## 3 Simulation Environment for Steering Feel Performance Prediction
### Extraction of EPS Control Logic Characteristics of Target Competior Vehicle  
The EPS logic in the target vehicle cannot be directly measured as a separate component, unlike the chassis. Therefore, we performed reverse engineering based on the EPS control system in our vehicle to extract the logic factors. As expected, when applying the EPS control system of our vehicle to the target vehicle, differences in steering feel performance were observed. Therefore, the control system was fine-tuned to achieve the same characteristics as the real car data of the target vehicle. However, only tuning the EPS control system could not achieve the desired outcome. Thus, we modified our EPS control system with regard to the function of damping. Regarding the transient steering characteristics, the initial steering torque gradient caused by friction is not properly offset, unlike in the measurement data, as shown in Figure 13b. This difference is primarily attributed to friction. To accurately implement the steering characteristics of the competitor vehicle, friction compensation should be enhanced. When significant steering torque is required at the initial steering stage due to friction, one approach is to increase the proportional gain to the high-frequency component generated by applying a high-pass filter to the torque sensor signal, as illustrated by the blue line in Figure 13b. However, too large a high-frequency assist distorts the torque response to a sinusoidal steering input, as shown in Figure 14a. This leads to an overly reduced damping characteristic and diminished friction feel. Additionally, in such cases, the steering wheel becomes more susceptible to disturbances and loses stability, as depicted in Figure 14b. To achieve stability and damping feel, the damping gain should be increased. However, increasing the damping also offsets the enlarged high-frequency assist. Thus, a significant trade-off between these factors exists, making it challenging to set the logic factors to match the steering feel characteristics of the competitor vehicle using high-frequency assist. To solve this problem, we proposed increasing the high-frequency assist with variable damping gain. First, the high-frequency assist gain was increased to improve the offset in  
Figure 14: (**a**) Steering torque route twist; (**b**) decreasing stability under high-frequency assist. Figure 13: (**a**) Comparison of weave test; (**b**) comparison of transition test. the transition test. Then, to fix the torque route twist problem, we modified the damping gain that could be set by steering angular velocity. The detailed parameter settings are as follows:  
As shown in Figure 15, the damping parameters were set by dividing the steering angular velocity into three sections. Unlike the conventional spec that defined only one gain regardless of the steering angular velocity, increasing the high-frequency assist gain did not cause the previously defined problem. Before setting up the configurations for each section, the damping gain in Section2 (0.1-0.3 rev/s) was increased to improve the damping feel and route twist that had been degraded by the increase in the high-frequency assist gain. The following section-specific configurations were then implemented. * Section1: Set a minimum level of resistance for the initial steering input and small-angle returnability (0-0.1 rev/s). When setting the damping gain to satisfy damping feel and stability, the level of resistance to driver steering increased, resulting in a delay in the initial response to rapid steering, such as step steer. However, applying the proposed method, the required torque at the initial steering phase was reduced, leading to the tire steering angle increasing, as shown in Figure 16a. In addition, the effect of damping on returnability was observed in hands-off situations, and improved returnability could be confirmed through the proposed methods, as shown in Figure 16b. * Section2: Set a consistent damping feel independent of angular velocity in sinusoidal steering (0.1-0.3 rev/s).