Function and Application of Laser Sensors

Catalog

Ⅰ Introduction

Laser sensors are sensors that use laser technology for measurement. It consists of a laser, laser detector, and measuring circuit. The laser sensor is a new type of measuring instrument. Its advantages are that it can achieve non-contact long-distance measurement with fast speed, high accuracy, large range, and strong anti-light interference and anti-electrical interference. 

Ⅱ Main classification of laser sensor 

laser sensor

Lasers can be divided into 4 types according to working materials. ①Solid laser: its working substance is solid. Commonly used working substances are ruby laser, neodymium-doped yttrium aluminum garnet laser (ie YAG laser), and neodymium glass laser. Their structure is roughly the same, with the characteristics of small and rugged, high power. Neodymium glass laser is the highest pulse output power device whose output power has reached tens of megawatts. ②Gas laser: its working substance is gas. Now there are various gas atoms, ions, metal vapor, gas molecular lasers. Commonly used gas lasers are carbon dioxide lasers, helium-neon lasers and carbon monoxide lasers, which are shaped like ordinary discharge tubes, are characterized by stable output, good monochromaticity, long life, but low power and low conversion efficiency. ③Liquid laser: It can be divided into a chelate laser, inorganic liquid laser, and organic dye laser, among which the most important one is organic dye laser whose biggest feature is the continuously adjustable wavelength. ④Semiconductor laser: It is a younger type of laser, and the more mature one is gallium arsenide laser. It is characterized by high efficiency, small size, lightweight, and simple structure, and is suitable for being carried on airplanes, warships, tanks and infantry. It can be made into a rangefinder and sight. However, its output power is small, and it is greatly affected by the ambient temperature.

Ⅲ How do laser sensors work?

When the laser sensor is working, the laser emitting diode emits laser pulses to the target. After being reflected by the target, the laser light is scattered in all directions. Part of the scattered light returns to the sensor receiver and is imaged on the avalanche photodiode after being received by the optical system. The avalanche photodiode is an optical sensor with an internal amplification function, so it can detect extremely weak optical signals and convert them into corresponding electrical signals. A commonly used laser sensor is the laser ranging sensor, which can measure the target distance by recording and processing the time from when the light pulse is sent to when it is received. The laser sensor must measure the transmission time extremely accurately because the speed of light is too fast.

Working of laser sensor

For example, the speed of light is about 3*10^8m/s. To achieve a resolution of 1mm, the electronic circuit of the transmission time ranging sensor must be able to distinguish the following extremely short time: 0.001m/(3*10^8m/s)=3ps

It is too expensive to distinguish the time of 3ps, which is an excessively high requirement for electronic technology. But today's laser ranging sensors cleverly avoid this obstacle, using a simple statistical principle, that is, the average rule to achieve a resolution of 1mm, and guarantee the response speed.

Ⅳ Main functions of laser sensor

The high directivity, high monochromaticity, and high brightness of the laser can realize the non-contact long-distance measurement. Laser sensors are often used for the measurement of physical quantities such as length, distance, vibration, speed, and orientation. They can also be used for flaw detection and monitoring of atmospheric pollutants.

1 Length measurement

Precise length measurement is one of the key technologies in the precision machinery manufacturing industry and optical processing industry. 

Most modern length measurements are made using the interference phenomenon of light waves, and its accuracy mainly depends on the quality of the monochromatic light. Laser is the most ideal light source. It is 100,000 times purer than the best monochromatic light source (Krypton-86 lamp) in the past. Therefore, the range of laser length measurement is large and the accuracy is high. According to the optical principle, the relationship between the maximum measurable length L of monochromatic light, the wavelength λ and the line width δ is L=λ/δ. With Krypton-86 lamp, the maximum length can be measured is 38.5 cm. For longer objects, it needs to be measured in sections and the accuracy is reduced. If a helium-neon gas laser is used, it can measure up to tens of kilometers. Generally measuring the length within a few meters, its accuracy can reach 0.1 microns.

