Programmable logic controllers (PLCs) have been a key part of industrial automation systems since their invention in the late 1960s. They consist of a special purpose real-time computer system interfaced to digital inputs and output, usually with the 24 volt standard used by sensors and actuators, often with relay outputs able to switch substantial current. Add-on modules provide analogue inputs and outputs, interfaces to serial communication and network connections, and other external devices.
PLCs are designed to be programmed by people without software development experience, and are often graphically programmed with ladder logic diagrams, which were originally developed as a means of specifying and documenting relay racks for electromechanical control. Programs can also be expressed in more conventional languages such as Structured Text, which can be translated back and forth to ladder logic.
PLCs are expensive, and many “makers” may say, “why not just use an Arduino and some sensors and relays?” That is, indeed, less expensive and more familiar to those used to programming, but the main advantage of PLCs is their great robustness and reliability (when failure of a PLC can shut down your automotive assembly line, you’re willing to pay for something that’s unlikely to break and can be easily replaced when it does). PLC manufacturers maintain rigid upward compatibility across their models, so there’s hardly ever a problem replacing a PLC with a newer model or deploying in-production programs to new installations.
In this episode of Tim Hunkin’s 2023 series on components, he explains why and how he uses PLCs in his machines and his experience buying used PLCs at bargain prices on eBay.