Application of detector deviation from focal plane in high-speed scanning lidar of Wuhan Aiwei Technology Co., Ltd

Lidar is an active detection technology that quickly obtains three-dimensional spatial information of targets. It is widely used in fields such as surveying, remote sensing, target detection, orbit detection, navigation, and autonomous driving. For LiDAR, when measuring targets at different distances, the returned echo energy will reach a very large dynamic range, and the echo energy of the same target at 10m and 1000m can differ by up to 10000 times. In addition, in high-speed scanning LiDAR, long-distance targets can also cause off target problems, leading to a further decrease in their farthest detection range. The dynamic range of ranging directly determines the effective detection range of LiDAR and its ability to simultaneously perceive light and dark objects in complex scenes. If the dynamic range of ranging is too small, it will lead to inaccurate measurement of objects with high reflectivity in close proximity by LiDAR, and even create blind spots, which will have adverse effects on subsequent applications. Off target phenomenon in high-speed scanning LiDAR

In commonly used scanning methods such as rotating mirrors and multi mirrors, there is a time difference between laser emission and echo reception. Within this time difference, the scanning mechanism has rotated a certain angle, resulting in a deviation between the laser foot and the instantaneous field of view angle center when the target echo is received by the LiDAR. In other words, the focus position of the echo through the receiving optical system is offset from the center of the detector target surface.

Figure 1: Off target phenomenon caused by time difference between laser emission and reception


The principle of detector deviation from the focal plane

1. Radial deviation

According to the characteristics of off target phenomenon, as shown in Figure 2, the detector target is artificially deviated radially towards the focal point of the distant echo, causing the distant echo to return to the field of view, thereby solving the off target problem of the distant echo under high speed scanning conditions.

Figure 2: Radial deviation of detector solves off target problem



Figure 3: The influence of radial deviation and target distance on the refractive index


2. Axial deviation

For echoes at different distances, the distant echoes can be approximated as parallel light, with the spot focused on the focal point of the optical system; Close range echo focuses at the position behind the focal point. When the size of the defocused spot is larger than the size of the detector target surface, only the echo energy inside the detector target surface is received, equivalent to the field of view aperture limiting the receiving aperture. Therefore, with a reasonable axial deviation, it is possible to receive all the weak echo energy of distant targets, attenuate the strong echo energy of close range targets, and thereby improve the dynamic range of the LiDAR system.

Figure 4: Principle of detector axial deviation


Figure 5: The influence of axial deviation and target distance on refractive index



Engineering application prospects

This article analyzes the off target problem in high-speed scanning LiDAR and proposes a method for the detector to deviate from the focal plane, which basically solves the problem of long-distance echo off target and improves the range dynamic range of high-speed scanning LiDAR. It has practical value and theoretical guidance for the engineering design of high-speed scanning range LiDAR.