A grating sensor is a sensor that measures displacement using the principle of stacked grating fringes. A grating is an optical device composed of a large number of parallel slits of equal width and spacing. Commonly used gratings are made by etching numerous parallel lines on a glass plate; the lines are opaque, while the smooth areas between the lines are transparent, acting as slits. High-precision gratings can have thousands or even tens of thousands of lines etched within a 1cm width.
A grating that utilizes the diffraction of transmitted light is called a transmission grating. There are also gratings that utilize the diffraction of reflected light between two notches, such as those formed by etching many parallel notches on a metal-coated surface. The smooth metal surface between the notches can reflect light; this type of grating is called a reflection grating. The overlapping fringes formed by gratings have optical magnification and error averaging effects, thus improving measurement accuracy.
The grating sensor consists of four parts: a scale grating, an indicator grating, an optical path system, and a measurement system. When the scale grating moves relative to the indicator grating, it forms alternating bright and dark grating fringes that are roughly distributed according to a sinusoidal pattern.
These stripes move at the relative speed of the grating and directly illuminate the photoelectric elements, generating a series of electrical pulses at their output terminals. These pulses are then amplified, shaped, oriented, and counted to produce a digital signal output, directly displaying the measured displacement.
Structure and working principle of grating sensor
The structure of a grating sensor consists of several main parts: a light source, a main grating, an indicator grating, a light-passing aperture, and photoelectric elements.
1. Light source: Tungsten filament bulbs have relatively low power, and when used in combination with photoelectric components, their conversion efficiency is low and their lifespan is short. Semiconductor light-emitting devices, such as gallium arsenide light-emitting diodes (LEDs), can operate within a certain range, and the peak wavelength of the emitted light is close to that of silicon phototransistors. Therefore, they have high conversion efficiency and fast response speed.
2. Grating Pair: Composed of a main grating and an indicator grating with equal grating pitch. The main grating and the indicator grating overlap but are not completely aligned. Their grating lines are offset by a small angle to produce moiré fringes. Generally, the main grating is movable; it can move independently or with the object being measured, and its length depends on the measurement range. The indicator grating is fixed relative to the photoelectric device.
3. Light Passage: The light passage is the pathway between the light emitter and the light receiver. It is generally strip-shaped, and its length is determined by the arrangement length of the light receivers, while its width is determined by the size of the light receivers. It is attached to the indicator grating plate.
4. Photosensitive element: The photosensitive element is used to sense the movement of moiré fringes produced when the main grating moves, thereby measuring the displacement. When selecting a photosensitive element, factors such as sensitivity, response time, spectral characteristics, stability, and size must be considered.
The main grating and the scale grating are placed overlapping, with a very small gap between them, and the lines of the two gratings are made to form a small angle θ, as shown in the figure.
When illuminated by a light source, alternating bright and dark stripes are formed in a direction roughly perpendicular to the grating lines due to the light-blocking effect (for gratings with a line density ≤ 50 lines/mm) or the diffraction of light (for gratings with a line density ≥ 100 lines/mm).
Where the lines of the two gratings overlap, light passes through the gap, forming a bright band; where the lines of the two gratings are misaligned, a dark band is formed; these alternating bright and dark stripes are called moiré fringes.
The relationship between the spacing of the moiré fringes and the grating pitch W and the angle θ (in rad) between the two grating lines is as follows:
(K is called the magnification factor).
When the indicator grating is stationary, the grating lines of the main grating and the indicator grating lines always maintain an angle θ. When the main grating is moved relative to the grating lines in the direction perpendicular to the grating lines, the moiré fringes will move along the direction of the grating lines. When the grating moves in the opposite direction, the moiré fringes will also move in the opposite direction.
For every movement of the principal grating by one pitch W, the moiré fringe also moves by one pitch S. Therefore, by measuring the movement of the moiré fringes, the magnitude and direction of the grating movement can be measured, which is much easier than directly measuring the grating.
When the main grating moves by a grating pitch W in a direction perpendicular to the grating lines, the moiré fringes move by a fringe spacing. When the angle θ between the two grating lines is small, according to the above formula, for a given W, the smaller θ is, the larger B is, which is equivalent to magnifying the grating pitch W by a factor of 1/θ. Therefore, the magnification of the moiré fringes is quite large, enabling highly sensitive displacement measurement.
Moiré fringes are formed by many lines of a grating and have an averaging effect on line errors. They can largely eliminate the influence of local and short-period errors caused by line errors, achieving measurement accuracy higher than that of the grating itself. Therefore, metrological gratings are particularly suitable for small displacement and high-precision displacement measurements.
