Programmable Logic Controllers (PLCs) have a history that dates back to the 1960s, yet there are still many in theautomation industry who've had little experience with them. When it comes to learning about these products, or making the jump from other areas of automation to applications that involve PLCs, thetransition can sometimes be difficult. It's hard to know where to begin, and if you've been charged with the task of selecting one, it can be even harder to know which manufacturer or model to choose. Tomake the switch to PLCs, it's important to have a basic understanding of what they are, what they do, and which PLC is right for your application.
What are PLCs and how do they work?
PLCs areoften defined as miniature industrial computers that contain hardware and software that is used to perform control functions. A PLC consists of two basic sections: the central processing unit (CPU) andthe input/output interface system. The CPU, which controls all PLC activity, can further be broken down into the processor and memory system. The input/output system is physically connected to fielddevices (e.g., switches, sensors, etc.) and provides the interface between the CPU and the information providers (inputs) and controllable devices (outputs).
To operate, the CPU "reads" input datafrom connected field devices through the use of its input interfaces, and then "executes", or performs the control program that has been stored in its memory system. Programs are typically created inladder logic, a language that closely resembles a relay-based wiring schematic, and are entered into the CPU's memory prior to operation. Finally, based on the program, the PLC "writes", or updatesoutput devices via the output interfaces. This process, also known as scanning, continues in the same sequence without interruption, and changes only when a change is made to the control program.