Incorporation Of Programmable Logic Controllers In Engineering: The Basics Of Ladder Logic

31 March 2015
 Categories: Industrial & Manufacturing, Articles

Share

Programmable logic controllers (PLC) have been around for over 40 years and rival other technology, like programmable automation controllers (PAC), due to their lower purchase cost, better user interface, ability to quickly execute commands and higher compatibility with simple machine controls. A PLC, like from Flodyne Incorporated, is a digital computer primarily used in automating the engineering and electromechanical processes of factory assembly lines and other industries. It functions using ladder logic, which will be explained in detail in this article. If you own a manufacturing plant that relies on electromechanical processes in the assembly line, dabbling in this field can help you enhance your plant.

The Basis of Ladder Logic

Although most people are still unfamiliar with ladder logic, it is actually one of the most rudimentary systems of control functions. Ladder logic relies on switch or relay contacts to implement Boolean expressions, which result in Boolean values of either true or false. Ladder logic is computed using specialized microprocessor-based devices, and can result in several different functions — logical functions, seal-in functions and more.

Ladder logic is better represented through ladder diagrams, which is a type of electrical notation that represents how the electromechanical switches and relays interconnect with one another. These diagrams resemble ladders in the sense that they are typically depicted by two vertical rails symbolizing supply power and "rungs," which are the horizontal lines representing control circuits. In ladder diagrams, the load device is almost always drawn at the right, and the wires are denoted with numbers for easy identification.

Scrutinizing the Ladder Diagram

Before setting the PLC up, it is important you take a good look at the ladder diagram. You will need to familiarize yourself with electrical notations in order to better understand the entire system. Each ladder diagram should be reviewed by several electricians in order for errors to be caught early on. Through the ladder diagram, you will be able to, at the very least, determine:

  • the type of motor control circuit being used. Depending on the type of load being handled, you will need to determine whether the motor phase being recommended will do a sufficient job. 
  • the operation of the control circuit and where it is located. The control circuit must have a relay and a seal-in contact. When the relay pulls in, it will cause the seal-in function to kick in. The seal-in function is responsible for sealing in the monetary start pushbuttons, so the motor will not stop even when the pushbutton is released. The ladder diagram will need to include information regarding the path to be developed around the switch, so the current can continue to flow.
  • the ideal location for temperature sensors and pressure switches to be installed in. These devices are responsible for providing input into the ladder logic circuit.
  • where timers and counters may be needed. These features are considered as enhancements and are installed for a more complex logic circuit.
  • which supply powers need to be connected with which control circuits for optimal efficiency in operation. 

The ladder diagram will also include information like the amount of current or voltage that will be supplied. By taking a good look at the ladder diagram, it is easy to determine how the electromechanical engineering part of the machine or assembly line will function. You can better detect energy inefficiencies, remove redundancies, improve and enhance overall operation productivity and integrate additional safety mechanisms that will reduce workplace risk in the event of machine failures.

Conclusion

With an efficient ladder diagram drawn up, you can easily understand how your assembly line will work before it is even set up. Learning ladder logic and how to read a ladder diagram will be beneficial for your business in the long run.