Position & Distance Sensors: Linear, Rotary, Laser & Selection

Position, displacement and distance are related but not identical measurements

Start by defining what the controller actually needs. Position is a location within a defined coordinate or stroke. Displacement is a change in position. Distance is the separation between the sensor and a target or between two reference points. The same sensor family may support more than one of these tasks, but the installation and error sources can be very different.

Linear position sensors differ mainly in contact method, reference behaviour and stroke

Linear position feedback may be mechanically linked to the moving part or measured without sliding electrical contact. Potentiometric sensors are simple and directly proportional over their electrical stroke, but the wiper is a wear element. LVDTs use a moving magnetic core inside a transformer structure and can provide frictionless measurement when the core is mechanically coupled without rubbing the coil assembly.

For hydraulic or pneumatic cylinders, a magnetostrictive rod sensor can be integrated into the cylinder so the position magnet follows the piston without mechanical contact with the sensing element. For short precision strokes, LVDTs and linear encoders are common alternatives. Choose among them from the required stroke, uncertainty, motion speed and installation geometry.

Incremental encoders measure relative motion; absolute encoders report shaft position

An incremental encoder produces pulses as the shaft moves. A controller counts those transitions to determine relative displacement and can use two phase-shifted channels to determine direction. An index channel may provide one reference event per revolution. An absolute encoder instead reports a code representing shaft position, so the controller can recover position without counting every pulse from a previously established origin.

Resolvers are electromagnetic rotary position transducers used where a rugged shaft-angle measurement is required, especially in motor and servo applications. They produce angle-dependent sine and cosine signals and require excitation plus resolver-to-digital or equivalent signal conditioning in the controller.

Laser triangulation and time-of-flight cover different distance ranges

Reflective laser displacement sensors commonly use triangulation for short, precise measurements. The sensor projects a spot on the target; a receiving lens images the reflected light onto a position-sensitive detector. As the target moves, the return angle changes and the image moves across the receiver. This geometry is well suited to displacement, height, thickness and run-out measurements where a small change in target position must be resolved quickly.

Ultrasonic sensors handle optically difficult targets; draw-wire sensors cover long strokes

An ultrasonic distance sensor emits a sound burst and measures the time until the echo returns. Because the measurement is acoustic, colour and visible transparency are far less important than for optical sensing. The target must still return a usable echo: steep angles can reflect sound away from the sensor, and soft, porous or highly irregular surfaces can reduce or scatter the signal.

A draw-wire sensor, also called a cable-extension or string sensor, converts linear movement into rotation. A spring-loaded spool keeps a measuring cable under tension while an internal potentiometer or encoder measures spool rotation. This makes long linear strokes possible from a compact housing and can simplify installation where a rigid rod or long scale is impractical.

Output type must match the cable run and controller interface

Continuous position sensors commonly provide analogue or digital measurement values. A simple threshold output such as PNP/NPN only indicates whether a programmed position or distance condition has been crossed; it does not by itself provide the full measured position.

Resolution, repeatability, linearity and bandwidth describe different limits

A position sensor can have fine resolution without high absolute accuracy. Accuracy is the closeness of the indicated value to the true position, repeatability is the spread when the same position is measured repeatedly, linearity describes deviation from the ideal transfer curve, and resolution is the smallest reportable change.

How do you choose a position or distance sensor?

Define the motion first: linear or rotary, absolute or relative, contact or non-contact, measuring range and maximum speed. Then set the required accuracy and repeatability, choose a sensing principle that works with the target and environment, and finally match the output, mounting, cable and controller interface.

1. Define the measurand. State whether you need absolute position, displacement from a reference, shaft angle, speed, gap, height or stand-off distance.
2. Set the range and mechanical limits. Include overtravel, end-stop tolerance, blind zones, cable travel and mounting clearances rather than using only the nominal working stroke.
3. Set the measurement requirement. Separate accuracy, repeatability, resolution and update rate. Use the worst-case machine tolerance rather than a display-digit target.
4. Choose the physical principle. Use LVDT, magnetostrictive, encoder, potentiometric, laser, ultrasonic, draw-wire or another technology according to motion, target, stroke and environment.
5. Check mounting geometry. Consider alignment, shaft coupling, bearing load, scale straightness, laser angle, acoustic cone, cable pull direction and vibration.
6. Check dynamics. Maximum shaft speed, acceleration, target speed, sampling rate, response time and controller input frequency must work together.
7. Match the electrical interface. Confirm analogue range, encoder electrical output, serial protocol, supply, grounding, shielding and PLC/DAQ input compatibility.
8. Verify the environment. Temperature, washdown, oil, dust, vibration, shock, welding fields, ambient light and target contamination can dominate the final error.
9. Commission against a reference. Check zero, span, end positions and repeatability on the actual machine before accepting the measurement for control or quality decisions.

Troubleshooting unstable or incorrect position and distance readings

Separate mechanical, sensing and electrical faults. A stable sensor reading can still be wrong because of coupling slip or mounting geometry, while a mechanically correct installation can produce unstable data through noise, reflections, poor target return or an overloaded interface.