Features of grating sensors
1. High precision.
In terms of large-range length or linear displacement measurement, grating sensors are second only to laser interferometers. However, in continuous measurement of circular division and angular displacement, grating sensors offer the highest accuracy.
2. Large range measurement with high resolution.
Inductive synchrotrons and magnetic grating sensors also have the characteristic of large-range measurement, but their resolution and accuracy are not as good as those of grating sensors.
3. It enables dynamic measurement and facilitates the automation of measurement and data processing;
4. It has strong anti-interference capabilities and is not as demanding on environmental conditions as laser interferometer sensors, but it is not as adaptable as inductive synchrotrons and magnetic grating sensors. Oil and dust can affect its reliability. It is mainly suitable for use in laboratories and workshops with relatively good environments.
Types of grating sensors
Gratings are mainly divided into two categories: one is Bragg grating (also known as reflection or short-period grating); the other is transmission grating (also known as long-period grating).
Fiber gratings can be classified into periodic and aperiodic structures in terms of structure, and into filtering gratings and dispersion compensation gratings in terms of function. Dispersion compensation gratings are aperiodic gratings, also known as chirp gratings.
Fiber Bragg grating sensor
Fiber Bragg gratings are made using the photosensitivity of optical fibers. The photosensitivity of optical fibers refers to the periodic change in refractive index along the fiber core axis that occurs when laser light passes through a doped fiber, thus forming a permanent spatial phase. The refractive index of the fiber grating will change accordingly with the spatial distribution of light intensity. The spatial phase grating formed within the fiber core essentially functions as a narrow-band (transmission or reflection) filter or mirror.
When a broadband beam of light passes through a fiber Bragg grating, wavelengths that satisfy the Bragg condition of the fiber Bragg grating will be reflected, while the remaining wavelengths will continue to propagate through the fiber Bragg grating. This property can be used to manufacture many fiber optic devices with unique properties.
Chirped fiber Bragg grating sensor
Similar to the working principle of fiber Bragg grating sensors, under the influence of external physical quantities, chirped fiber gratings will not only change ΔλB, but also cause spectral broadening.
This type of sensor is very useful in situations where both strain and temperature are present. Chirped fiber gratings experience broadening of the reflected signal and a shift in peak wavelength due to strain, while temperature changes, due to the temperature dependence of the refractive index (dn/dT), only affect the position of the center of gravity. By simultaneously measuring the spectral shift and broadening, both strain and temperature can be measured simultaneously.
Long-period fiber Bragg grating sensor
The period of a long-period fiber grating (LPG) is generally considered to be several hundred micrometers. An LPG couples light from the fiber core into the cladding at a specific wavelength: λi = (n0 - niclad) ? Λ. Where n0 is the refractive index of the fiber core, and niclad is the effective refractive index of the i-th order axisymmetric cladding mode. Light in the cladding will rapidly attenuate due to losses at the cladding/air interface, leaving a loss band.
A single LPG may exhibit numerous resonances over a wide wavelength range. The center wavelength of an LPG resonance primarily depends on the refractive index difference between the core and cladding. Any change caused by strain, temperature, or external refractive index variations can produce a large wavelength shift in the resonance. By detecting Δλi, information about changes in external physical quantities can be obtained. The response of an LPG to its resonance band at a given wavelength typically exhibits different amplitudes, thus making LPGs suitable for multi-parameter sensors.
Application areas of grating sensors
Because of their high measurement accuracy, wide dynamic measurement range, ability to perform non-contact measurements, and ease of system automation and digitization, grating sensors have been widely used in the machinery industry.
Applications of grating sensors in aerospace and ships
Advanced composite materials have good fatigue resistance and corrosion resistance, and can reduce the weight of ships or spacecraft, which is of great significance for rapid shipping or flight. Therefore, composite materials are increasingly used in the manufacture of aviation and marine vehicles (such as aircraft wings).
To comprehensively assess the condition of a ship's hull, it is necessary to understand the deformation torque, shear pressure, and impact force on the deck at different locations. A typical ship hull requires approximately 100 sensors, making fiber optic grating sensors with their high wavelength multiplexing capability ideally suited for hull inspection.
Fiber Bragg grating sensing systems can measure the bending stress of a ship's hull and the impact force of waves on a wet deck. A 16-channel fiber Bragg grating multiplexed system with interferometric detection capabilities has successfully achieved dynamic strain measurement with a bandwidth of 5 kHz and a resolution of less than 10ne/(Hz)1/2.
