PLC ladder logic for Level controlling



PLC ladder diagram logic program can be implemented easily by knowing the logic of the operation.


Process logic for level measurement is simple to know. There will be two sensors low-level sensor and high-level sensor. When the low-level sensor switch is high that means the tank is empty, the pumping motor turns ON and starts filling the tank. When the high-level switch turns ON it means that the tank is filled and the pumping motor turns off.

There are two operational modes at the level controlling Manual and Auto mode. Manual mode depends only on the low-level sensor, the High-level sensor doesn’t work for level control. In auto mode both the sensor work for level control as above mentioned.

There is an ON/OFF mode which does the basic option ON/OFF.

Ladder Logic:

Manual mode:

A manual operates only with the low-level switch. When the low sensor switch turns ON the motor turns ON.

And the motor operation is latched using unlatch coil. So the motor continues to work until it is switched OFF manually. And the tank starts draining until the low sensor switch turns ON. And the cycle repeats.

Auto mode:

In auto mode both the switches work together to control the level.

When the liquid level decreases below the low level, the low-level switch turns ON, now the motor turns on and starts filling the tank, the low-level switch is latched so that the process continues until the high-level switch turns ON.

When the desired setpoint is reached the High-level switch turns ON, the motor turns OFF and the water flows out from the tank, which is the only operation works all the time. The level starts decreasing, but the high-level switch is latched see the line 1 so that the level starts decreasing until the low-level switch is turned ON. Then the cycle repeats.

Boiler level control system