A displacement sensor converts an object's motion displacement into a measurable electrical quantity. It is commonly used to transform physical quantities that are difficult to quantify or process directly—such as displacement, position, deformation, vibration, and dimensions—into electrical signals that are easier to measure, transmit, and process.
Overview of Measurement Methods
Displacement measurement methods cover a wide range. Small displacements are typically measured with strain gauges, inductive sensors, linear variable differential transformers (LVDT), eddy current sensors, and Hall sensors. Large displacements are often measured with resolvers, optical encoders, capacitive encoders, magnetic scales, and magnetostrictive sensors. Displacement sensors are widely used in machine tools, inspection instruments, automotive manufacturing, and aerospace because they are relatively easy to digitize, offer high accuracy, have good noise immunity, tolerate harsh environments, and are convenient to install and operate.
1. Classification and Working Principles
1) Potentiometer-type displacement sensors
Potentiometer sensors convert mechanical displacement into resistance or voltage outputs that follow a linear or arbitrary functional relationship. Linear and rotary potentiometers can be used for linear and angular displacement measurement, respectively. Potentiometers designed for displacement measurement require a defined relationship between displacement and resistance. The movable wiper connects to the measured object, and object motion causes a change in resistance at the wiper. The magnitude and direction of resistance change indicate the amount and direction of displacement. A supply voltage is usually applied to convert resistance change into a voltage output.
Common types:
- Wire-wound potentiometers: Resistance changes in steps as the wiper moves across windings, producing a stepped output characteristic. Large step voltages can induce oscillation in servo systems used for feedback, so manufacturing aims to minimize per-turn resistance. Main drawbacks include wear due to contact. Advantages are simple structure, large output signal, ease of use, and low cost.
- Conductive-plastic potentiometers: Use a conductive polymer film applied and cured on an insulating substrate or thermoplastic conductive filler pressed into a substrate groove to form the resistive element.
- Metal-glass (thick-film) potentiometers: Produced by screen-printing resistive paste onto a ceramic substrate and firing. They offer a wide resistance range, good heat tolerance, overload capacity, moisture and wear resistance, but suffer from contact resistance and current noise.
- Metal-film potentiometers: Resistive elements made from alloy films, metal oxide films, or metal foils. They provide high resolution, high temperature tolerance, low temperature coefficient, low noise, and smooth output.
Advantages: Low cost, simple structure, good linearity and stability.
Disadvantages: Sensitive to environmental changes (temperature, etc.), limited resolution, and wear from mechanical contact.
2) Magnetostrictive displacement sensors
Magnetostrictive sensors use non-contact measurement to detect the absolute position of a movable magnetic ring. They operate on the magnetostrictive effect: two different magnetic fields intersect to produce a strain pulse that is detected. The sensing element is a waveguide made of a magnetostrictive material. An electronic module sends a current pulse through the waveguide, creating a circumferential magnetic field. When this field intersects the field from the movable magnetic ring, a strain mechanical wave pulse is generated in the waveguide. The pulse travels at a fixed acoustic velocity to the electronics, which detect it. The transit time of the pulse is proportional to the distance between the ring and the electronics, allowing precise absolute position measurement.
Because the output is an absolute value rather than a proportional or amplified signal, there is no signal drift and no need for frequent recalibration. The non-contact measurement between the ring and the sensor eliminates friction and wear, giving long service life, strong environmental tolerance, and high reliability. Magnetostrictive sensors can operate in harsh industrial environments (oil, dust, contaminants) and tolerate high temperature, high pressure, and vibration. Output is absolute, so data remain after power loss. Typical strokes reach 5 m or more, with nominal accuracies around 0.05% F.S., and for strokes above 1 m can reach 0.02% F.S.; repeatability can be as low as 0.002% F.S.
Advantages: High reliability, high resolution, oil and contamination tolerance, non-contact measurement, long life, strong environmental tolerance, and robust performance in harsh conditions. Useful for continuous, precise, real-time detection of displacement and velocity.
Disadvantages: Susceptible to external magnetic interference and not suitable for use with ferromagnetic materials near the sensing region.
