How is radar used for automotive in-cabin sensing?

Automotive in-cabin radar uses 60 (60 to 64 ISM band) GHz or 77 GHz mmWave sensors to monitor vehicle interiors, detecting, locating, and classifying passengers. By transmitting radio waves that reflect off surfaces, these systems can detect micro-movements like breathing and heart rates through blankets or clothing and can provide child presence detection (CPD), seatbelt reminders, and airbag performance optimization.

Basic in-cabin sensing for applications like occupant monitoring and child presence detection (CPD) previously relied on a variety of sensors including weight sensors, ultrasonic devices and simple ultrawide band (UWB) wireless sensors with a resolution of 10 to 3 cm. Modern in-cabin sensor systems use a single radar sensor to support multiple functions and deliver superior performance (Figure 1).

Figure 1. Comparison of in-cabin multiple sensor architecture (left) with the use of a single radar sensor (right). (Image: Lisleapex Electronic)

One of the factors driving the use of advanced sensor technologies are increasingly demanding safety standards from the European New Car Assessment Programme (Euro NCAP), the National Highway Traffic Safety Administration (NHTSA), and New Car Assessment Program (NCAP) in the U.S. That’s resulting in the development of more advanced sensors.

Common applications for radar sensors include (Figure 2):

Child presence detection (CPD) sends an alert if a child or pet is left alone in the car.
Advanced seatbelt reminder (SBR) detects seat occupancy without weight sensors.
Smart airbag deployment is used to change airbag force based on occupant size and position.
Vital signs monitoring can detect driver fatigue.
Gesture controls are used primarily with the infotainment system to reduce driver distractions.

Figure 2. Typical applications for in-cabin radar sensors. (Image: Infineon)

Architecture choices

Some of the considerations when designing in-cabin radar sensors include frequency selection, sensor placement, and the use of integrated sensors versus streaming data to a central electronic control unit (ECU).

60 GHz is currently the preferred choice. It has largely replaced 24 GHz since the higher frequency improves resolution (down to 5 cm) for distinguishing between adults and children and can accurately monitor vital signs. Compared with 24 GHz solutions, 60 GHz radar provides over 20x higher resolution due to a wider bandwidth (up to 5.5 GHz). Using advanced sensing algorithms, 60 GHz can detect sub-millimeter micro-movements, making it capable of sensing human breathing and even heartbeats.

Another choice is 77 GHz (76-81 GHz) that can provide even better resolution and angular accuracy but is currently used primarily in external advanced driver assistance system (ADAS) applications.

Placement of the sensor can be application dependent. Putting the sensor overhead in the headliner is most common and enables a single sensor to monitor the entire cabin and perform a variety of functions. Side mounting in the B-pillar is used for targeted occupant detection and for gesture controls. Vital signs can be monitored and occupants classified using under seat or dashboard mounted sensors.

The choice of physical location includes edge and satellite architecture. In an edge architecture, an intelligent sensor has integrated processing that sends finalized detection data to the ADAS ECU. Satellite architectures, a type of zonal architecture, uses a simpler and lower cost sensor and sends unprocessed data to a centralized ECU over a high-speed Ethernet connection.

Why not IR or RGB?

Infrared (IR) and visible light (RGB) imaging are also options for in-cabin applications, especially for driver alertness monitoring. They can be used to complement radar but are not generally considered to be substitutes for radar. It’s about more than imaging.

IR and RGB can provide high-resolution visual details like facial expressions and eye tracking that are useful for driver alertness monitoring, radar can support privacy protection while tracking vital signs.

IR imaging typically includes an IR lighting source and provides superior low-light performance but can be subject to interference under bright daylight conditions (Figure 3). RGB imaging can provide better context in daylight but has limitations under low-light or nighttime conditions, without introducing a light source that can be distracting to the driver.

Figure 3. IR sensors can operate effectively under low-light or nighttime conditions. (Image: Anyverse)

Engineers have developed sensors that capture both spectrums and that combine IR and RGB imaging, enabling systems that use RGB capability during daylight conditions and IR sensing when operating under lowlight or nighttime conditions. That can provide an option to radar for specific use cases, but radar supports the widest range of sensing requirements including CPD, SBR, gesture controls, and so on.

Summary

Automotive in-cabin radar monitors vehicle interiors, detecting, locating, and classifying occupants. These systems can detect micro-movements like breathing and heart rates through blankets or clothing and can provide CPD, SBR, airbag performance optimization, gesture controls and other functions. IR and RGB imaging can be used in certain cases, but are not generally considered to be substitutes for in-cabin radar.