Sensor basics: understanding industrial sensors
A sensor converts a physical quantity such as pressure, temperature, distance or flow into an electrical signal that a controller can work with. In industrial automation, sensors are the machine's sense organs: they supply the readings that plants use to control, switch and monitor. This guide covers the essentials you need to choose the right sensor, vendor-neutral and free of marketing.
How a sensor works
At its core, every sensor has three stages: a transducer that picks up the physical quantity, signal-conditioning electronics that clean up the raw signal, and an output stage that presents the result as a standardised signal. What matters most in practice is the underlying measuring principle, because it determines which materials and conditions a sensor can detect reliably.
The main measuring principles at a glance
Factory automation relies on a handful of physical principles. Inductive sensors use an electromagnetic field and detect metal only. Capacitive sensors respond to almost any material, because they evaluate a change in capacitance. Optical sensors work with light and cover long ranges. Ultrasonic sensors emit sound pulses and measure regardless of colour or transparency. Which principle is right depends on the material, distance and environment. For a detailed comparison, see our sensor selection guide.
Signal types and outputs
Sensors deliver their result in different ways. Analogue outputs such as 4–20 mA or 0–10 V transmit a continuous reading and suit pressure, temperature or level. Digital switching outputs (PNP/NPN, normally open/normally closed) simply report "detected" or "not detected". Increasingly, IO-Link is taking over: a digital point-to-point link that carries readings, parameters and diagnostic data over the same cable.
Protection rating and protection class
The protection rating, given as an IP code, describes how well a sensor is protected against dust and water. The first digit covers protection against solid objects, the second against water. IP67 means full dust protection and temporary immersion; IP69K adds resistance to high-pressure washdown, important in the food industry, for instance. If you measure in a harsh environment, always check the rating against the real load.
Means: dust-tight
Means: temporary immersion
The first digit covers protection against solids, the second against water. Pick the digits to see what each level protects against.
Not to be confused with the protection class: the protection rating (IP code, IEC 60529) is about the ingress of dust and water, whereas the protection class (I, II or III per IEC 61140) describes protection against electric shock. Most industrial sensors run on extra-low voltage and therefore fall under protection class III.