Video: The prototype of the robot leg is currently only hopping in circles and is tied up. But in the future, freely moving robots could be equipped with the “muscular” legs. © Robotic Materials Department at MPI-IS
Inspired by nature instead of motorized: Researchers have developed a robot leg for the first time that is powered by electro-hydraulic “muscles.” The tests show that the system could enable particularly efficient locomotion: The artificial muscle leg can confidently hop over different terrains without complex sensor technology and uses less energy than a motor drive. This shows considerable potential for the development of locomotion systems in robotics, say the researchers.
They have been in the service of humans for a long time and have been constantly developed further. But to this day, most robots are still characterized by an old basic technology: they are powered by motors. Even the movement elements of the walking robots inspired by animals and humans are still based on these elements. This is why they are still inferior to their models powered by muscle systems in some aspects. For this reason, technical concepts that imitate nature’s patent have been worked on for some time.
Electrohydraulic elements instead of motors
Researchers at the Swiss Federal Institute of Technology in Zurich (ETH) have already achieved promising results in recent years when developing artificial muscles. Now, in collaboration with international colleagues, they have used the units for the first time to set a robot leg in motion. As the team explains, an extensor and a flexor muscle ensure that movements in both directions are possible, analogous to the natural model. These artificial muscles, known as actuators, are connected to the plastic skeleton of the robot leg via tendon-like connections.
The actuators are plastic bags filled with oil. About half of the bag is coated on both sides with a conductive material. When voltage is applied, the differently polarized walls in the upper part of the bag contract due to the electrostatic attraction. The oil is then pressed into the lower part of the bag, which then swells. This in turn is associated with a shortening of the entire unit – the system can therefore provide tensile stress. Pairs of such actuator units consisting of several elements can thus cause movements in a similar way to the muscle systems in animals: when one muscle shortens, its counterpart lengthens.
Using this principle, the researchers now set their robot leg in motion using two joint units. A control system connected to a voltage sensor specifies which actuators should contract and which should extend. As tests with the prototype of the system show, the robot leg can perform very fast and powerful movements that allow it to move in a hopping manner.
Agile and energy efficient
The researchers were also able to show that the system can spontaneously adapt to changes in the surface thanks to the high elasticity of the artificial muscle system. “This is also the case with living beings: If we cannot bend our knees, for example, we have great difficulty walking on an uneven surface,” says senior author Katzschmann from ETH. With an electric motor, a sensor would have to constantly tell the electric motor what angle the robot leg is at. The artificial muscle system, on the other hand, can adapt flexibly through interaction with the environment, the researchers explain.
The electrohydraulic concept also scores points in terms of energy efficiency, the team reports. Above all, it can save energy when at rest, the studies showed. “In an infrared image, you can quickly see that the motor leg consumes a lot of energy when it has to be held in a bent position,” says lead author Thomas Buchner from ETH. This is because electricity has to flow through the DC motor that drives it to maintain the position, which means energy is lost in the form of heat. In contrast, the temperature in the electrohydraulically driven leg remains the same. This is because the artificial muscle works electrostatically and does not require a current flow.
However, the researchers emphasize that their concept is still in an early development phase. So far, the leg is attached to a pole, hops in circles, and cannot yet move freely. Nevertheless, they say that possible uses in robotics are already emerging. “Our publication shows how much potential there is for groundbreaking innovations in the introduction of new hardware concepts – such as in the case of the use of artificial muscles,” says co-author Christoph Keplinger from the Max Planck Institute for Intelligent Systems in Stuttgart. Katzschmann adds: “If we combine the technology of the robot leg to create a four-legged robot or a humanoid robot with two legs, we can one day use it as a rescue robot as soon as it is battery-operated,” the scientist hopes.
Source: Swiss Federal Institute of Technology in Zurich, specialist article: Nature Communications, doi: 10.1038/s41467-024-51568-3