Module1

1. Introduction: Need and comparison between traditional, non-traditional and micro & nanomachining process.2. Powder Metallurgy: Need of P/M - Powder Production methods:- Atomization,electrolysıs, Reduction of oxides, Carbonyls (Process parameters, characteristics of powderproduced in each method).3 Powder characteristics: properties of fine powder, size, size distribution, shape,compressibility, purity etc.4 Mixing – Compaction: - techniques, pressure distribution, HIP & CIP.5 Mechanism of sintering, driving force for pore shirking, solid and liquid phase sintering -Impregnation and Infiltration Advantages, disadvantages and specific applications of P/M.6 Programmable Logic Controllers (PLC): need – relays - logic ladder program –timers,simple problems only. 7 Point to point, straight cut and contouring positioning - incremental and absolute systems –open loop and closed loop systems - control loops in contouring systems: principle ofoperation.

INTRODUCTION:

Manufacturing processes can be broadly divided into two groups and they are primarymanufacturing processes and secondary manufacturing processes. The former ones providebasic shape and size to the material as per designer’s requirement. Casting, forming, powdermetallurgy are such processes to name a few. Secondary manufacturing processes provide thefinal shape and size with tighter control on dimension, surface characteristics etc. Materialremoval processes are mainly the secondary manufacturing processes.

Material removal processes once again can be divided into mainly two groups and they are“Conventional Machining Processes” and “Non-Traditional Manufacturing Processes”.Examples of conventional machining processes are turning, boring, milling, shaping,broaching, slotting, grinding etc. Similarly, Abrasive Jet Machining (AJM), UltrasonicMachining (USM), Water Jet and Abrasive Water Jet Machining (WJM and AWJM), Electro-discharge Machining (EDM) are some of the Non Traditional Machining (NTM) Processes.

Conventional machining method : Conventional machines requires direct contact of tool andwork piece/material. For example to cut an aluminium bar, an iron fast rotating cutter may beused. These methods involves a physical contact between the cutting tool and the materialbeing cut or processed.

Unconventional Machining processes: As the name suggest, unconventional machiningmethod involves the use of modern and advanced technology for machine processing. Thereis no physical contact between the tool and the work piece in such process. Tools used forcutting in unconventional methods are laser beams, electric beam, electric arc, infrared beam,Plasma cutting and so on depending on the type of working material.

Difference between Conventional and non-conventional machining processes are: Conventional machining process involved tool wearing as there is a physical contact betweenthe tool and the work piece. In non-conventional process, this is not the case.

1. Non-conventional tools are more accurate and precise than the conventional tool.

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2. No noise pollution is created as a result of non-conventional methods as these tools are much quieter.

3. Tool life is long for non-conventional processing.

4. Non-conventional tools are very expensive than the conventional tools.

5. Non-conventional tools have complex setup and hence requires a skilful operation by expert workers, whereas conventional tools do not require any special expert for its operation and are quite simple in set-up.

6. Spare parts of conventional machines are easily available but not for non-conventional machines.Which machine is more suitable for composite materials?For a composite material that is composed of different type of materials of different properties, I advise you to use non-conventional machining process as these are more precise and accurate; and will result in fine surface finish.

Sl

No.

Conventional Process Non Conventional Process

1 The cutting tool and work piece are

always in physical contact withrelative motion with each other,which results in friction and toolwear.

There is no physical contact between

the tool and work piece, In some non-traditional process tool wear exists.

2 Material removal rate is limited by

mechanical properties of work material.

NTM can machine difficult to cut and hard to cut materials like

titanium, ceramics, nimonics,

SST, composites, semiconductingmaterials3 Relative motion between the tool

and work is typically rotary or reciprocating. Thus the shape of work is limited to circular or flat shapes. In spite of CNC systems, production of 3D surfaces is still adifficult task.

Many NTM are capable of producing

complex 3D shapes and cavities

4 Machining of small cavities , slits , blind holes or through holes are difficult

Machining of small cavities, slits and Production of non-circular, micro sized, large aspect ratio, shall entry angle holes are easy using NTM

5 Use relative simple and inexpensive machinery and readily available cutting tools

Nontraditional processes requires expensive tools and equipment as well as skilled labor, which increase the production cost significantly

6 Capital cost and maintenance cost is low

Capital cost and maintenance cost is high

7. Traditional processes are well established and physics of process is well understood

Mechanics of Material removal of Some of NTM process are still under research

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8 Conventional process mostly uses mechanical energy

Most NTM uses energy in direct form For example: laser, Electron beam in its direct forms are used in LBM and EBM respectively.

9 Surface finish and tolerances are limited by machining inaccuracies

High surface finish(up to 0.1 micron) and tolerances (25 Microns)can be achieved

10 High metal removal rate. Low material removal rate.

Micro Machining process

Micro-machining is the most basic technology for the production of such miniature parts andcomponents. Micro machining is defined as the ability to produce features with thedimensions from 1 m to 999 m or when the volume of the material removed is at the microlevel. Lithography based micro-machining technology uses silicon as material to produceintegrated circuitry components and microstructures. However, these methods, in general,lack the ability of machining three-dimensional shapes because of poor machining control inthe Z axis.

Applications: In recent years, manufacturing industry has witnessed a rapid increasein demand for micro-products and micro-components in many industrial sectorsincluding the electronics, optics, medical, biotechnology and automotive sectors.Examples of applications include medical implants, drug delivery systems,diagnostic devices, connectors, switches, micro-reactors, micro-engines, micro pumps andprinting heads.

