Answer / md ziaul haque

PLC is programmable logic control its good for on off control there is no level these days our science is very developed. They developed PLC all DCS feature is there

Answer / md.nayeemuddin

A programmable logic controller (PLC) or programmable
controller is a digital computer used for automation of
electromechanical processes, such as control of machinery on
factory assembly lines, amusement rides, or light fixtures.
PLCs are used in many industries and machines. Unlike
general-purpose computers, the PLC is designed for multiple
inputs and output arrangements, extended temperature ranges,
immunity to electrical noise, and resistance to vibration
and impact. Programs to control machine operation are
typically stored in battery-backed-up or non-volatile
memory.
As an example, say a facility needs to
store water in a tank. The water is drawn from the tank by
another system, as needed, and our example system must
manage the water level in the tank.
Using only digital signals, the PLC has two digital inputs
from float switches (Low Level and High Level). When the
water level is above the switch it closes a contact and
passes a signal to an input. The PLC uses a digital output
to open and close the inlet valve into the tank.
When the water level drops enough so that the Low Level
float switch is off (down), the PLC will open the valve to
let more water in. Once the water level rises enough so that
the High Level switch is on (up), the PLC will shut the
inlet to stop the water from overflowing. This rung is an
example of seal-in (latching) logic. The output is sealed in
until some condition breaks the circuit.


| |
| Low Level High Level Fill Valve |
|------[/]------|------[/]----------------------(OUT)---------|
| | |
| | |
| | |
| Fill Valve | |
|------[ ]------| |
| |
| |


An analog system might use a water
pressure sensor or a load cell, and an adjustable
(throttled) control (e.g. by a valve) of the fill of the tank.

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