manufacturing technology (me461) lecture18

8/12/2019 Manufacturing Technology (ME461) Lecture18

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Manufacturing Technology

(ME461)

Instructor: Shantanu Bhattacharya


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Review of last lecture

Loading of program in CNC systems.

EIA and ASCII schemes of loading of program.

Punching system (odd and even parity checks).

Conventional numerical control.

Direct Numerical Control.

Distributed numerical control.

Programmable logic controllers. Logic control (Attribute type questions and

variable type questions)


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Logical ControlThe logical relationships can be expressed in a diagram form called a logic

network diagram whose symbols are expressed below.

The devices implementing thevarious logical relationships are

what we refer to as logic devices.

These logic devices are used in

constructing PLC

Although the logic network

diagrams could be used torepresent the logic in a PLC

control program, another logic

diagramming technique is known

as ladder logic diagram.These are more widely used

in the industries and they wereretained as PLCs got

introduced from a convenience

point of view of the technicians

who were involved in wiring the

various systems that were

being controlled.


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Ladder Logic DiagramThe ladder logic daigram makes use of representations similar to

electrical circuits in which a series of connections represents a

logical AND and a parallel connection represents a logical OR.A ladder logic diagram is made up of inputs and outputs connected

accroding to appropriate logic.

Each rung in the ladder represents a set of logical relationships

between the inputs that lead to a particular output.The output from one rung of ladder could be used as an input in

another rung of the same ladder.

Except when special provisions are made it is considered that all

rungs in a given ladder logic diagram are executed simultaneously,

so the order of the rungs on the ladder in general does not matter.


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Solved ExampleA robot is to be used to unload finished parts from a machine

onto an AGV and to load raw parts from the AGV to the machine.

Assume that there are sensors at the AGVs docking station to

indicate the arrival of the vehicle and onboard sensors on the

vehicle to indicate whether the vehicle has actually brought some

raw parts to be machined and whether the AGV has space to carry

away a finished part. Also, assume that there are sensors on themachine to indicate whether the machine is loaded with a part

and also a signal for the completion of part processing. The robot

is required to unload a processed part from the machine onto the

AGV, pick up a new part for processing from the AGV, and load it

onto the machine. The AGV is to be dispatched after the

completion of the cycle. Construct a ladder logic diagram.


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Solution

I/O Meaning/ Associated Action

01 AGV has arrived

02 AGV is carrying a new part to be

processed

03 AGV has space to store a

processed part

04 Machine has a finished part to

be unloaded

20 Unload old part from the

machine onto the AGV

21 Pick new part from the AGV and

load onto the machine

22 Dispatch the AGV


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Solution 1. The first rung states that if inputs 01, 03, and 04 are alltrue, then output 20 is true. This is interpreted as

meaning that if AGV has space to store a processed

part, and the machine has a finished 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 AND

input 02 is true), OR (input 01 is true, and input 04 is

not true, and input 02 is true), then output 21 is true.

This rung thus illustrates the use of OR in a ladder logic

diagram. The rung is interpreted as meaning that if the

machine has been unloaded, and the AGV is carrying anew part to be processed, then the robot should load

the new part from the AGV onto the machine. In this

case there are two scenarios in which the same output

may be obtained. Note in particular that output 20

from the previous rung is being used as an input on the

current rung.

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

station


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Counters and TimersCounters can be used in manufacturing to measure quantities such as production stock,

inventory, and packaging.

The counter accomplishes its task by counting voltage pulses, which can be generated 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 the production

stock can be counted with this technique.

With the timers the main difference is that they are used specifically to count clock pulses.

As a result, timers can be seen as clock driven whereas regular counters are 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 initial value

(usually zero).

Dependent on the intended application, the counter may have more than one output. For

example some counters may have CARRY (when the counter exceeds its maximum set limit)

or BORROW (when the counter goes beyond its lower minimum set limit).

Such counters can be connected in cascade fashion with a CARRY/BORROW from one

counter connected to the input of the next counter, increasing the overall range of

counting.


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Example Problem

During the powder metallurgy process, a punch is used to

press blended metal powder into a compact inside a die. A

push button is used to start the process. When the start

button is pressed, the die is filled with powder. The punch is

then advanced and it applies pressure to the powder for a

duration of 10s, after which it is retracted. The pressedcompact is then ejected from the die and the cycle repeats

itself. The cycle can be interrupted by pressing the stop

button. If the stop button is pressed the punch is required

to retract (if it had been advanced) before the process isstopped. We are required to construct a ladder logic

diagram.


