Position, Displacement and Speed Sensors

Choosing a position sensor

Position, displacement and length sensors report where something is (an analog or digital value), unlike the switches in Proximity Sensors that only say whether it is present. The choice depends on the stroke, the resolution, the speed, whether contact is allowed and the environment. Rotary position (angle) and the encoders on motors are covered in Encoders, Resolvers and Feedback.

Sensor	Typical stroke	Resolution / accuracy	Strengths	Limits
Potentiometer	10 mm to 1 m	Good for the cost: 0.1 to 1 percent	Cheap, simple	Wear of the wiper; noise; loading error
Draw-wire (string pot)	Up to 50 m	0.1 to 0.5 percent of range	Long strokes, easy mounting	Wire and spring wear; not for fast motion
LVDT	±0.5 mm to ±500 mm	Resolution limited by the electronics; 0.1 to 0.5 percent	No contact, infinite life, rugged, high resolution	Needs excitation and conditioning; length of coil
Magnetostrictive	25 mm to 20 m	1 to 10 µm; 0.01 to 0.05 percent	Absolute, long strokes, in hydraulic cylinders	Costly; the head and the magnet need space
Linear scale (optical or magnetic)	20 mm to many meters	0.1 to 5 µm	Highest accuracy, fast	Cost; dust, coolant and mounting
Laser triangulation	2 to 1 000 mm range	0.1 µm to 100 µm	Non-contact, fast, high resolution	Surface finish and angle; cost
Eddy-current	0.5 to 20 mm	Nanometers	Tiny, extremely precise on metal	Metal targets only; short range
Ultrasonic	5 cm to 10 m	About 0.1 to 1 mm	Any surface, simple	Temperature and soft targets (see Proximity Sensors)

Potentiometers and draw-wire sensors

A potentiometer is a resistive track with a wiper: the output voltage is a fraction of the supply proportional to the position. Conductive-plastic tracks last for millions of cycles; wire-wound and carbon tracks wear faster. A draw-wire sensor (string pot) winds a wire on a drum, coupled to a potentiometer, an encoder or a resolver, with a spring for return.

• Use the wiper as a voltage divider, with the track fed from a stable supply, not as a variable resistor (a poor contact then makes noise or a break).
• Linear (hybrid) potentiometers have a limited life (10⁶ to 10⁸ cycles) and a micro-motion limit: do not use them for a constant, tiny vibration that wears one spot.

LVDTs

A linear variable differential transformer has a primary coil excited with an AC signal and two secondary coils wired in opposition. A magnetic core (attached to the moving part, with no contact) couples the primary to the secondaries: at the center both secondaries give equal and opposite voltages, so the output is zero; moving the core makes one stronger, giving an output proportional to the displacement, with the phase showing the direction. A demodulator converts it to a DC voltage or current.

• Strengths: frictionless, almost infinite life, resolution limited only by noise, good in dirt, oil and radiation, and rugged against shock.
• Limits: the core must be aligned with the bore, the body is as long as the stroke, and a magnetic mounting or nearby steel can disturb the reading.

Magnetostrictive sensors

A magnetostrictive sensor has a long waveguide tube with a head at one end and a ring magnet on the moving part. The head sends a current pulse along the waveguide; where the pulse meets the magnet’s field a torsion pulse is created and travels back at about 2 800 m/s. The delay measures the distance: absolute position, with no contact between the magnet and the tube. This construction survives inside hydraulic cylinders at hundreds of bar.

Linear scales and encoders

A linear scale is the straight version of an encoder: an optical glass or a magnetic strip with a fine pattern read by a head. It is the most accurate way of measuring position, used on machine tools and measuring machines, with resolutions of 0.1 µm and better. It measures the load directly, avoiding the errors of a screw, a belt or a gearbox (see Encoders, Resolvers and Feedback for the signals, the decoding and the interfaces). Magnetic tapes are more tolerant of dirt, optical scales more accurate; both need protection from coolant and chips.

