Displacement sensors are devices that convert the motion displacement of an object into measurable electrical quantities. They are typically used to convert physical quantities such as displacement, position, deformation, vibration, and size, which are inconvenient for quantitative detection and processing, into electrical quantities that are easy to quantify and facilitate information transmission and processing.The range of displacement measurement methods is quite broad. Small displacements are usually detected using strain gauges, inductive sensors, differential transformers (LVDT), eddy current sensors, and Hall sensors; large displacements are commonly measured using inductive synchronizers, gratings, capacitive sensors, magnetic gratings, and magnetostrictive sensors. Due to their advantages of easy digitalization, high precision, strong anti-interference capability, adaptability to harsh environments, convenient installation, and reliable use, displacement sensors are widely used in industries such as machine tool processing, testing instruments, automotive manufacturing, and aerospace.
01Classification and Principles of Displacement Sensors1. Classification by working principle:(1) Potentiometric Displacement SensorsThese sensors convert mechanical displacement into a resistance or voltage output that has a linear or arbitrary functional relationship with it through a potentiometer element. Ordinary linear potentiometers and rotary potentiometers can be used as linear displacement and angular displacement sensors, respectively. However, potentiometers designed for measuring displacement require a definite relationship between displacement change and resistance change. The movable brush of the potentiometric displacement sensor is connected to the object being measured.The displacement of the object causes a change in the resistance at the moving end of the potentiometer. The change in resistance reflects the magnitude of the displacement, while an increase or decrease in resistance indicates the direction of displacement. Typically, a power supply voltage is applied to the potentiometer to convert the resistance change into a voltage output.① Wire-wound Potentiometers: As the brush moves, the resistance changes in a stepwise manner based on the winding resistance, resulting in a step-like output characteristic. If this type of displacement sensor is used as a feedback element in a servo system, excessive step voltage can cause system oscillation. Therefore, efforts should be made to minimize the resistance value per turn during the manufacturing of the potentiometer. Another major drawback of potentiometric sensors is their susceptibility to wear. Their advantages include: simple structure, large output signal, ease of use, and low cost.② Conductive Plastic Displacement Sensors: These sensors use a special process to coat DAP (di-2-ethylhexyl phthalate) resistive paste onto an insulating body, which is then heated to form a resistive film, or a solid body formed by thermoplastic pressing of DAP resistive powder into grooves in the insulating substrate.③ Metal Glass Uranium Displacement Sensors: These sensors are made by screen printing a specific pattern with metal glass uranium resistive paste onto a ceramic substrate, followed by high-temperature sintering. They feature a wide resistance range, good heat resistance, strong overload capacity, and excellent moisture and wear resistance, making them a promising type of potentiometer, although they have high contact resistance and current noise.④ Metal Film Displacement Sensors: The resistive element of metal film potentiometers can be composed of alloy films, metal oxide films, or metal foils. They are characterized by high resolution, high-temperature resistance, low temperature coefficient, low dynamic noise, and good smoothness.Advantages: inexpensive, simple structure, good linearity and stability.Disadvantages: significantly affected by changes in the external environment, such as temperature, and have lower resolution.(2) Magnetostrictive Displacement SensorsMagnetostrictive displacement sensors measure the actual displacement value of the detected product by accurately detecting the absolute position of a moving magnetic ring using internal non-contact measurement technology. They utilize the principle of magnetostriction to generate a strain pulse signal through the intersection of two different magnetic fields to accurately measure position. The measuring element is a waveguide tube, and the sensitive element inside the waveguide tube is made of a special magnetostrictive material.The measurement process involves generating a current pulse in the sensor’s electronic chamber, which travels through the waveguide tube, creating a circumferential magnetic field outside the waveguide tube. When this magnetic field intersects with the magnetic field generated by the moving magnetic ring that is fitted around the waveguide tube, a strain mechanical wave pulse signal is produced inside the waveguide tube due to the effect of magnetostriction. This strain mechanical wave pulse signal travels at a fixed sound speed and is quickly detected by the electronic chamber.The transmission time of this strain mechanical wave pulse signal within the waveguide tube is proportional to the distance between the moving magnetic ring and the electronic chamber. By measuring the time, this distance can be determined with high precision. Since