Production of Micro-Compressor

Production of Micro-Turbine Impeller

Nozzle for Diesel Fuel Injectors

Inkjet Printer Manufacturing

Cooling Holes in Turbine Blades

Micromachining Techniques

1. Micro-Ultrasonic Machining2. Mechanical Micromachining3. Micro-Electrochemical Machining4. Electrical Discharge Machining

Nano manufacturing process

Nano manufacturing is both the production of nano scaled materials, which can be powdersor fluids, and the manufacturing of parts "bottom up" from nano scaled materials or "topdown" in smallest steps for high precision, used in several technologies such as laser ablation,etching and others. Nano manufacturing differs from molecular manufacturing, which is themanufacture of complex, nanoscale structures by means of nonbiological mechanosynthesis.

the prefix "nano" means one-billionth, or 10-9; therefore one nanometer is one-billionth of ameter. It’s difficult to imagine just how small that is, so here are some examples:

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A sheet of paper is about 100,000 nanometers thick A strand of human DNA is 2.5nanometers in diameter There are 25,400,000 nanometers in one inch

A human hair is approximately 80,000- 100,000 nanometers wide A single gold atom is abouta third of a nanometer in diameter

On a comparative scale, if the diameter of a marble was one nanometer, then diameter of theEarth would be about one meter

One nanometer is about as long as your fingernail grows in one second

The illustration below has three visual examples of the size and the scale of nanotechnology,showing just how small things at the nanoscale actually are.

Within the top-down and bottom-up categories of nanomanufacturing, there are a growingnumber of new processes that enable nanomanufacturing. Among these are:

1. Chemical vapor deposition is a process in which chemicals react to produce very pure,high-performance films

2. Molecular beam epitaxy is one method for depositing highly controlled thin films3. Atomic layer epitaxy is a process for depositing one-atom-thick layers on a surface4. Dip pen lithography is a process in which the tip of an atomic force microscope is

"dipped" into a chemical fluid and then used to "write" on a surface, like an oldfashioned ink pen onto paper

5. Nanoimprint lithography is a process for creating nanoscale features by "stamping" or"printing" them onto a surface

6. Roll-to-roll processing is a high-volume process to produce nanoscale devices on aroll of ultrathin plastic or metal

7. Self-assembly describes the process in which a group of components come together toform an ordered structure without outside direction

Structures and properties of materials can be improved through these nanomanufacturingprocesses. Such nanomaterials can be stronger, lighter, more durable, water-repellent, anti-reflective, self-cleaning, ultraviolet- or infrared-resistant, antifog, antimicrobial, scratch-resistant, or electrically conductive, among other traits. Taking advantage of these properties,today's nanotechnology-enabled products range from baseball bats and tennis rackets tocatalysts for refining crude oil and ultrasensitive detection and identification of biological andchemical toxins.

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.

Powder metallurgy

IntroductionPowder metallurgy is a process of making components from metallic powders. Initially, itwasused to replace castings for metals which were difficult to melt because of high melting point.Thedevelopment of technique made it possible to produce a product economically, and today itoccupies an important place in the field of metal process.

Characteristics of Metal Powder

Following are the important characteristics of metal powders.

(a) Particle shape (b) Particle size

(c) Particle size distribution (d) Flow rate

(e) Compressibility (f) Apparent density

(g) Purity

(a) Particle Shape: The particle shape depends largely on the method of powdermanufacture. The shape may be special nodular, irregular, angular, and dendritic. The particleshape influences the flow characteristics of powders. Special particles have excellentsintering properties. However, irregular shaped particles are good at green strength becausethey will inter look on computing.(b) Particle Size: The particle size influences the control of porosity, compressibility andamount of shrinkage. It is determined by passing the powder through standard sieves or bymicroscopic measurement.(c) Particle Size Distribution: It is specified in term of a sieve analysis, the amount ofpowder passing through 100, 200 etc., mess sieves. Particle size distribution influences thepacking of powder and its behaviour during moulding and sintering.(d) Flow Rate: It is the ability of powder to flow readily and confirm to the mould cavity. Itdetermines the rate of production and economy.(e) Compressibility: It is defined as volume of initial powder (powder loosely filled in cavity)to the volume of compact part. It depends on particle size, distribution and shape.(f) Apparent Density: It depends on particle size and is defined as the ratio of volume toweight of loosely filled mixture.(g) Purity: Metal powders should be free from impurities as the impurities reduces the life ofdies and effect sintering process. The oxides and the gaseous impurities can be removed fromthepart during sintering by use of reducing atmosphere.BASIC STEPS OF THE PROCESSThe manufacturing of parts by powder metallurgy process involves the following steps:(a) Manufacturing of metal powders(b) Blending and mixing of powders(c) Compacting

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(d) Sintering(e) Finishing operationA .MANUFACTURING OF METAL POWDERSThere are various methods available for the production of powders, depending upon the type andnature of metal. Some of the important processes are:1. Atomization 2. Machining 3. Crushing and Milling 4. Reduction5. Electrolytic Deposition 6. Shotting7. Condensation

1. AtomizationIn this method as shown in Fig. (a), molten metal is forced through a small orifice andis disintegrated by a powerful jet of compressed air, inert gas or water jet. These smallparticles are then allowed to solidify. These are generally spherical inshape.Automation is used mostly for low melting point metals/alloy such as brass,bronze, zinc, tin, lead and aluminium powders.

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2. Reduction: Pure metal is obtained by reducing its oxide with a suitable reducing gasat an elevated temperature (below the melting point) in a controlled furnace. Thereduced product is then crushed and milled to a powder.Sponge iron powder is produced this wayFe3O4 + 4C = 3Fe + 4COFe3O4 + 4CO = 3Fe + 4CO2Copper powder byCu2O + H2 = 2Cu + H2OTungsten, Molybdenum, Ni and Cobalt are made by the method.

