A compact multi-legged mobile robot with leg-mounted adhesion mechanisms for climbing walls in narrow spaces

As cities grow older, the buildings and bridges that hold them together begin to show their age. In Japan, the average age of this infrastructure is rising quickly, creating a pressing need for regular inspections and repairs. Yet, sending human workers to check high walls, narrow tunnels, or tight spaces around heavy machinery is dangerous and increasingly difficult as the workforce shrinks. To solve this, engineers are turning to robots. While some robots roll on wheels or fly like drones, these methods struggle on rough, uneven surfaces or in tight corners where obstacles block their path. A more adaptable solution lies in the design of walking machines that can grip vertical surfaces, but building a robot that can climb a wall without falling requires a delicate balance of movement and grip. The challenge is not just to stick to the wall, but to move across it without dragging the sticky parts, which would tear them off or leave them behind.

Researchers at the Muroran Institute of Technology have developed a compact, four-legged robot designed specifically to tackle these difficult inspection tasks. Their approach combines a walking mechanism with a suction-based grip that works on many different materials, from smooth metal to rough concrete. Unlike previous six-legged designs that were bulky and limited in how they could move, this new robot uses a simpler four-legged frame where each leg has four moving joints. This extra flexibility allows the robot to swing its legs freely, placing its feet in new positions without having to drag its suction cups across the surface. The team focused heavily on the "feet" of the robot, testing various shapes and materials to find the one that could hold the most weight. They discovered that a flat, circular pad made of urethane rubber, thirty millimeters in diameter, provided the strongest and most reliable grip. By measuring the force of this suction carefully, they ensured the robot could hold on securely even when moving.

To make sure the robot could actually climb, the team built a prototype and tested its ability to move on both flat floors and vertical walls. They programmed the machine to use a diagonal walking pattern: two legs would stick to the wall to hold the body steady while the other two swung forward or backward to find new handholds. This method allowed the robot to shift its weight and step forward without losing its balance. A critical part of the design was ensuring that the suction pad at the tip of each leg stayed perfectly flat against the wall, even as the leg moved through complex angles. Using a single leg unit, the researchers verified that the mechanical links could keep the pad parallel to the surface during these movements, which is essential for maintaining a strong seal. The tests confirmed that the robot could move forward and backward on a wall, switching its grip and swinging its legs in a coordinated rhythm.

The study also looked back at older designs to understand why they fell short. Previous six-legged robots relied on passive suction cups that did not press hard enough against the wall, leading to weak grip and unstable movement. Those older models were also restricted to simple swinging motions that made it hard to navigate complex environments. The new four-legged design overcomes these issues by using an active vacuum pump to create a strong, consistent suction force and by giving each leg more freedom to move. While the initial tests were conducted on smooth acrylic panels, the researchers acknowledge that real-world walls, often made of rough concrete, present a different challenge. They plan to test the robot on concrete surfaces next to see how the suction holds up against the bumps and irregularities found in actual infrastructure. For now, the results show that a compact, four-legged robot with carefully chosen suction pads is a viable way to inspect hard-to-reach places, offering a safer and more adaptable alternative to human workers.