Experimental setup

All the instruments in our wide-angle LiDAR system use the plastic support structure and shell designed and made by ourselves to complete the system erection. Although this method of setting up the instrument is the most suitable design for the actual application, it will not be possible to modify the structure once the plastic model is completed, so pre-optical simulation will become very important. Our system has undergone a complete optical system simulation before implementation, including ray tracing analysis, light field intensity analysis in each area, scattering analysis and atmospheric influence factor analysis. In Zemax optical simulation software, a non-sequential mode with external environmental interference factors is used to construct a scanning system. The plastic support structure is drawn by Solidworks, which is a multi-functional drawing software for drawing three-dimensional graphics, and it can work with the optical simulation

software Zemax, that is, 3D objects designed by yourself can be imported into Zemax for optical simulation. As shown in Fig. 4.1, this is the result of drawing the plastic support structure of the LiDAR system in Solidworks. The laser rangefinder is directly placed on the support frame, followed by a lens group to adjust the beam size, and finally it scans the space after being reflected by the MEMS mirror. In Fig. 4.1, the wide-angle lens has not been added for experiments, so the scanning angle is small, so there is no need to modify the structure of the laser rangefinder to receive the reflected beam signal. In this experiment, we directly match the laser rangefinder with the MEMS mirror to get a small-scan LiDAR system. However, in order to test the performance of the wide-angle lens we designed, we will finally discuss how to modify the system to complete a large-angle scan in the following chapters.

Fig. 4.1 The plastic support structure of the LiDAR system is drawn in the computer aid graphic software Solidworks.

Fig. 4.2 The 3D printed lens support structure is designed with a tolerance of 0.04 mm, so that the lens can be inserted smoothly when the lens is installed. The blue part in

the picture is the lens.

Although today's 3D printers can print highly accurate plastic structures, the impact of tolerances on the system combination must still be considered. When we originally designed the support frame, we directly used the size of the lens to design a set of support frame and printed it out, but because it was too close to the size of the lens, the lens could not be installed into the system smoothly. After this failed experience, we compared the equipment specifications of commercially available lens and optical experiment equipment manufacturers and found that if a tolerance of 0.04mm is added to the design of the lens and the support frame, the assembly of the lens can be successfully carried out.

As shown in Fig. 4.2. Therefore, the final experimental framework took into account the design of tolerances, and successfully designed a support frame that can be inserted into the lens. But because the wavelength of the laser light source we use is 905 nm, which belongs to the infrared light band and cannot be seen directly by human eyes, the laser must be used to observe the card repeatedly when setting up the system to ensure that the laser beam is completely injected into the MEMS mirror.

In this LiDAR architecture, a wide-angle lens has not been added to the system, so the scanning angle is only 24 degrees of the mechanical deflection of the MEMS mirror.

We then designed a support structure that can add a corner mirror, and actually conducted experiments to test the wide-angle scanning capability. In this LiDAR structure, because the processing time of the wide-angle lens is long, the wide-angle lens in the system is borrowed from Ultimems to test the scanning capability of the wide-angle LiDAR. Fig.

4.3 is a MEMS mirror mounted on a plastic support frame. Behind the MEMS mirror is its control circuit board and Arduino microcontroller. The laser beam is first adjusted by the beam expender, reflected by the MEMS mirror, and then passed through the wide-angle lens to increase the scanning wide-angle.

However, after experiments, it was found that because the laser light transmitting and receiving modules in the LiDAR system were modified using a modular laser rangefinder, in addition to the beam size of the transmitting end, the structure of the receiving end must also be modified. , So that the large-angle light signal can enter the photodetector. Related research in the past found that if the lens and housing in front of the receiving end are removed, a wide-angle optical signal can be received[25]. As shown in Fig. 4.4, although the receiver lens can increase the detection range of LiDAR, that is, it can increase the receiving ability of weak reflected light signals, but it severely limits the optical signal receiving angle of the system. If the receiver lens is removed, the limitation on the receiving angle of the optical signal can be lifted. But doing so will limit the detection range of the LiDAR system, because after the receiver lens is released, the weak light signal returned from the original distance will not be received, and the scanning distance of our LiDAR system is limited to 200 cm. After experiments, our wide-angle LiDAR system can scan at an angle of 100 degrees, and finally generate a 3D point cloud image through a self-designed image processing program. The calculation

process of the point cloud will be discussed in Chapter 4.3.

Fig. 4.3 MEMS mirror mounted on a plastic support frame. The laser beam is first adjusted by the beam expender, reflected by the MEMS mirror, and then passed through

the wide-angle lens to increase the scanning angle.

Fig. 4.4 Removed the receiver of the lens. Although the receiver lens can increase the detection range of LiDAR, that is, it can increase the ability to receive weak reflected

light signals, but it severely limits the optical signal receiving angle of the system. If the receiver lens is removed, the limitation on the receiving angle of the optical signal can

be lifted.

Fig. 4.5 Wide-angle LiDAR system installed in a protective case.

As shown in Fig. 4.5, in order to better demonstrate the lightweight and small size characteristics of the wide-angle LiDAR, we designed its protective case to house all the messy wires and control circuit boards and other parts. By doing this, if you need to move the LiDAR system for other measurements, you don’t need to disassemble the originally set up system and then reassemble it on the optical table. You only need to remove the power cord of the USB interface. Pick up the entire system. In this paper, by introducing a MEMS mirror as a laser scanner, the weight of LiDAR is greatly reduced. A wide-angle lens is added in front of the MEMS mirror, which magnifies the original mechanical deflection of the MEMS mirror by four times to achieve a large-angle scan of 100 degrees.

In Table 4.2, the field-of-view required for different LiDAR applications are compared.

The LiDAR we designed has a scanning angle of 100 degrees, reaching the level of

application in automated driving systems. Most of the current LiDAR systems on the market use mechanical motors to drive the entire scanning lens, which makes the entire system very heavy. In Table 4.4, we have consolidated the weight of each part of the wide-angle LiDAR system we designed. Our system weighs only 230 grams, which is very light compared to the other commercially available LiDARs in Table 4.6 and can be widely used in many applications.

Table 4.2 Compare the FOV required by various LiDARs with other important parameters.

Table 4.3 The weight of each part of the wide-angle LiDAR we designed and the size of the entire system.

Table 4.4 The weight of a commercially available LiDAR system.