godfather (2)
TRANSCRIPT
A
PRACTICAL TRAINING REPORT
On
“CNC MACHINES AND THEIR MAINTAINENCE”
Submitted for
The partial fulfillment of degree of
BACHELOR OF TECHNOLOGY
In
ELECTRONICS & COMMUNICATION ENGINEERING
(Rajasthan Technical University, Kota)
1
2
SESSION 2010-11
Submitted to: Submitted by:Ajay Kumar Keshav SharmaFaculty, Training in-charge VII Semester (ECE)Department of ECE Roll No.:07ESTEC043Jaipur (Raj).
DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERINGSTANI MEMORIAL COLLEGE OF ENGENEERING AND TECHNOLOGY
PHAGI, JAIPURDEC.-2010-11
©Stani Memorial College of Engineering and Technology jaipurAll rights reserved.
A
PRACTICAL TRAINING REPORT
On
“CNC MACHINES AND THEIR MAINTAINENCE”
Submitted for
The partial fulfillment of degree of
BACHELOR OF TECHNOLOGY
In
ELECTRONICS & COMMUNICATION ENGINEERING
(Rajasthan Technical University, Kota)
3
SESSION 2010-11
Submitted to: Submitted by:Ajay Kumar Keshav SharmaFaculty, Training in-charge VII Semester (ECE)Department of ECE Roll No.:07ESTEC043Jaipur (Raj).
DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERINGSTANI MEMORIAL COLLEGE OF ENGENEERING AND TECHNOLOGY
PHAGI, JAIPURDEC.-2010-11
©Stani Memorial College of Engineering and Technology jaipurAll rights reserved.
4
ACKNOWLEDGEMENT
We are highly thankful to B.H.E.L. engineers and technical staff for providing us
vital and valuable information about the different facets of an industrial
management system.
We express our gratitude to Human Resource and Development department for
giving us a chance to feel the industrial environment and its working in B.H.E.L.
and we are thankful to Mr. B.S.GARG, Sr.Engineer for giving his precious time
and help us in understanding various theoretical and practical aspect of our project
on CNC under whose kind supervision we accomplished our project. We are also
thankful to Mr.VEDPRAKESH for his kind support.
I am deeply indebted to Mr. ABHISHEK SHARMA, Head ,Department of
Electronics and Communication for his valuable suggestion ,timely guidance and
providing the facilities in the department .
I wish to express my indebtedness and grateful thanks to my father
Shri. G.R.SHARMA , my mother Smt.KARUNA SHARMA and my family
members for their sacrifice during the training period.
KESHAV SHARMA
4TH yr EC
SMCET
5
PREFACE
At very outset of the prologue it becomes imperative to insist that vocational
training is an integral part of engineering curriculum. Training allows us to gain an
insight into the practical aspects of the various topics, with which we come across
while pursuing our B.Tech i.e. vocational training gives us practical
implementation of various topics we already have learned and will learn in near
future. Vocational training always emphasizes on logic and commonsense instead
of theoretical aspects of subject.
On my part, I pursued four weeks training at B.H.E.L. Haridwar. The training
involved a study of various departments of the organization as per the time
logically scheduled and well planned given to us.
The rotation in various departments was necessary in order to get an overall idea
about the working of the organization.
6
CONTENTS
1) PROFILE OF INDUSTRY
2) CNC SYSTEM
3) CONFIGURATION SYSTEM OF CNC
a.CENTRAL PROCESSING UNIT
b.SERVO CONTROL UNIT
c.OPERATOR CONTROL PANEL
d.PROGRAMMABLE LOGIC CONTROLLER
4) WORKING PRINCIPAL OF CNC MACHINES
5) VARIOUS CNC MACHINE
6) ADVANTAGE OF CNC SYSTEMS
7) BASIC PLC COMPONENTS
a. PLC OPERATION
i. RELAYS OUTPUT
ii. TRANSISTOR OUTPUT
b. PLC PROGRAMMING
i. CONFIGURATION OF HARDWARE AND SOFTWARE
ii. CONTROLLER COMMUNICATION SETUP
iii. CONFUGRING TIMER
iv. CONFUGRING COUNTER
v. LADDER COMMANDS
c. INPUT AND OUTPUT MODULES
i. SINK AND SOURCE INPUT AND OUTPUT
8) ADDRESSES
9) APPLICATION OF PLC
10) CONCLUSION
11) BIBLIOGRAPHY
7
8
PROFILE OF INDUSTRY
BHEL is the largest engineering and manufacturing enterprise in India in the energy-related/infrastructure sector, today. BHEL was established more than 40 years ago, ushering in the indigenous Heavy Electrical Equipment industry in India - a dream that has been more than realized with a well-recognized track record of performance. The company has been earning profits continuously since 1971-72 and paying dividends since 1976-77.
BHEL manufactures over 180 products under 30 major product groups and caters to core sectors of the Indian Economy viz., Power Generation & Transmission, Industry, Transportation, Telecommunication, Renewable Energy, etc. The wide network of BHEL's 14 manufacturing divisions, four Power Sector regional centres, over 100 project sites, eight service centres and 18 regional offices, enables the Company to promptly serve its customers and provide them with suitable products, systems and services -- efficiently and at competitive prices. The high level of quality & reliability of its products is due to the emphasis on design, engineering and manufacturing to international standards by acquiring and adapting some of the best technologies from leading companies in the world, together with technologies developed in its own R&D centres.
BHEL has acquired certifications to Quality Management Systems (ISO 9001), Environmental Management Systems (ISO 14001) and Occupational Health & Safety Management Systems (OHSAS 18001) and is also well on its journey towards Total Quality Management. BHEL's operations are organized around three business sectors, namely Power, Industry - including Transmission, Transportation, Telecommunication & Renewable Energy - and Overseas Business. This enables BHEL to have a strong customer orientation, to be sensitive to his needs and respond quickly to the changes in the market.
BHEL's vision is to become a world-class engineering enterprise, committed to enhancing stakeholder value. The company is striving to give shape to its aspirations and fulfill the expectations of the country to become a global player.
The greatest strength of BHEL is its highly skilled and committed 42,600 employees. Every employee is given an equal opportunity to develop himself and grow in his career. Continuous training and retraining, career planning, a positive work culture and participative style of management all these have engendered development of a committed and motivated workforce setting new benchmarks in terms of productivity, quality and responsiveness.
BHEL has Installed equipment for over 90,000 MW of power generation -- for Utilities, Captive and Industrial users.
Supplied over 2,25,000 MVA transformer capacity and other equipment operating in Transmission & Distribution network up to 400 kV (AC & DC).
Supplied over 25,000 Motors with Drive Control System to Power projects, Petrochemicals,
9
Refineries, Steel, Aluminum, Fertilizer, Cement plants, etc.
Supplied Traction electrics and AC/DC locos to power over 12,000 kms Railway network.
Supplied over one million Valves to Power Plants and other Industries.BHEL's operations are organised around three business sectors, namely Power, Industry - including Transmission, Transportation, Telecommunication & Renewable Energy - and Overseas Business. This enables BHEL to have a strong customer orientation, to be sensitive to his needs and respond quickly to the changes in the market.BHEL's vision is to become a world-class engineering enterprise, committed to enhancing stakeholder value. The company is striving to give shape to its aspirations and fulfill the expectations of the country to become a global player.The greatest strength of BHEL is its highly skilled and committed 42,600 employees. Every employee is given an equal opportunity to develop himself and grow in his career. Continuous training and retraining, career planning, a positive work culture and participative style of management all these have engendered development of a committed and motivated workforce setting new benchmarks in terms of productivity, quality and responsiveness.
MANUFACTURING UNITS OF B.H.E.L.
First Generation Units
BHOPAL Heavy Electrical Plant HARDWAR Heavy Electrical Equipment Plant HYDERABAD Heavy Electrical Power Equipment Plant TIRUCHY High Pressure Boiler Plant
Second Generation Units
JHANSI Transformer and Locomotive Plant HARDIWAR Central Foundry and Forge Plant TIRUCHY Seamless Steel Tube Plant
Unit Through Acquisition and Merger
BANGALORE Electronic Division Electro Porcelain Division
New Manufacturing Units
RANIPAT Boiler Auxiliaries Plant JAGDISHPUR Insulator Plant RUDRAPUR Component and Fabrication Plant BANGALORE Industrial System Group
10
POWER GENERATION
Power generation sector comprises thermal, gas, hydro and nuclear power plant
business as of 31.03.2001, BHEL supplied sets account for nearly 64737 MW or 65% of
the total installed capacity of 99,146 MW in the country, as against nil till 1969-70.
BHEL has proven turnkey capabilities for executing power projects from
concept to commissioning, it possesses the technology and capability to produce
thermal sets with super critical parameters up to 1000 MW unit rating and gas turbine
generator sets of up to 240 MW unit rating. Co-generation and combined-cycle plants
have been introduced to achieve higher plant efficiencies. to make efficient use of the
high-ash-content coal available in India, BHEL supplies circulating fluidized bed
combustion boilers to both thermal and combined cycle power plants.
The company manufactures 235 MW nuclear turbine generator sets and has
commenced production of 500 MW nuclear turbine generator sets.
Custom made hydro sets of Francis, Pelton and Kapian types for different head
discharge combination are also engineering and manufactured by BHEL.
In all, orders for more than 700 utility sets of thermal, hydro, gas and nuclear have been
placed on the Company as on date. The power plant equipment manufactured by BHEL
is based on contemporary technology comparable to the best in the world and is also
internationally competitive.
The Company has proven expertise in Plant Performance Improvement through
renovation modernisation and uprating of a variety of power plant equipment besides
specialised know how of residual life assessment, health diagnostics and life extension
11
of plants.
