Radar sensors enhance ADAS safety and realize driver convenience functions

In a world where automobiles are becoming more and more automated and semiconductor has become a key innovation factor, sensors have become crucial, especially when dealing with moving objects. Radar sensors use frequency-modulated continuous wave (FMCW) technology to reliably detect moving or stationary objects such as cars, trains, trucks, and loads even in extreme weather conditions. They are also ideal anti-collision solutions for mobile machines such as front cranes and forklifts, as well as port machinery such as trolleys, manipulators and loaders.

The application of FMCW in the field of automotive radar ranges from safety to comfort functions, including blind spot detection, lane change assistance, driver vital signs monitoring, free space detection and parking assistance. These functions are based on the ability of radar to accurately detect and locate obstacles, regardless of weather and ambient light conditions.

The typical high-resolution distance and speed (Doppler) performance of FMCW radar makes it suitable for gesture-based non-touch interfaces. Use cases in the automotive industry include gesture-based door/trunk openers and gesture-based infotainment system controls (such as waving your hand to switch between screens or rotating your finger to control volume). Continuous and accurate monitoring of the driver's vital signs (such as heart rate and breathing rate) is an important feature for improving road safety. The small size of these sensors makes it possible to implement this function non-intrusively. For example, the sensor can be incorporated into the backrest of the driver's seat.

Continuous wave FM

Although the frequency of the peak in the FFT diagram directly corresponds to the range of the object, the phase of the peak is extremely sensitive to small changes in the position of the object. This sensitivity is the basis of radar's ability to estimate the vibration frequency of an object, and it is also the basis for speed estimation.

Radar performance indicators depend on the choice of transmitted signal. For example：

The distance resolution increases (increases) the chirp bandwidth.

The speed resolution can improve (increase) the frame duration.

The maximum measurable speed is inversely proportional to the interval between adjacent chirps.

The number of Tx/Rx antennas limits the resolution of the angle.

application

Free space sensors use the resolution of radar at a long distance and the ability to detect obstacles (such as telephone poles, walls, or nearby parked cars) at close range. The free space sensor can also be used as a parking sensor.

The device processes data from the ADC by performing a 2D FFT, which can solve objects in distance and Doppler along a frame, and separate nearby moving objects from stationary obstacles. For moving radars, such as those mounted on doors, Doppler resolution also helps to detect stationary objects based on the relative speed difference between the radar and the object. The incoherent accumulation of the 2D FFT array creates a distance—Doppler heat map, which can be processed by a detection algorithm (Figure 2).

Antenna configuration and the field of view (FOV) of antenna elements are important design considerations in free space sensor applications. Generally, a compromise can be found between the FOV elevation angle and ground clutter suppression, as well as between the ability to estimate the elevation angle and the azimuth resolution.

The phase of the signal received in the FMCW radar is extremely sensitive to small changes in the position of the object. By using this characteristic, the vibration frequency of an object (such as vibration caused by heartbeat and breathing) can be estimated. For driver vital signs monitoring, the device sends a chirp sequence, and the peak in the range FFT recognizes a strong reflection from the driver's chest. The algorithm in the device tracks the phase of this peak in the chirp and performs spectral analysis of the step sequence to extract the driver's heart rate and breathing rate data.

For gesture-based recognition, the device performs a 2D FFT on the ADC data collected through the chirp of the frame (Figure 3). This solves the situation of distance and Doppler. Then calculate the 2D FFT matrix for each Rx antenna (or for each virtual antenna if the radar is in MIMO mode). The distance-Doppler heat map is created by the incoherent accumulation of the 2D FFT matrix of the antenna. The next step is to extract more features from the distance-Doppler heat map.

We know the dangers of leaving children and animals in enclosed vehicles in high temperatures, including fatal consequences. The FMCW radar installed in the passenger compartment can detect the presence of unattended vehicles for timely intervention. This application mainly depends on the radar's ability to achieve excellent speed resolution. The radar must separate objects so that even the faintest movement (such as a sleeping child) can be distinguished from the static clutter in the car.

Sensor ic

Texas Instruments' AWR1x and IWR1x series sensors are based on CMOS technology. The AWR series is designed for automotive applications, while the IWR series is suitable for industrial applications. Millimeter wave (millimeter wave band) radar products can transmit signals with wavelengths on the order of millimeters. The millimeter wave system operates at 76 to 81 GHz, with a corresponding wavelength of about 4 mm, and can detect movement as small as a fraction of a millimeter.

Each chip can realize intelligent and high-precision autonomous detection, with a resolution within 4 cm, on-site accuracy better than 50 μm, and a range of up to 300 meters. The goal of the millimeter wave AWR1x series is to help engineers overcome the obstacles they usually encounter when designing functions that meet advanced driver assistance system (ADAS) regulatory standards.

By allowing vehicles to identify dangerous situations, the 77-GHz radar system can prevent accidents and improve car safety. They are used to detect different types of obstacles, such as pedestrians and other vehicles, within a range of 30 to 250 meters, even in poor visibility. Their huge advantages include high precision and excellent extensibility from short to long distances. From a design point of view, the disadvantage is their high technical complexity, although development toolkits can be used to solve this challenge. Radar information is used in ADAS applications such as automatic emergency braking and adaptive cruise speed control. STMicroelectronics' STRADA 770 transceiver covers the millimeter wave band from 76 to 81 GHz and includes three transmitters, four receivers, and a frequency modulator.

The radar system IC (RASIC) series provides a high degree of integration for automotive radars in the 76 to 77 GHz range.

Improvements in microcontrollers and sensors have enabled the expansion of ADAS functions, such as electronic stability control, rear view cameras, and vision-based pedestrian detection. More advanced radar-based embedded solutions provide safety functions to complement ADAS functions in automotive design. The new radar sensor can be used to implement a cost-effective gesture recognition solution.