# Adapting Mechanisms for In-Pipe Inspection Robots: A Review
## 1 Introduction  
Pipe networks are a consequence of the increasingly intense development of today's society. The various problems that occur due to aging, corrosion, and cracks in the pipe walls have led to an increase in inspection, maintenance, and repair activities. Many sections of pipes are found underground, which makes access inside them even more difficult. Therefore, the use of robots for pipe inspection and/or maintenance is currently one of the most effective solutions. Their practicality proves to be even more important when the areas to be inspected are dangerous or difficult to access. Inspection robots are electromechanical devices that can move through the pipe and provide visual information and can identify various defects, or corrosion processes, thus they can provide solutions to specific problems. The simplest classification divides these robots into two main categories: passive robots that do not have traction systems, respectively active robots that benefit from their own traction system that ensures their movement through the pipe. Pipe networks with straight sections and relatively large and constant diameters can be easily inspected using so-called Pipeline Inspection Gauges or PIGs [1]. These inspection devices do not have their own propulsion system, so they are transported inside the pipes by the fluid flow. Pipeline inspection using these devices is accomplished by inserting a PIG into the pipe through a launcher hatch. Then the pipeline pressure pushes the PIG out of the launcher and along the pipeline until it reaches a receiver trap, on which it gets latched [2,3]. Therefore, PIGs are uncontrollable and unable to adapt to sudden changes in pipe direction and/or diameter. Despite these inconveniences most of the pipelines carrying natural gases, petrol or water can be inspected with PIGs. According to [1] these represent approximately 99% of the total existing pipelines. Inspection of fluid-free pipelines or those which have changes in diameter and/or orientation involves the use of robots equipped with their own traction systems. The use of the active in-pipe inspection robots has led to numerous challenges regarding their design and functionality. These include but are not limited to the autonomy of the robot, the traction systems, and the mechanisms they use to adapt to variations in pipe diameter. Depending on the method used to move inside the pipe, active inspection robots can be classified as follows: Wheel/track type [4; 5; 6; 7; 8], Wall pressed type [9; 10; 11; 12], Walking type [13; 14], Inchworm type [15; 16], Screw type [17; 18], Snake type [19; 20]. The wall pressed type is the most used category of inspection robots. They usually use linkages operated passively using springs or actively using actuators. Actuation by means of springs has the advantage of constructive simplicity and allows a smooth adaptation to the change of pipe diameter. Instead, it does not allow the control of the pressing force against the pipe wall, consequently, the movement through inclined or vertical pipes can be a problem. The pressing force can be controlled using an actuator, so the robots can adapt to the inner surface of the pipe or to its orientation, therefore their movement inside the pipe becomes more efficient. This category of robots can have two, three or more chains of wheels/tracks evenly distributed around the center axis, depending on the design chosen for the robot. In recent years several researchers have studied various types of linkages used to adapt the robot to variations in pipe diameter, e.g., [PERSON] et al. in [21; 22], and [PERSON] et al. in [23]. There are also various reviews related to the design of the in-pipe inspection robots. [PERSON] et al. present a review of wall pressing robots used in the inspection of unpiggable pipelines [1]. [PERSON] et al. present the state-of-the-art robotic solutions used in the onshore oil and gas industry [3]. [PERSON] et al. provide an extensive review of various approaches in the modelling of the in-pipe inspection robots [24].