# Driving Mechanisms, Motion, and Mechanics of Screw Drive In-Pipe Robots: A Review
## 3 Motion Behavior  
Conventional in-pipe robots are mostly applicable to inspection in a horizontal straight pipe or a straight pipe with small slope. Since urban gas pipelines have complicated structures due to complex distribution networks, researchers are increasingly likely to investigate in-pipe robots that are applicable to complex pipelines in recent years. Complex pipe structures include horizontal, vertical, curved, and branch. The robot must have a steering function to pass through the complex pipeline smoothly. At present, four types of steering mechanism are known, i.e., articulated, differential, variable helical pitch, and spring-connected types. Articulated steering mechanism is inspired by the movement of a snake or annelid. The guiding and driving parts of the single SDIR are often connected by joints (Figure 6), and the multi-robot modules are connected by joints (Figure 7) [43]. The robot body rotates relative to each other in a confined space. Figure 5: Transmission mechanism of the ASDIR. (**a**) Support-balanced and (**b**) self-balancing transmission structures. Articulated steering mechanisms are divided into two types, i.e., controllable and uncontrollable. An uncontrollable steering mechanism often relies on the bending and deformation of the pipe to force the robot's joint to rotate, enabling the robot to steer and adapt to the changes of the pipe [44]. However, the passive steering mechanism cannot select the path when encountering the branch pipes. Therefore, the uncontrollable steering mechanism is often only applicable to curved pipes. A controllable steering mechanism relies on the operators to control the robot actuators, such as a servo motor and an air pump, to realize active steering [45; 46; 47; 48; 49]. The controllable steering mechanism has an active path selectivity and a wide range of applications. The screw angle of the driving wheel can be adjusted by the adaptive mechanism through the differential principle and pipe binding force. In pipes with different radii of curvature, the robot can always adaptively find the corresponding screw angle [50]. As shown in Figure 8, the motor acts as an input and three level gears attached to the screw wheels act as outputs. The motor transmits power to the planet carrier \(H_{1}\) of the planetary gear train 1 through a worm gear mechanism. The output ring gear \(g_{\mathrm{r}3}\) of the planetary gear train 1 is connected to the input carrier \(H_{2}\) of the planetary gear train 2. The two sets of planetary gear trains form a three-axis differential mechanism. The planet carrier \(H_{1}\) is the input to the three-axle differential gear train, and the sun gears \(g_{\mathrm{s}1}\) and \(g_{\mathrm{s}6}\) and ring gear \(g_{\mathrm{r}5}\) are the three outputs. To convert these outputs into helix angles, a circular symmetrical output gear transmission was designed. For example, the rotational motion of the sun gear \(g_{\mathrm{s}6}\) is transmitted to the spur gear \(g_{\mathrm{s}7}\) through its coaxial spur gear \(g_{\mathrm{s}6}\), and the transmission direction is changed by two level gears \(g_{\mathrm{b}7}\) and \(g_{\mathrm{b}8}\). As the robot moves in the curved pipe, the force \(F_{\mathrm{w}1}\) on the outward screw roller  
Figure 6: Articulated steering mechanism (single robot). Figure 7: Articulated steering mechanism (multi-robot modules). will be greater than the force \(F_{\rm w2}\) on the inner screw roller. The differential mechanism adaptively adjusts the screw angle without any active control until the speed and force reach a new balance. Figure 9 shows the variable helical pitch mechanism steering in curved pipes by adjusting the helical angle of each driving wheel [39]. One driving motor drives the wheel carrier. The lifting and tilting degrees of the circular outer ring are controlled by the expansion and contraction of three linear stepping motor shafts, thereby controlling the link movement.