In addition, to monitor the strain, temperature, vibration, takeoff and landing status, ultrasonic field and acceleration of an aircraft, more than 100 sensors are usually required. Therefore, the weight of the sensors should be as light as possible and the size should be as small as possible. Thus, the most agile fiber optic grating sensor is the best choice.
In addition, there is actually strain in two directions in the composite materials of aircraft. Fiber grating sensors embedded in the material are ideal smart components for realizing multi-point and multi-axial strain and temperature measurement.
Application of grating sensors in civil engineering structures
Structural monitoring in civil engineering is the most active field for fiber optic grating sensors. For bridges, mines, tunnels, dams, buildings, and other structures, measuring the strain distribution of these structures can predict the local load and condition of the structure, facilitating maintenance and condition monitoring.
Fiber Bragg grating sensors can be attached to the surface of a structure or pre-embedded in it to simultaneously perform impact detection, shape control, and vibration damping detection, as well as monitor structural defects. Furthermore, multiple fiber Bragg grating sensors can be connected in series to form a sensor network for quasi-distributed structural monitoring, and the sensor signals can be remotely controlled via a computer.
One type of building structure that fiber Bragg grating sensors can detect is the bridge. In application, a set of fiber Bragg gratings is glued to the surface of the bridge's composite reinforcement, or a small groove is made on the surface of the beam so that the bare fiber core of the grating is embedded in the groove (for protection).
For more comprehensive protection, it is best to embed the optical gratings into the composite reinforcement during bridge construction. Additionally, to correct for strain caused by temperature effects, separate stress and temperature sensing arms can be used, with both arms installed on each beam.
Application of grating sensors in the power industry
Fiber Bragg grating (FBG) sensors are ideal for power industry applications due to their immunity to electromagnetic interference and ability to achieve long-distance, low-loss transmission. The load capacity of power lines, the temperature of transformer windings, and high currents can all be measured using FBG sensors.
In the power industry, current converters can transform current changes into voltage changes. These voltage changes cause deformation in piezoelectric ceramics (PZTs). By utilizing the wavelength shift of the fiber grating attached to the PZT, the deformation can be easily determined, and thus the current intensity can be measured. This is a relatively inexpensive method and does not require complex electrical isolation.
In addition, excessive pressure on power lines caused by heavy snow or other factors can lead to dangerous incidents. Therefore, it is very important to monitor power line pressure online, especially for power lines in mountainous areas where it is difficult to detect.
Fiber Bragg grating sensors can measure the load on a wire. The principle is to convert the change in load into a change in stress on a metal plate that is in close contact with the wire. This stress change can be detected by the fiber Bragg grating sensor attached to the metal plate.
This is an example of using fiber optic grating sensors to achieve measurements in harsh environments over long distances. In this case, the spacing between adjacent gratings is large, so rapid modulation and demodulation are not required.
In recent years, damage to transmission line towers caused by icing rain has occurred frequently. To monitor the tilt status of transmission towers, a common method is to use a GSM tower gauge to send the tilt information detected by the sensors to management personnel and a monitoring computer. The computer then processes the data and issues alarm messages based on the processing results. Another method is to directly attach resistance strain gauges to the structural components of the transmission tower for direct monitoring. Both of these methods are limited by certain factors, which negatively impact the monitoring work.
In recent years, research on the engineering applications of fiber optic sensors has developed rapidly. Among them, fiber Bragg grating sensors are functional fiber optic sensors that use fiber Bragg gratings as sensing elements. They can directly sense temperature and strain, as well as indirectly measure many other physical and chemical quantities related to temperature and strain. Stress change data from fiber Bragg grating sensors can reflect the tilt state of towers, and applying this method to tower tilt monitoring has significant advantages.
When using fiber optic sensing technology, specifically fiber optic Bragg gratings (FBRs), to monitor the tilt state of transmission line towers, the wavelength displacement information caused by stress changes on the FBR is utilized to obtain the stress change information sensed by the grating, thereby correspondingly obtaining the tilt state information of the tower and realizing the monitoring of the tower's tilt state.
Applications of grating sensors in medicine
In medicine, most sensors are electronic, which is unsuitable for many internal medicine procedures, especially in high-frequency microwave (radiation), ultrasound, or laser radiation-induced hyperthermic treatments. This is because the metal conductors in electronic sensors are easily affected by electromagnetic fields such as current and voltage, causing thermal effects around the sensor head or tumor, leading to erroneous readings.