3. Electrolytic Deposition or Electrolysis: This method is commonly used for producingiron and copper powders. This process is similar to electroplating. For making copperpowder, copper plates are placed as anodes in the tank of electrolyte, whereas thealuminium plates are placed into electrolyte to act as anode. When D. C. current ispassed through the electrolyte, the copper gets deposited on cathode. The cathodeplates are taken out from electrolyte tank and the deposited powder is scrapped off.The powder is washed, dried and pulverised to produce powder of the desired grainsize. The powder is further subjected to heat treatment to remove work hardnesseffect. The cost of manufacturing is high.

4. Mechanical Alloying: In this method, powders of two or more pure metals are mixed in a ball mill. Under the impact of the hard balls, the powders are repeatedly fractured and welded together by forming alloy under diffusion. A production of composites with controlled uniform distribution of second phase in the metallic matrix is its principle

5. Shotting: In this method, the molten metal is poured through a siever or orifice and is cooled by dropping into water. This produces spherical particles of large size. This method is commonly used for metals of law melting points.

6. Condensation: In this method, metals are boiled to produce metal vapours and thencondensed to obtain metal powders, This process is applied to volatile metals such aszinc, magnesium and cadmium.

(b) BLENDING AND MIXING OF POWDERSA single powder may not fulfil all the requisite properties and hence, powders of differentmaterials with wide range of mechanical properties are blended to form a final part.Blending is carried out for several purposes as follows.

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1. Blending imparts uniformity in the shapes of the powder particles,2. Blending facilitates mixing of different powder particles to impart wide rangingphysical and mechanical properties,3. Lubricants can be added during the blending process to improve the flowcharacteristics of the powder particles reducing friction between particles and dies,4. Binders can be added to the mixture of the powder particles to enhance the greenstrength during the powder compaction process.(c) Compacting

The main purpose of compacting is converting loose powder into a green compact of accurateshape and size. The following methods are adopted for compacting:

1. Pressing 2. Centrifugal compacting

3. Slip casting 4. Extrusion5. Gravity sintering

6. Rolling

7. Isostatic moulding

8. Explosive moulding

1. Pressing: The metal powders are placed in a die cavity and compressed to form a component shaped to the contour of the die as illustrated in Fig.. The pressure used for producing green compact of the component vary from 80 Mpa to 1400 Mpa, depending upon the material and the characteristics of the powder used. Mechanical presses are usedfor compacting objects at low pressure. Hydraulic presses are for compacting objects at high pressure.

2. Centrifugal Compacting: In this method, the moulder after it is filled with powder is centrifugal to get a compact of high and uniform density at a pressure of 3 Mpa. This method is employed for heavy metals such as tungsten carbide.3.Slip Casting: In this method, the powder is converted into slurry with water and pouredintothe mould made of plaster of paris. The liquid in the slurry is gradually absorbed by the

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mouldleaving the solid compact within the mould. The mould may be vibrated to increase thedensity ofthe compact.This technique is used for materials that are relatively incompressible byconventional die compaction. The main drawback of this process is relatively slowerprocess because it takes larger time for the fluid to be absorbed by the method.4. Extrusion: This method is employed to produce the components with high density.Both cold and hot extrusion processes are for compacting specific materials. In coldextrusion, the metal powder is mixed with binder and this mixture is compressed intobillet. The binder is removed before or during sintering. The billet is charged into acontainer and then forced through the die by means of ram. The cross-section of productdepends on the opening of the die. Cold extrusion process is used for cemented carbidedrills and cutters of ram. The cross-section of products depends on the opening of the die.Cold extrusion process is used for cemented carbide drills and cutters.In the hot extrusion, the powder is compacted into billet and is heated to extrudingtemperature in non-oxidising atmosphere. The billet is placed in the container andextruded through a die. This method is used for refractive berium and nuclear solidmaterials.5. Gravity Sintering: This process is used for making sheets for controlled porosity. Inthis process. the powder is poured on ceramic tray to form an uniform layer and is thensintered up to 48 hours in ammonia gas at high temperature. The sheets are then rolled todesired thickness.Porous sheet of stainless steel are made by this process and popularlyused for fitters.6. Rolling: This method is used for making continuous strips and rods having controlledporosity with uniform mechanical properties. In this method, the metal powder is fedbetween two rollswhich compress and interlock the powder particles to form a sheet of sufficient strengthas shownin Fig. It then situated, rerolled and heat treated if necessary. The metals that can be rolledare Cu, Brass, Bronze, Ni, Stainless steel and Monel.

7. Isostatic Moulding: In this method, metal powder is placed in an elastic mould which issubjected to gas pressure in the range of 65-650 Mpa from all sides. After pressing. Thecompact is removed from gas chamber. If the fluid is used as press medium then it is called ashydrostatic pressing. The advantages of this method are: uniform strength in all directions,

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higher green compact strength and low equipment cost. This method is used for tungsten,molybdenum, niobium etc.8. Explosive Compacting: In this method, the pressure generated by an explosive is used to compact the metal powder. Metal powder is placed in water proof bags which are immersed in water container cylinder of high wall thickness. Due to sudden deterioration of the charge at the end of the cylinder, the pressure of the cylinder increase. This pressure is used to press the metal powder to form green compact.SinteringSintering refers to the heating of the compacted powder perform to a specific temperature (below the melting temperature of the principle powder particles while well above thetemperature that would allow diffusion between the neighbouring particles). Sinteringfacilitates the bonding action between the individual powder particles and increase in thestrength of the final part. The heating process must be carried out in a controlled, inert orreducing atmosphere or in vacuum for very critical parts to prevent oxidation. Prior to thesintering process, the compacted powder perform is brittle and confirm to very low greenstrength. The nature and strength of the bond between the particles depends on themechanism of diffusion and plastic flow of the powder particles, and evaporation of volatilematerial from the in the compacted preform. Bonding among the powder particles takesplaces in three ways: (1) melting of minor constituents in the powder particles, (2) diffusionbetween the powder particles, and (3) mechanical bonding. The time, temperature and thefurnace atmosphere are the three critical factors that control the sintering process. Sinteringprocess enhances the density of the final part by filling up the incipient holes and increasingthe area of contact among the powder particles in the compact perform.

