# Driving Mechanisms, Motion, and Mechanics of Screw Drive In-Pipe Robots: A Review
## 1 Introduction  
Transporting hazardous fluids via pipeline is safer than through other ways of transportation. Therefore, pipelines are widely used for transporting fluids in various fields [1]. The fluid transported inside the pipeline will cause damage to its inner wall due to high temperature, high pressure, high flow rate, and corrosion [2; 3]. Hazardous fluids may leak if the pipeline is damaged, which may cause a large number of casualties, serious property damage, and environmental pollution [4]. Global pipelines have a large number of safety hazards which can be managed by conducting the regular inspection and maintenance of pipelines [5]. In-pipe robots are electromechanical systems carrying instruments and working tools that can perform inspections and repairs in the pipeline [6; 7]. At present, in-pipe robots are mainly divided into active and passive types. Passive in-pipe robots move passively in pipelines by fluid energy and have been widely used to detect large-diameter and long-distance pipelines [8]. However, small-diameter pipes (200 mm or less) often contain numerous special parts, such as elbows, branches, reducers, and valves. Passive in-pipe robots are uncontrollable and cannot adapt to sharp changes in pipe direction and diameter. Therefore, numerous studies have proposed active in-pipe robots which can travel autonomously in pipelines. At present, these robots are mainly divided into six types: Gravity walking wheel (track) type [9; 10; 11]; wall-press wheel (track) type [12; 13; 14; 15; 16]; screw type [17; 18; 19; 20; 21]; inchworm (caterpillar) type [22; 23]; walking type [24; 25]; and snake type [26; 27; 28]. Among the active in-pipe robots, the screw drive in-pipe robot (SDIR) has been increasingly researched due to its simple driving (single power source) and numerous advantages [29]. Figure 1 shows several prototypes of the SDIRs proposed and fabricated by the authors. In comparison with other types of in-pipe robots, the SDIR has the following advantages. 1. Climbs vertical pipelines with ease
2. Is easily sealed off
3. Does not damage the inner wall, as it does not drag its body
4. Has flexible movement
5. Does not block the fluid flow inside the pipe
6. Can adapt to changes in inner pipe diameter
7. Has wider contact area than other robot types, which may dissipate contact force to generate propulsive force
8. Is generally difficult to back-drive (e.g., screw locomotion robots) due to its angled wheels or tracks and is thus effective in high-flow networks  
As shown in Figure 2, the driving wheel rotates around the pipe axis under the action of torque. An SDIR requires only one motor to drive, which greatly simplifies the structure and control system. SDIRs are wall-pressed, which allows them to easily climb vertical pipes [30]. Figure 1: Prototypes of the screw drive in-pipe robots (SDIRs) proposed and fabricated by the authors. (**a**) Variable helical pitch SDIR, (**b**) Wheel type PSDIR, (**c**) Inchworm type SDIR, (**d**) Support-balanced type ASDIR, (**e**) Track type PSDIR, and (**f**) Self-balancing type ASDIR. In recent years, more and more researchers have begun to study different types of SDIRs. [PERSON] et al. presented an intermediate articulated passive screw drive in-pipe robot (PSDIR) [31]. [PERSON] et al. designed an active turning SDIR with a controllable shaft inclination [32]. [PERSON] et al. presented a crab-shaped active screw drive in-pipe robot (ASDIR) [33]. [PERSON] et al. studied a differential-driven screw in-pipe robot [34; 35] and an ASDIR that can travel in circular and square pipes [36]. [PERSON] et al. proposed an ASDIR based on compound planetary gearing [37; 38], a variable-pitch SDIR [39], an inchworm SDIR [40], and a helical-contact deformation measuring method in pipelines [41]. The abovementioned SDIRs have different structures and driving principles, and each has its own advantages and application scope.