# Adapting Mechanisms for In-Pipe Inspection Robots: A Review
## 3 Analysis of Adapting Mechanisms
### Adapting Mechanisms Based on Slider-Crank Linkage
The spring between the nut and slider ensures the ability to mitigate the impact with small obstacles inside the pipe. Thus, adjusting the position of the slider, pipes with a diameter between 330 and 372 mm can be inspected. The mathematical model for the relationship between traction force, mechanism geometry and the spring force through the slider to element 2 is available in [35]. Another inspection robot with a multiaxial differential gear is presented by [PERSON] et al. in [36]. The proposed robot has an active adapting mechanism and three pairs of wheel sets, one active and the other passive. A multiaxial differential gear receives power from a single drive motor and distributes it to each active wheel via a bevel gear and the gear train. The schematic diagram of the robot's adapting mechanism is presented in Figure 7.  
A screw driven by the preload motor moves the nut \(1\) back and forth. This movement causes the elements 3 and 3 that support the wheels to rotate around the joints \(D\) and \(E\) thus, the robot is adapted to pipes with diameters between 130 and 180 mm. Even though the robot has an active adapting mechanism, the pressure against the pipe wall cannot be controlled. As each wheel arm has its own spring damper, the wheels can maintain the required wall pressing force. Additionally, each wheel can individually withstand small obstacles. The authors of the cited work performed several experiments that proved the robot's ability to move through horizontal and vertical pipes with elbows and different diameters. Another application of the slider-crank linkage is in the support and/or self-centering units. [PERSON] et al. in [37] use a slider-crank linkage to a support unit for a self-balancing robot with a helical drive. In this case, the linkage is similar to the one shown in Figure 1b with the mention that the supporting wheel is mounted at joint \(B\), and the necessary force is provided by a spring on slider \(1\). The support unit contains four radially arranged linkages and is designed to prevent the robot from rotating and to provide an anchor point so that the robot can center itself inside the pipe. In the examples presented above, the adapting mechanism has 1 DOF, being driven by a single actuator or spring. This causes all sets of wheels to move simultaneously. Additional mobility due to the presence of suspensions cannot be controlled. However, there are inspection robots that use, in the structure of the adapting mechanism, two independently driven slider-crank linkages which lead to 2 DOF. This gives the robot greater  
Figure 7: Adapting mechanism with dampers. flexibility in adapting to different pipe diameters and also allows the use of track-based locomotion systems. Such an approach is presented by [PERSON] et al. in [11]. The proposed robot has a passive adapting mechanism and three pairs of wheel sets. Each set of three wheels can be moved separately by means of slider-crank linkages. The drive wheels receive the required power from a single motor through a combination of cylindrical and worm gears. The prototype of the inspection robot is shown in Figure 3a and the schematic diagram of the adapting mechanism and the drive system is presented in Figure 8.  
The lengths of elements 2 and 2\({}^{\prime}\) are 53 mm and 95 mm while elements 3 and 3\({}^{\prime}\) have a length of 58 mm. The robot has a total length of about 300 mm and was designed to move in pipes with a diameter between 140 and 190 mm. The variation of the friction force between the wheels and the pipe wall is compensated only by the force generated by the two springs. Since only one drive motor is used, the speeds of the drive wheels cannot be controlled independently. This makes it difficult for the robot to move through curved pipes. However, the experiments performed by the authors confirm that the robot can easily move through straight or slightly inclined pipes and can easily overcome small obstacles.