Sensors and Field Devices

Sensors are the eyes and ears of an automated machine: they tell the controller whether a part is there, how far an axis has moved, how hot a process is, how full a tank is. A good choice and a good installation are the difference between a machine that runs for years and one that stops for a dirty lens or a loose wire. This guide covers the sensor families used in industry, how each works and where it fits, then the wiring, installation, troubleshooting and the special case of safety sensors, with worked numbers and diagrams. It connects to the PLC inputs in I/O Wiring and Signals, the networks in Industrial Communication, and the motion feedback devices in Encoders, Resolvers and Feedback.

Start here: what do you need?

I need to…	Go to
understand accuracy, resolution, response time, or choose between sensors	Sensor Basics and Selection
detect a metal part, or a part of any material, at a short distance	Proximity Sensors
detect objects at a distance with light, read color, or use a laser sensor	Photoelectric and Vision Sensors
measure the position, length or speed of a moving part	Position, Displacement and Speed Sensors
measure temperature with a thermocouple, RTD or thermistor	Temperature Sensors
measure pressure, a tank level or a flow rate	Pressure, Level and Flow Sensors
weigh something, measure force or torque, or monitor vibration	Force, Load Cells and Vibration
wire a sensor to a PLC: NPN or PNP, 2, 3 or 4 wires, 4-20 mA, IO-Link	Sensor Outputs and Wiring
mount a sensor, stop noise, or find out why it does not work	Installing and Troubleshooting Sensors
protect people with a light curtain, an interlock or an emergency stop	Safety Sensors and Devices

A suggested order

If you are new to industrial sensors, read the pages in this order. The first gives the vocabulary, the next six cover the families, and the last three are about connecting, installing and protecting.

1. Sensor Basics and Selection: What a sensor is, the signal chain, accuracy, resolution and repeatability, response time, and a method for choosing the right sensor.
2. Proximity Sensors: Inductive, capacitive, magnetic and ultrasonic sensors: how they work, sensing distance, correction factors, mounting and hysteresis.
3. Photoelectric and Vision Sensors: Through-beam, retro-reflective and diffuse sensors, excess gain, background suppression, laser sensors, color and vision.
4. Position, Displacement and Speed Sensors: Potentiometers, LVDTs, magnetostrictive and laser sensors, linear scales, Hall sensors and speed measurement.
5. Temperature Sensors: Thermocouples, RTDs, thermistors and infrared sensors: types, accuracy, wiring, cold junction and lead-resistance corrections.
6. Pressure, Level and Flow Sensors: Gauge, absolute and differential pressure, level by hydrostatic, ultrasonic and radar methods, and flow by differential pressure, magnetic and vortex meters.
7. Force, Load Cells and Vibration: Strain gauges and bridges, load cell ratings and mounting, torque sensing, accelerometers and vibration monitoring.
8. Sensor Outputs and Wiring: NPN and PNP outputs, two-, three- and four-wire sensors, analog 4-20 mA and 0-10 V, IO-Link, and connecting sensors to PLC inputs.
9. Installing and Troubleshooting Sensors: Mounting and alignment, protection, noise and shielding, calibration and a fault-finding flowchart with a symptom table.
10. Safety Sensors and Devices: Light curtains, interlock switches, safety mats and scanners, safety distance, performance level and how a safety circuit is built.

Which sensor, at a glance

Need	Sensor	Typical range	Watch out for
Metal present	Inductive proximity	1 to 40 mm	Correction factors for non-steel metals
Any object present	Photoelectric (diffuse, retro, through-beam)	mm to 60 m	Dirt, color and reflectivity
Liquid or powder level (point)	Capacitive, float switch, vibrating fork	Probe length	Build-up on the probe, foam
Position, long stroke	Magnetostrictive, linear scale	Up to many meters	Cost, shock
Position, short stroke	LVDT, laser triangulation	mm to hundreds of mm	Target surface
Temperature	Thermocouple, RTD	−200 to 1 800 °C	Cold junction, lead resistance, response time
Pressure	Piezoresistive or capacitive transmitter	mbar to 1 000 bar	Overpressure, temperature, vented or sealed reference
Flow	Magnetic, vortex, Coriolis, ultrasonic, differential pressure	Pipe-size dependent	Straight run, conductivity, fouling
Weight, force	Load cell with a bridge amplifier	N to MN	Mounting, side loads, overload
People	Light curtain, scanner, interlock switch	Per the risk assessment	Must be a certified safety device

Quick reference: formulas

Topic	Formula	Page
4-20 mA to percent	% = (I − 4 mA) ÷ 16 mA × 100	Sensor Basics and Selection
ADC step	span ÷ 2N	Sensor Basics and Selection
RSS error budget	√(e1² + e2² + …)	Sensor Basics and Selection
Inductive sensing distance	Sreal = Sn × correction factor	Proximity Sensors
Excess gain	signal received ÷ signal needed; use at least 1.5 to 10	Photoelectric and Vision Sensors
Pt100	R = 100 (1 + 3.9083×10-3 T − 5.775×10-7 T²)	Temperature Sensors
Hydrostatic level	P = ρ g h	Pressure, Level and Flow Sensors
Strain gauge	ΔR ÷ R = GF × ε; Vout = Vex × GF × ε ÷ 4	Force, Load Cells and Vibration
Safety distance	S = K × T + C	Safety Sensors and Devices

Calculators and related guides

• 4-20 mA / 0-10 V Analog Scaling Tool: converting a sensor signal to engineering units.
• Voltage Divider and LED Resistor Calculator and the Resistor Network Calculator: dividers for thermistors and level shifting.
• Sensors and Signal Conditioning: the electronics behind thermistors, RTDs, bridges and 4-20 mA loops.
• Motion Control and Actuators: encoders and limit sensors on moving axes.
• I/O Wiring and Signals: connecting sensors to PLC input cards.

Safety

• A standard sensor is not a safety device. People must be protected with certified safety components and a safety circuit (see Safety Sensors and Devices).
• Lock out and de-energize before working on wiring or replacing a sensor on moving machinery.
• Laser sensors have power classes: avoid looking into the beam, and follow the class label.
• Hazardous areas (explosive gas or dust) need certified sensors and barriers (ATEX, IECEx, Class/Division).
• High pressure and temperature sensors can fail violently: depressurize and cool the line before removing them.

FAQ

Where should I start?

With Sensor Basics and Selection for the vocabulary and the error-budget method, then the page for the quantity you measure.

What happened to the Sensor Wiring page?

It is now Sensor Outputs and Wiring and Installing and Troubleshooting Sensors, part of this guide.

Inductive or photoelectric for detecting a metal part?

Use an inductive sensor when the part is metal and close: it is robust, ignores dirt, oil and light, and needs no alignment. Use a photoelectric sensor for longer ranges, for non-metal parts or for transparent or small objects.

Why is my sensor reading wrong only some of the time?

Usually the installation, not the sensor: loose connectors, noise on the cable, vibration, a changing temperature or a build-up of dirt. Installing and Troubleshooting Sensors has a flowchart for it.

References

• Fraden, J., Handbook of Modern Sensors, Springer.
• Hesse, D. and Leppert, R., Industrial Sensors and the application guides of Sick, Pepperl+Fuchs, Banner, Balluff, Omron and Keyence.
• IEC 60947-5-2 (proximity switches), IEC 60584 (thermocouples), IEC 60751 (industrial platinum resistance thermometers), ISO 13849-1 (safety-related control systems).
• ISA, Instrument Engineers’ Handbook, CRC Press.

Last reviewed: October 2026. Values are typical: check the manufacturer’s data for the sensor you use, and follow the safety standards for your machine.