2 Distance measurement

 Laser ranging sensor

Its principle is the same as that of radio radar. After the laser is emitted at the target, its round-trip time is measured, and then multiplied by the speed of light to get the round-trip distance. Because lasers have the advantages of high directivity, high monochromaticity, and high power, these are very important for measuring long distances, determining target azimuth, improving the signal-to-noise ratio of the receiving system, and ensuring measurement accuracy.  The laser radar developed on the basis of the laser range finder can not only measure the distance but also measure the target azimuth, transportation speed, and acceleration, etc. It has been successfully used in the ranging and tracking of artificial satellites, such as the laser using ruby laser radar, ranging from 500 to 2000 kilometers, with an error of only a few meters. The LDM series of distance measuring sensors developed by Zhenshen's research and development center can achieve micron-level accuracy in the measurement range of several kilometers. Ruby lasers, neodymium glass lasers, carbon dioxide lasers, and gallium arsenide lasers are often used as light sources for laser rangefinders.

3 Vibration measurement

A laser sensor measures the vibration speed of an object based on the Doppler principle. The Doppler principle refers to If the wave source or the observer receiving the wave moves relative to the medium propagating the wave, the frequency measured by the observer depends not only on the vibration frequency emitted by the wave source but also on the magnitude and direction of the motion speed of the wave source or observer. The difference between the measured frequency and the frequency of the wave source is called the Doppler shift. When the vibration direction is consistent with the direction, the Dopp 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 reciprocation of light. During the measurement, the optical part converts the vibration of the object into the corresponding Doppler frequency shift by the optical part and converts the frequency shift into an electrical signal by the photodetector. And then the signal will be sent to the Doppler signal processor. The processor converts the Doppler frequency shift signal into an electrical signal corresponding to the vibration speed and finally records it on the magnetic tape. This vibrometer uses a HeNe laser with a wavelength of 6328 Angstroms (┱), an acousto-optic modulator for optical frequency modulation, a quartz crystal oscillator plus a power amplifier circuit as the driving source for the acousto-optic modulator, a photomultiplier tube for photoelectric detection, and the frequency tracker to process the Doppler signal. Its advantage is that it is easy to use; it does not require a fixed reference frame; it does not affect the vibration of the object itself; it has a wide measurement frequency range, high accuracy, and a large dynamic range. The disadvantage is that the measurement process is greatly affected by other stray light.

4 Speed measurement

It is also a laser speed measurement method based on the Doppler principle. The laser Doppler flowmeter is used more. It can measure the wind tunnel airflow speed, rocket fuel flow rate, aircraft jet flow rate, atmospheric wind speed, and particle size and aggregation speed in chemical reactions.

Ⅴ Applications of laser sensors

1 Over-limit detection of vehicle width and height

The laser sensor is used for rapid measurement, the network core of the PC industrial control computer, and the visual programming software VB and the sensor are used for real-time transmission and processing of data, and the friendly interface computer control software is also designed. Field test data shows that the system has good real-time performance and high measurement accuracy, and has certain practical value.

2 Highway toll stations

The laser sensor is used in highway toll stations to count and protect vehicles. Malaysian Teras has applied hundreds of BEA laser sensors to its manual and automatic toll station systems. The laser sensor uses the time-of-flight (TOF) measurement principle, which can form 4 planes in the detection area to detect the vehicle. At the same time, the product also has functions such as anti-collision and vehicle safety protection. Compared with the traditional light curtain, the laser sensor has the advantages of high sensitivity, high accuracy, easy installation, high-cost performance, and strong stability.

3 Google's second-generation unmanned vehicles

Google's unmanned vehicles

In addition to the laser sensor on the top, Google's second-generation driverless car prototype is still quite obvious, and the other sensors are set very concealed. The front, rear, and sides of the vehicle are clearly marked with the Google unmanned vehicle logo. The driving principle of Google's unmanned vehicle is to continuously collect various precise data of the vehicle itself and the surroundings through many sensors installed around the car, analyze and calculate it through the processor in the car, and then control the driving of the car according to the calculation results. Unmanned vehicles will use GPS devices and sensors to accurately locate the vehicle's position and speed, and judge pedestrians, vehicles, bicycles, signal lights, and many other objects around it.