Laser, eddy-current and other non-contact sensors

• Laser triangulation: a laser spot is imaged onto a line sensor from an angle; the spot position shifts with the distance. High speed and resolution for thickness, height, flatness and vibration, with surface finish, color and angle that matter. Use a blue or a red laser to suit the target.
• Confocal and white-light sensors measure transparent and shiny surfaces to sub-micron resolution.
• Time-of-flight laser rangefinders measure long distances (meters to hundreds of meters) with millimeter accuracy for cranes, vehicles and stores.
• Eddy-current displacement sensors measure the gap to a metal target with nanometer resolution over a few millimeters: shaft position and runout in turbines, and thin films.
• Inclinometers measure the angle to gravity with MEMS accelerometers: crane booms, solar trackers, leveling.
• Analog Hall and magnetoresistive sensors give a position signal from a magnet over a short stroke: pedals, valves, lever positions.

Measuring speed

Speed can be derived from position: count the pulses of an encoder or of a gear-tooth sensor over a known time (frequency counting), or time the interval between pulses (period measurement). Frequency counting is best at high speed; period measurement is best at low speed. A tachogenerator gives a DC voltage proportional to speed; Doppler radar and laser sensors measure the speed of a surface (vehicles, conveyors, webs) without contact.

Outputs and interfacing

Output	Notes
0 to 10 V, 4 to 20 mA	Common; scale with the Analog Scaling Tool. Mind the resolution of the input card.
SSI, BiSS	Synchronous serial absolute position; clock from the controller; fast; differential RS-422 lines
CANopen, EtherCAT, PROFINET, IO-Link	Fieldbus sensors with configuration and diagnostics (see Fieldbuses and Device Networks)
Quadrature pulses (A, B, Z)	Incremental scales; need a counter and homing
Start/stop pulses	Older magnetostrictive systems measure the interval with a timer module

Troubleshooting

Symptom	Likely cause	Check
Noisy or jumping reading	Electrical noise on the cable, a poor connection, a worn potentiometer track	Shielding and grounding (see Installing and Troubleshooting Sensors), the supply ripple, replace a worn pot
Reading offset or not linear	Core or magnet misaligned, a magnetic part nearby, wrong calibration	Re-align; keep steel away; recalibrate zero and span
No output	No excitation or supply, a broken wire, a failed conditioner	Measure the supply and the excitation at the sensor
Drift with temperature	Thermal expansion of the mounting, a cable or electronics drift	Mount on a stable frame; allow warm-up; use the temperature-compensated type
Lost position after power-off	An incremental device	Use an absolute sensor, or home at start-up
Wrong speed reading	Gate time too short, a missed pulse, wrong pulses per revolution	Check the settings and the counter input rating

Related

• Sensors and Field Devices: the overview of this guide.
• Encoders, Resolvers and Feedback and Mechanical Transmission: Screws, Belts and Gearboxes: encoders on motors, and where the position is measured.
• Proximity Sensors and Photoelectric and Vision Sensors: switching sensors.
• Sensor Outputs and Wiring: analog and digital connections.

FAQ

LVDT or linear potentiometer?

A potentiometer is cheaper and simpler for low-duty positioning. An LVDT has no contact and so a far longer life, a better resolution and is much more rugged in dirt and shock, at the cost of the electronics.

How accurate is a draw-wire sensor?

Typically 0.1 to 0.5 percent of the range: very good for the cost, especially on long strokes, but not for precision work. The wire needs to run straight, without side pull.

Absolute or incremental?

Absolute devices know the position at power-up; incremental ones count and must be referenced. For long strokes or critical positions an absolute sensor avoids a homing move.

What limits the resolution of an analog position sensor?

The input converter and the noise: a 12-bit input on a 100 mm sensor resolves about 25 µm at best, while a 16-bit input resolves 1.5 µm if the sensor and wiring are quiet enough.

References

• Fraden, J., Handbook of Modern Sensors, Springer (LVDTs, magnetostrictive and optical sensors).
• MTS Sensors, Temposonics and Heidenhain and Renishaw linear encoder technical information; TE Connectivity / Measurement Specialties, LVDT application notes.
• Doebelin, E. O., Measurement Systems: Application and Design, McGraw-Hill.

Last reviewed: October 2026. Sensor figures are typical; use the manufacturer’s specifications for real accuracy and resolution.