Question

Packing three articles in a manufacturing process. A manufacturing process packs three articles in a box. Using a counter develop a ladder logic circuit that meets this requirement. Construct the mnemonic program and simulate this system using the internet TRiLOGI software. Explain the operation of the ladder logic circuit.

Answer 1

For this application, we use S7-300 PLC and TIA portal software for programming. We can implement this logic by using other PLC also.

Network 1 :

In the first network, we used latching circuit for cycle ON. Here we used to START PB (I0.0 ) to start the cycle and STOP PB (I0.1) to stop the cycle.

Network 2 :

Here when an empty box is detected (I0.3) and the cycle is ON, conveyor 1 (Q0.0) will be ON.

Network 3 :

The counter is used to count the objects moving on the conveyor 1. Objects are moving on the conveyor 1 (Q0.0) and mounted proximity sensor (I0.2) is detecting the objects. Now counter counts in an incremental way. Total counted objects will be stored in the memory word or register (MW10).

Network 4 :

The counter is used to count the objects moving on the conveyor 1. Objects are moving on the conveyor 1 (Q0.0) and mounted proximity sensor (I0.2) is detecting the objects. Now counter counts in an incremental way. Total counted objects will be stored in the memory word or register (MW10).

Network 5:-

When three boxes completely packed in the box, the counter output will be ON and as per logic relay coil (M0.2) will be latched for timer operation.

Timer instruction is executed when the relay coil (M02.) is activated.

Network 6:-

The box is filled with three objects or counter output is ON and the empty box is not detected, conveyor belt 2(Q0.2) will be activated.

When objects are moving on the conveyor 1, counter will count the objects. We need to pack three objects in the box. So when a box is filled with three objects, the timer will stop the conveyor belt 1 and conveyor belt 2 will start automatically.

Here we have not considered all interlocks for simplicity. Belts synchronization is assumed because we have used timer-based synchronization.

Program to stimulate the system using Internet TRiLOGY software:

Ladder Logic Programming

Ladder logic is a programming language that is used to program a PLC (Programmable Logic Controller). It is a graphical PLC programming language which expresses logic operations with symbolic notation using ladder diagrams, much like the rails and rungs of a traditional relay logic circuit. It is used by engineers and electricians to execute logical, sequential, counting, timing and arithmetic tasks in order to carry industrial automation applications.

The first thing you will see when you create a new piece of ladder logic is two vertical lines. It is in between these two lines your ladder logic goes. When you draw ladder logic, you will draw vertical connections between these two lines. Each of those is called a rung. Just like on a physical ladder.

In these rungs, you can put any of the ladder logic symbols to create the logic you want. As you can see above, I have put numbers on each rung. This is to understand how the PLC will execute the ladder logic. You may be familiar with the PLC scan time or scan cycle.
One rung at a time.

This might be one of the most important rules of ladder logic. The PLC only executes one rung at a time, then executes the next. In fact, the PLC only executes one symbol at a time.