Responsive soft robots inspired by spraying

Cleverly designed pressure valve allows soft robots to react to their environment without the need for computer control, AMOLF researchers reveal in their journal paper Question. This brings robots with natural movements and tactile responses similar to those of living organisms in reality. These developments make soft robots more suitable for exploring rough and unknown terrain or for medical applications.

Video: https://youtu.be/wHYL4cFTAY0

Robots are still often thought of as rigid machines controlled by a central computer that precalculates each step. Conversely, living organisms move smoothly because intelligent behavior is ingrained in their bodies. This would be ideal for robots that need to continuously interact with people, such as those used in medical care. Researchers in the field of soft robotics are therefore developing robots made from soft and flexible materials that can respond to changes in their environment without outside guidance.

Bas Overvelde, leader of the Soft Robotic Matter group at AMOLF: “We want to create robots without a central computer that can move and react to their environment thanks to the reflexes integrated into the robot’s body.” In the review Question, the Overvelde team presents such a soft robot that works on air pressure without the need for electronics. He can walk and change his pace by reacting to his environment thanks to a cleverly designed valve.

Ketchup bottle
At the heart of the new soft robot is a “hysteretic valve”, which is how the researchers refer to their invention in their publication. Outsiders may recognize the valve as a variant of a ketchup bottle opener. “The valve ensures that you can easily dose the ketchup and that the liquid does not leak out of the bottle,” says Overvelde: “But if you turn the bottle upside down and squeeze it really hard, the ketchup sometimes spits out and does a mess. .’ This occurs when the aperture opens and closes repeatedly and rapidly.Modern bottles have been adapted to sputter as little as possible, but Overvelde and his colleagues investigated whether this normally undesirable behavior could allow a robot to move.

The researchers started with a computer model to design features such as the stiffness of the tiny flaps of the valve opening. After that, the valves were produced by pouring silicone rubber into a precise 3D printed mold. With a laser, they then cut small notches in the valves. As a result of this, they produced valves that are normally closed and suddenly open when the pressure gets high enough. The valves only close when the pressure has dropped considerably. If you connect this type of valve to a pump and a reservoir, you automatically create a rhythm of building up pressure and releasing air. With this, the muscles of the flexible robot are alternately inflated and relaxed.

The new component looks simple, but it has been discovered to have hidden powers, says Luuk van Laake, a doctoral student at AMOLF. “When we built a computer model of two such valves connected to the same tank, they started to open alternately with great precision. This was completely unexpected, but later proved to be equally effective in practice. Based on this principle, the researchers were able to build a four-legged robot with a natural gait that did not require any external commands. And they also ran a gentle robotic hand demonstrator with five fingers tapping in rhythm.

Respond to the environment
The ultimate dream of soft robot designers is a robot that not only moves, but also reacts to its environment. Van Laake: “Ultimately, you want a robot that automatically changes its gait if it encounters an obstacle simply because the muscles and joints react to the change in pressure.”

In the laboratory, the robot’s responsiveness to the environment has already been proven. When the researchers briefly closed a flexible tube by squeezing it a little, the pressure in the robot’s body changed and the robot then switched to a different gait. “Our soft robots react to the environment without a central computer needing to control it,” says Van Laake.

Efficient
We still have a long way to go before we have soft robots at the bedside. But the researchers are optimistic about the prospects. Overvelde: “With our robot, you can see how simple components can be used to produce complex behavior without the need for a computer to calculate every move.”

According to Van Laake, a flexible robot that moves through natural reflexes will not only save computing power, but also energy. “In living organisms, many movements and behaviors arise from body shape and muscle mechanics, rather than the brain directing all movement.” For example, our heart automatically balances the pressure in the left and right ventricles and we re-use the elastic energy stored in our tendons with every step we take. ‘It’s incredibly effective. There are still a lot of behaviors to explore in this area.

REFERENCE
Lucas C. van Laake, Jelle de Vries, Sevda Malek Kani, Johannes TBOvervelde, A fluid relaxation oscillator for reprogrammable sequential actuation in soft robots, QuestionJuly 8 (2022).
https://doi.org/10.1016/j.matt.2022.06.002

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