Sintering Sequence on a Microscopic Scale (1) Particle bonding is initiated at contact points; (2) Contact points grow into "necks"; (3) Pores between particles are reduced in size;

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(4) Grain boundaries develop between particles in place of necked regions.

Finishing OperationsThese are secondary operations intended to provide dimensional tolerances, physical and better surface finish. They are:

1. Sizing 2. Coining 3. Machining 4. Impregnation5. Infiltration6. Heat treatment7. Plating

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ADVANTAGES OF POWDER METALLURGY1. Although the cost of making powder is high there is no loss of material. The componentsproduced are clean, bright and ready for use.2. The greatest advantage of this process is the control of the composition of the product.3. Components can be produced with good surface finish and close tolerance.4. High production rates.5. Complex shapes can be produced.6. Wide range of properties such as density, porosity and particle size can be obtained forparticular applications.7. There is usually no need for subsequent machining or finishing operations.8. This process facilitates mixing of both metallic and non-metallic powders to give products of

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special characteristics.9. Porous parts can be produced that could not be made any other way.10. Impossible parts (cutting tool bits) can be produced.11. Highly qualified or skilled labour is not required.LIMITATION OF POWDER METALLURGY1. The metal powders and the equipment used are very costly.2. Storing of powders offer great difficulties because of possibility of fire and explosion hazards.3. Parts manufactured by this process have poor ductility.4. Sintering of low melting point powders like lead, zinc, tin etc., offer serious difficulties.APPLICATIONS OF POWDER METALLURGYPowder metallurgy techniques are used for making large number of components. Some of theapplication are as follows:1. Self-Lubricating Bearing and Filters: Porous bronze bearings are made by mixing copperand tin powder in correct proportions, cold pressed to the desired shape and then sintered. These bearings soak up considerable quantity of oil. Hence during service, these bearings produce a constant supply of lubricant to the surface due to capillary action. These are used where lubricating is not possible. Porous filters can be manufactured and are used to remove, undesirable materials from liquids and gases.2. Friction Materials: These are made by powder metallurgy. Clutch liners and Brake bandsare the example of friction materials.3. Gears and Pump Rotors: Gears and pump rotor for automobile oil pumps are manufacturedby powder metallurgy. Iron powder is mixed with graphite, compacted under a pressure of 40 kg/cm and sintered in an electric furnace with an atmosphere and hydrocarbon gas. These areimpregnated with oil.4. Refractor Materials: Metals with high melting points are termed as refractory metals.These basically include four metals tungsten, molybdenum, tantalum and niobium. Refractory metals as well as their alloys are manufactured by powder metallurgy. The application are not limited to lamp filaments and heating elements, they also include space technology and the heavy metal used in radioactive shielding.5. Electrical Contacts and Electrodes: Electrical contacts and resistance welding electrodesare made by powder metallurgy. A combination of copper, silver and a refractory metal like tungsten. Molybdenum and nickle provides the required characteristics like wear resistant, refractory and electrical conductivity.6. Magnet Materials: Soft and permanent magnets are manufactured by this process. Softmagnets are made of iron, iron-silicon and iron-nickle alloys. These are used in D.C. motors, or generators as armatures and in measuring instruments. Permanent magnets known as Alnico which is a mixture of nickle, aluminium, cobalt, copper and iron are manufactured by this technique.7. Cemented Carbides: These are very important products of powder metallurgy and findwide applications as cutting tools, wire drawing dies and deep drawing dies. These consist ofcarbides of tungsten, tantalum, titanium and molybdenum. The actual proportions of various carbides depend upon its applications, either cobalt or nickle is used as the bonding agent while sintering.

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Introduction

What type of task might a control system have? It might berequired to control a sequence of events or maintain somevariable constant or follow some prescribed change. Examplein case of a four wheeler, the different controls are used forcontrol the vehicle. In this case steering wheel, break etc arecontrollers for vehicle. The break controls the speed andsteering controls the direction. So every system role ofcontroller is very important. The controllers controls thedifferent parameters which are affecting the systemperformance.

The logical controllers are which controls differentparameters with certain logic. Example: - We have a controller with elevator, if you are in the4th floor and if you want to go to ground floor, then you will press the ground floor button.And same process will do it from fifth floor and second floor at same time. So the elevatorcontroller will give the priority for the first come first bases. So the lift will get the personfrom 4th floor and go to ground floor, on the way it will stop at 2nd because the controllercome to know that the 2nd floor comes after 4th floor. So lift will stop at second floor and getperson from second floor, then go to ground floor .So controlling is performing with the helpof certain logics.

A programmable logic controller (PLC) is an industrial grade computer that is capable ofbeing programmed to perform control functions. The programmable controller has eliminatedmuch of the hardwiring associated with conventional relay control circuits. Other benefitsinclude fast response, easy programming and installation, high control speed, networkcompatibility, troubleshooting and testing convenience, and high reliability.

