# SambotII: A New Self-Assembly Modular Robot Platform Based on Sambot
## 6 Docking Experiments  
In this section, docking experiments are performed between two robots to verify the entire hardware and software system.  
Before the final structure was fixed, two prototypes (see Figure 16) of SambottII are built by adding the camera, laser tube, LED lights, and other components into Sambott. Due to the limitation of size, the hooks in the active docking surface are removed from these prototypes. The coincidence degree of the two docking surfaces is chosen as the evaluation index. In the final structure of SambottII, after the customized camera has been delivered, the hooks are equipped (as shown in Figure 2b).  
Figure 16: A docking process experiment. It takes 29 s in total.  
In Figure 17, L = 80 mm is the width of each surface, C is the coincident width of the two surfaces and E is the docking error. The coincidence degree \(\eta\) is defined as \(\eta=(C/L)\times 100\%\). As shown in Table 2, the permission deviation along the Z axis is \(\pm 19.5\) mm. Thus, when E is less than 19.5 mm or \(\eta\geq[(L-19.5 mm)/L]\times 100\%\approx 75\%\), the docking process can be regarded as successful.  
The experiment was carried out on a 60 \(\times\) 60 cm platform with an enclosure being 40 cm high. Because the enclosure surface is rough, the reflection of light is so weak that the recognition of the laser and LED lights cannot be affected. Because only two robots are applied in the experiments, only the LEDs on one passive docking surfaces are turned on.  
The passive robot is placed on one side of the platform and stay still, while the active docking robot is placed on the other side. The docking process is repeated 10 times to evaluate the success rate of the first docking. In each time, if the active robots misses the passive docking surface, the docking process will end, and this experiment is then counted as a failure.  
Experiment indicate that the success rate of the first docking is approximately 80%. Thus, the feasibility and validity of the docking algorithm is verified. The experimental results are shown in Table 5[18].  
There are two failed dockings in the experiments. One failure occurs due to compound errors. When the accurate alignment step ends, the angle that was calculated by the active robot is less than 5\({}^{\circ}\), but the actual angle might be 6\({}^{\circ}\) or 7\({}^{\circ}\). Besides, the speed difference between the two wheels may also lead to angle error. Influenced by these two errors, the final coincidence degree is between 65% and 75%. Another failure may be caused by incorrect LED recognition. When the accurate alignment step ends, the angle between the two robots exceeds the expected value. So, the active robot misses the passive docking surface finally.  
The failure caused by errors are inevitable. They can be reduced by improving the measurement accuracy and decreasing the speed difference between the two wheels. The failure cause by LED misrecognition may occur of the light reflected by LED is incorrectly identified as a LED's light. So, the algorithm should be further optimized to deal with the problem of reflection.  
When compared with Sambolt, SamboltI has higher efficiency and a larger range (it is 50 cm, but for Sambolt it is just 20 cm).  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Coincidence Degree** & **Times** \\ \hline
0–64\% & 1 \\
65–74\% & 1 \\
75–84\% & 1 \\
85–94\% & 4 \\
95–100\% & 3 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Result of autonomous docking experiment.  
Figure 17: Diagram of the coincidence degree from the top view of the two docking surfaces.