Customizable wheel overcomes obstacles with water droplet technology

Wheels have a decisive disadvantage: if they lack the right tread, they get stuck in mud and sludge (or snow and ice). The same applies to overcoming (too) large obstacles, over which the cross-section of the tire simply cannot pass – the vehicle gets stuck. A team of researchers at the Korea Institute of Machinery and Materials is therefore currently working on a "morphing wheel" that can adapt to the surface. The study by Jae-Young Lee's group, which was published in Science Robotics in August, compares the method to a drop of liquid whose surface tension changes and which does not simply burst when it rolls over sharp objects.

Overcoming stones and steps

Instead of liquid, however, the wheel consists of a "smart chain", i.e. an intelligent chain structure that can reconfigure itself based on external pressure. The structure consists of a chain of blocks that extend over the outermost side of the wheel and are each connected by wire spokes to the opposite sides of a hub on the inside of the wheel. By changing the distance between the two sides of the hub, the researchers were able to vary the length of the wire spokes and thus the shape of the outer chain. The resulting movement is not very elegant, but it is useful. In tests, the system was also capable of riding over steps and even larger stones, as can be seen in a video.

The prototype has already been mounted under a wheelchair. The adjustable strength of the tires would also be suitable for robots that need to move faster than humanoids on two legs – and still be able to cope with difficult surfaces. The system can currently overcome obstacles that make up up to 40 percent of the tire radius. This was tested with both a two-wheeled prototype, which weighed 120 kilograms, and a four-wheeled prototype. However, the two-wheeled system is likely to feel quite wobbly with a person on it.

3D printing and Kevlar

The Smart Chain for the Morphing Wheel was produced using a Stratasys 3D printer with ABS as the base material. Each chain block is connected by an aluminum rod with a radius of 2 mm. A sponge structure and a honeycomb structure form soft supports. The materials are ABS and liquid urethane rubber. The material used for the wire spoke structure is Kevlar fiber with a thickness of 2 mm. For the small-sized wheel used for the four-wheeled vehicle, the diameter was 300 mm and the tire width was 40 mm. The large wheel used for the wheelchair system had a diameter of 560 mm and a tire width of 90 mm. So both are by no means car-sized.

The useful thing about this comparatively simple technology, which was previously investigated in a computer model, is that it does not require a complex control or sensor system. Obstacles are overcome simply by increasing the contact surface, as is also known from tank tracks –, except that these run much slower than wheel-based systems. There is currently no automatic switch between normal tire operation and the morphing wheel; the researchers switch manually. Whether the South Korean scientists can really reinvent the wheel with their idea is therefore still unclear. So far, the speed in smart operation is still low.

In addition, particles and dust can get caught in the chains, which interferes with the mechanics, and encapsulated operation is not yet an option. It remains to be seen whether there will be a commercial application. The idea of innovative tires is nothing new in South Korean research. Three years ago, the local tire manufacturer Hankook demonstrated an origami-style tire in the Seoul metropolitan region that could even make itself smaller and larger to allow a vehicle to drive under narrow spaces. However, this system is also too sensitive for practical applications and therefore remained nothing more than a – prototype, albeit a spectacular one –.

(nie)