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
Rung 0000:
Start/Stop PB latched with memory B3:0/0.
Rung 0001:
B3:0/0 enabled to turn on agitator (O: 0/0), Solenoid valve 1 (O:0/1) and Solenoid valve 2 (O:0/2).
Rung 0002:
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.
Rung 0003:
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.
Rung 0004:
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.
Conclusion
We can use this example to understand the programming logic in Allen Bradley Programmable Logic Controller (PLC).
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