POWER TRANSMISSION & DISTRIBUTION (T & D)
BHEL offer wide ranging products and systems for T & D applications. Products
manufactured include power transformers, instrument transformers, dry type
transformers, series – and stunt reactor, capacitor tanks, vacuum – and SF circuit
breakers gas insulated switch gears and insulators.
A strong engineering base enables the Company to undertake turnkey delivery of
electric substances up to 400 kV level series compensation systems (for increasing
power transfer capacity of transmission lines and improving system stability and voltage
regulation), shunt compensation systems (for power factor and voltage improvement)
and HVDC systems (for economic transfer of bulk power). BHEL has indigenously
developed the state-of-the-art controlled shunt reactor (for reactive power management
on long transmission lines). Presently a 400 kV Facts (Flexible AC Transmission
System) project under execution.
INDUSTRIES
BHEL is a major contributor of equipment and systems to industries. Cement, sugar,
fertilizer, refineries, petrochemcials, paper, oil and gas, metallurgical and other process
industries lines and improving system stability and voltage regulation, shunt
compensation systems (for power factor and voltage improvement) and HVDC systems
(for economic transfer of bulk power) BHEL has indigenously developed the state-of-
the-art controlled shunt reactor (for reactive power management on long transmission
lines). Presently a 400 kV FACTS (Felxible AC Transmission System) projects is under
execution.
12
Industries
BHEL is a major contributor of equipment and systems to industries, cement, sugar,
fertilizer, refinances, petrochemicals, paper, oil and gas, metallurgical and other process
industries. The range of system & equipment supplied includes: captive power plants,
co-generation plants DG power plants, industrial steam turbines, industrial boilers and
auxiliaries. Wate heat recovery boilers, gas turbines, heat exchangers and pressure
vessels, centrifugal compressors, electrical machines, pumps, valves, seamless steel
tubes, electrostatic precipitators, fabric filters, reactors, fluidized bed combustion boilers,
chemical recovery boilers and process controls.
The Company is a major producer of large-size thruster devices. It also supplies digital
distributed control systems for process industries, and control & instrumentation
systems for power plant and industrial applications. BHEL is the only company in India
with the capability to make simulators for power plants, defense and other applications.
The Company has commenced manufacture of large desalination plants to help
augment the supply of drinking water to people.
Transportation
BHEL is involved in the development design, engineering, marketing, production,
installation, maintenance and after-sales service of Rolling Stock and traction
propulsion systems. In the area of rolling stock, BHEL manufactures electric
locomotives up to 5000 HP, diesel-electric locomotives from 350 HP to 3100 HP,
both for mainline and shunting duly applications. BHEL is also producing rolling
stock for special applications viz., overhead equipment cars, Special well wagons,
13
Rail-cum-road vehicle etc., Besides traction propulsion systems for in-house use,
BHEL manufactures traction propulsion systems for other rolling stock producers of
electric locomotives, diesel-electric locomotives, electrical multiple units and metro
cars. The electric and diesel traction equipment on India Railways are largely powered
by electrical propulsion systems produced by BHEL. The company also undertakes
retooling and overhauling of rolling stock in the area of urban transportation systems.
BHEL is geared up to turnkey execution of electric trolley bus systems, light rail
systems etc. BHEL is also diversifying in the area of port handing equipment and
pipelines transportation system.
Telecommunication
BHEL also caters to Telecommunication sector by way of small, medium and
large switching systems.
Renewable Energy
Technologies that can be offered by BHEL for exploiting non-conventional and
renewable sources of energy include: wind electric generators, solar photovoltaic
systems, solar lanterns and battery-powered road vehicles. The Company has taken up
R&D efforts for development of multi-junction amorphous silicon solar cells and fuel
based systems.
International Operations
BHEL has, over the years, established its references in around 60 countries of the
world, ranging for the United States in the West to New Zealand in the Far East. These
references encompass almost the entire product range of BHEL, covering turnkey
power projects of thermal, hydro and gas-based types, substation projects,
14
rehabilitation projects, besides a wide variety of products, like transformers, insulators,
switchgears, heat exchangers, castings and forgings, valves, well-head equipment,
centrifugal compressors, photo-voltaic equipment etc. Apart from over 1110MW of
boiler capacity contributed in Malaysia, and execution of four prestigious power projects
in Oman, Some of the other major successes achieved by the Company have been in
Australia, Saudi Arabia, Libya, Greece, Cyprus, Malta, Egypt, Bangladesh, Azerbaijan,
Sri Lanka, Iraq etc.
The Company has been successful in meeting demanding customer's requirements in
terms of complexity of the works as well as technological, quality and other
requirements viz extended warrantees, associated O&M, financing packages etc. BHEL
has proved its capability to undertake projects on fast-track basis. The company has
been successful in meeting varying needs of the industry, be it captive power plants,
utility power generation or for the oil sector requirements. Executing of Overseas
projects has also provided BHEL the experience of working with world renowned
Consulting Organisations and inspection Agencies.
In addition to demonstrated capability to undertake turnkey projects on its own, BHEL
possesses the requisite flexibility to interface and complement with International
companies for large projects by supplying complementary equipment and meeting their
production needs for intermediate as well as finished products.
The success in the area of rehabilitation and life extension of power projects has
established BHEL as a comparable alternative to the original equipment manufactures
(OEMs) for such plants.
15
Technology Upgradation and Research & Development
To remain competitive and meet customers' expectations, BHEL lays great emphasis on
the continuous upgradation of products and related technologies, and development of
new products. The Company has upgraded its products to contemporary levels through
continuous in house efforts as well as through acquisition of new technologies from
leading engineering organizations of the world.
The Corporate R&D Division at Hyderabad, spread over a 140 acre complex, leads
BHEL's research efforts in a number of areas of importance to BHEL's product range.
Research and product development centers at each of the manufacturing divisions play
a complementary role.
BHEL's Investment in R&D is amongst the largest in the corporate sector in India.
Products developed in-house during the last five years contributed about 8.6% to the
revenues in 2000-2001.
BHEL has introduced, in the recent past, several state-of-the-art products developed in-
house: low-NQx oil / gas burners, circulating fluidized bed combustion boilers, high-
efficiency Pelton hydro turbines, petroleum depot automation systems, 36 kV gas-
insulated sub-stations, etc. The Company has also transferred a few technologies
developed in-house to other Indian companies for commercialisation.
Some of the on-going development & demonstration projects include: Smant wall
blowing system for cleaning boiler soot deposits, and micro-controller based governor
for diesel-electric locomotives. The company is also engaged in research in futuristic
areas, such as application of super conducting materials in power generations and
industry, and fuel cells for distributed, environment-friendly power generation.
16
CNC SYSTEMS
Development of computerized numerical controlled (CNC) machines is an outstanding
contribution to the manufacturing industries. It has made possible the automation of the
machining process with flexibility to handle small to medium batch of quantities in part
production.
Initially, the CNC technology was applied on basic metal cutting machine like lathes, milling
machines, etc. Later, to increase the flexibility of the machines in handling a variety of
components and to finish them in a single setup on the same machine, CNC machines capable of
performing multiple operations were developed. To start with, this concept was applied to
develop a CNC machining centre for machining prismatic components combining operations like
milling, drilling, boring and taping. Further, the concept of multi-operations was also extended
for machining cylindrical components, which led to the development of turning centers.
17
Numerical control (NC) is a method employed for controlling the motions of a machine tool
slide and its auxiliary functions with input in the form of numerical data. A computer numerical
control (CNC) is a microprocessor-based system to store and process the data for the control of
slide motions and auxiliary functions of the machine tools. The CNC system is the heart and
brain of a CNC machine which enables the operation of various machine members such as slides,
spindles, etc. as per the sequence programmed into it, depending on the machining operations.
The main advantage of a CNC system lies in the fact that the skills of the operator hitherto
required in the operation of a conventional machine is removed and the part production is made
automatic.
The CNC systems are constructed with a NC unit integrated with a programmable logic
controller (PLC) and some times with an additional external PLC (non-integrated). The NC
controls the spindle movement and the speeds and feeds in machining. It calculates the traversing
path of the axes as defined by the inputs. The PLC controls the peripheral actuating elements of
the machine such as solenoids, relay coils, etc. Working together, the NC and PLC enable the
machine tool to operate automatically. Positioning and part accuracy depend on the CNC
system's computer control algorithms, the system resolution and the basic mechanical machine
accuracy. Control algorithm may cause errors while computing, which will reflect during
contouring, but they are very negligible. Though this does not cause point to point positioning
error, but when mechanical machine inaccuracies are present, it will result in poorer part
accuracy.
This chapter gives an overview of the configuration of the CNC system, interfacing and
introduction to PLC programming
18
diagram of a CNC machine tool
A CNC system basically consists of the following:
Central processing unit (CPU)
Servo-control unit
Operator control panel
Machine control panel
Other peripheral device
Programmable logic controller (PLC)
Central Processing Unit (CPU)
The CPU is the heart and brain of a CNC system. It accepts the information stored in the
memory as part program. This data is decoded and transformed into specific position control and
velocity control signals. It also oversees the movement of the control axis or spindle whenever
this does not match the programmed values, a corrective action is taken.
19
All the compensations required for machine accuracy (like lead screw pitch error, tool
wear out, backlash, etc.) are calculated by the CPU depending upon the corresponding inputs
made available to the system. The same will be taken care of during the generation of control
signals for the axis movement. Also, some safety checks are built into the system through this
unit and the CPU unit will provide continuous necessary corrective actions. Whenever the
situation goes beyond control of the CPU, it takes the final action of shutting down the system in
turn the machine.