PLCs have the great advantage that the same basic controller can be used with a wide rangeof control systems. To modify a control system and the rules that are to be used, all that isnecessary is for an operator to key in a different set of instructions. There is no need torewire. The result is a flexible, cost effective, system which can be used with control systemswhich vary quite widely in their nature and complexity. PLCs are similar to computers butwhereas computers are optimised for calculation and display tasks, PLCs are optimised forcontrol tasks and the industrial environment.

Thus PLCs are:

Programmablelogic

controllers

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1 Rugged and designed to withstand vibrations, temperature, humidity and noise.2 Have interfacing for inputs and outputs already inside the controller.3 Are easily programmed and have an easily understood programming language which is primarily concerned with logic and switching operations.

Initially the PLC was used to replace relay logic, but its ever-increasing range of functions means that it is found in many and more complex applications. Because the structure of a PLC is based on the same principles as those employed in computer architecture, it is capablenot only of performing relay switching tasks but also of performing other applications such astiming, counting, calculating, comparing, and the processing of analog signals.

In case of relays we have to hardwire to perform a specific function. When the systemrequirements change, the relay wiring has to be changed or modified. So PLC are inexpensiveand give more flexibility compare to relay based system.

Other Benefits of PLC’s

Increase reliability: Once a program has been written and tested, it can be easilydownloaded to other PLCs. Since all logic is contained in the PLC’s memory, there isno chance of making a logic wiring error. It will reduce the use external wiring forcontrol the process.

More Flexibility: It is easy to create and update a program in a PLC. With a PLCrelationship b/w the input and output are determined by the user programs. Originalequipment manufacturers can provide system updates by sending out a new program.

Low cost : PLCs were originally designed to reduce relay control logic and the costsavings have been so significant that relay control is becoming obsolete except forpower applications.

Communications Capability: It can communicate with other controllers or computerequipment to perform such functions as supervisory control, data gathering,monitoring devices, process parameters etc.

Faster Response Time: It is designed for high speed real-time applications. Theprogrammable controller operations in real time, which means that an event takingplace in the field will result in the execution of an operation or output.

Easy to troubleshoot: PLCs have resident diagnostics and override functions thatallow user to easily trace and correct software and hardware problems.

Easier to Test field Devices: A PLC control panel has the ability to check fielddevices at common point. By having each device wired back to a common point on aPLC module, each device could be checked for operation fairly quickly.

Hardware

Typically a PLC system has the basic functional components of processor unit, memory, power supply unit, input/output interface section, communications interface and the programming device. Figure shows the basic arrangement.

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The PLC system

1 The processor unit or central processing unit (CPU) is the unit containing themicroprocessor and this interprets the input signals and carries out the control actions,according to the program stored in its memory, communicating the decisions as action signalsto the outputs.

2 The power supply unit is needed to convert the mains a.c. voltage to the low d.c. voltage (5V) necessary for the processor and the circuits in the input and output interface modules.

3 The programming device is used to enter the required program into the memory of theprocessor. The program is developed in the device and then transferred to the memory unit ofthe PLC.

4 The memory unit is where the program is stored that is to be used for the control actions tobe exercised by the microprocessor and data stored from the input for processing and for theoutput for outputting.

5 The input and output sections are where the processor receives information from externaldevices and communicates information to external devices. The inputs might thus be fromswitches, as illustrated in Figure 1.1(a) with the automatic drill, or other sensors such asphoto-electric cells, as in the counter mechanism in Figure 1.1(b), temperature sensors, orflow sensors, etc. The outputs might be to motor starter coils, solenoid valves, etc. Input andoutput devices can be classified as giving signals which are discrete, digital or analogue(Figure 1.5). Devices giving discrete or digital signals are ones where the signals are eitheroff or on. Thus a switch is a device giving a discrete signal, either no voltage or a voltage.Digital devices can be considered to be essentially discrete devices which give a sequence ofon-off signals. Analogue devices give signals whose size is proportional to the size of thevariable being monitored. For example, a temperature sensor may give a voltage proportionalto the temperature.

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Input and output devices can be classified as giving signals which are discrete, digital oranalogue .Devices giving discrete or digital signals are ones where the signals are either offor on. Thus a switch is a device giving a discrete signal, either no voltage or a voltage. Digitaldevices can be considered to be essentially discrete devices which give a sequence of on-offsignals. Analogue devices give signals whose size is proportional to the size of the variablebeing monitored. For example, a temperature sensor may give a voltage proportional to thetemperature.

6 The communications interface is used to receive and transmit data on communicationnetworks from or to other remote PLCs (Figure 1.6).It is concerned with such actions asdevice verification, data acquisition, synchronisation between user applications andconnection management.

With the relay type, the signal from the PLC output is used to operate a relay and is able toswitch currents of the order of a few amperes in an external circuit. The relay not only allowssmall currents to switch much larger currents but also isolates the PLC from the externalcircuit. Relays are, however, relatively slow to operate. Relay outputs are suitable for a.c. andd.c. switching. They can withstand high surge currents and voltage transients.

Programming PLCsProgramming devices can be a hand-held device, a desktop console or a computer. Only

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when the program has been designed on the programming device and is ready is it transferredto the memory unit of the PLC.1 Hand-held programming devices will normally contain enough memory to allow the unit to retain programs while being carried from one place to another.2 Desktop consoles are likely to have a visual display unit with a full keyboard and screen display.3 Personal computers are widely configured as program development work-stations. Some PLCs only require the computer to have appropriate software; others require special communication cards to interface with the PLC. A major advantage of using a computer is that the program can be stored on the hard disk or a CD and copies easily made.

