288 CONTROL OF MACHINES

288 CONTROL OF MACHINES

switch .  In each zone ,  limit switches are provided for sensing the product in that particular

zone .  As seen from Fig .  11 . 51 there are five zones and LS1 to LS5 are provided in these zones .

There is an interlocking in the system due to which the conveyor cannot be started if a product is lying in any of the zones and has not been picked by the workers of that zone .  Another

provision in the control circuit is that if the product is desired in zone 1 then conveyor does not shift to fast speed but continues to run in slow speed .  Control circuit for the above system is

shown in Fig .  11 . 52 .

Explanation of the circuit is as follows :

(1) Suppose it is desired to take the product to zone 5 .  First of alk ,  the operator will set the selector switch to position 5 .  Terminal 5 of encoder thus goes low and binary coded decimal

number 0101 appears at the output terminal of the encoder .  This number also appears at the input of decade down counter .

(2) When operator presses the conveyor START-push button ,  terminal 1 of AND1 goes high .  If all the limit switches from LS1 to LS5 are in de-actuated condition ,  a high state appears

at terminal 2 of AND1 .  Its output goes high .  This causes terminal T of OS1 to go low .  Its output Q ,  which is connected to load terminal of decade down counter ,  also goes low .  Decade-down

counter thus gets loaded to 0101 (5) which is present at its input terminals .  Output of the down counter gets connected to the decoder .  Thus ,  decoder’s terminal 5 will be high .

(3) As soon as AND1 output goes high it also turns on FF1 .  Its output Q goes high and energises the main contactor of conveyor motor .  A high also appears at terminal 2 of NAND

gate but its terminal 1 is low so its output will be high and appear at terminal 1 of OR1 causing its output to go high .  Slow contactor gets energised through the amplifier and thus the conveyor

motor runs in slow speed .  After some time delay ,  the timer T output goes high and appears at

terminal 1 of NAND gate .  As both the inputs of the NAND gate are now high ,  its output goes

low .  OR1 is disabled as its terminal 2 is already low .  Slow contactor of the motor is de-energised

while the fast contactor gets energised and the conveyor runs at fast speed with the product

loaded on it .              

(4) When LS1 is actuated a low appears at CK terminal of the down counter and its output decreases from 0101(5) to 0100(4). Now the decoder output terminal 4 goes high .  As the

product on the conveyor proceeds further ,  limits switches LS2, LS3 and LS4 are actuated in succession . The decoder output terminals 3, 2 and 1 become high in succession . When terminal

1 of decoder becomes high ,  it appears at terminal 2 of OR1 and thus its output goes high .  Fast contactor is de-energised and slow contactor is energised .  Conveyor again shifts to slow speed .

When limit switch LS5 is actuated ,  fifth count pulse is delivered to the down counter and its output becomes 0000 and decoder terminal 0 becomes high .  Immediately trigger terminal T of

OS2 goes low and its output Q becomes high which resets flip-flop FF1 .  When FF1 resets ,  its Q goes low and conveyor main contactor M is de-energised .  Conveyor thus stops .  Now the product can be lifted from the conveyor for further work on it in zone 5 .

(5) Similarly the product can be taken to any zone by selecting the required position on the selector switch .

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11 . 26 PRODUCT DISPERSION CONVEYOR SYSTEM USING A

11 . 26 PRODUCT DISPERSION CONVEYOR SYSTEM USING A

DOWN-COUNTER ,  AN ENCODER ,  A MONO-SHOT AND A TIMER

The conveyor system ,  used for dispensing a product to any desired zone is illustrated in

                                                                                       Fig .  11 . 51 .

Fig .  11 . 51 Plan view of a product dispersion system

In this system ,  a product is loaded on the conveyor in the loading zone by the operator .

Depending upon the zone in which this product is desired ,  operator selects the zone number on the selector switch .  After selecting the zone ,  operator presses the conveyor START-push button . First ,  the conveyor runs in slow speed for a small time as set by a timer .  Then it runs in fast speed ,  and again it becomes slow when it reaches a zone prior to the zone set on the selector WITH REGARD

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STATIC CONTROL OF MACHINES 287

STATIC CONTROL OF MACHINES 287

encoder (terminal 3 being connected to a low). Active high input encoders may also be used from logic families which interpret floating input as low

Fig .  11 . 50 Combination of a 10-position selector switch ,  a decimal-to-BCD encoder and a decade down counter

In this text ,  active low convention has been followed . WITH REGARD

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286 CONTROL OF MACHINES

286 CONTROL OF MACHINES

11 . 25 ENCODER

An encoder is the inverse of a decoder .  In this ,  input is in the decimal form and output appears in a binary equivalent number .  There are various types of encoders ,  but in this text only 1-of-

10 decimal input to BCD output type is discussed .  Symbols for Encoders with active low inputs is shown in Fig .  11 . 49 ( a )

Fig .  11 . 49 ( a ) Symbol for 1-of-10 decimal encoder ( b ) Truth table for encoder In an active low input encoder any one of the lines 0, 1, 2, 3 etc .  going low is considered to be the desired input number .  When input 0 is low while all others are high ,  output of the

encoder is 0000, and when 1 becomes low while others are high ,  output becomes 0001 .  Truth table shown in Fig .  11 . 49 ( b ) indicates the output states for various inputs .  Generally ,  the encoders receive their input from a 10-position selector switch .  Selector switch can be set at any desired number by the operator and equivalent BCD number will appear at the output .

This output is then loaded into a down counter when its load terminal goes low .  Arrangement of selector switch ,  encoder and decade down counter is shown in Fig .  11 . 50 .

The encoder discussed here is supposed to belong to the logic family which interprets a floating input as High i . e . , when all terminals from 0 to 9 except 3 appear as high  WITH REGARD

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STATIC CONTROL OF MACHINES 285

STATIC CONTROL OF MACHINES 285

(2) In case the output ,  1 to 3 of the decoder goes high in succession ,  AND 1, AND 2 and AND 3 are enabled partially in succession ,  as their terminal 1 goes high in succession .

If low level limit switch of any of the silos is closed ,  then the corresponding AND gate will be fully enabled .  Let us assume in this case that lower limit switch of silos 2 is actuated and thus AND2 is enabled .  Its output therefore goes high and brings FF2 in set mode .  Now due to output Q of FF2 going high ,  three actions take place .

(i) Gate of main silos opens and conveyor also starts running with material falling from main silos on it .

(ii) Diverter 2 falls down so as to divert material from conveyor into silos 2 due to energisation of diverter 2 solenoid .

(iii) Q output of FF2 also appears at terminal 2 of OR2 and its output goes high which brings FF4 in set mode .  Thus Q of FF4 goes low and AND4 is disabled and counter does not receive count pulses .  The counter therefore freezes in its present

state .       

(3) When the level in silos 2 rises ,  lower limit switch opens ,  thus disabling AND2 but FF2

remains in set mode due to its memory ability .  Thus filling of material continues till upper level limit switch is actuated which resets FF2 .  FF2 output Q goes low which

disables OR1 and OR2 and de-energises the solenoid of Diverter 2 .  Disabling of OR1 stops the conveyor and main silos gate also closes .  Disabling of OR2 removes high state from the S terminal of FF4 .

(4) Also ,  when Q of FF2 goes low ,  it delivers a negative edge to the trigger terminal of OS2 causing it to fire .  Its output goes high for a few microseconds enabling OR3 .  The output of OR3 goes high and resets FF4 thus high state appears at terminal 1 of

AND4 and thus clock pulses again appear at CK terminal of the counter .  The counter starts counting from where it had stopped .

In the above circuit ,  observe the utility of using mono shots ,  1 to 3 for resetting FF4 .