the output signal is a true absolute value, rather than a proportional or amplified signal, there is no signal drift or variation, and periodic recalibration is unnecessary.Magnetostrictive displacement sensors are high-precision, long-stroke absolute position measurement sensors manufactured based on the principle of magnetostriction. They employ an internal non-contact measurement method, which prevents friction and wear between the moving magnetic ring used for measurement and the sensor itself, thus ensuring a long service life, strong environmental adaptability, high reliability, good safety, and ease of automation in systems. They can operate normally even in harsh industrial environments (such as those prone to oil spills, dust, or other contaminants).The sensor utilizes high-tech materials and advanced electronic processing technology, allowing it to be used in high-temperature, high-pressure, and high-vibration environments. The output signal of the sensor represents the absolute displacement value, ensuring that data is not lost even if the power supply is interrupted or reconnected, and there is no need for re-zeroing.Since the sensitive element is non-contact, repeated measurements do not cause any wear on the sensor, significantly improving measurement reliability and service life. The stroke can reach 5 meters or longer, with a nominal accuracy of 0.05% F·S, and for strokes over 1 meter, the sensor accuracy can reach 0.02% F.S, with repeatability up to 0.002% F·S, making it widely applicable.Advantages: high reliability, high resolution, oil and dirt resistance, non-contact measurement method, long service life, strong environmental adaptability, good safety; can operate normally even in harsh industrial environments, continuously and accurately detecting the displacement (position) and speed of various moving components in real-time. Additionally, it can withstand high temperatures, high pressures, and strong vibrations.Disadvantages: susceptible to magnetic field interference, cannot be used with ferromagnetic materials.(3) Grating Displacement SensorsGrating displacement sensors measure displacement using the principle of grating interference. The grating consists of densely spaced parallel lines etched onto a strip of optical glass, with a line density of 10 to 100 lines/mm. The interference pattern formed by the grating has optical amplification and error averaging effects, thus improving measurement accuracy.The sensor consists of a scale grating, an indicator grating, an optical system, and a measurement system. When the scale grating moves relative to the indicator grating, a pattern of alternating light and dark fringes is formed, which approximately follows a sine distribution. These fringes move at the relative speed of the grating and are directly illuminated onto a photoelectric element, producing a series of electrical pulses at their output. These pulses are amplified, shaped, and counted to generate a digital signal output that directly displays the measured displacement.There are two types of optical paths for the sensor: one is a transmissive grating, which is etched onto transparent materials (such as industrial white glass, optical glass, etc.); the other is a reflective grating, which is etched onto highly reflective metals (stainless steel) or glass coated with a metallic film (aluminum film). The advantages of this type of sensor are a large measurement range and high accuracy.Grating sensors are used in programmable and numerical control machine tools and coordinate measuring machines to measure static and dynamic linear displacements and circular angular displacements. They are also applied in mechanical vibration measurement, deformation measurement, and other fields.Advantages: large detection range, high measurement accuracy, fast response speed.Disadvantages: contact-type measurement, with a typical measurement speed of up to 1.5 m/s, suitable only for static measurements.(4) LVDT Displacement SensorsLVDT displacement sensors are linear differential transformer displacement sensors. The working principle of LVDT displacement sensors involves winding three sets of coils around a hollow core: one primary coil and two secondary coils. When the primary coil is energized, a magnetic field is generated inside the hollow core. When a ferromagnetic core is inserted into the hollow core, it cuts the magnetic field lines, inducing a weak alternating voltage in the two secondary coils. When the ferromagnetic core moves closer to one of the secondary coils, the voltage output of that coil becomes greater than that of the other coil. By following this principle, the output voltages of the two secondary coils are amplified, and the difference between the two voltages is calculated. This difference is linearly proportional to the displacement of the ferromagnetic core within the hollow core, and this difference can be processed into analog signals such as -5V, 0-10V, 4-20mA, or digital signals like Modbus SSI, which are the normal output signals of LVDT.Advantages: non-contact principle, long service life; fast response speed; high linearity; good repeatability; wide measurement range; low failure rate/low power consumption/diverse input and output options; good dynamic characteristics, suitable for high-speed online detection, automatic measurement, and automatic control; can be used in harsh environments such as humidity and dust; can operate under special conditions such as high pressure, high temperature, radiation, and underwater; can withstand shocks of up to 150g/11ms and vibration frequencies of 2kHz with accelerations of 20g; compact size, low cost, and high performance-to-price ratio.Disadvantages: for very large strokes (over 1 meter), production is challenging, and the sensor and pull rod length may exceed 2 meters, making it inconvenient to use, and linearity may not be high.