Speed Control Unit
This unit acts in unison with the CPU for the movement of the machine axes. The CPU sends the
control signals generated for the movement of the axis to the servo control unit and the servo
control unit convert these signals into the suitable digital or analog signal to be fed to the
machine tool axis movement. This also checks whether machine tool axis movement is at the
same speed as directed by the CPU. In case any safety conditions related to the axis are overruled
during movement or otherwise they are reported to the CPU for corrective action
Servo-Control Unit
The decoded position and velocity control signals, generated by the CPU for the axis movement
forms the input to the servo-control unit. This unit in turn generates suitable signals as command
values. The servo-drive unit converts the command values, which are interfaced with the axis
and the spindle motors .
The servo-control unit receives the position feedback signals for actual movement of the
machine tool axes from the feedback devices (like linear scales, rotary encoders, resolves, etc.).
The velocity feedback is generally obtained through tacho generators. The feedback signals are
passed on to the CPU for further processing. Thus the servo-control unit performs the data
communication between the machine tool and the CPU.
As explained earlier, the actual movements of the slides on the machine tool is achieved
through servo drives. The amount of movement and the rate of movement are controlled by the
CNC system depending upon the type of feedback system used, i.e. closed-loop or open-loop
system .
20
Closed-loop System
The closed-loop system is characterized by the presence of feedback. In this system, the CNC
system send out commands for movement and the result is continuously monitored by the system
through various feedback devices. There are generally two types of feedback to a CNC system --
position feedback and velocity feedback.
PositionFeedback
A closed-loop system, regardless of the type of feedback device, will constantly try to achieve
and maintain a given position by self-correcting. As the slide of the machine tool moves, its
movement is fed back to the CNC system for determining the position of the slide to decide how
much is yet to be traveled and also to decide whether the movement is as per the commanded
rate. If the actual rate is not as per the required rate, the system tries to correct it. In case this is
not possible, the system declares fault and initiates action for disabling the drives and if
necessary, switches off the machine.
21
Vertical borer machine
Closed loop positioning control
22
Open loop postioning control
Velocity feedback
In case no time constraint is put on the system to reach the final programmed position, then the
system may not produce the required path or the surface finish accuracy. Hence, velocity
feedback must be present along with the position feedback whenever CNC system are used for
contouring, in order to produce correct interpolation and also specified acceleration and
deceleration velocities. The tacho generator used for velocity feedback is normally connected to
the motor and it rotates whenever the motor rotates, thus giving an analog output proportional to
the speed of motor. The analog voltage is taken as speed feedback by the servo-controller and
swift action is taken by the controller to maintain the speed of the motor within the required
limits.
23
Open-loop system
The open loop system lacks feedback. In this system, the CNC system send out signals for
movement but does not check whether actual movement is taking place or not. Stepper motors
are used for actual movement and the electronics of these stepper motors is run on digital pulses
from the CNC system. Since system controllers have no access to any real time information
about the system performance, they cannot counteract disturbances appearing during the
operation. They can be utilized in point to point system, where loading torque on the axial motor
is low and almost constant.
Servo-drives
The servo-drive receives signals from the CNC system and transforms it into actual movement
on the machine. The actual rate of movement and direction depend upon the command signal
from CNC system. There are various types of servo-drives, viz., dc drives, ac drives and stepper
motor drives. A servo-drive consists of two parts, namely, the motor and the electronics for
driving the motor.
Operator Control Panel
The operator control panel provides the user interface to facilitate a two-way communication
between the user, CNC system and the machine tool. This consists of two parts:
Video Display Unit (VDU)
Keyboard
Video Display Unit (VDU)
The VDU displays the status of the various parameters of the CNC system and the machine tool.
It displays all current information such as:
Complete information of the block currently being executed
Actual position value, set or actual difference, current feed rate, spindle speed
Active G functions
24
Main program number, subroutine number
Display of all entered data, user programs, user data, machine data, etc.
Alarm messages in plain text
Soft key designations
In addition to a CRT, a few LEDs are generally provided to indicate important operating modes
and status.
Video display units may be of two types:
1. Monochrome or black and white displays
2. Color displays
25
26
SYSTEM 3
SINUMERIK SIEMENS
Z -
X -
Z+
X+
POWER ON
Emergency Stop
Cycl
Fig.2 Typical numerical control configuration of Hinumerik 3100 CNC system
Tape Tape
Power
Suppl
NC PLC1
Logic Unit
MachineControlPanelExpansi
MachineControl
LSM2
LSM1
PLC 2,
LSM-Logic Sub
27
Keyboard
A keyboard is provided for the following purposes:
Editing of part programs, tool data, and machine parameters.
28
SYSTEM 3
SINUMERIK SIEMENS
Z -
X -
Z+
X+
POWER ON
Emergency Stop
Z -
X -
Z+
X+
POWER ON
Emergency Stop
Fig.4 Operator control panel of Hinumerik 3100 systemOperator's and machine panel
29
SYSTEM 3
SINUMERIK SIEMENS
Z -
X -
Z+
X+
POWER ON
Emergency Stop
Cycl
Control elements and indicators of the operator's panel
Program in progressFeed holdPosition not yet reached(Machine in motion)Basic displayTool compensationZero offset
TestPart program
CRT
LED-indicatorFor
Change to actual value display
Change of display
Leaf forwards
Reset changeover
Assignment of keysCancel word
Alter word
Enter wordChange over to customer displayOperator guidance Yes,No
Address Keys/Numerical keyboard
Selection of different pages for viewing.
Selection of operating modes, e.g. manual data input.
Selection of feed rate override and spindles speed override.
Execution of part programs.
Execution of other toll functions.
Machine Control Panel (MCP)
It is the direct interface between operator and the NC system, enabling the operation of the
machine through the CNC system. Fig.5 shows the MCP of Hinumerik 3100 system.
During program execution, the CNC controls the axis motion, spindle function or tool function
on a machine tool, depending upon the part program stored in the memory. Prior to the starting
of the machine process, machine should first be prepared with some specific tasks like,
Establishing a correct reference point
Loading the system memory with the required part program
Loading and checking of tool offsets, zero offsets, etc.
For these tasks, the system must be operated in specific operating mode so that these preparatory
functions can be established.
Control elements of the machine control panel
30
Z -
X -
Z+
X+
POWER ON
Emergency Stop
Cycle
Mode selector Switch
Spindle speed override
Feedrate/rapid traverse override
Rapid traverse activate
Direction keys
SpindleOFF ON
FeedHold/StartCycle start
NC ON
Single
block
Dry Run
Block
DeletRapid TraverseOverride
Modes of operation
Generally, the CNC system can be operated in the following modes:
Manual mode
Manual data input (MDI) mode
Automatic mode
Reference mode
Input mode
Output mode, etc.
Manual mode: In this mode, movement of a machine slide can carried out manually by pressing
the particular jog button (+ or -). The slide (axis) is selected through an axis selector switch or
through individual switches (e.g., X+, X-, Y+, Y-, Z+, Z-, etc.). The feed rate of the slide
movement is prefixed. CNC system allows the axis to be jogged at high feed rate also.
The axis movement can also be achieved manually using a hand wheel interface instead of jog
buttons. In this mode slides can be moved in two ways:
Continuous
Incremental
Continuous mode: In This mode, the slide will move as long as the jog button is pressed.
Incremental mode: Hence the slide will move through a fixed distance, which is selectable.
Normally, system allows jogging of axes in 1, 10, 100, 1000, 10000, increments. Axis movement
is at a prefixed feed rate. It is initiated by pressing the proper jog+ or jog- key and will be limited
to the no of increments selected even if the jog button is continuously pressed. For subsequent
movement the jog button has to be released and once again pressed.
31
Manual Data Input (MDI) Mode
In this mode the following operation can be performed:
Building a new part program
Editing or deleting of part program stored in the system memory
Entering or editing or deleting of:
------ Tool offsets (TO)
------ Zero offsets (ZO)
------ Test data, etc.
Teach-in
Some system allows direct manual input of a program block and execution of the same. The
blocks thus executed can be checked for correctness of dimensions and consequently transferred
into the program memory as part program.
Playback
In setting up modes like jog or incremental, the axis can be traversed either through the direction
keys or via the hand wheel, and the end position can be transferred into the system memory as
command values. But the required feed rates, switching functions and other auxiliary functions
have to be added to the part program in program editing mode.
Thus, teach-in and playback operating method allows a program to created during the first
component prove out.
Automatic Mode (Auto and Single Block)
In this mode the system allows the execution of a part program continuously. The part program
is executed block by block. While one block is being executed, the next block is read by the
system, analyzed and kept ready for execution. Execution of the program can be one block after
another automatically or the system will execute a block, stop the execution of the next block till
it is initiated to do so (by pressing the start button). Selection of part program execution
continuously (Auto) or one block at a time (Single Block) is done through the machine control
32
panel.
Many systems allow blocks (single or multiple) to be retraced in the opposite direction. Block
retrace is allowed only when a cycle stop state is established. Part program execution can resume
and its execution begins with the retraced block. This is useful for tool inspection or in case of
tool breakage. Program start can be effected at any block in the program, through the BLOCK
SEARCH facility.
Reference Mode
Under this mode the machine can be referenced to its home position so that all the
compensations (e.g., pitch error compensation) can be properly applied. Part programs are
generally prepared in absolute mode with respect to machine zero. Many CNC systems make it
compulsory to reference the slides of the machine to their home positions before a program is
executed while others make it optional.
Input Mode and Output Mode (I/O Mode)
In this mode, the part programs, machine setup data, tool offsets, etc. can be loaded/unloaded
into/from the memory of the system from external devices like programming units, magnetic
cassettes or floppy discs, etc. During data input, some systems check for simple errors (like
parity, tape format, block length, unknown characters, program already present in the memory,
etc.). Transfer of data is done through a RS232C or RS485C port.
