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Dec 31 2019

The Function And Principle Of Laser Sensor

Ⅰ. Introduction

Laser technology and laser is one of the most important science and technology in the 1960s. It has the characteristics of strong direction, high brightness and good monochromaticity. It is widely used in industrial and agricultural production, national defense and military, medical and health, scientific research and other aspects, such as distance measurement, precision detection, positioning, etc., as well as length benchmark and optical frequency benchmark.

Laser technology

With the rapid development of laser technology and application, it has been combined with many disciplines to form new interdisciplinary subjects, such as optoelectronics, information optics, laser spectroscopy, nonlinear optics, ultrafast laser science, quantum optics, optical fiber optics, guided wave optics, laser medicine, laser biology, laser chemistry, etc. The emergence of these cross technologies and new disciplines has greatly promoted the development of traditional industries and emerging industries, making the application of lasers expand to almost all areas of the national economy.

Laser sensor is a kind of sensor which uses laser technology to measure. It is generally composed of laser, optical parts and photoelectric devices. It can convert the measured physical parameters (such as length, flow, speed, etc.) into optical signals, and then use photoelectric converter to convert the optical signals into electrical signals. Through the filtering, amplification and rectification of corresponding circuits, the output signals can be obtained, so as to calculate the measured quantity.

Laser sensor has the following advantages: simple and reliable principle and structure, strong anti-interference ability, suitable for all kinds of harsh working environment, high resolution ( it can reach several nanometers when measuring the length), small measurement error, good stability, suitable for rapid measurement.

. What is laser?

Laser light is different from ordinary light. We need to use laser to produce laser light. In the normal state, most of the atoms in the laser are in stable low energy level E1. Under the action of appropriate frequency of external light, the atoms in low energy level absorb photon energy to excite and transition to high energy level E2. The photon energy E = e2-e1 = h V, where h is the Planck constant and V is the photon frequency. On the contrary, when the frequency of light is V, the atom in level E2 will jump to the low energy level to release energy and emit light, which is called stimulated radiation. First of all, the laser makes the atoms of the working materials abnormally in the high-energy level (i.e. inversion distribution of the particle number ), which can make the stimulated radiation process dominant, so that the induced light with the frequency of V can be enhanced, and the large stimulated radiation light can be produced through the avalanche amplification of the parallel reflector, which is called laser light for short.

Single Blue Laser

. Important characteristics of laser

1. High directivity, small divergence angle of light speed, the laser beam extends only a few centimeters from a few kilometers away.

2. High monochromaticity, the frequency width of laser light is more than 10 times smaller than that of ordinary light.

3. High brightness, laser beam convergence can produce temperatures up to several million degrees.

. Types of laser

Laser can be divided into four types according to working substance:

1. Solid state laser

Its working substance is solid. Ruby laser, neodymium doped yttrium aluminum garnet laser (i.e. YAG laser) and neodymium glass laser are commonly used. Their structures are basically the same, characterized by small and solid, high power. At present, neodymium glass laser is the device with the highest pulse output power, which has reached tens of megawatts.

2. Gas laser

Its working substance is gas. Now there are various kinds of gas atoms, ions, metal vapor, gas molecular lasers. Commonly used are carbon dioxide laser, helium neon laser and carbon monoxide laser, whose shape is like a common discharge tube, characterized by stable output, good monochromaticity, long life, but small power, low conversion efficiency.

3. Liquid laser

It can be divided into chelate laser, inorganic liquid laser and organic dye laser, the most important of which is organic dye laser. Its main feature is that the wavelength is continuously adjustable.

4. semiconductor laser

It is a younger laser, and the more mature one is GaAs laser. It is characterized by high efficiency, small size, light weight and simple structure, and is suitable for carrying on airplanes, warships, tanks and infantry. It can be made into range finder and sighting device. However, the output power is small, the directivity is poor, and it is greatly affected by the ambient temperature.

