Ladder logic is one of the most popular PLC programming language used by electrical engineers. The inputs, outputs, and logic functions used in ladder logic are similar to those used in electrical circuits, making it easy for electrical engineers and technicians to understand and use. This article will provide an overview of the basics of ladder logic, including its components, functions, benefits, and limitations. By the end of this article, readers will have a better understanding of ladder logic and how it can be used in industrial automation systems.
What is Ladder Logic?
Ladder logic is a programming language in the form of a graphical representation of a control circuit that is made up of several rungs, where each rung contains a combination of inputs, outputs, and logic functions that define the operation of the control system.
In ladder logic programming, the term “rung” refers to the horizontal lines that make up the diagram. These rungs can be thought of as individual circuits that are activated when certain conditions are met.
Each rung consists of two vertical rails that represent the power supply and the ground, and a series of horizontal lines that connect the inputs and outputs of the control system. Take a look at the below diagram, it resembles a ladder and that is why it is termed as a ladder diagram.
How to read a ladder logic diagram?
To read the ladder logic diagram, you must be aware of the symbols of logic instructions that are used in PLCs.
The inputs in ladder logic are represented by normally open (NO) or normally closed (NC) contacts. A normally open contact represents a switch that is open when there is no signal present, and it closes when the signal is applied. Conversely, a normally closed contact represent a switch that is closed when there is no signal present, and it opens when the signal is applied.
The output in ladder logic are represented by coils. Coils are used to control devices such as motors, solenoids, and valves. When a coil is energized, it activates the device that it is connected to.
In addition to inputs and outputs, ladder logic includes various logic functions that are used to control the operation of the system. These functions include AND, OR, NOT, and XOR. AND and OR functions are used to combine multiple inputs, while the NOT function is used to invert a signal. The XOR function is used to create exclusive OR logic.
How to draw a ladder diagram?
To create a ladder logic diagram, a programmer must first determine the inputs and outputs of the control system, as well as logic functions that are required to control the system. The programmer then creates rungs by connecting the inputs and outputs using the appropriate logic functions. The rungs are then connected in series to create a ladder diagram that represents the entire control system.
Let us make a small ladder logic.
Ladder logic example –
When we press the start push button the motor should get ON. When we press the stop push button the motor should get OFF.
While making a ladder logic diagram first and foremost step is listing down the I/Os. In the above example, we have two inputs and one output.
The condition is that – when we press the start push button the motor should get ON. For the start push button, we would take NO contact in PLC. When normally open contact receives a signal the NO contact will become closed.
The other condition is when we press the stop push button the motor should get OFF. Here, we will take NC contact in PLC. When normally close contact receives a signal it will become open and would interrupt the entire circuit. The motor relay is output so we will take a coil to represent it. Take a look at the below figure.
Moving further, with the above ladder logic the motor would remain ON till you retain the start push button in close position. Once you release it, the motor would get stop which we do not want. Our condition is that the motor should remain ON till we press the stop push button. So, we would add a latch of the motor relay with NO contact.
Now the motor would remain ON even though we release the start push button. It would remain ON through the latch as shown in the above diagram with green lines.
Advantages of Ladder Logic
- One of the benefits of ladder logic is that it is easy to read and understand. Because it is a graphical language, the logic diagrams can be easily interpreted by operators and maintenance personnel. This can help to reduce the amount of training that is required for this personnel, and it can also make it easier to troubleshoot problems with the control system.
- Another benefit of ladder logic is that it is flexible and can be easily modified. Changes to the control system can be made by adding, deleting, or modifying rungs in the ladder diagram. This can help to reduce downtime and increase productivity in the manufacturing process.
- Ladder logic is also a cost-effective solution for control systems. Because it is a graphical language, it does not require the use of expensive software or hardware. Additionally, the programming can be done by maintenance personnel or other non-specialists, which can help to reduce the cost of programming and maintenance.
Disadvantages of Ladder Logic
- Despite its benefits, ladder logic does have some limitations. One of the main limitations is that it is not well-suited for complex control systems. As the control system becomes more complex, the ladder diagram can become difficult to read and maintain. Additionally, ladder logic is not well-suited for systems that require high-speed processing or real-time control.
Summarizing
In conclusion, ladder logic is a programming language that is widely used in the field of industrial automation. It is a graphical language that is used to create logic diagrams that control various processes in manufacturing and other industries. Ladder logic is easy to read and understand, flexible, and cost-effective, making it an ideal solution for many control systems. However, it does have limitations and is not well-suited for complex control systems or systems that require high-speed processing or real-time control.
