Manufacturing Technology (ME461) Lecture17

8/12/2019 Manufacturing Technology (ME461) Lecture17

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Manufacturing Technology

(ME461)

Instructor: Shantanu Bhattacharya


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Review of Previous Lectures

1.CNC programming using wordformat.

2.Example problems withdifferent geometries for CNC

codes.


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Example problem of NC Code


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Solution

5

4


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Solution

5

4


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Advanced Features With the progress in controller capabilities, several

advanced features are becoming available on NCmachines.

Among these are the following:1. Ability to take a portion of the program and execute it in

a rotated or mirrored position.2. Ability to scale the program and produce larger orsmaller features than those programmed.

3. Capabilities such as three dimensional circular

interpolation, helical interpolation, parabolic and cubicinterpolation for producing free form designs.

4. Ability to execute part of the program more than once.


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Loading the program For the NC program to be executed, it has to be loaded into

the MCU . One of the most widely used methods of program storage

among the various methods used in the 1in.-wide punchedpaper tape .

Punched tape uses a binary based representation code that isefficient for the machine to read and provides a compactstorage medium .

Numbers in the program are represented in a modified binaryform called binary coded decimal . Non numeric characters arealso assigned specific codes in this representation scheme.

The digit 1 in the BCD representation of a number isrepresented by a punched hole on the tape , whereas the digit0 is represented by the absence of a hole.


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Loading the program BCD is slightly less efficient for data storage than pure binary

but is easier for humans to interpret if need arises. Two standard coding schemes have emerged and are widelyused in industry.

One is the electronic Industries Association (EIA) standard

RS-244A and the other is the American standard code forinformation interchange (ASCII) standard. With each standard a character is represented by a series of

holes punched across the width of the tape .

Each character is represented by a specific pattern of holes .A total of eight tracks are available across the width of thetape for punching holes.


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Loading of the program In each coding scheme , one of the eight channels isreserved for checking the accuracy of the punchedinformation.This is done by using a scheme called parity check .The EIA scheme uses odd parity which requires every

row of the tape to have an odd number of punchedholes.If the character represented in that row in fact has anodd number of ones , nothing needs to be done.

If, however the actual no. of holes (or ones) for aparticular character is even, an extra hole is punchedin the parity channel so that this, together with theholes for the character , results in an odd no. of holes.


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Loading of the program

Thus, the system can make a rough checkon the accuracy of the punched data bycounting the no. of holes.For example, if an even number of holesis found in a given row of the tape in thiscase, it is recognized as an error.The ASCII scheme follows the same logicbut uses even parity .


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Conventional Numerical Control With the conventional numerical control, once the program is on tape, it is read intothe MCU by a tape recorder on the controller .The program is read and executed one block at a time .At the end of the program, the tape is physically rewound to get ready for machiningthe next component . (Remember the % and EOB as indicators)The tape then has to be stored if it is to be used for making more parts in the future .Perforated paper tape is fragile and can be difficult to handle and store conveniently,especially at a facility with large number of programs of different lengths.Another problem is that if a program has to be altered for example because ofprogram errors or engineering changes, the complete tape must be replaced.Once a program has been verified and there are no errors in it, a more durablematerial such as Mylar is sometimes used for the production tape.Another important aspect of conventional numerical control is that, in general, theMCU has only a limited range of capabilities .Most of the functionality of the controller is fixed in the way the controller is built; inother words, the various capabilities are hardwired and cannot be changed.

For example, some early NC controllers were capable of incremental programming butnot absolute programming; others were capable of linear interpolation but not circularinterpolation.The greater the range of capabilities desired in a given controller, the more expensive

it was to buy.


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Direct Numerical ControlIn the 1960s a solution to the problems associated with punched tape was found withthe use of a central time-sharing computer to store NC programs .The central computer was hardwired to the controller of one or more NC machines onthe shop floor.The part program would then be downloaded from the central computer directly to theNC machine, during the actual machining of a component, one block at a time .Several machines could be controlled simultaneously in this way because of the highspeed capabilities of the computer.This eliminated the need to use punched tapes .Among the other advantages a large no. of programs could be stored more convenientlyon the computer, and if a change in the program was required, it could be accomplishedeasily using a text editor. There were several disadvantages with this type of direct numerical control (DNC)

however.For example, if a problem developed in the central computer, all the machines wouldshut down.Also, the wiring between the central computer and the machines was vulnerable tosignificant voltage variations in the manufacturing environment , and the centralcomputer itself was very costly.