(5) Laser Displacement SensorsLaser displacement sensors are devices that utilize laser technology for measurement. They consist of a laser source, a laser detector, and measurement circuitry. They can accurately measure the position, displacement, and other changes of the target object without contact. They can measure displacement, thickness, vibration, distance, diameter, and other precise geometric measurements.Advantages: lasers have excellent linearity, and laser displacement sensors offer higher precision compared to ultrasonic sensors.Disadvantages: the laser generation device is relatively complex and large, which imposes strict requirements on the application range. It requires a larger measurement space and is not suitable for small spaces.(6) Eddy Current Displacement SensorsEddy current sensors can measure the distance between a metallic conductor and the probe surface both statically and dynamically, with high linearity and high resolution. They are a type of non-contact linear measurement tool. Eddy current sensors can accurately measure the relative displacement changes between the measured object (which must be a metallic conductor) and the probe’s end face. In the analysis of the state of high-speed rotating machinery and reciprocating machinery, they can continuously and accurately collect various parameters of rotor vibration states, such as radial vibration, amplitude, and axial position.Advantages: non-contact measurement, smaller size, good reliability, wide measurement range, high sensitivity, high resolution; high resolution and high sampling rate; adjustable zero point, gain, and linearity; options for extended cables, temperature compensation, and other functions; can measure all metallic materials, both ferromagnetic and non-ferromagnetic; supports multi-sensor synchronization; unaffected by humidity and dust, with low environmental requirements; widely used in online monitoring and fault diagnosis of large rotating machinery.Disadvantages: cannot measure non-metallic materials.(7) Capacitive Displacement SensorsCapacitive displacement sensors are precision measuring instruments based on non-contact capacitive principles. The capacitor plates of capacitive displacement sensors are mostly made of metal, and the dielectric between the plates is often inorganic materials such as air, glass, ceramics, or quartz; thus, they can operate for long periods under high temperatures, low temperatures, strong magnetic fields, and strong radiation, especially solving detection challenges in high-temperature and high-pressure environments. They are widely used in research institutes, universities, factories, and military industries, becoming an indispensable testing instrument in scientific research, teaching, and production.This sensor can also be connected to secondary instruments or controllers in the control room for online, continuous, real-time detection of various data, which can then be displayed directly, allowing for remote control and alarm functions. It enables data storage, transmission, and control functions. It is widely used in various injection molding machines.Capacitive displacement sensors are particularly suitable for measuring slow changes or small quantities; generally, capacitive sensors are more appropriate for detection. The performance of capacitive displacement sensors is bound to expand their application range. They are mainly used to solve measurement problems in piezoelectric micro-displacement, vibration tables, electronic microscope fine-tuning, astronomical telescope lens fine-tuning, and precision micro-displacement measurements.Advantages: in addition to the common advantages of non-contact instruments such as no friction, no wear, and no inertia, they also have a large signal-to-noise ratio, high sensitivity, low zero drift, wide frequency response, low non-linearity, good accuracy stability, strong electromagnetic interference resistance, and ease of operation.Disadvantages: limited measurement range, typically only a few tens of millimeters, and easily affected by external interference and distributed parameters.(8) Hall Effect Displacement SensorsHall effect displacement sensors mainly consist of two semi-circular magnetic steel components that create a gradient magnetic field and a Hall plate (sensitive element) made of germanium material located at the center of the magnetic field. Additionally, they include measurement circuits (bridges, differential amplifiers, etc.) and display components.They form a direct current magnetic circuit system with two identical structures that create a gradient magnetic field along the x-axis. To ensure a good linear distribution of the magnetic field in the air gap, special forms of pole shoes are installed at the ends of the magnetic poles. Displacement sensors made using this method are highly sensitive. The Hall plate is placed between the two magnetic fields, and careful adjustment of its initial position can ensure that the initial Hall potential is