In recent years, the use of high-frequency current, microwave radiation, and lasers for hyperthermia as an alternative to surgery has received increasing attention in the medical community. Furthermore, the small size of sensors is crucial in medical applications because smaller sizes minimize damage to human tissue. Fiber Bragg grating sensors are currently the smallest sensors achievable. They can measure precise local information about temperature, pressure, and sound wave fields within human tissue with minimal invasiveness.
To date, fiber optic grating sensing systems have successfully detected the temperature and ultrasonic field of diseased tissues, achieving measurement results with a resolution of 0.1℃ and an accuracy of ±0.2℃ within the range of 30℃ to 60℃. The measurement resolution of the ultrasonic field is 10⁻³ atm/Hz¹/², which provides useful information for the study of diseased tissues.
Fiber Bragg grating sensors can also be used to measure cardiac efficiency. In this method, a doctor inserts a thermodilution catheter embedded with a fiber Bragg grating into the right atrium of the patient's heart and injects a cold solution. The temperature of the blood in the pulmonary artery can be measured, and combined with pulse power, the cardiac output can be determined, which is very important for cardiac monitoring.
Application of grating sensors in the safety technology of high-speed rail operation in my country
When we ride trains, we sometimes feel a lot of vibration and feel uncomfortable. This is because flat spots or polygonal markings have appeared on the train wheels. Although these flat spots are only a few micrometers long, they can cause significant vibrations to the train due to its high speed. The role of sensors is to detect where these flat spots appear on the train.
A fiber optic grating monitoring system is essentially a sensor that draws carbon fiber into optical fibers, then etchs them into gratings for installation on trains and tracks. When the grating receives a laser signal, it reflects a wavelength. By analyzing the wavelengths reflected back from the grating at different locations on the train, train safety can be monitored in real time.
High-speed rail lines are complex and diverse, making sensor placement a major challenge. The brilliance of fiber optic sensors lies in their ability to monitor trains using the rails. By placing sensors on a short section of the rail, and ensuring the sensor length is slightly longer than the circumference of a train wheel, all train wheels passing through that section can be monitored. Similarly, sensors placed on the train wheels can be used to monitor the rails.
Regarding the advantages of fiber Bragg grating sensors, on the one hand, traditional sensors using electrical signals are subject to electromagnetic interference from trains and rails, while fiber optics do not have this problem; on the other hand, the sensors developed by the center are small in size and can be directly installed on high-speed trains without affecting the normal operation of the train.
Application of grating sensors in CNC machine tools
As a position detection element of the linear axis of a CNC machine tool, the grating sensor is equivalent to the "eyes" of a person, which "monitors" whether the linear axis actually moves accurately to the position required by the CNC system after executing the movement command of the CNC system.
If a CNC machine tool is not equipped with a grating sensor, whether the linear axis can reach the position required by the CNC system after the CNC system issues a movement command depends entirely on the accuracy of the CNC system's calibration and the accuracy of the mechanical transmission.
After a period of use, due to changes in electrical debugging parameters and increased mechanical errors, the linear axis of a CNC machine tool may deviate significantly from the position required by the CNC system commands. At this point, neither the CNC system nor the maintenance and operation personnel are aware of this discrepancy. To determine this difference, maintenance personnel must perform precision testing on the machine tool.
Therefore, if a CNC machine tool is not equipped with a grating sensor, its accuracy must be checked regularly. If the accuracy of the CNC machine tool is forgotten, it may lead to the processed products having out-of-tolerance accuracy or even being scrapped.
If a linear axis of a CNC machine tool is equipped with a grating sensor, the above problems will no longer need to be addressed by humans, as the grating sensor will take care of the task.
If the linear axis does not reach the correct position due to mechanical or other reasons, the grating sensor, as a position detection element, will send a command to the CNC system to enable the linear axis to reach a more accurate position, until the resolution of the grating sensor can no longer distinguish it.
At this point, the grating sensor acts as a monitoring function independent of the machine tool, like a human eye, constantly "monitoring" the position of the linear axis to ensure that the linear axis can reach the position required by the CNC system.
In general, fiber optic grating sensors have become a research hotspot in fiber optic sensors. With advancements in fiber optic grating manufacturing technology, performance improvements, and the continuous emergence of application development research results, fiber optic grating sensors have secured an increasingly important position in the sensor field. They have wide applications in civil engineering structures, aerospace, shipbuilding, power, petrochemical, medical, and nuclear industries. Many promising and marketable technologies are under research, and the maturity of these technologies will greatly promote national economic development.