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Computer Numerical Control With the computer numerical control (CNC), a local dedicated microcomputer with itsown memory is used to control the operation of each machine .This is in contrast to the centrally controlled computer used in direct numerical control(DNC).Perhaps even more significant , the fall in cost of computing power that led to the adventof CNC also made it possible to write many of the MCU functions into the software thatcontrolled the MCU rather than having them physically hardwired into the design .This software called the executive program, is loaded into the CNCs memory andcontrols how a particular controller operates.The executive program of a CNC unit is analogous to the operating system of a computer.It thus became possible to provide far greater flexibility in the MCU by writing moresophisticated executive programs.For example the controller could be switched between absolute and incrementalprogramming, or linear and circular interpolation, rather than having these as hardware

functions that could not be changed.With the CNC approach, the NC program is read from a storage medium such as punchedtape and stored in the memory of the local computer.The program can then be executed by the MCU reading one instruction at a time frommemory, rather than from the original storage medium .With the use of computer diskettes, it became possible to store multiple programs more

conveniently.


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Distributed Numerical ControlDistributed numerical control is known by the same acronym as Direct NumericalControl.As mentioned earlier, the original concept of DNC was superseded with thedevelopment of CNC.In its current incarnation as distributed, DNC is an attempt to combine the bestfeatures of CNC and those of the original (i.e., direct: DNC).Modern DNC still uses a central time-sharing computer capable of communicatingwith several machines on the shop floor and storing part programs that are

downloaded to individual machines as needed.The important difference is that because modern NC machines have CNC capability,they have memory and therefore a complete program can be downloaded into thememory of the local computer of the CNC machine rather than one block at a time asin original DNC.Thus, the local machine can run autonomously and does not depend on the centralcomputer for execution for each step of the program .

The central computer need only play a supervisory role, monitoring the operationof the various machines, archiving programs, and downloading appropriateprograms to scheduled machines as needed .Thus, the individual machines can continue to operate even if the central computerfails.


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Programmable logic controllersUptil now we have seen the control of manufacturing at process level.

We were interested in the technical details of how the manufacturingoperation isaccomplished on a step by step basis.

There is a different level of control,however, at which our focus is not on thedetails of the process itself but rather on issues such as whether a given operationhas been successfully completed, whether the retinent conditions for the nextoperations has been satisfied, whether the parts required to fill an order have beenmade etc.

At this level of control we are more interested in governing the operation of thesystem than in technology.

The logical relationship between various parameters determine what the nextaction should be.

Logical relationships can be represented by logical operators, which in turn canbe implemented by using hardware components called logic devices.


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Logical ControllersPLCs are used widely used in manufacturing as a means of implementing logicalcontrol of manufacturing systems.Control actions are taken by making decisions depending on the valuesassociated with various inputs or variables and the control logic in the program.If a particular decision can be made by answering yes or no, go or nogo or 0 and 1(binary system) they are referred to as decision by attributes. These may includethe following:1. Is the machine turned on?2. Is the gate to the work cell open?3. Has the AGV arrived at the docking station? Etc.

If a particular decision cannot be made by answering yes or no to a question, itis referred to as decision by variables. Generally these types of decisions arebased on values that can vary widely. Examples include:

1. How deep is the hole?2. What is the cutting speed?3. What is the required surface finish?4. How many parts are to be made?

L i l C l


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Logical Control Attribute type questions can generally be considered as candidates for automationusing logical control, whereas variable type questions would be less amenable tological control.

A variable type question can be made suitable for logical automation, however, itcan be broken down into a series of elementary attribute type questions.

An attribute type question (having a yeas/no kind of answer) can always beexpressed as a logical variable and relationships between such variables constitutea logical expression and they are governed by the laws of Boolean algebra.

Three basic logical operator exist and these are the AND, OR and NOT asdescribed in the table below as a relationship between variables X and Y.

Relationship MeaningX AND Y Both variables are true

X OR Y Either one of the variable is true

NOT X The specified variable is not true


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Logical Control The logical relationships can be expressed in a diagram form called a logicnetwork diagram whose symbols are expressed below.

The devices implementing thevarious logical relationships arewhat we refer to as logic devices.These logic devices are used inconstructing PLC Although the logic network

diagrams could be used torepresent the logic in a PLCcontrol program, another logicdiagramming technique is knownas ladder logic diagram.

These are more widely used

in the industries and they wereretained as PLCs gotintroduced from a conveniencepoint of view of the technicianswho were involved in wiring thevarious systems that were

being controlled.