Suppose ,  if FF4 is reset by the output Q of flip-flops from FF1 to FF3 then if FF4 is reset due to Q of FF2 going high it will remain in reset mode even if Q of FF1 or FF3 goes low as Q of FF2

will hold in reset mode .  Thus it is necessary to temporarily apply a high to the terminal R of FF4 when any of the flip flops ,  FF1 to FF3 goes into reset mode . WITH REGARD

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11 . 24 DOWN COUNTER

11 . 24 DOWN COUNTER

In a down counter every time a count pulse is delivered the number stored in the counter decreases by one .  This type of counter is especially required in circuits where an output signal

after a preset number of counts ,  and another signal earlier to a fixed number of counts ,  is desired .  Symbol and table showing states of down counter after each input pulse are shown in

Fig .  11 . 48 .  Down counters have input terminals DCBA ,  in addition to the output terminals ,  so as to preset a number into the counter in BCD form .  When the load terminal of the counter goes

low ,  BCD number at input is loaded into the counter and it appears at output terminals DCBA .

Assuming that number 9(1001) is present at input terminals when load terminal goes low ,  the output is preset to 1001 as shown in the table of Fig .  11 . 48 ( b ). When the first pulse appears at

CK terminak ,  output becomes 1000(8). At 9th pulse the output becomes 0000 .  At 10th pulse the counter again resets to 1001(9).

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284 CONTROL OF MACHINES

284 CONTROL OF MACHINES

Control circuit for the above system is shown in Fig .  11 . 47, and the explanation for the circuit is as follows:

(1) If the level in all the silos 1, 2 and 3 are above the upper limit ,  FF4 remains OFF .  Its Q output is high ,  and so ,  input 1 of AND4 is high .  At its terminal 2, clock is delivering

square-wave pulses and ,  therefore ,  the output from AND4 is also a square-wave of same frequency as the clock .  These pulses appear at CK terminal of decade counter

and it starts counting .

The counted number appears in BCD form at its output which is given to the decoder .

Thus ,  terminal 1 of decoder goes high first and then 2, 3 and 4 .  When 4th output goes high it appears as a negative going edge at trigger terminal T of OS4 due to a NOT gate in series .  OS4 fires for a few microseconds ,  applying a low signal to CL terminal of the counter ,  and counter immediately resets to zero .  Counter again starts counting from 1 .  This is because when count pulse appears through AND4, the clear pulse has

long departed from counter terminal CL .  This is the example of a mono shot resetting the counter and then removing the clear signal in time for the counter to resume

counting again . Thus the counter continuously counts from 1 to 3 WITH REGARD

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282 CONTROL OF MACHINES

282 CONTROL OF MACHINES

Mono shots are frequently used to reset a counter or a flip-flop in circuits where the conditions are such that a long term level change cannot be used to reset the counter or the

flip-flop (since the counter is to resume counting again after a short duration). Long term level change as compared to a small time change by a mono shot ,  is shown in Fig .  11 . 44 ( c ) . Suppose

the counter is to reset at instant A when Vin changes to low ,  and has to resume counting at instant B .  If Vin is used to clear the counter by negative going edge at instant A ,  then even at

instant B ,  the counter will remain reset and hence cannot resume counting .  Thus Vin cannot be used to clear the counter .  Instead ,  if a mono shot is triggered by Vin and its Q output is used

to clear the counter ,  then counters can be resumed at instant B ,  since by that time Q of the mono shot will be back to its high state after firing time tf (refer Fig .  11 . 44 ( c ) ). The above will

become more clear when a practical industrial circuit is discussed WITH REGARD

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STATIC CONTROL OF MACHINES 283 11 . 23 AUTOMATIC FILLING OF SILOS USING A CLOCK AND A MONO SHOT

STATIC CONTROL OF MACHINES 283

11 . 23 AUTOMATIC FILLING OF SILOS USING A CLOCK AND A MONO SHOT

The schematic diagram for the system is shown in Fig .  11 . 46 .

In this system ,  certain material is to be distributed in three small silos 1, 2, 3 which feed individual machines 1, 2, and 3 .

Material is fed to silos from the main silos through a belt conveyor and diverters .  Level of material in small silos is sensed by a lower level and an upper level limit switch as shown in the figure .  Filling of any of the silos starts when the level of material in it falls below certain level as determined by the position of the lower level limit switch .  When material in all the silos is above the upper levek ,  conveyor is at standstill and diverters are in up position . Gate of

main silos is also closed .

When level of material in any of the silos falls below the lower limit ,  the conveyor starts moving and gate of the main silos opens .  Diverter above the particular silos falls down so that

material is diverted into that silos from the conveyor .  When level rises to the upper limit ,  up limit switch gets actuated which causes the conveyor to stop, gate of the main silos to close ,

and the diverter to go up . In the diagram ,  diverter 2 is shown in down position and material is shown falling in to silos No .  2 .

In the diagram ,  the diverters are shown connected in series ,  so that only one silos may be filled at a particular time .

Gate of main silos and diverter 1, 2, 3 are operated by solenoid valves .  Special limit switches are used for sensing low and high level in silos .

Material for processing in machines can be taken out from the silos by manual operation of the gate of a silos by the operator of a particular machine . WITH REGARD

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11 . 22 CLOCK

11 . 22 CLOCK

The formal name of a clock in digital text books is astable multivibrator or free-running multivibrator .  A clock generates a continuous train of pulses which may be utilised to

synchronise various digital devices in an industrial circuit .  It may also be required to provide count pulse to an industrial system which does not generate count pulses naturally .  Symbol of

a clock is shown in Fig .  11 . 45 .

The frequency of the waveform is set by values of resistor ,  capacitor ,  and inductor which are internally connected to the circuit .  A lower frequency can be obtained by connecting a

frequency divider circuit as shown in Fig .  11 . 45 ( b ). A frequency divider divides the clock pulse by an integer and produces a lower frequency pulse train . Some systems require clock pulses

of different frequency in order to synchronise various events properly . WITH REGARD

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11 . 21 MONO SHOT

11 . 21 MONO SHOT

Mono shot is a very useful digital element used in industrial controls .  Its formal name is monostable multivibrator .  The symbol for a mono shot is shown in Fig .  11 . 44 ( a ).

It has two outputs Q and Q similar to a flip-flop . In this text ,  it is assumed that mono shot are triggered by a negative edge at trigger terminal T .  However ,  they can be triggered by

a positive edge also .  When one shot is triggered ,  the Q output goes high while the Q output goes low .

After a preset period of time ,  called firing time tf , the output Q returns to low and Q returns to high .  Triggering of the mono shot by a short pulse and the resulting output Q and Q

are shown in Fig .  11 . 44 ( b ). This way ,  a mono shot may be used as a pulse stretcher ,  or for delaying the negative edge of a pulse by a firing time tf . WITH REGARD

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STATIC CONTROL OF MACHINES 281

STATIC CONTROL OF MACHINES 281

(2) For loading ,  Start loading/unloading push-button is pressed .  Output appears at AND1

gate as its input terminal 2 and 3 are already high .

Terminal 3 is high as R-S flip-flop is in reset mode .  Output is fed back to terminal 1 for sealing the circuit .

(3) As soon as the AND1 output goes high ,  entry and exit door opens due to energisation of solenoids S1 and S2 .

(4) When both the doors are open ,  LS1 and LS2 get actuated and AND2 output goes high .

This output runs the furnace conveyor and loading conveyor by energising contactor C1 and C2 respectively .

