Only 10S takes you to understand the principle and application of millimeter wave radar sensors. The so-called millimeter wave is a part of radio waves. We call electromagnetic waves with wavelengths of 1 to 10 mm as millimeter waves. It lies in the wavelength range of microwave and far-infrared waves, so it has two spectral properties. The theory and technology of millimeter wave are the extension of microwave to high frequency and the development of light wave to low frequency.
The so-called millimeter wave radar sensor refers to the radar whose working frequency band is in the millimeter wave frequency band. The ranging principle is the same as that of ordinary radar, that is, it sends radio waves (radar waves), then receives echoes, and measures the target position data according to the time difference between sending and receiving. Millimeter wave radar is a radio wave frequency in the millimeter wave band.
Since the wavelength of millimeter wave is between centimeter wave and light wave, millimeter wave has the advantages of microwave guidance and photoelectric guidance. Compared with the centimeter wave guide, the millimeter wave guide has the characteristics of small size, light weight and high spatial resolution. Compared with optical seekers such as laser infrared and TV, the millimeter-wave seeker has strong ability to penetrate fog, smoke and dust, and has the characteristics of all-weather. In addition, the anti-interference performance of the millimeter-wave seeker is also very strong. Millimeter wave radar is to measure the relative distance and speed of the measured object. With the development and progress of radar technology, millimeter wave radar sensors have begun to be used in automotive electronic products, unmanned aerial vehicles, intelligent transportation and other fields.
1. Characteristics of millimeter wave radar
1. The frequency band is extremely wide. The available bandwidth of the two atmospheric windows currently used, 24G and 77G, is greater than 10G, which is suitable for various broadband signal processing;
2. A narrow beam can be obtained under a small antenna aperture, with good directivity, high spatial resolution and high tracking accuracy;
3. The Doppler bandwidth is high, the Doppler effect is obvious, the Doppler resolution is good, and the speed measurement accuracy is high;
4. The influence of ground clutter and multipath effect is small, and the tracking performance is good;
5. The millimeter wave scattering characteristics are sensitive to the details of the target shape, which can improve the ability and imaging quality of multi-target recognition and target recognition;
6. Since the millimeter-wave radar sensor transmits with a narrow beam, the interception performance is low and the anti-electronic interference performance is good;
7. Millimeter waves have the ability to penetrate smoke, dust and fog.
Second, the advantages of millimeter wave radar sensor ranging
1. High precision anti-interference
Compared with microwaves, millimeter waves are small in size, light in weight, and have high spatial resolution. When the antenna diameter is the same, the millimeter-wave radar has a narrow beam (generally in the order of milliradians), which can improve the angular resolution and angular measurement accuracy of the radar, which is beneficial to resist electronic interference, clutter interference and multipath reflection interference.
2. 24/7
Compared with infrared, laser and other optics, the millimeter-waveguide head has a strong ability to penetrate fog, smoke and dust, and has the characteristics of all-weather.
3. High-resolution multi-target
Due to the high operating frequency, a large signal bandwidth (such as the order of gigahertz) and Doppler frequency shift can be obtained, which is beneficial to improve the measurement accuracy and discrimination ability of distance and speed, and analyze the detailed characteristics of the target. At the same time, millimeter-wave radar can identify small targets and multiple targets, so it has strong spatial discrimination and imaging capabilities.
4. High sensitivity, low false positives
The system has high sensitivity, low false alarm rate, and is not easily interfered by external electromagnetic noise.
5. High frequency and low power
The transmission frequency is higher and the transmission power is lower.
6. Can measure speed and distance
Using FMCW frequency-modulated continuous wave can measure the distance and speed of multiple targets at the same time, and can track the target continuously, even if it is a stationary target, it can keep tracking without losing.
3. The working principle of millimeter wave radar sensor
Taking the vehicle-mounted millimeter-wave radar as an example, the radar transmits millimeter-wave outward through the antenna, receives the reflected signal of the target, and quickly and accurately obtains the physical environment information around the vehicle body (such as the relative distance, relative speed, angle, and movement between the car and other objects). direction, etc.), then target tracking, identification and classification are carried out according to the information of the found objects, and then data integration is carried out in combination with the dynamic information of the vehicle body, and finally intelligent processing is carried out through the central processing unit (ECU). After making a reasonable decision, deal with it with sound. .Notify or warn the driver by means of notification or warning, or actively intervene in the car in time to ensure the safety and comfort of the driving process and reduce the probability of accidents.
