LiDAR serves as a crucial perception sensor for autonomous driving. It emits laser beams into the surrounding environment, captures the reflected signals from objects, and computes the time taken for the light to travel back and forth. This process enables it to ascertain the distance, shape, and position of objects, thereby constructing a three-dimensional “point cloud map” of the vehicle's vicinity.

This map information is of paramount importance for autonomous driving systems, as it aids in evaluating road conditions, steering clear of obstacles, and charting routes. Compared to cameras, it is less susceptible to lighting variations and offers higher-resolution data than millimeter-wave radar. LiDAR can be categorized into several types based on its internal scanning mechanism. Today, our focus will be on mechanical LiDAR and semi-solid LiDAR.

How Does Mechanical LiDAR Operate?

Mechanical LiDAR, the earliest type employed by most autonomous driving test vehicles, is typically installed on the roof in a compact barrel- or disk-shaped configuration. The defining characteristic of this design is the rotation of the laser emission and reception modules via a mechanical assembly. It encompasses motors and rotating devices that perpetually spin the optical components, enabling the laser beams to scan the surrounding environment in a 360° arc. This rotation facilitates the acquisition of point cloud data from all directions around the vehicle.

Mechanical LiDAR does not indiscriminately emit laser beams in all directions; rather, it employs a mechanical structure to rotate the beam akin to a scanner. The laser emitter, receiver, or mirrors rotate in unison, and with the assistance of multi-beam or other mechanical components, achieve 360° horizontal coverage and a specific vertical scanning range. Owing to its extensive detection coverage, a single device can perceive the entire environment surrounding the vehicle without the need for additional units, while also attaining high point cloud density and ranging accuracy.

This design endows LiDAR with a broad field of view, comprehensive coverage, and relatively high data precision, rendering it the most prevalent solution for numerous advanced autonomous driving prototype vehicles and early self-driving car platforms.

Nevertheless, this design also presents certain drawbacks. Relying on mechanical moving parts, such as motors, bearings, and mirrors that are in constant rotation, these components are prone to wear and tear. Over prolonged usage, they may malfunction due to factors like vibration and temperature fluctuations, making it challenging to meet automotive-grade reliability standards of tens of thousands or even over a hundred thousand hours.

The mechanical rotating structure of mechanical LiDAR necessitates a certain amount of space, hence it is generally installed solely on the roof to ensure adequate visibility. This not only alters the vehicle's aesthetics but also impacts its aerodynamics and overall design. Furthermore, the intricate mechanical structure of mechanical LiDAR complicates the achievement of low-cost mass production. These issues render mechanical LiDAR less advantageous.

How Does Semi-Solid LiDAR Function?

Currently, numerous autonomous driving vehicles are outfitted with semi-solid or solid-state LiDAR. The term “semi-solid” implies that, unlike traditional mechanical LiDAR, which mandates the entire radar to rotate mechanically, it secures the core module responsible for emitting and receiving light and permits only some minor components to move mechanically. This design substantially reduces the number of moving parts in comparison to traditional mechanical radar, resulting in a simpler and more stable structure.

A prevalent implementation of semi-solid LiDAR is the rotating mirror structure, wherein the emission/reception module remains stationary, but a small mechanical mirror is utilized to alter the direction of the laser beam. This enables more flexible scanning than pure solid-state designs but with significantly less movement than rotating the entire module. This approach curtails the number and range of motion of mechanical components, leading to substantial enhancements in cost, reliability, and packaging volume in contrast to mechanical LiDAR.

Fundamental Structural Disparities Between Mechanical and Semi-Solid LiDAR

Mechanical LiDAR attains full-range scanning through the extensive mechanical rotation of the entire emission/reception module, whereas semi-solid LiDAR secures the emission/reception module and manipulates the laser direction via small-scale mechanical components or mirror adjustments.