3) Optical encoder (grating) displacement sensors
Optical encoder sensors measure displacement using the principle of overlapping grating fringes. A grating consists of dense, evenly spaced parallel lines etched on optical glass with line densities from about 10 to 100 lines/mm. The overlapping fringe pattern has optical magnification and averaging effects that improve measurement accuracy.
The sensor comprises a scale grating, a reference (index) grating, an optical path system, and a measurement system. Relative motion between the scale and index gratings produces alternating bright and dark fringe patterns that move with the grating speed. These patterns illuminate photodetectors, producing pulse trains that are amplified, shaped, and counted to generate digital displacement outputs that directly indicate the measured displacement.
Two optical formats exist: transmissive gratings etched on transparent substrates, and reflective gratings etched on highly reflective metal or metal-coated glass. Advantages include large range and high accuracy. Optical encoders are used in programmable and CNC machine tools, coordinate measuring machines, and for static and dynamic linear and rotary displacement measurements. They are also used in vibration and deformation measurement.
Advantages: Large measurement range, high precision, fast response.
Disadvantages: Typically contact-guided, measurement speed usually below about 1.5 m/s, and suitability mainly for static or relatively low-speed measurements.
4) LVDT (linear variable differential transformer) sensors
An LVDT is a linear differential transformer that contains three coils wound on a hollow form: one primary and two secondary coils. When the primary is energized, a magnetic field is established in the hollow core. Inserting a ferromagnetic core into the hollow form induces small AC voltages in the two secondary coils. When the core moves closer to one secondary coil, that coil's output voltage becomes larger relative to the other. By amplifying and subtracting the two secondary voltages, a differential signal proportional to core displacement is obtained. That differential value can be conditioned to standard analog outputs such as ±5 V, 0–10 V, 4–20 mA, or to digital protocols like Modbus or SSI.
Advantages: Non-contact principle, long service life, fast response, high linearity, good repeatability, wide measurement range, low failure rate and power consumption, flexible input/output options, good dynamic properties for high-speed online measurement and control, tolerant of humid and dusty environments. Special versions can withstand high voltage, high temperature, radiation, and full sealing for underwater use. They tolerate shock up to 150 g/11 ms and vibration up to 2 kHz at 20 g, while offering small size and a favorable cost-performance ratio.
Disadvantages: Manufacturing and handling become difficult for very long strokes (above about 1 m) because the sensor and rod lengths increase and linearity can degrade.
5) Laser displacement sensors
Laser displacement sensors use laser sources, photodetectors, and measurement electronics to perform precise non-contact measurement of position and displacement. They can measure displacement, thickness, vibration, distance, and diameter for precision geometric measurements.
Advantages: Laser sources provide excellent linearity and higher precision compared with many ultrasonic sensors.
Disadvantages: Laser emitting and detection systems are relatively complex and can be bulky, imposing constraints on application space. They are less suitable for very small measurement spaces.
6) Eddy current (electromagnetic induction) displacement sensors
Eddy current sensors measure the distance between a metallic conductor and the probe surface with non-contact high linearity and high resolution. They are linearization tools for metal targets and can accurately measure static and dynamic relative displacement between a metal object and the probe tip. These sensors are widely used in analysis and measurement of high-speed rotating machinery and reciprocating systems, vibration research, and condition monitoring. They can continuously and accurately capture rotor vibration parameters such as radial vibration, amplitude, and axial position.
Advantages: Non-contact, compact, reliable, wide measurement range, high sensitivity and resolution, high sampling rate, adjustable zero and gain, optional long cable, temperature compensation, and compatibility with ferromagnetic and non-ferromagnetic metals. They support multi-sensor synchronous operation and are tolerant of humidity and dust, making them suitable for online monitoring and fault diagnosis of large rotating machinery.
Disadvantages: Cannot measure non-metallic targets.
7) Capacitive displacement sensors
Capacitive sensors are non-contact precision instruments based on capacitive principles. Capacitor plates are typically metal, with dielectric materials such as air, glass, ceramic, or quartz between them, allowing long-term operation in high temperature, low temperature, strong magnetic field, and strong radiation environments. They are widely used in research institutes, universities, factories, and military sectors in China, serving as essential instruments in research, teaching, and production.
