PLC

In a factory, something constantly needs to be switched. A conveyor belt should stop when a crate arrives at the end. A pump should start when a tank gets too empty. It is exactly these kinds of decisions that a PLC makes. This is a small, highly resilient computer permanently installed in a machine or in a control cabinet. It receives measurements from sensors and switches, compares them against the rules in its program, and then switches motors, valves, or lamps accordingly. The German term for this is speicherprogrammierbare Steuerung, abbreviated SPS.

Why factories grind to a halt without these boxes

Almost everything manufactured industrially passes through a PLC at some point. Beverage filling plants, elevators, sewage treatment plants, roller coasters, wind turbines, traffic light systems: such controllers are found everywhere. Several hundred million devices are in use worldwide. If even a single one fails, an entire production line can come to a stop.

The reason for this widespread use is reliability. A PLC is built to run for years without being restarted. It withstands heat, dust, vibration, and voltage fluctuations. It often has no fan and no moving hard drive, since both could break. A normal office computer would fail in this environment after just a few months.

Also important is reliability in time. A PLC not only reacts quickly, but above all predictably quickly. Experts call this real-time capability: the response is guaranteed to arrive within a fixed time span, usually a few milliseconds. For a press that must evaluate an emergency stop switch, this is exactly what matters.

The cycle of reading, computing, and switching

A PLC operates in an always identical loop, the cycle. First, it reads in all inputs, meaning the current values from sensors, buttons, and limit switches. Then it runs through its program once from top to bottom. Finally, it writes the results to the outputs, for example switching a relay. After that, it immediately starts over from the beginning, often several hundred times per second.

A PLC is rarely programmed in languages like Python. Widely used is ladder logic, a graphical representation that looks like a wiring diagram with contacts and coils. This has historical reasons: before the PLC, controllers were built from hard-wired relays. Electricians were meant to be able to keep their familiar way of thinking. Alongside this, there are text-based languages and function block diagrams, all standardized in IEC 61131-3.

A common misconception is that a PLC is simply a cheap small computer. A Raspberry Pi has more computing power than many controllers. The difference lies not in speed but in guarantees: rugged inputs and outputs for industrial voltages, an operating system free of surprises, and certifications for safety functions.

From the control cabinet to the business news

In everyday life, one practically never sees a PLC, since it sits behind a metal door. Nevertheless, one encounters it indirectly every day. The traffic light at the intersection, the escalator in the train station, and the plant that fills a row of yogurt cups are all controlled this way. The heating system of a large office building is also usually run by such a controller.

In business news, the term comes up in relation to the automation industry. Major manufacturers include Siemens, Rockwell Automation, Mitsubishi Electric, and Schneider Electric. Their order books are considered a leading indicator of whether industrial companies are currently investing in new manufacturing capacity.

A second topic is security. Because PLCs are increasingly connected to networks, they become targets for attacks. The malware Stuxnet manipulated Siemens controllers in Iranian facilities in 2010 and destroyed centrifuges. Also new is the connection to AI: sensor data from PLCs is collected and analyzed to predict defects before a machine fails. This predictive maintenance is one of the most tangible AI use cases in industry.