Things you need to know about LiDAR: solid-state and hybrid solid-state, what’s the difference?


Solid-state, hybrid solid-state, MEMS, and rotating mirror are all commonly used terms in the lidar industry, but do you know what they all mean?

This article will examine the different technical routes of lidar.

Due to different scanning methods, lidar can be categorized as mechanical, semi-solid(hybrid solid-state), and solid-state.

Two standard solutions for hybrid solid-state lidar are 1D Scanning and 2D Scanning. And solid-state lidar mainly has two mainstream solutions: OPA (optical phased array) and Flash.

Mechanical 360-degree all-round perception

Transmit-receive plus 360-degree scanning

Let’s start with the mechanical lidar consisting of the most extended history. This type of lidar drives the optomechanical structure rotating at 360 degrees through a motor. It can scan the environment in all directions to form a point cloud, achieving the best performance.

Real point cloud of Hesai’s Pandar128 lidar

As the most classic and mature solution, mechanical lidar is often used in the testing and iteration of Robotaxi. However, the traditional discrete design is large in size and high in cost, making it difficult for large-scale automotive lidar production.

With the development of lidar technology, discrete transmit-receive modules have been integrated through semiconductor processes, reducing the number of components and costs, and thus improving mass production's reliability.

In other words, with the integrated transmit-receive modules and the 360-degree spinning scanning module, a "new" mechanical lidar is born. This new generation of mechanical lidar with high performance, high reliability, and lower cost is expected to enter a broader range of automotive-grade applications.

Hybrid solid-state: the mainstream solution for mass-produced vehicles

Transmit-receive plus 1D or 2D Scanning

At present, mass-produced vehicles are mainly equipped with hybrid solid-state lidar. It has two methods: 1D Scanning and 2D Scanning. What they have in common is that they change the direction of the laser through an internal moving mirror.

2D Scanning

This solution has two kinds of methods: MEMS and a 2D Rotating Mirror.

The core of the MEMS solution is a centimeter-scale vibrating mirror, through which the cantilever beam moves at a high speed on the horizontal and vertical axes, thereby changing the direction of the laser reflection and allowing scanning.

Compared with the traditional mechanical lidar, the MEMS solution simplifies the scanning structure. The scanning path can be changed by controlling the deflection angle of the micro-vibration mirror. Only a few lasers are needed to achieve the equivalent mechanical lidar's coverage and point cloud density.

However, the technical difficulty of this solution is that the rotation angle of the cantilever beam is limited, so the field of view covered by a single galvanometer is small. Multiple splicing is often required to achieve a large field of view coverage, which may cause irregular distortion and overlapping of the point cloud, increasing algorithm processing difficulty.

Let’s look at the 2D Rotating Mirror solution, which consists of a polygonal prism rotating continuously on the horizontal axis and a mirror swinging on the vertical axis. Below is a schematic diagram of a 2D Rotating Mirror. The continuously rotating polygonal prism enables the laser beam to scan horizontally while the oscillating mirror can change the vertical scanning direction of the light beam.

This design requires only one laser beam to complete the scanning. But as there is only one laser beam, to ensure the high-resolution scanning of the three-dimensional world, high scanning frequency, and power are required, which brings challenges to the reliability of the scanning device.

1D Scanning

Compared with the 2D Scanning structure, the 1D Scanning uses a mirror that only rotates at a low speed in the horizontal direction to change the direction of the light, obtaining coverage of the field of view and gaining higher stability and reliability.

In 2017, Valeo adopted the 1D Scanning method and revealed the first automotive-grade lidar in the industry. However, the low number of channels makes it hard to achieve high resolution.

This problem has already been solved. As mentioned in the previous article, the number of channels in the 1D Rotating Mirror structure equals the number of lasers. To increase the number of channels, the lidar needs enough lasers. With 128 transmit-receive modules integrated, it is enough to ensure high channels and high resolution.

Solid-state: an essential development direction

Area array transmit-receive plus no moving parts

Whether a mechanical or hybrid solid-state, it is a transceiver module paired with a scanning module that performs the mechanical movement. As long as moving parts are inside, they cannot be seen as a pure solid state. This means that the scanning module is just a mechanical component, and the "essence" that determines the performance of the lidar is its transmit-receive modules.

The ones that don't have any moving parts inside are pure solid-state lidars. This kind of lidar has the simplest structure and the highest integration. At present, solid-state lidar mainly has two technical routes: OPA and Flash. The principle of OPA is to change the emitting angle of the laser beam by adjusting the phase difference of each unit of the emitting array. Flash uses a high-density laser source array to emit laser light and cover an area quickly using a high-sensitivity receiver to construct a three-dimensional image.

Hesai’s FT120 solid-state lidar

As the solid-state lidar gives up the complex and high-frequency rotating mechanical structure, it reduces the cost of materials and mass production, improves reliability, efficiency, and production consistency. It can be well applied in automotive-grade mass production.

Currently, the shortcomings of solid-state lidar are low power density, short detection distance, and it can not be mass-produced and used as the main lidar. However, the near-range blind spot solid-state lidar can be combined with the long-range hybrid solid-state lidar to create a complete automotive-grade lidar solution.

To sum up, different lidar technical routes have their advantages. Mechanical radar can conduct 360-degree scanning of the surroundings. Hybrid solid-state lidar, whether a 1D or 2D structure, has moving parts inside. The design of pure solid-state lidar is the simplest with no moving parts inside.

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