PLC Learning series 15: Applications – Level measurement

Consider a simple application of water tank level measurement application using sensors. We put one near the bottom and one near the top, as shown in the picture below:

The final element is configured to be a highly reliable valve and to retain strong shut-off capability under all circumstances to maintain the integrity of the operation of the components. We need 3 I/Os (i.e. inputs/outputs) here. Three are the inputs (sensors) and one is the output (fill motor). Both of our outputs will be fibre-optic level sensors NC (normally closed). They’ll be ON when they’re NOT submerged in water. They’ll be OFF when they’re submerged in water.

We will provide an address for each input and output unit. It helps the plc to know where they are linked physically. The addresses are shown below:


Low level: 0000

High level: 0001


Motor (M): 0500

Internal Relay: 1000

Below is the real look of the schematic of the ladder. Notice that in this instance we use an internal utility relay. The contacts of these relays can be used as many times as necessary.

We are used twice here to describe a relay of 2 contact sets. Note, these relays do NOT exist in the plc actually, but they are bits in a register you can use to SIMULATE a relay.

It should be that replacing real-world relays is the most common reason for using PLCs in our applications. The relays of internal utility allow this action. How many internal relays are included with each plc product is difficult to say. Many of them include 100’s while others have 1000’s while others have 10’s of 1000’s.

The above-shown picture is of the PLC ladder logic program for water tank level gauge, now let’s see how the program scan line by line.

Scanning of the program:

Initially, the tank is empty. Therefore, input 0000 is TRUE and input 0001 is also TRUE:

The tank fills slowly as 500(fill motor) is on. The oil level rises above the low-level sensor after 100 scans and it becomes accessible. (That is, FALSE).

Remember that there is still a direction of true reasoning from left to right, even if the low level sensor is incorrect. That’s why we used a relay inside. Relay 1000 lets the production (500) lock. It will stay this way until there is no true path of logic from left to right (i.e. when 0001 is false).

After 1000 scans, the oil level is also open (i.e. false) above the high-level sensor.

Since the logic path no longer exists, output 500 is no longer energized (true) and therefore the engine is switched off. The water level drops below the high-level sensor after scans 1050 and it will again become real.

Remember that although the high-level sensor became correct, the true logic path is NOT continuous and therefore coil 1000 remains false.

The water level drops below the low-level sensor after 2000 scans and it will again become real. At this point, the logic will appear the same as above, and the logic will be repeated as shown above.

PLC Learning series:

PLC learning Series I: What is PLC? Functions of PLC
PLC learning series 2: PLC programming languages
PLC learning series 3: PLC Architecture and wiring of PLC
PLC learning series 5: 8 Rules for ladder diagram programming
PLC learning series 6: PLC process Scan basics
PLC learning series 8: Instruction List programming
PLC learning series 9: PLC selection criteria.
PLC learning Series 10: PLC timers
PLC learning series 11 : How to interface PLC with SCADA?
PLC learning series 12: Troubleshooting PLC
PLC learning Series 13: Counters in PLC
PLC learning series 14: Ladder logic Instructions
PLC Learning series 15: Water tank application







Instrumentation Engineer

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