Lidar - System Parameters
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1、 System composition

Lidar is mainly composed of four systems: laser emission, reception, rotation mechanism, and information processing, forming a sensing loop to achieve target detection, tracking, and recognition.Lidar (Light Detection and Ranging, abbreviated as LiDAR), as an advanced remote sensing technology, has been widely used in fields such as unmanned driving, robot navigation, terrain surveying, and meteorological observation. Lidar measures parameters such as distance and velocity between targets and detectors by emitting laser beams and receiving reflected signals, and has advantages such as high precision, high resolution, and strong anti-interference ability. This article will provide a detailed introduction to the system parameters of LiDAR, including wavelength, far-field capability, frequency, point frequency, angular resolution, field of view angle, number of lines, etc., and explore the impact of these parameters on the performance of LiDAR.


1. Laser emission

There are four types of commonly used lasers: semiconductor, solid-state, fiber, and carbon dioxide gas lasers. Among them, unmanned driving mostly uses semiconductor lasers, which are small in size, coherent, and highly reliable.



2. Laser reception

After the laser emitted by the laser is irradiated on an obstacle, the reflected light will converge onto the receiver through the lens group through the reflection of the obstacle. The core device of laser detection is a photodetector, which converts light energy into a semiconductor device that facilitates the measurement of physical quantities (voltage or current) based on the photoelectric effect. It mainly has characteristics such as frequency bandwidth, high sensitivity, wide linear output range, and low noise.

From the perspective of detection technology, there are mainly two types of detection techniques: direct detection and coherent detection.



Direct detection method (energy detection) utilizes the photoelectric conversion function of the detector to directly demodulate the information of the optical signal. The system is simple, but also has low accuracy and is sensitive to noise.
Coherent detection method (heterodyne detection) has an additional laser output compared to direct detection method, achieving signal mixing analysis. Sensitivity and accuracy are high, but the system is relatively complex.

In summary, LiDAR achieves target detection and other functions through a combination of laser emission and reception devices.


3. Rotating mechanism

The rotating mechanism is responsible for rotating the laser emitting and receiving mechanisms at a certain frequency to achieve scanning of two-dimensional/three-dimensional space and generate real-time point cloud information.
During the process of rotation, institutions require the transmission of energy and data. Therefore, most non solid state LiDARs are designed with an electric rotary connector (commonly known as a "slip ring", consisting of a stator and rotor, with several brushes inside) installed at the center of the mechanism's rotation to transmit energy and data information (if wires are directly connected, rotation can cause entanglement problems). It is precisely due to the existence of slip rings that the lifespan of mechanical rotary LiDAR is greatly affected. Currently, some manufacturers use technologies such as optical magnetic fusion to achieve non-contact power supply and data transmission, which to some extent increases the lifespan of the radar.


4、 Lidar system parameters

wavelength

The wavelength of LiDAR is one of the important parameters that affect its performance. At present, the commonly used laser radar wavelengths are mainly 905nm and 1550nm. The laser radar with a wavelength of 905nm has high detection sensitivity and fast response speed, but is greatly affected by atmospheric attenuation and has relatively weak penetration ability. The 1550nm wavelength lidar has good penetration ability and can maintain high detection performance in harsh environments such as rain, fog, and dust. In addition, lasers with a wavelength of 1550nm are less likely to propagate in human eye fluids, resulting in less harm to the eyes and greater safety and reliability.


Telemetry capability

Distance measurement capability refers to the farthest detection distance of a target by a LiDAR. This parameter is influenced by various factors, including the emission power of the laser, the sensitivity of the receiver, atmospheric attenuation, etc. In practical applications, the ability to measure distance is usually related to the reflectivity of the target. For targets with low reflectivity (such as white paper), the ranging ability of LiDAR will correspondingly decrease. Therefore, when choosing a LiDAR, it is necessary to choose the appropriate far-field measurement capability based on the actual application scenario and target characteristics.

frequency

Frequency refers to the reciprocal of the time required for a complete scan of a LiDAR, also known as scanning speed or frame rate. The higher the frequency of LiDAR, the more target information can be obtained per unit time, thereby improving the real-time and accuracy of detection. However, high frequency also means higher power consumption and more complex control system design. Therefore, when selecting the frequency of a LiDAR, it is necessary to comprehensively consider factors such as power consumption, real-time performance, and accuracy.

