KIMM develops wheel that alters stiffness in real time based on situation

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 noble technology is anticipated to find wide applications in various moving vehicles equipped with wheels, where overcoming terrain obstacles is essential.

The Korea Institute of Machinery and Materials (President Seog-Hyeon Ryu, hereinafter referred to as KIMM), an institute under the jurisdiction of the Ministry of Science and ICT, announced that it has developed, for the first time in the world, a variable-stiffness morphing wheel inspired by the surface tension of a liquid droplet, and successfully applied the technology to a mobile system

The morphing wheel developed by the research team lead by Principal Researcher Sung-Hyuk Song and Dong Il Park, Head of the Advanced Robotics Research Center of the KIMM’s Research Institute of AI Robotics, is a wheel and modularization system capable of altering the stiffness of the wheel by simply changing the surface tension applicable to the 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 switch to a soft, deformable state when rolling over obstacles, depending on the height and shape of those obstacles.

There already have been technologies for designing the inside of wheels as flexible structures, like non-pneumatic tires. However, 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’s research team can be adjusted between a rigid state for fast movement on flat ground and a soft, deformable state for overcoming obstacles.

The morphing wheel developed by the KIMM’s research team is composed of a smart chain block and a flexible structure. The outermost edge of the wheel consists of a smart chain block, and 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.
^{* Wire spokes: Wire spokes connect the wheel hub, which is located at the center of rotation of the wheel and generates rotational force, to the wheel’s outermost structure.}

If the wire spokes pull the smart chain blocks inward, the traction force at the outermost smart chain structure increases. 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 KIMM’s research team recently succeeded in developing the modularization technology for easily applying the wheel to a wide range of mobile systems. A miniaturized, light-weight 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 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-centimeter-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.

Principal Researcher Sung-Hyuk Song of the KIMM commented, “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.” Song added, “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.”

Dong Il Park, Head of the KIMM’s Advanced Robotics Research Center, was quoted as saying, “This new technology for morphing wheels inspired by surface tension can help to overcome the limitations of conventional mechanical devices such as leg-type robots and wheel clusters for overcoming obstacles.” Park added, “It will likely find applications in a wide range of fields including wheelchairs, mobile robots, and personal mobility capable of overcoming obstacles.”

Meanwhile, this research was published as the cover article in the August 2024 issue of “Science Robotics (IF 26.1),” a leading journal in the robotics sector and a sister journal of “Science.” The research was supported by the project “Independent activity supporting robot for the paraplegic people (led by Dong Il Park, Head of the KIMM’s Advanced Robotics Research Center),” and funded by a major project of the KIMM (NK250F).

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The Korea Institute of Machinery and Materials (KIMM) is a non-profit government-funded research institute under the Ministry of Science and ICT. Since its foundation in 1976, KIMM is contributing to economic growth of the nation by performing R&D on key technologies in machinery and materials, conducting reliability test evaluation, and commercializing the developed products and technologies.