Robotic crawlers for inspection of vertical or even overhanging structures are both an active area of Research as well as a field of intense industrial development. As the typical structure in ships are made of (magnetic) carbon steel, magnetic adhesion is typically used for generating the positive contact force required to provide both adhesion and traction against gravity.
In this project, other adhesion principles such as vacuum or electrostatic adhesion are not considered. Magnetic crawlers typically run on magnetic wheels or tracks, providing both adhesion and propulsion. These wheels can easily deliver magnetic forces 2 to 5 times the weight of the robot, thus providing a reasonable
safety margin against detachment and finally falling, even under sub-optimum surface conditions, such as paint and dirt layers or rust. Problematic is the quick contamination of the wheels/magnets w/ magnetic particle. Thus, the surfaces have to be sufficiently clean in order to guaranteed sufficient inspection time between cleaning procedures.
The main problem of crawlers based on magnetic wheels is the limited ability to pass non-flat obstacles. As soon as one (or more) wheel touch two surfaces, e.g. in 90° corners, tremendous forces are needed to free the wheel from the first surface and then move on.
Often the motor forces and/or the friction at the contact surface are not sufficient to escape this locked-in situation. Several approaches have been proposed to overcome this limit: multiple wheel configurations, moveable field magnets instead of magnetic wheels, active lifters, passive lifters.
For ship inspections, several crawlers have been developed to clean and inspect ship hulls. However, due to size and limited obstacle handing ability they are not suited to inspect other parts of the ship, such as structured cargo holds, stiffeners or (ballast) tanks. Additionally, they lack robustness and stability during handling such obstacles.
Due to the large magnetic forces (up to several kN), placing the robots on a magnetic structure and even more removing it can become a difficult and dangerous task. Dedicated placement tools or adjustable magnet positions are used to enable the operator user friendliness and safe handling.
It is industrial practice to manually control the robotic crawlers along the surface, thereby requiring fulltime, uninterrupted attention of a well-trained operator. Limited support for dedicated scenarios support comes from builtin sensors: inclinometers enable straight vertical motions and line-following sensors control the robot along a given feature, such as an edge, a weld or an artificially placed guidance structure.