Other Peripherals
These include sensor interface, provision for communication equipment, programming units,
printer, tape reader/puncher interface, etc.
Fig.6 gives an overview of the system with few peripheral devices.
33
Programmable Logic Controller (PLC)
A PLC matches the NC to the machine. PLCs were basically introduced as replacement for hard wired relay control panels. They were developed to be reprogrammed without hardware changes when requirements were altered and thus are reusable. PLCs are now available with increased functions, more memory and large input/output capabilities. Fig.7 gives the generalized PLC block diagram. In the CPU, all the decisions are made relative to controlling a machine or a process. The CPU receives input data, performs logical decisions based upon stored programs and drives the outputs. Connections to a computer for hierarchical control are done via the CPU. The I/O structure of the PLCs is one of their major strengths. The inputs can be push buttons, limit switches, relay contacts, analog sensor, selector switches, proximity switches, float switches, etc. The outputs can be motor starters, solenoid valves, position valves, relay coils, indicator lights, LED displays, etc.The field devices are typically selected, supplied and installed by the machine tool builder or the end user. The voltage level of the field devices thus normally determines the type of I/O. So, power to actuate these devices must also be supplied external to the PLC. The PLC power supply is designated and rated only to operate the internal portions of the I/O structures, and not the field devices. A wide variety of voltages, current capacities and types of I/O modules are available.
Fig.6 System with peripheral devices
34
Programming Units
Tape Reader
PrintersTape Puncher
Fig.7 Generalized PLC block diagram
INTRODUCTION AND GENERAL WORKING OF CNC MACHINE
CNC machine tools have been widely accepted as time proven manufacturing technique all over
the globe. In India too, CNC machine have picked up momentum and Indian Industries are going
for more and more CNC machine.
Numerical control turns machine tools into a flexible production unit with a multitude of possible
application. Although at first they were mainly used for manufacture of geometrically
complicated parts, numerical control were later used for added enhanced efficiency in the
medium batch production of turned and milled parts. The next step is the introduction of
numerical controls in all sections of productions. The aim in all the cases is to combine high
productivity with flexible possibilities of NC technology.
Other machine processes are being added to turning and milling which may be already described
as classical NC applications. Conventional methods are being replaced by NC for sheet metal-
working processes of punching, nibbling and cutting. Productivity increases multifold using
numerical control with grinder, gear hobbers and spark erosion.
35
Processor
Logic memor
y
Storage
memorPowerSupply
Input
Outp
Power
Suppl
Programmer
Field Devic
es
ELECTRONIC REVOLUTION AND COMPUTER GROWTH
Modern day computerized numerical control system (CNC) is the result of advancement and sky
rocketing microelectronics with every day breaks its own record and grow by leaps and bounds.
Initially NC machines had as many as 280 printed circuit board (PCB) and quite
extensive wiring which consequently made maintenance very difficult and reliability was poor.
First generation NC system had transistor elements.
With advent of integrated circuits only 40 PCBs were used with reduced wiring for the system.
Further integration called ‘Medium Scale Integration’ (MSI) replaced nearly 10 ICs and system
involved only 5 to 10 PCBs. With arrival of what is known as age of “Microprocessors and Very
Large Scale Integration (VLSI) technique” ,modern day CNC machines use only 2 to 4 PCBs.
The trend with microprocessors helps in lowering cost of jobs and increment in reliability to
appreciable label.
Flexibility as inherent feature of minicomputer and microcomputer gave new technology
–“Software Oriented System”.
Unlike rigid hardwire system which had been used hitherto, this new system comes to market as
COMPUTER NUMERICAL CONTROL (CNC) system.
With this latest technology, hardware cost of CNC system lowered considerably whereas
flexibility increased multifold due to software capabilities
WORKING PRINCIPLE OF NC MACHINES
Electronic industries association defines numerical control as “A system in which action is
controlled by direct insertion of numerical data. The system must automatically interpret at least
some potion of data.”
In simple word numerical control means control by numbers. In NC machine tools the main
function is to control the displacement and positioning of slides, spindle, speed, feed rate,
selection of tool and many other auxiliary functions. NC directs the machine tool to achieve all
36
these function in a very controlled and systematic manner .The major elements that comprises
NC machine tools are :-
Control system – CNC
The machine tool
Servo drive units
Feedback devices
Operator control
Electrical cabinet
37
Schematic diagram representation of NC machine tool
In CNC system, tape instructions are read by tape reader. These instructions undergo
electronic processing and system gives output in the form of electrical signal to servo drive of the
machine tool to determine the length of movement and feed rates. System also directs commands
to various relays, solenoids etc to initiate operation of the machine tools such as spindle motor
starting and stopping , coolant supply , auto tool change and other miscellaneous functions.
Once the machine tool has commenced its operation and operative element and moving , it
become necessary to ensure that required lengths of movements have taken place or a particular
function has been accomplished. This is done by feedback devices. Position feedback devices
like linear scales, encoder, resolvers, inductosyn feedback status of actual position of slides to
control system. A velocity feedback transducer known as ‘Tachogenertor’ is used for velocity
control as warranted during contouring operation . Feed back of auto tool change function etc are
taken from proximity sensors or limit switches. Thus all operations of machine are monitored
continuously with appropriate feedback devices. In case of failure or adverse feedback received
by system , machine stops and system displays ‘Fault message’ in clear English text.
TYPES OF NC
Based on feedback, NC system can be broadly classified in two types as ‘Open loop’ and ‘closed
loop’. The open loop system has no feedback, whereas closed loop system utilizes feedback
transducers which continuously monitor the position of slides. This enable machine to achieve a
very high degree of accuracy in slide displacement
38
Figure above showing open loop system
From metal removal point of view, the classification can be made as point to point, straight cut
and contouring system. In point to point system, the machine performs machining operations at
specific positions and does not affect work piece while moving from one point to another. An
example of this type is NC drilling machine.
Figure showing closed loop system.
Straight cut or straight line system provides movement at controlled feed rate in one axis
direction at one time. The examples of this are face milling, pocket milling etc.
39
Figure demonstrating NC system’s some of different paths and cut.
The continuous path control system calls for co-ordinate movement of the tool and work piece
along different axes. This enables machining of complex profiles, contours and curved surfaces.
MACHINE TOOL REQUIREMENT
In a NC machine certain design features are desirable on machine tool. Simultaneous movement
of 2 or more axes and high removal capability of NC machines demand high dynamic stiffness in
drives and also stiff structural elements. The structure of NC machines should be very rigid to
withstand heavy cuts and it must be maintained for long time to obtain high accuracies. High
positioning accuracy needed in NC machine make it essential to have backlash free screw and
40
nut and slides with a very low friction of co-efficient. This is achieved by ground re circulating
ball screw and nut, tachoway bearings, hardened and ground guideways and friction reducing
linears such as Turcite and PTFE.
Electronic spindle drives on NC machines facilitate step less speed over a wide range of RPMs.
All axes are driven by powerful DC Servo drives controlled by PWM or SCR controllers through
preloaded ball screw and nuts.
Automatic tool changer, centralized lubrication system, index table or fourth axis is common
features on NC machines.
ACCURACIES
CNC machines ensure better and consistent accuracies on job compared to conventional
machines. Positioning accuracies to be extent of +10 or -10 microns and repeatability of +5 or-5
microns can be achieved in NC machines depending on the elements used.
Parts Suitable For CNC Machines:-
To utilize the CNC machine effectively and economically the suitable pats selection is very
important, the following guidelines should be observed:
High number of operation per component.
Complexity of operation.
Size of batches medium.
Repetitions of batches are often
Labor cost of component is high.
Requires substantial tooling.
Requires 100% inspection.
Setup and inspection time is high.
Ration of cutting time to non-cutting time is high.
Varieties of components produced are more.
Skilled required by operator is high.
GUIDELINES FOR EFFECTIVE UTILIZATION OF CNC MACHINES
When a company or organization first decides to buy CNC machine, Orientation at all material
level is required to get maximum return out of huge investment made. It is not sufficient that
management understands the benefit of machine and decided to buy one, instead a deep insight
41
of CNC technology is must and will quite obviously helps in buying suitable one and keeping its
idle time minimum. Following Are few point for effective utilization of CNC machine.
It is vital that time taken for machine to become productive is as short as possible. The
key to good CNC machine lies in prior planning, which should start the day management
decides to buy CNC machine.
Programmers, maintenance people and operators should be selected and trained before
CNC arrives.
Foundation and electric supply requirement should ready before machine arrives.
For small organizations where experts and maintenance staffs are generally not available,
proper support from supplier should be ensured.
It should be ensured that suppliers of NC machines provide with all relevant technical
documents for the machine.
While NC can be out product its manual counterpart 3 to 4 times, you can just easily
loose this multi machine capacity during breakdown. To keep downtime minimum,
sufficient quantities of spare are recommended by suppliers of machine should be
maintain. Also it is always make sure that supplier of machine stocks critical parts and
accessories.
Sufficient thought should be given to tooling, accessories and all other peripherals that
surround CNC.
CNC machines must be placed at convenient places in the shop considering work flow
and material handling. We can’t keep NC machine waiting while we are searching for
tools or waiting for availability of raw materials.
Future expandability should always be in decision making while buying a CNC machine.
Clean and dust free environment should be ensured in shop in order to minimize
breakdown efficient working and consequently longer life of CNC machine.
VARIOUS FUNCTIOS OF CNC
42
1) Axes position and velocity control
2) Spindle speed
3) Miscellaneous functions.
