The principle and application of automotive UAV obstacle avoidance millimeter wave radar sensor technology, as we all know, radar uses electromagnetic waves, which determines that microwave radar is different from other methods such as ultrasonic wave and sonar. Electromagnetic waves are alternating electromagnetic fields that propagate in free space. The alternating frequency of the electromagnetic field determines the fundamental properties of the radar. Of course, wavelength and frequency are an equivalent concept. Electromagnetic waves have several typical frequency bands according to the frequency: the radio frequency used at ordinary times is less than 300Mhz frequency band, mainly used by AM and FM broadcasting. The microwave frequency band is the main frequency band used by communication and radar. This is a very wide frequency band, ranging from 300Mhz to 300GHz. Millimeter wave is a sub-band of microwave. What you can see is that visible light, infrared, lasers, etc., these are also a type of electromagnetic wave, but due to the difference in frequency, it is very different from the characteristics of the microwave frequency band.
Therefore, methods based on visible light, infrared, or laser are generally not called radar, although the mechanism of laser may be similar to that of radar. So what are the main differences between electromagnetic waves in different frequency bands? This is the following characteristic. When an electromagnetic wave propagates in space, the medium it propagates changes, and phenomena such as reflection, absorption, transmission, and diffraction occur. For electromagnetic waves in different frequency bands, the proportions of these phenomena vary greatly.
Therefore, we know that whether it is radar, active infrared, or lidar, it is based on the characteristics of reflection, and electromagnetic waves of different frequencies have great differences in reflection characteristics. On the one hand, it depends on the medium, that is, the material of the reflective surface. For example, metallic materials reflect microwaves more easily, and water mainly absorbs electromagnetic waves, so we rarely use radar underwater. At the same time, this reflection, transmission and other properties also depend on the frequency of the electromagnetic waves. For example, our broadcasts can usually be received in the house, but they can also be received in the house, but the wifi may be weak across a few walls, and infrared and light may not pass through a sheet of paper. This is because the longer the wavelength, the easier it is to transmit and diffract, and the shorter the wavelength, the easier it is to reflect.
Generally speaking, if the relationship between the wavelength of the electromagnetic wave and the size of the medium is larger, it is easy to pass and diffract, and if it is smaller, it is easy to reflect. Of course, rays in particular here, since it's basically a particle property, can't be considered a wave. The millimeter-wave wavelengths we're talking about are 1cm to 1mm in between, very short wavelengths, close to terahertz or infrared, but much longer than both wavelengths are difficult to develop early, and used about a decade later.
As we just said, the frequency of electromagnetic waves that we can now communicate and process with is getting higher and higher, and now we are talking about terahertz, visible light communication. The millimeter wave radar sensor band is 30-300GHz, and the frequency is very high, but the electromagnetic waves in many frequency regions of this frequency band are easily propagated by water molecules and absorbed by oxygen in the air, so several typical frequency bands can be used, namely 24, 60, 77, 120GHz. Of course, 24GHz, in particular, is not a millimeter wave, strictly speaking, because its wavelength is about 1cm. But it was the first to be used. Now all countries have divided 24GHz for civilian use, 777GHz is divided into automotive anti-collision radar, and 24Ghz is also used in automobiles at the beginning.
Millimeter-wave radar sensors differ from radio and low-frequency microwaves because of their short wavelengths. According to the reflection characteristics just mentioned, first of all, it is very close to the propagation characteristics of light, which can better reflect smaller reflective surfaces (objects). In addition, due to the high frequency, the adjustable bandwidth is very large. Also, as we'll say later, because the wavelength is so short, the antenna can be small. However, due to the small wavelength, it is easy to be blocked and absorbed in space propagation, so its action distance cannot be too far. Of course, compared to other bands, this distance is generally within 1km.
Millimeter wave radar sensors can detect ranging, speed and angle. We know that radar is an electronic device that emits electromagnetic waves and detects whether the target is far or near by detecting echoes. Active infrared lasers are the same. Millimeter waves, like most microwave radars, have the concept of beams, that is, the emitted electromagnetic waves are cone beams, not laser lines. This is because antennas in this band mainly emit electromagnetic radiation rather than light particle emission. Radar is the same as ultrasonic, and this beam leads to its pros and cons. The advantage is that it is reliable, because the reflective surface is large, and the disadvantage is that the resolution is not high. Can millimeter wave radar detect targets? Distance measurement, speed measurement and bearing measurement.
It is very simple to judge whether there is a target or not, to judge whether there is an echo. Ranging is also very simple, based on the TOF principle, but we say that the propagation speed of electromagnetic waves is the speed of light, so this brings certain challenges. Just now we said that the millimeter-wave radar is too far. For example, if we say a car or a drone, then the detection distance is very close, and the interval between the echo and the transmitted wave is very short, so it is generally not suitable to use a simple transmission pulse, so the FMCW method is mainly used now. many.
The speed measurement of millimeter wave radar sensor is the same as that of ordinary radar. One is based on the principle of ordinary radar dopler. When the emitted electromagnetic wave moves relative to the detected target, the frequency of the echo is different from the frequency of the emitted wave. The speed at which the target is moving relative to the radar can be measured by detecting this frequency difference. But this method cannot detect the tangential velocity, the second method is to obtain the velocity by tracking the position differentiation.
