For nearly 70 years, inventors and researchers have been dedicated to developing robots. However, these machines have always been powered by motors, which are 200-year-old technology. As a result, robots lack the adaptability and mobility found in living creatures.
Now, researchers at ETH Zurich and the Max Planck Institute for Intelligent Systems have created a breakthrough: a robotic leg powered by artificial electro-hydraulic muscles. Modeled after living creatures, this innovative robotic leg is not only more energy efficient than conventional designs but also capable of high jumps, fast movements, and obstacle detection and reaction – all without the need for complex sensors.
The robotic leg utilizes extensor and flexor muscles, just like in humans and animals, to enable movement in both directions. These electro-hydraulic actuators, known as HASELs, are connected to the skeleton via tendons. They consist of oil-filled plastic bags with black electrodes on either side, which, when voltage is applied, creates movement similar to how static electricity attracts objects.
By using a computer code to communicate with high-voltage amplifiers, researchers can precisely control the contraction and extension of these actuators, resulting in coordinated muscle movements similar to those found in living creatures: as one muscle shortens, its counterpart lengthens.
The researchers compared the efficiency of their robotic leg’s energy consumption with that of a traditional robotic leg driven by an electric motor. They examined how much energy is converted into heat unnecessarily.
“On the infrared image, it’s easy to see that the motorized leg consumes much more energy if, say, it has to hold a bent position,” doctoral student Thomas Buchner says.
In contrast, the electro-hydraulic leg maintains a consistent temperature because of its electrostatic artificial muscle. Buchner illustrated this by comparing it to the example of a balloon and hair, where the hair remains attached to the balloon for a prolonged period.
“It’s like the example with the balloon and the hair, where the hair stays stuck to the balloon for quite a long time,” Buchner adds. “Typically, electric motor-driven robots need heat management, which requires additional heat sinks or fans for diffusing the heat to the air. Our system doesn’t require them,” Toshihiko Fukushima says.
The robotic leg’s capacity to jump explosively is based on its ability to lift its own weight. The researchers also demonstrated the robotic leg’s high adaptability, which is particularly crucial for soft robotics. Katzschmann explained that sufficient elasticity in the musculoskeletal system is essential for flexible adaptation to different terrains. He emphasized that similar to living creatures, navigating uneven surfaces becomes significantly more challenging if the knees cannot bend, such as when stepping down from pavement onto the road.
In contrast to electric motors, which need sensors to constantly determine the robotic leg’s angle, the artificial muscle adjusts to the appropriate position through environmental interaction. Fukushima clarified that it is driven by just two input signals: one for bending the joint and one for extending it.
Fukushima explains: “Adapting to the terrain is a key aspect. When a person lands after jumping into the air, they don’t have to think in advance about whether they should bend their knees at a 90-degree or a 70-degree angle.”
The same principle applies to the robotic leg’s musculoskeletal system, as upon landing, the leg joint adaptively moves into a suitable angle based on the hardness or softness of the surface.
The field of electrohydraulic actuators is relatively new, having emerged just six years ago. It’s essential to recognize the potential for disruptive innovation that comes from introducing new hardware concepts, such as the use of artificial muscles. While electrohydraulic actuators may not be suitable for heavy machinery on construction sites, they do offer specific advantages over standard electric motors, especially in applications such as grippers.
“Compared to walking robots with electric motors, our system is still limited. The leg is currently attached to a rod, jumps in circles, and can’t yet move freely,” said Robert Katzschmann at ETH Zurich.
Future work should focus on overcoming these limitations to develop real walking robots with artificial muscles. “If we combine the robotic leg in a quadruped robot or a humanoid robot with two legs, maybe one day, when it is battery-powered, we can deploy it as a rescue robot.”
Journal reference:
- Thomas J. K. Buchner, Toshihiko Fukushima, Amirhossein Kazemipour, Stephan-Daniel Gravert, Manon Prairie, Pascal Romanescu, Philip Arm, Yu Zhang, Xingrui Wang, Steven L. Zhang, Johannes Walter, Christoph Keplinger & Robert K. Katzschmann. Electrohydraulic musculoskeletal robotic leg for agile, adaptive, yet energy-efficient locomotion. Nature Communications, 2024; DOI: 10.1038/s41467-024-51568-3