1.1 Various functional codes
1) Preparatory function: these are commands which prepare the machine for different modes of
movements like position countering, thread cutting etc.
2) Dimensional Data: Movement of machine tool slides in one or more axes is determined by
dimensional data entered in the program.
3) Miscellaneous Function: Some of the important miscellaneous function which worth to be
considered here are coolant on or off, Spindle CW or CCW, program stop etc.
4) Speed Function(S): this function pertains to speed of spindle.
5) Feed Function (F): It pertains to feed rates of the slides.
6) Tool Function (T): this function pertains to the selection of required tool for the particular
operation.
CODES USED IN CNC PROGRAMS
43
G-code is the name of any word in a CNC program that begins with the letter G and generally is
a code telling machine tool what type of action to perform. Such as: Rapid Move etc.
Controlled feed move in straight line or are series of controlled feed moves that would result in
whole being bored, a work piece cut (routed) to a specific dimension, or a decorative profile
shape added to edge of work piece.
There are other codes; the type codes can be thought of registers in the computer
X absolute position
Y absolute position
Z absolute position
A position (rotary around X)
B position (rotary around Y)
C position (rotary around Z)
U Relative parallel axis to X
V Relative parallel axis to Y
W Relative parallel axis to Z
M code (another “action” register or machine code (*))
(Otherwise refers to as miscellaneous function)
F feed rate
S spindle speed
44
N line number
R arc radius
P dwell time
T tool selection
I arc data X axis
J arc data Y axis
J arc data Z axis
D cutter diameter
H tool length offset.
(*)M codes control the overall machine, causing it to stop, start, turn on coolant, etc., whereas
other code pertain to the path traversed by cutting tool may use same code to perform different
functions; even machines that use the same CNC control.
COMMON FANUE G CODES
Code Description
G00 Rapid positioning
G01 Linear interpolation
G02 CW circular interpolation
45
G03 CCW circular interpolation
G04 Dwell
G10/G11 Data writing/Data write cancel
G17 X-Y plane selection
G18 X-Z plane selection
G19 Y-Z plane selection
G20 Programming in inches
G21 Programming in mm
G28 Return to home position
G31 Skip function
G33 Constant pitch threading
G34 Variable pitch threading
G40 Tool radius compensation off
G41 Tool radius compensation left
G42 Tool radius compensation right
G81 Simple drilling cycle
46
G82 Drilling cycle with dwell
G83 Peck drilling cycle
G84 Tapping cycle
G90 Absolute programming
G91 Incremental programming
G94/G95 Inch per minute/Inch per revolution feed
G96/G97 Constant cutting speed/Constant rotation speed
BASIC ISO CNC CODES
M00 Program stop
M01 Optional stop
M02 Program stop
M03 Spindle CW
M04 Spindle CCW
M05 Spindle stop
M08 Coolant/Lubricant on
M09 Coolant/Lubricant off
M30 Program end
M98 Subprogram call
47
M99 Subprogram end
G96 Constant surface speed
G97 Constant spindle speed
G50 Maximum spindle speed
G95 Feed mm per revolution
G94 Feed mm/min
G00 Rapid movement
G01 Linear interpolation
F Feed
S Spindle speed.
48
CNC MACHINES
There are different CNC machines in haridwar unit, which serve some special purposes.
CNC OXY-ACETYLENE FLAME CUTTING
This machine is in bat-0 & is used for M.S sheet. This machine can cut up to 300mm thick sheet.
It has four burners, which can work simultaneously.
• Control system : ESAB German (NCE-510)
• Axes: There are two axes in machine X and Y axis. In X axis tool can move up to 7 meters and
in Y axis tool can move up to 3.5 meters
• Drive: D.C.
• Feedback rotary encoders
CNC CROPPING LINE
There are two cropping line CNC machine in bay-5. These are used to cut CRZO sheets for
construction of core of transformer.
First machine has been made by George German with control system from Siemens 810D. This
machine mainly consists of two tools: punch and swing shear for cutting lamination as required
by program.
The other machine has been manufactured by Sooner Company. It has two punches. One fixed
shear and one movable shear, which can shear straight as well as 45 degree. It consists of one tip
cut and one V cut also.
(a) Control System 810D
(b) Axis: One axis
(c) Drive Way
49
(d) Feedback linear Scale
ASQUITH CNC BOGIE MACHINE CENTRE
This machine is in boogie shop. it is used for all operation in boogie manufacturing like
milling, drilling and boring. All the operation can be done in the single machine.
CONTROL SYSTEM : GE FANUC 15M
Axis: It works three axis X, Y and Z axis. It can travel up to 8000 mm in X-axis. 4000 mm in
Y-axis and 800 mm in Z-axis.
Feedback linear scale.
It has auto tool changer, which can change tools automatically. According to program its spindle
diameter is 180 mm and 40 KW power is required to operate the spindle.
(A)HMT SB CNC LATHE
This is used for turning the job.
1) Control system: Sinumeric 3T
2) Axis: It works in two axis X and Z axis. Tool can traverse up to 1000 mm in X-axis and 300
mm in Z-axis.
3) Drive D.C.
4) Feedback: Rotary Encoder.
(B) HMT CNC VERTICLE MILLING MACHINE
This machine is used for milling purpose.
1) Control system : Sinumeric 800 M
2) Axis: it works in three axes X, Y and Z axis. Tool can travel 1200 mm in X-axis, 600 mm in
50
Y-axis, 400 mm in Z-axis.
3) Drive D.C.
4) Feedback rotary encoder.
(C) COOPER CNC VERTICLE BORING MACHINE
This machine is used for boring purpose.
1) Control System: Cruceder
2) Axis: Tool can move in two axes: X and Z- axis. It can move 600 mm in X- axis and 400 mm
in Z-axis.
3) Drive D.C.
4) Feedback : Linear Sale
ADVANTAGES OF CNC MACHINES
1. Productivity
Since cutting tool is brought to its machining position much more efficiently than it was done
manually by the machine operator, NC machine is spending much more time per shift cutting
than in past. Conventional machines very seldom remove metal for more than 15% of total
available time under normal batch production conditions. Whereas CNC machine tools should be
capable of removing metal for between 50% and 75% of available time. When working on
medium batch production, CNC machining has around 4 to 1 productivity advantage over
conventional machine. The actual advantage may vary from batch to batch depending upon the
complexity of components to be produced and is normally proportional to the number of
conventional operation required to produce the components.
51
2. Flexibility in design and production
Machine can switch over to different job as set up times are low and sudden changes in sales
requirement are much more easily catered for. This enables the formulation of more aggressive
marketing plans. The use of CNC machines also give designers freedom to design components
which, by conventional means, are often impossible to produce. Change of design can also be
easily incorporated as it means change of tape.
3. Inspection
High position accuracies and repeatability are inherent features of CNC machines and reduce
inspection time considerably.
Normally a 100% inspection of the first component produced by a new tape is all that is
necessary to prove the tape and tooling. Subsequently it is required to have only sample
inspection. In process gauging and inspection is also provided on modern CNC machines.
4. Floor space
One CNC machine can replace five to six conventional machines. Thus manufacturing activities
of a company can be expanded without increasing the floor area proportionately.
5. Inventory
By using CNC machine, procurement sizes and batch sizes can be reduced because of shorter lead time’s .This results in substantial saving. Lead time is time taken to progress a batch of component through a batch of production shop and is proportional to number of operation required by conventional methods. For example a component which requires 112 set ups by conventional methods may requires only 1 or 2 set ups in CNC machining center reducing total product flow times.
6. Material Handling
Handling of component from machine to machine which is necessary on conventional machine is
significantly reduced on CNC machine, as all the operations are performed on one machine. This
obviously reduces labor cost.
7. Tooling
52
This ability to complete machine part in a single setup means that fewer and simpler fixtures are
required, which in turns requires less storage space and maintenance. The simpler a fixture is, the
less expensive is to manufacture it.
8. Operator’s Skill
Dependence on skilled labor can be dispensed with. The accuracy of part produced with CNC
machines machine depend upon accuracy and ability of machine and tape – and not on individual
operator.
9. Scrap and Rework
Drastic reduction in scrap is achieved because of the inherent accuracy and repeatability of CNC
machine.
10. Costing
Time required to produce a component is a function of machining cycle of CNC machines and is
not influenced by operator’s efficiency or variation in labor’s rate, a great stability of prices can
often be achieved throughout the life cycle of the respective product. Also cost accounting
becomes very precise.
11. Better Management Information and Control
With various advantages of CNC machines, decisions effecting unit cost, delivery and quality are
firmly placed in the hands of management and not of the machine operator.
Programmable Logic Controller (PLC)
A programmer logic controller(PLC) is a digital computer used for automation of industrial
process such as control of machinery on factory assembly line. A plc is an example of real time
system .since output result must be produced to input condition.A PLC matches the NC to the
machine. PLCs were basically introduced as replacement for hard wired relay control panels.
They were developed to be reprogrammed without hardware changes when requirements were
53
altered and thus are reusable. PLCs are now available with increased functions, more memory
and large input/output capabilities.
In the CPU, all the decisions are made relative to controlling a machine or a process. The CPU
receives input data, performs logical decisions based upon stored programs and drives the
outputs. Connections to a computer for hierarchical control are done via the CPU.
Generalized PLC block diagram
The I/O structure of the PLCs is one of their major strengths. The inputs can be push buttons,
limit switches, relay contacts, analog sensor, selector switches, proximity switches, float
switches, etc. The outputs can be motor starters, solenoid valves, position valves, relay coils,
indicator lights, LED displays, etc.
