# Physiological Driver Monitoring Using Capacitively Coupled and Radar Sensors
###### Abstract  
Unobtrusive monitoring of drivers' physiological parameters is a topic gaining interest, potentially allowing to improve the performance of safety systems to prevent accidents, as well as to improve the driver's experience or provide health-related services. In this article, two unobtrusive sensing techniques are evaluated: capacitively coupled sensing of the electrocardiogram and respiration, and radar-based sensing of heartbeat and respiration. A challenge for use of these techniques in vehicles are the vibrations and other disturbances that occur in vehicles to which they are inherently more sensitive than contact-based sensors. In this work, optimized sensor architectures and signal processing techniques are proposed that significantly improve the robustness to artefacts. Experimental results, conducted under real driving conditions on public roads, demonstrate the feasibility of the proposed approach. R peak sensitivities and positive predictivities higher than 98% both in highway and city traffic, heart rate mean absolute error of 1.02 bpm resp. 2.06 bpm in highway and city traffic and individual beat R-R interval 95% percentile error within \(\pm\)27.3 ms are demonstrated. The radar experimental results show that respiration can be measured while driving and heartbeat can be recovered from vibration noise using an accelerometer-based motion reduction algorithm. advanced driver assistance systems; capacitively-coupled ECG; contactless driver monitoring; heartbeat; radar remote sensing; respiration; unobtrusive health monitoring; vibration compensation; vital signs monitoring 2019 9 3994 3990 3994 3990 3994 3990 3990 3990 390 399 When such physiological sensors are combined with complementary non-physiological sensing systems inside a vehicle (e.g., steering wheel and pedal sensors, speed, lane-keeping, collision avoidance, etc.) it potentially allows to improve the performance of safety systems to prevent accidents, as well as to improve the driver's experience and well-being or provide new services such as for example daily health check-ups during commuting. For such a system to be successfully adopted and used it is essential that the sensors are unobtrusive, i.e., the driver should not have to take any action to connect any sensors to their body. Two sensing techniques that meet this requirement are capacitive-based sensors, which can measure physiological signals through clothing, and radar-based sensors, which can measure remotely. As will be shown below, these sensors have complementary characteristics. They do not rely on a driver-facing camera, avoiding what may be considered a potential privacy concern for some users. The main challenge for the use of such unobtrusive sensors in vehicles is that the weak physiological signals need to be accurately detected while being reliably distinguished from motion artefacts due to unavoidable vibrations of the car while driving as well as the driver's body motions. The capacitive-based sensing system considered in this work can measure capacitively coupled ECG (ccECG) and capacitively coupled bioimpedance (ccBioZ). The ccECG measures the electrical signals generated by the heart (ECG) coupled capacitively through clothing to a conducting sensor plate. The ccBioZ is measured by capacitively (through the clothing) injecting a small a.c. current into the body from two conducting plates and measuring the corresponding voltage signal (also capacitively) through the clothing using two conducting sensor plates. The bio-impedance obtained in this way measures the respiration using the principle of impedance pneumography [8]. The ccECG can be measured through multiple layers of clothing [9] but not through e.g., thick jackets. The ccBioZ can be measured through very thick clothing layers or even small air gaps due to its 4-point measurement principle and due to the high frequency of the injected signal.