# Reliable Visual Exploration System with Fault Tolerance Structure
## 7 Discussion  
As the experimental results demonstrate, reliable performance is achieved with a couple of fault tolerance strategies. To improve the reliability of exploration, besides VLM, an active controller and exploration evaluation are introduced as a top layer of the mobile robot system to overcome the failure cases of visual tracking and map. A demonstration of autonomous exploration is provided in the attached video \"Autonomous Visual Exploration\".  
Regarding the designed active controller, the results of simulations and experiments are provided, which show the ability of environment coverage, as well as the feasibility of obstacle avoidance. Evaluations have been done in the Gazebo and in real experiments, and the experimental results are shown in Figures 9-13. The active controller is able to drive the mobile platform properly without global  
Figure 16: Convergence of termination condition in density-based evaluation is verified, and the density-based evaluation can be satisfied after a certain time of exploration.  
Figure 17: Completeness evaluation in distribution-based evaluation.  
localization in different situations, to achieve fault recovery by being able to move continuously until re-localization is obtained.  
In the VLM modules, regarding the fault recovery for maintaining tracking and mapping, as is indicated in Table 3, the A-KFs of VLM are much larger than ORB-SLAM. As for the frame rate by parallel design, which is analyzed in Table 4, P-MC can improve the efficiency compared with the serial version and the P-F&B version, and the precision is similar. Such a performance strengthens the fault prevention by reliable data association.  
With respect to maintaining mapping by submap-based back-end, as is shown in Figure 14, the error increases when visual tracking fails, and the error decreases when loop-closure is detected. Accounting for the whole exploration process, as the number of vertices in the map and the number of submaps increase, mapping is maintained all the time and the improved global consistency can be obtained. In other words, the use of submap-based mapping can to some extent overcome tracking failure by allowing the robot to operate within one of the submaps.  
Considering the termination condition, we examine the effect of completeness evaluation on visual re-localization. The improvement introduced by completeness evaluation is indicated in Figure 15, where a higher re-localization percentage is obtained. The convergence of \(s_{n\upsilon}\) is provided in Figure 16. Such a convergence verifies the feasibility of density-based evaluation. In addition, as is indicated in Figure 17, \(n^{grid}\) increases during exploration, and the upward trend becomes obvious after density-based evaluation is satisfied. This verifies the feasibility of the proposed Coarse-to-Fine mechanism.  
As can be seen in Table 1, our proposed solutions to the failure cases of visual tracking and mapping have been successfully verified, and the superiority with fault tolerance design in all the modules is shown quantitatively by different performance metrics.