Wheels have played a major role in the development of mobile robots. They are simple, efficient, reliable, and capable of moving quickly across smooth surfaces. In places such as factories, warehouses, and paved environments, traditional wheels are often the best choice for robotic mobility.
However, wheels have an important weakness: they struggle when the ground becomes uneven.
A small step, raised platform, rock, or other obstacle can stop a wheeled robot completely. To overcome such challenges, engineers have developed legged robots and special wheel-leg systems called “whegs.” These designs can climb over larger obstacles, but they often require more complex mechanisms, consume more energy, and produce greater vibrations.
Now, researchers led by Godden have introduced an innovative solution called the PaTS-Wheel, or Passively-Transformable Single-part Wheel. This unusual wheel is designed to combine the smooth, energy-efficient movement of a traditional wheel with the obstacle-climbing ability of a legged robot.
The most impressive part is that it can do this without sensors, motors, actuators, or complex control systems.
A Wheel That Changes Shape When It Meets an Obstacle
The PaTS-Wheel looks and operates like a normal wheel while moving across flat ground. However, when it encounters a stepped obstacle, its unique geometry allows it to passively transform.
As the wheel pushes against the obstacle, parts of its structure interact with the edge of the step. This interaction causes hook-like features to appear and engage with the obstacle. These hooks provide additional grip and help the robot pull itself upward.
The transformation does not require a motor or an electronic command.
Instead, the obstacle itself guides the transformation.
This is what makes the design particularly interesting. The wheel automatically responds to the shape and position of the obstacle through its mechanical geometry. In other words, the physical structure of the wheel performs the work that would normally require sensors, software, actuators, and a control system.
This approach is known as passive mechanical adaptation.
Combining the Best Features of Wheels and Legs
Traditional wheels are excellent at moving efficiently across flat surfaces. They provide a smooth ride and require relatively little energy. However, their ability to climb obstacles is limited.
Legged robots, on the other hand, can step over and climb large obstacles. Their legs allow them to adapt to highly uneven environments. But this flexibility usually comes with increased mechanical complexity, higher energy consumption, and more vibrations.
Whegs, which combine features of wheels and legs, are another solution. They can climb obstacles better than traditional wheels, but their movement can be less smooth and may produce more vibration.
The PaTS-Wheel was designed to bridge this gap.
On flat terrain, it behaves much like a conventional wheel. When it encounters a significant obstacle, it uses its special geometry to create hook-like contact points and climb over the obstacle.
The result is a hybrid system that attempts to deliver the efficiency and smooth movement of a wheel while offering the obstacle-climbing capability of a legged mechanism.
Impressive Performance in Real-World Testing
To test the new design, the researchers compared the PaTS-Wheel with a traditional wheel and an equivalent wheg.
All three designs had the same diameter of 120 millimetres, allowing the researchers to make a fair comparison.
The results were particularly impressive.
The PaTS-Wheel successfully climbed obstacles approximately 83 millimetres high, which is around 70% of its own diameter. It achieved a 100% success rate when traversing these stepped obstacles.
By comparison, the equivalent traditional wheel could successfully overcome obstacles of approximately 30 millimetres, or about 25% of its diameter.
The wheg performed better than the standard wheel, reaching obstacles of approximately 73 millimetres, or around 61% of its diameter. However, the PaTS-Wheel still achieved a greater climbing height.
These results demonstrate the major advantage of the new design. A wheel with a relatively simple structure was able to climb obstacles significantly taller than those handled by a conventional wheel and even slightly higher than the tested wheg.
Energy Efficiency Similar to a Standard Wheel
Climbing ability is not the only important factor in mobile robotics. Energy consumption is also critical, especially for robots that operate using batteries.
A robot that consumes too much energy may need frequent charging or a larger, heavier battery. This can reduce its operating time and increase its overall weight.
According to the testing, the PaTS-Wheel maintained energy efficiency comparable to a standard wheel of the same size.
This is important because the wheel does not need additional motors or actuators to activate its obstacle-climbing mechanism. Its transformation is powered by the interaction between the wheel and the obstacle itself.
As a result, the design can provide enhanced mobility without introducing the energy demands normally associated with more complex robotic systems.
Smoother Movement and Reduced Vibrations
Another important advantage of the PaTS-Wheel is its vibration performance.
Wheg-based systems can produce more vibrations because their movement involves uneven contact with the ground. Excessive vibration can create several problems for robots.
It can reduce the lifespan of mechanical components, damage sensitive payloads, and affect the performance of onboard sensors. For robots carrying cameras, scientific instruments, or other precision equipment, smooth movement is especially important.
The researchers found that the PaTS-Wheel experienced fewer vibrations than the wheg during testing.
This could potentially improve the durability of the robot and its payload. It may also help sensors collect more accurate data while the robot is moving through difficult environments.
A Simple Design With Fewer Points of Failure
The PaTS-Wheel also benefits from its single-part construction.
Because the wheel does not depend on additional actuators, sensors, or complex control systems, there are fewer components that can fail. This could make the system more reliable in challenging environments.
A robot operating outdoors, in disaster zones, on rough terrain, or in remote locations may benefit greatly from a mechanism that can adapt mechanically without relying entirely on electronics.
The simplicity of the design could also make it easier to integrate into different types of mobile robots.
Potential Applications in Challenging Environments
The PaTS-Wheel could be useful in many areas where robots need to move across uneven or cluttered terrain.
Potential applications include search-and-rescue robots navigating damaged buildings, exploration robots operating in natural environments, agricultural robots crossing uneven fields, and planetary robots exploring rocky surfaces.
Small delivery robots and autonomous vehicles could also benefit from improved obstacle traversal. A raised pavement edge or unexpected step that would stop a normal wheeled robot might become manageable with a passively transformable wheel.
The technology could be especially useful in situations where low energy consumption, mechanical reliability, and simple control are important.
A New Direction for Robotic Mobility
The PaTS-Wheel demonstrates how clever mechanical design can sometimes replace complicated electronics.
Rather than using sensors to detect an obstacle, a computer to analyse it, and a motor to change the wheel's shape, the PaTS-Wheel uses its own geometry to respond automatically.
This approach combines the efficiency and smoothness of a traditional wheel with the climbing ability of a legged system.
With a 100% success rate on obstacles approximately 70% of its diameter, energy efficiency comparable to a standard wheel, and lower vibration than a wheg, the PaTS-Wheel represents an exciting development in robotic locomotion.
The research suggests that the future of mobile robots may not always depend on adding more sensors, motors, and software. Sometimes, a smarter mechanical design can provide a simpler and more efficient answer.
The PaTS-Wheel is a powerful example of this idea: a single, passive wheel that can adapt to challenging terrain and climb obstacles simply through the way it is designed.
Reference: T. Godden, B. W. Mulvey, E. Redgrave and T. Nanayakkara, "PaTS-Wheel: A Passively-Transformable Single-Part Wheel for Mobile Robot Navigation on Unstructured Terrain," in IEEE Robotics and Automation Letters, vol. 9, no. 6, pp. 5512-5519, June 2024, doi: 10.1109/LRA.2024.3389828.
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