54
Processor
Logic memor
y
Storage
memoryPower
Supply
Inputs
Outputs
Power
Supply
Programmer
Field Devic
es
The field devices are typically selected, supplied and installed by the machine tool builder or the
end user. The voltage level of the field devices thus normally determines the type of I/O. So,
power to actuate these devices must also be supplied external to the PLC. The PLC power supply
is designated and rated only to operate the
internal portions of the I/O structures, and not the field devices. A wide variety of voltages,
current capacities and types of I/O modules are available.
Basic PLC Components:-
Processor or central processing unit
Rack or mountain
Input assembly
Output assembly
Power Supply
Programming unit, Device or PC Software
INPUT RELAYs-(contacts)These are connected to the outside world. They physically
exist and receive signals from switches, sensors, etc. Typically they are not relays but
rather they are transistors.
INTERNAL UTILITY RELAYS-(contacts) These do not receive signals from the outside
world nor do they physically exist. They are simulated relays and are what enables a PLC
to eliminate external relays. There are also some special relays that are dedicated to
performing only one task. Some are always on while some are always off. Some are on
55
only once during power-on and are typically used for initializing data that was stored.
COUNTERS-These again do not physically exist. They are simulated counters and they
can be programmed to count pulses. Typically these counters can count up, down or both
up and down. Since they are simulated they are limited in their counting speed. Some
manufacturers also include high-speed counters that are hardware based. We can think
of these as physically existing. Most times these counters can count up, down or up and
down.
TIMERS-These also do not physically exist. They come in many varieties and
increments. The most common type is an on-delay type. Others include off-delay and
both retentive and non-retentive types. Increments vary from 1ms through 1s.
OUTPUT RELAYS-(coils)These are connected to the outside world. They physically exist
and send on/off signals to solenoids, lights, etc. They can be transistors, relays, or triacs
depending upon the model chosen.
DATA STORAGE-Typically there are registers assigned to simply store data. They are
usually used as temporary storage for math or data manipulation. They can also typically
be used to store data when power is removed from the PLC. Upon power-up they will still
have the same contents as before power was removed. Very convenient and necessary!!
56
Overview of twidosoft PLC
57
58
PLC OPERATION
A PLC works by continually scanning a program. We can think of this scan cycle as consisting
of 3 important steps. There are typically more than 3 but we can focus on the important parts and
not worry about the others. Typically the others are checking the system and updating the current
internal counter and timer values.
Step 1-CHECK INPUT STATUS-First the PLC takes a look at each input to determine if it is
on or off. In other words, is the sensor connected to the first input on? How about the second
input? How about the third... It records this data into its memory to be used during the next step.
59
Step 2-EXECUTE PROGRAM-Next the PLC executes your program one instruction at a time.
Maybe your program said that if the first input was on then it should turn on the first output.
Since it already knows which inputs are on/off from the previous step it will be able to decide
whether the first output should be turned on based on the state of the first input. It will store the
execution results for use later during the next step.
Step 3-UPDATE OUTPUT STATUS-Finally the PLC updates the status of the outputs. It
updates the outputs based on which inputs were on during the first step and the results of
executing your program during the second step. Based on the example in step 2 it would now
turn on the first output because the first input was on and your program said to turn on the first
output when this condition is true. After the third step the PLC goes back to step one and repeats
the steps continuously. One scan time is defined as the time it takes to execute the 3 steps listed
above.
RELAYS
Now that we understand how the PLC processes inputs, outputs, and the actual program we are
almost ready to start writing a program. But first let’s see how a relay actually works. After all,
the main purpose of a plc is to replace "real-world" relays.
We can think of a relay as an electromagnetic switch. Apply a voltage to the coil and a magnetic
field is generated. This magnetic field sucks the contacts of the relay in, causing them to make a
connection. These contacts can be considered to be a switch. They allow current to flow
between2 points thereby closing the circuit.
A typical industrial relay
60
61
RELAY OUTPUT
Some common forms of a load are a solenoid, lamp, motor, etc. These "loads" come in all sizes.
Electrical sizes, that is. Always check the specifications of your load before connecting
it to the plc output. You always want to make sure that the maximum current it will consume is
within the specifications of the plc output. If it is not within the specifications (i.e. draws too
much current) it will probably damage the output. When in doubt, double check with the
manufacturer to see if it can be connected without potential damage . Some types of loads are
very deceiving. These deceiving loads are called "inductive loads" .These have a tendency to
deliver a "back current" when they turn on. This back current is like a voltage spike coming
through the system.
The relay is internal to the plc. Its circuit diagram typically looks like
that shown above. When ourladder diagram tells the output to turn on, the plc will
internally apply a voltage to the relay coil.This voltage will allow the proper contact to close.
62
When the contact closes, an external current isallowed to flow through our external circuit. When
the ladder diagram tells the plc to turn off theoutput, it will simply remove the voltage from the
internal circuit thereby enabling the outputcontact to release. Our load will than have an open
circuit and will therefore be off.
TRANSISTOR OUTPUT
The next type of output we should learn about is our transistor type outputs. It is important to
note that a transistor can only switch a dc current. For this reason it cannot be used with an AC
voltage.
We can think of a transistor as a solid-state switch. Or more simply put, an electrical switch.
Asmall current applied to the transistors "base" (i.e. input) lets us switch a much larger
currentthrough its output. The plc applies a small current to the transistor base and the transistor
output"closes". When it's closed, the device connected to the plc output will be turned on. The
above isa very simple explanation of a transistor. There are, of course, more details involved but
we don'tneed to get too deep.
We should also keep in mind that as we saw before with the input circuits, there are generally
more than one type of transistor available. Typically a plc will have either NPN or PNP
typeoutputs. The "physical" type of transistor used also varies from manufacturer to
manufacturer.
Some of the common types available are BJT and MOSFET. A BJT type (Bipolar Junction
Transistor) often has less switching capacity (i.e. it can switch less current) than a MOS-FET
(Metal Oxide Semiconductor- Field Effect Transistor) type. The BJT also has a slightly faster
switching time. Once again, please check the output specifications of the particular plc you are
going to use.
63
Shown below is how we typically connect our output device to the transistor output. Please
note that this is an NPN type transistor. If it were a PNP type, the common terminal would
most likely be connected to V+ and V- would connect to one end of our load. Note that since this
is a DC type output we must always observe proper polarity for the output. One end of the load is
connected directly to V+ as shown above.
Let's take a moment and see what happens inside the output circuit. Shown below is a typical
output circuit diagram for an NPN type output.
64
Notice that as we saw with the transistor type inputs, there is a photocoupler isolating the "real
world" from the internal circuit. When the ladder diagram calls for it, the internal circuit turns
onthe photocoupler by applying a small voltage to the LED side of the photocoupler. This makes
theLED emit light and the receiving part of the photocoupler will see it and allow current to
flow. Thissmall current will turn on the base of the output transistor connected to output 0500.
Therefore,whatever is connected between COM and 0500 will turn on. When the ladder tells
0500 to turnoff, the LED will stop emitting light and hence the output transistor connected
between 0500 andCOM will turn off.
65
One other important thing to note is that a transistor typically cannot
switch as large a load as a relay. Check the manufacturers specifications to find the largest load it
can safely switch. If the load current you need to switch exceeds the specification of the output,
you can connect the plcoutput to an external relay. Then connect the relay to the large load. You
may be thinking, "whynot just use a relay in the first place"? The answer is because a relay is not
always the correct choice for every output. A transistor gives you the opportunity to use external
relays when and only when necessary.
66
PLC PROGRAMMING
The principle of operation of a PLC is determined essentially by the PLC program memory,
processor, inputs and outputs.
The program that determines PLC operation is stored in the internal PLC program memory. The
PLC operates cyclically, i.e. whe at the beginning of the program. At the beginning of each
cycle, the processor examines the signal status at all inputs as well as the external timers and
counters and are stored in a process image input (PII). During subsequent program scanning, the
processor the accesses this process image.
To execute the program, the processor fetches one statement after another from the
programming memory and executes it. The results are constantly stored in the process image
output (PIO) during the cycle. At the end of a scanning cycle, i.e. program completion, the
processor transfers the contents of the process image output to the output modules and to the
external timers and counters. The processor then begins a new program scan.
Configuring Hardware and Software
Configuring Twido programmable controllers consists of selecting options for thehardware and
software resources of the controller. These resources can beconfigured at any time while creating
a program.
Hardware resources are: the controller itself, hardware that connects to the
controller, and the connections to the hardware.
A PLC matches the NC to the machine. PLCs were basically introduced as replacement for hard
wired relay control panels. They were developed to be reprogrammed without hardware changes
when requirements were altered and thus are reusable. PLCs are now available with increased
functions, more memory and large input/output capabilities. Fig.7 gives the generalized PLC
block diagram.
In the CPU, all the decisions are made relative to controlling a machine or a process. The CPU
67
receives input data, performs logical decisions based upon stored programs and drives the
outputs. Connections to a computer for hierarchical control are done via the CPU.
The I/O structure of the PLCs is one of their major strengths. The inputs can be push buttons,
limit switches, relay contacts, analog sensor, selector switches, proximity switches, float
switches, etc. The outputs can be motor starters, solenoid valves, position valves, relay coils,
indicator lights, LED displays, etc.
The field devices are typically selected, supplied and installed by the machine tool builder or the
end user. The voltage level of the field devices thus normally determines the type of I/O. So,
power to actuate these devices must also be supplied external to the PLC. The PLC power supply
is designated and rated only to operate the
internal portions of the I/O structures, and not the field devices. A wide variety of voltages,
current capacities and types of I/O modules are available.
INTERFACING
Interconnecting the individual elements of both the machine and the CNC system using cables
and connectors is called interfacing.