Point frequency

Point frequency refers to the total number of points scanned by a LiDAR per second. The higher the point frequency, the richer the target information obtained by LiDAR, thereby improving the resolution and accuracy of detection. However, an increase in point frequency can also increase the difficulty and computational complexity of data processing. Therefore, when selecting the point frequency of a LiDAR, it is necessary to balance it based on practical application requirements and data processing capabilities.

Angular resolution

Angular resolution refers to the minimum angle that a LiDAR can distinguish in the horizontal or vertical direction. The higher the angular resolution, the more accurate the detection of targets by LiDAR. The angular resolution is usually determined by the scanning method and scanning angle of the LiDAR. In practical applications, it is necessary to choose the appropriate angular resolution based on the detection target and detection range.

Field of view angle

The field of view angle refers to the range angle that the LiDAR can detect. The larger the field of view angle, the wider the detection range that LiDAR can cover. However, increasing the field of view angle also increases the complexity and cost of LiDAR. Therefore, when choosing the field of view angle of a LiDAR, it is necessary to weigh it based on actual application requirements and cost budget.

The number of lines in LiDAR

The number of lines in a LiDAR refers to the number of laser beams in the vertical direction (equal to the number of laser transceiver modules). The more lines there are, the richer the target information obtained by LiDAR, thereby improving the resolution and accuracy of detection. However, an increase in the number of lines will also increase the complexity and cost of LiDAR. Therefore, when selecting the number of lines for LiDAR, it is necessary to weigh the actual application requirements and cost budget.

5、 The influence of system parameters on the performance of LiDAR

The Influence of Wavelength on Detection Performance


Wavelength is one of the important factors affecting the detection performance of LiDAR. Lidars of different wavelengths have different detection capabilities in different environments and target conditions. When selecting a LiDAR, it is necessary to choose the appropriate wavelength based on the actual application scenario and target characteristics.

The impact of distance measurement capability on detection range

The ability to measure distance is one of the key factors determining the detection range of LiDAR. The greater the distance measurement capability, the wider the detection range that LiDAR can cover. However, the ability to measure distance is also affected by factors such as target reflectivity and atmospheric attenuation. Therefore, when choosing a LiDAR, it is necessary to choose the appropriate telemetry capability based on practical application requirements and target characteristics.
The impact of frequency and point frequency on the real-time and accuracy of detection

Frequency and point frequency are important factors that affect the real-time and accuracy of LiDAR detection. High frequency and high spot frequency LiDAR can obtain more target information, thereby improving the real-time and accuracy of detection. However, high frequency and high spot frequency also mean higher power consumption and more complex control system design. Therefore, when choosing a LiDAR, it is necessary to comprehensively consider factors such as power consumption, real-time performance, and accuracy.

The influence of angular resolution and field of view angle on detection accuracy and range

Angular resolution and field of view are important factors that affect the detection accuracy and range of LiDAR. High angle resolution LiDAR can obtain more accurate target information, while a larger field of view can cover a wider detection range. However, the improvement of angular resolution and field of view angle will also increase the complexity and cost of LiDAR. Therefore, when choosing a LiDAR, it is necessary to weigh the actual application requirements and cost budget.

6、 Conclusion

Lidar, as an advanced remote sensing technology, has broad application prospects in fields such as unmanned driving, robot navigation, terrain surveying, and meteorological observation. The performance of LiDAR is influenced by various system parameters, including wavelength, far-field capability, frequency, point frequency, angular resolution, field of view angle, number of lines, etc