and

sets it high drives Q0.3 i.e. executing B- return stroke of stamping cylinder.

Reset control:This is activated when the next step output M0.3 is activated in the sequence or when

the electronic reset push button (PB2-I0.1) is pressed.

Network 4:

Set control: A-Activated when the previous output M0.2is actuated and sensor S3 is activated

by backward stroke of the stamping cylinder, the signal passes to set side of M0.3 and

sets it high, drives Q0.1 i.e. executing A- return stroke of clamping cylinder.

Reset control:This is activated when the next step output M0.4 is activated in the sequence

or when the electronic reset push button (PB2-I0.1) is pressed.

34

Network 5:Set control: C+

Activated when the previous output M0.3 is activated and sensor S1 is

activated by backward stroke of the clamping cylinder, the signal passes to set side of

M0.4 and sets it high, drives Q0.4 i.e. executing C+ Forward stroke of Eject

cylinder.

Reset control:This is activated when the next step output M0.5 is activated in the sequence

or when the electronic reset push button (PB2-I0.1) is pressed.

Network 6:

Set control C-:Activated when the previous output M0.4 is activated and sensor S6 is

activated by forward stroke of the eject cylinder, the signal passes to set side of M0.5

and sets it high drives Q0.5 i.e. executing C- return stroke of eject cylinder.

Reset control:This is activated when the next step output M0.0 is activated in the sequence

or when the Mechanical Reset push button (PB3-I0.2) is pressed.

MR: PB3 (I 0.2)Mechanical Reset is given to SET side of all Memory elements that control

Return stroke of all cylinders. So all the cylinders execute return stroke and the

mechanical system attains initial state.

ER: PB2 (I 0.1)Electronic Reset is used to bring back all the memory elements to reset

condition low state and the electronic control system attains initial state.

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7.4 Pneumatic circuit

Fig 7.3: Pneumatic circuit

36

For speed control

Double solenoid

Fig 7.4: PLC software circuit 1

37

Start I0.0 S5

M0.1

Q0.0M0.0

ER

M0.1 S2

M0.2

ER

M0.1

M0.1 S4

Q0.2

M0.2

M0.3

Q0.3

M0.4

M0.2

ER

M0.3 Q0.1

ER

S3

Fig 7.5: PLC software circuit 2

38

M0.3

M0.5

S1 Q0.4M0.4

ER

S6 M0.5M0.4

M0.0

Q0.5

ER

CONCLUSIONS

P L C systems have within the industrial process and as such have established

a strong presence in modern industry. Continuous development of pneumatic power

technology has significantly expanded and increased the applications of PLC to many

areas hitherto unknown for adopting pneumatics. Among many applications and users

of PLC along with pneumatic systems are:

Aircraft manufacturers

Cement plants

Chemical plants

Coal mines

Cotton mills

Dairies

Distilleries

Forge shops

Foundries and steel mills

Oil refineries

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