Question

a)Constract the use of electromechanical relays against a PLC in the implementation of an automatic solution, giving the merits of using the PLC.
b) Two digital outputs y and z need to controlled as follows:
Y turns on if inputs x1 and x2 are high or if input x3 is low and input x4 is high. Y stays on for 7s and then turns off, while simultaneously setting the output z to high. Z stays high for 5s, following which the sequence is repeated. The process is started by pressing a start pushbutton and is stopped by pressing a stop pushbutton.
Draw a ladder logic diagram to program a PLC for this application.

Answer 1

Ans) Electromechanical relays operate by using a coil which on getting energized by voltage. Once it is energized it will mechanically operate a switch. So here lot of process is involved to make a switch to operate where when we consider a PLC , turning ON and OFF a switch is just by applying voltage to the circuit. No coil is required and therefore PLC is more suited for implementing automatic solution when compared to electromechanical relay.

Merits of PLC

In Programmable Logic Controller, logic can be modified as per the requirements without much cost which is not the case with electromechanical relay where entire hardware needs to be modified therefore PLC is more suitable for implementing automatic solution.

b)

In the ladder diagram, when switch x1 and x2 are high or when switch x3 is low and x4 is high the timer TON (T4;0) is energised having a preset value of 7 seconds. As soon as the timer is energised and timer starts timing, the timing bit(TT) gets energised and remains till the accumulated value becomes equal to preset value.

This means the timing bit keeps the output Y high for 7 seconds and then turns off. When the timer accumulated value becomes equal to preset value the timer done bit gets energised leading to energising the second timer (T4:1).

The second timer T4:1 timing bit (TT) remains high for 5 seconds (preset Value) and keeps the output Z high for 5 seconds and this process is repeated again. The entire process is controlled by two switches Start push button and stop push button.