## 3 Experiment Results and Observations
### Simulation and HILS Experiment Results
#### 3.2.2 The Longitudinal Driving Simulation and HILS Experiment Results  
Figure 7 illustrates the control output results of the power distribution system controller when the drive motor was operated at 20 and 100 rpm in flat (0%) road conditions. The control output maintained values between 40:60 and 60:40, which correspond to the front/rear power distribution ratios obtained by previous simulations. However, there was a time delay of approximately 0.02 s as the torque value output by the experiment equipment's torque sensor was inputted into the PC via the DAQ, and it was confirmed that oscillation occurred in the CarSim controller's output. Nevertheless, the error rate was less than 3%, and it is not expected to have a significant effect on control.  
Figure 6: Graph of the torque by current value and drive RPM.  
Figure 7: Power distribution ratio: (**a**) 20 rpm. (**b**) 100 rpm.  
Figure 8 shows the front/rear driveshaft torque results according to the power distribution ratio based on the front/rear driveshaft torque values outputted by the CarSim vehicle when the output of the controller was inputted as the vehicle's rear-wheel driveshaft torque ratio. Owing to the characteristics of the program, the interval at 0-0.1 s was deemed to be an unstable interval. Therefore, the stabilized region from 0.2 to 1.0 s was analyzed. During the interval from 0.3-0.5 s, which indicates the moment when the frictional coefficient changed from \(Mu=0.5\) to \(Mu=0.2\), the front wheel experienced a relative loss of friction force, and the rear-wheel driveshaft torque increased by 11.06 Nm at 20 rpm and 6.34 Nm at 100 rpm to transmit additional power to the rear wheels. Moreover, when considering the front/rear driveshaft torque during the interval from 0.5-0.7 s, which corresponds to the moment when the friction coefficient changed from \(Mu=0.2\) to \(Mu=0.9\), there were torque differences of 43.22 Nm and 45.51 Nm, respectively. Even when the road's state varied rapidly, there was no significant impact on the power transmission.  
Figure 9 presents a graph that shows the longitudinal slip ratio of the wheels, and it confirms that the longitudinal slip ratio of the wheel's changed according to the rapid changes in the road friction coefficient. During the interval from 0.3-0.5 s, which corresponds to the instance when the friction coefficient changed from \(Mu=0.5\) to \(Mu=0.2\), the changes in the slip ratio at both 20 rpm and 100 rpm were not large. However, during the interval from 0.5-0.7 s, which corresponds to the instance when the friction coefficient changed from \(Mu=0.2\) to \(Mu=0.9\), there were momentary increases of 0.014907 and 0.014631, which are factors of 1.4 and 1.44, respectively, and they were found in the rear right wheel (RR). In addition, owing to the time delay, the rear right wheel's (RL) slip ratio output had a time delay of 0.1 s, and it was found that a slip ratio difference between the front wheel and the rear wheel occurred in the moments when the road friction coefficient changed.  
Figure 10 illustrates the control output results of the power distribution system controller when the drive motor was operated at 20 rpm and 100 rpm on an inclined road (30%). Since the vehicle drove on a road with a fixed gradient of 30%, the CarSim controller's output was fixed at 0.6 after 0.175 s and 0.172 s, respectively. Furthermore, when the CarSim controller output was 0.5 or greater, the MR fluid multi-plate clutch changed to friction mode to transmit the required torque. Accordingly, it was determined that the sub-controller controlling the MR multi-plate clutch performed smoothly.  
Figure 8: Front/rear driveshaft torque: (a) 20 rpm. (b) 100 rpm.