Think of a conveyor belt system in a bottling facility. Bottles must be filled, capped, and labeled in order. If the machinery is controlled by individual momentary push buttons for each function and an input signal is lost, the process will be interrupted. If the operator releases the button too early, the machine may stop in the middle of its cycle dumping the contents and making quite the mess.
This is where latching can help. Latching in PLC logic means that an output will stay true even after the input signal has been removed. This is very beneficial in keeping processes going without human interaction.
To address these challenges, I’ll explain the concept of latching when programming PLCs. I’ll touch on the theory, and give a real world example of latching.
What is Latching in PLC Programming
How Latching Works
There are two basic ways to perform latching in PLC programming:
- Parallel Output Latching
- Set and Reset Latching (Latch & Unlatch)
These are necessary instructions for a situation where a process needs to be initiated and will run without the constant intervention of the operator. Let’s look at how to code each type with some real world examples.
Method 1: Parallel Output Latching
In Parallel Output Latching involves connecting output coil to the input coil parallelly. This causes the circuit to form a loop which sustains itself.
Example: Parallel Output Latching in a Conveyor Belt System
Now, to add flesh to these bones, let’s take an example of a conveyor belt system in a manufacturing plant. This system has three steps:
1. Start the Conveyor: Triggered when a start button is pressed.
2. Run the Conveyor: The conveyor stays running until it is instructed to stop.
3. Stop the Conveyor: Triggered when a stop button is pressed.
Step-by-Step Implementation
Step 1: Define Inputs and Outputs
Firstly define each inputs and outputs in PLC Program:
- Start Button (Input) (I0.0)
- Stop Button (Input) (I0.1)
- Conveyor Motor (Output) (Q0.0)
Step 2: Create the Ladder Logic
Now you can add the ladder logic to get latching functionality
In this case, when the Start Button (I0.0) is pushed, it will turn on the Conveyor Motor (Q0.0). Output (Q0.0) is in parallel with input, so the input is always enabled as long as the Stop Button (I0.1) is Not Pressed. Releasing the Start Button (I0.0) will not turn the motor off. Once Stop Button is Pressed Output Turns OFF.
Explanation:
In this circuit:
- When the Start Button (I0.0) is pushed, it will enable the Conveyor Motor (Q0.0) and because the Conveyor Motor (Q0.0) is connected in parallel with the Start Button (I0.0), the motor will stay enabled.
- If the Start Button (I0.0) is released, the Conveyor Motor (Q0.0) will still stay enabled.
- When the Stop Button (I0.1) is pushed, it will disable the signal going to the Conveyor Motor (Q0.0) and the conveyor belt will stop.
Method 2: Set and Reset (Latch and Unlatch)
The second method to latch the output is using the Set and Reset technique (Latch and Unlatch). Here, we use two instructions.
1. Set (Latch): It activates the output and maintains its activation, even if the input signal that activated it, turns off.2. Reset (Unlatch): It deactivates the output regardless of the input signal.
Example: Set and Reset in a Conveyor Belt System
Continuing with the conveyor belt example, let's implement the Set and Reset method.
Step 1: Define Inputs and Outputs
Firstly define each inputs and outputs in PLC Program:
- Start Button (Input) (I0.0)
- Stop Button (Input) (I0.1)
- Conveyor Motor (Output) (Q0.0)
Step 2: Create the Ladder Logic
Now you can add the ladder logic to get latching functionality
Rung 1: Start the Conveyor
In this case, When start push button (I0.0) is pressed Conveyor Motor (Q0.0) sets to High.
Rung 2: Stop the Conveyor
In this case, When Stop push Button (I0.1) is pressed Conveyor Motor (Q0.0) Resets to Low.
Explanation:
In the example
- When Start push button is pressed (I0.0) the Set function latches the state from Low to High Keeping the Conveyor Motor (Q0.0) in ON regardless of input signal
- In Second Rung when push button is pressed (I0.1) the Reset function gets enabled and it changes the Conveyor motor state from HIGH to LOW.
Practical Considerations and Benefits
Latching in PLC programming offers several advantages:
- Better Process Control : Allows machinery to continue operating without the need of constant input signals.
- Greater Safety : Avoids operator mistakes such as accidental button releases.
- Greater Efficiency : Prevents unnecessary interruptions in automated processes.
As always, it is important to carefully design and test the latching logic to avoid undesirable behavior such as a machine running forever, should the reset condition fail.
Real-World Application: Packaging Industry
For example, take a packaging line in a food processing plant where individual products are sealed in bags. Once the sealing machine is started it must run continuously and the operator must ensure that all products are sealed before transferring to the next department. Here is an example of how latching can be used:
- Start Sealing: Operator pushes a
start button to initiate the sealing cycle.
- Run Sealing Machine: Operator does not
need to keep pushing start button as the machine will continue to run and seal
the bags.
- Stop Sealing: Operator pushes a
stop button once all bags are sealed.
Conclusion
If you’re looking to add extra control and reliability to your process, then latching is for you. Latching is commonly used in PLC programming and often creates a very robust method of controlling an automated process. Used correctly, latching will help your machine run nonstop without creating any unnecessary interruptions or errors.
From basic conveyor belt systems to more complex automated factories, latching offers a solution that easily maintains this continuous process. I hope the examples I have shown and the case studies within this guide have proved to you the practicality of latching and how it is implemented in PLC programming.
Productivity, safety and the general smooth running of any automated process are just a few of the benefits you can gain from understanding how to use latching. So the next time you’re writing a PLC program and you find yourself with a process that needs to remain active long after its original input has disappeared.
Q1. Can ChatGPT write ladder logic?
Unfortunately, no. Although ChatGPT can provide a very detailed and comprehensive text-based description of ladder logic, it does not have the capability to generate a visual ladder diagram. However, ChatGPT can assist you in generating the ladder logic by describing the rungs and symbols needed for the ladder in well-structured text form. You can then use this text-based description to draw the ladder diagram manually or enter the logic into the PLC program using your programming software.
Q2. What is Ladder Logic Diagram?
A ladder diagram is a type of flowchart that represents the logic needed to control a machine or machines in a factory. It is called a ladder diagram because it looks like a ladder with a number of steps or rungs. Each step represents a condition or action in the overall process.
The steps in a ladder diagram usually include inputs (such as switches, sensors, and valves) on the left and outputs (such as motors, relays, and lights) on the right. Various connections and contacts are represented by simple symbolic notations, making it easy for engineers to understand how the different parts should interconnect and work as an integrated automated system.
Q3. How to learn Ladder logic?
If you want to learn ladder logic, first of all understand what PLCs are and their functionality. Then, look for beginner friendly videos or books that can help you understand the basics of ladder logic. Use the simulation software to practice writing ladder logic programs and testing them.
Always start with small programs like lighting up a lamp and gradually move on to more advanced applications. Don’t forget to ask questions and reach out for help to online forums or communities. The most important thing is to keep practicing, experimenting and learning from your mistakes.
Q4: How to read Ladder Logic Schematics?
To read ladder logic schematics first learn all basic and advanced symbols used in ladder logic program, Understand how logic flow works, Writing Rules . After that you can easily read and Understand the Ladder Logic Schematics.
If you have any Questions, Comments, Please let me know in the below Comment Box .