Principles of Operation

Here a mixer motor is to be used to automatically stir the liquid in a vat when the temperatureand pressure reach preset values. In addition, direct manual operation of the motor isprovided by means of a separate pushbutton station. The process is monitored withtemperature and pressure sensor switches that close their respective contacts when conditionsreach their preset values.

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The PLC ladder logic program would be constructed and entered into the memory of theCPU. A typical ladder logic program for this process is shown in Figure 1-20. The formatused is similar to the layout of the hardwired relay ladder circuit. The individual symbolsrepresent instructions, whereas the numbers represent the instruction location addresses. Toprogram the controller, you enter these instructions one by one into the processor memoryfrom the programming device. Each input and output device is given an address, which letsthe PLC know where it is physically connected. Note that the I/O address format will differ,depending on the PLC model and manufacturer. Instructions are stored in the user programportion of the processor memory. During the program scan the controller monitors the inputs,executes the control program, and changes the output accordingly.

During the program scan, the controller monitors the inputs, executes the control program,and changes the output accordingly. Each symbol (looks like a normally open contact) is aninstruction. The symbol is considered to represent a coil that, when energized, will energizethe device that is wired to the respective output. In the ladder logic program of Figure 1-20,the coil O/1 is energized when contacts I/1 and I/2 are closed or when contact I/3 is closed.Either of these conditions provides a continuous logic path from left to right across the rungthat includes the coil.

Following sequence of events:• First, the pressure switch, temperature switch, and pushbutton inputs are examined and theirstatus is recorded in the controller’s memory.• A closed contact is recorded in memory as logic 1 and an open contact as logic 0.• Next the ladder diagram is evaluated, with each internal contact given an OPEN or CLOSED status according to its recorded 1 or 0 state.• When the states of the input contacts provide logic continuity from left to right across the rung, the output coil memory location is given a logic 1 value and the output module interface contacts will close.

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• When there is no logic continuity of the program rung, the output coil memory location is set to logic 0 and the output module interface contacts will be open.• The completion of one cycle of this sequence by the controller is called a scan. The scan time, the time required for one full cycle, provides a measure of the speed of response of the PLC.• Generally, the output memory location is updated during the scan but the actual output is not updated until the end of the program scan during the I/O scan.

Fundamentals of Logic

The Binary Concept: The term binary principle refers to the idea that many things can be thought of as existing in only one of two states. These states are 1 and 0. There is no in between state so when information is processed the outcome is either yes or no.

A logic gate is a circuit with several inputs but only one output that is activated by particular combinations of input conditions. The two-state binary concept, applied to gates, can be the basis for making decisions. The high beam automobile lighting circuit of Figure 4-1 is an example of a logical AND decision. For this application, the high beam light can be turned ononly when the light switch AND the high beam switch are closed.

The AND Function: The basic rules that apply to an AND gate are:• If all inputs are 1, the output will be 1.• If any input is 0, the output will be 0.The AND logic gate operates similarly to control devices connected in series,

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The OR Function

The symbol drawn in Figure 4-6 is that of an OR gate. An OR gate can have any number of inputs but only one output.

Figure 4-7 illustrates the four possible combinations of inputs for a 2-input OR gate. The basic rules that apply to an OR gate are:• If one or more inputs are 1, the output is 1.• If all inputs are 0, the output will be 0.

The NOT Function

The symbol drawn in Figure 4-9 is that of a NOT function.Unlike the AND and ORfunctions, the NOT function can have only one input. The NOT output is 1 if the input is0.The output is 0 if the input is 1. The result of the NOT operation is always the inverse of theinput, and the NOT function is, therefore, called an inverter.

The NOT symbol placed at the output of an AND gate would invert the normal output result.An AND gate with an inverted output is called a NAND gate. The NAND gate symbol andtruth table are shown in Figure 4-12. The NAND function is often used in integrated circuitlogic arrays and can be used in programmable controllers to solve complex logic.

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The same rule about inverting the normal output result applies if a NOT symbol is placed atthe output of the OR gate. The normal output is inverted, and the function is referred to as aNOR gate. The NOR gate symbol and truth table are shown in Figure 4-13.

The Exclusive-OR (XOR) Function

An often-used combination of gates is the exclusive-OR (XOR) function. The XOR gatesymbol and truth table are shown in Figure 4-14. The output of this circuit is HIGH onlywhen one input or the other is HIGH, but not both.The exclusive-OR gate is commonly usedfor the comparison of two binary.

Boolean algebra

The mathematical study of the binary number system and logic is called Boolean algebra. Thepurpose of this algebra is to provide a simple way of writing complicated combinations oflogic statements. Figure 4-15 summarizes the basic operators of Boolean algebra as theyrelate to the basic AND, OR, and NOT functions.

PLC digital systems may be designed using Boolean algebra. Circuit functions arerepresented by Boolean equations. Figure 4-16 illustrates how logic operators AND, NAND,OR, NOR, and NOT are used singly to form logical statements. Figure 4-17 illustrates howbasic logic operators are used in combination to form Boolean equations.

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COMMUTATIVE LAWA + B = B + AA ⋅ B = B ⋅ A

ASSOCIATIVE LAW(A + B) + C = A + (B + C)(A ⋅ B) ⋅ C = A ⋅ (B ⋅ C )DISTRIBUTIVE LAWA ⋅ (B + C) = (A ⋅ B) + (A ⋅ C)

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A + (B ⋅ C) = (A + B) ⋅ (A + C)This law holds true only in Boolean algebra.

