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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