Issue 254: it takes about 4 minutes and 1657 words
[sensory system] autonomous driving: dissecting the sensory system of a car
Recommendation: introduce the eyes, nose, ears and other tactile organs of the car, including digital cameras, lidar, radar, ultrasonic sensors, millimeter wave radar sensors. It is precisely because of them that automatic driving is possible.
Autonomous driving relies on sensors to obtain the surrounding environment of the car body. Each sensor has its own advantages and limitations. It needs to combine the advantages of various sensors and carry out technical complementarity to complete the understanding and response to the surrounding environment.
1、 Digital camera
Adding it to the car is like equipping it with a pair of eyes that can observe the world. This provides an important data source for the "middle level control system" mentioned in our previous article. Through these collected environmental data, the deep learning algorithm can carry out object recognition, trajectory prediction and other operations.
Professional digital cameras used in the field of driverless are also far from simply recording pixel values. It can automatically analyze the picture data, process the "content" contained in the picture, automatically detect various objects in the picture, convert them into numerical tables, list the object information list, and provide preprocessing data for further identification and judgment.
However, unlike the "stereo vision" of organisms, digital cameras can convert 3D spatial data into 2D pixel matrix, but they will miss the most critical element information - depth value. At present, a solution is: by configuring multiple digital cameras for the car to capture a unified visual lens in view of the shape of the organism, we can reconstruct the 3D scene through the computer, and have a clearer in-depth understanding of the surrounding environment of the car body.
2、 Lidar
It continuously emits pulsed laser beams around, and then completes the ranging task according to the time it takes for the beam to turn back. The launch frequency of millions of times per second enables lidar sensor to calculate the three-dimensional digital model of the surrounding environment, truly realize the visualization of the 3D world, and complete the rendering of high-precision 3D models.
There was a vivid metaphor for the working principle of lidar: it's like you are in a room full of invisible objects. Lidar is like the red paint in your hand. You rotate your posture and spray the red paint around. Soon, those invisible objects will show their shape and show up in front of you because they are contaminated with paint. When the paint is sprayed far enough and covered completely enough, all invisible objects in the whole room will be exposed without hiding.
Here we need to introduce the concept of "point cloud" to explain. The point data set of the product appearance surface obtained by measuring instruments is also called point cloud. The larger the distance between points, we call it "sparse point cloud". On the contrary, the point cloud obtained by three-dimensional laser scanner is called "dense point cloud" because there are many points to describe the appearance of objects.
3、 Radar
Its working principle is to transmit electromagnetic waves to the target area according to a certain frequency. The electromagnetic waves expand outward at a ring speed with a propagation speed equal to the speed of light. The echo formed by the reflection of electromagnetic waves when touching obstacles is used to detect the surrounding real environment and object layout.
For autonomous vehicle, the application of radar sensors can monitor the surrounding environment of the car body in real time, identify large obstacles, deal with abnormal weather such as heavy rain and fog, and is not affected by dust on the road. It can accurately detect the position of objects away from the car body, as well as the direction and speed of each object.
4、 Ultrasonic sensor
Similar to radar, ultrasonic sensor monitors environmental information by transmitting a certain waveform signal and detecting the echo frequency. Unlike the electromagnetic wave of radar, ultrasonic sensor emits ultrasonic wave just like its name. Ultrasonic sensor is also divided into two parts: transmitter and sensor.
The transmitter generates machine waves that are higher than the range that human beings can listen to. The receiver senses the echo of sound waves and monitors the position and speed of objects according to the time, frequency and shape of sound waves reflected by objects. Similarly, it also has the ability to penetrate the dense fog and dust weather environment, and can observe objects under the dazzling sun.
Compared with the electromagnetic wave of radar, the propagation speed of sound wave is relatively slow, which means that it can see small objects with high resolution, but it decays faster under the influence of wind and distance - that is, its monitoring range is relatively small, and it is often complementary to radar sensors in daily applications.
5、 Millimeter wave radar sensor
In short, it is a radar working in the millimeter wave band, and millimeter wave is essentially a band of electromagnetic waves. Millimeter wave refers to the electromagnetic wave with the wavelength between 1mm and 10mm, which is located in the overlapping wavelength range of microwave and far-infrared wave, and the corresponding frequency range is 10~200ghz. Its frequency is higher than radio and lower than visible light and infrared.
According to the wave propagation theory, the higher the frequency, the shorter the wavelength, the higher the resolution and the stronger the penetration ability, but the greater the loss in the propagation process and the shorter the transmission distance; Relatively, the lower the frequency, the longer the wavelength, the stronger the diffraction ability, and the longer the transmission distance.
From the above, we can see the great advantages and application value of millimeter wave radar. Compared with cameras or ultrasonic sensors that are highly susceptible to weather and other natural environments, millimeter wave radar sensors have won the crown of "all-weather, all-weather" with their absolute advantage of being free from dust, fog, rain, snow and other bad weather, and become the only choice for many car driving assistance systems.
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