## 3 A Taxonomy of Measuring Methods, Techniques, and Sensors in Physiological-Based DMSs
### Electrocardiography (ECG)
#### 3.1.1 Contact ECG Sensors  
The classical contact method for real-time ECG signal acquisition involves three disposable electrodes attached to the driver's skin. This method is reliable, using adhesive electrodes made of silver with a thin coating of silver chloride (Ag/AgCl) placed around the chest, as shown on Figure 2a. A small portable device with an embedded ECG module records the voltage changes and transmits the signal via Bluetooth to a smartphone or a laptop [26]. Direct and close contact with the driver's cardiac region allows the generation of a reliable and high-quality ECG signal, which can be used also as a gold standard for evaluation of other methods. However, during long periods of driving, the self-adhesive electrodes can cause discomfort to the driver or peel off when sweating. They are sometimes perceived as intrusive by the driver. A promising alternative to wearable ECG modules is a wearable wrist-worn device with a built-in ECG sensor (Figure 2b), such as the smart watches that have been on the market in recent years (e.g., HUAWEI WATCH D2, Xiaomi Watch H1, etc.). Compared to the classical contact method for ECG recording, the advantage of the wrist-worn device is the lack of skin irritation and discomfort from stuck electrodes. Various types of electrodes and their placements have been studied, developed, and implemented for ECG signal acquisition with contact non-wearable sensors. The placement of electrodes on the steering wheel is one of the early research ideas (Figure 2c). In an attempt to quantify the stress level of a driver, [PERSON] et al. [27] in 2007 investigated the mental workload of a driver and the corresponding stress response by examining HRV in the ECG signal obtained from a set of dry ECG electrodes placed symmetrically on both sides of the steering wheel at the level of the central horizontal axis. In the same year, [PERSON] et al. [28] presented their study using dry electrodes made of copper tape integrated into the steering wheel symmetrically on both sides at a level above the central horizontal axis. In 2010, [PERSON] et al. [29], and later in 2014, [PERSON] et al. [30], presented steering wheel electrodes based on conductive fabric. In 2010, [PERSON] et al. [31] published their multi-sensor system, allowing measurement of ECG, skin conductance, and skin temperature with dry contact electrodes placed on the steering wheel symmetrically on both sides at the level of the  
Figure 2: ECG method and relevant techniques and sensors for acquisition of the driver’s ECG signal. central horizontal axis of a test car. At the same time, [PERSON] et al. [32; 33] demonstrated a multi-sensor system integrated into the steering wheel of a test car. In their design, the conductive electrodes for ECG recording are integrated into the steering wheel, gear lever, and left armrest. The authors reported collected ECG data from the steering wheel sensor electrodes during 81% of the driving time. In 2012, [PERSON] and [PERSON] [34] presented a steering-wheel-mounted ECG demonstrator that used four dry ECG electrodes. The two-electrode pairs were mounted symmetrically on both sides of the steering wheel at the level of the central horizontal axis. In the same year, [PERSON] et al. [35] demonstrated that dry Ag/AgCl electrodes mounted on the steering wheel could capture ECG signals with high quality, similar to the classical contact method. In 2014, [PERSON] et al. [30] proposed a design including two couples of conductive fabric electrodes placed on the steering wheel symmetrically on both sides at a level below the central horizontal axis. In 2015, [PERSON] et al. [36] developed a system for continuous acquisition of ECG signals by sewing a conductive fabric to a standard steering wheel cover. Further, the company CardioID developed an ECG sensor integrated into the steering wheel for monitoring the ECG signal (CardioWheel). In 2025, [PERSON] et al.