(5) As the conveyors start ,  the castings placed on loading conveyor also move and are shifted over to the furnace conveyor .  Number of casting loaded on the furnace conveyor

is counted by de-actuation of limit switch LS3 installed on the end of the loading conveyor .  Every time LS3 is deactuated ,  CK terminal of units counter goes low and registers the count at its output-binary coded decimal form .  Decoder converts it into

decimal form .  At the second count the ouput of units selector switch will go high and appear at input terminal 1 of AND3 but no further action takes place as the other input terminal 2 of AND3 is low .

(6) Units counter will count upto 9 and when 10th count pulse appears ,  its output resets to 0000 from 1001 (9). Negative going transition of terminal D causes the tens counter

to register a count and its output becomes 0001 .

Thus a high signal appears at input terminal 2 of AND3 on the 10th ,  count ,  but now the input at terminal 1 of AND3 is absent as unit counter is reset .

(7) From 11th count ,  units counter again starts counting from 1 onwards .  At the 12th count pulse ,  common terminal of Decoder selector switch goes high .  Now both the

inputs of AND3 are high so its output becomes high .  This brings the R-S flip-flop in set mode and its output becomes high .  This causes the input terminal 3 of AND1 to

go low due to the presence of a NOT gate in series .  Thus AND1 output goes low .

Conveyors 1 and 2 stop immediately and furnace door starts closing due to deenergisation of door solenoids .

(8) As the output of AND3 goes high ,  a low signal appears at CL terminal of units and tens counter due to the presence of a NOT gate in series with output of AND3 gate . WITH REGARD

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280 CONTROL OF MACHINES

 

280 CONTROL OF MACHINES

Fig .  11 . 43 Control circuit for loading of castings in an annealing furnace WITH REGARD

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STATIC CONTROL OF MACHINES 279

 

STATIC CONTROL OF MACHINES 279

In many industrial applications ,  where a certain action has to be taken after a fixed ,  count ,  a ten position selector switch is connected to the output of decoder as shown in Fig .  11 . 41

( b ). If it is desired to initiate some action after a count of say 5, selector switch is placed in position 5 as shown in Fig .  11 . 41 .  After 5 number of count pulses ,  DCBA of counter will be

0101, which would also appear at DCBA terminals of Decoder .  Decoder terminal 5 will go high and a high output will appear at common terminal of the switch to initiate the desired action .

Here ,  the BCD number has been referred to the decimal decoder .  There are other types of decoders available such as BCD to seven segment ,  grey code to decimak ,  excess three code to

decimal etc .  In this text ,  the term decoder will refer to the BCD to decimal decoder .  Interested students may refer to a text book on digital electronics for study of inbuilt structure of a decoder . WITH REGARD

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11 . 19 DECODER

11 . 19 DECODER

A counter records pulses in binary coded decimal numbers .  But this information is not understood by a layman . A decoder is thus required which converts BCD number into decimal

 number .  Schematic representation of a Decoder is shown in Fig .  11 . 41 ( a ).

If the binary information represents the decimal digit 3 ( dCBA = 0011), the decoder drives the output line No .  3 high . WITH REGARD

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278 CONTROL OF MACHINES

278 CONTROL OF MACHINES

CK DCBA

1 000 1

2 00 10

3 00 11    

4 0 100

5 0 101

6 0 110

7 0 111

8 1 000

9 1 001

When clear terminal CL goes to its active low ,  all four bits clear to 0 .  The counter output becomes 0000 .

From the above table ,  it is clear that a decade up-counter counts upto 9 .  However ,  counting pulses greater than 9 is done by cascading the counters as shown in Fig .  11 . 40 ( b ). At the tenth

pulse all the bits DCBA of counter 1 clear to 0 .  As the terminal D of the unit counter is connected to CK terminal of counter 2, the negative going edge of D triggers the counter and its output

becomes 0001 .  Thus second counter registers ,  tens ,  and similarly ,  third counter will register hundreds .  For example ,  when 15 number of pulses have appeared the output of counter will be :

Counter 3 Counter 2 Counter 1 (hundreds) (tens) (unit)

BCD number 0000 0001 0101 Numerical number 0 1 5

i . e . , 00010101 = 15 WITH REGARD

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11 . 18 COUNTER

11 . 18 COUNTER

A digital counter can count the number of pulses appearing at its input terminal .  Counting ,  however ,  is done in Binary coded decimal number since the counter circuit is made of flip flops

and logic gates .  Schematic representation of a decade counter is shown in Fig .  11 . 40 ( a ).

Input count pulses apear at terminal CK and the count number in binary code appears at terminal DCBA with corresponding numerical values of 8, 4, 2, 1 .  Input count pulse number

versus the corresponding BCD output is given below: WITH REGARD

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STATIC CONTROL OF MACHINES 277

STATIC CONTROL OF MACHINES 277

Working of the circuit is as follows:

(1) If the Reject-button is pressed while the bad piece is in zone 1, the preset terminal of flip-flop FF1 becomes low and it goes into set mode ( assuming that the flip flop responds

to low preset signal). Its output Q1 becomes high and appears at input terminal J of FF2 .

(2) When the rejected piece leaves the zone 1 and enters zone 2, a limit switch is actuated .

The limit switch gives a low signal to all CK terminals .  Flip flop FF2 goes into set mode and its output Q2 becomes high while FF1, FF3 and FF4 get reset .  FF1 gets

reset due to its J terminal being grounded .  Thus we see that as the rejected part

moves into zone 2, the information about its non-acceptance enters flip flop 2 .

(3) When the rejected piece enters zone 3, limit switch is again actuated ,  by the piece

following it i . e . , by the one entering zone 2 .  Shift command thus appears at all the CK terminals .

(4) On application of signal to CK terminals ,  FF3 gets in set mode as its input is high (Q2 = 1), the information of a bad piece in zone 3 enters flip flop FF3 .  Similarly ,  when a

bad piece enters zone 4 the limit switch is again actuated and another shift command

occurs ,  which turns on FF4 .

                                 

 (5) When output Q4 of FF4 goes high it energises the solenoid which applies a push to the work piece .  Pusher moves forward and the bad piece is thrown into the reject

bin .

(6) When the next piece enters zone 4, FF4 gets reset if the part is good and the pusher immediately moves backward before the part can run into it . WITH REGARD

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11 . 17 CONVEYOR SYSTEM USING A SHIFT REGISTER

11 . 17 CONVEYOR SYSTEM USING A SHIFT REGISTER

Shift registers are widely used in industrial applications involving conveyor systems .  Each flip flop in shift register represents one zone on the conveyor system .  The state of a particular flip

flop, ON or OFF ,  stands for some characteristic of the piece in that zone .  The most obvious WITH REGARD

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276 CONTROL OF MACHINES ,

276 CONTROL OF MACHINES ,

example which can be represented in binary 1 or 0 is pass or faik ,  i . e . , either the piece passes inspection and is routed to the next zone or it fails inspection and is rejected .

Let us discuss a practical application of shift register in control of a production set-up in which evenly spaced parts come down a conveyor for inspection . Schematic representation of

the system is shown in Fig .  11 . 38 .

Assume that the conveyor is divided in four zones .  Inspection of the piece is done in zone 1 .  In zones 2 and 3 some work is done on the piece if it passes inspection ,  while the piece which

fails ,  is allowed to pass through unattended .  For identification of the rejection piece ,  it is marked with paint by an inspector of zone 1 for the benefit of workers of zones 2 and 3 .  A reject

push button is also pressed while the rejected piece is still in zone 1 .  When the piece reaches the fourth zone ,  a pneumatic pusher extends and pushes the rejected piece into the reject bin

while the good parts move past the fourth zone .

Control circuit for the above system is shown in Fig .  11 . 39 .  Shift register in the circuit has been used to keep track of the progress of a bad piece on the system and to operate the

pusher when the bad piece reaches zone 4 WITH REGARD

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STATIC CONTROL OF MACHINES 275

 

STATIC CONTROL OF MACHINES 275

                                                                                         

The schematic diagram of flip flop represents how pre-packaged shift registers are internally built however ,  they are illustrated in a different way as shown in Fig .  11 . 37 ( a ).

