Application of the Wall Climbing Robot for Magnetic Particle Detection of Ship Welds

Currently, many wall-climbing robots are used in shipbuilding, particularly for weld inspections, to ensure sailing safety.

The rapid development of maritime trade has led to a surge in the demand for ships, significantly stimulating the upgrade of the shipbuilding industry. Many welds are used in sectional structures and hull closures, and the speed and accuracy of weld inspections directly affect construction quality and maritime safety. Presently, a common method is to rely on handheld inspection equipment, which has issues such as low efficiency, high risk, and an inability to store inspection results digitally.

Thus, a magnetic wheel wall climbing robot that can stably move on ships, perform weld inspections according to standard procedures, and digitally store inspection results has become a great solution to address these problems.

Magnetic particle inspection has become the most common weld inspection method due to its high efficiency and sensitivity. For the climbing robot to meet inspection requirements, it must solve three key issues: a movement mechanism with secure adsorption and flexible mobility, an inspection mechanism that can flexibly adapt and closely conform to the wall, and a workflow that mirrors standard human procedures.

In a previous discussion, we mentioned one of the clear advantages of the wheel-based climbing robot, which is its satisfactory movement performance and wall adaptability. Today, we will focus on the permanent magnetic adsorption part of this robotic solution.

Magnetic Wheel Design

This solution designs a permanent adsorption wheel in the form of spliced magnets, which can adjust magnetic force and quickly demagnetize. Based on the traditional four-wheel drive format, combined with a flexible hinge mechanism, a wall-climbing system capable of stable and flexible movement on circular walls was developed. Based on the parallel-series flexible adaptation structure, a passive flexible detection mechanism was created, utilizing its multi-degree-of-freedom (DOF) flexible deformation to tightly attach the yoke to the wall surface, ensuring the required magnetic field strength for weld inspections.

Robot Structure

The wall-climbing robot, which integrates stable adsorption and weld detection, solves issues related to reliable wall-climbing adsorption, flexible movement, dynamic adaptability of the detection mechanism, and an efficient, intelligent workflow. Below is a schematic of the detection robot's overall structure.

A reliable adsorption mechanism is a crucial condition for safe movement on external walls. Current magnetic wheels generally place magnets on the outside of the wheels, allowing direct contact with the wall to achieve adsorption, but these wheels easily scratch the wall surface and are difficult to remove after work. Based on an analysis of magnetic circuit conduction mechanisms, an adsorption device composed of several magnets was arranged inside the wheel to protect the wall. Considering that a smaller tangential force could relatively rotate the magnets to reduce adsorption, a rotating magnetic unloading mechanism was designed to facilitate the removal of the wheels from the wall. The structure of the magnetic wheel is as shown.

The magnetic wheel mainly consists of an adsorption mechanism, a wheel module, and a rotating magnetic unloading mechanism. The adsorption mechanism is formed by stitching three magnets with different poles together with a yoke iron. The diagram shows the magnetic poles of the magnets arranged clockwise and fixed on the yoke iron. By using the internal conduction mechanism of each magnet, the magnetic circuit length is shortened, providing strong adsorption force. At the same time, the yoke collects surrounding magnetic energy to improve the utilization efficiency of the magnets. The rotating magnetic unloading mechanism is fixed to the adsorption mechanism and attached to the axle so that it does not rotate relative to the wheel, allowing it to generate constant adsorption. By using the lever principle to rotate the magnets actively, the adsorption force can be reduced, making it easier to remove the magnetic wheels from the wall after detection. The wheel module, fixed on the outer side of the axle, can wrap the adsorption mechanism inside the wheel hub and is connected to the motor output, providing power to the robot. The wheel module is composed of an aluminum wheel hub and an inflatable tire, ensuring the adsorption force remains within an appropriate range to avoid scratching the wall surface and increasing the friction coefficient between the wall and the wheels.

By optimizing the parameters that affect the performance of the magnetic wheel, the optimal magnetic wheel was obtained. The shape parameters of the magnets are directly related to adsorption and the thickness of the yoke iron is related to the magnetic energy in the circuit. The width and outer diameter of the magnets, as well as the width and thickness of the yoke iron, are certainly constrained by the wheel's limitations, so the length of the middle magnet was optimized. Magnetic simulation software was used to determine the changes in adsorption force (F) and wall height (h) under various parameter influences.

Mechanics of the Robot

The robot’s weight and adsorption force directly affect the safety and efficiency of its inspection during ship operations. Achieving a balance between adsorption and weight and resolving the contradiction between safe adsorption and flexible movement is a challenging task. A variable, the robot’s posture angle, was introduced, and a unified system analysis was performed by changing parameter values to find the optimal adsorption force. The impact of the front wheel’s angle with the chassis on movement flexibility was analyzed, and a flexible motion control system was achieved through the establishment of a dynamic model. By analyzing the mechanical model on the ship, the robot can achieve safe adsorption and flexible movement.

The mechanical model of the robot on the vertical surface, as shown, mainly involves gravity, the magnetic wheel’s adhesion force, wall support force, and frictional force. Gravity acts downward constantly, the wall support force and the magnetic wheel’s adhesion force are perpendicular to the wall, and the frictional force is parallel to the robot’s forward movement direction.

During the robot’s wall climbing process, if adsorption is insufficient, instability phenomena such as falling, slipping, tipping, and overturning may occur. To ensure safe adsorption and stable movement, mechanical analyses of each instability condition were performed to obtain the minimum adsorption force.

Each magnetic wheel can achieve the maximum adsorption force required to cling to the wall (each wheel can provide stable adsorption).

During the weld inspection on ships, due to irregular obstacles, gravity, or weld deviations, deviations in the movement trajectory relative to the weld may be observed. The robot needs to adjust its posture through flexible movements to achieve accurate weld tracking and inspection efficiency. The movement mechanism uses a flexible hinge structure, which adjusts the angle between the front wheel module and the body frame to obtain a smaller turning radius, allowing for quick and flexible adjustments. This enables the robot to actively track the weld and perform efficient inspections.

There are still many applications where understanding the functionality we want the robot to achieve and the problems it may encounter are necessary before designing and adjusting solutions. What issues have you encountered when using a wall climbing robot for maintenance? I'm looking forward to hearing your voice.