# Unobtrusive Vital Sign Monitoring in Automotive Environments--A Review
## 4 Ballistocardiography  
Ballistocardiography (BCG) is an old medical sensing technique, with early BCG recordings dating back more than 100 years [66; 67]. BCG allows monitoring of cardiac (and usually respiratory) activity unobtrusively. Typically relying on external pressure or strain-gauge sensors, a ballistocardiograph records the vibrations which are caused by the mechanical activity of heart and lung, as well as the momentum of the blood pulse traveling down the aorta. Hence, the diagnostic information is \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**System** & **Year** & **References** & \multicolumn{2}{c}{**Electrode Properties**} & **Special Properties** \\ \hline  & & & \# & **Shape** & **Ground** & \\ \hline SMART Test Vehicle & 2008 & [42] & 2 rectangular & in backrest & isolated (coated) metal electrodes \\ Daimier Sedas seat & 2008 & [43] & 2 round & in seat plane & \\ Ford S-Max seat & 2011 & [44] & 6 rectangular & in seat plane & multi-electrode system \\ Daimier Test Vehicle & 2011 & [58] & 2 rectangular & textile electrodes \\ Audi QS & 2012 & [48] & 2 rectangular & in backrest & textile electrodes, removable seat cover \\ Car seat & 2012 & [59] & 2 rectangular & in seat plane & \\ Ford S-Max seat & 2014/2017 & [53; 60] & 2 rectangular & in seat plane & textile electrodes, release of humidity \\ Plessey PICT-M seat & 2014 & [59] & 3 round & in seat plane & deep-drawn electrodes \\ Ford S-Max Test Vehicle & 2015 & [61] & 6 round & backrest, built-into main unit & removable seat cover \\ Plesssey WARDENTM & 2016 & [52] & 6 rectangular & backrest, built-into main unit & up to 64 electrodes possible \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of different cECG systems integrated into car seats. completely different from a classical electrocardiogram (ECG) which records electrical activity of the heart muscle. In the late 1950s, [PERSON] et al. introduced different categories of BCG measurements [68] and defined three axes of measurement for translational BCG recordings: longitudinal (head-foot, also called _craniocaudal_), _transversal_ (side to side), and _dorsoventral_ (back to chest). We note that typically the craniocaudal momentum is much bigger than the transversal and the dorsovental component. We also note that, to measure a BCG signal, contact (mechanical coupling) is required. The early BCG recording systems were typically of the longitudinal kind [66; 67]. For a standing person, this corresponds to the \(z\)-axis (see Figure 5a). Weighing scales which recently received increased attention [69] also measure along this axis. However, bed-based BCG systems [70] measure along a combination of the transversal and dorsoventral axes (see Figure 5b). If the coordinate system is not rotated with the person, but considered a fixed base coordinate system, then the dorsoventral axis corresponds to the \(z\)-axis as well. For the bed, sensors can be placed above or between mattresses (for example, quasi-piezoelectric ferroelectret foils), inside the mattress (optical sensors), on the slatted frame (employing e.g., strain-gauge sensors) or in the bedposts (using, for example, pressure sensors). If pressure or accelaration sensors are attached to the chest to record the heart motion (\"thoracic sensors\" in Figure 5), the resulting special form of BCG signal is referred to either as  
* Phonocardiogram, or
* Apexcardiogram, or
* Kinetocardiogram, or
* Seismocardiogram. BCG has gained renewed interest in recent years [71], mainly because of the simple and cheap instrumentation and the potential of this technique being used outside classical medical scenarios, for example in sleep analysis. In 2015, a review on recent advances was provided by [PERSON] et al. [72]. However, the BCG signal is much more variable than the ECG signal and also dependent on the position of a (sleeping) body. Due to this high inter- and intravariability, proper heart rate detection is more difficult than ECG analysis.