# Adapting Mechanisms for In-Pipe Inspection Robots: A Review
## 3 Analysis of Adapting Mechanisms
### Adapting Mechanisms Based on Pantograph Linkages  
Pantographs usually have a larger stroke than other linkages with the same lengths of elements. A linear radial travel for the drive wheels can also be provided, which can be an advantage when the robot passes through the uneven surfaces of the pipes. [PERSON] et al. in [43] propose one of the first inspection robots in this category. These types of robots, although they have a very simple structure, can face problems with stability and maintaining a stable position while moving through the pipes. Figure 10: Adapting mechanism with inner springs. [PERSON] et al. in [44] propose an inspection robot whose three wheels are placed in the marginal joints of a planar pantograph linkage. In this case, the adapting mechanism actually represents the entire mechanical structure of the robot. A single drive wheel receives the required torque from a DC motor via a gear train. The prototype of the inspection robot is shown in Figure 3d, and the schematic diagram of the adapting mechanism is presented in Figure 11.  
The force produced by a spring determines the rotation of elements \(1\) and \(2\) around the joint D. This causes the expansion of the mechanism and therefore the increase of the wall pressing force. The lengths of elements \(2\) and \(3\) are 50 and 25 mm, respectively, the robot being able to inspect horizontal or vertical pipes with diameters between 50 and 70 mm. By using wider wheels, it was possible to increase the friction force and thus avoid slipping. As all wheels are in the same plane, the robot is not able to maintain its orientation toward the inner walls of the pipe. The equation for determining the pressing force is available in [44]. [PERSON] et al. in [45] present the development of a steerable in-pipe robot. The robot consists of two frames and a pantograph linkage that connects the centers of the two frames. The frames are identical, the wheels are independently steered, but the drive power is given only to the front wheels. Since the lower frame can rotate around the vertical axis, both frames can twist. This ability is useful for turning the robot around and avoiding obstacles. The proposed robot is able to move in three different ways: a straight line in which the wheel frames are parallel; a spiral movement in which the frames twist with a constant angle without steering and a steering movement in which all wheels are actively steered. Thus, the robot can avoid obstacles, and enter the joint spaces of L- and T-shaped pipes. The length of the robot is about 200 mm and the height is adjustable from 140 to 210 mm. Figure 12 shows the schematic diagram of the adapting mechanism for this robot. There is a helical spring between joints C and D. Its force causes the pantograph to expand and the wheels of the two frames to be pressed against the inside wall of the pipe. [PERSON] et al. in [46; 47] propose a track-based inspection robot that moves through horizontal, vertical, and curved pipes with various diameters. Each of the three-track modules is connected to a pantograph linkage through a revolute joint so that it can rotate freely. This and the presence of two spring dampers helps the tracks to have improved contact with the uneven surface of the pipe. Figure 13 presents the structure of the active adapting mechanism. Figure 11: Inspection robot with pantograph linkage. A gear train transfers the power of a DC motor to three screws so that they can rotate synchronously and ensure the movement of the pantograph linkages. The pantograph sliders \(1\) and \(2\) are moved with a left/right threaded screw and are pushed by the nuts by means of two springs. These springs give the adapting mechanism a certain compliance with the movement in the radial direction. The robot estimates the pressing force by observing the relationship between the forces acting on the pantograph and the movement of the two sliders. The slider displacement is calculated using the DC motor encoder counting value and the measured value of angle \(\theta\).