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New wheel design alters stiffness in real time

The variable-stiffness morphing wheel takes on a rock obstacle. [Credit: Korea Institute of Machinery and Materials (KIMM)]

 

 

A new technology for wheels and mobile systems, necessary for overcoming various obstacles in daily life such as stairs or rocks by adjusting the stiffness of the wheel in real time, has been developed for the first time in the world. This novel technology is anticipated to find wide applications in various moving vehicles equipped with wheels, where overcoming obstacles in the terrain is essential.

The shape-shifting wheel was developed at the Korea Institute of Machinery and Materials (KIMM) inspired by the surface tension of a liquid droplet.

The wheel was created by a research team lead by Principal Researcher Sung-Hyuk Song and Dong Il Park, head of the Advanced Robotics Research Center at KIMM's Research Institute of AI Robotics. The system is capable of altering the stiffness of the wheel by simply changing the surface tension applicable to its smart chain blocks without having to use complex machinery or sensors. The wheel can operate as a rigid, circular wheel in a regular driving mode and then can switch to a soft, deformable state when rolling over obstacles, depending on the height and shape of those obstacles.

Principal Researcher Sung-Hyuk Song (right) and Head of the Advanced Robotics Research Center Dong Il Park (left) at KIMM's Research Institute of AI Robotics developed a variable-stiffness morphing wheel inspired by surface tension. [Credit: Korea Institute of Machinery and Materials (KIMM)]

 

 

 

 

Although there are already technologies for designing the inside of wheels as flexible structures, such as non-pneumatic tires, the limitation of these conventional technologies is that the wheels are continuously deformed even when driving on flat surfaces, which hinders driving efficiency and stability and generates noise. On the other hand, the stiffness of the wheel developed by the KIMM research team can be adjusted between a rigid state for fast movement on flat ground and a soft, deformable state for overcoming obstacles.

KIMM's morphing wheel is composed of a smart chain block and a flexible structure. The outermost edge of the wheel consists of a smart chain block. A wire spoke structure for controlling the surface tension of the smart chain block is connected to the hub structure of the wheel. When the hub structure rotates or the distance changes, the connected wire spoke structure is either pulled tightly or loosened, thereby altering the surface tension of the smart chain block structure.

If the wire spokes pull the smart chain blocks inward, the traction force at the outermost smart chain structure increases. For the researchers, this is similar to a situation where an increment in the surface tension of a liquid droplet leads to an increase in the net force pulling the outermost liquid molecules, which results in the liquid droplet stably maintaining a circular shape. Conversely, if the wire spoke structure loosens, the rigidity declines.

The variable-stiffness morphing wheel design encounters a curb when attached to a self-balancing two-wheel wheel chair.

 

 

The KIMM's research team has also developed the modularization technology for easily applying the wheel to a wide range of mobile systems. A miniaturized, lightweight version of the variable-stiffness mechanism was inserted into the interior of the wheel. Then, a modularized version of the wheel was applied to various mobile systems such as a self-balancing two-wheeled wheelchair.

In this two-wheeled wheelchair system, the stiffness of the wheel can be changed in real time, allowing the wheelchair to move in a stable manner and alter directions even when space is limited. Additionally, the wheelchair can roll over obstacles such as rocks or 18-cm-high stairs. Meanwhile, by applying the wheel to a four-wheeled mobile system, the KIMM's research team confirmed that the wheel can also stably overcome obstacles that are 1.3 times higher than the wheel radius.

"One of the problems associated with quadrupedal and bipedal walking robots for overcoming obstacles is that movement efficiency is relatively low on flat surfaces, and that shaking inevitably occurs in the moving state," said researcher Song. "The newly developed morphing wheel is significantly meaningful in that it is capable of overcoming obstacles while at the same time maintaining high movement efficiency comparable to conventional wheels."

This research was published as the cover article in the August 2024 issue of Science Robotics.

Source: Korea Institute of Machinery and Materials

Published September 2024

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