. Characteristics and application of laser 

Laser measurement of length

Precise measurement of length is one of the key technologies in precise machinery manufacturing industry and optical processing industry. Modern length measurement is mostly based on the interference phenomenon of light wave, and its accuracy mainly depends on the monochromaticity of light. Laser is the most ideal light source. It is 100 thousand times purer than the best monochromatic light source (krypton-86 lamp). Therefore, the laser measurement range of length is large and the accuracy is high. According to the optical principle, the relationship between the maximum measurable length L of monochromatic light and wavelength λ and spectral line width δ is L = λ 2 / δ. The maximum measurable length of krypton-86 lamp is 38.5cm. For a long object, it is necessary to measure in sections to reduce the accuracy. If He-Ne gas laser is used, it can measure tens of kilometers at most. Generally, the length within several meters can be measured with an accuracy of 0.1 μ M.

Laser Measure

Laser measurement of distance

Its principle is the same as that of the radio radar. After the laser is aimed at the target, the round-trip time is measured, and then the round-trip distance is obtained by multiplying the speed of light. Because of the advantages of laser, such as high directivity, high monochromaticity and high power, these are very important for the measurement of long distance, the determination of target orientation, the improvement of signal-to-noise ratio of the receiving system, and the guarantee of measurement accuracy, so the laser rangefinder is paid more and more attention. The lidar developed on the basis of the laser rangefinder can not only measure the distance, but also the azimuth, velocity and acceleration of the target. It has been successfully used in the ranging and tracking of the artificial satellite. For example, the lidar using ruby laser has a distance measuring range of 500-2000 km with an error of only a few meters. At present, ruby laser, neodymium glass laser, carbon dioxide laser and GaAs laser are often used as the light source of laser rangefinder.

Measuring Distance with Laser Sensor Technology

Laser measurement of thickness

Based on the principle of triangle ranging, a precise laser ranging sensor is divided at the upper and lower part of the C-frame. The modulated laser emitted by the laser hits the surface of the measured object. By sampling the signal of the linear CCD, the distance between the measured object and the C-frame is synchronously obtained by the linear CCD camera under the control of the control circuit. The thickness of the middle measured object is calculated by the data fed back by the sensor. Because the detection is continuous, the continuous dynamic thickness of the measured object can be obtained.

Thickness measuring laser sensor

1. Thickness measurement by single laser displacement sensor

Put the measured body on the measuring platform, measure the distance from the sensor to the platform surface, then measure the distance from the sensor to the measured body surface, and measure the thickness after calculation. It is required that there is no air gap between the measured body and the measuring platform, and the measured body is not cocked. These strict requirements can only be achieved offline.

2. Thickness measurement by double laser displacement sensor

A laser displacement sensor is installed above and below the measured body respectively, and the thickness of the measured body is d = C - (a + b). Among them, C is the distance between two sensors, a is the distance between the upper sensor and the measured body, and B is the distance between the lower sensor and the measured body. The advantage of this method for on-line thickness measurement is that it can eliminate the influence of the vibration of the measured body on the measurement results. But at the same time, there are requirements for sensor installation and performance. The conditions to ensure the accuracy of measurement are that two sensor beams must be coaxial and that two sensor scans must be synchronous. Coaxiality is realized by installation, and synchronization depends on the selection of laser sensor with synchronization end.


Laser vibration measurement

It is based on the principle of Doppler to measure the vibration velocity of objects. Doppler principle refers to that if the observer of wave source or receiving wave moves relative to the medium of propagation wave, the frequency measured by the observer not only depends on the vibration frequency emitted by the wave source, but also depends on the size and direction of the motion speed of the wave source or observer. The difference between the measured frequency and the frequency of the source is called Doppler shift. When the vibration direction is the same as the direction, the multi frequency shift FD = V / λ, where V is the vibration speed and λ is the wavelength. In the laser Doppler vibration velocity measuring instrument, FD = 2V / λ due to the reason of light back and forth. In the measurement, the vibration of the object is converted into the corresponding Doppler frequency shift by the optical part, and the frequency shift is converted into the electrical signal by the optical detector, and then it is sent to the Doppler signal processor for proper processing by the circuit part to transform the Doppler frequency shift signal into the electrical signal corresponding to the vibration speed, and finally recorded on tape. The vibrometer uses a He-Ne laser with a wavelength of 6328 angstroms, an acousto-optic modulator for optical frequency modulation, a quartz crystal oscillator and a power amplifier as the driving source of the acousto-optic modulator, a photomultiplier tube for photoelectric detection and a frequency tracker for Doppler signal processing. It has the advantages of convenient use, no fixed reference system, no impact on the vibration of the object itself, wide measurement frequency range, high accuracy and large dynamic range. The disadvantage is that the measurement process is greatly affected by other stray light.

