Laser is a kind of visible light or invisible light discovered by people in recent decades. Although it was discovered late, people's research and application of laser is really quite rapid. At present, laser is not only used in common measurement and information input, but also has been used in medical and military fields with people's attention to laser energy, such as the most common laser knife, laser myopia repair surgery and laser cannon.

principle of operation

When the laser sensor works, the laser emitting diode is aimed at the target to emit laser pulses. The laser is reflected by the target and 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. Avalanche photodiode is an optical sensor with internal amplification function, so it can detect extremely weak optical signals and convert them into corresponding electrical signals. Laser ranging sensor is common, which can measure the target distance by recording and processing the time from sending optical pulse to its return and reception. Laser sensors must measure the transmission time extremely accurately, because the speed of light is too fast.

For example, the speed of light is about 3 * 10 8m/s, and if the resolution reaches 1mm, the electronic circuit of the transmission time ranging sensor must be able to distinguish the following extremely short time: 0.001m/(3 *108m/s) = 3ps.

The time to distinguish 3ps is too high for electronic technology and the implementation cost is too high. Today's laser ranging sensor cleverly avoids this obstacle, and uses a simple statistical principle, namely the average rule, to achieve the resolution of 1mm and ensure the response speed.

Advantages and disadvantages of laser displacement sensor;

Next, Bian Xiao will introduce the advantages and disadvantages of laser displacement sensor. First of all, Bian Xiao will talk about the advantages of laser displacement sensor. Because the linearity of laser is very good, it is very accurate to measure whether something is displaced in a flat place. But for today's life, many factors in the construction site are interfered by the outside world, so it is impossible to be so idealistic. At the same time, the devices generated by laser are complicated, and the volume of laser displacement sensor is relatively large, so laser displacement sensor is used.

strong function

laser length measure

Precision length measurement is one of the key technologies in precision machinery manufacturing and optical processing industry. Modern length measurement mostly uses the interference phenomenon of light waves, and its accuracy mainly depends on the monochromaticity of light. Laser is the most ideal light source, which is 654.38+ million times purer than the best monochromatic light source in the past (Krypton -86 lamp). Therefore, laser length measurement has a wide range and high precision. 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/δ. The maximum length of krypton lamp -86 measurement is 38.5 cm, and the measurement of long objects needs to be segmented, which reduces the accuracy. If he-ne gas laser is used, it can measure dozens of kilometers at most. Generally, the length within a few meters can be measured to an accuracy of 0. 1 micron.

laser ranging

Its principle is the same as that of radio radar. After the laser is aimed at the target, the round-trip time is measured and multiplied by the speed of light to get the round-trip distance. From radar sensor to laser range finder has the advantages of high directivity, high monochromaticity and high power, which is very important for measuring distance, determining target orientation, improving signal-to-noise ratio of receiving system and ensuring measurement accuracy, so laser range finder is paid more and more attention. The laser radar developed on the basis of laser rangefinder can not only measure the distance, but also measure the target orientation, transportation speed and acceleration. It has been successfully used in satellite ranging and tracking, such as lidar with ruby laser, ranging from 500 km to 2000 km, with an error of only a few meters. Not long ago, LDM series ranging sensors developed by some R&D centers can achieve micron-level accuracy in the measurement range of several kilometers. The light sources of laser rangefinder are usually ruby laser, Nd: glass laser, carbon dioxide laser and gallium arsenide laser.

Laser vibration measurement

It measures the vibration speed of objects based on Doppler principle. Doppler principle means that if the wave source or the observer who receives the wave moves relative to the medium that propagates 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 movement speed of the wave source or the observer. The difference between the measured frequency and the frequency of the wave source is called Doppler shift. When the vibration direction is consistent with the direction, the Doppler frequency shift fd=v/λ, where V is the vibration speed and λ is the wavelength. In the laser Doppler vibrometer, fd=2v/λ due to the round trip of light. In this vibrometer, the optical part converts the vibration of the object into a corresponding Doppler frequency shift, and the optical detector converts this frequency shift into an electrical signal, which is sent to the Doppler signal processor after being properly processed by the circuit part, and the Doppler frequency shift signal is converted into an electrical signal corresponding to the vibration speed, and finally recorded on the magnetic tape. The vibrometer uses a He-Ne laser with a wavelength of 6328 angstrom (┱), uses an acousto-optic modulator for optical frequency modulation, uses a quartz crystal oscillator and a power amplifier circuit as driving sources of the acousto-optic modulator, uses a photomultiplier tube for photoelectric detection, and uses a frequency tracker to process Doppler signals. Its advantages are convenient use, no need to fix the reference system, no influence on the vibration of the object itself, wide measuring frequency range, high precision and large dynamic range. The disadvantage is that the measurement process is greatly influenced by other stray light.

Laser speedology

It is also a laser velocimeter method based on Doppler principle. Laser Doppler velocimeter (see laser flowmeter) is widely used, which can measure wind tunnel airflow speed, rocket fuel flow speed, aircraft jet airflow speed, atmospheric wind speed, particle size and convergence speed in chemical reaction, etc.