The latter is the lateral direction of the millimeter-wave radar. The detection of the direction of the target by the radar is mainly based on one method, that is, the use of a narrower beam. Because when the target is in the beam, we usually can't tell the specific direction of the target in the beam, so we have to narrow the beam, of course preferably like a laser, but it is difficult. So there are several ways to narrow down the beam, one uses a directional antenna, such as a horn antenna or a lens antenna. Another approach is to use multiple antennas + an array signal processing approach. For millimeter-wave radar sensors, because the wavelength is very short, the cost of making many antennas is small (this cost refers to price and size), so millimeter-wave radars use array antennas to form narrow beams. How narrow can it be? For example, 3 degrees and 5 degrees are commonly used in automobiles. Of course, this can't compare to a laser, but it's good enough.
One application direction of civilian millimeter-wave radar sensor technology is automotive applications. Around 199X, millimeter-wave radar was used for automotive ACC function (adaptive cruise), that is to follow the car in front at high speed, he is slow, you are slow, he is fast Hurry up and keep your distance. This depends on the distance detection function with a wavelength of more than 200 meters in millimeters, which is difficult to achieve by other means. Later, it developed into other functions such as collision avoidance and blind spot detection, but the technology has always been very expensive. It was not until 2012 that chip-level millimeter-wave radio frequency chips appeared, the threshold for this technology was lowered, and all applications opened a window.
Millimeter-wave radar generally has several components: antenna, radio frequency, baseband, and possibly a control layer.
Let's talk one by one, the antenna first. Just now we said that the wavelength of millimeter-wave radar is a few millimeters. Because the antenna size and wavelength are the same, the millimeter-wave radar antenna can be very small, so multiple antennas can be used to form an array antenna to achieve the purpose of narrow beams. As the number of antennas increases, the beam Can be very narrow. Another factor is that due to the small wavelength, millimeter waves can use a "microstrip patch antenna" which is what it looks like in the picture, in the picture, the ground on the pcb board lays a few microstrip lines on the layer, you can Do the antenna. This antenna leading to millimeter wave radar can be made into a pcb board. The pcb antennas for wifi and bluetooth are very similar to those common to everyone. Of course, due to the high frequencies of millimeter waves, high frequency boards are usually required.
The digital signal processing part of the millimeter wave radar sensor. This part is some algorithms, mainly including array antenna beamforming algorithm, signal detection, measurement algorithm, classification and tracking algorithm. This is not the beginning, because there are so many faces involved. The principle of radar is simple, but to do it well, the effort should be in this place. In addition, there are solutions from some manufacturers that are integrated from RF band to baseband, and we can predict that in the near future, the level of integration will be higher, and then single-chip solutions.
Automotive mmWave radar sensor technology
Due to the long distance and high reliability of mmWave radar, it is not affected by light and dust. Compared with cameras, the characteristics of distances beyond 150 meters are much better. Compared with the laser, the price of about 1,000 yuan is also a big win. So it's still a mainstream technology. Of course, we just mentioned that it has a slightly lower resolution, so integration with the camera must be a trend. Of course, lidar is working hard to innovate the technology in hopes of lowering the price. Due to the rapid popularization of technology and price, there are only more than 500,000 millimeter-wave radars, and now more than 100,000 cars are slowly starting to be installed. ADAS such as telsa also began to tap technical directors from automotive radar manufacturers, and began to assemble in September to electric vehicles. It can be seen that the application of millimeter-wave radar in automobiles is still the mainstream technology.
UAV millimeter wave radar sensor technology
We often say that cars and drones are actually very similar: high-speed movement, safety first, high-speed, inevitable requirements, long enough detection distance, safety, must require the robustness of the detection method, and it is less affected by the environment. Of course, in some applications, the environment for drones is a bit more complex than for cars. Millimeter-wave radar has been widely used in drones.
Its first application in unmanned aerial vehicles, and currently the largest in the market, is the height-fixing application of plant protection unmanned aerial vehicles. We know that gps barometers measure altitude, and during plant protection, we want drones to fly at a fixed altitude above crops regardless of whether the ground and vegetation are undulating. This is also called flight simulation. This app has many solutions such as Ultrasound, Laser, Infrared, Eye and many more. But because most plants have poor protection environment, there is a lot of dust, and there is a lot of water mist, ultrasonic and optical-based ultrasonic waves will be greatly disturbed.
Currently, altimeters based on millimeter-wave radar sensor technology are stable. First, it can penetrate dust and water mist, largely undisturbed. Based on beams, not point reflections, the height simply reflects the height of the vegetation leaves.
The second application of drones is obstacle avoidance
It's also a battleground for a variety of sensors. But we say that millimeter-wave radar has the advantages of not being affected by light, large working distance, and reliability, and it has been proven in military manned aircraft, automobiles, and unmanned aerial vehicles. Of course, the resolution of radar is really low. But we said that this can be greatly improved due to the advantages of the array antenna, a resolution of 3-5 degrees is possible. Therefore, we say that millimeter-wave radar has a lot of room for adjustment, such as beam width, working distance, price, etc. We believe that millimeter-wave radar has obvious advantages in UAV altimetry and obstacle avoidance, but there are also places where optics need to be supplemented. Therefore, we propose an architecture that uses mmWave radar for 360-degree obstacle avoidance and height measurement. Of course, under this architecture, stronger flight control processing platforms and technologies are needed.