Extreme care should be taken during interfacing. Proper grounding in electrical installation is
most essential. This reduces the effects of interference and guards against electronic shock to
personnel. It is also essential to properly protect the electronic equipment.
Cable wires of sufficiently large cross-sectional area must be used. Even though proper
grounding reduces the effect of electrical interference, signal cable requires additional protection.
This is generally achieved by using shielded cables. All the cable shields must be grounded at
control only, leaving other end free. Other noise reduction techniques include using suppression
devices, proper cable separation, ferrous metal wire ways, etc. Electrical enclosures should be
designed to provide proper ambient conditions for the controller.
MONITORING
In addition to the care taken by the machine tool builder during design and interfacing, basic
control also includes constantly active monitoring functions. This is in order to identify faults in
68
the NC, the interface control and the machine at an large stage to prevent damages occurring to
the work piece, tool or machine. If a fault occurs, first the machining sequence is interrupted, the
drives are stopped, the cause of the fault is stored and then displayed as an alarm. At the same
time, the PLC is informed that an NC alarm exits. In Hinumerik CNC system, for example, the
following can be monitored:
Read-in
Format
Measuring circuit cables
Position encoders and drives
Contour
Spindle speed
Enable signals
Voltage
Temperature
Microprocessors
Data transfer between operator control panel and logic unit
Transfer between NC and PLC
Change of status of buffer battery
System program memory
User program memory
Serial interfaces
DIAGNOSTICS
The control will generally be provided with test assistance for service purposes in order to display some status on the CRT such as:
Interface signals between NC and PLC as well as between PLC and machine Flags of the PLC Timers of the PLC Counters of the PLC Input/output of the PLC
For the output signals, it is also possible to set and generate signal combinations for test purposes
69
in order to observe how the machine react to a changed signal. This simplifies trouble shooting considerably.
MACHINE DATA
Generally, a CNC system is designed as a general-purpose control unit, which has to be matched with the particular machine to which the system is interfaced. The CNC is interfaced to the machine by means of data, which is machine specific. The NC and PLC machine data can be entered and changed by means of external equipment or manually by the keyboard. These data are fixed and entered during commissioning of the machine and generally left unaltered during machine operations. Machine data entered is usually relevant to the axis travel limits, feed rates, rapid traverse speeds and spindle speeds, position control multiplication factor, Kv factor, acceleration, drift compensation, adjustment of reference point, backlash compensation, pitch error compensation, etc. Also the optional features of the control system are made available to the machine tool builder by enabling some of the bits of machine data.
COMPENSATIONS FOR MACHINE ACCURACY
Machine accuracy is the accuracy of the movement of the carriage, and is influenced by:
(a) Geometric accuracy in the alignment of the slide ways(b) Deflection of the bed due to load(c) Temperature gradients on the machine(d) Accuracy of the screw thread of any drive screw and the amount of backlash (lost motion)(e) Amount of twist (wind up) of the shaft which will influence the measurement of rotary
transducers
The CNC systems offer compensation for the various machines' accuracy. These are detailed below:
Lead Screw Pitch Error Compensation
To compensate for movements of the machine slide due to in accuracy of the pitch along the length of the ball screw, pitch error compensation is required. To begin with, the pitch error curve for the entire length of the screw is built up by physical measurement with the aid of an external device (like laser). Then the required compensation at predetermined points is fed in to the system. Whenever a slide is moved, these compensation are automatically added up by the CNC system (Fig.8)
Backlash Compensation
70
Whenever a slide is reversed, there is some lost motion due to backlash between nut and the
screw; a compensation is provided by the CNC system for the motion lost due to reversal, i.e.
extra movement is added into the actual movement whenever reversal takes place. This extra
movement is equal to backlash between the screw and the nut. This has to be measured in
advance and fed to the system. This value keeps on varying due to wear of the ball screws, hence
the compensation value has to be updated regularly from time to time
Inaccuracy due to sag in the slide can be compensated by the system. Compensations required
along the length of the slide have to be physically measured and fed to the system. The system
automatically adds up the compensation to the movement of the slide.
Tool Nose Compensation
Tool nose compensation normally used on tool for turning centers. While machining
chamfers, angles or turning curves, it is necessary to make allowance for the tool tip radius; this
radius is known as radius compensation. If the allowance is nit made, the edges of the tool tip
radius would be positioned at the programmed X and Z coordinates, and the tool will follow the
path AB and the taper produced will be incorrect. In order to obtain correct taper, tool position
has to be adjusted.
It is essential that the radius at the tip of the tool is fed to the system to make an automatic
adjustment on the position and movement of the tool to get the correct taper on the work.
Typical error curve
Cutter Diameter Compensation
71
Reference point
Positive end limit
Pitch error (um)
To negative end limit
The diameter of the used tool may be different from the actual value because of regrinding of the
tool or due to non-availability of the assumed tool. It is possible to adjust the relative position of
cutter size and this adjustment is known as cutter diameter compensation.
Tool Offset
A part program is generated keeping in mind a tool of a particular length, shape and thickness as
a reference tool. But during the actual mounting of tools on the machine, different tools of
varying lengths, thickness and shapes may be available. A correction for dimension of the tools
and movements of the work piece has to be incorporated to give the exact machining of the
component. This is known as tool offset. This is the difference in the positions of the centre line
of the tool holder for different tools and the reference tool. When a number of tools are used, it is
necessary to determine the tool offset of each tool and store it in the memory of the control unit.
Fig.11 explains the function of the tool offset.
Normally, it is found that the size of the work piece (diameter or length) is not within tolerance
due to wear of the tool; it is the possible to edit the value of offsets to obtain the correct size.
This is known as tool wear compensation.
Fig.11 Tool offsets
72
Reference tool
Tool no.1
XR=Setting distance for
reference tool
X offset for tool
no.2Z offset for tool
no.2
Tool no.2
ZR
XR
X0
HARDWARE RESOURCES
Types of hardware resources:
Base and Remote controllers
Expansion I/O
AS-Interface V2 bus interface module and its slave devices
Options
The principle of operation of a PLC is determined essentially by the PLC program memory,
processor, inputs and outputs.
The program that determines PLC operation is stored in the internal PLC program memory. The
PLC operates cyclically, i.e. when a complete program has been scanned, it starts again at the
beginning of the program. At the beginning of each cycle, the processor examines the signal
status at all inputs as well as the external timers and counters and are stored in a process image
input (PII). During subsequent program scanning, the processor the accesses this process image.
To execute the program, the processor fetches one statement after another from the . program
completion, the processor transfers the contents of the process image output to the output
modules and to the external timers and counters. The processor then begins a new program scan.
STEP 5 programming language is used for writing user programs for SIMATIC S5
programmable controllers. The program can be written and entered into the programmable
controller as in:
Statement list (STL), Fig.12 (a)
Control system flowchart (CSF), Fig.12 (b)
Ladder diagram (LAD), Fig.12 (c)
73
O
Fig.12 Programmable controller
The statement list describes the automation task by means of mnemonic function designations.The control system flowchart is a graphic representation of the automation task.The ladder diagram uses relay ladder logic symbols to represent the automation task.The statement is the smallest STEP 5 program component. It consists of the following:
Operation, i.e. what is to be done?
E.g. A = AND operation (series connection)O= OR operation (parallel connection)
74
AND
OR
I 2.3
I 4.1
I 3.2 Q 1.6
(b) Control system flow chart CSF
Statement list STL
A I 2.3A I 4.1O I 3.2= Q 1.6
A I 2.3
A I 2.3
I 2.
Statement
Operand
Operation
Operand identifier
Parameter
(c) Ladder diagram LAD
I 2.3
I 4.1
I 3.2
S= SET operation (actuation)
Operand, i.e. what is to be done with?
E.g. I 4.5, i.e. with the signal of input 4.5
The operand consists of:
Operand identifier (I = input, Q = output, F = flag, etc.) Parameter, i.e. the number of operand identifiers addressed by the statement. For inputs, outputs and flags (internal relay equivalents), the parameter consists of the byte and bit addresses, and for timers and counter, byte address only.
The statement may include absolute operands, e.g. I 5.1, or symbolic operand, e.g. I LS1. Programming is considerably simplified in the later case as the actual plant designation is directly used to describe the device connected to the input or output.Typically, a statement takes up one word (two bytes) in the program memory.
75
Fig. Ladder Logic Element OR
Fig.Ladder Logic Element AND
76
STRUCTURED PROGRAMMING
The user program can be made more manageable and straightforward if it is broken down into
relative sections. Various software block types are available for constructing the user program.
Program blocks (PB ) contain the user program broken down into technologically or functionally
related sections (e.g. program block for transportation, monitoring, etc.). Further blocks, such as
program blocks or function blocks can be called from a PB.
Organization blocks (OB ) contain block calls determining the sequence in which the PBs are to
be processed. It is therefore possible to call PBs conditionally (depending on certain conditions).
In addition, special OBs can be programmed by the user to react to interruptions during cyclic
programming processing. Such an interrupt can be triggered by a monitoring function if one or
several monitored events occur.
Function block (FB ) is block with programs for recurrent and usually complex function. In
addition to the basic operations, the user has a extended operation at his disposal for developing
function blocks. The program in a function block is usually not written with absolute operands
(e.g. I 1.5) but with symbolic operands. This enables a function block to be used several times
over with different absolute operands.
For even more complex functions, standard function blocks are available from a program library.
Such FBs are available, e.g. for individual controls, sequence controls, messages, arithmetic
operations, two step control loops, operator communications, listing, etc. These standard FBs for
complex functions can be linked it the user program just like user written FBs simply by means
of a call along with the relevant parameters.