Most common length for pre-package shift registers are 4, 5 and 8 bits .  If a longer shift register is required ,  two or more small ones can be cascaded as shown in Fig .  11 . 37 ( b ). There

are many different types of shift registers but the basic function of all is to shift binary bits from one location to the next .  To avoid confusion only the type discussed above will be used

subsequently . WITH REGARD

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11 . 16 SHIFT REGISTER

11 . 16 SHIFT REGISTER

Shift register consists of JK flip flops connected in series which transfer their contents from one to another .  Schematic diagram of a shift register having four flip flops is shown in Fig .  11 . 36 .

Output of the first is connected to input of the second and so on . Clock and clear terminals of all are connected in parallel .

Working:

When a low clear signal is applied ,  all flip flops get reset i . e . , Q1, Q2, Q3, Q4 becomes low while

Q1, Q2, Q3, Q4 becomes high .  The state of the register is Q1, Q2, Q3, Q4 = 0000 .

Now suppose a high signal is applied to J terminal of the first flip flop FF1 and is maintained .  When clock pulses ,  also called shift commands are applied to the CK terminals of

all flip flops ,  then on negative edge of first pulse ,  FF4, FF3, FF2 remain in reset mode as their J input terminal is low .  Flip flop FF1, however gets into set mode and its output terminal Q1

becomes high .  Thus state of the register after first pulse becomes 1000 .  When second negative edge appears ,  FF2 will go into set mode as it has a high at its J terminal .  FF1 remains in set

mode since a high is maintained at its input terminal J .  FF4 and FF3 remain in reset mode as they have a low at their J terminal .  Thus after the second pulse the state of register becomes

1100 .  If high input at J terminal of FF1 is maintained ,  then on application of third negative edge to CK terminals FF3 will also go into set mode and the state of register becomes 1110 .

Similarly on fourth negative edge the state of the register becomes 1111 .  On subsequent pulses the state of register remains unchanged as 1111, if constant high is maintained at J terminal

of FF1 .  Registers continue to lose 1 from the extreme right flip flop while another 1 is added in the extreme left flip flop FF1 .  Hence ,  the information in the flip flops is shifted one place to the

right whenever a shift command occurs ,  while an external circuit keeps feeding 1’s into the leading flip flop FF1 . WITH REGARD

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274 CONTROL OF MACHINES

274 CONTROL OF MACHINES

(7) Output of AND3 also appears at input terminal 1 of AND4 after a small delay through timer T3 .  Output of AND4 therefore goes high and energises the solenoid to open the gate of the silos .  Thus ,  sand from silos starts falling on conveyor 1 after a small delay from the starting of conveyor 1 .

(8) Now ,  when output of AND1 goes low either due to pressing of OFF-push button or tripping of some over-load relay or actuation of sand hopper limit switch ,  the input at terminal 1 or OR1 goes low ,  but its output remains high as its input terminal 2 is

high (output from timer T1). Similarly ,  output of OR2 remains high due to a high

signal at its input 2 although its terminal 1 goes low .  Thus ,  conveyor 3 and 2 continue

to run . Conveyor 1, however ,  stops immediately when AND1 output goes low as input terminal 2 of AND3 goes low .  Silos gate also closes immediately as output of

AND4 goes low due to output of AND1 going low .

(9) As output of AND3 goes low ,  input to timer T2 becomes low .  Timer output goes low only after a delay of 2 minutes and makes the input terminal 2 of OR2 also low .  The output of OR2 thus also becomes low .  This leads to stopping of conveyor 2 after a

delay of 2 minutes from the stopping of conveyor 1 .

(10) As output of OR2 goes low ,  input of T1 also becomes low ,  and therefore its output

becomes low after 2 minutes .  This makes the input terminal 2 of OR1 low and its

output also becomes low .  As a result contactor 3 gets de-energised .  Conveyer 3 thus

stops after a delay of 2 minutes from the stopping of conveyor 2 . WITH REGARD

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STATIC CONTROL OF MACHINES 273

STATIC CONTROL OF MACHINES 273

Control circuit for the above requirements is shown in Fig .  11 . 35 .

Fig .  11 . 35 Control circuit for three conveyors

In this control circuit ,  timer elements T1 and T2 have been prefixed and post fixed by NOT elements to make the timer action equivalent to an OFF-delay timer with a normally

open delayed contact .  Students are advised to clearly understand the operation of an OFF delay timer ,  if they have any doubt ,  before proceeding further .

Operations of the control circuit is as follows:

(1) The conveyors can be started if the contact of the sand hopper limit switch LS1 is closed (i . e . , sand level is lower than the limit in the sand hopper) and all the overload

relay contacts are closed .  If the above two conditions are true then there will be high signals at input terminal 2 and 3 of AND1 .

(2) On pressing the ON-push button ,  a high signal appears at input terminal 1 of AND1

and its output goes high .  This output is fed back to terminal 1 for sealing .  High output from AND1 appears at terminal 2 of AND2, AND3 and AND4 .

(3) Output of AND1 also appears at input terminal 1 of OR1 .  Its output therefore goes high and energises contactor 3 to run conveyor 3 .  Output of OR1 also appears at

input terminal 1 of AND2 and its output becomes high and appears at terminal 1 of OR2 .  Output of OR2 thus also goes high and energises contactor 2 to run conveyor 2 .

(4) Output from OR2 appears at input terminal 1 of timer T1 and its output immediately goes high (off delay timer action) and appears at input terminal 2 of OR1 .

(5) Output from OR2 also appears at input terminal of AND3 and makes its output high to energise contactor 1 .

(6) Output from AND3 also appears at input terminal of timer T2 and its output immediately goes high and appears at input terminal 2 of OR2 . WITH REGARD

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270 CONTROL OF MACHINES

270 CONTROL OF MACHINES

In this machine ,  the work piece or the job placed on the table moves to and fro by rack and pinion arrangement mounted on the shaft of a squirrel cage motor .  The cutting tool is fixed while the job placed on the table is worked upon by movement of the table .  Movement of the table is controlled between two limits on left and right by limit switches 1LS and 2LS .

When table moves from left to right ,  tool works on the job while it remains ideal during right to left motion of the table .  At the end of right to left motion ,  tool gets feed for the next cut on

the job .  Various control requirements for the machine are as follows:

(1) The motor is to be started manually by pressing START-push button . Once the motor starts it is reversed automatically at the end of right or left stroke by limit switches

2LS and 1LS .

(2) There should be provision for jogging the motor by a Jog-push button .

(3) If the machine table is lying in between extreme positions ,  machine should fail to start .  Selection of initial direction of travel should be possible through right and left traverse push button ,  PBR and PBL .

(4) There should be a delay in starting the motor in left to right stroke so as to allow the tool to get the feed for a fresh cut on the job .

(5) The machine should stop on pressing of STOP-push button or on over-load tripping of motor .

(6) Interlocking of coolant pump motor (running) should be provided as a pre-condition for starting of the machine .

Control circuit for the machine adhering to the above mentioned control requirements is shown in Fig .  11 . 33

Sand is fed to conveyor 1 by opening the gate of silos which is operated by a pneumatic cylinder .  The gate opens only when all the three conveyors are running .  Conveyor 1 dumps

sand on to conveyor 2 which in turn dumps sand onto conveyor 3 .  Sand from conveyor 3 falls in the sand hopper .  Conveyor can be started and stopped through the ON or OFF push button

provided near the mould making machine .  Conveyors stop automatically when the sand hopper is full .  Various control requirements for this system are as follows:

(1) ON-push button is to start all conveyor motors in sequence from 3 to 1, i . e . , from the last to the first .