The roof of this Lexus is equipped with a 360° rotating laser holographic sensor, which can sense the front, side, and rear conditions of the car almost simultaneously. The data collected by the sensor will be input into the processor located on the rear right side of the vehicle through the green data line. This laser sensor can also enable unmanned vehicles to perform accurate global positioning. The original L-shaped Lexus logo in front of the car was also removed and replaced with a radar sensor. To determine the condition of the vehicle in front and control the acceleration and deceleration of the vehicle, it was used to measure the distance in front and the speed of the vehicle. 

The wheel hub of the tire is also equipped with a position sensor, which is used to detect wheel rotation. The heart of Google's unmanned vehicles-the processor is located on the right rear side of the vehicle. The data information from each sensor will be transmitted here through the data wire and analyzed and processed through the software to accurately sense and judge the difference between the unmanned vehicle's object. In addition to analyzing and judging the current position of objects around the unmanned vehicle, the unmanned vehicle also needs to be calculated by software to accurately predict the possible next position of each object. Finally, the unmanned car will make safe driving decisions based on all the collected data, including controlling the speed of the car and the surrounding distance.

Ⅵ Development of global sensors

The global sensor industry market scale exceeded 200 billion in 2018. Due to the widespread attention and investment in countries around the world, the sensor exhibition is very fast. At present, there are more than 6,500 units engaged in research and production. The United States, Europe, and Russia are each engaged in more than 1,000 sensor research and production manufacturers, and more than 800 in Japan.

In recent years, the global sensor market has maintained rapid growth. As the economic environment continues to improve, the market demand for sensors will continue to increase. By 2017, the global sensor industry market size will increase to 195.5 billion US dollars, an increase of 12.29% year-on-year. According to preliminary calculations, the global sensor industry market size will exceed US$200 billion in 2018, reaching approximately US$205.9 billion, a year-on-year increase of 5.3%.

The major manufacturers in the global sensor market include GE sensors, Emerson, Siemens, Bosch, STMicroelectronics, Honeywell, ABB, Yokogawa, Omron, Schneider Electric, E+H, etc. In the global consumer inertial sensor (accelerometer + gyroscope) market, STMicroelectronics is in the market leader position, occupying about 40% of the market. Bosch is the world's largest manufacturer of MEMS sensors.

In terms of product structure, CMOS image sensors (CIS) accounted for 27% of the sensor market in the global sensor market in 2018, MEMS sensors accounted for 25% of the market, and voltage-based fingerprint recognition sensors accounted for 8% of single-material sensors. In addition, RF sensors and radar sensors have applications in long-distance detection and information transmission in communications, automobiles, and the military, accounting for 15% and 11%, respectively.

It is foreseeable that the Internet of Things will have more and more application needs in various industries, and will become the most noticeable long-term trend in the next 10 or 20 years. According to estimates by authoritative institutions, the size of the downstream market that the global Internet of Things is expected to affect will exceed US$3 trillion in 2020, with more than 25 billion systems/devices connected, and a total of 4.4 billion users using the Internet at the same time; McKinsey also predicts that the scale of the downstream application market that is expected to penetrate the Internet will grow to US$3.9-11.1 trillion by 2025, reaching a global economic share of about 11%.

Therefore, as the global Internet of Things enters a substantial development stage, the sensor manufacturing industry will benefit from the explosion of the Internet of Things. Specifically, the potential scale of the automotive sensor market is 5.7 billion, which is more than 14 times the current; the potential scale of the logistics sensor market is more than 10 billion, which is more than ten times the current; the potential scale of the coal mine security inspection sensor market is tens of billions of yuan; The growth rate of the security sensor market will be synchronized with the growth rate of the security industry's output value. The global sensor market will maintain a growth rate of about 8% in the next 5 years, and the market size will reach 328.4 billion US dollars by 2024.

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