ADAS – RADAR an Important Component
The automobile industry is entering a new paradigm with rapidly advancing technologies where vehicles of the future will incorporate mobile communications technologies and AI implemented as the CASE (Connected, Autonomous, Shared and Electric) revolution. Making CASE a reality will not only solve social issues, such as traffic congestion, energy consumption, and environmental pollution but will also lead to creation of new mobility services contributing to industrial and regional revitalization.
To bring self-driving vehicles and connected cars to market, automobile makers must accurately and reliably test the safety of mission-critical advanced technologies supporting CASE. To reduce road traffic accidents caused by driver errors, countries worldwide are progressing with development of Advanced Driver-Assistance Systems (ADAS) as well as selfdriving vehicles. This progress is supported by various sensor technologies, including radar.
Implementation of self-driving vehicles uses mmWave and highfrequency communication systems for transferring more data, and automobile radar is transitioning to higher frequencies. Consequently, developers require better solutions, such as wideband spectrum analyzers and signal generators. This article cover use of VNA based measurement for car radar. With continued customer demand for advanced automotive safety systems like automatic emergency braking, blind-spot detection, lane-change assist, and adaptive cruise control (to name just a few), manufacturers continue to utilize automotive radar systems to enable these functionalities. Radar systems have proven to have inherent advantages that make them an ideal solution (all-weather, better measurements, detection of more objects, easily incorporated in to the design of the car, and more). These radar systems typically fall into three categories: short-range radar (SRR), ultra-wideband SRR (UWB SRR), and long-range radar (LRR)- Figure 1.
For example, with a range of 10 to 250 meters, LRR systems are often used for adaptive cruise control (ACC) systems.
These impact driver safety and convenience, as well as increasing capacity of roads by maintaining optimal separation between vehicles and reducing driver errors. Utilizing LRR-provided data (transverse, longitudinal, and relative velocity), ACC is then able to make necessary adjustments based on the preselected car speed by moderately influencing breaking and acceleration. Typically, the LRR used is designed for an angular range of up to ±10° and a distance range of 10 to 150 m, which enables the adjustment of the automobile’s velocity between 30 to 180 km/h.
These impact driver safety and convenience, as well as increasing capacity of roads by maintaining optimal separation between vehicles and reducing driver errors. Utilizing LRR-provided data (transverse, longitudinal, and relative velocity), ACC is then able to make necessary adjustments based on the preselected car speed by moderately influencing breaking and acceleration. Typically, the LRR used is designed for an angular range of up to ±10° and a distance range of 10 to 150 m, which enables the adjustment of the automobile’s velocity between 30 to 180 km/h.
Car Radar Technology
In almost all ACC systems, a 77 GHz LRR system is used and typically mounted behind the car emblem. Transmit and receive patch antennas of the radar are focused by a dielectric lens and operate at a 4 mm wavelength range. The radar beam looks through the car emblem and the reflected signal from the target is thus exposed twice to the influence of the radome. While the measurement principle seems intuitively simple, the requirements for automation and safety are enormous. The advantage of using a frequency modulated continuous wave radar (FMCW radar2) is that not only can it measure the distance of other vehicles, but more importantly, it can directly measure the speed at which they are travelling.
The Principle of FMCW Radar
When using an FMCW radar for distance measurements, the output signal frequency continuously changes over the transmission time, usually around 76-77 GHz, and it is linearly modulated to reach a maximum of 81 GHz over a given time period. This waveform is called a chirp. A frame consists of N number of chirps, each lasting for a given chirp time (TChirp). The bandwidth and slope of each chirp also crucial to the performance of the FMCW radar. These parameters have a direct influence on the maximum range, maximum velocity, and their corresponding resolutions.
In FMCW radars, the chirp configurations control all the basic requirements like maximum range, range resolution, maximum velocity, and velocity resolution. As the range resolution is dependent just on the chirp bandwidth (FChirp), it becomes clear that range accuracy requirements below 10 cm necessitate a bandwidth of 4 GHz (or even larger). This large bandwidth is extremely beneficial as it increases range and velocity resolution, thereby enabling the distinction of objects that are closely spaced. This makes it ideal for features such as automated parking. Whereas the maximum range is mainly dependent on the maximum “designed” IF frequency, the range resolution is determined by chirp time and the chirp bandwidth. Below equation shows the fundamental math behind it.
mmwave Measurement using Anritsu Shockline VNA:
VNAs for mmWave applications have been historically large, heavy, complicated, and very expensive. A new approach that shows significant advantages for these car radar radome and bumper measurements can be seen with the Anritsu ShockLine MS46522B VNA (Figure 10) with option 82 or 83, a dedicated E-band VNA for 55-92 GHz applications. For antenna characterization and material measurement, this kind of VNA is best suited for industrial applications where ease of use along with ruggedized handling and interfacing are major requirements. The ShockLine MS46522B E-band VNAs with options 82 or 83 configuration consists of small tethered source/receiver reflectometer modules and a base chassis. The modules are attached to the chassis with the option of either one or five meter cables that are permanently attached to the unit, making this a compact, ready-touse E-band VNA. The remote, small form factor reflectometer modules (6 x 10 x 4 cm) have native WR12 waveguide interfaces for convenient connection to typical waveguide devices. In addition to their miniature size, these reflectometers provide highly attractive features such as: short/long term thermal stability due to the vanishing thermal gradient across the modules; high amplitude and phase stability; and, raw directivity (to mention a few). Most importantly, placing the sampling directional bridge closest to the AUT/DUT provides long-term amplitude and phase stability. The ShockLine MS46522 has a 3U high chassis and uses simple & easy GUI, software, command syntax, drivers, and programming environments as the rest of the Anritsu ShockLine VNA family. Compatibility to the Interchangeable Virtual Instruments (IVI) Foundation allows users to get the maximum possible measurement speed for their own SCPI-based programs. This enables a reduction in test time, which is an important parameter for antenna or material characterization.
Transmission measurement (S12) -Attenuation and phase shift is important parameter to characterise radome’s cover transmission behaviour. The phase shift of the electromagnetic wave refers to a time delay, which is caused by longer transit time through the dielectric material. The attenuation of the wave is caused by loss of energy inside the material. The attenuation has an influence on the detection range of the radar.
It is possible to visualize the impact of materials in front of the radar by conducting VNA-based transmission and phase measurements. As a result, it is possible to judge if such material might have an influence upon the radar detection range.
Reference:
- i) Anritsu Application Note E-Band Based Car Radar Emblem Measurements