(2) Sand gate of the silos should open automatically when all the three conveyors are running .

(3) An over-load on any of the conveyors should stop all these conveyors .

(4) When sand hopper of the machine is fulk ,  a limit switch should actuate and stop all the conveyors .

(5) Stopping of the conveyors should follow the sequence 1, 2, 3 i . e . , from first to last .

(6) An additional requirement during stoppage is that there be a delay of 2 minutes between the stopping of subsequent conveyors ,  in order that the sand on the following conveyor might clear before it is stopped .

(7) A STOP-push button near the mould making machine should also be able to stop the conveyors . WITH REGARD

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11 . 14 CONTROL OF PLANER MACHINE

11 . 14 CONTROL OF PLANER MACHINE

The machine has already been discussed in the chapter on industrial applications of magnetic control .  Here it will be shown how the same machine is controlled using digital logic elements .

Schematic representation of a planer machine is shown in Fig .  11 . 32 WITH REGARD

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STATIC CONTROL OF MACHINES 269

STATIC CONTROL OF MACHINES 269

Working of the circuit is as follows:

(1) When push button for the lowest speed is pressed ,  memory element 1 goes into set mode ,  its output Q becomes high which appears at terminal 1 of OR1 .  Output of OR1 thus becomes high and contactor M is energised through the amplifiers .  Motor starts

with all resistances in rotor circuit at a slow speed .

(2) When the motor is desired to run in next higher speed ,  the corresponding push button is pressed which brings the memory element 2 in set mode .  Its output appears at input terminal 2 of OR1 and also at terminal 1 of OR2 .  Output of OR1 goes high and

energises contactor M while OR2 output goes high and appears at input terminal 2 of AND1 .  Besides energising contactor M ,  output of OR1 also appears at terminal 1 of AND1 .  Now both the terminals of AND1 are high ,  thus output of AND1 becomes high

and timer T1 is energised .  After a pre set delay ,  timer output goes high and energises contactor 1S .  Energisation of 1S shorts one bank of resistance R1 from the rotor circuit and thus motor accelerates to second higher speed .

(3) When the third speed push button is pressed ,  memory element 3 goes into set mode and its output goes high and appears at terminal 3 of OR1 and energises contactor M .  Then ,  as described earlier ,  contactor 2S gets energised after a fixed delay ,  set by

timer T1 .  When T1 has timed out its output energises contactor 1S and also appears at input terminal 1 of AND2 .  Output of AND2 goes high as its terminal 2 is already high due to output from memory element 3 .  Output of AND2 energises timer T2 and

its output goes high after a fixed delay .  This energises contactor 2S ,  which shorts resistance R2 from the rotor circuit .  The motor thus accelerates to the final i . e . , the third speed .

(4) When STOP-push button is pressed or over load relay trips the output of NOT1 goes  high which appears at reset terminal R of all the three memory elements and resets them .  This leads to the dropping of all the three contactors M ,  1S and 2S . WITH REGARD

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270 CONTROL OF MACHINES

 

270 CONTROL OF MACHINES

In this machine ,  the work piece or the job placed on the table moves to and fro by rack and pinion arrangement mounted on the shaft of a squirrel cage motor .  The cutting tool is

fixed while the job placed on the table is worked upon by movement of the table .  Movement of the table is controlled between two limits on left and right by limit switches 1LS and 2LS .

When table moves from left to right ,  tool works on the job while it remains ideal during right to left motion of the table .  At the end of right to left motion ,  tool gets feed for the next cut on

the job .  Various control requirements for the machine are as follows:

(1) The motor is to be started manually by pressing START-push button . Once the motor starts it is reversed automatically at the end of right or left stroke by limit switches

2LS and 1LS .       

(2) There should be provision for jogging the motor by a Jog-push button .

(3) If the machine table is lying in between extreme positions ,  machine should fail to

start .  Selection of initial direction of travel should be possible through right and left

traverse push button ,  PBR and PBL .

(4) There should be a delay in starting the motor in left to right stroke so as to allow the

tool to get the feed for a fresh cut on the job .

(5) The machine should stop on pressing of STOP-push button or on over-load tripping

of motor .

(6) Interlocking of coolant pump motor (running) should be provided as a pre-condition

for starting of the machine .

Control circuit for the machine adhering to the above mentioned control requirements

is shown in Fig .  11 . 33 WITH REGARD

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268 CONTROL OF MACHINES

268 CONTROL OF MACHINES

lower speed is desired ,  the stop button should be pressed before pressing the required speed push button . Recall that speed control for a wound rotor motor is obtained by

placing a resistance in the rotor circuit as shown in Fig .  11 . 30

In this starter contactor M is used to connect the stator windings of motor to the supply mains .  When contactor M is energised the motor starts with all resistances in the rotor circuit ,

and accelerates to its first speed .  For the second higher speed ,  contactor 1S closes and shorts resistance bank R1 from rotor circuit .  Third speed can be obtained by closing contactor 2S

which cuts R2 also from the rotor circuit .  This shorts the rotor winding terminals and the motor operates as a squirrel cage motor at third speed .  Control circuit for the above mentioned

operation is shown in Fig .  11 . 31 WITH REGARD

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11 . 13 SPEED CONTROL OF WOUND ROTOR INDUCTION MOTOR

11 . 13 SPEED CONTROL OF WOUND ROTOR INDUCTION MOTOR

Let us consider a wound rotor induction motor which is to run on three different speeds .  Separate

push buttons are used to select the speed of operation . The motor accelerates automatically to the selected speed .  For example ,  if the second speed is selected ,  the motor first starts running

on low speed and then accelerates to the second higher speed .  If the third speed is selected ,  the motor must start in first speed ,  accelerate to second speed and then accelerate to third speed .

The various control requirements for the control circuit are as follows:

(1) One STOP-push button to stop the motor regardless of the speed of rotation of the motor .

(2) An overload protection for the motor .

(3) Three separate push buttons to select the first ,  second or third speed .

(4) A 5 seconds time delay between acceleration from one speed to another .

(5) If the motor is in operation and a higher speed is desired ,  it should be obtained by pushing the respective push button . On the other hand if the motor is running  WITH REGARD

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STATIC CONTROL OF MACHINES 267

STATIC CONTROL OF MACHINES 267

(3) Closed contact of thermostat T3 will give a high signal to terminal 1 of AND2 while its other terminal 2 is already high due to closure of FS1 .  AND2 thus gives a high output which is applied to both OR1 and OR2 .  As one terminal each of OR1 and OR2

is now high ,  their outputs are also high .  Output from OR1 leads to energisation of contactor C1 and output from OR2 leads to energisation of contactor C2 through their repsective amplifiers .  Thus ,  compressors 1 and 2 will run when temperature in the

hall is above 28°C .

(4) When the hall temperature is below 28°C but more than 24°C ,  contact of thermostat T3 will be open while contacts of T1 and T2 are closed .  Due to open contact of T3 there

is low signal at terminal 1 of AND2 and therfore its output is low .  Output of AND1 is also low as closed contact of T2 gives a low signal at terminal 3 due to a NOT gate in

series .  In this case AND3 will have high output as its input terminal 1 has high signal from the closed contact of thermostat T2 .  It is to be noted that supply to terminal 1 of AND3 is taken prior to the NOT gate .  High output from AND3 goes to terminal 2

of OR2 which then gives a high output .  This output energises contactor C2 through

the amplifier .  Thus compressor 2 will run only when the temperature is above 24°C

but below 28°C .                          