In mechanical LiDAR, the laser emitter and receiver rotate in tandem with the mechanical structure, necessitating the entire device to endure vibrations, friction, and mechanical wear induced by high-speed rotation. To uphold structural stability, additional balancing structures, power drives, and packaging systems are required, all of which augment design complexity, manufacturing costs, and vehicle integration difficulty.

The core design of semi-solid LiDAR is to maintain the sensing components in a stationary state, with only mirrors or minuscule mechanical structures tasked with guiding the laser beam's direction. This results in fewer mechanical parts, fewer wear points, and simpler vehicle integration, even permitting concealed installation.

The structural differences also imply that the manufacturing process of semi-solid LiDAR can more readily modularize and standardize key components, facilitating large-scale production. For car companies, this structure is more conducive to meeting automotive-grade reliability standards and is easier to integrate with the vehicle's warranty and maintenance systems.

Why Do Car Companies Prefer Semi-Solid Over Mechanical LiDAR?

When selecting LiDAR for autonomous driving systems, car companies take into account performance, cost, reliability, and vehicle integration difficulty. During the prototype or experimental vehicle stage, mechanical LiDAR, with its comprehensive coverage and high precision, was once the premier choice for attaining overall perception capabilities. However, as industrialization advances, issues such as its bulky size, exorbitant cost, and limited lifespan of mechanical components render it challenging to apply on a large scale in vehicles.

In contrast, semi-solid LiDAR is more pragmatic. With fewer mechanical parts, it offers heightened reliability. Vehicles encounter a myriad of complex environments during actual use, including extreme temperatures, bumps, vibrations, dust, and humidity. The more intricate the mechanical structure, the more susceptible it is to wear and tear caused by environmental stress. In semi-solid designs, reduced mechanical movement results in greater overall stability, superior long-term reliability, and easier compliance with automotive-grade certifications.

The cost of LiDAR has long been a pivotal bottleneck restricting the widespread application of autonomous driving. Mechanical radar, with its convoluted structure, necessitates customization of all components, as well as precise assembly and calibration. Semi-solid LiDAR, on the other hand, boasts a relatively simpler structure, substantially lower manufacturing difficulty, and material costs, making it highly appealing to car companies striving for greater sales volume and lower vehicle costs.

Moreover, mechanical LiDAR often requires protruding installation, frequently observed mounted on the roof, which not only mars the vehicle's body lines but may also augment air resistance and wind noise. Semi-solid designs, owing to their compact structure, permit embedded installation, offering a more aesthetically pleasing appearance and superior sensor concealment and protection, which is particularly crucial for mass-produced vehicles.

At present, some companies are pursuing even more advanced objectives by researching fully solid-state LiDAR. This design further diminishes mechanical obstacles, relying entirely on electronic optics to control the laser direction. However, current fully solid-state solutions confront numerous challenges in terms of field of view coverage, detection range, and manufacturing processes. In contrast, semi-solid LiDAR, which strikes a balance between cost, reliability, and scanning capabilities under existing technological conditions, has emerged as the mainstream choice for many car companies in mass-produced autonomous driving models in the short term.

Car companies opt for semi-solid radar not because it outperforms mechanical radar in every aspect, but because it best satisfies commercial production requirements under comprehensive standards of reasonable performance, controllable costs, high reliability, and vehicle integration friendliness.

Final Thoughts

Mechanical LiDAR is essentially a “trade mechanical complexity for field of view” solution. It achieves complete spatial coverage with a limited number of laser channels through continuous and stable mechanical scanning. Since the scanning path and field of view are inherently closed, the system can more readily obtain uniform and stable point cloud distributions, which is a distinct advantage for environmental modeling and target detection. However, this advantage is predicated on continuous mechanical motion, with field of view, precision, and reliability inextricably linked to structural complexity. This also renders mechanical LiDAR more of an engineering feasible but product-wise challenging solution, which is why semi-solid LiDAR is being adopted more extensively.

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