Hardwired Logic versus Programmed Logic

The term hardwired logic refers to logic control functions that are determined by the waydevices are electrically interconnected. Hardwired logic can be implemented using relays andrelay ladder schematics. Relay ladder schematics are universally used and understood inindustry. Figure 4-22 shows a typical relay ladder schematic of a motor stop/start controlstation with pilot lights. The control scheme is drawn between two vertical supply lines. Allthe components are placed between these two lines, called rails or legs, connecting the twopower lines with what look like rungs of a ladder—thus the name, relay ladder schematic.Hardwired logic is fxed; it is changeable only by altering the way devices are electricallyinterconnected.

In contrast, programmable control is based on the basic logic functions, which areprogrammable and easily changed. These functions (AND, OR, NOT) are used either singlyor in combinations to form instructions that will determine if a device is to be switched on oroff. The form in which these instructions are implemented to convey commands to the PLC iscalled the language. The most common PLC language is ladder logic. Figure 4-23 shows atypical ladder logic program for the motor start/stop circuit. The instructions used are therelay equivalent of normally open (NO) and normally closed (NC) contacts and coils.

PLC contact symbolism is a simple way of expressing the control logic in terms of symbols. These symbols are basically the same as those used for representing hardwired relay control circuits. A rung is the contact symbolism required to control an output. Some PLCs allow a rung to have multiple outputs while others allow only one output per rung.

Because the PLC uses ladder logic diagrams, the conversion from any existing relay logic toprogrammed logic is simplified. Each rung is a combination of input conditions (symbols)connected from left to right, with the symbol that represents the output at the far right. Thesymbols that represent the inputs are connected in series, parallel, or some combination of thetwo to obtain the desired logic. The following group of examples illustrates the relationshipbetween the relay ladder schematic, the ladder logic program, and the equivalent logic gatecircuit.

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PLC ladder programming

A very commonly used method of programming PLCs is based on the use of ladder diagrams.Writing a program is then equivalent to drawing a switching circuit. The ladder diagramconsists of two vertical lines representing the power rails. Circuits are connected as horizontallines,i.e. the rungs of the ladder, between these two verticals.

In drawing a ladder diagram, certain conventions are adopted:1 The vertical lines of the diagram represent the power rails between which circuits are connected. The power flow is taken to be from the left-hand vertical across a rung.2 Each rung on the ladder defines one operation in the control process.3 A ladder diagram is read from left to right and from top to bottom, Figure 5.3 showing the scanning motion employed by the PLC. The top rung is read from left to right. Then the second rung down is read from left to right and so on. When the PLC is in its run mode, it goes through the entire ladder program to the end, the end rung of the program being clearly denoted, and then promptly resumes at the start. This procedure of going through all the rungs of the program is termed a cycle. The end rung might be indicated by a block with the word END or RET for return, since the program promptly returns to its beginning.

4 Each rung must start with an input or inputs and must end with at least one output. The terminput is used for a control action, such as closing the contacts of a switch, used as an input to the PLC. The term output is used for a device connected to the output of a PLC,e.g. a motor.

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5 Electrical devices are shown in their normal condition. Thus a switch which is normally open until some object closes it, is shown as open on the ladder diagram. A switch that is normally closed is shown closed.6 A particular device can appear in more than one rung of a ladder. For example, we might have a relay which switches on one or more devices. The same letters and/or numbers are used to label the device in each situation.7 The inputs and outputs are all identified by their addresses, the notation used depending on the PLC manufacturer. This is the address of the input or output in the memory of the PLC.

NUMERICAL CONTROL (NC) Machine Tools.

Introduction:Numerical Control (NC) refers to the method of controlling the manufacturingoperation by means of directly inserted coded numerical instructions into the machine tool. Itis important to realize that NC is not a machining method, rather, it is a concept of machinecontrol.Although the most popular applications of NC are in machining, NC can be applied tomany other operations, including welding, sheet metalworking, riveting, etc.

History:The invention of numerical control has been due to the pioneering works of John T.Parsons in the year 1940, when he tried to generate a curve automatically by milling cuttersby providing coordinate motions. In the late 1940s Parsons conceived the method of usingpunched cards containing coordinate position system to control a machine tool. The machinedirected to move in small increments and generate the desired finish. In the year, 1948,Parons demonstrated this concept to the US Air Force, who sponsored the series of project atlaboratories of Massachusetts Institute of Technology (MIT). After lots of research MIT wasable to demonstrate first NC prototype in the year 1952 and in the next year they were able toprove the potential applications of the NC.

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Concept:

PRINCIPLE OF NUMERICAL CONTROL

“a system in which actions are controlled by direct insertion of numerical data at some point.The system must automatically interpret this data.”

Numerical control, popularly known as the NC is very commonly used in the machine tools.Numerical control is defined as the form of programmable automation, in which the processis controlled by the number, letters, and symbols. In case of the machine tools thisprogrammable automation is used for the operation of the machines.

In numerical control method the numbers form the basic program instructions for differenttypes of jobs; hence the name numerical control is given to this type of programming. Whenthe type of job changes, the program instructions of the job also change. It is easier to writethe new instructions for each job, hence NC provides lots of flexibility in its use.

The NC technology can be applied to wide variety of operations like drafting, assembly,inspection, sheet metal working, etc. But it is more prominently used for various metalmachining processes like turning, drilling, milling, shaping etc. Due to NC all the machiningoperations can be performed at the fast rate resulting in bulk manufacturing becoming quitecheaper.

Components of the Numerical Control System:There are three important components of the numerical control or NCsystem. These are:1) Program of instructions2) Controller unit, also called as the machine control unit (MCU) and3) Machine toolAll these have been shown in the figure below and also described in thesubsequent sections.

