# Adapting Mechanisms for In-Pipe Inspection Robots: A Review
## 2 Linkages for Adapting Mechanisms  
There are many types of linkages that can theoretically be used to increase the pressure between the drive wheels/tracks and the pipe wall. Regardless of their structure and type, they must ensure that the robot adapts as easily as possible to variations in the diameter of the pipe. Furthermore, it must provide a constant and sufficient pressure to allow the robot to move. For many categories of in-pipe inspection robots, the linkages used for adapting mechanisms actually determine their mechanical structure and size. Therefore, the type of linkage used and the correct way of designing it can significantly change the performance of the robot.  
The adapting mechanism should have a small number of elements, reduced complexity and take up as little space as possible inside the pipe. From this standpoint, the majority of them have as base units, the four-bar and slider-crank linkages shown in Figure 1.  
These are simple and planar linkages with 1 DOF consisting of three mobile elements and four lower pair revolute and/or prismatic joints. In all the following schematic diagrams, the mobile elements of the linkages are denoted by numbers starting from 1, and the joints with the letters \(A\), \(B\), etc. The movements of the linkage's elements are relative to an element denoted by 0. A force, denoted by \(\overset{\rightarrow}{F}_{m}\), produced by a spring or an actuator can be applied on element 1 to move the rest of the elements in the linkage.  
In order to adapt to the diameter of the pipe and to improve the stability of the movement, the axial dimension of the mechanism is usually larger than the cross dimension. In most cases, the force required to adapt the robot to the inside diameter of the pipe is applied in the axial or near axial direction. This ensures a higher applied pressure on the pipe wall and a sufficient linear displacement at the point of application of the force. Therefore, taking the four-bar linkage as the base unit to design the adapting mechanism, a slider-crank linkage can be added to it. According to [21], 14 topological combinations are possible between the two base units shown in Figure 1.  
The input of the horizontal displacement should be small to reduce the size of the adapting mechanism and the output of the vertical displacement should be as large as possible to increase the diameter adjustment capability. A few possible configurations of linkages used for adapting mechanisms are shown in Figure 2.  
Figure 1: Base units for adapting mechanisms: (**a**) four-bar linkage; (**b**) slider-crank linkage.  
Figure 2: Linkages used for adapting mechanisms: (**a**) pantograph-based; (**b**) combined linkages.  
Linkages shown in Figure 2 will be analyzed in this paper. Being that the topic is quite broad, the authors chose only those configurations that appear more frequently in relatively recent articles.  
Several prototypes of wall pressed inspection robots developed by the authors are presented in Figure 3. Adapting the structure of the robots to the inner surface of the pipe is conducted passively, using pre-stressed compression springs.  
In all prototypes shown in Figure 3a,b,d, the movement along the pipe is ensured using a single drive motor. Its torque is transmitted to the drive wheels by means of gears. Alternately, for the prototype shown in Figure 3c, there is one gearmotor for each drive wheel. This slightly complicates the motion control system but increases the traction force and ensures better adaptability of the robot, especially when moving through curved pipes.