Engineers at the University of California San Diego have created a four-legged soft robot that does not need electronics to work. The robot instead requires a constant source of pressurised air for all its functions, including its controls and mobility systems.
Led by Michael T. Tolley, a professor of mechanical engineering at the Jacobs School of Engineering at UC San Diego, the team detailed its findings in the 17 February 2021 issue of the journal, Science Robotics.
“This work represents a fundamental yet significant step towards fully-autonomous, electronics-free walking robots,” says Dylan Drotman, a Ph.D. student in Tolley’s research group and the paper’s first author.
Applications for the robot include low-cost entertainment, such as toys, and robots that can operate in environments where electronics cannot function, such as MRI machines or mine shafts. Soft robots are of particular interest because they easily adapt to their environment and operate safely near humans.
A majority of soft robots are powered by air but, controlled by electronic circuits. This approach requires complex components such circuit boards, valves and pumps — which are often outside the robot’s body. According to the researcher, these components constitute the robot’s brains and nervous system — which are typically bulky and expensive.
The UC San Diego researchers state the robot is controlled by a light-weight, low-cost system of pneumatic circuits, made up of tubes and soft valves, onboard the robot itself. The robot can walk on command or in response to signals it senses from the environment.
“With our approach, you could make a very complex robotic brain,” says Tolley, the study’s senior author. “Our focus here was to make the simplest air-powered nervous system needed to control walking.”
The robot’s computational power approximately mimics mammalian reflexes, driven by a neural response from the spine rather than the brain. The team was inspired by neural circuits found in animals, called central pattern generators, made of simple elements that can generate rhythmic patterns to control motions such as walking and running.
To mimic the generator’s functions, the engineers built a system of valves that act as oscillators, controlling the order in which pressurised air enters air-powered muscles in the robot’s four limbs. The researchers built a component that coordinates the robot’s steps by delaying the injection of air into the robot’s legs — with the steps inspired by sideneck turtles.
The robot is equipped with mechanical sensors — small soft bubbles filled with fluid placed at the end of booms protruding from the robot’s body. When the bubbles are depressed, the fluid flips a valve in the robot that causes it to reverse direction.
The Science Robotics paper builds on previous work by other research groups that developed oscillators and sensors based on pneumatic valves, and adds the components necessary to achieve high-level functions like walking.
How it works
The robot is equipped with three valves which act as inverters that cause a high pressure to spread around the air-powered circuit, with a delay at each inverter.
Each of the robot’s four legs has three degrees of freedom powered by three muscles. The legs are angled downward at 45° and are composed of three parallel, connected pneumatic cylindrical chambers with bellows. When a chamber is pressurised, the limb bends in the opposite direction — providing multi-axis bending required for walking. The researchers paired chambers from each leg diagonally, which they claim simplify the control problem.
A soft valve switches the direction of rotation of the limbs between counter clockwise and clockwise. The valve acts as a latching double pole, double throw switch. This is a switch with two inputs and four outputs, with each input having two corresponding outputs it’s connected to. The mechanism is similar to taking two nerves and swapping their connections in the brain.
Next steps
In the future, the researchers want to improve the robot’s step so it can walk on natural terrains and uneven surfaces. This would allow the robot to navigate over a variety of obstacles. However, the improvements require a more sophisticated network of sensors more complex pneumatic system.
Additionally, the team aims to research how the technology could be used to create robots which are in part controlled by pneumatic circuits and traditional electronic circuits for higher functions.