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Ladder Logic Diagram The ladder logic daigram makes use of representationssimilar to electrical circuits in which a series of connections

represents a logical AND and a parallel connection representsa logical OR. A ladder logic diagram is made up of inputs and outputsconnected accroding to appropriate logic.Each rung in the ladder represents a set of logicalrelationships between the inputs that lead to a particular

output.The output from one rung of ladder could be used as an inputin another rung of the same ladder.Except when special provisions are made it is consideredthat all rungs in a given ladder logic diagram are executedsimultaneously, so the order of the rungs on the ladder ingeneral does not matter.


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Solved Example A robot is to be used to unload finished parts from amachine onto an AGV and to load raw parts from the AGV tothe machine. Assume that there are sensors at the AGVs

docking station to indicate the arrival of the vehicle andonboard sensors on the vehicle to indicate whether thevehicle has actually brought some raw parts to be machined

and whether the AGV has space to carry away a finishedpart. Also, assume that there are sensors on the machine toindicate whether the machine is loaded with a part and alsoa signal for the completion of part processing. The robot isrequired to unload a processed part from the machine ontothe AGV, pick up a new part for processing from the AGV,and load it onto the machine. The AGV is to be dispatchedafter the completion of the cycle. Construct a ladder logicdiagram.


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Solution

I/O Meaning/ Associated Action

01 AGV has arrived

02 AGV is carrying a new part to beprocessed

03 AGV has space to store aprocessed part

04 Machine has a finished part tobe unloaded

20 Unload old part from themachine onto the AGV

21 Pick new part from the AGV andload onto the machine

22 Dispatch the AGV

S l ti 1 The first rung states that if inputs 01 03 and 04


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Solution 1. The first rung states that if inputs 01, 03, and 04are all true, then output 20 is true. This isinterpreted as meaning that if AGV has space tostore a processed part, and the machine has afinished part to be unloaded, then the robot should

unload the old part from the machine onto the AGV.2. The second rung states that if (input 20 is true ANDinput 02 is true), OR (input 01 is true, and input 04is not true, and input 02 is true), then output 21 istrue. This rung thus illustrates the use of OR in aladder logic diagram. The rung is interpreted as

meaning that if the machine has been unloaded,and the AGV is carrying a new part to beprocessed, then the robot should load the new partfrom the AGV onto the machine. In this case thereare two scenarios in which the same output maybe obtained. Note in particular that output 20 from

the previous rung is being used as an input on thecurrent rung.

3. The third rung contains the logic for dispatching the AGV after it arrives at thedocking station

C d Ti


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Counters and TimersCounters can be used in manufacturing to measure quantities such as productionstock, inventory, and packaging.The counter accomplishes its task by counting voltage pulses, which can begenerated by a sensor set to detect the event whose occurrence is to be noted.Every time a pulse is received the count is changed by one. For example theproduction stock can be counted with this technique.

With the timers the main difference is that they are used specifically to count clockpulses. As a result, timers can be seen as clock driven whereas regular countersare event driven.

A counter can be constructed to count up or down or both.Counters typically may have another input line to reset the counter to its initialvalue (usually zero).Dependent on the intended application, the counter may have more than oneoutput. For example some counters may have CARRY (when the counter exceedsits maximum set limit) or BORROW (when the counter goes beyond its lowerminimum set limit).Such counters can be connected in cascade fashion with a CARRY/BORROWfrom one counter connected to the input of the next counter, increasing the overall

range of counting.

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Example ProblemDuring the powder metallurgy process, a punch isused to press blended metal powder into a compactinside a die. A push button is used to start the process.Whn the start button is pressed, the die is filled withpowder. The punch is then advanced and it applies

pressure to the powder for a duration of 10s, afterwhich it is retracted. The pressed compact is thenejected from the die and the cycle repeats itself. Thecycle can be interrupted by pressing the stop button. If

the stop button is pressed the punch is required toretract (if it had been advanced) before the process isstopped. We are required to construct a ladder logicdiagram.

S l i


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SolutionI/O Meaning

01 Start Button

02 Stop ButtonT1 Timer (with a limit of 10s)

30 Fill die

31 Advance punch

32 Retract punch34 Eject part (i.e., compact)

35 Stop cycle

S l ti


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SolutionWhen the start button is pushed to initiatethe process, or if a part has been ejectedsuccessfully from the die, and if the stop

button has not been pushed, the die is filledwith predetermined amount of powder. After the die is filled, the punch is advancedto start applying pressure on the powder.The pressing time is to last for 10s, afterwhich the timer resets itself.

After the 10s, or any time the stop button ispressed, the punch retracts. After the punch retracts and provided thestop button has not been pressed, the part isejected. After the part is ejected, the cycle repeats

from the beginning.When ever the stop button is pressed thepunch gets retracted and the cycle isstopped.

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Manufacturing Technology (ME461) Lecture17