(5) When temperature falls below 24°C contact of thermostat T2 opens and output of

AND3 will then go low due to a low signal at its input terminal .  The open contact of T2 will however give a high signal to terminal 3 of AND1 ( due to the NOT gate in

series). It will get switched ON as its terminal 1 and 2 are already high .  The high

output from AND1 then goes to terminal 1 of OR1 which then gives a high output to energise contactor C1 .  Thus ,  one compressor will run when temperature is below

24°C but above 20°C .  When temperature falls below 20° C ,  contact of thermostat T1

also opens and terminal 1 of AND1 goes low and it is switched off .  Thus compressor 1

also stops when temperature falls below 20°C .

Compressor 1 continues to run if temperature in the hall remains between 20-24°C .  If

due to more viewers in the halk ,  compressor 1 is unable to maintain temperature below 24°C ,

compressor 2 will be switched ON while compressor 1 will be switched off .  If the load is still

more and compressor 2 alone can not cope up and temperature goes above 28°C then compressor

1 will also start to bring down the temperature . WITH REGARD

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266 CONTROL OF MACHINES

266 CONTROL OF MACHINES

A three stage air conditioning system is used for conservation of electrical energy an optimum utilisation of the installed capacity of the compressor .  There are two compressor

motors in this system .  One motor is of smaller horsepower ,  and the second one is of higher horsepower rating .  These motors are designated a comp . 1 and comp . 2 in this case .  The system

is installed in a cinema hall for maintaining the hall temperature between 20-24° C .  Depending upon the number of viewers in the hall and the ambient atmospheric conditions ,  comp . 1 and

comp . 2 motors can be run in three different combinations .  The three combinations described below are also the control requirements of the air conditioning system .

(1) Compressor 1 and compressor 2 should run when the temperature in the hall is above 28°C .

(2) Only compressor 2 should run if temperature in the hall rises above 24°C but is below 28°C .

(3) Only compressor 1 should run if temperature in the hall is above 20°C but below 24°C .

A pre-condition for running any compressor is that chilling water flow switch FS1 should be closed .  Chilling water flow is necessary to take away heat from the compressed cooling gas .

Three thermostats with different settings are used for the control of compressor motors running in three different stages described above .  The three thermostats T1, T2, T3 are set at

20°C ,  24°C and 28°C respectively .

The control of three stages with three thermostats and flow switch (FS1) of air

conditioning system can be understood from the control circuit shown in Fig .  11 . 29 .  The STARTpush

button ,  STOP-push button ,  and over load contacts for compressor motors have not been shown in the circuit for sake of simplicity .

Working of the control circuit is as follows:

(1) When chilled water flow is maintained ,  flow switch FS1 will actuate and close its contacts .  Closing of contact FS1 causes application of a high logic signal to terminal 2 of all AND gates .

(2) When temperature in the cinema hall is above 28°C ,  contacts of all the three thermostats T1, T2, T3 close .  Closed contacts of T1 will give a high input to terminal 1 of AND1 but closed contact of T2 will give low logic signal to terminal 3 of AND1 as

there is a NOT gate in series .  The output of AND1, therefore ,  becomes low WITH REGARD

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11 . 12 CONTROL OF THREE STAGE AIR CONDITIONING SYSTEM

11 . 12 CONTROL OF THREE STAGE AIR CONDITIONING SYSTEM

A simple air conditioner system consists of a single compressor motor which gets switched off when temerature of the space being controlled falls below the setting on the thermostat .

 Thermostats are provided with differential setting to avoid frequent switching ON and OFF of the compressor motor .

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STATIC CONTROL OF MACHINES 265

 

STATIC CONTROL OF MACHINES 265

piston should move to the right until it actuates the limit switch 2LS .  Thus ,  at position 2, on actuation of 2LS the piston should immediately move backward to position 1 .  There should

also be provision to return the piston back from any point in its forward travel (from left to right). Control circuit shown in Fig .  11 . 28 fulfills all the control requirements listed above .

In this control circuit normally open (NO) contact of limit switch 1LS and normally closed (NC )  contact of limit switch 2 LS have been used .  A Reverse push button is used to bring back the piston to position 1 from any point in its forward travel .  Working of the control circuit is as follows:

When piston is in position 1, limit switch 1LS is actuated ,  its normally open (NO) contact is closed ,  thus providing a high signal to input terminal 2 of AND1 .  On pressing the START-push button a high signal appears at terminal 1 of AND1 .  Its output therefore goes high which appears at input terminal 2 of OR1 .  Thus ,  output of OR1 goes high which is fed bac to the terminal 1 of AND2 .  Terminals 2 and 3 of AND2 already have high signals as Reverse push button and contact of 2LS are closed .  Thus when START-push button is pressed ,  all three inputs to AND2 become high and it gives a high output .  This output is amplified and applied to the solenoid valve coil .  Valve shifts its spool and piston starts moving in forward direction from left to right towards position 2 .  Signal from output of AND2 is fed back to terminal 1 of

 OR1 for sealing ,  as the input at terminal 2 of OR1 will disappear as soon as START-push button is released .  Contact of limit switch 1LS will also open as soon as piston leaves the

position 1 .  The output of AND2 is maintained till limit switch 2 LS operates and cuts off the input at its terminal 3 .  Loss of one input to AND2 brings its output low which leads to deenergisation

of the solenoid .  As the solenoid valve is spring controlled the spool will come back to its original position and the piston will move back to position 1 .

The piston can also be made to reverse at any point from its forward movement by pressing the Reverse-push button . When the Reverse-push button is pressed the high signal at

terminal 2 of AND2 goes low which causes output of AND2 also to go low .  As output of AND2

goes low ,  the solenoid valve is de-energised and sealing/holding of circuit is also broken . The piston thus moves back to position 1 WITH REGARD

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264 CONTROL OF MACHINES

264 CONTROL OF MACHINES

gets applied to the primary of a pulse transformer connected to the base of UJT .  This pulse is transferred to the secondary side of the transformer and gets applied between gate and cathode

of SCR .  The SCR will trigger on and will start conducting if its anode is positive with respect to cathode .  To achieve current flow through load on both half cycles of ac voltage ,  a bridge rectifier

is used to provide a continuous dc supply to the SCR WITH REGARD

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11 . 10 OUTPUT DEVICES FOR SOLID STATE LOGIC

11 . 10 OUTPUT DEVICES FOR SOLID STATE LOGIC

The actuating devices like solenoids ,  motor starter coils etc .  can not be energised directly from gates as they are low voltage ,  low power devices .  Thus ,  an output amplifier is required between

the logic gate output and the actuating devices .  The purpose of the output amplifier is to increase the low-voltage low current logic power to high voltage high current output power .

Most of the output amplifiers are designed to drive 110 V or 220 V ac load ,  since most industrial solenoid valves and starter coils are designed for this voltage .  An output amplifier

using a reed relay is shown in Fig .  11 . 25 .  In this circuit ,  when the output from the logic circuit is high ,  transistor T1 will conduct and thus relay R will operate .  Its contact R will close and

energise the solenoid coil .  This type of arragement provides electrical isolation between the logic circuit and the output circuit

Modern output amplifiers are built using an SCR ,  triggered by a UJT .  A typical design of such output amplifier is shown in Fig .  11 . 26 .  Due to the closing of limit switch ,  when a high

input appears at RC potential divider ,  a capacitor starts charging .  At a particular value of capacitor voltage the UJT is turned on and capacitor gets discharged through it .  A pulse thus WITH REGARD

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262 CONTROL OF MACHINES

262 CONTROL OF MACHINES

In recent years ,  optically coupled signal converters have become very popular due to their light weight ,  low cost and more reliablility .  The optically coupled signal converters work

equally well on either ac or dc high voltage signals .  They are therefore also referred to as the universal signal converters .  In these converters no transformer or relay is required .  One type

of opto coupled signal converters ,  which is built using a light emitting diode and photo transistor ,  is shown in Fig .  11 . 24 .