The Sequence block (SB ) contain the step enabling conditions, monitoring times and conditions
for the current step in sequence cascade. Sequence blocks are employed, for example, to organise
the sequence cascade in communication with a standard FB.
The data blocks (DB ) contain all fixed or variable data of the user program.
CYCLIC PROGRAM PROCESSING
The blocks of the user program are executed in the sequence in which they specified in the
77
organisation block.
INTERRUPT DRIVEN PROGRAM PROCESSING
When certain input signal changes occur, cyclic processing is interrupted at the next block
boundary and an OB assigned to this event is started. The user can formulate his response
program to this interrupt in the OB. The cyclic program execution is the resumed from the point
at which it was interrupted.
TIME CONTROLLED PROGRAM EXECUTION
Certain Obs are executed at the predetermined time intervals (e.g. every 100ms, 200ms, 500ms,
1s, 2s, and 5s). For this purpose, cyclic program execution is interrupted at the block boundary
and resumed again at this point, once the relevant OB has been executed. Fig.13 gives the
organisation and execution of a structured user program.
78
PB1
PB2
FB3
FB2OB
1
Structured programming
PB
FB
PB
FB
Organisation block (OB)
Program block (PB)
Function block (PB)
Cycle execution
OB
Fig.13 Organisation and execution of a structured user program
79
PB
FBO
B
Interrupt-driven execution
Points at which interrupt-driven program can be insertedStart and finish of interrupt-driven program execution
EXAMPLES OF PLC PROGRAM
Before attempting to write a PLC program, first go through the instruction set of the particular
language used for the equipment, and understand the meaning of each instruction. Then study
how to use these instructions in the program (through illustration examples given in the manual).
Once the familiarization task is over, then start writing the program.
Follow the following steps to write a PLC program.
List down each individual element (field device) on the machine as Input/Output.
Indicate against each element the respective address as identifier during electrical interfacing
of these elements with the PLC.
Break down the complete machine auxiliary functions that are controlled by the PLC into
individual, self contained functions.
Identify each individual function as separate block (PBxx/FBxx)
Once the PBs and FBs for each function are identified, take them one by one for writing the
program.
List down the preconditions required for the particular function separately.
Note down the address of the listed elements.
Write down the flow chart for the function.
Translate the flow chart into PLC program using the instructions already familiarized.
Complete the program translation of all individual functions in similar lines.
Check the individual blocks independently and correct the program to get the required
results.
Organize all the program blocks in the organization block depending upon the sequence in
which they are supposed to be executed as per the main machine function flow chart.
Check the complete program with all the blocks incorporated in the final program.
80
Example 1: Spindle ON
Preconditions Feedback elements Address Fault indicationAddress Remark
Tool clamp Pressure switch I 2.4 Lamp Q 2.1Job clamp Proximity switch I 3.2 Lamp Q 1.7Door close Limit switch I 5.7 Lamp Q 4.0Lubrication ON PLC output bit Q 1.0 Lamp Q 7.7Drive ready Input signal from I 4.6 Lamp Q 0.4
Drive unit
PB 12 written is the individual function module for spindle ON for all the preconditions checked
and found satisfactory. This function is required to be executed only when the spindle rotation is
requested by the NC in the form of a block in the part program.
Whenever NC decodes the part program block, it in turn informs the PLC through a fixed
buffer location that spindle rotation is requested. Say Flag bit F 100.0 is identified for this
information communication. With this data, spindle ON function module can be recalled in the
organisation block OB1 as follows.
OB 1
……
A F 100.0
JC PB12
……
……
BE
Now, spindle ON function module PB12 will be executed only when F 100.0 is set. Otherwise
the function execution will be bypassed.
81
FLOW CHART
82
START
TOOL CLAMP
JOB CLAMP
DOOR CLOSED
LUBRICATION ON
DRIVE READY
ANY FAULT
INDICATE FAULT
INDICATE FAULT
INDICATE FAULT
INDICATE FAULT
INDICATE FAULT
STOP SPINDL
PB12
AN I 2.4 Tool not clamped= Q 2.1 Display fault
AN I 3.2 Job not clamped= Q 1.7 Display fault
AN I 5.7 Door not closed= Q 4.0 Display fault
AN Q 1.0 Lubrication not on= Q 7.7 Display fault
AN I 4.6 Drive not ready= Q 0.4 Display fault
Comments
Exit
YES
YES
YES
YES
YES
NO
NO
NO
NO
NO
NO
YES
APPLICATIONS OF PLC
In the present industrial world, a flexible system that can be controlled by user at site is
preferred. Systems, whose logic can be modified but still, used without disturbing its connection
to external world, is achieved by PLC. Utilizing the industrial sensors such as limit switches,
ON-OFF switches, timer contact, counter contact etc., PLC controls the total system. The drive
to the solenoid valves, motors, indicators, enunciators, etc are controlled by the PLCs.
The above said controlling elements (normally called as inputs of PLCs) and controlled
elements (called as outputs of PLCs) exist abundantly in any industry. These inputs, outputs,
timers, counters, auxiliary contacts are integral parts of all industries. As such, it is difficult to
define where a PLC cannot be used.
Proper application of a PLC begins with conversion of information into convenient parameters to
save money, time and effort and hence easy operation in plants and laboratories.
The areas where PLC is used maximum are as follows:
1. The batch processes in chemical, cement, food and paper industries which are
sequential in nature, requiring time of event based decisions is controlled by PLCs.
83
DO SPINDLE
END
2. In large process plants PLCs are being increasingly used for automatic start up and
shut down of critical equipment. A PLC ensures that equipment cannot be started
unless all the permissive conditions for safe start have seen established. It also
monitors the conditions necessary for safe running of the equipment and trips the
equipment whenever any abnormality in the system is detected.
3. The PLC can be programmed to function as an energy management system for boiler
control for maximum efficiency and safety.
4. In automation of blender reclaimers
5. In automation of bulk material handling system at ports.
6. In automation for a ship unloader.
7. Automation for wagon loaders.
8. For blast furnace charging controls in steel plants.
9. In automation of brick moulding press in refractories.
10. In automation for galvanizing unit.
11. For chemical plants process control automation.
12. In automation of a rock phosphate drying and grinding system.
13. Modernization of boiler and turbogenerator set.
14. Process visualization for mining application.
84
15. Criteria display system for power station.
16. As stored programmed automation unit for the operation of diesel generator sets.
17. In Dairy automation and food processing.
18. For a highly modernized pulp paper factory.
19. In automation system for the printing industry.
20. In automation of container transfer crane.
21. In automation of High-speed elevators.
22. In plastic moulding process.
23. In automation of machine tools and transfer lines.
24. In Mixing operations and automation of packaging plants.
25. In compressed air plants and gas handling plants.
26. In fuel oil processing plants and water classification plants.
27. To control the conveyor/classifying system.
Thus PLC is ideal for application where plant machine interlock requirements are finalized at a
later stage and need changes during engineering trial runs, commissioning or normal use. It can
be used extensively to replace conventional relay controls in power stations, refineries, cement,
steel, fertilizer, petrochemical, chemical industries etc.
Applications can thus be extended from monitoring to supervision, control and management.
85
CONCLUSION
It is true that CNC machine costs more to install initially. But higher initial cost is set off by the
direct and indirect gains resulting from various advantages of CNC machines. In most cases,
careful techno-economic evaluation of a given manufacturing situation will clearly bring out that
unit cost of production is definitely less tools with that of so called conventional machines.
To conclude numeric control is the most sophisticated form of automatic control of machine tool.
It has high degree of precision and reliability. The control system has undergone several stage of
development.
Some of the special features offered by CNC machine manufacture are:
Thermal stabilization
Axis calibration
Lost machine compensation
With the various above qualities of CNC machine there are numerous advantages. They are
High accuracy
High reliability
Less scrap and network
Better machine utilization
Computer control of manufacture capability of integration into distribution numeric control
(DNC) etc.
The programs written for CNC are easy to write and understand. These programs use either G-
cod or M-code that runs the program. The codes are simple to understand.
No wonder CNC machines tools are becoming more and more popular day by day in modern
industries. In longer run CNC machine pays for itself with such outstanding qualities.
86
Plc is a simple computer which used for automation of real world processes such as
controlling of machinery in a industry. Plc is a microprocessor based device with input and
output circuitry that monitors the status of the field connected "sensor" inputs and controls
attached output "actuators" according to user created logic program .application: maintaining the
water level in a tank between two float switches by controlling the electric valve. Ladder
diagram is a language which composes of program using a relay logic symbols as a base in an
image similar to a hard wired relay logic sequence. Advantages flexibility ,pilotrunning ,space
efficiency, correcting errors, visual observation. The PLC offers a compromise between advance
control techniques and present day technology. It is extremely difficult to forecast the rate and
form of progress of PLCs, but there is strong evidence that development is both rapid and
cumulative. Though a PLC is not designed to replace a computer, it is useful and cost effective
for medium sized control systems. With the capability of functioning as local controllers in
distributed control systems. PLCs will retain their application in large process plants.
A further development of PLCs leads to the development of programmable function
controller (PFC) is compatible to PCs and directly controls the desired functions.
In India every process industry is replacing relay control systems by PLCs and will go for
PFCs in near future. In the near future every flats and offices may possess PFCs to control room
temperature, as elevator controller, maintain water tank levels, as small telephone exchange etc.
87
Bibliography
1. CNC Programming handbook by Mr. Peter Smid.
2. Managing CNC operations by Mike Lynch
3. Wikipedia.
www. wikipedia .org/ wiki /Main
4. B.H.E.L. senior engineer Mr.B.K.Garg
5. CNC information and easy CNC by Mr. David Benson.
6. http://www.cnccncmachines.com
*******************************
88