We will look at a PLC basic tutorial of a paint spraying station. Following the 5 steps to program development this PLC programming example should fully explain the procedure for developing the PLC program logic. Ladder will be our PLC programming language.
We will be using the Do-more Designer software which comes with a simulator. This fully functional program is offered free of charge at automation direct.
Define the task:
What has to happen?
Paint spraying system where boxes are fed by gravity through a feeder magazine one at a time onto a moving conveyor belt. Upon the start signal, boxes are pushed towards the conveyor by valve 1. This is a cylinder which extends and retracts which operates switches S1 and S2 respectfully. A spraying nozzle paints each box as it passes under the paint spray controlled by valve 2. A sensor (S3) counts each box being sprayed. When 6 boxes have been painted the valve 2 shuts off (paint spray) and valve 1 (cylinder) stops moving boxes onto the conveyor. Three seconds later the conveyor stops moving and the hopper with its load moves forward (valve 3) where it is emptied. Ten seconds later the hopper returns to the original position. The cycle is then complete and waits for a start signal again.
Define the Inputs and Outputs:
Inputs:
Start Switch – On/Off (Normally Open) – NO Stop Switch – On/Off (Normally Closed) – NC S1 – Valve 1 (cylinder retract) On/Off – NO S2 – Valve 1 (cylinder extend) On/Off – NO S3 – Box Detected- On/Off – NO Outputs: Motor – On/Off (Conveyor Run) Valve 1- Cylinder to feed boxes – On/Off Valve 2- Paint Spray – On/Off Valve 3- Cylinder to move hopper – On/Off
Develop a logical sequence of operation:
Sequence Table: The following is a sequence table for our paint spraying application.
1 – Input / Ouput ON 0 – Input / Output OFF x – Input / Output Does not Matter When the power goes off and comes on the sequence will continue. This means that we must use memory retentive areas of the PLC. The stop pushbutton will stop the sequence. The start will resume until the end.
Develop the PLC program:
The best way to see the development of the programmable logic controller program is to follow the sequence table along with the following program. You will see the direct correlation between the two and get a good understanding of the process.
This is the main process to start and stop bit. V0:0 is used because it is memory retentive.
Control of the Motor (Conveyor) and the paint spray is done with the V0:0 contacts in front of the actual PLC output. The conveyor and paint spray will stop when the timer 0 is done. This is the delay after the last box is detected to allow the box to be painted and loaded onto the hopper.
Control of the box movement onto the conveyor. As long as we have the process start and the hopper count is not complete this will allow the cylinder to put boxes on the conveyor.
Count number of boxes in the hopper via S3. The counter is memory retentive.
A timer to stop the conveyor and spray after the last box is detected for the hopper. This will allow time for the box to be sprayed and loaded into the hopper.
Hopper movement to load and unload the boxes.
The hopper unload timer is to unload the boxes and will then trigger the reset conveyor timer, box counter and the process start bit (V0:0).
Test the program:
Test the program with a simulator or actual machine. Make modifications as necessary. Remember to follow up after a time frame to see if any problems arise that need to be addressed with the program.
Watch on YouTube: PLC Programming Example – Paint Spraying
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Today, I will explain to you about the Ladder Diagram in PLC programming and different parts of the Ladder Diagram.
There are different standards programming languages defined for Programmable Logic Controller (PLC). Some of the internationally accepted standards are…
Generally, Ladder diagram is most popular all over the world (including India). This language is easy to learn and it recognizes and looks similar to the electrical circuit diagram.
So I will be focusing only on the Ladder Diagram for PLC.
In this article, I will explain to you more about the Ladder Diagram in PLC programming.
Let’s start with some basics introduction.
What is the Ladder Diagram?
The ladder diagram is the universal programming language of PLC. It has a short abbreviation as ‘LD’ and also known as ‘Ladder Logic’.
It is one of the oldest programming languages for PLC.
In the ladder diagram, the programming language that used to create the program to control the PLC system is known as ‘Ladder Diagram Language’ or ‘Ladder Logic Language’.
It has signified by the graphical representation, just like electrical wiring for logic control. (At the end of this article, I will share LD examples, that will get you a clear understanding.)
Different Symbols used in Ladder Diagram:
This programming uses different graphic elements. These graphic elements are also called as ‘Symbols’.
Basic Important Parts of Ladder Diagram in PLC Programming:
So let us see one by one.
Rungs
What is Rung Ladder?
In Ladder diagram, the horizontal lines called as ‘Rungs’. You can put as many numbers of rungs as per your project requirements.
The above diagram is shown with the ‘N’ number of rungs.
Plc Ladder Diagram Simple ExamplesBranches
There are three types of branches. They are as follows.
1) Series Branch:
In the series branch, inputs or outputs are connected in the series.
2) Parallel Branch:
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In the parallel branch, inputs or outputs are connected parallelly.
3) Nest Branch:
The combination of series and parallel branches in the same or different rungs are called as Nest Branch.
Inputs and Outputs for PLC Programming
For writing the program, Inputs and outputs are play most important role.
Input is Normally Open (NO) Contact or Normally Closed (NC) Contact.
Addressing Inputs and Outputs
Another next most important thing in PLC programming is addressing of inputs and output for writing the program. Various addressing schemes are used for the different brands of PLC.
Let us take examples from different Brands of PLC
Different Brands of PLC and their Addressing
I explain brands of PLC with their instruction.
1) ABB PLC:
The format for the digital I/O address is as below,
2) Siemens PLC:Plc Ladder Diagram Examples
The format for the digital I/O address is as below,
Consider, I/O address for the 1 Byte-
3) AB PLC:
The format for the digital I/O address is as below,
Consider, I/O address for the 1 Byte-
4) Delta PLC & Mitsubishi PLC:
In both PLC, the function’s addresses for input, output, and memory are the same. The format for the digital I/O address is as below,
Instruction:
The types of instruction used for PLC’s controlling purpose.
I explain the list of instruction –
What’s Next?
To understand these aspects more clear, check out the PLC programming example for Running Motor. I have explained it in detail.
Next, go through the rules for writing the PLC program using Ladder Diagram.
This is all from this tutorial about the introduction and different parts of the Ladder Diagram in PLC. I will explain further details in upcoming articles about PLC programming. Till now, if you have any question, you can ask in the comment section.
Myself Dipali Chaudhari. I am a master in Electrical Power System. Sharing my knowledge on this blog makes me happy. Apart from that, I love playing badminton. And sometimes I dwell on the Python programming.
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