# Adapting Mechanisms for In-Pipe Inspection Robots: A Review
## 4 Discussion  
The robot's adapting mechanisms must ensure a possibility of pushing against the pipe wall with adequate pressing forces. Furthermore, the pressing forces must not have significant changes during navigation, in order to provide stable traction and flexible locomotion. Last but not least, the mechanism should be simple and small in size, to take up as little space as possible inside the pipes and not significantly increase the mass of the robot [48]. The specifications for the inspection robots presented in this paper and for their adapting mechanisms are summarized in Table 1.  
Table 2 shows a comparison between the linkages used in the adapting mechanisms presented in the paper. Slider-crank-based adapting mechanisms have a simple design and construction and ensure good maneuverability for the robot. Multiple modules will add the advantage of having greater stability and traction and the ability to carry inspection tools. The main disadvantage is the slippage when moving vertically. Inspection robots whose adapting mechanisms are based on independently driven slider-crank linkages or combined linkages, generally have greater structural complexity. However, they can travel along reducers, elbows and also steer in the branches by using various steering systems. For wheeled robots, the most used is the three-axis differential, which performs steering by modulating the speed of the drive wheels. The steerability for track-based robots is achieved by using differential speed on each track. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Ref. No.**} & \multirow{2}{*}{**Authors Type of Linkage**} & \multirow{2}{*}{**Adapting Mechanism Type**} & \multirow{2}{*}{**Robot Length Length Type**} & \multirow{2}{*}{**Locomotion Type**} & \multirow{2}{*}{**Traction Force (N)**} & \multirow{2}{*}{**Diameter Range (mm)**} & \multirow{2}{*}{**Max Slope (\(\lx@math@degree\))**} \\ \cline{6-9}  & & & & & & & & & & \\ \hline \([32]\) & [PERSON] et al. & & Passive & Unk. & Wheel & 341.4 & 270–380 & 20 & No & Unk. \\ \hline \([33]\) & [PERSON] et al. & & & & & & & & & \\ \cline{2-9} \([34]\) & [PERSON] et al. & Slider-crank (1 DOF) & & & & & & & & \\ \cline{2-9} \([35]\) & [PERSON] et al. & & Active & 370 & Wheel & Unk. & 330–372 & Unk. & No & Unk. \\ \hline \([36]\) & [PERSON] et al. & & Active & 128 & Wheel & Unk. & 130–180 & Unk. & No & Unk. \\ \hline \([11]\) & [PERSON] et al. & & & Passive & 300 & Wheel & 10.4 & 140–190 & 90 & No & 45 \\ \hline \([38]\) & [PERSON] et al. & Independently driven slider-crank (2 DOF) & & Passive & 235 & Track & Unk. & 136–226 & 30 & No & Unk. \\ \hline \([41]\) & [PERSON] et al. & & Passive & 150 & Wheel & 9.8 & 85–109 & 90 & T & 90 \\ \hline \([42]\) & [PERSON] et al. & & & & & & & & & \\ \cline{2-9} \([44]\) & [PERSON] et al. & & & & & & & & & \\ \hline \([45]\) & [PERSON] et al. & & & & & & & & & \\ \cline{2-9} \([46,47]\) & [PERSON] et al. & & & & & & & & & \\ \cline{2-9} \([48]\) & [PERSON] et al. & & & & & & & & & \\ \hline \([49]\) & [PERSON] et al. & & & & & & & & & \\ \hline \([50]\) & [PERSON] et al. & Combined (Four-bar and slider-crank linkages) & & & & & & & & \\ \cline{2-9} \([52]\) & [PERSON] et al. & & & & & & & & & \\ \cline{2-9} \([53]\) & [PERSON] et al. & & & & & & & & & \\ \hline \([55]\) & [PERSON] et al. & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison between the adapting mechanisms of the robots presented in the paper. One of the main advantages of pantograph-based adapting mechanisms is a larger stroke. This can be useful for inspecting pipes with large diameter variations. A linear radial travel for the drive wheels/tracks can be an advantage when the robot passes through the uneven surfaces of the pipes. The wheeled versions of these robots, although they have a simple structure, have problems maintaining a constant orientation along the pipe.