Vibration Measurements by Laser

Laser velocity measurement

It is also a laser velocity measurement method based on the kidople principle, which is mostly used as a laser Doppler velocimeter. It can measure the wind speed of wind tunnel, the velocity of rocket fuel, the velocity of jet flow of aircraft, the atmospheric wind speed, the size and convergence speed of particles in chemical reaction, etc.

Principle of Doppler velocity measurement system

The distance between the sound waves heard by the locomotive was compressed, as if a person were turning off the accordion. The result of this action is a significantly higher pitch. When the train leaves, the sound waves spread, and there is a lower sound. This phenomenon is known as the "Doppler" effect.

This Doppler effect is also used by the radar velocimeter to check the speed of motor vehicles. A beam of radiation is emitted from the velocimeter, which is sent to the car and then returned to the velocimeter. The micro information processor in the tachometer compares the returned wavelength with the original wavelength. The closer the return wavelength is, the faster the car is moving forward, which proves that drivers are more likely to over speed.

Laser Doppler velocimeter is to measure the Doppler signal of the tracer particles through the laser probe, and then get the velocity according to the relationship between velocity and Doppler frequency. Because of the laser measurement, there is no interference to the flow field, and the speed measurement range is wide. Because the Doppler frequency is linear with the speed and has no relationship with the temperature and pressure of the point, it is the instrument with the highest speed measurement accuracy in the world.

The principle of Doppler velocity measurement can be explained by interference fringes. When the focus lens converges the two incident beams at a certain angle, the interference fringes between light and dark are formed on the converging point by the good coherence of the dry laser beam. The fringes spacing is directly proportional to the wavelength of the dry light wave, and inversely proportional to the sine value of the dry half angle. When the particles in the fluid pass through the direction of the fringes, they will successively scatter a series of scattered light waves with time varying light intensity, which is called Doppler signal. The frequency at which the light intensity changes is called Doppler shift. The higher the velocity of particles passing through the fringes, the higher the Doppler shift. The linear relationship between Doppler shift and fluid velocity can be obtained by dividing the particle velocity perpendicular to the fringes by the fringes interval and considering the refractive index of the fluid. Doppler velocity measurement system uses the linear relationship between velocity and Doppler frequency shift to determine velocity. The phase difference of Doppler frequency in each direction is directly proportional to the particle diameter. The particle size can be determined by the phase difference detected.

. Main principle and application of two kinds of laser sensors

By using the characteristics of laser, such as high directivity, high monochromaticity and high brightness, the contactless remote measurement can be realized. Laser sensors are often used to measure physical quantities such as length, distance, vibration, speed, orientation, etc. they can also be used for detection and monitoring of air pollutants. In a word, the application field of laser sensor is more and more extensive. The main principles and applications of two kinds of laser sensors are introduced below.

Laser displacement sensor

The laser displacement sensor can make use of the characteristics of laser, such as high directivity, high monochromaticity and high brightness to realize non-contact remote measurement. Laser displacement sensor (magnetostrictive displacement sensor) is a new type of measuring instrument made of these advantages of laser. Its appearance greatly improves the accuracy and reliability of displacement measurement, and also provides an effective measurement method for non-contact displacement measurement.