zero. Its displacement measurement is relatively small, making it suitable for measuring micro-displacements and mechanical vibrations.There are two types of Hall effect displacement measurements: one uses a linear Hall sensor to measure the distance between the sensor and a magnet, allowing the output signal of the linear Hall element to indicate the distance from the magnet. This method is used for testing paper thickness, deformation of metallic materials, and other small displacements, as well as applications like throttle pedal distance; the other uses a switch-type Hall element for mechanical angle or displacement positioning, such as detecting the gear position of a car’s gear lever, where a Hall sensor detects the position when the gear lever reaches the corresponding position. This type of application is very common.Characteristics:(1) The control current of the sensor is 1 to 5mA, with low power consumption, high sensitivity, and high resolution;(2) Simple principle, easy implementation, high reliability, and good repeatability;(3) Small size, lightweight, and long lifespan;(4) Stable and reliable detection circuit, with stable test results up to 5-digit readings and high accuracy;(5) Hardware compensation can largely eliminate temperature effects on the sensor;(6) Easy to extend to other non-electrical measurements such as vibration, flow, pressure, and differential pressure, with certain promotional application value.Advantages: large output variation, high sensitivity, high resolution, lightweight, low inertia, fast response speed; Hall elements have a wide frequency response range, making them suitable for dynamic displacement testing.(9) Ultrasonic Displacement SensorsUltrasonic displacement sensors use the principle of ultrasonic echo distance measurement, employing precise time difference measurement technology to detect the distance between the sensor and the target object.Advantages: non-contact measurement, hygienic, accurate measurement, no contact, waterproof, and capable of detecting highly corrosive media. They can be used for liquid level and material level detection, with a unique method that ensures stable output even when foam or significant agitation is present on the liquid surface, making it difficult to detect echoes.Disadvantages: the generation and reception of ultrasonic waves require specific environmental conditions, making ultrasonic sensors less suitable in harsh environments.2. Classification by motion type(1) Linear Displacement SensorsThe function of linear displacement sensors is to convert linear mechanical displacement into electrical signals. To achieve this, a variable resistor slider is typically fixed in the sensor, and the displacement of the slider along the rail is used to measure different resistance values. The sensor’s rail is connected to a steady-state DC voltage, allowing a small current in microamperes to flow, which is proportional to the length of the slider’s movement. Using the sensor as a voltage divider minimizes the accuracy requirements for the total resistance of the rail, as resistance changes due to temperature variations do not affect the measurement results.(2) Angular Displacement SensorsAngular displacement sensors are used for obstacle detection: using angular sensors to control your wheels can indirectly detect obstacles. The principle is very simple: if the motor’s angular sensor operates while the gears do not turn, it indicates that your machine is blocked by an obstacle.This technology is very simple to use and very effective; the only requirement is that the moving wheels must not slip on the floor (or slip too much), otherwise, you will not be able to detect obstacles.02Selection of Displacement SensorsWhen selecting displacement sensors, the following criteria must be met:1. Sensitivity technical indicators: For an instrument, generally, the higher the sensitivity, the better, as higher sensitivity makes it easier to detect changes in acceleration in the surrounding environment. A significant change in acceleration naturally leads to a larger change in output voltage, making measurement easier and the resulting data more accurate.2. Zero point temperature: Changes in environmental temperature can cause zero point balance changes. This is generally expressed as the percentage change in zero point balance per 10°C temperature change relative to the rated output, indicating the drift caused by temperature changes when the sensor is not under pressure.3. Bandwidth technical indicators: Bandwidth refers to the effective frequency range that the sensor can measure. For example, a sensor with a bandwidth of hundreds of Hz can measure vibrations, while a sensor with a bandwidth of fifty Hz can effectively measure tilt.4. Output method technical indicators: There are two output methods: digital output and analog output. Digital sensors input digital signals to instruments, such as quantity, weight, etc.; analog sensors input analog signals to instruments, such as voltage, current, etc.5. Measurement range technical indicators: Different measurements require different ranges, which should be assessed based on actual conditions.6. Limit overload: The maximum load that the sensor can withstand without losing its operational capability. This means that exceeding this value will permanently damage the sensor.7. Sensor gain: The amplification factor of the sensor’s original signal output.Source: Sensor Expert Network