## 4 Experimental Results
### Experimental Results Discussion
#### 4.2.2 Experiments in Curved Condition  
In the curved condition experiments, the desired longitudinal velocity of the platform was set to 1.8 km/h for 0-1.5 s, 5 km/h for 1.5-13 s, and 0 for 13-13.5 s, as shown in Figure 13. The desired yaw rate was set to 2.87 degrees/s from 3 to 11 s and 0 in other periods. The longitudinal and lateral tracking in the curve condition more rigorously verified the performance of the driving force distribution control algorithm compared to in the straight conditions. Therefore, the purpose of these experiments was to verify the maneuverability and stability performance of the proposed driving force distribution algorithm.  
Figure 11: Comparison of sideslip angles.  
Figure 12: Comparison of phase planes.  
Figure 13 compared the longitudinal velocity tracking. The longitudinal velocity of the UGV platform with the Ref1 controller diverged when the desired yaw rate was not zero. At 3 s, the desired yaw rate was set to 2.87 deg/s. Then, the Ref1 controller distributed the torque on each wheel to control the platform to track the desired longitudinal velocity and yaw rate.  
Since the torque on the same side of the vehicle was equal with the Ref1 controller, the vehicle's longitudinal velocity could not converge to the desired value, and the experiment was stopped at 5.8 s. Figure 14a,b present the longitudinal velocity tracking error and its percentage in stable tracking from 3 to 12 s with Ref2 and the proposed controller. There was little difference in terms of the longitudinal speed tracking accuracy between the two controllers. Figure 15 compared the yaw rate tracking. However, according to Figures 15 and 16, the yaw rate tracking error with the proposed controller was smaller than that with the Ref2 controller in stable tracking from 5 to 11 s. In order to observe the tracking error and accuracy more directly and clearly, the mean absolute error (MAE), root mean square error (RMSE), and standard deviation (SD) values of the errors with two controllers were calculated and are listed in Table 2 From Table 2, it can be observed that the MAEs of the longitudinal velocity and yaw rate tracking with the proposed controller were smaller than those with the Ref2 controller. According to the MAEs, compared to Ref2, the longitudinal velocity tracking error and yaw rate tracking error decreased by 33.9% and 54.5% with the proposed controller in the tracking stabilization stage, respectively. The RMSE of the yaw rate tracking with the proposed controller was smaller than that with the Ref2 controller, and the other two metrics including the RMSE of the longitudinal velocity tracking and SDs of the longitudinal and yaw rate tracking with the proposed controller were all smaller than those with the Ref2 controller. This indicates that the proposed controller has better performance in terms of longitudinal and lateral tracking accuracy.  
Figure 13: Longitudinal velocity tracking.  
Figure 16: Yaw rate tracking error.  
Figure 14: Comparison of longitudinal velocity tracking with two controllers: (**a**) Comparison of longitudinal velocity tracking error; (**b**) Comparison of percentage of longitudinal velocity tracking error.  
Figure 15: Yaw rate tracking.