CLASSIFICATION OF NC SYSTEMS:

The classification of NC machine tool systems can be done in four ways:

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1. According to the type of machine: Point-to-point versus contouring (continuous path)

2. According to the structure of the controller: Hardware-based NC versus CNC

3. According to the programming method: incremental versus absolute

4. According to the type of control loops: Open-loop versus closed-loop

1. Point-to-Point and Contouring: Point to Point positioning:

In PTP, the objective of the machine tool control system is to move the cutting tool to apredefined location. The speed or path by which this movement is accomplished is notimportant in point to point NC. Once the tool reaches the desired location, the machiningoperation is performed at that position. NC drill presses are a good example of PTP systems.The spindle must first be positioned at a particular location on the work piece. This is doneunder PTP control. Then the drilling of the hole is performed at the location, and so forth.Since no cutting is performed between holes, there is no need for controlling the relativemotion of the tool and work piece between hole locations. Figure 1 illustrates the point topoint type of control. Point-to point (PTP) is also sometimes called a positioning system.Positioning systems are the simplest machine tool control systems and are therefore the leastexpensive of the three types. However, for certain process, such as drilling operations andspot welding. PTP is perfectly suited to the task and any higher level of control would beunnecessary.

Contouring systems (Continuous path systems)

Other type of machine tools involves motion of work piece with respect to the cutter whilecutting operation is taking place. These machine tools include milling, routing machines etc.and are known as contouring machines as shown in figure and the controls required for theircontrol are known as contouring control.

Contouring machines can also be used as point-to-point machines, but it will beuneconomical to use them unless the work piece also requires having a contouring operationto be performed on it. These machines require simultaneous control of axes. In contouringmachines, relative positions of the work piece and the tool should be continuously controlled.The control system must be able to accept information regarding velocities and positions ofthe machines slides. Feed rates should be programmed.

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Incremental and Absolute systems

CNC systems are further divided into incremental and absolute systems (Figure 8). Inincremental mode, the distance is measured from one point to the next. For example, if youwant to drill five holes at different locations, the x-position commands are x + 500, + 200, +600, - 300, -700, -300. An absolute system is one in which all the moving commands arereferred from a reference point (zero point or origin) . For the above case, the x-positioncommands are x 500,700, 1300, 1000, 300, 0. (Figure 8) . Both systems are incorporated inmost CNC systems. For an inexperienced operator, it is wise to use incremental mode.

The absolute system has two significant advantages over the incremental system :

1,Interruptions caused by, for example, tool breakage (or tool change, or checking the parts),would not affect the position at the interruption .

If a tool is to be replaced at some stage, the operator manually moves the table, exchanges thetool, and has to return the table to the beginning of the segment in which the interruption hasoccurred . In the absolute mode, the tool is automatically returned to the position . Inincremental mode, it is almost impossible to bring it precisely to that location unless yourepeat the part program

2. Easy change of dimensional data

The incremental mode has two advantages over the absolute mode.

1. Inspection of the program is easier because the sum of position commands for eachaxismust be zero. A nonzero sum indicates an error. Such an inspection is impossible with theabsolute system.

2. Mirror image programming (for example, symmetrical geometry of the parts) is simple bychanging the signs of the position commands.

Open Loop System

In an open loop system the machine slides are displaced according to the information loadedfrom the part program into the control system. Hence there is 14 CNC Machines nomeasurement of slide position and no feedback signals for comparison with the input signal.

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The correct movement of slide entirely depends upon the ability of the drive systems to movethe slide through the required exact distance. The most common method of driving the leadscrew is by a stepper motor. The stepper motors are the simplest way for converting detailelectrical signals into proportional movement. As there is no check on the slide position, thesystem accuracy depends upon the motors ability to step through the exact number of stepsprovided at the input as shown in fig.

Closed Loop System

A closed loop system is sends back a signal to the control unit from a measuring device calledas transducer. The transducer is attached to the slide ways. The signal indicates the actualmovement and position of the slides. The control unit continues to adjust the position of theslide until it arrives it’s destination, this system has feedback. Although more costly andcomplex than open loop system, these system gives more accurate positioning. For this typeof system, servomotors are used.

Control of Contouring Systems

In contouring systems the tool is cutting while the machine axes are moving. The contour ofthe part is determined by the ratio between the velocities, along the two axes. The control incontouring systems operates in closed loop. Therefore, a contouring system uses a cascadecontrol structure involving an inner velocity loop and an outer position loop for each feedaxis improved dynamic response. In such systems the interpolator generate reference signals(in form of a sequence pulses or position words) for each axis of motion, in a coordinatedmanner so that a desired contour is generated. Typical cascade control structure of contouringsystems is shown in Fig. 24.9. It uses an inner velocity feedback loops incorporating atachometer usually mounted directly on the motor shaft and an outer position feedback loopwhich is capable of measuring incremental (such as from an incremental encoder) or absoluteangular position of the leadscrew shaft (such as a resolver or inductosyn).

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In encoder-based systems each pulse indicates a motion of 1 BLU of axistravel. Therefore, thenumber of pulses over a period represents incremental change in positionover the period and the encoder pulse frequency is proportional to theaxis velocity. In such a system, fed from areference pulse interpolator the comparison is done by an up-downcounter which is fed by twosequences of pulses: reference pulses from the interpolator and feedbackpulses generated by the encoder. The counter produces a numberrepresenting the instantaneous position error in pulse units. This numbercan be converted by the DAC and fed to an analog position controlsystem. A typical electronic PLC function module board for CNC drives isdescribed below.

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