When limit switch LS is open ,  LED does not conduct .  Photo transistor is cut off ,  while transistor T1 is conducting as its base to emitter is forward biased due to voltage appearing

across the photo transistor .  As transistor T1 is conducting ,  a low voltage appears at terminal Vout .  When limit switch closes ,  LED will conduct and emit light which would fall on the base of

photo transistor and it would start conducting .  Due to the conduction of photo transitor ,  WITH REGARD

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STATIC CONTROL OF MACHINES 263

STATIC CONTROL OF MACHINES 263

base to emitter bias for transistor T1 falls and it goes to cut off state .  When T1 is cut off ,  a high

 voltage i . e . , (+ 5 V) will appear at the output terminal WITH REGARD

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TATIC CONTROL OF MACHINES 261

 

TATIC CONTROL OF MACHINES 261

When the limit switch contact closes the following action takes place in the bounce eliminator switch:

(i) The NC contact of limit switch opens first ,  causing the number 2 input of NOR2 to be low .  NOR2 does not change state as its output 1 is still high .

(ii) The NO contact of limit switch closes momentarily on the first closure .  This puts a momentary high pulse on input 1 of NOR1 causing its output to go low .  The inverter drives the final output high .  The NOR1 output feeds input 1 of NOR2 and ,  therefore ,

NOR2 now has two inputs .  Its output therefore goes high which is applied to input 2 NOR1 .  NOR1 now has two high inputs .

(iii) When the NO contact of limit switch opens on first rebound ,  a low voltage appears at input 1 of NOR1, but its input 2 maintains its high voltage .  Therefore NOR1 does not change states and the final output remains high .

(iv) Several more bounces of limit switch contact takes place ,  which changes the voltage level of input of NOR1 .  However ,  NC contact of the limit switch does not close during

bounces and a high input is maintained on input 2 of NOR1 from the output of NOR2 .  Thus the output of NOR1 remains steady at high value .

(v) When the limit switch is released ,  the bounce eliminator reverses the above action resulting in one time transition of low level at the final output of the bounce eliminator

switch . WITH REGARD

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11 . 9 . 2 BOUNCE ELIMINATORS

 

11 . 9 . 2 BOUNCE ELIMINATORS
Bounce eliminators are manufactured using gates as shown in Fig .  11 . 22 .  In bounce eliminators ,  the output appears at its terminal on the first contact bounce ,  instead of waiting for the final
closure of limit switch contact as in capacitive filter circuits .  The disadvantages of this circuit is that it requires a double throw switch instead of a single NO contact .
When the limit switch contact is open as shown in Fig . 11 . 22 ( a ), high input is applied to R2 and also to input 2 of NOR2 .  The output of NOR2 is thus low causing input 2 of NOR1 to be
low .  Input 1 of NOR1 is also low because contact of the limit switch is open . Output of NOR1 is thus high ,  as both its inputs are low .  The output of NOR1 is then inverted to make the final
output low WITH REGARD
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11 . 9 . 3 Signal Converters

11 . 9 . 3 Signal Converters

The function of a signal converter ,  as described earlier ,  is to step down a high (110 or 220 V) ac or dc voltage fed to information gathering devices to a suitable low dc voltage for feeding to

logic elements .  Brief description of different types of signal converters is given below:

In a dc to dc signal converter ,  high dc voltage (110 V) from the information gathering devices is given to a potential divider circuit .  Low voltage (+ 5 or + 15 volts) is taken from the

potential divider as shown in Fig .  11 . 23 ( a ). This simple circuit can be improved by adding a transistor circuit to the voltage divider network to make the output insensitive to electric

noise and line disturbance .

A simple ac signal converter is shown in Fig .  11 . 23 ( b ). In this signal converter ,  electric isolation is provided between high voltage input circuit and low voltage logic circuit due to

magnetic coupling provided by transformer windings .  A center tapped transformer is used to step down ac voltage from the sensing switch ,  a limit switch in this case .  Step down voltage is

rectified and applied to a potenitial divider .  The output voltage from the potentiometer is fed to a logic element .  Transistor circuit can be used so that the output is not affected due to noise

and line disturbance .  The signal converter shown in Fig .  11 . 23 ( c )  uses a reed relay ,  to switch on and off the output logic supply to the signal converter output line .  When limit switch LS

closes ,  rectified dc voltage will energise realy RR .  Its contact will close and switch + 5 V to the logic gate .  The reed relay ,  in this case can provide electric isolation between high voltage and

low dc logic circuit supply .

As the above two types of signal converters use a capacitor to smoothen the dc voltage from rectifier ,  no separate bounce eliminator is needed .  The capacitor in the signal converter

takes care of the bouncing of switches and line disturbance .

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260 CONTROL OF MACHINES

260 CONTROL OF MACHINES

When the limit switch first closes ,  capacitor C starts getting charged through the venin

resistance of R1 and R2 .  As the limit switch contact stays closed only for a very short time during the bouncing action ,  the capacitor does not get charged to such an appreciable value as

to trigger the logic gates .  When permanent closure occurs the capacitor will get charged to the full voltage required to turn on the digital element .  This filter circuit also helps in arresting

high speed noise pulses on the load coming from the switch .  The final value of voltage appearing across the capacitor is less than the supply voltage due to the divider action of resistance R1

and R2 .  This ,  however ,  does not cause any problem ,  as digital elements can operate reliably with an input voltage less than the full supply voltage .  For example ,  for a TTL gate although

the high voltage signal is 5 V ,  it can operate with as low a voltage as 2 . 4 V WITH REGARD

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11 . 9 . 1 Capacitive Switch Filters

11 . 9 . 1 Capacitive Switch Filters

A capacitive switch filter is shown connected in between a limit switch contact and a logic gate in Fig .  11 . 21 . WITH REGARD

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STATIC CONTROL OF MACHINES 259

STATIC CONTROL OF MACHINES 259

The second statement is: Statement 1 will be true only if LS2 and T2 are open . The graphical picture for second statement is obtained by ORing inputs from LS2 and T2 followed by a NOT element as shown in Fig .  11 . 19 ( b ).

The next step is to combine the circuits for the above two statements .  This is done as shown in Fig .  11 . 19 ( c ) . Further ,  next step is to simplify the circuit ,  if possible .  In this case it is

done by using a NOR gate instead of an OR gate followed by a NOT gate .  The last step is to analyse the circuit whether it works according to the specifications .  According to Fig . 11 . 19 ( d ),

when PB1 is pressed ,  it sends a signal to OR gate through its signal converter .  There is an output from OR gate ,  which is fed through an amplifier for energisation of contactor M .

If LS1 and T1 are both closed ,  there are two inputs at the AND gate .  The third input is provided by the inputs from LS2 and T2 .  If these two are open ,  there is no input to the NOR

element ,  and therefore it will have an output which is the third input to the AND gate .  When all the three inputs are present at the AND element it will have an output .  The output of AND

gate provides an input to the OR element and thus there is an output for energising contactor M .  WITH REGARD

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11 . 9 INPUT DEVICES FOR SOLID STATE LOGIC

11 . 9 INPUT DEVICES FOR SOLID STATE LOGIC

In addition to signal converters mentioned in last section ,  another input device used in series with pilot and sensing device is the bounceless switch .  It is necessary to use this because

mechanical switches never make a clean contact closing in a single action . Most of the contacts

of pilot and sensing devices are snap action type i . e . , they are spring loaded .  When the movable

contacts meet the stationary contact ,  there is often a fast bouncing action . This means ,  the movable contact may make and break the contact three or four times in succession before the

 switch finally closes .  The action of contact bounce is shown in Fig11 . 20 .

