A Programmable Logic Controller, or PLC, is a ruggedized computer used for industrial automation. These controllers can automate a specific process, machine function, or even an entire production line.
There are a few key features that set PLCs apart from industrial PCs, microcontrollers, and other industrial control solutions:
• I/O – The PLC’s CPU stores and processes program data, but input and output modules connect the PLC to the rest of the machine; these I/O modules are what provide information to the CPU and trigger specific results. I/O can be either analog or digital; input devices might include sensors, switches, and meters, while outputs might include relays, lights, valves, and drives. Users can mix and match a PLC’s I/O in order to get the right configuration for their application.
• Communications – In addition to input and output devices, a PLC might also need to connect with other kinds of systems; for example, users might want to export application data recorded by the PLC to a supervisory control and data acquisition (SCADA) system, which monitors multiple connected devices. PLCs offer a range of ports and communication protocols to ensure that the PLC can communicate with these other systems.
• HMI – In order to interact with the PLC in real time, users need an HMI, or Human Machine Interface. These operator interfaces can be simple displays, with a text-readout and keypad, or large touchscreen panels more similar to consumer electronics, but either way, they enable users to review and input information to the PLC in real time.
In today’s world of the Industrial Internet of Things (iIoT), and Industry 4.0 programmable controllers are called upon to communicate data via Web browser, connect to databases via SQL, and even to the cloud data via MQTT.
An All-in-One PLC integrates the controller with the HMI panel, creating a compact, easy-to-use automation solution. Users no longer need to establish PLC to panel communications and can program both the Ladder Logic and HMI design in a single software environment. An all-in-one approach saves time, reduces wiring, and cuts the cost of purchasing multiple devices.
A PLC program is usually written on a computer and then is downloaded to the controller
Most PLC programming software offers programming in Ladder Logic, or “C”. Ladder Logic is the traditional programming language. It mimics circuit diagrams with “rungs” of logic read left to right. Each rung represents a specific action controlled by the PLC, starting with an input or series of inputs (contacts) that result in an output (coil). Because of its visual nature, Ladder Logic can be easier to implement than many other programming languages.
“C” programming is a more recent innovation.
Some PLC manufacturers supply control programming software.
In addition to the traditional PLC described above, there are variations, including PLC + HMI controllers.