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The flexible spheres, created by roboticists at the University of California San Diego, could allow machines to quickly explore hazardous areas in applications including search-and-rescue missions and potentially even space exploration.
“Robots need to be able to walk fast and efficiently on natural, uneven terrain so they can go everywhere humans can go, but maybe shouldn't,” said Emily Lathrop, PhD student and the paper's first author.
“Usually, robots are only able to control motion at specific joints,” said senior author Professor Michael T. Tolley. “In this work, we showed that a robot that can control the stiffness – and hence the shape – of its feet outperforms traditional designs and is able to adapt to a wide variety of terrains.”
The feet are flexible spheres made from a latex membrane filled with coffee grounds. Structures inspired by plant roots and man-made stabilising structures were embedded in the coffee grounds to increase stability.
The spheres helped improve speed and grip thanks to a mechanism called granular jamming, which allows the coffee grounds to go between behaving like a solid and a liquid. When the feet hit the ground they tense up, conforming to the ground underneath and providing solid footing. The support structures help the flexible feet remain stiff while jammed. They then unjam and loosen up between steps.
The project was reportedly the first time such feet have been tested on uneven terrain, such as gravel and wood chips.
The feet were installed on a commercially available hexapod robot. Researchers designed and built an onboard system that generates positive and negative pressures to control the jamming of the feet. They can be ‘actively jammed’, with a vacuum pump removing air from between the coffee grounds, or ‘passively jammed’ by the weight of the robot pushing out air from between the particles.
The team tested the robot on flat ground, wood chips and pebbles, with and without the feet. They found that passive jamming performs best on flat ground, while active jamming is better on loose rocks. The feet also helped the robot's legs grip the ground better, increasing its speed. The improvements were particularly significant when the robot walked up sloped, uneven terrain, the researchers said.
“The natural world is filled with challenging grounds for walking robots – slippery, rocky, and squishy substrates all make walking complicated,” said study co-author Professor Nick Gravish. “Feet that can adapt to these different types of ground can help robots improve mobility.”
In a companion paper co-authored by Tolley and Gravish with PhD student Shivam Chopra as first author, researchers quantified exactly how much improvement each foot generated. For example, the feet reduced the depth of penetration in sand by 62%, and reduced the force required to pull the foot out by 98% compared to a rigid foot.
Next steps for the team include incorporating soft sensors on the bottom of the feet to allow an electronic control board to identify what kind of ground the robot is about to step on, and whether the feet need to be jammed actively or passively. The researchers also hope to improve design and control algorithms to make the feet more efficient.
The team will present its findings at the virtual RoboSoft conference between 15 May and 15 July.
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