As a result a series of pulses are generated till the final closure of contact takes place .

Each of the pulse generated due to bounce can act as a command .  As the digital elements are

very fast acting ,  they will respond to these commands and control circuit will malfunction . An

intermediate circuit is thus required which should not allow the bouncing action to affect the

control circuit .  Bouncing of contacts ,  however ,  does not pose any problem in relay circuits as

relays are relatively slow acting devices as compared to digital logic elements .  In digital circuits

the bouncing action of contacts can be prevented from effecting the control circuit by using

capacitive switch filters or Bounce eliminators . WITH REGARD

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258 CONTROL OF MACHINES

258 CONTROL OF MACHINES

The first step in developing a logic circuit is to convert the above specifications into logic statements .  The first statement is:

Contactor M will be energised when PB1 or LS1 and T1 are closed .  The above statement is translated into graphical picture as shown in Fig .  11 . 19 ( a ). The inputs from LS1 and T1 are

ANDed and then ORed with inputs from PB1 in the diagram WITH REGARD

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STATIC CONTROL OF MACHINES 257

STATIC CONTROL OF MACHINES 257

This amplifier has been used because output of AND gate is a low voltage dc and can not directly energise the output devices like contactor ,  solenoid ,  etc .

In Fig .  11 . 17 ( c )  an RS flip flop has been used for obtaining a holding circuit .  As STOPbutton

is always closed ,  there is always a low input at terminal R of flip flop due to a NOT gate in series with the STOP-button . When START-button is pressed ,  input at terminal S becomes

high and flip flop is set .  A high output appears at terminal Q which energises contactor M .

Output Q remains high even when input S goes to low value due to release of START – button .

When contactor M is to be de-energised or sealing of the circuit is to be broken ,  the STOP button

is pressed .  This applies a high input at terminal R of the flip flop and it gets reset .

Output Q becomes low and contact M gets de-energised .  Another holding circuit can be built using OR gate and AND gate as shown in Fig .  11 . 7 ( d ).

An important difference between a relay circuit shown in Fig . 11 . 17 ( a ) and a digital logic circuit shown in Figs .  11 . 17 ( b ), ( c )  and ( d ) is that in a relay circuit ,  current flow can be traced from line 1, through the control devices ,  contactor coil and back to line 2 and is easy to understand .  On the other hand ,  a digital logic circuit is only a block diagram of the control functions of the circuit .  In the logic circuit actual wiring is omitted and only symbolic representation is done .  The logic diagram ,  therefore ,  does not represent the actual circuit .  The common bus to which all input and output voltages are referred to is also omitted so as to avoid unnecessary cluttering of circuit .  Also ,  it adds nothing to the information given by the logic

diagram .

In the above discussed circuits ,  pilot devices like push buttons have been shown connected

directly to the logic elements .  However ,  in practice they are connected to the digital elements

through signal converters as shown in Fig .  11 . 18 .

Logic elements are low voltage low power devices .  They operate at say + 5 V dc (TTL).

The control components may be even 500 metres away from the circuit .  If the push buttons or

limit switches in control ciruits are directly connected through logic elements ,  there will be appreciable voltage loss .  Therefore ,  the practice is to feed high voltage i . e . , 110 V and 220 V ac

to the sensing device and then convert back to low dc voltage by signal converters for feeding to digital logic elements .  On the output side ,  output devices also work on high voltage .  Therefore  

the output from logic elements is amplified and then fed to these devices .

Before control circuits are discussed ,  it will be worthwhile to stydy an example on how the specifications for a control operation are first written in logic statments in English ,  and then converted into graphical logic diagrams .

The specifications for a control circuit are as follows:

A contactor coil M is to be energised whenever a normally open push button PB1 is pressed or whenever both limit switch LS1 and temperature switch T1 are closed and extreme

limit switch LS2 and extreme temeperature switch T2 are open . WITH REGARD

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256 CONTROL OF MACHINES

256 CONTROL OF MACHINES

To start with the three wire holding sealing control circuit which has so frequently been

used in the study of magnetic controls is taken . This circuit is shown in Fig .  11 . 17 ( a ). Equivalent

digital logic circuit for holding/sealing function are shown in Figs .  11 . 17 ( b ), ( c )  and ( d ). The same function is designed in three different ways by using different logic elements like OR

gate ,  AND gate ,  and MEMORY element .  At this stage there is no need of explaining the first circuit as the reader is already familiar with relay circuits .

Now refer to the logic diagram in Fig . 11 . 17 ( b ), one input is always provided to the AND gate through STOP-button . When START-button is pressed ,  the second and final input to

AND element is provided and output is obtained .  Once an output is obtained ,  the internal feed back loop maintains the input even though the START-button is released .  This is equivalent to

holding or sealing function of Fig . 11 . 17 ( a ). When the STOP-button is pressed ,  AND gate loses one input and so ,  the output goes to the low (O) value and the contactor M is de-energised .

Observe that an amplifier (OA ) is used in between output of the AND gate and contactor M . WITH REGARD

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11 . 8 DEVELOPMENT OF LOGIC CIRCUITS

11 . 8 DEVELOPMENT OF LOGIC CIRCUITS

Digital logic elements and their circuitry can be understood better by comparing them with their equivalent relay circuits .  However ,  once the fundamentals of logic design are understood ,

one should try to design the circuit directly using digital elements ,  instead of first developing the circuit with relays and then converting it into digital circuitry .  Developing circuit directly

in digital logic will avoid wastage of time and labour .  No universal theory can be given on how to approach the circuit design as circuit details vary considerably from job to job .  Inspection

and common sense are the most important tools for designing the control circuits .  However ,  a few suggestions are given here on how to approach the design of logic circuit for a specific

application . Design work is greatly simplified if the following steps are followed:

(1) Study the various machine operations and write them in their proper sequence .

(2) Identify the control devices used for controling each operation . Write down the relation between control devices and resulting operation in simple English logic statements .

(3) Draw logic diagram of each statement by translating the word picture of statement

into logic element block diagram .

(4) Combine the individual logic sequence to form a complete control circuit .

(5) Inspect the diagram ,  minimise circuits and save devices by simplifying circuits . WITH REGARD

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questionnair from m2pack company

Dear customer , welcome in m2pack company . we supply all packaging machine and materials  . 

we need To understand your purchasing requirements, and then better our services, please provide your opinions and suggestions by completing this questionnaire.
we hope 50 buyers will be selected to know your opinion in our company and need to improve our service as like service of sales and after services .
Thanks again for your reading , we hope take your opinion

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STATIC CONTROL OF MACHINES 255

STATIC CONTROL OF MACHINES 255

Relay equivalent for retentive memory is the mechanically held or permanent magnet

latch type relay .  In the memory element when input A = 1 (high) and B = 0 (Low), the output

X is high (1) and Y is low (0). This output state is maintained even when control supply is restored after a failure .  When input states change i . e . , A = 0 and B = 1, then output X becomes low (0) and Y becomes high (1). Considering the relay circuit for analogous operation it is observed that closing of contact A (equivalent to A = 1) will energise latch coil (LC ) , giving output at X through closing of its normally open (NO) contact .  Once the latch coil is energised  

it remains latched even when control supply fails .  Supply at X can be cut off by dropping the relay .  Relay drops when its mechanical latch is disengaged or the core is demagnetised by

energising another coil i . e . , ULC .  It is clear from Fig .  11 . 16 that coil ULC is energised when contact B closes .

This is equivalent to A = 0 and B = 1 for the static element .  Both A and B cannot be high

as it is the forbidden input and leads to unpredictable output (Race condition). WITH REGARD

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