Inspired by the surface tension of a liquid droplet, researchers have created an adjustable wheel that changes shape in real time and can easily cope with uneven surfaces and tall obstacles. This opens the door to developing wheelchairs or mobile robots that can better navigate rough terrain.
Wheels, a key component of cars, trucks, and bikes, are so common that we often take them for granted. But when it comes to wheelchairs, the poor performance of wheels in moving over uneven surfaces or surfaces with obstacles means that wheelchair users are often limited to staying ‘on the road.’ The same goes for mobile robots.
Now, a group of Korean researchers has literally reinvented the wheel, developing a configurable wheel that changes shape in real time to enable unconventional travel by bypassing obstacles on the ground.
OK, “reinventing the wheel” is probably an exaggeration. A few years ago, tire giant Hankook teamed up with Seoul National University and Harvard to create origami-inspired transforming wheels. Years before that, graduate student Ackeem Ngwenya developed a “Trackless” tire system whose treads can be adjusted from narrow to wide with the turn of a screw. And in the meantime, NASA has created tires for rovers made of a nickel-titanium alloy that can deform down to the axle and then return to its original shape.
The problem of uneven or obstacle-filled ground can be overcome by using a track-based motion system, a specially designed mechanism that increases the surface area of the wheel. However, a track system is limited to relatively low speeds and consumes more energy than a wheeled system due to the high friction created by the increased contact between the system and the ground.
On difficult terrain, using airless or non-pneumatic tyres is an advantage, especially since they are resistant to punctures, leaks and bursts. However, their rigidity means that they are not very good at overcoming obstacles, especially high ones, because they are less deformable or less able to adapt to the ground they are travelling on.
Lee et al.
So, inspired by the surface tension of liquid droplets, scientists created a variable-stiffness wheel that can traverse rough terrain and obstacles while maintaining the driving advantages of a regular wheel on a flat surface. The surface tension of a liquid is caused by an imbalance in the attractive or cohesive forces between molecules. While a molecule in a bulk liquid experiences cohesive forces with other molecules in all directions, a molecule at the surface of the liquid experiences only net inward cohesive forces. In a droplet, as the cohesive force of the surface molecules increases, the net force pulling the liquid molecules inward also increases, resulting in the droplet returning to a circular shape.
Test drive a two-wheel wheelchair on a flat track
A key aspect of the deformable wheel is its ‘smart chain structure’, which consists of a chain of blocks around the outside of the wheel, connected by wire spokes to opposite sides of a central hub. By changing the space between the two sides of the hub, the researchers were able to change the length of the spokes and therefore the shape of the outer chain of blocks. Increasing the hub space shortened the spokes, forcing the chain block inwards and creating a circular wheel for fast movement. Decreasing it lengthened the wire spokes and loosened the chain block, allowing it to deform and pass over obstacles it encountered.
Test driving the two-wheel wheelchair on the grass
The researchers tested the adaptive wheel on a two-wheeled wheelchair and a four-wheeled vehicle and found that each could adapt to and roll over large steps and irregularly shaped rocks that were 1.2 times higher than the wheel radius.
The video above shows how the 120kg (265lb) two-wheel wheelchair system easily covers uneven, grassy ground – but the chair's extreme forward tilt eventually raises concerns about the safety of its occupant. The researchers haven't released any video of the four-wheeled vehicle in action, but there are images below.
Lee et al.
The researchers noticed that dust and particles were getting into the spaces between the smart chain blocks and damaging the wheel, so they are working on adding a wheel cover structure to future iterations. They see great potential in their adaptive wheels.
“This work demonstrates real-time stiffness variation at the scale of actual wheels used in wheelchairs and suggests broader common applications in wheel-based robots and transportation systems for efficient driving on rough terrain,” the researchers said.
The study was published in the journal Science Robotics.
Source: EurekAlert!