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SolutionI/O Meaning

01 Start Button

02 Stop Button

T1 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 i


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SolutionWhen the start button is pushed to initiate the

process, or if a part has been ejected successfully

from the die, and if the stop button has not been

pushed, the die is filled with predeterminedamount of powder.

After the die is filled, the punch is advanced to

start applying pressure on the powder.

The pressing time is to last for 10s, after which

the timer resets itself.

After the 10s, or any time the stop button ispressed, the punch retracts.

After the punch retracts and provided the stop

button has not been pressed, the part is ejected.

After the part is ejected, the cycle repeats from

the beginning.

When ever the stop button is pressed the punchgets retracted and the cycle is stopped.


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Sample problemThis example illustrates the use of both the timing and

counting functions. Consider a production line in which

parts requiring processing are brought to a machine by aconveyor. A robot is used to load parts from the conveyor

onto the machine and, after the part has been processed,

unload the part from the machine and place it on the

pallet. The cycle time for processing each part is 10mins.

The robot is to palletize the parts by placing 125 parts on

each pallet. The parts are to be arranged on the pallet in

five layers of 25 parts each. Once the pallet is complete, itis dispatched and a new pallet is started. An allowance of

30 s is to be made for pallet dispatching and presentation

of a new pallet. Construct a ladder logic diagram to effect

the required control.


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SolutionI/O Meaning/ AssociatedAction

10 Machine has a part to be

unloaded

30 Load machine with a newpart

40 Unload a process part

from the machine

45 The current pallet layer is

full

48 Increment layer being

palletized by 1

50 The pallet is full-dispatch

60 Present a new pallet

C100 Up counter with a limit

set to 25

C200 Up counter with a limit

set to 5

T250 Timer with limit set to

600s


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Description of the various rungsNotice that no identifying

number for a counter or timer

has been used more than once.

This is a good practice as most

of the PLCs use the same

memory location for storing

counter and timer data.

Within the ladder logic

diagram, the reader should notethat a counter or timer function

is set up on one rung and then

references on another rung to

trigger its function.

Each of the counters and

timers has two inputs and oneoutput.

One input triggers counting/

timing and the other input

resets the device.


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Description of various rungs If the machine has a part to beunloaded and there is a pallet

onto which to place it, the part is

unloaded and placed on the

pallet.

After the part is unloaded or if

there were no part in the machine

to begin with a new part is loaded

onto the machine.

Timer T250 is used to time the

processing operation, and after

the 10min it takes to process the

part, the robot is signaled that

there is a part to be unloaded

from the machine, T250 resets

itself, and the cycle is repeated.

Counter C100 is used to countparts as they are unloaded from

the machine.

When C100 counts 25 parts a

signal is generated indicating that

the current layer in the pallet is full

and C100 is reset.

D i ti f i


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Description of various rungs Counter C200 is used to countthe number of full layers on the

pallet.

If the layer just completed is not

the fifth layer, pallet loading is

continued.When the no. of layers on a

pallet reaches 5, a signal that the

pallet is full is generated, the

pallet is dispatched, and a signal

is generated to present a new

pallet.Timer T300 is used to time the

dispatch and presentation of

pallets, for which a time of 30

secs. is allowed.

After the 30 secs, T300 resets

itself and C200 also resets. The

whole cycle can now be repeated

with the newly presented pallet.

G li d P C t l


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Generalized Process ControlWe now see the concept of computer control of multiple manufacturing processes.

By this we mean the use of a computer to monitor, control and coordinate the

operations of various types of physical equipment on the sop floor in a concurrent

manner.

This type of control is an extension of the capabilities of the logical control

associated with PLCs.

Specially, a control computer can be used to control several processes or pieces of

equipment at the same time.

This is in contrast with the PLC, which is dedicated to a single process, or the NC

controller which is dedicated to a single machine.

A control computer can be distinguished from the general purpose computer by its

ability to communicate directly with the process or equipment being controlled.

This capability for direct communication between the computer and process is

called on-line-control.

The main rationale for using a computer in manufacturing process is its response

speed to the changing process parametres.

manufacturing technology (me461) lecture18

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