Laser Displacement Sensor

The laser emitter shoots the visible red laser to the object surface through the lens, and the laser reflected by the object passes through the receiver lens, which is accepted by the internal CCD linear camera. According to different distances, the CCD linear camera can "see" this light point at different angles. According to the distance between the laser and the camera known from this angle, the digital signal processor can calculate the distance between the sensor and the measured object.

At the same time, the position of the beam in the receiving element is processed by analog and digital circuits, and the corresponding output value is calculated by microprocessor analysis, and the standard data signal is output in proportion in the analog quantity window set by the user. If switching value output is used, it will be conducted in the set window and cut off outside the window. In addition, independent detection window can be set for analog quantity and switch quantity output.

Measurement principle of laser echo analysis

The laser displacement sensor can achieve a certain degree of accuracy by using the echo analysis principle to measure the distance. The sensor is composed of processor unit, echo processing unit, laser transmitter and laser receiver. The laser displacement sensor emits one million pulses per second through the laser transmitter to the detector and returns to the receiver. The processor calculates the time required for the laser pulse to meet the detector and return to the receiver, so as to calculate the distance value. The output value is the average output of thousands of measurement results.

Application of laser displacement sensor

1. Dimension measurement: position identification of small parts; monitoring of whether there are parts on the conveyor belt; detection of material overlapping and covering; control of manipulator position (tool center position); device state detection; detection of device position (through the small hole); monitoring of liquid level; thickness measurement; vibration analysis; collision test measurement; automobile related test, etc.

2. Thickness measurement of sheet metal: laser sensor measures the thickness of sheet metal. Thickness change detection can help to detect wrinkles, small holes or overlaps to avoid machine failure.

3. Cylinder measurement: angle, length, eccentricity of inner and outer diameter, conicity, concentricity and surface profile.

Displacement Sensors Application

4. Length measurement: place the measured component on the conveyor belt at the designated position, the laser sensor detects the component and simultaneously measures it with the triggered laser scanner, and finally obtains the length of the component.

5. Uniformity check: place several laser sensors in a row in the tilt direction of the workpiece movement to be measured, and directly output the measurement value through one sensor. In addition, a software can be used to calculate the measurement value and read out the result according to the signal or data.

6. Inspection of electronic components: two laser scanners are used to place the tested components between them. Finally, the data is read out by the sensor, so as to detect the accuracy and integrity of the component size.

7. Inspection of filling level in production line: laser sensor is integrated into the production and manufacturing of filling products. When the filling products pass through the sensor, it can detect whether the filling is full. The sensor can accurately identify whether the filling product is qualified and the quantity of the product by using the extended program of laser beam reflecting surface.

Distance measuring laser sensor

The principle of laser ranging sensor is the same as that of wireless radar. After the laser is aimed at the target, the round-trip time is measured, and then the round-trip distance is obtained by multiplying the speed of light. Because of the advantages of laser, such as high directivity, high monochromaticity and high power, these are very important for measuring long distance, determining the target orientation, improving the performance to noise ratio of the receiving system, and ensuring the measurement accuracy.

Distance measuring laser sensor

Principle of distance measuring laser sensor

Laser ranging is actually an active optical detection method. The detection mechanism of active optical detection is that the beam is transmitted from the detection system to the target (in optical detection, it is usually infrared or visible light), and the beam is radiated by the target surface to generate the echo signal. The echo signal directly or briefly contains the information to be measured. The receiving and signal processing system obtains the measured signal by receiving and analyzing the echo signal.

Application of distance measuring laser sensor

Automobile anti-collision detector

Generally speaking, most of the existing laser ranging sensors of the vehicle collision prevention system use the laser beam to identify the distance between the target vehicles in the front or rear situation in a non-contact way. When the distance between vehicles is less than the predetermined safety distance, the vehicle anti-collision system will brake the vehicle in an emergency, or give an alarm to the driver, or make real-time judgment and response to vehicle driving by integrating the target vehicle speed, Vehicle distance, vehicle braking distance, response time and so on, which can greatly reduce traffic accidents. Its advantages are more obvious when it is used on highway.


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