## 4 Fault-Tolerance Control Strategy
### Fault-Tolerant Decision Making  
Based on the resulting steering-motor failure information, a decision is made about the next moment of vehicle response based on the location and number of steering motor failures. If all four steering motors fail at the same time, the vehicle is uncontrollable at that point and needs to stop quickly to enter the emergency braking state. When one or more steering motors fail, they can continue to work in some cases. Therefore, the failure of the motors is analyzed, as shown in Table 4.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Left Front** & **Right Front** & **Left Rear Steering** & **Right Rear** & **Decision-Making Results** \\  & **Steering Motor** & **Steering Motor** & **Motor** & **Steering Motor** & **Decision-Making Results** \\ \hline \hline \multirow{3}{*}{Single motor failure} & \(\otimes\) & \(\surd\) & \(\surd\) & \(\surd\) & Fault tolerance control \\  & \(\surd\) & \(\otimes\) & \(\surd\) & Fault tolerance control \\  & \(\surd\) & \(\surd\) & \(\otimes\) & Fault tolerance control \\ \hline \multirow{3}{*}{Dual motors failure} & \(\otimes\) & \(\otimes\) & \(\surd\) & \(\surd\) & Rear wheel steering \\  & \(\otimes\) & \(\surd\) & \(\otimes\) & \(\surd\) & Emergency brake \\ \cline{1-1}  & \(\otimes\) & \(\surd\) & \(\otimes\) & \(\surd\) & Front wheel steering \\ \cline{1-1}  & \(\surd\) & \(\otimes\) & \(\otimes\) & \(\surd\) & Emergency brake \\ \cline{1-1}  & \(\surd\) & \(\otimes\) & \(\surd\) & \(\otimes\) & Emergency brake \\ \hline \multirow{3}{*}{Triple motors failure} & \(\otimes\) & \(\otimes\) & \(\otimes\) & \(\surd\) & Emergency brake \\ \cline{1-1}  & \(\otimes\) & \(\otimes\) & \(\surd\) & \(\otimes\) & Emergency brake \\ \cline{1-1}  & \(\otimes\) & \(\surd\) & \(\otimes\) & \(\otimes\) & Emergency brake \\ \cline{1-1}  & \(\prime\) & \(\otimes\) & \(\otimes\) & \(\otimes\) & Emergency brake \\ \hline \hline Four motors failure & \(\otimes\) & \(\otimes\) & \(\otimes\) & \(\otimes\) & Emergency brake \\ \hline \hline \end{tabular}
\end{table}
Table 4: Fault-tolerant decision making.  
Figure 13: Fault detection strategy diagram.  
As shown in Table 4, \(\otimes\) for a failed motor condition, \(\surd\) for a normal motor operation. Faults can be classified as single motor faults, dual motors faults, triple motors faults and four motors faults.  
1. Single motor failure: When a single steering motor failure occurs, differential drive control is performed.
2. Dual motors failure: 1. Two front motors' failure: When the front two steering motors fail, the faulty motor stops working and the vehicle continues to drive through the rear wheel steering.
2. Two rear motors' failure: When the two steering motors at the rear end fail, the rear axle steering motor can be made to stop working and drive using the front wheel steering form.
3. Same side or opposite side motor failure: To ensure the security of the vehicle, use the emergency brake command to bring the vehicle to a quick stop.
3. Triple motors' failure: To ensure the security of the vehicle, use the emergency brake command to make the vehicle stop quickly.
4. Four motors' failure: To ensure the security of the vehicle, use the emergency brake command to make the vehicle stop quickly.