1. Location
The millimeter wave radar sensor transmits directional millimeter waves in the corresponding band through the transmitting antenna. When the millimeter wave encounters an obstacle target, the reflected millimeter wave is received by the receiving antenna. According to the millimeter wave band, the flight time of the millimeter wave on the way is calculated by the formula, combined with the driving speed of the preceding vehicle and the driving speed of the vehicle, the relative distance between the millimeter wave radar) and the target can be known, and the position of the target can also be known.
2. Speed
In addition, according to the Doppler effect, the frequency of the millimeter-wave radar changes, and the relative speed of the vehicle is closely related to the tracked target. According to the change of the reflected millimeter wave frequency, we can know the relative movement speed of the obstacle tracked in front in real time compared with the car. Therefore, when the sensor sends out a safe distance alarm, if the car continues to accelerate, or the front monitoring target decelerates, or when the current monitoring target is stationary, the frequency of millimeter wave reflection echoes will become higher and higher, and vice versa.
3. Azimuth
Regarding the measurement of the azimuth angle of the monitoring target, the detection principle of the millimeter-wave radar sensor is: after transmitting the millimeter-wave through the millimeter-wave radar transmitting antenna, the parallel receiving antenna of the millimeter-wave radar is used to calculate the phase difference of the millimeter-wave reflected by the same monitoring target. Monitoring the azimuth, position, velocity and azimuth of the monitoring target is the advantage of the millimeter wave radar. Combined with the strong anti-jamming capability of millimeter-wave radar, it can work stably around the clock. Therefore, millimeter-wave radar was selected as the core sensing technology for automobiles.
In short, millimeter-wave radar is a hard-to-replace sensor, and its all-weather working status is a big advantage. The accuracy of its speed measurement and distance measurement is much higher than that of visual sensors, and better than lidar. But in general, this is not a conflict, because the future will move towards integration, especially for autonomous driving, there is no doubt that these three sensors will integrate with each other.
4. Main applications of millimeter wave radar sensors
1. Guidance radar: At present, the main reason for choosing fire control radar in the millimeter wave band is to improve the detection capability, reduce the size of the radar, reduce the weight and volume, and facilitate integration.
2. Target detection radar: mainly through mechanical/electronic beam scanning to achieve target distance, detection speed and angle in the observation area, equipped with corresponding data processing units, scattering characteristics), tracking and prediction (kalman filter, particle filter, etc.).
3. Millimeter-wave ground observation radar, mainly millimeter-wave synthetic aperture radar (Synthetic Aperture Radar, SAR), this type of radar mainly realizes ground imaging observation and obtains SAR images of ground areas.
4. Millimeter-wave short-range detection radar, mainly to achieve two-dimensional or three-dimensional imaging detection of targets within 2 meters. Currently, the band of the system is 30~37.5GHz, and 94~200GHz or THz has all the bands. For example, the current three-dimensional scanning radar for human security inspection at American airports uses millimeter waves instead of X-rays to achieve the detection and imaging of objects inside and outside the skin of human clothing to enhance security; some special materials are realized through 35GHz band radar or 94GHz and THz band radar. of nondestructive 3D inspection.
5. Automotive radar, install radar sensors on the car to realize car collision avoidance, automatic parking, pedestrian detection, etc. At present, the mainstream automotive radar is 24GHz radar, but it is limited to frequency band control, radio astronomy 5Km band radar is not allowed to use and its own volume Due to the large size (mainly due to the large size of the antenna) and other reasons, the 77GHz automotive radar is being productized and equipped with some high-end vehicles. The main advantages of the 77GHz automotive radar are that the distribution frequency band is wider, the distance resolution is higher, and the volume is smaller than the 24GHz radar. , The target detection ability is strong, but the production and processing technology of 77GHz radar is relatively high, but at present, this problem is no longer an industry barrier.
