Design a PLC program to control the liquid level in a tank. A level transmitter is used to measure the tank level into a standard current signal from 4 to 20mA as illustrated in the below process diagram.
Tank Level Control in PLC
PLC Program Logic
- The agitator motor is to be started and stopped by a pushbutton station.
- The solenoid valves are to be open (Energized) when the agitator is running.
- Pump N0.1 starts at the 75% liquid level and stops at the 25% level. The pump will run only if the agitator is running.
- On a high-level alarm (Tank 90%), an alarm light will come ON and stay on even if the tank level drops. An operator must press the reset button to turn off the light. On the high-level alarm, both the solenoid valve will also close (de-energize).
- A low-level alarm light shall be provided (Tank 10%), similar to the high level, and reset using the same pushbutton.
Scaling Tabular Column:
PLC Programming for Tank Level Control
PLC Program Description
Start/Stop PB latched with memory B3:0/0.
B3:0/0 enabled to turn on agitator (O: 0/0), Solenoid valve 1 (O:0/1) and Solenoid valve 2 (O:0/2).
Scaling block is used, Level transmitter gives output in mA (N7:0), to convert that into PLC range (N7:1), and scaling block is used.
Refer to the above tabular column for a respective range of values.
According to the program logic, we need to turn on the PUMP (O:0/3) at tank reaching 75%
And the pump still is in ON condition up to 25%, so comparator blocks are used to get this condition.
Agitator motor contact also connected in series as mentioned in program logic.
Memory coil connected to turn ON pump motor output.
Rung 0005 & 0006:
High-level alarm should turn ON when tank reaching 90% and should not turn off until alarm reset (I:0/2) is pressed.
Greater than or equal block is used to compare the real scaling value(N7:1) and register value.
Rung 0007 & 0008:
Low-level alarm should turn ON when tank reaching 10% and should not turn off until alarm reset (I:0/2) is pressed.
Less than or equal block is used to compare the real scaling value(N7:1) and register value.
We can use this example to understand the programming logic in Allen Bradley Programmable Logic Controller (PLC).
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