A pressure valve that resembles the opening of a ketchup bottle allows soft robots to move and react to their environment. These robots don’t need electronics to move their legs in any particular order.
Five elongated, angular robotic fingers – made of a spongy material – smoothly curl one by one. It is as if the robot hand, developed by the Amsterdam research institute AMOLF, is drumming on an invisible table. The soft, flexible fingers work on air pressure and require no electronics.
When you think of robots, you might think of hard, steel machines that move jerkily, like functional devices in factories, or the friendly robots from movies, like Wall-E or R2-D2. But today researchers are also working on robots that are soft like living things and that respond smoothly and reflexively to their environment. This is necessary to allow robots to work safely with and for people, for example in healthcare.
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on the shoulders
ketchup bottle
AMOLF researchers present in their latest research article a soft robot that works on air pressure. Without the use of electronics, he can walk and change rhythm in response to the environment.
To do this, the robots use a pressure valve whose design resembles the opening of a ketchup bottle. If you turn the ketchup bottle upside down, this valve is closed. Only if you squeeze the bottle hard enough, the lid will open and ketchup will come out.
Squeezing a bottle too hard can cause the ketchup to sputter as the opening snaps open and shut quickly. ‘I saw this phenomenon a few years ago’, says AMOLF physicist Bas Overvelde† “Then I thought ‘this is interesting’.”
Recently, for fun, the team tried to replicate the ketchup experiments in the lab. But that turned out not to be easy. Overvelde: ‘It appears that they have adapted the design of the cap so that it sputters less. But for us it is precisely this normally undesirable behavior that is interesting.’
moving rhythmically
The researchers made their own valve opening from silicone rubber in which they cut small notches. They connected this to a pump, a kind of balloon and a soft robotic finger.
They inflate the balloon with the pump. When the pressure in it is high enough, the valve to the robot finger opens, causing the finger to move.
‘When we connected two robotic fingers with valves to a balloon, something unexpected happened,’ says team member Luke van Laake† ‘The fingers then move exactly back and forth.’
This is because one valve, which responds at a slightly lower pressure, opens first. This causes the pressure in the balloon to drop so that the second valve valve cannot open. If the pressure then builds up again, the first valve will not open again, because the finger is still slightly bent. The first valve has, as it were, remembered that it has just been open and skips a turn. This gives the second valve a chance to open and the second finger moves.
This effect creates a rhythm where one finger after the other moves. This also works with four or five soft robotic fingers. For example, you can let a soft robotic hand drum rhythmically.
Walking robot and artificial heart
You also see this alternating movement in natural movements, such as walking creatures. Thanks to this rhythm, a soft robot with four air pressure legs could walk by itself, without external control.
The rhythm adapts as the environment changes. If the researchers briefly closed one of the air tubes, the robot hand would switch to a different drum rhythm, for example. According to this principle, in the future a walking soft robot could change its walking rhythm by itself when it goes faster or when it encounters an obstacle.
The researchers are also thinking further ahead. Van Laake: ‘Soft robotics is still at the beginning of its development. We are now working on parts that last longer and are more efficient, because we are thinking of applications in the human body, such as an artificial heart.’ The aim is to make a soft artificial heart beat without complex, hard electronics.