# Risk-Sensitive Rear-Wheel Steering Control Method Based on the Risk Potential Field
## 4 Experimental Study Using a Driving Simulator
### Discussion  
Figure 20a,b depicts the RMS values of the lateral deviations for each subject and the mean of all the data. However, the data for each subject exhibited an average of three times for each condition. Similarly, Figure 21a,b illustrates the integral of the squared steering wheel angle, while Figure 22a,b presents the EAPI. Figure 23 depicts the integrated risk potential values. Figure 20: Root mean square (RMS) of the lateral deviation at (**a**) \(V=60\ \mathrm{km}/\mathrm{h}\) and (**b**) \(V=80\ \mathrm{km}/\mathrm{h}\). 4 WS was applied, the steering burden of the driver reduced for all subjects compared with the cases of the 2 WS and zero-sideslip angle 4 WS. Figure 22 validates that the application of the risk potential field 4 WS reduced the EAPI significantly for all subjects in comparison with the cases of the 2 WS and zero-sideslip angle 4 WS. Furthermore, the comparison of Figure 22a,b indicates that the decrease in EAPI when the risk potential field 4 WS was applied was larger when the vehicle velocity was 80 km/h than that observed for 60 km/h. In other words, the higher the vehicle velocity, the more difficult it is to conduct emergency avoidance without control. However, the emergency avoidance performance was significantly improved, owing to the application of the proposed method. Finally, Figure 23 indicates that the risk potential of the vehicle trajectory could  
1. The vehicle could generate a yaw rate equivalent to that of the reference driver and derive a safe and smooth trajectory in a double lane change test by applying the risk potential field 4 WS. 2. The experiments were conducted three times under each condition, and the trajectory variation became smaller when the risk potential field 4 WS was applied. 3. The corrective steering and steering burden of the driver were reduced by applying the risk potential field 4 WS in comparison with the 2 WS and conventional 4 WS. Consequently, the handling quality and emergency avoidance performance were enhanced. 4. The amount of steering support for the rear wheels in the proposed method was adjusted while considering the driving characteristics. Hence, the system could guide the driver to safety while maintaining the driver's vehicle control authority. 5. In this study, the simulation was conducted under the assumption that the risk potential field was generated accurately from the road environment information. However, for practical use, it was necessary to conduct experimental verification while actually obtaining information from the sensors and maps in real time. In addition, there were obstacles such as pedestrians and other vehicles in the actual driving environment. Therefore, if the effectiveness of the proposed control could be confirmed after defining the obstacle potential, its feasibility would be further enhanced. In the future, the application of the proposed risk-sensitive 4 WS system in the other driving scenarios to support driving safety will be studied, and the driver-vehicle interaction and shared control characteristics will be discussed. As for recent deployment of 4 WS-equipped production vehicles [14], experimental tests in real vehicles will also be investigated in the near future. This study has shown one example of adding an active chassis control system in the vehicle for enhancing automated driving functions (e.g., lane keeping and steering assistance for collision avoidance). Aside from the four-wheel steering system, the four-wheel drive system (4 WD) is also one of the promising chassis control technologies which can be used in advanced vehicle automation with high stability and vehicle dynamics capability [23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36]. Especially in the era of vehicle electrification, further developments of the active chassis control applied to vehicle automation will be intensively studied in the near future. Conceptualization, T.K. ; formal analysis, T.K. and P.R. ; funding acquisition, P.R. ; methodology, T.K. and P.R. ; software, T.K. ; supervision, T.K. ; validation, T.K.