Capacitive sensors can be connected to control room instruments or controllers for online, continuous, real-time data display, remote control, alarm, data storage, transmission, and control. They are widely used in injection molding machines. They are especially suitable for slow or very small displacement measurements, such as piezo microdisplacement, vibration tables, fine adjustment in electron microscopes, telescope lens micro-adjustments, and other precision microdisplacement tasks.
Advantages: Typical non-contact benefits—no friction or wear—along with high signal-to-noise ratio, high sensitivity, low zero drift, wide frequency response, low nonlinearity, stable accuracy, strong electromagnetic interference rejection, and convenient operation.
Disadvantages: Small range (usually only a few tens of millimeters) and susceptibility to external interference and distributed parameter effects.
8) Hall-effect displacement sensors
Hall-effect displacement sensors commonly use two half-ring magnet assemblies that create a gradient magnetic field along an axis, with a semiconductor Hall element (sensitive element) placed at the center. Measurement electronics (bridge circuits, differential amplifiers, etc.) and display circuitry complete the device. The two identical magnet circuits form a gradient field; pole pieces are often shaped to improve linearity. The Hall element is positioned so that its output is zero in the initial state. Hall sensors are suitable for measuring small displacements and mechanical vibration.
Two measurement approaches exist: using a linear Hall element to detect distance from a magnet (useful for paper thickness, small material deformation, and moderate distances such as throttle pedal position), or using a switch-type Hall element for angular or position limit detection (common in gearshift position detection and many other applications).
Characteristics:
- Low control current (1–5 mA), low power, high sensitivity and resolution.
- Simple principle, easy implementation, high reliability, and good repeatability.
- Small size, lightweight, and long life.
- Stable detection circuits and high-precision results.
- Hardware compensation can largely eliminate temperature effects.
- Applicable to measurement of non-electrical quantities such as vibration, flow, pressure, and differential pressure.
Advantages: Large output change, high sensitivity and resolution, light weight, low inertia, fast response, and wide frequency response suitable for dynamic displacement testing.
9) Ultrasonic displacement sensors
Ultrasonic displacement sensors measure distance using ultrasonic echo ranging and precise time-difference measurement techniques to detect the distance between the sensor and the target.
Advantages: Non-contact, hygienic, accurate, waterproof, and capable of detecting highly corrosive media. Commonly used for liquid level and material level detection; unique methods allow stable outputs even with foam or significant surface agitation that can make echo detection difficult.
Disadvantages: Ultrasonic transmitters and receivers have high environmental requirements, making them less suitable in very harsh conditions.
2. Classification by Motion Type
Linear displacement sensors
Linear displacement sensors convert linear mechanical displacement into electrical signals. A variable resistance track is often fixed inside the sensor, and a slider moves along the track. The slider is powered with a steady DC voltage, and the output voltage proportional to slider position is taken from the slider. Using the sensor as a voltage divider minimizes the influence of total track resistance changes due to temperature.
Angular displacement sensors
Angular sensors are used in obstacle detection by monitoring wheel or shaft angle. If a motor actuator registers rotation but the driven gear does not move, an obstacle is likely blocking motion. The method is simple and effective but assumes the driven wheel does not slip frequently; excessive slip prevents reliable obstacle detection.
3. Sensor Selection Criteria
When selecting a displacement sensor, consider the following technical requirements:
- Sensitivity: Higher sensitivity improves the ability to detect small changes and yields larger output variations for easier and more accurate measurement.
- Zero-point temperature drift: The change in zero balance caused by environmental temperature variation, often expressed as a percentage of rated output per 10°C.
- Bandwidth: The effective frequency band the sensor can measure. For example, sensors with hundreds of Hz bandwidth can measure vibration; sensors with around 50 Hz bandwidth can measure tilt effectively.
- Output type: Choose between digital and analog outputs. Digital sensors provide digital data to instruments; analog sensors provide voltage or current signals.
- Range: Select a sensor with an appropriate measurement range for the expected motion.
- Overload limit: The maximum load the sensor can endure without permanent damage.
- Sensor gain: The amplification factor applied to the sensor's raw output signal.
