Industrial sensors are devices that turn pressure, position, strain, gas, heat or vibration into an electrical signal. Newer ones also include software that analyses the signal and sends alerts. A sensor is the first link in every automated system, because a controller can act only on what it can measure.
Many industrial sensors send their reading as a current between 4 and 20 milliamps. Consider a pressure sensor on a water line that can measure from 0 to 10 bar. At 0 bar, it sends 4 mA. At 10 bar, it sends 20 mA. At 5 bar, it sends 12 mA, which is half-way. The PLC converts the current back to a pressure. A wire that is broken gives 0 mA, which cannot be a real reading, so the PLC knows at once that the wire is broken.
Different sensors measure different things. A strain sensor measures how much a metal part stretches. A gas sensor detects a leak. A vibration sensor shows the health of a motor. Sensors are also used inside products, such as the cardiac implants that measure a heartbeat, and the safety electronics of a car.
A pressure sensor shows how a sensor works. Inside it is a thin metal diaphragm. When the pressure rises, the diaphragm bends by a tiny amount, and a circuit on the diaphragm turns this bending into a change in electrical resistance. The sensor's electronics convert the change into the 4 to 20 mA signal. Sensors slowly drift from their true readings with age and heat. So plants calibrate them at fixed intervals by comparing them with a reference instrument. A sensor that has drifted is dangerous, because the controller acts on its wrong reading with full confidence.
Sensor makers win a place in a product early, as a design win, and then supply it for years. They buy and sell through electronic component distributors. The sensors are joined to machines by connectors and cables.
A sensor that only measures is useful. A sensor that also analyses is more useful: it runs a small edge AI program that flags a pattern, and sends it over a cellular IoT module to an industrial IoT platform. The readings are used by asset performance management and, in a process plant, by the distributed control system. A sensor worn by a worker to detect gas is described under portable gas detectors.